[Federal Register Volume 77, Number 100 (Wednesday, May 23, 2012)]
[Proposed Rules]
[Pages 30765-30818]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2012-12212]
[[Page 30765]]
Vol. 77
Wednesday,
No. 100
May 23, 2012
Part III
Department of Transportation
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National Highway Traffic Safety Administration
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49 CFR Part 571
Federal Motor Vehicle Safety Standards; Electronic Stability Control
Systems for Heavy Vehicles; Proposed Rule
Federal Register / Vol. 77 , No. 100 / Wednesday, May 23, 2012 /
Proposed Rules
[[Page 30766]]
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DEPARTMENT OF TRANSPORTATION
National Highway Traffic Safety Administration
49 CFR Part 571
[Docket No. NHTSA-2012-0065]
RIN 2127-AK97
Federal Motor Vehicle Safety Standards; Electronic Stability
Control Systems for Heavy Vehicles
AGENCY: National Highway Traffic Safety Administration (NHTSA),
Department of Transportation (DOT).
ACTION: Notice of proposed rulemaking (NPRM).
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SUMMARY: This document proposes to establish a new Federal Motor
Vehicle Safety Standard No. 136 to require electronic stability control
(ESC) systems on truck tractors and certain buses with a gross vehicle
weight rating of greater than 11,793 kilograms (26,000 pounds). ESC
systems in truck tractors and large buses are designed to reduce
untripped rollovers and mitigate severe understeer or oversteer
conditions that lead to loss of control by using automatic computer-
controlled braking and reducing engine torque output.
In 2012, we expect that about 26 percent of new truck tractors and
80 percent of new buses affected by this proposed rule will be equipped
with ESC systems. We believe that ESC systems could prevent 40 to 56
percent of untripped rollover crashes and 14 percent of loss-of-control
crashes. By requiring that ESC systems be installed on truck tractors
and large buses, this proposal would prevent 1,807 to 2,329 crashes,
649 to 858 injuries, and 49 to 60 fatalities at less than $3 million
per equivalent life saved, while generating positive net benefits.
DATES: Comments: Submit comments on or before August 21, 2012.
Public Hearing: NHTSA will hold a public hearing in the summer of
2012. NHTSA will announce the date for the hearing in a supplemental
Federal Register document. The agency will accept comments to the
rulemaking at this hearing.
ADDRESSES: You may submit comments electronically [identified by DOT
Docket Number NHTSA-2012-0065] by visiting the following Web site
Federal eRulemaking Portal: Go to http://www.regulations.gov. Follow the online instructions for submitting
comments.
Alternatively, you can file comments using the following methods:
Mail: Docket Management Facility: U.S. Department of
Transportation, 1200 New Jersey Avenue SE., West Building Ground Floor,
Room W12-140, Washington, DC 20590-0001
Hand Delivery or Courier: West Building Ground Floor, Room
W12-140, 1200 New Jersey Avenue SE., between 9 a.m. and 5 p.m. ET,
Monday through Friday, except Federal holidays.
Fax: (202) 493-2251
Instructions: For detailed instructions on submitting comments and
additional information on the rulemaking process, see the Public
Participation heading of the SUPPLEMENTARY INFORMATION section of this
document. Note that all comments received will be posted without change
to http://www.regulations.gov, including any personal information
provided. Please see the Privacy Act heading below.
Privacy Act: Anyone is able to search the electronic form of all
comments received into any of our dockets by the name of the individual
submitting the comment (or signing the comment, if submitted on behalf
of an association, business, labor union, etc.). You may review DOT's
complete Privacy Act Statement in the Federal Register published on
April 11, 2000 (65 FR 19477-78).
Docket: For access to the docket to read background documents or
comments received, go to http://www.regulations.gov. Follow the online
instructions for accessing the dockets.
FOR FURTHER INFORMATION CONTACT: For technical issues, you may contact
George Soodoo, Office of Crash Avoidance Standards, by telephone at
(202) 366-4931, and by fax at (202) 366-7002. For legal issues, you may
contact David Jasinski, Office of the Chief Counsel, by telephone at
(202) 366-2992, and by fax at (202) 366-3820. You may send mail to both
of these officials at the National Highway Traffic Safety
Administration, 1200 New Jersey Avenue SE., Washington, DC 20590.
SUPPLEMENTARY INFORMATION:
Table of Contents
I. Executive Summary
II. Safety Problem
A. Heavy Vehicle Crash Problem
B. Contributing Factors in Rollover and Loss-of-Control Crashes
C. NTSB Safety Recommendations
D. Motorcoach Safety Plan
E. International Regulation
III. Stability Control Technologies
A. Dynamics of a Rollover
B. Description of RSC System Functions
C. Description of ESC System Functions
D. How ESC Prevents Loss of Control
E. Situations in Which Stability Control Systems May Not Be
Effective
F. Difference in Vehicle Dynamics Between Light Vehicles and
Heavy Vehicles
IV. Research and Testing
A. UMTRI Study
B. Simulator Study
C. NHTSA Track Testing
1. Effects of Stability Control Systems--Phase I
2. Developing a Dynamic Test Maneuver and Performance Measure To
Evaluate Roll Stability--Phase II
(a) Test Maneuver Development
(b) Performance Measure Development
3. Developing a Dynamic Test Maneuver and Performance Measure To
Evaluate Yaw Stability--Phase III
(a) Test Maneuver Development
(b) Performance Measure Development
4. Large Bus Testing
D. Truck & Engine Manufacturers Association Testing
1. Slowly Increasing Steer Maneuver
2. Ramp Steer Maneuver
3. Sine With Dwell Maneuver
4. Ramp With Dwell Maneuver
5. Vehicle J Testing
(a) EMA Testing of Vehicle J
(b) NHTSA Testing of EMA's Vehicle J
E. Other Industry Research
1. Decreasing Radius Test
2. Lane Change on a Large Diameter Circle
3. Yaw Control Tests
V. Agency Proposal
A. NHTSA's Statutory Authority
B. Applicability
1. Vehicle Types
2. Retrofitting In-Service Truck Tractors, Trailers, and Buses
3. Exclusions From Stability Control Requirement
C. ESC System Capabilities
1. Choosing ESC vs. RSC
2. Definition of ESC
D. ESC Disablement
E. ESC Malfunction Detection, Telltale, and Activation Indicator
1. ESC Malfunction Detection
2. ESC Malfunction Telltale
3. ESC Activation Indicator
F. Performance Requirements and Compliance Testing
1. Characterization Test--SIS
2. Roll and Yaw Stability Test--SWD
(a) Roll Stability Performance
(b) Yaw Stability Performance
(c) Lateral Displacement
3. Alternative Test Maneuvers Considered
(a) Characterization Maneuver
(b) Roll Stability Test Maneuvers
(c) Yaw Stability Test Maneuvers
(d) Lack of an Understeer Test
4. ESC Malfunction Test
5. Test Instrumentation and Equipment
(a) Outriggers
(b) Automated Steering Machine
(c) Anti-Jackknife Cables
(d) Control Trailer
(e) Sensors
6. Test Conditions
(a) Ambient Conditions
(b) Road Test Surface
(c) Vehicle Test Weight
(d) Tires
(e) Mass Estimation Drive Cycle
(f) Brake Conditioning
7. Data Filtering and Post Processing
G. Compliance Dates and Implementation Schedule
VI. Benefits and Costs
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A. System Effectiveness
B. Target Crash Population
C. Benefits Estimate
D. Cost Estimate
E. Cost Effectiveness
F. Comparison of Regulatory Alternatives
VII. Public Participation
VIII. Regulatory Analyses and Notices
A. Executive Order 12866, Executive Order 13563, and DOT
Regulatory Policies and Procedures
B. Regulatory Flexibility Act
C. Executive Order 13132 (Federalism)
D. Executive Order 12988 (Civil Justice Reform)
E. Protection of Children From Environmental Health and Safety
Risks
F. Paperwork Reduction Act
G. National Technology Transfer and Advancement Act
H. Unfunded Mandates Reform Act
I. National Environmental Policy Act
J. Plain Language
K. Regulatory Identifier Number (RIN)
L. Privacy Act
I. Executive Summary
The agency proposes to reduce rollover and loss of directional
control of truck tractors and large buses by establishing a new
standard, Federal Motor Vehicle Safety Standard (FMVSS) No. 136,
Electronic Stability Control Systems for Heavy Vehicles. The standard
would require truck tractors and certain buses \1\ with a gross vehicle
weight rating (GVWR) of greater than 11,793 kilograms (26,000 pounds)
to be equipped with an electronic stability control (ESC) system that
meets the equipment and performance criteria of the standard. ESC
systems use engine torque control and computer-controlled braking of
individual wheels to assist the driver in maintaining control of the
vehicle and maintaining its heading in situations in which the vehicle
is becoming roll unstable (i.e., wheel lift potentially leading to
rollover) or experiencing loss of control (i.e., deviation from
driver's intended path due to understeer, oversteer, trailer swing or
any other yaw motion leading to directional loss of control). In such
situations, intervention by the ESC system can assist the driver in
maintaining control of the vehicle, thereby preventing fatalities and
injuries associated with vehicle rollover or collision. Based on the
agency's estimates regarding the effectiveness of ESC systems, we
believe that an ESC standard could annually prevent 1,807 to 2,329
crashes, 649 to 858 injuries, and 49 to 60 fatalities, while providing
net economic benefits.
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\1\ As explained later in this notice, the applicability of this
proposed standard to buses would be similar to the applicability of
NHTSA's proposal to require seat belts on certain buses. These buses
would have 16 or more designated seating positions (including the
driver), at least 2 rows of passenger seats that are rearward of the
driver's seating position and forward-facing or can convert to
forward-facing without the use of tools. As with the seat belt NPRM,
this proposed rule would exclude school buses and urban transit
buses sold for operation as a common carrier in urban transportation
along a fixed route with frequent stops.
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There have been two types of stability control systems developed
for heavy vehicles. A roll stability control (RSC) system is designed
to prevent rollover by decelerating the vehicle using braking and
engine torque control. The other type of stability control system is
ESC, which includes all of the functions of an RSC system plus the
ability to mitigate severe oversteer or understeer by automatically
applying brake force at selected wheel-ends to help maintain
directional control of a vehicle. To date, ESC and RSC systems for
heavy vehicles have been developed for air-braked vehicles. Truck
tractors and buses covered by this proposed rule make up a large
proportion of air-braked heavy vehicles and a large proportion of the
heavy vehicles involved in both rollover crashes and total crashes.
Based on information we have received to date, the agency has
tentatively determined that ESC and RSC systems are not available for
hydraulic-braked medium or heavy vehicles.
Since 2006, the agency has been involved in testing truck tractors
and large buses with stability control systems. To evaluate these
systems, NHTSA sponsored studies of crash data in order to examine the
potential safety benefits of stability control systems. NHTSA and
industry representatives separately evaluated data on dynamic test
maneuvers. At the same time, the agency launched a three-phase testing
program to improve its understanding of how stability control systems
in truck tractors and buses work and to develop dynamic test maneuvers
to challenge roll propensity and yaw stability. By combining the
studies of the crash data with the testing data, the agency is able to
evaluate the potential effectiveness of stability control systems for
truck tractors and large buses.
As a result of the data analysis research, we have tentatively
determined that ESC systems can be 28 to 36 percent effective in
reducing first-event untripped rollovers and 14 percent effective in
eliminating loss-of-control crashes caused by severe oversteer or
understeer conditions.\2\ As a result of the agency's testing program
and the test data received from industry, the agency was able to
develop reliable and repeatable test maneuvers that could demonstrate a
stability control system's ability to prevent rollover and loss of
directional control among the varied configurations of truck tractors
and buses in the fleet.
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\2\ See Wang, Jing-Shiam, ``Effectiveness of Stability Control
Systems for Truck Tractors'' (January 2011) (DOT HS 811 437); Docket
No. NHTSA-2010-0034-0043.
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In order to realize these benefits, the agency is proposing to
require new truck tractors and certain buses with a GVWR of greater
than 11,793 kilograms (26,000 pounds) to be equipped with an ESC
system. This proposal is made pursuant to the authority granted to
NHTSA under the National Traffic and Motor Vehicle Safety Act (``Motor
Vehicle Safety Act''). Under 49 U.S.C. Chapter 301, Motor Vehicle
Safety (49 U.S.C. 30101 et seq.), the Secretary of Transportation is
responsible for prescribing motor vehicle safety standards that are
practicable, meet the need for motor vehicle safety, and are stated in
objective terms. The responsibility for promulgation of Federal motor
vehicle safety standards is delegated to NHTSA.
This proposal requires ESC system must meet both definitional
criteria and performance requirements. It is necessary to include
definitional criteria in the proposal and require compliance with them
because developing separate performance tests to cover the wide array
of possible operating ranges, roadways, and environmental conditions
would be impractical. The definitional criteria are consistent with
those recommended by SAE International and used by the United Nations
(UN) Economic Commission for Europe (ECE), and similar to the
definition of ESC in FMVSS No. 126, the agency's stability control
standard for light vehicles. This definition would describe an ESC
system as one that would enhance the roll and yaw stability of a
vehicle using a computer-controlled system that can receive inputs such
as the vehicle's lateral acceleration and yaw rate, and use the
information to apply brakes individually, including trailer brakes, and
modulate engine torque.
The proposal requires that the system be able to detect a
malfunction and provide a driver with notification of a malfunction by
means of a telltale. This requirement would be similar to the
malfunction detection and telltale requirements for light vehicles in
FMVSS No. 126. An ESC system on/off switch is allowed for light
vehicles; however, there is no provision in this proposal for allowing
an ESC system to be deactivated. For truck tractors and large buses, we
do not believe such controls are necessary.
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After considering and evaluating several test maneuvers, the agency
is proposing to use two test maneuvers for performance testing: The
slowly increasing steer (SIS) maneuver and the sine with dwell (SWD)
maneuver. The SIS maneuver is a characterization maneuver used to
determine the relationship between a vehicle's steering wheel angle and
the lateral acceleration. This test serves both to normalize the
severity of the SWD maneuver and to ensure that the system has the
ability to reduce engine torque. The SIS maneuver is performed by
driving at a constant speed of 48 km/h (30 mph), and then increasing
the steering wheel angle at a constant rate of 13.5 degrees per second
until ESC system activation occurs. Using linear regression followed by
extrapolation, the steering wheel angle that would produce a lateral
acceleration of 0.5g is determined.
Using the steering wheel angle derived from the SIS maneuver, the
agency would conduct the sine with dwell maneuver. The SWD test
maneuver challenges both roll and yaw stability by subjecting the
vehicle to a sinusoidal input. To conduct the SWD maneuver, the vehicle
is accelerated to 72 km/h (45 mph) and then turned in a clockwise or
counterclockwise direction to reach a set steering wheel angle in 0.5
seconds. The steering wheel is then turned in the opposite direction
until the same steering wheel angle is reached in the opposite
direction in one second. The steering wheel is then held at that
steering wheel angle for one second, and then the steering wheel angle
returned to zero degrees within 0.5 seconds. This maneuver would be
repeated for two series of test runs (first in the counterclockwise
direction and then in the clockwise direction) at several target
steering wheel angles from 30 to 130 percent of the angle derived in
the SIS maneuver.
The lateral acceleration, yaw rate, and engine torque data from the
test runs would be measured, recorded, and processed to determine the
four performance metrics: Lateral acceleration ratio (LAR), yaw rate
ratio (YRR), lateral displacement, and engine torque reduction. The LAR
and YRR metrics would be used to ensure that the system reduces lateral
acceleration and yaw rate, respectively, after an aggressive steering
input, thereby preventing rollover and loss of control, respectively.
These two metrics can effectively measure what NHTSA's testing has
found to be the threshold of stability. The lateral displacement metric
would be used to ensure that the stability control system is not set to
intervene solely by making the vehicle nonresponsive to driver input.
The engine torque reduction metric would be used to ensure that the
system has the capability to automatically reduce engine torque in
response to high lateral acceleration and yaw rate conditions. The
manner in which the data would be filtered and processed is described
in this proposal.
The agency considered several test maneuvers based on its own work
and that of industry. In particular, the agency's initial research
focused on a ramp steer maneuver (RSM) for evaluating roll stability.
In that maneuver, a vehicle is driven at a constant speed and a
steering wheel input that is based on the steering wheel angle derived
from the SIS maneuver is input. The steering wheel angle is then held
for a period of time before it is returned to zero. A stability control
system would act to reduce lateral acceleration, and thereby wheel lift
and roll instability, by applying selective braking. A vehicle without
a stability control system would maintain high levels of lateral
acceleration and potentially experience wheel lift or rollover.
The proposed rule also sets forth the test conditions that the
agency would use to ensure safety and demonstrate sufficient
performance. All vehicles would be tested using outriggers for the
safety of the test driver. The agency would use an automated steering
controller to ensure reproducible and repeatable test execution
performance. Truck tractors would be tested with an unbraked control
trailer to eliminate the effect of the trailer's brakes on testing.
Because the agency tests new vehicles, the brakes would be conditioned,
as they are in determining compliance with the air brake standard. The
agency would also test to ensure that system malfunction is detected.
This proposed rule would take effect for most truck tractors and
covered buses produced two years after publication of a final rule. We
believe that this amount of lead time is necessary to ensure sufficient
availability of stability control systems from suppliers of these
systems and to complete necessary engineering on all vehicles. For
three-axle tractors with one drive axle, tractors with four or more
axles, and severe service tractors, we would provide two years
additional lead time. We believe this additional time is necessary to
develop, test, and equip these vehicles with ESC systems. Although the
agency has statutory authority to require retrofitting of in-service
truck tractors, trailers, and large buses, the agency is not proposing
to do so, given the integrated aspects of a stability control system.
Based on the agency's effectiveness estimates, the adoption of this
proposal would prevent 1,807 to 2,329 crashes per year resulting in 649
to 858 injuries and 49 to 60 fatalities. The proposal also would result
in significant monetary savings as a result of prevention of property
damage and travel delays.
Based on information obtained from manufacturers, the agency
estimates that 26.2 percent of truck tractors manufactured in model
year 2012 will be equipped with an ESC system and that 80 percent of
covered buses manufactured in model year 2012 will be equipped with an
ESC system. Information obtained from manufacturers indicates that the
average unit cost of an ESC system is approximately $1,160. In
addition, 16.5 percent of truck tractors manufactured in model year
2012 will be equipped with an RSC system. The incremental cost of
installing an ESC system in place of an RSC system is estimated to be
$520 per vehicle. Based upon the agency's estimates that 150,000 truck
tractors and 2,200 buses covered by this proposed rule will be
manufactured in 2012, the agency estimates that the total cost of this
proposal would be approximately $113.6 million.
The agency believes that this proposal is cost effective. The net
benefits of this proposal are estimated to range from $228 to $310
million at a 3 percent discount rate and from $155 to $222 million at a
7 percent discount rate. As a result, the net cost per equivalent live
saved from this proposal ranges from $1.5 to $2.0 million at a 3
percent discount rate and from $2.0 to $2.6 million at a 7 percent
discount rate. The costs and benefits of this proposal are summarized
in Table 1.
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Table 1--Estimated Annual Cost, Benefits, and Net Benefits of the Proposal
[In millions of 2010 dollars]
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Property damage Cost per
Costs Injury and travel delay equivalent Net benefits
benefits savings live saved
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At 3% Discount............... $113.6 $328-405 $13.9-17.8 $1.5-2.0 $228-310
At 7% Discount............... 113.6 257-322 11.0-14.1 2.0-2.6 155-222
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The agency considered two regulatory alternatives. First, the
agency considered requiring truck tractors and large buses to be
equipped with RSC systems. When compared to this proposal, RSC systems
would result in slightly lower cost per equivalent life saved, but
would produce net benefits that are lower than the net benefits from
this proposal. This is because RSC systems are less effective at
preventing rollover crashes and much less effective at preventing loss-
of-control crashes. The second alterative considered was requiring
trailers to be equipped with RSC systems. However, this alternative
would save fewer than 10 lives at a very high cost per equivalent life
saved and would provide negative net benefits.
The remainder of this notice will describe in detail the following:
(1) The size of the safety problem to be addressed by this proposed
rule; (2) how stability control systems work to prevent rollover and
loss of control; (3) the research and testing separately conducted by
NHTSA and industry to evaluate the potential effectiveness of a
stability control requirement and to develop dynamic test maneuvers to
challenge system performance; (4) the specifics of the agency's
proposal, including equipment and performance criteria, compliance
testing, and the implementation schedule; and (5) the benefits and
costs of this proposal.
II. Safety Problem
A. Heavy Vehicle Crash Problem
The Traffic Safety Facts 2009 reports that tractor trailer
combination vehicles are involved in about 72 percent of the fatal
crashes involving large trucks, annually.\3\ According to FMCSA's Large
Truck and Bus Crash Facts 2008, these vehicles had a fatal crash
involvement rate of 1.92 crashes per 100 million vehicle miles traveled
during 2007, whereas single unit trucks had a fatal crash involvement
rate of 1.26 crashes per 100 million vehicle miles traveled.\4\
Combination vehicles represent about 25 percent of large trucks
registered but travel 63 percent of the large truck miles, annually.
Traffic tie-ups resulting from loss-of-control and rollover crashes
also contribute to in millions of dollars of lost productivity and
excess energy consumption each year.
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\3\ DOT HS 811 402, available at http://www-nrd.nhtsa.dot.gov/Pubs/811402.pdf (last accessed May 9, 2012).
\4\ FMCSA-RRA-10-043 (Mar. 2010), available at http://www.fmcsa.dot.gov/facts-research/ltbcf2008/index-2008largetruckandbuscrashfacts.aspx (last accessed May 9, 2012).
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According to Traffic Safety Facts 2009, the overall crash problem
for tractor trailer combination vehicles is approximately 150,000
crashes, 29,000 of which involve injury. The overall crash problem for
single-unit trucks is nearly as large--approximately 146,000 crashes,
24,000 of which are injury crashes. However, the fatal crash
involvement for truck tractors is much higher. In 2009, there were
2,334 fatal combination truck crashes and 881 fatal single-unit truck
crashes.
The rollover crash problem for combination trucks is much greater
than for single-unit trucks. In 2009, there were approximately 7,000
crashes involving combination truck rollover and 3,000 crashes
involving single-unit truck rollover. As a percentage of all crashes,
combination trucks are involved in rollover crashes at twice the rate
of single-unit trucks. Approximately 4.4 percent of all combination
truck crashes were rollovers, but 2.2 percent of single-unit truck
crashes were rollovers. Combination trucks were involved in 3,000
injury crashes and 268 fatal crashes, and single-unit trucks were
involved in 2,000 injury crashes and 154 fatal crashes.
According to FMCSA's Large Truck and Bus Crash Facts 2008, cross-
country intercity buses were involved in 19 of the 247 fatal bus
crashes in 2008, which represented about 0.5 percent of the fatal
crashes involving large trucks and buses, annually. The bus types
presented in the crash data include school buses, intercity buses,
cross-country buses, transit buses, and other buses. These buses had a
fatal crash involvement rate of 3.47 crashes per 100 million vehicle
miles traveled during 2008. From 1998 to 2008, cross-country intercity
buses, on average, accounted for 12 percent of all buses involved in
fatal crashes, whereas transit buses and school buses accounted for 35
percent and 40 percent, respectively, of all buses involved in fatal
crashes. Most of the transit bus and school bus crashes are not
rollover or loss-of-control crashes that ESC systems are capable of
preventing. The remaining 13 percent of buses involved in fatal crashes
were classified as other buses or unknown. Fatal rollover and loss-of-
control crashes are a subset of these crashes.
There are many more fatalities in buses with a GVWR greater than
11,793 kg (26,000 lb) compared to buses with a GVWR between 4,536 kg
and 11,793 kg (10,000 lb and 26,000 lb).\5\ In the 10-year period
between 1999 and 2008, there were 34 fatalities on buses with a GVWR
between 4,536 kg and 11,793 kg (10,000 lb and 26,000 lb) compared to
254 fatalities on buses with a GVWR greater than 11,793 kg (26,000 lb).
Among buses with a GVWR of greater than 11,793 kg (26,000 lb), over 70
percent of the fatalities were cross-country intercity bus occupants.
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\5\ This data was taken from the FARS database and was presented
in the NPRM that would require seat belts on certain buses. See 75
FR 50,958, 50,917 (Aug. 18, 2010).
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Furthermore, the size of the rollover crash problem for cross-
country intercity buses is greater than in other buses. According to
FARS data from 1999 to 2008, there were 97 occupant fatalities as a
result of rollover events on cross-country intercity buses with a GVWR
of greater than 11,793 kg (26,000 lb), which represents 52 percent of
cross-country intercity bus fatalities.\6\ In comparison, rollover
crashes were responsible for 21 occupant fatalities on other buses with
a GVWR of greater than 11,793 kg (26,000 lb) and 9 occupant fatalities
on all buses with a GVWR between 4,536 kg and 11,793 kg (10,000 lb and
26,000 lb). That is, 95 percent of bus occupant rollover fatalities on
buses over 4,536 kg (10,000 lb) were occupants on buses with a GVWR of
over 11,793 kg (26,000 lb).
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\6\ See U.S. Department of Transportation Motorcoach Safety
Action Plan, DOT HS 811 177, at 13 (Nov. 2009), available at http://www.fmcsa.dot.gov/documents/safety-security/MotorcoachSafetyActionPlan_finalreport-508.pdf (last accessed May
9, 2012).
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[[Page 30770]]
B. Contributing Factors in Rollover and Loss-of-Control Crashes
Many factors related to heavy vehicle operation, as well as factors
related to roadway design and road surface properties, can cause heavy
vehicles to become yaw unstable or to roll. Listed below are several
real-world situations in which stability control systems may prevent or
lessen the severity of such crashes.
Speed too high to negotiate a curve--The entry speed of
vehicle is too high to safely negotiate a curve. When the lateral
acceleration of a vehicle during a steering maneuver exceeds the
vehicle's roll or yaw stability threshold, a rollover or loss of
control is initiated. Curves can present both roll and yaw instability
issues to these types of vehicles due to varying heights of loads (low
versus high, empty versus full) and road surface friction levels (e.g.,
wet, dry, icy, snowy).
Sudden steering maneuvers to avoid a crash--The driver
makes an abrupt steering maneuver, such as a single- or double-lane-
change maneuver, or attempts to perform an off-road recovery maneuver,
generating a lateral acceleration that is sufficiently high to cause
roll or yaw instability. Maneuvering a vehicle on off-road, unpaved
surfaces such as grass or gravel may require a larger steering input
(larger wheel slip angle) to achieve a given vehicle response, and this
can lead to a large increase in lateral acceleration once the vehicle
returns to the paved surface. This increase in lateral acceleration can
cause the vehicle to exceed its roll or yaw stability threshold.
Loading conditions--The vehicle yaw due to severe over-
steering is more likely to occur when a vehicle is in a lightly loaded
condition and has a lower center of gravity height than it would have
when fully loaded. Heavy vehicle rollovers are much more likely to
occur when the vehicle is in a fully loaded condition, which results in
a high center of gravity for the vehicle. Cargo placed off-center in
the trailer may result in the vehicle being less stable in one
direction than in the other. It is also possible that improperly
secured cargo can shift while the vehicle is negotiating a curve,
thereby reducing roll or yaw stability. Sloshing can occur in tankers
transporting liquid bulk cargoes, which is of particular concern when
the tank is partially full because the vehicle may experience
significantly reduced roll stability during certain maneuvers.
Road surface conditions--The road surface condition can
also play a role in the loss of control a vehicle experiences. On a
dry, high-friction asphalt or concrete surface, a tractor trailer
combination vehicle executing a severe turning maneuver is likely to
experience a high lateral acceleration, which may lead to roll or yaw
instability. A similar maneuver performed on a wet or slippery road
surface is not as likely to experience the high lateral acceleration
because of less available tire traction. Hence, the result is more
likely to be vehicle yaw instability than vehicle roll instability.
Road design configuration--Some drivers may misjudge the
curvature of ramps and not brake sufficiently to negotiate the curve
safely. This includes ramps with decreasing radius curves as well as
curves and ramps with improper signage. A decrease in super-elevation
(banking) at the end of a ramp where it merges with the roadway causes
an increase in vehicle lateral acceleration, which may increase even
more if the driver accelerates the vehicle in preparation to merge.
C. NTSB Safety Recommendations
The National Transportation Safety Board (NTSB) has issued several
safety recommendations relevant to ESC systems on heavy and other
vehicles. One is H-08-15, which addresses ESC systems and collision
warning systems with active braking on commercial vehicles.
Recommendations H-11-07 and H-11-08 specifically address stability
control systems on commercial motor vehicles and buses with a GVWR
above 10,000 pounds. Two other safety recommendations, H-01-06 and H-
01-07, relate to adaptive cruise control and collision warning systems
on commercial vehicles, and are indirectly related to ESC on heavy
vehicles because all these technologies require the ability to apply
brakes without driver input.
H-08-15: Determine whether equipping commercial vehicles
with collision warning systems with active braking \7\ and electronic
stability control systems will reduce commercial vehicle accidents. If
these technologies are determined to be effective in reducing
accidents, require their use on commercial vehicles.
---------------------------------------------------------------------------
\7\ Active braking involves using the vehicle's brakes to
maintain a certain, preset distance between vehicles.
---------------------------------------------------------------------------
H-11-07: Develop stability control system performance
standards for all commercial motor vehicles and buses with a gross
vehicle weight rating greater than 10,000 pounds, regardless of whether
the vehicles are equipped with a hydraulic or pneumatic brake system.
H-11-08: Once the performance standards from Safety
Recommendation H-11-07 have been developed, require the installation of
stability control systems on all newly manufactured commercial vehicles
with a GVWR greater than 10,000 pounds.
D. Motorcoach Safety Plan
In November 2009, the U.S. Department of Transportation Motorcoach
Safety Action Plan was issued.\8\ Among other things, the Motorcoach
Safety Action Plan includes an action item for NHTSA to assess the
safety benefits for stability control on large buses and develop
objective performance standards for these systems.\9\ Consistent with
that plan, NHTSA made a decision to pursue a stability control
requirement for large buses.
---------------------------------------------------------------------------
\8\ See supra, note 6.
\9\ Id. at 28-29.
---------------------------------------------------------------------------
In March 2011, NHTSA issued its latest Vehicle Safety and Fuel
Economy Rulemaking and Research Priority Plan (Priority Plan).\10\ The
Priority Plan describes the agency plans for rulemaking and research
for calendar years 2011 to 2013. The Priority Plan includes stability
control on truck tractors and large buses, and states that the agency
plans to develop test procedures for a Federal motor vehicle safety
standard on stability control for truck tractors, with the
countermeasures of roll stability control and electronic stability
control, which are aimed at addressing rollover and loss-of-control
crashes.
---------------------------------------------------------------------------
\10\ See Docket No. NHTSA-2009-0108-0032.
---------------------------------------------------------------------------
E. International Regulation
The United Nations (UN) Economic Commission for Europe (ECE)
Regulation 13, Uniform Provisions Concerning the Approval of Vehicles
of Categories M, N and O with Regard to Braking, has been amended to
include Annex 21, Special Requirements for Vehicles Equipped with a
Vehicle Stability Function. Annex 21's requirements apply to trucks
with a GVWR greater than 3,500 kg (7,716 lb), buses with a seating
capacity of 10 or more (including the driver), and trailers with a GVWR
greater than 3,500 kg (7,716 lb). Trucks and buses are required to be
equipped with a stability system that includes rollover control and
directional control, while trailers are required to have a stability
system that includes only rollover control. The directional control
function must be demonstrated in one of eight tests, and the rollover
control function must be demonstrated in one of two tests. For
[[Page 30771]]
compliance purposes, the ECE regulation requires a road test to be
performed with the function enabled and disabled, or as an alternative
accepts results from a computer simulation. No test procedure or pass/
fail criterion is included in the regulation, but it is left to the
discretion of the Type Approval Testing Authority in agreement with the
vehicle manufacturer to show that the system is functional. The
implementation date of Annex 21 is 2012 for most vehicles, with a
phase-in based on the vehicle type.
III. Stability Control Technologies
A. Dynamics of a Rollover
Whenever a vehicle is steered, the lateral forces that result from
the steering input lead to one of the following results: (1) Vehicle
maintains directional control; (2) vehicle loses directional control
due to severe understeer or plowing out; (3) vehicle loses directional
control due to severe oversteer or spinning out; or (4) vehicle
experiences roll instability and rolls over.
A turning maneuver initiated by the driver's steering input results
in a vehicle response that can be broken down into two phases. Phase 1
is the yaw response that occurs when the front wheels are turned. As
the steering wheel is turned, the displacement of the front wheels
generates a slip angle at the front wheels and a lateral force is
generated. That lateral force leads to vehicle rotation, and the
vehicle starts rotating about its center of gravity.
This rotation leads to Phase 2. In Phase 2, the vehicle's yaw
causes the rear wheels to experience a slip angle. That causes a
lateral force to be generated at the rear tires, which leads to vehicle
rotation. All of these actions establish a steady-state turn in which
lateral acceleration and yaw rate are constant.
In combination vehicles, which typically consist of a tractor
towing a semi-trailer, an additional phase is the turning response of
the trailer. Once the tractor begins to achieve a yaw and lateral
acceleration response, the trailer begins to yaw as well. This leads to
the trailer's tires developing slip angles and producing lateral forces
at the trailer tires. Thus, there is a slight delay in the turning
response of the trailer when compared to the turning response of the
tractor.
If the lateral forces generated at either the front or the rear
wheels exceed the friction limits between the road surface and the
tires, the result will be a vehicle loss-of-control in the form of
severe understeer (loss of traction at the steer tires) or severe
oversteer (loss of traction at the rear tires). In a combination
vehicle, a loss of traction at the trailer wheels would result in the
trailer swinging out of its intended path. However, if the lateral
forces generated at the tires result in a vehicle lateral acceleration
that exceeds the rollover threshold of the vehicle, then rollover will
result.
Lateral acceleration is the primary cause of rollovers. Figure 1
depicts a simplified rollover condition. As shown, when the lateral
force (i.e., lateral acceleration) is sufficient large and exceeds the
roll stability threshold of the tractor-trailer combination vehicle,
the vehicle will roll over. Many factors related to the drivers'
maneuvers, heavy vehicle loading conditions, vehicle handling
characteristics, roadway design, and road surface properties would
result in various lateral accelerations and influences on the rollover
propensity of a vehicle. For example, given other factors are equal, a
vehicle entering a curve at a higher speed is more likely to roll than
a vehicle entering the curve at a lower speed. Also, transporting a
high center of gravity (CG) load would increase the rollover
probability more than transporting a relatively lower CG load.
[GRAPHIC] [TIFF OMITTED] TP23MY12.002
Stability control technologies help a driver maintain directional
control and help to reduce roll instability. Two types of heavy vehicle
stability control technologies have been developed. One such technology
is roll stability control or RSC, which is designed to help prevent on-
road, untripped rollovers by automatically decelerating the vehicle
using brakes and engine control. The other technology is electronic
stability control, or ESC,\11\ which is designed to
[[Page 30772]]
assist the driver in mitigating severe oversteer or understeer
conditions by automatically applying selective brakes to help the
driver maintain directional control of the vehicle. On heavy vehicles,
ESC also includes the RSC function described above.
---------------------------------------------------------------------------
\11\ In light vehicles, the term ESC generally describes a
system that helps the driver maintain directional control and
typically does not include the RSC function because these vehicles
are much less prone to untripped rollover.
---------------------------------------------------------------------------
B. Description of RSC System Functions
Currently, RSC systems are available for air-braked tractors with a
GVWR of greater than 11,793 kilograms (26,000 pounds) and for trailers.
A tractor-based RSC system consists of an electronic control unit (ECU)
that is mounted on a vehicle and continually monitors the vehicle's
speed and lateral acceleration based on an accelerometer, and estimates
vehicle mass based on engine torque information.\12\ The ECU
continuously estimates the roll stability threshold of the vehicle,
which is the lateral acceleration above which a combination vehicle
will roll over. When the vehicle's lateral acceleration approaches the
roll stability threshold, the RSC system intervenes. Depending on how
quickly the vehicle is approaching the estimated rollover threshold,
the RSC system intervenes by one or more of the following actions:
Decreasing engine power, using engine braking, applying the tractor's
drive-axle brakes, or applying the trailer's brakes. When RSC systems
apply the trailer's brakes, they use a pulse modulation protocol to
prevent wheel lockup because tractor stability control systems cannot
currently detect whether or not the trailer is equipped with ABS. Some
RSC systems also use a steering wheel angle sensor, which allows the
system to identify potential roll instability events earlier.
---------------------------------------------------------------------------
\12\ RSC systems are not presently available for large buses.
---------------------------------------------------------------------------
An RSC system can reduce rollovers, but is not designed to help to
maintain directional control of a truck tractor. Nevertheless, RSC
systems may provide some additional ability to maintain directional
control in some scenarios, such as in a low-center-of-gravity scenario,
where an increase in a lateral acceleration may lead to yaw instability
rather than roll instability.
In comparison, a trailer-based RSC system has an ECU mounted on the
trailer, which typically monitors the trailer's wheel speeds, the
trailer's suspension to estimate the trailer's loading condition, and
the trailer's lateral acceleration. When a high lateral acceleration
that is likely to cause the trailer to rollover is detected, the ECU
commands application of the trailer brakes to slow the combination
vehicle. In this case, the trailer brakes on the outside wheels can be
applied with full pressure since the ECU can directly monitor the
trailer wheels for braking-related lockup. The system modulates the
brake pressure as needed to achieve maximum braking force without
locking the wheels. However, a trailer-based RSC system can only apply
the trailer brakes to slow a combination vehicle, whereas a tractor-
based RSC system can apply brakes on both the tractor and trailer.
C. Description of ESC System Functions
Currently, ESC systems are available for heavy vehicles, including
truck tractors and buses, equipped with air brakes. An ESC system
incorporates all of the inputs of an RSC system. In addition, an ESC
system monitors steering wheel angle and yaw rate of the vehicle.\13\
These system inputs are monitored by the system's ECU, which estimates
when the vehicle's directional response begins to deviate from the
driver's steering command, either by oversteer or understeer. An ESC
system intervenes to restore directional control by taking one or more
of the following actions: Decreasing engine power, using engine
braking, selectively applying the brakes on the truck tractor to create
a counter-yaw moment to turn the vehicle back to its steered direction,
or applying the brakes on the trailer. An ESC system enhances the RSC
functions because it has the added information from the steering wheel
angle and yaw rate sensors, as well as more braking power because of
its additional capability to apply the tractor's steer axle brakes.\14\
---------------------------------------------------------------------------
\13\ Because ESC systems must monitor steering inputs from the
tractor, ESC systems are not available for trailers.
\14\ This is a design strategy to avoid the unintended
consequences of applying the brakes on the steering axle without
knowing where the driver is steering the vehicle.
---------------------------------------------------------------------------
D. How ESC Prevents Loss of Control
Like an RSC system, an ESC system has a lateral acceleration
sensor. However, it also has two additional sensors to monitor a
vehicle for loss of directional control, which may result due to either
understeer or oversteer. The first additional sensor is a steering
wheel angle sensor, which senses the intended direction of a vehicle.
The other is a yaw rate sensor, which measures the actual turning
movement of the vehicle. When a discrepancy between the intended and
actual headings of the vehicle occurs, it is because the vehicle is in
either an understeering (plowing out) or an oversteering (spinning out)
condition. The ESC system responds to such a discrepancy by
automatically intervening and applying brake torque selectively at
individual wheel ends on the tractor, by reducing engine torque output
to the drive axle wheels, or by both means. If only the wheel ends at
one corner of the vehicle are braked, the uneven brake force will
create a correcting yaw moment that causes the vehicle's heading to
change. An ESC system also has the capability to reduce the engine
torque output to the drive wheels, which effectively reduces the
vehicle speed and helps the wheels to regain traction. This means of
intervention by the ESC system may occur separate from or simultaneous
with the automatic brake application at selective wheel ends. An ESC
system is further differentiated from an RSC system in that it has the
ability to selectively apply the front steer axle brakes while the RSC
system does not incorporate this feature.
Figure 2 illustrates the oversteering and understeering conditions.
While Figure 2 may suggest that a particular vehicle loses control due
to either oversteer or understeer, it is quite possible that a vehicle
could require both understeering and oversteering interventions during
progressive phases of a complex crash avoidance maneuver such as a
double lane change.
[[Page 30773]]
[GRAPHIC] [TIFF OMITTED] TP23MY12.003
Oversteering. The right side of Figure 2 shows that the truck
tractor whose driver has lost directional control during an attempt to
drive around a right curve. The rear wheels of the tractor have
exceeded the limits of road traction. As a result, the rear of the
tractor is beginning to slide. This would lead a vehicle without an ESC
system to spin out. If the tractor is towing a trailer, as the tractor
in the figure is, this would result in a jackknife crash. In such a
crash, the tractor spins and may make physical contact with the side of
the trailer. The oversteering tractor in this figure is considered to
be yaw-unstable because the tractor rotation occurs without a
corresponding increase in steering wheel angle by the driver. In a
vehicle equipped with ESC, the system immediately detects that the
vehicle's heading is changing more quickly than appropriate for the
driver's intended path (i.e., the yaw rate is too high). To counter the
leftward rotation of the vehicle, it momentarily applies the right
front brake, thus creating a rightward (clockwise) counter-rotational
force and turning the heading of the vehicle back to the correct path.
It will also cut engine power to gently slow the vehicle and, if
necessary, apply additional brakes (while maintaining the uneven brake
force to create the necessary yaw moment). The action happens quickly
so that the driver does not perceive the need for steering corrections.
Understeering. The left side of Figure 2 shows a truck tractor
whose driver has lost directional control during an attempt to drive
around a right curve, except that in this case, it is the front wheels
that have exceeded the limits of road traction. As a result, the
tractor is sliding at the front (``plowing out''). Such a vehicle is
considered to be yaw-stable because no increase in tractor rotation
occurs when the driver increases the steering wheel angle. However, the
driver has lost directional control of the tractor. In this situation,
the ESC system rapidly detects that the vehicle's heading is changing
less quickly than appropriate for the driver's intended path (i.e., the
yaw rate is too low). In other words, the vehicle is not turning right
sufficiently to remain on the right curve and is instead heading off to
the left. The ESC system momentarily applies the right rear brake,
creating a rightward rotational force, to turn the heading of the
vehicle back to the correct path. Again, it will also cut engine power
to gently slow the vehicle and, if necessary, apply additional brakes
(while maintaining the uneven brake force to create the necessary yaw
moment).
E. Situations in Which Stability Control Systems May Not Be Effective
A stability control system will not prevent all rollover and loss-
of-control crashes. A stability control system has the capability to
prevent many untripped on-road rollovers and first-event loss-of-
control events. Nevertheless, there are real-world situations in which
stability control systems may not be as effective in avoiding a
potential crash. Such situations include:
Off-road recovery maneuvers in which a vehicle departs the
roadway and encounters an incline too steep to effectively maneuver the
vehicle or an unpaved surface that significantly reduces the
predictability of the vehicle's handling
Entry speeds that are much too high for a curved roadway
or entrance/exit ramp
Cargo load shifts on the trailer during a steering
maneuver
Vehicle tripped by a curb or other roadside object or
barrier
Truck rollovers that are the result of collisions with
other motor vehicles
Inoperative antilock braking systems--the performance of
stability control systems depends on the proper functioning of ABS
Brakes that are out-of-adjustment or other defects or
malfunctions in the ESC, RSC, or brake system.
Maneuvers during tire tread separation or sudden tire
deflation events.
F. Difference in Vehicle Dynamics Between Light Vehicles and Heavy
Vehicles
On April 6, 2007, the agency published a final rule that
established FMVSS No. 126, Electronic Stability Control Systems, which
requires all passenger cars, multipurpose passenger vehicles, trucks
and buses with a GVWR of 4,536 kg (10,000 lb) or less to be equipped
with an electronic stability control system beginning in model year
2012.\15\ The rule also requires a phase-in of 55 percent, 75 percent,
and 95 percent of vehicles produced by each manufacturer during model
years 2009, 2010, and 2011, respectively, to be equipped with a
compliant ESC system. The system must be capable of applying brake
torques individually at all four wheels, and must comply with the
performance criteria established for stability and responsiveness when
subjected to the sine with dwell steering maneuver test.
---------------------------------------------------------------------------
\15\ 72 FR 17236.
---------------------------------------------------------------------------
For light vehicles, the focus of the FMVSS No. 126 is on addressing
yaw instability, which can assist the driver in preventing the vehicle
from leaving the roadway, thereby preventing fatalities and injuries
associated with crashes involving tripped rollover, which often occur
when light vehicles run off the road. The standard does not include any
equipment or performance requirements for roll stability.
The dynamics of light vehicles and heavy vehicles differ in many
respects. First, on light vehicles, the yaw stability threshold is
typically lower than the roll stability threshold. This means that a
light vehicle making a crash avoidance maneuver, such as a lane change
on a dry road, is more likely to reach its yaw stability threshold and
lose directional control before it reaches its roll stability threshold
and rolls over. On a heavy
[[Page 30774]]
vehicle, however, the roll stability threshold is lower than the yaw
stability threshold in most operating conditions, primarily because of
its higher center of gravity height.\16\ As a result, there is a
greater propensity for a heavy vehicle, particularly in a loaded
condition, to roll during a severe crash avoidance maneuver or when
negotiating a curve, than to become yaw unstable, as compared with
light vehicles.
---------------------------------------------------------------------------
\16\ One instance where a heavy vehicle's yaw stability
threshold might be higher than its roll stability threshold is in an
unloaded condition on a low-friction road surface.
---------------------------------------------------------------------------
Second, a tractor-trailer combination unit is comprised of a power
unit and one or more trailing units with one or more articulation
points. In contrast, although a light vehicle may occasionally tow a
trailer, a light vehicle is usually a single rigid unit. The tractor
and the trailer have different center of gravity heights and different
lateral acceleration threshold limits for rollover. A combination
vehicle rollover frequently begins with the trailer where the rollover
is initiated by trailer wheel lift. The trailer roll torque is
transmitted to the tractor through the vehicles' articulation point,
which subsequently leads to tractor rollover. In addition to the
trailer's loading condition, the trailer rollover threshold is also
related to the torsional stiffness of the trailer body. A trailer with
a low torsional stiffness, such as a flatbed open trailer, would
typically experience wheel lift earlier during a severe turning
maneuver than a trailer with a high torsional stiffness, such as a van
trailer. Hence, compared with a light vehicle, the roll dynamics of a
tractor trailer combination vehicle is a more complex interaction of
forces acting on the units in the combination, as influenced by the
maneuver, the loading condition, and the roadway.
Unlike with light vehicles, there is a large range of loading
scenarios possible for a given heavy vehicle, particularly for truck
tractors towing trailers. A tractor-trailer combination vehicle can be
operated empty, loaded to its maximum weight rating, or loaded anywhere
in between the two extremes. The weight of a fully loaded combination
vehicle is generally more than double that of the vehicle with an empty
trailer. Furthermore, the load's center of gravity height can vary over
a large range, which can have substantial effects on the dynamics of a
combination vehicle.
Third, due to greater length, mass, and mass moments of inertia of
heavy vehicles, they respond more slowly to steering inputs than do
light vehicles. The longer wheelbase of a heavy vehicle, compared with
a light vehicle, results in a slower response time, which gives the
stability control system the opportunity to intervene and prevent
rollovers.
Finally, the larger number of wheels on a heavy vehicle, as
compared to a light vehicle, results in making heavy vehicles less
likely to yaw on dry road surface conditions.
As a result of the differences in vehicle dynamics between light
vehicles and heavy vehicles, the requirements in FMVSS No. 126 for
light vehicle ESC systems cannot translate directly into requirements
for heavy vehicles. Nevertheless, many requirements in FMVSS No. 126
are pertinent to heavy vehicles because they do not relate to any
difference in vehicle dynamics between light vehicles and heavy
vehicles. For example, the ESC system malfunction detection and
telltale requirements already developed for light vehicles can be
translated to heavy vehicles.
IV. Research and Testing
NHTSA has been studying ways to prevent untripped heavy vehicle
rollovers for many years. In the mid-1990s, the agency sponsored the
development of a prototype roll stability advisor (RSA) system that
displayed information to the driver regarding the truck's roll
stability threshold and the peak lateral acceleration achieved during
cornering maneuvers. This was followed by a fleet operational test
sponsored by the Federal Highway Administration, under the Department
of Transportation's Intelligent Vehicle Initiative. The tractors were
equipped with a RSA system using an engine retarder, which was an early
configuration of an RSC system. As that test program was concluding,
industry developers of stability control systems began to add tractor
and trailer foundation braking capabilities to increase the
effectiveness these systems.
In 2006, the agency initiated a test program at the Vehicle
Research and Test Center (VRTC) to conduct track testing on RSC- and
ESC-equipped tractors and semitrailers. The initial testing focused
only on roll stability testing and provided comparative data on the
performance of the different stability control systems in several test
maneuvers. Subsequent testing focused on refining test maneuvers and
developing performance metrics suitable for a safety standard. The
agency studied a slowly increasing steer maneuver that would
characterize a tractor's steering system and verify the ability of a
tractor-based system to control engine torque. The agency also
developed a ramp steer maneuver to evaluate the roll stability
performance of a stability control system, and investigated a sine with
dwell maneuver to evaluate both yaw and roll stability performance. In
addition to tests conducted on combination unit trucks, the VRTC
research program included testing of three large buses equipped with
ESC using these test maneuvers. As part of the research at VRTC, the
agency also developed data collection and analysis methods to
characterize the performance of stability control systems.
NHTSA researchers began updating their vehicle dynamics simulation
programs to include a stability control model, and coordinated with
researchers at the National Advanced Driving Simulator (NADS) at the
University of Iowa to add stability control modeling capability to
their tractor trailer simulations. NHTSA sponsored a research program
with the NADS to evaluate potential RSC and ESC effectiveness in
several tractor-trailer driving scenarios involving potential rollover
and loss of control, using sixty professional truck drivers who were
recruited as test participants.
NHTSA purchased three tractors equipped with ESC or RSC systems for
testing: A Freightliner 6x4 \17\ tractor that had ESC as a production
option, a Sterling 4x2 tractor that had RSC as a production option, and
a Volvo 6x4 tractor that had ESC included as standard equipment. NHTSA
also obtained a RSC control unit that could be retrofitted on the
Freightliner 6x4 tractor so that it could be comparatively tested with
both ESC and RSC. The agency also purchased a Heil 9,200-gallon tanker
semitrailer that was equipped with a trailer-based RSC system, and
retrofitted a Fruehauf 53-foot van semitrailer with a trailer-based RSC
system. NHTSA also obtained three large buses equipped with stability
control systems: A 2007 MCI D4500 (MCI 1), a 2009 Prevost H3,
and a second 2007 MCI D4500 (MCI 2). The MCI buses were
equipped with a Meritor WABCO ESC system and the Prevost was equipped
with a Bendix ESC system.
---------------------------------------------------------------------------
\17\ The 6x4 description for a tractor represents the total
number of wheel positions (six) and the total number of wheel
positions that are driven (four), which means that the vehicle has
three axles with two of them being drive axles. Similarly, a 4x2
tractor has four wheel positions, two of which are driven, meaning
that the vehicle has two axles, one of which is a drive axle.
---------------------------------------------------------------------------
Although the manufacturers of truck tractors and large buses and
the suppliers of stability control systems have performed extensive
development
[[Page 30775]]
work to bring these systems to the market, there are few sources of
objective evaluations for testing on stability control systems in the
public domain beyond the research programs described above. The agency
coordinated with truck, bus, and stability control system manufacturers
throughout the VRTC test program so that industry organizations had the
opportunity to contribute additional test data and other relevant
information on test maneuvers that the agency could consider for use
during the research program. Potential maneuvers suggested by industry
included a decreasing radius test from the Truck & Engine Manufacturers
Association (EMA),\18\ a sinusoidal steering maneuver and a ramp with
dwell maneuver from Bendix, and a lane change maneuver (on a large
diameter circle) from Volvo.\19\ In late 2009, the EMA provided results
from their tests of the ramp steer, sine with dwell, and ramp with
dwell maneuvers to NHTSA. The agency evaluated these data from a
measures-of-performance perspective. EMA provided data in December 2010
discussing additional testing with the sine with dwell, J-turn, and a
wet-Jennite drive through maneuver. Additional details on these
research programs are included in the sections below.
---------------------------------------------------------------------------
\18\ EMA was formerly known as the Truck Manufacturers
Association (TMA). Many docket materials refer to EMA as TMA.
\19\ Presentations from briefings NHTSA had with EMA have been
included in the docket. See Docket Nos. NHTSA-2010-0034-0025 through
NHTSA-2010-0034-0031; Docket Nos. NHTSA-2010-0034-0041 and NHTSA-
2010-0034-0042. Research notes provided by EMA, Bendix, and Volvo
Trucks have also been included in the docket. See Docket Nos. NHTSA-
2010-0034-0032 through NHTSA-2010-0034-0040.
---------------------------------------------------------------------------
A. UMTRI Study
NHTSA sponsored a research program with Meritor WABCO and the
University of Michigan Transportation Research Institute (UMTRI) to
examine the potential safety effectiveness of stability control systems
for five-axle tractor-trailer combination vehicles. The systems
investigated included both RSC and ESC.\20\ The research results are
provided in the report ``Safety Benefits of Stability Control Systems
for Tractor-Semitrailers.'' A copy of this report has been included in
the docket.\21\
---------------------------------------------------------------------------
\20\ A similar study has been initiated with respect to straight
trucks over 10,000 pounds GVWR.
\21\ DOT HS 811 205 (Oct. 2009), Docket No. NHTSA-2010-0034-0006
---------------------------------------------------------------------------
The objectives of the study were: (1) To use the Large Truck Crash
Causation Study (LTCCS) to define typical pre-crash scenarios and
identify factors associated with loss-of-control and rollover crashes
for tractor-trailers; (2) to study the effectiveness of RSC and ESC in
a range of realistic scenarios through hardware-in-the-loop simulation
testing, and through case reviews by a panel of experts; (3) to apply
the results of this research to generate national estimates from the
Trucks Involved in Fatal Accidents (TIFA) and General Estimates System
(GES) crash databases of the safety benefits of RSC and ESC in
preventing tractor-trailer crashes; and (4) to review crash data from
2001 through 2007 from a large trucking fleet that had started
purchasing RSC on all of its new tractors starting in 2004, to
determine if there was an influence of this system on reducing crashes.
The LTCCS was a joint study undertaken by the Federal Motor Carrier
Safety Administration (FMCSA) and NHTSA, based on a sample of 963
crashes between April 2001 and December 2003 with a reported injury or
fatality involving 1,123 trucks with a GVWR over 10,000 pounds. The
LTCCS crash data formed the backbone for this study because of the high
quality and consistent detail contained in the case files. Included in
the LTCCS are categorical data, comprehensive narrative descriptions of
each crash, scene diagrams, and photographs of the vehicle and roadway
from various angles. This information allowed the researchers to
achieve a high level of understanding of the crash mechanics for
particular cases. The LTCCS was used to help develop the crash
scenarios for modeling (hardware-in-the-loop) performed as part of the
engineering analyses for this stability control project. In addition,
LTCCS cases of interest with respect to stability control systems were
also reviewed by a panel of three experts (two from UMTRI and one from
industry) to help estimate the safety benefits of RSC and ESC.
One method for assessing the safety benefits of vehicle
technologies is to analyze crash datasets containing data on the safety
performance of vehicles equipped with the subject technology. However,
because the deployment of the stability control technologies for large
trucks is still in its early stages, national crash databases do not
yet have sufficient cases that can be used to evaluate the safety
performance of stability control technology. Given this limitation,
this study used an indirect method to estimate the safety performance
of stability control technologies based on probable outcome estimates
derived from hardware-in-the-loop simulation, field test experience,
expert panel assessment, and crash data from trucking fleets.
UMTRI's study made several conclusions. First, identifying relevant
loss-of-control and rollover crashes within the national databases
proved a difficult task because the databases are developed for general
use and this project required very precise definitions of loss-of-
control and rollover (e.g., tripped versus untripped). Relying on the
general loss-of-control or rollover categories captures a wide range of
crashes, many of which cannot be prevented by the stability control
technology. Furthermore, many of the crashes involved vehicles that
were not equipped with ABS. Because ABS is now mandatory for the target
population of vehicles, the researchers had to factor in what effect
the presence of ABS on the vehicle may have reduced the likelihood of
or prevented the crash.
Second, the LTCCS was highly valuable in providing a greater level
of detail concerning rollover and loss-of-control crashes, which was
used to construct a number of relevant crash scenarios so that the
technical potential of the candidate RSC and ESC technologies could be
estimated systematically. However, the inability to determine with
confidence if a vehicle lost control and the lack of detailed
information on driver input and vehicle state placed limitations on the
ability to assess the potential for stability control technologies to
alter the outcome of a particular crash scenario. In contrast, for
rollover crashes, it was clear that rollover occurred. Tire marks and
road alignment provide strong evidence of the vehicle path and the
point of instability.
Third, UMTRI concluded that ESC systems would provide more overall
safety benefits than RSC systems. The difference between the estimated
effectiveness of RSC and ESC varied among crash scenarios. ESC systems
were slightly more effective at preventing rollovers than RSC systems
and much more effective at preventing loss-of-control crashes.
Finally, the safety benefits estimates derived from this study were
limited to five-axle tractor-trailer combination vehicles, which
constitute a majority of the national tractor fleet. However, the study
did not include benefits estimates for multi-trailer combinations or
for tractors not towing a trailer.
B. Simulator Study
NHTSA sponsored a research study with the University of Iowa to
study the effectiveness of heavy truck electronic stability control
systems in reducing jackknife and rollover incidents using the NADS-1
National Advanced Driving Simulator. The NADS-1 is a high-fidelity,
full motion driving simulator with a 360-degree visual display system
[[Page 30776]]
that is typically used for the study of driver behavior. Sixty
professional truck drivers were recruited to participate in the study.
The participants drove a typical tractor-semitrailer in five scenarios
designed to have a high potential for rollover or jackknife. The study
used the NADS heavy truck cab and vehicle dynamics model to simulate a
typical 6x4 tractor-trailer combination vehicle in a baseline (ABS-
only), RSC-equipped, and ESC-equipped configurations, using twenty
truck drivers per configuration. The purpose of the study was to
determine the effectiveness of both roll stability control and yaw
stability control systems, to demonstrate driver behavior while using
stability control systems, and to help NHTSA refine safety benefits
estimates for heavy truck stability technologies.\22\
---------------------------------------------------------------------------
\22\ The final report is available in the docket. ``Heavy Truck
ESC Effectiveness Study Using NADS'' (DOT HS 811 233, November
2009), Docket No. NHTSA-2010-0034-0007.
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The NADS truck model performance was compared with test track data
from VRTC. The test maneuver used was a ramp steer maneuver with a
steering wheel angle of 190 degrees and an angular steering rate of 175
degrees per second. The steering angle was held constant for five
seconds after reaching 190 degrees, and then returned to zero. Steering
inputs on the NADS were performed manually rather than by using an
automated steering machine. The RSM was performed in the NADS to both
the right and left directions to check for any simulation
abnormalities, and was performed for the baseline, RSC, and ESC test
conditions. Exact matching of values to the test track data was not
possible because the NADS model was developed by simulating the braking
properties of a Freightliner tractor while using the inertial
properties of a Volvo tractor. Also, the NADS was modeled with rigid
body tractor and trailer vehicle models that did not include the
torsional chassis compliance that is a variable in actual vehicles. The
result of the testing was that the NADS model tractor-semitrailer
experienced wheel lift at slightly lower speeds in the RSM in all three
conditions (baseline, RSC, and ESC) than in the VRTC track tests. An
additional comparison of VRTC track test data and the NADS ESC model
was performed for lane change maneuvers at 45 and 50 mph and showed
that the NADS ESC system responses closely matched the responses of the
actual test vehicle.
The maneuvering events used to assess the influence of ESC systems
consisted of lane incursion from the left side on a snow-covered road
and from the right side on a dry road surface, with each event
necessitating a sudden lane change to avoid collision. These events
provided a greater challenge for the stability control systems due to
the aggressive steering and braking inputs by the drivers. Neither
stability control system showed benefits in preventing rollover on the
dry road surface. ESC systems did provide improved vehicle control on
the snow-covered surface; however, two jackknife events still occurred
with the ESC system. A large number of jackknife events occurred on the
snow-covered surface with the RSC system (11 loss-of-control events in
20 runs) which may have been a result of the aggressive RSC braking
strategy found in the model interfering with the driver's ability to
maintain steering control of the tractor.
The NADS research study indicated that the RSC system showed a
statistically significant benefit in preventing rollovers on both
curves and exit ramps on dry, high-friction road surfaces. The tractors
equipped with RSC and ESC systems showed a benefit over the baseline
tractor in assisting drivers to avoid a jackknife on a low-friction
road surface and a rollover on a high-friction road surface when
encountering a directional change due roadway geometry. However, in
several instances the ESC system was found to activate at abnormally
high levels of lateral acceleration in a curve with a high-friction
road surface. Although the reason for this was not determined, there
may have been problems with the mass estimation algorithm or vehicle
parameter inaccuracies in the model.
C. NHTSA Track Testing
NHTSA researchers at VRTC in East Liberty, Ohio, initiated a test
program in 2006 to evaluate the performance of stability control
systems under controlled conditions on a test track, and to develop
objective test procedures and measures of performance that could form
the basis of a new FMVSS. Researchers tested three truck tractors, all
of which were equipped with an RSC or ESC system (one vehicle was
tested with both an RSC and ESC system), one trailer equipped with a
trailer-based RSC system, and three large buses equipped with an ESC
system. Additionally, the agency tested five baseline semi-trailers not
equipped with a stability control system, including an unbraked control
trailer that is used to conduct tractor braking tests as prescribed by
FMVSS No. 121, Air brake systems.
The testing was conducted in three phases. Phase I research focused
on understanding how stability control systems performed. Phase II
research focused on the development of a dynamic test maneuver to
evaluate the roll stability of tractor semitrailers and large buses.
Phase III research focused on the development of a dynamic test
maneuver to evaluate the yaw stability of truck tractors and large
buses.
The Phase I and II research results are documented in the report
``Tractor Semi-Trailer Stability Objective Performance Test Research--
Roll Stability.'' \23\ The Phase III research results for truck
tractors are documented in the report ``Tractor Semitrailer Stability
Objective Performance Test Research--Yaw Stability.'' \24\ The
information provided in sections IV.C.1, IV.C.2, and IV.C.3 below is
based on these two reports. The motorcoach research is documented in
the report ``Test Track Lateral Stability Performance of Motorcoaches
Equipped with Electronic Stability Control Systems.'' \25\ The
information in section IV.C.4 is based on this report.
---------------------------------------------------------------------------
\23\ DOT HS 811 467 (May 2011), Docket No. NHTSA-2010-0034-0009.
Results from Phase I are also summarized in the paper ``NHTSA's
Class 8 Truck-Tractor Stability Control Test Track Effectiveness''
(ESV 2009. Paper No. 09-0552). Docket No. NHTSA-2010-0034-0008.
\24\ Docket No. NHTSA-2010-0034-0046.
\25\ Docket No. NHTSA-2010-0034-0045.
---------------------------------------------------------------------------
1. Effects of Stability Control Systems--Phase I
The test vehicles used in Phase I included a 2006 Freightliner 6x4
tractor equipped with air disc brakes and a Meritor WABCO ESC system as
factory-installed options, a 2006 Volvo 6x4 tractor with S-cam drum
brakes and a Bendix ESC system included as standard equipment, and a
2000 Fruehauf 53-foot van trailer that was retrofitted with a Meritor
WABCO trailer-based RSC system. Tests were conducted by enabling and
disabling the stability control systems on the tractor and the trailer
to compare the individual performance of each system, evaluate the
performance of the combined tractor and trailer stability control
systems, establish the baseline performance of each tractor-trailer
combination without any stability control system. All tests were
conducted with the tractor connected to the trailer, in either the
unloaded condition (lightly loaded vehicle weight (LLVW)) or loaded to
a 80,000 pound combination weight with the ballast located to produce
either a low or high center of gravity height (low CG or high CG)
loading condition. During testing, all
[[Page 30777]]
combination vehicles were equipped with outriggers.
The first test maneuver evaluated in Phase I was a constant radius
circle test (either a 150 foot or a 200 foot radius) conducted on dry
pavement. In this constant radius circle test, the driver maintained
the vehicle on the curved path while slowly increasing the vehicle
speed until the stability control system activated, wheel lift
occurred, or the tractor experienced a severe understeer condition.
With the stability control systems disabled, no cases of wheel lift
were observed under the LLVW or low CG condition. Under these load
conditions, both tractors went into a severe understeer condition. The
LLVW tractor did not reach a velocity greater than 40 mph and the low
CG tractor did not reach a velocity greater than 34 mph. However, in
the high CG condition with the tractor ESC systems disabled, wheel lift
occurred in every test that resulted in a lateral acceleration greater
than 0.45g at 30 mph.
With the tractor ESC systems enabled, the performance of the two
ESC-equipped vehicles improved during the constant radius tests. Both
ESC systems limited the maximum lateral acceleration of the tractor by
reducing the engine output torque and prevented wheel lift and severe
tractor understeer with the different loads tested. With ESC systems
enabled, both tractors tested allowed higher maximum lateral
accelerations for the LLVW condition compared to the low CG and high CG
conditions. There was little difference in peak lateral acceleration
for the low CG and high CG conditions.
The trailer-based RSC system limited the maximum lateral
acceleration by applying the trailer brakes, which mitigated wheel lift
and understeer with the different loads tested. The maximum lateral
acceleration of both tractors was limited by the trailer RSC system to
below 0.50g for the LLVW condition, 0.40g to 0.50g for the low CG
condition, and 0.35g to 0.40g for the high CG condition.
When both tractor- and trailer-based stability control systems were
enabled, results were similar to the results of the tractor-based
stability control system for the low CG and high CG conditions. Under
the LLVW condition, results were similar to the trailer-based RSC
system values observed.
The second maneuver evaluated in Phase I was a J-turn, also
conducted on dry pavement, in which the test driver accelerated the
vehicle to a constant speed in a straight lane and then negotiated 180
degrees of arc along a 150-foot radius curve. The initial maneuver
entrance speed was 20 mph and it was incrementally increased in
subsequent runs, until a test termination condition was reached. The
test terminated upon the occurrence of one of the following: The
trailer outriggers making contact with the ground, indicating that
wheel lift was occurring; the tractor experiencing a severe understeer
condition; a stability control system brake activating; or the maneuver
entry speed reaching 50 mph.
For both tractors in the baseline configuration (stability control
disabled), trailer wheel lift occurred in all load combinations except
for the Freightliner in the LLVW condition, which went into a severe
understeer condition at a maneuver entry speed of 50 mph. For the Volvo
in the LLVW load condition, trailer wheel lift was observed when the
tractor's maximum lateral acceleration exceeded 0.75g at 48 mph. With
stability control disabled in the low CG load condition, trailer wheel
lift was observed when the tractor's maximum lateral acceleration was
greater than 0.67g at 40 mph for the Freightliner and 0.60g at 38 mph
for the Volvo. For the high CG load condition, trailer wheel lift was
observed when the tractor's maximum lateral acceleration was
approximately 0.45g at 33 mph for the Freightliner and 0.42g at 31 mph
for the Volvo.
Tractor ESC systems limited the maximum lateral acceleration for
both the tractor and the trailer. Wheel lift was not observed for the
range of speeds evaluated. For both tractors tested in the low CG and
high CG loading conditions, the tractor's ESC intervened at a speed
that was well below the speed that would produce trailer wheel lift.
With the trailer in the LLVW load condition, the tractor's maximum
lateral acceleration was limited to approximately 0.60g for the
Freightliner and the Volvo. With the trailer tested in either the low
CG or high CG load conditions, the tractor's lateral acceleration was
limited to 0.50g and 0.40g for the Freightliner and Volvo respectively.
The trailer-based RSC system also improved the baseline vehicle's
roll stability in the J-turn maneuver. For the LLVW load condition, the
trailer-based RSC system activated at speeds similar to those of the
tractor-based systems. For the low CG and high CG load conditions, the
tractor-based systems activated at approximately a 3 mph lower speed
than the trailer-based RSC system. With both systems enabled, the
tractor-based system activated and mitigated the roll propensity before
the trailer RSC system activated.
The third maneuver evaluated in Phase I was a double-lane-change
maneuver, in which the test driver accelerated the vehicle up to a
constant speed on a dry road surface and then negotiated a lane change
maneuver followed by a return to the original lane within physical
boundaries (gates) marked by cones. The maneuver entry speed was
incrementally increased in subsequent test runs. Although the top speed
in this maneuver was intended to be limited to 50 mph for safety
reasons, the test driver performed runs at speeds as high as 51 mph.
In the baseline configuration, both tractors completed the maneuver
at 50 mph without wheel lift or yaw instability in the LLVW and the low
CG loading conditions. In the high CG loading condition, the
Freightliner experienced trailer wheel lift at a maneuver entry speed
of 41 mph and the Volvo experienced trailer wheel lift at a maneuver
entry speed of 45 mph.
With the ESC system, the Freightliner's stability control system
was observed to limit peak lateral acceleration to approximately 0.50g,
which prevented trailer wheel lift in the high CG load condition for
tests performed up to 50 mph. Tests performed at 51 mph resulted in
trailer wheel lift. The Volvo's stability control system limited the
tractor's maximum lateral acceleration to approximately 0.40g and
prevented trailer wheel lift for the high CG condition up to a maximum
test speed of 51 mph.
With only a trailer-based RSC system, trailer wheel lift was
observed during the high CG load condition when the system was
overdriven at 41mph when tested with the Freightliner, which
represented no improvement over the baseline condition. Trailer wheel
lift was observed at 50 mph when tested with the Volvo, which
represented a 5 mph improvement over the baseline condition. When
tested with this maneuver in the high CG load condition, the trailer-
based RSC system activated the trailer brakes at entrance speeds of 30
and 33 mph for the Freightliner and Volvo, respectively.
All stability control systems tested improved the roll stability of
the vehicle over the baseline condition. For each maneuver, the
tractor-based stability control systems were able to mitigate trailer
wheel lift at the same or higher maneuver entrance speeds than trailer-
based systems. The trailer-based RSC system was typically able to
mitigate trailer wheel lift at a higher maneuver entry speed than the
baseline condition, with the exception of the double-lane-change
maneuver with one of the tractors. In the tests with both tractor-
[[Page 30778]]
based ESC systems and trailer-based RSC systems enabled, the tractor-
based ESC system was often found to be the first system to intervene to
reduce wheel lift or understeer.
Based on the results of Phase I, the agency determined that a
performance test based on the J-turn was suitable to evaluate tractor
and trailer stability control systems. The J-turn maneuver generates a
sufficient amount of lateral acceleration to provide a challenging test
at reasonable test speeds. The J-turn maneuver is also more
representative of the real-world conditions, such as curved off-ramp,
that could generate untripped rollover. Because the results from Phase
I showed that tractor-based stability control systems increased the
roll stability by a larger margin than trailer-based RSC systems, NHTSA
concluded that Phase II research should focus on tractor-based
stability control systems.
2. Developing a Dynamic Test Maneuver and Performance Measure To
Evaluate Roll Stability--Phase II
(a) Test Maneuver Development
The researchers at VRTC conducted Phase II to develop test methods
that could evaluate stability control system performance objectively
and measures of performance that would ensure that a stability control
system could prevent rollover effectively. After Phase I test results
demonstrated that a test driver's steering input variation could affect
test outcome, an automated steering machine was used for subsequent
research. The testing focused on tractor-based stability control
systems that were determined to be most effective in preventing
rollovers from the Phase I research.
Both the Freightliner and Volvo 6x4 tractors equipped with an ESC
system from Phase I were tested, and an RSC electronic control unit was
also obtained for the Freightliner. A Sterling 4x2 equipped with a
Meritor WABCO RSC system was also tested in Phase II. In addition to
the Fruehauf 53-foot van trailer used in Phase I (its trailer-based RSC
system was disabled throughout the Phase II testing), five additional
trailers were tested, including a second 53-foot van trailer, two 48-
foot flatbed trailers, a 9200-gallon tanker trailer, and a 28-foot
flatbed trailer which is used as a control trailer in FMVSS No. 121
brake system testing.
The first maneuver evaluated in Phase II was a slowly increasing
steer maneuver. The SIS maneuver has been used by the agency and the
industry to determine the unique dynamic characteristics of each
vehicle. This maneuver is included in the FMVSS No. 126 test procedure
for ESC systems on light vehicles. The maneuver provides the steering
wheel angle to lateral acceleration relationship for each vehicle,
accounting for the differences in steering gear ratios, suspension
systems and wheelbases among vehicles. It also normalizes test
conditions to account for variations in test conditions, such as road
surface friction. The steering wheel angle derived from the SIS test
was used to program the automated steering machine for the ramp steer
maneuver discussed below.
To initiate the SIS maneuver, the test driver accelerated the
vehicle to a constant speed of 30 mph on a dry road surface. The driver
then activated the steering machine to input a steadily increasing
steering wheel angle up to 270 degrees at a rate of 13.5 degrees per
second. The test driver manually maintained constant speed using the
accelerator pedal while the tractor's path radius steadily decreased
and the tractor's lateral acceleration steadily increased. The SIS
maneuvers were conducted with the tractor in the bobtail condition (no
trailer attached). The SIS maneuver also demonstrated that tractor-
based stability control systems are capable of detecting a high lateral
acceleration condition and intervening by reducing the engine output
torque.
The SIS maneuver was used to determine the steering wheel angle
projected to generate 0.5g of lateral acceleration when traveling at 30
mph. This value varied depending on characteristics of the tractor such
as its wheelbase and steering ratio. For tractors, that steering wheel
angle and lateral acceleration data was found to have a linear
relationship at the lateral acceleration values between 0.05 and 0.3g.
Over this range of data a linear regression method followed by linear
extrapolation was used to estimate the steering wheel angle at 0.5g
lateral acceleration for each SIS maneuver. The final steering wheel
angle was then calculated by averaging the values from tests conducted
while turning to the left and while turning to the right. The resulting
calculated steering wheel angles were 193 degrees for the Freightliner,
199 degrees for the Volvo, and 162 degrees for the Sterling. This
indicates that the Sterling, which was a 4x2 configuration, had a
higher steering wheel gain than the other tractors which were 6x4
configurations.
The SIS testing was repeated for the three tractors throughout the
test program to determine the consistency of the steering wheel angle
calculations and the test speeds. The resulting standard deviations in
steering wheel angle were 2.5 degrees for the Sterling, 7.4 degrees for
the Freightliner, and 10.2 degrees for the Volvo, although the
replacement of the tires on the Volvo may have contributed to an
increase in steering wheel angle during one of the repeat tests. The
tractor speed at the beginning of the SIS steering input ranged from
29.6 to 32.2 mph for all of the tests.
After the SIS testing, tests were conducted using a ramp steer
maneuver to assess the roll stability of tractor-trailer combinations
and the effectiveness of both types of tractor-based stability control
systems. The RSM was derived from and is similar to the J-turn
maneuver, but instead of the driver controlling the steering wheel to
follow a fixed path, the steering controller turns the steering wheel
to an angle determined from the results of the SIS test. One advantage
of the RSM over the J-turn maneuver is that the RSM uses a steering
machine, which allows for a more consistent and repeatable steering
input.
To conduct the RSM, the test driver accelerated the vehicle to a
constant speed of one to two mph above the target maneuver entry speed
on a dry surface and then released the throttle and de-clutched the
engine. Once the vehicle coasted down to the desired maneuver entry
speed, the automated steering controller initiated a steering input, at
a constant rate of 175 degrees per second, up to the steering wheel
angle that was derived for the tractor in the SIS test. Once the
steering wheel angle was reached (the end of ramp input), it was held
constant for five seconds, and then the controller returned the
steering wheel angle back to zero at a steering rate of 175 degrees per
second. The initial maneuver entry speed was 20 mph and it was
incrementally increased in subsequent runs until a test termination
condition was met. The termination conditions were as follows: Two
inches of wheel lift occurring at either the tractor drive wheels or
the trailer wheels; the tractor reaching a severe oversteer condition
(safety cables were installed to limit the tractor-trailer articulation
angle for testing safety); or the maneuver entry speed reached 50 mph
without a roll or yaw instability condition. Although the intent of the
RSM was to evaluate combination vehicle roll stability, testing with
the trailers in the unloaded condition resulted in several occurrences
of tractor yaw instability.
For all of the RSM tests, each tractor was tested with all six
trailers and the trailers were either unloaded, or loaded to a high CG,
on-highway combination
[[Page 30779]]
weight appropriate for the number of axles on the combination vehicle.
For the flatbed and van trailers, the load ballast was placed on 24-
inch high tables to produce a high CG height, and the tanker trailer
was loaded with water.
The purpose of the RSM test is not to cause a rollover, but to
create a high lateral acceleration condition to demonstrate that a
stability control system has the capability to reduce the likelihood of
a rollover. Typically, wheel lift occurred first at the trailer wheels
although the flatbed trailer combinations had tractor drive wheel lift
occurring first or in unison with the trailer wheels. In the RSM tests
with the stability control system disabled and the trailer in the high
CG condition, wheel lift occurred at entry speeds between 25 and 31 mph
for all combinations of tractors and trailers. The peak tractor lateral
acceleration at wheel lift was in the range of 0.45 to 0.50g, showing
that the high CG loading condition was representative of fully loaded
tractor-trailers with a medium density cargo.
Tractor-based stability control systems applied the foundation
brakes on the tractor and trailer, which reduced the vehicle speed and
lateral acceleration during the RSM. The entry speed at which wheel
lift was first visible improved to between 31 and 42 mph for three of
the four tractors tested (Freightliner RSC, Freightliner ESC, and Volvo
ESC).
In tests with the trailer brakes disabled, the entry speed at which
wheel lift was detected was between 29 and 41 mph, which showed that
the contribution of trailer braking to prevent wheel lift was evident,
but that it was relatively small in comparison to the deceleration
resulting from tractor braking. The Sterling tractor equipped with an
RSC system had wheel lift with three of the trailers at the same speed
as with the stability control system disabled, and with the other three
trailers at speeds between two and four mph over the disabled test
condition. In all of the RSM tests, the Sterling tractor's RSC system
was not as effective at mitigating wheel lift for this maneuver.
The results indicated that, in general, the ESC systems provided a
higher level of deceleration compared to the RSC systems and typically
had the higher maneuver entry speeds prior to wheel lift. However,
there were individual trailer combinations in which the RSC system
performed as well or slightly better than the ESC system on the
Freightliner. We believe the better performance by the RSC system in
some tests is attributable to the RSC system having a more aggressive
braking strategy than the ESC system tested.
The RSM was then performed with each of the six trailers in the
unloaded condition, with the tractor stability control system enabled
with the trailer brakes disabled. Tests were not conducted with the
systems disabled. The initial maneuver entry speed was 20 mph and was
incrementally increased in subsequent runs until the speed reached 50
mph, severe oversteer occurred, or wheel lift occurred. The tractors
with ESC systems enabled were able to complete all but one of the RSM
tests up to 50 mph without any tractor instability or wheel lift. The
Volvo tractor towing the empty tanker trailer resulted in wheel lift of
the tractor drive wheels and the trailer wheels at a speed of 47 mph.
In comparison, most of the tests with the tractors equipped with
RSC systems towing unloaded trailers resulted in severe tractor
oversteer, with the tractor-trailer articulation angle typically
reaching the limits allowed by the safety cables. This occurred at
speeds between 35 and 39 mph for the Freightliner 6x4 tractor and
between 34 and 42 mph for the Sterling 4x2 tractor. However, both of
these tractors were able to complete the RSM up to 50 mph when coupled
to the unloaded 28-foot control trailer, and the Freightliner reached
50 mph without wheel lift or severe understeer when coupled to the
unloaded tanker trailer.
In summary, the goal of the Phase II research was to develop a test
maneuver to challenge the roll propensity of a truck tractor. The RSM
is similar in test severity to the J-turn and demonstrates that the
stability control systems are able to mitigate wheel lift in most cases
that occurred when the stability control systems were disabled. In the
high CG load condition, the ESC systems were observed to mitigate wheel
lift at or above the speed observed with RSC-equipped vehicles, with
the exception of a few instances with the Freightliner's ESC system.
When tested with the unloaded test trailer, substantial improvements in
tractor yaw stability were evident in the tractors equipped with ESC
systems during RSM tests.
(b) Performance Measure Development
NHTSA's Phase II testing also examined possible performance
measures to evaluate roll stability. In situations where the vehicle's
stability limits are approached in a gradual manner, engine/power unit
control can improve stability in these situations. However, in
situations where stability limits of the vehicle are approached
rapidly, application of the vehicle's foundation brakes may be a more
appropriate means of improving stability.
The agency investigated four measures for development as metrics
for engine/power unit control. They were truck tractor speed, truck
tractor lateral acceleration, truck tractor longitudinal acceleration,
and actual engine torque and driver requested engine torque.
The forward speed of a truck tractor appears to be directly related
to the lateral forces generated during an untripped rollover. Test data
from four different vehicles with stability control enabled indicated
that forward speed was reduced from the target maneuver entrance speed
of 30 mph. However, due to the nature of the roll maneuver, it is
possible for the vehicle to lose traction on the inside wheels, which
results in a reduction in vehicle speed but does not necessarily
enhance vehicle stability.
Lateral acceleration was a possible measure of performance because
of its direct relationship in producing the forces associated with
untripped rollover. Data from four different tractors with the
stability control system enabled indicate that each combination of
tractor and stability control system had a different lateral limit that
the system has allowed. This shows that the control strategy used by
the manufacturer is different depending on the vehicle and system used.
One strategy allows the vehicle to build lateral acceleration to a set
threshold level and then allows that level to be maintained throughout
the maneuver. The other strategy allows lateral acceleration to build
and then the stability control system reduces the lateral acceleration.
Both of these strategies were observed to increase lateral stability.
Because the lateral acceleration limits were different for vehicles
using these control strategies, lateral acceleration alone was not
found to be a good measure for stability control performance.
Longitudinal acceleration of a vehicle is reduced when a vehicle's
stability control system is enabled and is directly related to a
reduction in forward speed. On the four vehicles tested, the stability
control activation had measurable differences in longitudinal
acceleration, but had similar disadvantages to forward speed in being
used as a performance metric.
Engine torque measures were observed to be a direct way to
determine ESC activation during the SIS tests. Engine torque refers to
two different measures. The first relates to the torque output from the
engine and is expressed
[[Page 30780]]
as a percentage of maximum engine output. The second relates to the
throttle pedal used by the driver to control engine torque output. This
value is also expressed as a percentage of maximum engine output and is
referred to as the ``driver requested torque.'' During normal operation
the ``driver requested torque'' and ``engine torque'' measures were
observed to be equal to each other. However, during ESC activation when
engine control intervened, the two measures were observed to be
separate. In every case, the ``engine torque'' was much less than the
``driver requested torque'' and continued to reduce until vehicle
stability was regained. After careful review of the data the torque
separation activity was confirmed for all the SIS test series in which
stability control was enabled for each vehicle. This led the agency to
conclude that this measure was a good candidate for further analysis
and development as a measure of performance for truck tractors equipped
with a stability control system.
The engine torque data analysis was based on the test driver
attempting to maintain a constant vehicle speed at the point of
stability control engine torque intervention by making a substantial
increase in driver-requested engine torque. For the four vehicles
tested, the driver requested engine torque after stability control
intervention was between 60 percent and 100 percent of engine output
whereas the engine torque output after stability control intervention
ranged from zero to 60 percent. The analysis of engine torque
differentials was limited to the first four seconds after stability
control engine torque intervention since none of the SC systems were
observed to make substantial reapplications of engine torque output
during this initial time- frame. On two vehicles engine torque
interventions reduced engine output torque to zero during the first
four seconds, and both systems allowed engine torque to be momentarily
reapplied to over 50 percent of engine torque output. The Volvo had the
highest engine torque output during the first four seconds after
intervention, which ranged from 23 percent to 18 percent of maximum
engine torque.
The agency also investigated several other measures for development
for foundation braking in rollover tests because stability control
systems were observed to improve the vehicle's roll stability by
applying the foundation brakes. The measures investigated were wheel
lift, lateral acceleration, lateral acceleration ratio, trailer lateral
acceleration ratio, and trailer roll angle ratio.
Wheel lift is a direct measure of performance with minimal
calculations needed to determine its value. The measure is simple and
directly represents the pre-crash condition that immediately precedes a
rollover. If wheel lift can be prevented, a rollover cannot occur. For
our research, wheel lift was considered to occur upon two inches of
lift for the tractor drive axle wheels or the trailer wheels. Wheel
lift does not always indicate that rollover is imminent, particularly
because certain suspension designs will lift a wheel during hard
cornering. We estimated the vehicle speed that produced wheel lift
during the ramp steer maneuver and found that between 29 mph and 32
mph, there is a high probability of wheel lift occurring on the
combination vehicles tested. Given that only four different truck
tractors and six different test trailers were used, we believed that
the data may not be sufficient to assess the real world service of
tractors with ESC expected to function with different trailers having
different torsional stiffness and loads.
Using lateral acceleration as a performance metric is based on the
principle that a tractor-trailer combination vehicle with a high center
of gravity that achieves a certain level of lateral acceleration would
roll over. Tests performed on the Freightliner in combination with all
trailers configured with a high-CG load, at a mean entrance speed of 28
mph generated a lateral acceleration. The data showed that using
tractor maximum lateral acceleration as a performance criteria would
not discriminate between vehicles equipped with stability control and
those without it. However, it did show that a ratio-based metric could
be more appropriate for such a performance metric.
Lateral acceleration ratio is calculated by dividing the tractor's
lateral acceleration at a given time interval by the measured lateral
acceleration at the end of ramp input, which is the end of the steering
maneuver and the point near which the vehicle experiences its peak
lateral acceleration. The LAR was plotted at five equal one-second
intervals for several truck tractors and test trailers. The plots
indicated sharp decreases in LAR caused by activation of the stability
control system.
A similar ratio metric for trailers, trailer lateral acceleration
ratio, also showed the ability to discriminate between vehicles with
stability control systems and those without. A third ratio metric was
considered, trailer roll angle ratio based on a test trailer roll
angle, but it did not clearly discriminate between vehicles with
stability control systems and those without.
3. Developing a Dynamic Test Maneuver and Performance Measure To
Evaluate Yaw Stability--Phase III
(a) Test Maneuver Development
The purpose of the Phase III research was to develop maneuvers to
evaluate the yaw stability performance of stability control systems on
tractors. Although we have examined several maneuvers to evaluate yaw
stability, two maneuvers were fully investigated because other
maneuvers were not able to provide a consistent, repeatable performance
test. We fully considered a sine with dwell test maneuver that is
similar to the test maneuver used in FMVSS No. 126 for light vehicles;
and a half-sine with dwell (HSWD) test maneuver. The steering inputs
for the SWD and HSWD maneuvers are depicted in the figures below, and
as discussed in additional detail, variations on the steering wheel
angle, the frequency of the sine wave (cycles per second, Hz), and the
dwell time were evaluated for both maneuvers. A steering machine was
used to achieve consistent steering wheel inputs for these maneuvers.
[[Page 30781]]
[GRAPHIC] [TIFF OMITTED] TP23MY12.004
The test vehicles used in Phase III included: A 2006 Freightliner
6x4, which was tested with both ESC and RSC systems; a 2006 Volvo 6x4
tractor with an ESC system; and a Sterling 4x2 tractor equipped with an
RSC system. Although most of the testing was performed using the 28-
foot flatbed control trailer, each tractor was also tested with a 53-
foot Strick van trailer, a 48-foot Fontaine spread axle flatbed
trailer, and a 9600-gallon Heil tanker trailer. Tests were conducted
with the trailer brakes both enabled and disabled.
Two tractor loading conditions were used for both the SWD and HSWD
testing. Each tractor was tested in the bobtail condition (no trailer
attached) and using a trailer loaded over the fifth wheel so that the
tractor drive axle(s) was loaded to 60 percent of its gross axle weight
rating (GAWR). The yaw instability that occurred in the RSM testing
showed that the unloaded 28-foot control trailer was too light to
produce yaw instability. Therefore, additional weight was added for
these tests. Testing was conducted on two test surfaces: A high-
friction dry road surface and a slippery wet Jennite road surface.
Additional SIS tests were performed, similar to the bobtail SIS
tests described in Phase II, conducted with each tractor coupled to the
28-foot control trailer and loaded to the 60 percent GAWR condition.
The steering wheel angles from these tests were 197 degrees for the
Freightliner with ESC, 200 degrees for the Freightliner with RSC, 200
degrees for the Volvo, and 153 degrees for the Sterling. The average
tractor lateral acceleration at engine torque intervention in the SIS
tests was 0.40g for the Freightliner with ESC, 0.34g for the
Freightliner with RSC, 0.35g for the Volvo, and 0.4g for the Sterling.
For the SWD and the HSWD test maneuvers, the maneuver entrance
speed for the bobtail tractor tests was 50 mph, and for the tests at 60
percent GAWR the entry speed was 45 mph. The driver accelerated the
test vehicle up to a speed slightly over the desired speed in a
straight lane, then released the throttle and de-clutched the engine.
Once the vehicle coasted down to the desired speed, the automated
steering machine initiated either the sinusoidal or half-sine steering
input, at a specified test frequency as described below (e.g., 0.3 Hz,
0.5 Hz, etc.), with the steering wheel angle held constant during the
dwell, as depicted in the figures. Two dwell times were evaluated as
described below, 0.5 and 1.0 second. The initial test run began with a
steering wheel angle equal to 30 percent of the angle determined from
an SIS test. The test severity was increased in subsequent runs by
increasing the steering wheel angle in 10 percentage point increments
until reaching 130 percent of the SIS steering wheel angle. Thus, 11
test runs were needed to complete a test series. If severe oversteer or
wheel lift greater than two inches was detected, then the test was
repeated using the previous steering wheel angle in which the systems
was observed to be stable. If the tractor-trailer was stable during the
repeated run, additional tests were performed by increasing the
steering wheel angle in 5 percent increments until instability was
observed.
Tests were conducted on baseline tractors in the 60 percent GAWR
condition on dry pavement to evaluate frequency and dwell time for the
SWD and HSWD test maneuvers. Frequencies between 0.3 and 0.7 Hz were
evaluated. A frequency of 0.5 Hz was found to require the lowest
steering scalar to produce severe oversteer in the Sterling and Volvo
tractors in the SWD maneuver, and 0.4 Hz was found to require the
lowest steering scalar to produce severe oversteer in the Freightliner
tractor (and 0.5 Hz was the second-most severe frequency for this
tractor). A dwell time of 1.0 second was found to result in severe
tractor oversteer at lower steering scalars. Thus the researchers
selected a 0.5 Hz frequency and 1.0 second dwell time as the parameters
for the SWD and HSWD maneuvers. However, the researchers also found
that the SWD maneuver was less sensitive to differences in steering
frequency compared to the HSWD maneuver.
In tests conducted with baseline tractors in the bobtail condition,
no yaw instability occurred; however, in both the SWD and HSWD tests
the Sterling tractor experienced wheel lift at the tractor drive
wheels. Seventy test series were conducted on the baseline tractors in
the 60 percent GAWR load condition, with fifteen of the series
terminated due to roll instability and 28 due to severe tractor
oversteer.
In tests conducted with the tractor stability control system
enabled and in the 60 percent GAWR load condition, all of the tractors
with an ESC system were able to complete the SWD maneuver at test
scalars up to 130 percent. However, the tractors equipped with RSC
systems experienced severe oversteer in 12 of 15 test series at the
steering scalars of 120 and 130 percent. In tests conducted using the
HSWD maneuver, the ESC-equipped tractors completed seven of eight test
series without tractor yaw instability, and the RSC-equipped tractors
experienced
[[Page 30782]]
severe oversteer at steering scalars ranging from 80 to 125 percent. In
both test maneuvers, the RSC systems improved tractor yaw stability
compared to the baseline tractor, but they could not maintain yaw
stability at the higher steering scalars.
Additional SWD tests were conducted with the 53-foot van trailer
and the 48-foot flatbed trailer using the 60 percent GAWR loading
condition. In eight test series conducted with the tractor stability
control systems enabled, seven were completed without wheel lift or
tractor yaw instability, but the Sterling tractor equipped with an RSC
system tested with the 48-foot flatbed reached a termination condition
at a steering scalar of 105 percent. In tests with stability control
enabled, all of the tractors coupled to the tanker trailer experienced
wheel lift in the SWD maneuver at scalars between 60 and 95 percent.
SWD tests were also conducted on a low-friction wet Jennite surface
using a lower maneuver entry speed of 30 mph. In the baseline condition
with the tractor stability control systems disabled, 43 test series
were conducted and a termination condition was reached in only four
test series. Testing on the dry, high-friction surface was found to
result in more yaw instabilities than the testing conducted on the low-
friction, wet Jennite surface.
In summary, the purpose of Phase III research was to develop a
maneuver to evaluate the yaw stability of a tractor trailer combination
vehicle. VRTC researchers found that the SWD maneuver with a one-second
dwell time based on a single cycle of steering input with a frequency
of 0.5 Hz conducted on a high friction surface appropriately assessed
the ability of an ESC system to improve yaw stability. From this
maneuver, performance measure were investigated for lateral stability
and responsiveness: the lateral acceleration ratio, which is directly
correlated to roll stability and the yaw rate ratio, which the
performance metric used in FMVSS No. 126 for light vehicle ESC systems
and was found to be a direct performance measure of yaw stability. A
responsiveness measure was also studied to evaluate the lateral
displacement of a vehicle during SWD maneuvers.
(b) Performance Measure Development
Phase III of NHTSA's research also examined potential measures of
yaw instability prevention performance. In light of the conclusion in
Phase II that lateral acceleration ratio was a suitable metric to
measure a stability control system's ability to prevent lateral
acceleration, the agency examined a yaw rate ratio metric. The YRR
expresses the lateral stability criteria for the sine with dwell test
to measure how quickly the vehicle stops turning, or rotating about its
vertical axis, after the steering wheel is returned to the straight-
ahead position. Similar to the LAR, the YRR metric is the percent of
peak yaw rate that is present at a designated time after completion of
steer. This performance metric is identical to the metric used in the
light vehicle ESC system performance requirement in FMVSS No. 126.
Phase III research found that both LAR and YRR were capable of
measuring stability during the SWD maneuver. However, while LAR was
better at predicting roll instability, YRR was better at predicting yaw
instability.
4. Large Bus Testing
Researchers at VRTC tested three large buses equipped with
stability control systems: A 2007 MCI D4500 (MCI 1), a 2009
Prevost H3, and a second 2007 MCI D4500 (MCI 2). The MCI buses
were equipped with a Meritor WABCO ESC system and the Prevost was
equipped with a Bendix ESC system. RSC systems were not offered on
large buses and, consequently, were not evaluated. All of the buses
were equipped with air disc brakes. Both the MCI 1 and the MCI
2 had a GVWR of 48,000 lb and a wheelbase of 317 in., and the
Prevost had a GVWR of 53,000 lb and a wheelbase of 317 in. Each of the
buses had three axles: A steer axle, a drive axle, and a non-driven tag
axle.
The MCI 1 was equipped with outriggers supplied by MCI and
Meritor WABCO. The outriggers limited the use of higher maneuver entry
speeds for tests without the ESC system enabled. At higher speeds, the
lower support portion of the outrigger would dig into the test surface
and influence the dynamics of the vehicle. Therefore, tests of the MCI
1 at higher speeds had no baseline performance to compare to.
The Prevost and MCI 2 buses were tested using NHTSA-
designed outriggers. The outriggers designed for combination vehicles
were adapted for installation on the mid-section of each bus, just in
front of its drive axle and slightly behind its longitudinal center of
gravity. Using these outriggers, the vehicles were able to complete
testing for all speeds, with or without ESC enabled.
Each bus was tested using two primary simulated load conditions.
The first condition was a lightly loaded vehicle weight (LLVW) that
included the weight of the test instrumentation, outriggers, and
driver. The second load condition, gross person occupancy weight
(GPOW), included the LLVW weight plus the addition of 175-lb water
dummies in each available passenger seat without exceeding the GVWR of
the vehicle. This condition was used to represent a high CG load that a
bus may experience while in service. A third loading condition was
conducted with the Prevost, which added ballast to the cargo holds
under the mid-section of the bus. This condition loaded the vehicle to
its GVWR.
Test maneuvers that were conducted included the 150 ft. constant
radius increasing velocity test, SIS, RSM, HSWD, and SWD. Tests were
conducted using an automated steering machine, except for the constant
radius maneuvers. The severity for each test maneuver was increased
either by increasing vehicle speed or steering angle.
SIS maneuvers were conducted under both loading conditions, with
ESC systems enabled and disabled, and in both left and right directions
in order to characterize each vehicle. Initially, the maneuver was
executed exactly as it was for the tractor testing. However it was
observed that steering to a maximum steering wheel angle of 270 degrees
generated barely over 0.3g of lateral acceleration. From this, it was
clear that large buses have a larger steering ratio, and it would take
a larger steering input to achieve the appropriate lateral acceleration
levels. The steering wheel angle necessary to achieve 0.5g in the LLVW
loading condition was 405 degrees for the MCI 1, 352 degrees
for the Prevost, and 407 degrees for the MCI 2. In the GPOW
loading condition, steering wheel angles were found to be 405 degrees
for the MCI 1, 383 degrees for the Prevost, and 461 degrees
for the MCI 2.
SIS tests were conducted at GPOW to evaluate the ability of the ESC
system to reduce speed by limiting engine torque. For the three buses
tested the average speed at activation for each SIS maneuver ranged
between 29.8 and 30.6 mph. At four seconds following SC activation the
average speed for each SIS had been reduced to 27.9 mph for the MCI
1, 26.5 mph for the Prevost, and 26.6 mph for the MCI
2. Without stability control enabled, speeds did not decrease.
The average lateral acceleration for a test series observed at
activation was 0.32g for MCI 1, 0.27g for the Prevost, 0.31g
for MCI 2.
RSM testing was completed for each bus to evaluate their roll
propensity while loaded in the LLVW and GPOW conditions. Tests were
conducted using the same RSM protocol as the one developed for
tractors. Using an
[[Page 30783]]
automated steering machine programmed with the steering wheel angle
calculated from the SIS maneuver, tests were conducted with ESC systems
enabled and disabled. The initial maneuver entry speed was 20 mph and
was incrementally increased in subsequent runs until two inches of
wheel lift occurred at any of the wheels, the vehicle went into a
severe oversteer condition, or the entry speed reached 50 mph without a
roll or yaw instability condition.
For RSM tests with ESC systems disabled and the buses loaded in the
LLVW condition, wheel lift was observed in both MCI test vehicles at
speeds of 41 to 45 mph, and no wheel lift was observed for tests with
the Prevost for the speeds tested. When tested in the GPOW condition,
wheel lift was observed at 35 to 39 mph for all vehicles tested.
For RSM tests with ESC systems enabled and the buses loaded in the
LLVW condition, no instances of wheel lift were observed over the range
of speeds tested. During tests in the GPOW condition wheel lift was not
observed in either MCI over the range of speeds tested, but was
observed in some of the Prevost tests at speeds between 42 and 48
mph.\26\
---------------------------------------------------------------------------
\26\ Initial tests conducted with the Prevost demonstrated that
the vehicle was able to complete the RSM at up to 48 mph without
wheel lift for the GPOW condition. The Prevost was not tested to 50
mph because there was not enough test area to bring the vehicle up
to this speed and allow the driver to recover safely if the test
needed to be aborted. RSM tests under the same conditions were
repeated less than a week later. During these tests, wheel lift
greater than 2 inches was observed at speeds of 42 to 44 mph with
ESC enabled. Upon further investigation when preparing to de-
instrument the vehicle, a broken roll stabilizer bar was discovered.
Researchers attributed the change in performance observed to the
broken stabilizer bar.
---------------------------------------------------------------------------
SWD testing was completed for each bus to evaluate its yaw
propensity while loaded in the LLVW and GPOW conditions. All tests were
conducted with the ESC systems enabled and disabled. Using an automated
steering machine, the SWD tests were run using steering frequencies of
0.3, 0.4, 0.5, and 0.6 Hz, dwell times of 0.5 and 1.0 seconds, and a
maneuver entry speed of 45 mph. Test severity was increased by
increasing the steering wheel angle by a scalar from 30 to 130 percent
in 10 percent increments. A test series was terminated if the vehicle
experienced wheel lift greater than 2 inches, the vehicle spun out, or
the steering input reached a terminating scalar of 130 percent.
No instances of spinout were observed during this testing, but
tests at higher steering wheel angles produced drift. Although the
buses were yaw stable in the maneuvers, the test results demonstrated
that the SWD maneuver was challenging the buses' roll propensity.
Several SWD test series with the GPOW condition produced wheel lift
when the ESC system was disabled. When the ESC systems were enabled,
all vehicles were able to complete their series without exceeding
either roll or yaw stability thresholds.
The SWD test data from the GPOW load condition were analyzed to
determine a frequency and dwell time for a candidate performance
maneuver. For all tests with ESC disabled, maneuvers with a 1.0-second
dwell time required an equal or lower steering scalar (0 to 50 percent
lower) to exceed a threshold of 6 degrees of yaw angle. As with the
tractor testing, this suggested that the 1.0-second dwell time was more
challenging to large buses because it required less steering to exceed
the threshold.
Using only the 1.0-second dwell time tests, analysis to determine
the optimum frequency for the SWD test was completed by evaluating the
roll and yaw angles. Review of the test data indicated that the largest
roll and yaw angles were produced in the maneuvers using 0.4 and 0.5 Hz
frequencies.
The large buses were also tested using the HSWD maneuver. Like the
SWD, the test results for the HSWD indicated that the longer dwell time
was more challenging to stability. Unlike the SWD, the lower
frequencies were observed to produce wheel lift at lower steering wheel
angle scalars. Tests results from both the SWD and HSWD maneuvers
indicated that both maneuvers generated dynamic responses from the
vehicles. There were clear differences in lateral acceleration and yaw
rate between test series conducted with ESC systems enabled compared to
test series with ESC systems disabled. The data showed that ESC systems
were reducing both rollover and spinout propensities. However, the SWD
maneuver was favored over the HSWD maneuver because the SWD maneuver
could be conducted in a smaller area, would be representative of a
crash avoidance or lane change maneuver, and its use in FMVSS No. 126
accelerated performance measure research.
This research indicates that large buses equipped with ESC systems
can use the same objective performance maneuver as was developed for
tractors. Testing also indicates that the same performance measures can
be used to assess lateral stability and responsiveness, but the
performance measures must be tailored for the vehicle differences.
D. Truck & Engine Manufacturers Association Testing
The Truck & Engine Manufacturers Association (EMA) performed tests
on ten tractors listed in the following table equipped with stability
control systems using the three test maneuvers developed at VRTC.
Table 2--EMA Test Tractors Including Type, GVWR, and Wheelbase
----------------------------------------------------------------------------------------------------------------
Wheelbase
Tractor configuration (EMA Vehicle I.D.) Stability control type GVWR (lb) (inches)
----------------------------------------------------------------------------------------------------------------
6x4 Typical Tractor (Vehicle A)............... ESC............................. 52,000 228
4x2 (Vehicle B)............................... ESC............................. 32,000 140
4x2 (Vehicle C)............................... RSC with steering wheel angle 34,700 152
sensor.
6x4 Severe Service (Vehicle D)................ ESC............................. 66,000 220
6x4 w/Pusher Axle (Vehicle E)................. ESC............................. 86,000 270
8x6 Tridem Drive Axle (Vehicle F)............. ESC............................. 89,000 263
6x4 w/Pusher Axle (Vehicle G)................. ESC............................. 92,000 243
6x4 Severe Service (Vehicle H)................ RSC............................. 60,600 246
6x4 (Vehicle I)............................... ESC............................. 52,000 232
6x4 (Vehicle J)............................... ESC............................. 52,350 245
----------------------------------------------------------------------------------------------------------------
[[Page 30784]]
EMA provided its test data to the agency.\27\ Although the tractors
were not identified by make or model, EMA provided the configuration
and weight ratings for each tractor. Eight tractors were subjected to
the SIS and RSM to evaluate rollover prevention, and three tractors
were subjected to the SWD maneuver, and the ramp with dwell (RWD)
maneuver on a low-friction surface to evaluate yaw stability. Two of
the tractors were equipped with RSC systems and seven tractors were
equipped with ESC systems. EMA also submitted test data for several
maneuvers in which the test parameters were varied. With the exception
of Vehicle J, EMA did not submit baseline test data--that is, EMA
submitted data only for maneuvers with ESC or RSC systems enabled.
---------------------------------------------------------------------------
\27\ Data from Vehicles A through I are included have been
placed in the docket. Docket Nos. NHTSA-2010-0034-0011 through
NHTSA-2010-0034-0021 and Docket No. NHTSA-2010-0034-0024. Vehicle J
testing is discussed in detail in a later section.
---------------------------------------------------------------------------
1. Slowly Increasing Steer Maneuver
For all tractors, test data were provided for the SIS tests used to
derive the steering wheel angle with each tractor in the bobtail
condition. In the first SIS series conducted on eight of the tractors,
three SIS tests were conducted in each direction on a dry road surface,
and a best fit linear regression was used to project the steering wheel
angle for a lateral acceleration of 0.5g. The average of the absolute
value of each of the six runs was calculated for the final angle.
Compared to the steering wheel angles that were derived for the
three VRTC tractors, a much wider range in SWA was seen among EMA's
results. The steering wheel angles generally increased with the
tractor's wheelbase from an angle of 126 degrees for the 140-inch
wheelbase 4x2 to an angle of 291 degrees for the 270-inch wheelbase 6x4
with a pusher axle. For Vehicle H, EMA also provided data from direct
measurement of the steering wheel angle from driving the tractor at
0.5g of lateral acceleration. This angle was 290 degrees, which is
slightly larger than the calculated value of 281 degrees extrapolated
from the SIS test data in the 0.05 to 0.30g operating region. The EMA
data provided for these SIS tests did not include information on
stability control engine torque reduction.
Additional SIS tests were conducted on three tractors that were to
be subsequently tested using the SWD maneuver to evaluate tractor yaw
stability. The SIS test conditions were identical to the prior SIS
tests. A best fit linear regression was used to project the steering
wheel angle for a lateral acceleration of 0.5g, and the average of the
absolute value of each of the six runs was calculated for the final
angle as in the prior SIS tests. Comparing these data to the prior SIS
test results, Vehicle B, which had the smallest angle of 126 degrees in
the prior SIS tests, showed a ten degree reduction of its angle in this
test series. Vehicle G's angle was nearly identical (203 degrees in the
first series vs. 205 degrees in the second series).
2. Ramp Steer Maneuver
For the RSM tests on eight tractors, the tractors were attached to
a FMVSS No. 121 control trailer and were loaded to their GVWR by
placing the ballast over the fifth wheel, with the ballast placed
directly on the trailer deck resulting in a low center of gravity
height. The weight on the FMVSS No. 121 control trailer's single axle
ranged between 5,720 and 5,930 lb for all eight tractor tests, and the
trailer brakes were not enabled. While the weight on the trailer axle
is nominally 4,500 lb when the trailer is used for FMVSS No. 121
stopping distance tests, the increased weight in these RSM tests
reflects the added weight of the outriggers installed on the trailer.
In general, each of the tractors was loaded to its GVWR with the steer,
drive, and auxiliary axles loaded to, or very close to, their
respective GAWRs. The only exception was the 140-inch wheelbase 4x2
which only had 9,950 lb on the steer axle, although it was rated for
12,000 lb.
In the tests, the stability control systems automatically applied
the tractor's foundation brakes to reduce speed and lateral
acceleration. The initial vehicle deceleration generally coincided with
the end of ramp steer input, indicating that the stability control
systems were effective at reducing the lateral acceleration. The speed
at wheel lift for EMA's tests ranged from 33 to 38 mph, as compared to
31 to 39 mph for the VRTC tests that used a similar unbraked trailer,
but with a higher center of gravity loading condition and a higher
overall vehicle test weight. Both 4x2 tractors tested by EMA
experienced oversteer in addition to the wheel lift.
3. Sine With Dwell Maneuver
EMA provided test results for the SWD maneuver for four tractors
equipped with ESC systems. The sinusoidal steering frequency used for
testing was 0.5 Hz and the dwell time was one second. The amplitude of
the steering wheel inputs started at 30 percent of the steering wheel
angle derived from SIS testing, and in subsequent test runs was
increased by 10 percent increments up to 130 percent of the steering
angle. The SWD tests were conducted with two tractor loading
conditions: Loaded to 60 percent of the drive axle(s) GAWR with the
FMVSS No. 121 unbraked control trailer attached (loaded tests), and in
the unloaded condition with no trailer attached (bobtail tests). The
maneuver entrance speed was 45 mph and the test was conducted on dry
pavement.
The results of the loaded tests for Vehicles G and I indicated that
both tractors remained roll and yaw stable through the full range of
testing, and there were no indications of tractor wheel lift in the
test comments or the unprocessed data. The largest steering wheel angle
produced the highest peak lateral acceleration, which occurred during
the dwell portion of the maneuver for both tractors. Vehicle I reached
approximately 0.75g and Vehicle G reached just under 0.6g. Although
both tractors were close in wheelbase and tested with similar steering
wheel angles, Vehicle G, tested with its liftable axle in the lowered
position, was either less responsive in the SWD maneuver or its ESC
performed slightly better than the ESC on Vehicle I. Both tractors had
similar overall vehicle decelerations; however, the ESC on Vehicle G
commanded higher steer axle braking pressures than the ESC on Vehicle
I. Vehicle I appeared to have more lateral sliding in the maneuver, as
its yaw rate decay was slower at the end of steering input.
Vehicle B (140-inch wheelbase 4x2) exhibited yaw instability in the
SWD maneuver. This tractor had high lateral acceleration that was
attained at lower steering wheel angles than for the 6x4 tractors. For
example, the peak tractor lateral acceleration was already reaching
0.70g at 80 percent of the SIS-derived steering wheel angle, compared
to Vehicle I which reached 0.60g and Vehicle G which reached 0.45g at
this steering wheel angle scalar. The yaw rate decay after completion
of steer was also much slower than for the 6x4 tractors, which appears
to indicate that the vehicle was sliding much more and taking longer to
return to the straight-ahead position. This is most evident in the
testing at 130 percent of the SIS-derived steering wheel angle, in
which the decay yaw rate decay was about 3.5 seconds.
The maneuver entrance speed was reduced to 30 mph in the bobtail
SWD tests, which were conducted on a low-friction wet Jennite surface.
The short wheelbase 4x2 tractor, Vehicle B, appeared to complete all of
the test series without any observed instability
[[Page 30785]]
or control issues, and the peak tractor lateral acceleration was
limited to approximately 0.3g in all tests. However, both 6x4 tractors
(Vehicles G and I) appeared to have steering responsiveness issues that
were particularly noticeable at higher steering wheel angles. At the
reversal in steering wheel angle direction, the yaw rate and lateral
acceleration response was delayed, indicating severe understeer. During
the dwell portion of the maneuver at higher steering wheel angles,
Vehicle I slowly built lateral acceleration up to 0.3g, while Vehicle G
achieved similar but slightly lower acceleration levels. Vehicle G's
yaw rate also was slower to respond at the completion of steer, taking
as long as 2.5 seconds to decay to zero for the test conducted at the
highest steering wheel angle tested.
4. Ramp With Dwell Maneuver
The three tractors equipped with ESC systems tested in the SWD
maneuvers were also tested to the RWD maneuver. Once the initial
steering wheel angle and test speed were attained, the steering machine
increased the steering wheel angle to 180 degrees in one second, held
that steering wheel angle constant for three seconds (the dwell portion
of the maneuver), and then reduced the steering wheel angle to zero in
one second. In subsequent RWD test runs, the steering wheel angle was
increased in 90 degree increments up to 540 degrees.
The test results show that for Vehicles B and I, the steady-state
lateral acceleration (prior to the ramp steer) was approximately 0.2g,
and for Vehicle G the steady-state tractor lateral acceleration was
approximately 0.1g. When the steering wheel angle was increased during
the initial steering ramp input, the lateral acceleration and yaw rate
increased slightly and in many of the test runs was then observed to
drop off, indicating that the tractor was not responsive to the
steering input. During the first two seconds of the steering dwell
portion of the maneuver, the tractor lateral acceleration typically
remained at 0.25g or less for all tests. During the last one second of
the steering dwell, all of the test runs for Vehicles G and I showed
steadily increasing lateral acceleration, as high as 0.5g, even as the
steering wheel angle was reduced to zero. This indicates that the
tractors were in a severe oversteer condition, and the agency
speculates that the relatively high lateral acceleration may have been
a result of the tractor running off of the low friction wet Jennite
surface and onto a higher friction road surface. The test data show
that this was always accompanied by braking on the steer axle, which is
indicative of oversteer corrections being commanded by the ESC. Vehicle
B had much less increase in lateral acceleration at the end of the
maneuver and appeared to be under control. Late in the maneuver the
commanded brake pressures for Vehicle B showed that both front and rear
brake applications were made on the right side of the tractor, and the
application pressures were nearly identical. Whether this is a data
collection anomaly or stability control braking strategy is not
certain, but Vehicle B was the vehicle that exhibited the least amount
of oversteer.
The RWD test results demonstrated that the stability control
systems on these tractors correctly identified the vehicle loss of
control problems (severe oversteer and understeer) and took corrective
action, including engine output torque intervention and commanding
individual applications of the tractor's foundation brakes. However,
the severity of the RWD test maneuver was sufficiently high to
overdrive the capability of the stability control systems to mitigate
severe understeer.
In summary, EMA provided test data for nine tractors each tested
for the three maneuvers developed by NHTSA researchers. The nine
tractors included a wider variety of tractor configurations than those
tested by the agency, and included severe service tractors, tractors
with auxiliary lift axles, a tridem drive axle tractor, and a very
short wheelbase two-axle tractor. Slowly increasing steer vehicle
characterization tests were conducted on all nine tractors (two with
RSC and seven with ESC) in the bobtail condition and the test data were
used to extrapolate the steering wheel angle that would provide 0.5g of
lateral acceleration at 30 mph. These data produced a wider range of
steering wheel angles than had been seen from the agency's tests on its
three tractors, with the short wheelbase 4x2 having an angle of only
116 degrees, and a 6x4 tractor with a liftable pusher axle having the
highest angle at 291 degrees.
EMA provided ramp steer maneuver test results for eight tractors
that were loaded to their GVWRs using an unbraked 28-foot control
trailer. Data were only provided for tests with the stability control
system enabled, and the RSM was conducted up to speeds at which the
system could successfully intervene. The range of speeds achieved at
the point of overdriving the stability control systems was similar to
the range of speeds from the VRTC RSM tests, although the loading
conditions were slightly different. The two 4x2 tractors (one with RSC,
and one with ESC) tested by EMA experienced oversteer and wheel lift,
while the other tractors all experienced wheel lift.
SWD test results were provided for three tractors, each equipped
with ESC, using a 0.5 Hz sinusoidal steering input frequency and a 1.0
second dwell time, and the tractors were tested in the bobtail
condition and loaded to 60 percent of drive axle(s) GAWR. In the tests
on dry pavement at a maneuver entrance speed of 45 mph, the typical 6x4
completed all tests, while the 6x4 equipped with a lift axle (tested in
the lowered position) also completed all tests but appeared to be
slower to respond to the steering inputs. The short wheelbase 4x2
tractor appeared to exhibit control problems and at the highest
steering wheel angle tested. The sine with dwell tests on the three
tractors in the bobtail condition were conducted on a low-friction wet
Jennite test surface with a lower maneuver entrance speed of 30 mph. In
these tests, the short wheelbase 4x2 tractor completed all tests, while
the two 6x4 tractors appeared to experience severe understeer at the
higher steering wheel angles tested.
5. Vehicle J Testing
(a) EMA Testing of Vehicle J
In December 2010, EMA provided testing data on a tenth vehicle they
tested.\28\ Vehicle J was intended to be representative of a typical
6x4 tractor, with a 245 inch wheelbase and a GVWR of 52,350 pounds. EMA
subjected Vehicle J to four different test maneuvers: The slowly
increasing steer test; the sine with dwell test; a J-turn maneuver, and
a wet Jennite drive through test.
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\28\ Vehicle J data provided to the agency has been placed in
Docket No. NHTSA-2010-0034-0022 and Docket No. NHTSA-2010-0034-0023.
---------------------------------------------------------------------------
EMA first conducted the slowly increasing steer test maneuver with
a steering controller on Vehicle J to determine the steering wheel
angle that would produce a lateral acceleration of 0.5g. EMA conducted
two series of test runs, one in each direction. A best fit linear
regression was used to determine that the average steering angle on the
six runs that would produce a lateral acceleration of 0.5g was 197
degrees. This value was used for subsequent testing.
EMA next conducted sine with dwell testing. EMA conducted two
series of SWD tests--one with the ESC system on and one with the ESC
system off. EMA equipped the vehicle with an FMVSS No. 121 control
trailer and loaded the
[[Page 30786]]
vehicle so that the drive axles were loaded to 60 percent of the GAWR,
which resulted in the vehicle being loaded to approximately 78.6
percent of its GVWR.
EMA provided data on six runs of the SWD maneuver. EMA conducted
the test at scalars from 0.8 to 1.3 of the SIS-derived steering wheel
angle. EMA also provided data on three runs of the SWD maneuver with
the system deactivated. Those tests were conducted at scalars of 1.0
and 1.3, and 1.5.
Each test run with the system enabled showed a 20- to 25-mph
reduction of speed during the test maneuver. In contrast, tests
conducted with the system off indicated only limited speed reduction of
less than five mph. This indicated that the ESC system acted to reduce
vehicle speed.
Each test run with the system enabled conducted at scalars between
0.8 and 1.2 resulted in a peak lateral acceleration between 0.6g and
0.7g. The lateral acceleration then quickly dropped to zero within 0.3
to 0.4 seconds after the completion of the steer. Yaw rate during the
dwell portion of the maneuver peaked at approximately 18 to 22 degrees
per second, except at a scalar of 1.2 where yaw rate peaked at
approximately 24 degrees per second) and showed a downward trend during
the dwell, dropping by approximately five degrees per second. The yaw
rate dropped to zero within 0.2 seconds after completion of steer. The
vehicle's ESC system used selective braking to reduce the speed,
lateral acceleration, and yaw rate responses.
With the system disabled, the test run at a scalar of 1.0 resulted
in a peak lateral acceleration of approximately 0.8g. A 0.2g drop in
lateral acceleration was observed at the beginning of the dwell portion
of the maneuver followed by a sudden rise of the same amount,
indicating possible oversteer. The lateral acceleration dropped to zero
less quickly than in tests with the system on (approximately 0.5
seconds) after completion of steer. This was largely due to the drop in
lateral acceleration starting later with the system off than with the
system on. The yaw rate peaked at approximately 21 degrees per second.
Unlike with the system on, there was not a clear drop in yaw rate
during the dwell portion of the maneuver. The yaw rate also dropped to
zero slower than in tests with the system off (approximately 0.25
seconds after completion of steer).
For test runs at steering wheel angle scalars of 1.3, the peak
lateral acceleration was slightly lower with the system on
(approximately 0.75g) in comparison to the test run with the system off
(over 0.8g). Momentary variability in lateral acceleration was observed
in both tests, indicating possible tractor instability. Again, with the
system on, the lateral acceleration decayed faster at the completion of
steer (approximately 0.4 seconds) than it did with the system off (over
0.6 seconds). This was largely due to the reduction in lateral
acceleration starting later with the system off than with the system
on. The yaw rate peaked for both tests at approximately 25 degrees per
second. Again, however, the yaw rate decreased by approximately five
degrees during the dwell portion of the maneuver with the system on
while no clear decay was observed with the system off. Also, the yaw
rate decreased to zero slower after completion of steer with the system
off (0.25 seconds) than it did with the system on (less than 0.2
seconds).
EMA also submitted data on one SWD test run with the system off at
a steering wheel angle scalar of 1.5. Peak lateral acceleration
observed during this test run was nearly 0.9g. The lateral acceleration
rate dropped to zero in slightly over 0.5 seconds after completion of
steer. The yaw rate peaked at approximately 24 degrees per second.
Unlike in runs with lower steering wheel angles, a reduction in yaw
rate was observable during the dwell portion. However, that reduction
was much sharper, occurring entirely within a 0.5 second period rather
than throughout the entire 1.0 second dwell period. Like in prior
tests, the yaw rate dropped to zero within approximately 0.25 seconds.
EMA's SWD maneuver test data from Vehicle J demonstrated that the
ESC system activated to lower lateral acceleration and yaw rate during
the SWD maneuver. However, even with the ESC system turned off, the
lateral acceleration and yaw rates dropped relatively quickly at the
end of the test maneuver, indicating that the vehicle did not become
unstable during testing. Although EMA only provided test data from
three runs with the system off compared to six runs with the system
enabled, the runs with the system off did include a run with a steering
wheel angle scalar of 1.5, which was higher than any run in NHTSA's
testing, and no severe incidents of instability were observed.
EMA next conducted J-turn testing both with the system enabled and
disabled. The test was conducted on a 150-foot fixed radius curve. The
vehicle was tested with an FMVSS No. 121 control trailer and was loaded
to the FMVSS No. 121 loading conditions. The tests were conducted at
initial entry speeds of 30 to 36 mph, in increments of two mph.
In tests conducted with the ESC system enabled, system activation
occurred at each test speed. The system commanded brake activations to
reduce vehicle speed to 18 mph from initial speeds of 30 mph and 32
mph, down to 10 mph from an initial speed of 34 mph, and down to 6 mph
at an initial speed of 36 mph. The vehicle was able to maintain the
lane at all speeds tested. Lateral acceleration peaked at 0.4 to 0.5g
at 30 and 32 mph and peaked at 0.6g at 34 mph and 36 mph. Yaw rate
peaked at approximately 15 degrees per second at 30 and 32 mph and
peaked at approximately 20 degrees per second at 34 mph and 36 mph. At
the higher speeds tested, lateral acceleration and yaw rate were
observed to drop coincident with speed.
With the system disabled, no reduction in speed during the maneuver
was observed. Thus, lateral acceleration and yaw rates remained
relatively constant throughout the maneuver. At test speeds of 30 and
32 mph, lateral acceleration peaked at approximately 0.55 to 0.65g and
yaw rate peaked at approximately 20 degrees per second. At 34 mph, the
lateral acceleration peaked at approximately 0.9g and the steering
wheel angle necessary to maintain the lane decreased substantially. Yaw
rate peaked at approximately 22 degrees per second and dropped to
approximately 15 degrees per second, indicating the vehicle was
starting to plow out. At 36 mph, the vehicle plowed out of the lane.
The fourth maneuver EMA performed on Vehicle J was a wet Jennite
drive-through (WJDT) maneuver. This maneuver was intended to test yaw
stability. The WJDT maneuver is identical to method for determining the
maximum drive-through speed when testing vehicles for compliance with
S5.3.6.1 of FMVSS No. 121. The vehicle is driven through a 500-foot
radius curve with a wet surface having a peak coefficient of friction
of approximately 0.5 at successively increasing speeds (up to 40 mph)
to determine the maximum speed at which the vehicle can maintain the
curve.\29\
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\29\ To conduct the FMVSS No. 121 stability and control during
braking compliance test, the vehicle is driven at the lesser of 30
mph or 75 percent of the maximum drive-through speed. A full brake
application is made and a vehicle must stop at least three times out
of four within the 12-foot lane.
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EMA performed this test with both the stability control system
enabled and disabled in two load configurations. First, the vehicle was
tested in the bobtail (unloaded) configuration.
[[Page 30787]]
Second, the vehicle was loaded to the FMVSS No. 121 test loading
condition.
In the bobtail configuration with the ESC system enabled, test runs
at 30 and 32 mph yielded no system activation. At 33 mph, system
activation occurred as both engine torque reduction and selective
braking to improve yaw stability occurred. As a result, the vehicle
speed decreased to approximately 29 mph during the maneuver and the
driver responded by rapidly straightening the steering wheel. Vehicle
yaw rate peaked at approximately 10 degrees per second. A second run at
33 mph showed only brief system activation and a minimal reduction in
speed. During two runs at 34 mph, ESC system intervention was again
observed as torque reduction and selective braking reduced vehicle
speed to 28 to 29 mph and the driver again responded by rapidly
straightening the steering wheel. Yaw rate peaked at near 10 degrees
per second and again, as the driver responded, decreased. During two
runs at 35 mph, the vehicle was unable to maintain the lane due to
understeer, despite system intervention.
In the bobtail configuration with the system disabled, at 32 mph,
the driver had to adjust steering by adding steering input during both
runs attempted at this speed, indicating substantial understeer. During
two runs at 33 mph, the vehicle was unable to maintain the lane,
despite large steering inputs from the driver.
In the loaded configuration with the ESC system enabled, system
activation occurred at a speed of 30 mph, though only slight (1 to 2
mph) reduction in speed was observed. The driver had to increase his
steering input, but there was no corresponding increase in yaw rate,
indicating understeer. At 32 mph, both engine torque reduction and
selective braking occurred to improve yaw stability occurred. As a
result, the vehicle speed decreased to approximately 27 to 28 mph
during the maneuver. At 34 mph, the ESC system intervened more
substantially, resulting in a reduction of speed to approximately 26
mph. Nevertheless, the vehicle was able to maintain the lane. At 35
mph, the vehicle was unable to maintain the lane due to understeer,
despite system intervention.
In the loaded configuration with the system disabled, understeer
was observed at 32 mph, as evident by substantial increase in steering
input by the driver; however, the vehicle was able to maintain the
lane. At 33 mph, the vehicle was unable to maintain the lane.
The maximum drive through speed in both vehicle configurations was
only 32 mph with the system off, compared to 34 mph with the system on.
This demonstrates that an ESC system has some ability to mitigate
understeer when navigating a curve on a low-friction surface, and allow
the driver to maintain control at higher curve entrance speeds.
(b) NHTSA Testing of EMA's Vehicle J
At NHTSA's request, EMA provided Vehicle J to NHTSA for NHTSA to
duplicate EMA's testing.\30\ In particular, the agency was interested
in the performance of Vehicle J during the sine with dwell maneuver.
NHTSA's two 6x4 tractors that were tested in with the SWD represented
the upper and lower size bounds of what would be considered a typical
6x4 tractor and both tractors could not maintain stability during a SWD
maneuver with the ESC system disabled. Vehicle J's size is within the
bounds of the two typical 6x4 tractors tested by NHTSA.
---------------------------------------------------------------------------
\30\ A copy of NHTSA's Vehicle J test data has been placed in
the docket. Docket No. NHTSA-2010-0034-0044.
---------------------------------------------------------------------------
NHTSA conducted 20 test runs of Vehicle J in the SWD maneuver at
steering wheel angle scalars of 0.4 to 1.3 of the SIS-derived steering
wheel angle attached to VRTC's FMVSS No. 121-style control trailer.
When tested with the ESC system disabled at a steering wheel angle
scalar of 1.2, NHTSA was able to detect lateral instability that
continued for almost two seconds after completion of the SWD
maneuver.\31\
---------------------------------------------------------------------------
\31\ NHTSA was able to conduct 19 test maneuvers with Vehicle J
that did not result in substantial roll instability. NHTSA did not
find any yaw instability in any of the 20 test maneuvers.
---------------------------------------------------------------------------
It was discovered that EMA conducted its testing of Vehicle J with
a control trailer with different specifications than NHTSA used. NHTSA
then attempted to duplicate EMA's Vehicle J's testing using the control
trailer used by EMA.\32\ The results of NHTSA's tests with EMA's
control trailer were not meaningfully different than the results of
EMA's testing. That is, there were no instances of substantial roll or
yaw instability in 20 test runs conducted by NHTSA.
---------------------------------------------------------------------------
\32\ NHTSA's test data identifies the trailer used by EMA as a
``Link'' trailer and the trailer used by NHTSA as the ``NHTSA'' or
``VRTC'' trailer.
---------------------------------------------------------------------------
As a result of NHTSA's testing of Vehicle J, the agency discovered
that there exist three areas of variability in FMVSS No. 121-style
control trailers and loading which, while not necessarily relevant to
FMVSS No. 121 testing, could affect the results of stability control
system testing if the specifications for an FMVSS No. 121-style control
trailer were simply carried over to a stability control standard.
First, EMA's control trailer had a wider track width \33\ than NHTSA's
trailer, which made EMA's trailer, and thereby the combination vehicle,
more stable during SWD testing. Second, EMA's control trailer had a
lower deck height than NHTSA's trailer, which contributed to a lower
center of gravity on EMA's trailer. Third, EMA loaded its trailer with
steel for ballast, whereas NHTSA loaded its trailer with concrete for
ballast, which also contributed to the lower center of gravity on EMA's
trailer because steel would not have to be stacked as high to achieve a
full load.
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\33\ The track width is the distance between the centerlines of
a vehicle's left and right tires. In vehicles with dual tires, the
track width would be measured from between the dual tires on each
side of the vehicle.
---------------------------------------------------------------------------
E. Other Industry Research
The SAE Truck and Bus Control Systems Task Force (renamed as the
Truck and Bus Stability Control Committee) was formed in 2007 to
facilitate information sharing among the industry and government
regarding heavy vehicle stability control systems.\34\ The information
shared included proposed test maneuvers that could potentially be used
to evaluate the performance of stability control systems. Although the
Task Force has not published any formal documents describing these test
maneuvers, the following provides an overview of the maneuvers that
have been discussed.
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\34\ See http://www.sae.org/events/cve/presentations/2007truckbus.pdf for an overview of the SAE Truck and Bus Council
organizational chart.
---------------------------------------------------------------------------
1. Decreasing Radius Test
A decreasing radius test (DRT) was developed to evaluate the roll
stability performance of a heavy vehicle stability control system.\35\
With the DRT, the test conditions could also be adjusted to evaluate
yaw stability as well. In the DRT, the vehicle is accelerated to a
constant speed of 29 mph on a dry road surface, and an initial steering
input is made to follow a curve with a 150-foot radius. Once the
initial curve radius is achieved, the radius is linearly reduced to a
radius of 90 feet as the vehicle negotiates 120 degrees of arc. Thus,
it is similar to the J-turn maneuver. The speed of 29 mph was derived
based on a vehicle dynamics simulation, which estimated that the
maneuver would produce 0.3g of lateral acceleration during the initial
steering input and this would steadily increase to 0.6g at the 90-foot
radius curve.
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\35\ See Docket No. NHTSA-2010-0034-0036.
---------------------------------------------------------------------------
Tests would be conducted in a loaded condition with the tractor
coupled to a trailer and an unloaded condition in a
[[Page 30788]]
bobtail configuration. Because actual vehicle testing had not been
conducted using this maneuver, pass/fail criteria have not yet been
developed. Simulations of this test have been run using driver-
controlled steering inputs; however, parameters could also be developed
to conduct this maneuver using an automated steering controller.
2. Lane Change on a Large Diameter Circle
Volvo provided information on the Lane Change on a Large Diameter
Circle (LC-LDC) maneuver that they have used to evaluate stability
control system performance.\36\ In this maneuver the vehicle is driven
at a constant speed, just below the threshold speed for rollover or
loss of control, around the inside lane of an 800-foot radius curve
that has two lanes. The driver then drifts to the outside lane, and
steers back into the inside lane. For rollover testing the asphalt road
surface is dry and for yaw testing the surface is wet. The test can be
conducted using a bobtail tractor, a tractor towing an FMVSS No. 121
control trailer, or a tractor towing any other type of trailer in a
fully loaded condition. Volvo evaluated the roll stability performance
during this maneuver based on whether the trailer outrigger made
contact with the ground. Volvo considers this maneuver to be
representative of certain highway segments that are encountered, and
that the maneuver is severe enough to fully challenge a stability
control system.
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\36\ See Docket No. NHTSA-2010-0034-0042.
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3. Yaw Control Tests
Bendix developed two yaw stability test maneuvers to evaluate the
ability of stability control systems to prevent severe oversteer and
understeer conditions. The first test maneuver is a Sinusoidal Steering
Maneuver (SSM) to evaluate oversteer prevention.\37\ The first step in
this test is to identify the steering wheel angle that produces a
tractor lateral acceleration of 0.5g at 30 mph on dry pavement with the
tractor in the bobtail condition. Bendix recommended that this angle be
derived by either a slowly increasing steer test (SIS test described in
section IV.D.2 above) or an equation developed by Bendix for estimating
the angle based on the tractor's wheelbase:
\37\ See Docket No. NHTSA-2010-0034-0037.
---------------------------------------------------------------------------
Steering Wheel Angle ([delta]) = (35.5 x (tractor wheelbase in meters))
+ 30.94
The Sinusoidal Steering Maneuver test is then conducted with the
tractor in the bobtail condition using a low-friction wet Jennite road
surface (nominal peak friction coefficient of 0.5). The vehicle is
driven at a constant speed of approximately 30 mph and, as a sinusoidal
steering input is initiated (continuous left and right steering inputs
using the steering wheel angle determined above), the driver increases
the throttle position to request 100 percent of engine torque.
The second test maneuver developed by Bendix was the ramp with a
dwell maneuver discussed in section IV.D.4 above.\38\ The RWD maneuver
is intended to evaluate understeer prevention, though oversteer can
also occur during the maneuver. The RWD test is conducted with the
tractor in the bobtail condition and using a wet Jennite road surface.
The first step in this test is to characterize the vehicle's steering
by conducting a series of drive-through speed evaluations at a constant
speed on a 500-foot radius curve. Once the maximum constant travel
speed is determined (typically between 28 and 32 mph, but not to exceed
35 mph), the steering wheel angle is measured for negotiating the curve
at that speed. The RWD test maneuver speed is then conducted at the
maximum drive-through speed. Bendix suggested that manual steering by a
test driver or an automated steering machine could be used. Once the
vehicle has been accelerated to the test maneuver speed, the speed is
held constant by the driver and he inputs the drive-through steering
wheel angle. After the vehicle reaches a constant lateral acceleration
condition, the steering wheel angle is increased to 180 degrees in a
period of one second. That increased angle is held constant for three
seconds, and then the angle is reduced to zero in a period of one
second. Subsequent test runs are conducted by increasing the steering
wheel angle in increments of 90 degrees up to 540 degrees.
---------------------------------------------------------------------------
\38\ See Docket No. NHTSA-2010-0034-0038.
---------------------------------------------------------------------------
The RWD test performance measures would be based upon test data
showing that the vehicle's stability control system successfully
identified a vehicle control problem (understeer or oversteer) and
intervened by reducing the engine torque output and commanding the
application of individual foundation brakes in a manner that is
suitable to mitigate the control problem. Bendix did not believe that
vehicle yaw or path-following pass/fail criteria would be appropriate
for this test maneuver.
Two maneuvers that the industry has developed to evaluate the
performance of stability control systems, lane change on a large
diameter circle and sinusoidal steering, can be used to demonstrate
that a stability control system is capable of preventing a rollover or
a yaw instability condition. The RWD maneuver may exceed the
capabilities of stability control systems but provides brake
application data that can be reviewed to determine if a stability
control system provides the correct control responses to address a
severe oversteer or understeer condition.
V. Agency Proposal
Based upon the foregoing research, the agency is proposing a new
FMVSS to require ESC systems be installed on truck tractors and buses
with a GVWR of greater than 11,793 kilograms (26,000 pounds).\39\ There
are several issues raised by this proposed rule on which the agency
seeks public comment, each of which is discussed in detail in the
following sections.
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\39\ To distinguish this new FMVSS from the light vehicle ESC
requirement in FMVSS No. 126, we are proposing to revise the title
FMVSS No. 126 to reflect that it is applicable only to light
vehicles.
---------------------------------------------------------------------------
A. NHTSA's Statutory Authority
NHTSA is proposing today's NPRM under the National Traffic and
Motor Vehicle Safety Act (``Motor Vehicle Safety Act''). Under 49
U.S.C. Chapter 301, Motor Vehicle Safety (49 U.S.C. 30101 et seq.), the
Secretary of Transportation is responsible for prescribing motor
vehicle safety standards that are practicable, meet the need for motor
vehicle safety, and are stated in objective terms. ``Motor vehicle
safety'' is defined in the Motor Vehicle Safety Act as ``the
performance of a motor vehicle or motor vehicle equipment in a way that
protects the public against unreasonable risk of accidents occurring
because of the design, construction, or performance of a motor vehicle,
and against unreasonable risk of death or injury in an accident, and
includes nonoperational safety of a motor vehicle.'' ``Motor vehicle
safety standard'' means a minimum performance standard for motor
vehicles or motor vehicle equipment. When prescribing such standards,
the Secretary must consider all relevant, available motor vehicle
safety information. The Secretary must also consider whether a proposed
standard is reasonable, practicable, and appropriate for the types of
motor vehicles or motor vehicle equipment for which it is prescribed
and the extent to which the standard will further the statutory purpose
of reducing traffic accidents and associated deaths. The
[[Page 30789]]
responsibility for promulgation of Federal motor vehicle safety
standards is delegated to NHTSA. In making the proposals in today's
NPRM, the agency carefully considered all the aforementioned statutory
requirements.
B. Applicability
1. Vehicle types
Vehicles with a GVWR greater than 10,000 pounds include a large
variety of vehicles ranging from medium duty pickup trucks to different
types of single unit trucks, buses, trailers and truck tractors.
Vehicles with a GVWR of greater than 10,000 pounds are divided into
Classes 3 through 8. Class 7 vehicles are those with a GVWR greater
than 11,793 kilograms (26,000 pounds) and up to 14,969 kilograms
(33,000 pounds), and Class 8 vehicles are those with a GVWR greater
than 14,969 kilograms (33,000 pounds).
The vast majority of vehicles with a GVWR of greater than 4,536
kilograms (10,000 pounds) for which stability control systems are
currently available are truck tractors. Approximately 150,000 truck
tractors with a GVWR of greater than 11,793 kilograms (26,000 pounds)
are manufactured each year. In 2009, about 20 percent of Class 7 and 8
truck tractors were equipped with a stability control system.
About 85 percent of truck tractors sold annually in the U.S. are
air-braked three-axle (6x4) tractors with a front axle that has a GAWR
of 14,600 pounds or less and with two rear drive axles that have a
combined GAWR of 45,000 pounds or less, which we will refer to as
``typical 6x4 tractors.'' Two-axle (4x2) tractors and severe service
tractors (those with three axles that are not ``typical 6x4 tractors''
or those with four or more axles) represent about 15 percent of the
truck-tractor market in the U.S.
The majority of the research on the effectiveness of stability
control systems to date has been performed on typical 6x4 tractors. As
a result, the agency's research included two typical 6x4 tractors. The
agency also included one 4x2 tractor in its testing because two-axle
tractors represent the next largest segment of the truck-tractor
market. No severe service tractors were tested. EMA performed tests on
nine tractors equipped with stability control systems. The tractors
included two 4x2 tractors, two typical 6x4 tractors, two severe service
6x4 tractors, two 6x4 tractors with a liftable auxiliary axle in front
of the drive axles, and one 8x6 tractor.
This proposal would also require certain buses to be equipped with
an ESC system. We intend the applicability of this proposed requirement
to be similar to the applicability of the agency's proposal that
certain buses be equipped with seat belts.\40\ That proposal was
applicable to buses with a gross vehicle weight rating (GVWR) of 11,793
kilograms (26,000 pounds) or greater, 16 or more designated seating
positions (including the driver), and at least 2 rows of passenger
seats that are rearward of the driver's seating position and are
forward-facing or can convert to forward-facing without the use of
tools.'' That proposal excluded school buses and urban transit buses
sold for operation in urban transportation along a fixed route with
frequent stops. The agency is proposing a very similar applicability in
this NPRM. We have not made this proposal applicable to buses with a
GVWR of exactly 11,793 kilograms (26,000 pounds) in order to exclude
Class 6 vehicles from this proposal. We believe that this proposal
encompasses the category of ``cross-country intercity buses''
represented in the FARS and FMCSA data (identified in section II.A
above) that had a higher involvement of crashes that ESC systems are
capable of preventing.
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\40\ 75 FR 50,958 (Aug. 18, 2010).
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The agency tested three buses, all of which had a GVWR over 14,969
kg (33,000 pounds). There are seven manufacturers or distributors of
Class 8 buses covered by this proposal for the U.S. market: Prevost,
MCI, VanHool, Daimler/Setra, CAIO, BlueBird, and BCI. Three of them
(Prevost, MCI, and VanHool), have stated that an ESC system is a
standard feature on their buses sold in the U.S. Daimler/Setra
indicated that an ESC system will be available as an option on its
buses beginning in model year 2011 and that no decision has been made
to make it a standard feature. No official information is available
from CAIO, Bluebird, and BCI regarding ESC system availability.
There are also at least nine manufacturers of Class 7 buses covered
by this proposal for the U.S. market: Champion, ElDorado National,
Federal Coach, Glaval, IC Bus, MCI, Rexhall, Stallion, and VanHool.
Many Class 7 buses are built on chassis similar to those of single unit
trucks for which ESC has not been widely developed, and we are not
aware of any Class 7 bus that is equipped or currently available with
ESC. Class 7 buses represent less than 20 percent of the market.
Although the agency is not aware of any Class 7 bus currently available
with ESC, we are aware that stability control systems are available on
a limited number of Class 8 single unit trucks, such as ready mix
concrete trucks, refuse trucks, and other air-braked trucks, and that
the same technology could be developed for use on Class 7 buses, which
we believe are also air-braked vehicles.
Although this proposal would not apply to all buses with a GVWR of
greater than 11,793 kilograms (26,000 pounds), we seek comment on
whether this proposal should be applied to the types of buses that are
excluded from the proposed rule such as school buses and transit buses.
We also seek comment on the feasibility of including the Class 7 buses
described in the prior paragraph that are built on chassis similar to
those of single unit trucks within two years. In particular, we believe
that ESC systems are readily available for air-braked buses; however,
system availability for any hydraulically braked buses that may be
covered by this proposed rule may be more limited. If hydraulically
braked buses are covered by this proposal, we request comment on
manners in which hydraulically braked buses may be differentiated for
exclusion or a different phase-in period.
The agency is not proposing to include single unit trucks with a
GVWR over 4,536 kg (10,000 pounds) at this time. There are substantial
differences in the complexity of the single unit truck population
compared to the truck-tractor population. The single unit truck
population has wide variations in vehicle weight, wheelbase, number of
axles, center of gravity height, and cargo type, among other things
that affect the calibration and performance of stability control
systems. While some variation exists in the truck tractor market, the
degree of complexity and diversity is substantially less.
Further, the single unit truck market is structurally different
than the truck tractor market in that the chassis supplier, who is
generally responsible for the brake systems and therefore would likely
provide stability control systems, is often different than the final
body builder. Hence, the chassis supplier may not have knowledge of
critical vehicle design parameters that would affect stability control
system calibration. In contrast, manufacturers of truck tractors have
more complete control of the final, delivered vehicle.
The complexity of the single unit truck population and the limited
crash data available present a significant challenge to determining the
effectiveness of stability control on these vehicles. We believe that
approximately 1 percent of newly manufactured single-unit trucks are
equipped with stability control systems, and that few, if any, of those
are for
[[Page 30790]]
vehicles with hydraulic brakes. The development of stability control
system for vehicles over 10,000 pounds GVWR has been focused on air-
braked vehicles, which include the truck tractors and buses addressed
in this proposal. Because we are concerned about the availability of
production-ready systems on these vehicles, they are not included in
the proposal. However, we seek comment on these observations.
The agency has initiated a safety benefit study to determine the
safety need for stability control on single-unit trucks, and has also
initiated vehicle research, similar to the research conducted on truck
tractors and large buses described in part IV.C above, which is
expected to be completed in 2012. However, the agency proposes to
require stability control systems on truck tractors without waiting for
the study on the effectiveness of stability control systems on single-
unit trucks to be completed. Waiting for that study to be completed
would unnecessarily delay the benefits of having stability control
systems on truck tractors and large buses, for which testing has been
completed the benefits of stability control systems identified.
The agency is not proposing to include a requirement for stability
control systems on trailers, primarily because trailer-based RSC
systems were judged by the agency research to be much less effective
than tractor-based RSC or ESC systems in preventing rollover. Trailer-
based RSC systems are capable of applying braking only on the trailer's
brakes. Tractor-based systems can command more braking authority by
using both the tractor and trailer brakes. As a result, trailer-based
RSC systems do not appear to provide additional safety benefits when
used in combination with tractor-based RSC or ESC systems. The trailer-
based RSC systems provide some improvement in roll stability compared
to a base trailer without an RSC system, but a vehicle could still be
overdriven at a lower speed with trailer-based RSC systems than with a
tractor-based system. This means that the maneuver entrance speed
beyond which the stability control system is unable to reduce the
vehicle speed to prevent a rollover was lower for the trailer-based
system than for the tractor-based system. In addition, the typical
service life of a trailer is 20 to 25 years compared with about 8 to 10
years for a truck tractor. Because new tractors are added to the U.S.
fleet at a faster rate than new trailers, the safety benefits from
stability control systems would be achieved at a faster rate by
requiring stability control systems to be installed on a tractor.
Therefore, the agency proposes to require stability control systems
on truck tractors and buses with a GVWR of greater than 11,793
kilograms (26,000 pounds).
2. Retrofitting In-Service Truck Tractors, Trailers, and Buses
NHTSA has considered proposing to require retrofitting of in-
service truck tractors, trailers, and large buses with stability
control systems proposed to be required by this NPRM. The Secretary has
the statutory authority to promulgate safety standards for ``commercial
motor vehicles and equipment subsequent to initial manufacture.'' \41\
The Secretary has delegated authority to NHTSA to ``promulgate safety
standards for commercial motor vehicles and equipment subsequent to
initial manufacture when the standards are based upon and similar to
[an FMVSS] promulgated, either simultaneously or previously, under
chapter 301 of title 49, U.S.C.'' \42\ Additionally, the Federal Motor
Carrier Safety Administration (FMCSA) is authorized to promulgate and
enforce vehicle safety regulations, including those aimed at
maintaining commercial motor vehicles so they continue to comply with
the safety standards applicable to commercial motor vehicles at the
time they were manufactured. Although this NPRM does not propose
requiring truck tractors, trailers, or large buses to be equipped with
stability control systems ``subsequent to initial manufacture,'' we are
requesting public comment on several issues related to retrofitting in-
service truck tractors, trailers, and buses:
---------------------------------------------------------------------------
\41\ See Motor Carrier Safety Improvement Act of 1999, sec.
101(f), Pub. L. 106-159 (Dec. 9, 1999).
\42\ See 49 CFR 1.50(n).
---------------------------------------------------------------------------
The extent to which a proposal to retrofit in-service
vehicles with stability control systems would be complex and costly
because of the integration between a stability control system and the
vehicle's chassis, engine, and braking systems.
The changes necessary to an originally manufactured
vehicle's systems that interface with a stability control system, such
as plumbing for new air brake valves and lines and a new electronic
control unit for a revised antilock brake system.
The additional requirements that would have to be
established to ensure that stability control components are at an
acceptable level of performance for a compliance test, given the
uniqueness of the maintenance condition for vehicles in service,
particularly for items such as tires and brake components that are
important for ESC performance.
The original manufacture date of vehicles that should be
subject to any retrofitting requirements.
Whether the performance requirements for retrofitted
vehicles should be less stringent or equally stringent as for new
vehicles, and, if less stringent, the appropriate level of stringency.
The cost of retrofitting a stability control system on a
vehicle, which we believe would exceed the cost of including stability
control on a new vehicle.
In light of these questions, the agency is not proposing that in-
service vehicles be required to be retrofitted with stability control
systems. Instead, this proposed requirement would be applicable only to
newly manufactured vehicles. However, the comments we receive on the
issue of retrofitting will help us determine whether we should issue a
separate supplemental NPRM to require a retrofit.
3. Exclusions From Stability Control Requirement
Our proposed rule excludes certain types of low-volume, highly
specialized vehicle types. In these cases, the vehicle's speed
capability does not allow it to operate at speeds where roll or yaw
instability is likely to occur.
Specifically, FMVSS No. 121, Air brake systems, excludes certain
heavy air-braked heavy vehicles from that standard. For truck tractors
and buses, these exclusions include:
Any vehicle equipped with an axle that has a gross axle
weight rating of 29,000 pounds or more.
Any truck or bus that has a speed attainable in two miles
of not more than 33 mph.
Any truck that has a speed attainable in two miles of not
more than 45 mph, an unloaded vehicle weight that is not less than 95
percent of its GVWR, and no capacity to carry occupants other than the
driver and operating crew.
We believe that the vehicles that are excluded from the
requirements of FMVSS No. 121 should also be excluded from the proposed
stability control requirements because the speed at which these
vehicles operate would make it unlikely that roll or yaw instability
would occur. Accordingly, the proposed stability control requirement
excludes these vehicles.
C. ESC System Capabilities
1. Choosing ESC vs. RSC
We are proposing to require that truck tractors and large buses be
equipped
[[Page 30791]]
with ESC systems rather than RSC systems. An ESC system is capable of
all of the functions of an RSC system. In addition, an ESC system has
the additional ability to detect yaw instability, provide braking at
front wheels, and detect the steering wheel angle. These additions, as
demonstrated by NHTSA's testing, allow an ESC system to have better
rollover prevention performance than an RSC system in addition to the
yaw instability prevention component. This is because the steering
wheel angle sensor allows the ESC system to anticipate changes in
lateral acceleration based upon driver input and to intervene with
engine torque reduction or selective braking sooner, rather than
waiting for the lateral acceleration sensors to detect potential
instability.
As discussed in greater length in Section VI, mandating ESC systems
rather than RSC systems will prevent more crashes, injuries, and
fatalities. The additional benefits from ESC systems can be attributed
to both the ESC's system's ability to intervene sooner and its ability
to prevent yaw instability that would lead to loss-of-control crashes.
Mandating ESC systems rather than RSC systems will result in higher
costs to manufacturers. Moreover, our benefit and cost estimates lead
to the preliminary conclusion that mandating RSC systems would be more
cost-effective than mandating ESC systems. However, these extra costs
are more than offset by higher net benefits that would accrue by
mandating ESC systems rather than RSC systems.
2. Definition of ESC
Definitional requirements in an FMVSS define and describe the type
of system that can be used to meet the performance requirements of a
particular FMVSS. However, the inclusion of a definitional requirement
in an FMVSS may be design restrictive because it would be based on
currently available technology. Limiting the equipment that can be used
to satisfy an FMVSS may limit future technological advancements and
innovation. As stability control technologies are developed even
further, a definitional requirement could be a hindrance to safety
improvements if it limits the use of a newly developed equipment or
technology that is not addressed by the specified definitional
requirement. On the other hand, relying solely on performance-based
tests without mandating any specific equipment may require a battery of
tests to cover the complete operating range of the vehicle. Given the
wide array of possible configurations and operating ranges for heavy
vehicles, the agency does not believe it is practical to develop
performance tests that would address the full range of possibilities
and remain cost-effective. Accordingly, the agency is proposing to
include a definitional requirement in this proposed rule that includes
equipment that would be required as part of a compliant ESC system. We
note that, when developing the ESC requirement for light vehicles, the
agency chose to include such a requirement in FMVSS No. 126.
SAE International has a Recommended Practice on Brake Systems
Definitions--Truck and Bus, J2627 (Aug. 2009), which includes a
definition of Electronic Stability Control and Roll Stability Control.
SAE International's definition of an ESC system requires that a system
have an electronic control unit that considers wheel speed, yaw rate,
lateral acceleration, and steering angle and that the system must
intervene and control engine torque and auxiliary brake systems to
correct the vehicle's path.
The UN ECE Regulation 13 definition for the electronic stability
control system, promulgated in Annex 21, includes the following
functional attributes for directional control: sensing yaw rate,
lateral acceleration, wheel speeds, braking input and steering input;
and the ability to control engine power output. For vehicles with
rollover control, the functions required by the stability control
include: sensing lateral acceleration and wheel speeds; and the ability
to control engine power output.
In developing a definition for ESC, the agency has reviewed the
functional attributes contained in the SAE and the ECE definitions, and
has incorporated portions of both of these definitions in this NPRM. We
have developed a definition that is similar in wording to the
definition from FMVSS No. 126, which specifies certain features that
must be present, that ESC be capable of applying all the brakes
individually on the vehicle, and that it have a computer using a
closed-loop algorithm to limit vehicle oversteer and understeer when
appropriate. Unlike the light vehicle standard, which focuses on yaw
stability, this NRPM proposes to require a stability control system
that also helps to mitigate roll instability conditions. As a result,
we have expanded the definition from the one in FMVSS No. 126 to
include a requirement that the system be capable of sensing impending
rollover and reducing the vehicle's lateral acceleration to prevent
rollover.
Furthermore, we believe that the ESC system must be operational
during all phases of driving, including acceleration, coasting,
deceleration, and braking, except when the vehicle is below a low-speed
threshold where loss of control or rollover is unlikely. According to
information the agency has obtained from vehicle manufacturers and ESC
suppliers, this low speed threshold for a stability control system is
10 km/h (6.2 mph) for yaw stability control and 20 km/h (12.4 mph) for
roll stability control. For the purposes of a proposed regulation, we
believe that setting a single low speed threshold would be preferable
since the yaw and roll stability functions during a test maneuver are
closely intertwined, which could make it difficult to differentiate
when the roll or yaw function ends. Therefore, we propose a single
threshold of 20 km/h (12.4 mph) as the speed below which ESC is not
required to be operational.
Therefore, the agency proposes to require the installation of an
ESC system on truck tractors and large buses, which has all of the
following attributes:
1. Augments vehicle directional stability by applying and adjusting
vehicle brake torques individually at each wheel position on at least
one front and at least one rear axle of the vehicle to induce
correcting yaw moment to limit vehicle oversteer and to limit vehicle
understeer;
2. Enhances rollover stability by applying and adjusting the
vehicle brake torques individually at each wheel position on at least
one front and at least one rear axle of the vehicle to reduce lateral
acceleration of a vehicle;
3. Computer-controlled with the computer using a closed-loop
algorithm to induce correcting yaw moment and enhance rollover
stability;
4. Has a means to determine the vehicle's lateral acceleration;
5. Has a means to determine the vehicle's yaw rate and to estimate
its side slip or side slip derivative with respect to time;
6. Has a means to estimate vehicle mass or, if applicable,
combination vehicle mass;
7. Has a means to monitor driver steering input;
8. Has a means to modify engine torque, as necessary, to assist the
driver in maintaining control of the vehicle; and
9. When installed on a truck tractor, has the means to provide
brake pressure to automatically apply and modulate the brake torques of
a towed semi-trailer.
The benefit of an ESC system is that it will reduce vehicle
rollovers and loss of control under a wide variety of vehicle
operational and environmental conditions. However, the performance
[[Page 30792]]
tests proposed in this NPRM would only evaluate ESC system performance
under very specific environmental conditions. To ensure that a vehicle
is equipped with an ESC system that meets the proposed definition, we
are proposing that vehicle manufacturers make available to the agency
documentation that would enable us to ascertain that the system
includes the components and performs the functions of an ESC system.
We are proposing that the vehicle manufacturer provide a system
diagram that identifies all ESC system hardware; a written explanation,
with logic diagrams included, describing the ESC system's basic
operational characteristics; and a discussion of the pertinent inputs
to the computer and how its algorithm uses that information to prevent
rollover and limit oversteer and understeer. Because the proposed
definition for ESC systems on truck tractors includes the capability to
provide brake pressure to a towed vehicle, the agency is proposing to
require that, as part of the system documentation, the manufacturer
include the information that shows how the tractor provides brake
pressure to a towed trailer under the appropriate conditions.
It is common practice for the NHTSA's Office of Vehicle Safety
Compliance to request relevant technical information from a
manufacturer prior to conducting many of its compliance test programs.
The agency included such a requirement in the light vehicle ESC
standard. Prior to conducting any of the FMVSS No. 126 compliance
tests, NHTSA requires manufacturers to provide the documentation
required by that standard, including identification of all ESC system
hardware and an explanation of the system operational characteristics.
We also request additional information about the ESC system including
manufacturer make and model, telltale(s), pertinent owner's manual
excerpts and suggested malfunction scenarios. All of the requested
information allows NHTSA to verify that the ESC system meets the
definitional and operational requirements that cannot necessarily be
verified during the performance test. Furthermore, this information
aids the test engineers with execution and completion of the compliance
test.
D. ESC Disablement
The agency has also considered whether to allow a control for the
ESC to be disabled by the driver; however, heavy vehicles currently
equipped with ESC systems do not include on/off controls for ESC that
would allow a driver to deactivate or adjust the ESC system. Given the
lack of on/off switches on heavy vehicles equipped with ESC, we do not
propose to allow an on/off switch for ESC systems in this NPRM.
Nevertheless, we seek comment on the need to allow an on/off switch.
Such comments should address why manufacturers might need this
flexibility and how manufacturers would implement a switch in light of
the ABS requirements for truck tractors and large buses.
E. ESC Malfunction Detection, Telltale, and Activation Indicator
1. ESC Malfunction Detection
This proposed rule would require that vehicles be equipped with an
indicator lamp, mounted in front of and in clear view of the driver,
which is activated whenever there is a malfunction that affects the
generation or transmission of control or response signals in the
vehicle's ESC system. Heavy vehicles presently equipped with ESC
generally do not have a dedicated ESC malfunction lamp. Instead, they
share that function with the mandatory ABS malfunction indicator lamp
or the traction control activation lamp. The agency proposes requiring
a separate ESC malfunction lamp because it would alert the driver to
the malfunction condition of the ESC and would help to ensure that the
malfunction is corrected at the earliest opportunity.
We believe that there are safety benefits associated with such a
warning. An ESC malfunction indicator warns the driver in the event of
an ESC system malfunction so that the system can be repaired. ESC
system activations on a heavy vehicle will be infrequent events in
panic situations, and drivers should not experience the activation of a
stability control system during the normal operation of the vehicle.
Because most steering maneuvers performed during the normal operation
of a heavy vehicle are not severe enough to activate the ESC system, a
vehicle may be operated for long periods without an ESC activation
event. Without such a malfunction indicator, a driver might have no way
of knowing that an ESC system is malfunctioning until a loss of control
or rollover event occurs. For example, the agency received a complaint
recently in which a heavy truck had an inoperative ESC system, but the
driver was unaware of the malfunction, primarily due to the lack of a
malfunction indicator lamp. The agency believes that such a warning is
important to ensure that the driver could have the malfunction
corrected at the earliest opportunity in order to continue to realize
the system's safety benefits.
The ESC malfunction telltale would be required to remain
illuminated continuously as long as the malfunction exists whenever the
ignition locking system is in the ``On'' (``Run'') position. The ESC
malfunction telltale must extinguish after the malfunction has been
corrected. These proposed requirements are identical to the
requirements established in the light vehicle ESC standard, FMVSS No.
126, and help to ensure that the system provides a warning indication
in the event of a malfunction.
Because many malfunctions cannot be detected when the vehicle is
stationary, this NPRM includes a test that would allow the engine to be
running and the vehicle to be in motion as part of the diagnostic
evaluation. We are aware that some malfunctions are not time-based, but
instead require comparisons of sensor outputs generated when the
vehicle is driven. Hence, some malfunctions would require certain
driving motions to make the ESC system's malfunction detection
possible. We believe that an ESC malfunction should be detected within
a reasonable time of starting to drive. As a result, we propose that
the malfunction telltale illuminate within two minutes after attaining
a test speed of 48 km/h (30 mph) so that the parts of a system's
malfunction detection capability that depend on vehicle motion can
operate. This two-minute period is identical to the period included in
the test procedure in FMVSS No. 126 for ESC malfunction detection.
We anticipate that FMCSA will issue a companion proposal to NHTSA's
proposal to require ESC on truck tractors and large buses, which would
require that the ESC system on a commercial vehicle be maintained in a
fully operating condition. In addition, we expect that the roadside
inspection procedures developed for commercial vehicle ESC systems
would be facilitated by the ESC malfunction telltale and the format
that is required to indicate whether or not the system is operational.
2. ESC Malfunction Telltale
The ESC malfunction lamp requirement in this NPRM states that each
truck tractor and large bus must be equipped with a telltale that
provides a warning to the driver when one or more malfunctions that
affect the generation of control or response signals in the vehicle's
electronic stability control system is detected. Specifically, the ESC
malfunction telltale will be required to
[[Page 30793]]
be mounted in the driver's compartment in front of and in clear view of
the driver and be identified by the symbol shown for ``ESC Malfunction
Telltale'' or the specified words or abbreviations listed in Table 1 of
FMVSS No. 101, Controls and displays. FMVSS No. 101 includes a
requirement for the telltale symbol, or abbreviation, and the color
required for the indicator lamp to show a malfunction in the ESC
system.
The agency believes that the symbol used to identify ESC
malfunction should be standardized with the symbol used on light
vehicles. The symbol established in FMVSS No. 126 is the International
Organization for Standardization (ISO) ESC symbol, designated J.14 in
ISO Standard 2575. The symbol shows the rear of a vehicle trailed by a
pair of ``S'' shaped skid marks, shown below in Figure 5. The agency
found that the ISO J.14 symbol and close variations were the symbols
used by the greatest number of vehicle manufacturers that used an ESC
symbol before the requirement was established. Furthermore, FMVSS No.
126 allows, as an option, the use of the text ``ESC'' in place of the
telltale symbol. This same option is being proposed.
[GRAPHIC] [TIFF OMITTED] TP23MY12.005
The color of the ESC malfunction telltale specified in Table 1 of
FMVSS No. 101 for light vehicles equipped with ESC is yellow, which is
the color used to communicate to the driver the condition of a
malfunctioning vehicle system that does not require immediate
correction. The agency chose to associate indication of an ESC system
malfunction with a yellow telltale color as a warning to the driver
because we believe that it communicates the level of urgency with which
the driver must seek to remedy the malfunction of the ESC system.
For this proposed rule, we believe that the ESC malfunction
telltale and color designation developed for light vehicles would be
appropriate for use on heavy vehicles. Accordingly, the agency proposes
that the ESC malfunction telltale symbol and color requirements of
FMVSS No. 101 be proposed for use on truck tractors and buses, and that
the abbreviation ``ESC'' should be allowed as an option instead of the
symbol.
In addition to the ESC malfunction telltale being used to warn the
driver of a malfunction in the ESC, the telltale is also used as a
check of lamp function during vehicle start-up. We believe that the ESC
malfunction telltale should be activated as a check of lamp function
either when the ignition locking system is turned to the ``On''
(``Run'') position whether or not the engine is running. This function
provides drivers with the information needed to ensure that the ESC
system is operational before the vehicle is driven. It also provides
Federal and State inspectors with the means to determine the
operational status of the ESC system during a roadside safety
inspection.
Accordingly, this NPRM proposes that the ESC malfunction telltale
must be activated as a check of lamp function either when the ignition
locking system is turned to the ``On'' (``Run'') position when the
engine is not running or when the ignition locking system is in a
position between the ``On'' (``Run'') and ``Start,'' which is
designated by the manufacturer as a check position.
3. ESC Activation Indicator
The agency is requesting comment on whether there is a safety need
for an ESC activation indicator. In the light vehicle ESC rulemaking,
the agency considered the safety need for an ESC activation indicator
to alert the driver during an emergency situation that the ESC is
activating. NHTSA conducted a study using the National Advanced Driving
Simulator (NADS), which included experiments to gain insight into the
various possibilities regarding ESC activation indicators. The study
compared the performance of 200 participants in driving maneuvers on a
wet pavement, and used road departures and eye glances to the
instrument panel as measures of driver performance. The significant
finding was that the drivers who received various ESC activation
indicators did not perform better than drivers who were given no
indicator. That finding formed the basis for the agency's decision not
to require an ESC activation indicator for light vehicles.
F. Performance Requirements and Compliance Testing
The agency's research initially focused on a variety of maneuvers
which we could use to evaluate the roll stability performance and the
yaw stability performance of truck tractors and large buses. Several of
these maneuvers were also tested by industry and some of them are
allowed for use in testing for compliance to the UN ECE stability
control regulation. The agency's goal was to develop one or more
maneuvers that showed the most promise as repeatable and reproducible
roll and yaw performance tests for which objective pass/fail criteria
could be developed.
As the research program progressed, the data indicated that the
ramp steer maneuver to evaluate roll stability performance and the sine
with dwell maneuver to evaluate yaw stability performance were the most
promising. The slowly increasing steer maneuver was developed to
normalize testing conditions for each vehicle so that the level of
stringency for each test vehicle would be similar. The agency also
found that the SIS maneuver could also be used to evaluate the engine
torque reduction capability of a vehicle's ESC system, which is
important because engine torque reduction may bring a vehicle under
control before brakes are applied. After further testing, the agency
was able to develop test parameters for the SWD maneuver so that both
roll stability and the yaw stability could be evaluated using a single
maneuver and loading condition. This development eliminated the need
for the ramp steer maneuver to evaluate roll stability performance.
Therefore, based on testing at VRTC and the results from industry-
provided test data, two stability proposed performance tests have been
chosen to
[[Page 30794]]
evaluate ESC systems on truck tractors and large buses--the SIS test
and the SWD test.
The agency also considered the ECE performance tests for heavy
vehicle stability control systems, which are included in the brake
systems regulation, ECE Regulation 13. The performance test for a heavy
vehicle with a directional control function includes meeting the
requirements in one of eight tests allowed for compliance. The eight
tests are as follows: Reducing radius test (which is identical to the
decreasing radius test discussed above), step steer input test, sine
with dwell, J-turn, mu-split lane change, double lane change, reversed
steering test or ``fish hook'' test, and asymmetrical one period sine
steer or pulse steer input test. No test procedure or pass/fail
criteria are included in ECE Regulation 13, but it is left to the
discretion of the Type Approval testing authority in agreement with the
vehicle manufacturer to show that the system is functional.
The issue of whether the U.S. should adopt the stability control
requirements similar to those in ECE Regulation 13 is addressed in the
context of whether a definitional requirement specifying required
equipment along with a performance test that does not include a test
procedure or pass/fail criteria would be considered sufficiently
objective for a safety standard. The agency considered several of the
eight ECE tests that we believed showed the most promise for
repeatability and reproducibility, and decided to focus on the SWD
test, which is one of the eight tests allowed for compliance testing to
ECE Regulation 13. However, in light of the requirement in the Motor
Vehicle Safety Act that FMVSSs be stated in objective terms, NHTSA is
required to develop objective performance criteria for the SWD test to
be set forth in the regulatory text.
1. Characterization Test--SIS
The agency is proposing to conduct compliance testing
characterization using a slowly increasing steer to determine the
steering wheel angle needed to achieve 0.5g of lateral acceleration at
30 mph and also to evaluate the capability of the ESC system to reduce
engine torque. The SIS maneuver has been used for many years by the
agency and the industry to determine the unique dynamic characteristics
of a vehicle. This maneuver allows the agency to determine the
relationship between the steering wheel angle and lateral acceleration
for a vehicle, which varies due to different steering gear ratios,
different suspension systems, and wheelbase and other dimensions, among
other things. To normalize the severity of the SWD maneuver that
follows, each vehicle is tested based on its steering wheel angle
determined in the SIS maneuver. The agency is proposing a 0.5g lateral
acceleration target because our test results indicated that a truck
tractor or large bus is highly likely to experience instability at that
level of lateral acceleration. Even though the vast majority of truck
tractors are typical 6x4 tractors, there are other configurations, such
as those with 2-axle or 4-axle configurations and buses, which would
require a different steering wheel angle to normalize the test
conditions for each different vehicle.
To perform the SIS maneuver, the tractor or bus is driven at a
constant speed of 30 mph, and then the steering controller increases
the steering wheel angle at a slow, continuous rate of 13.5 degrees per
second. The steering wheel angle is increased linearly from zero to 270
degrees and then held constant for one second, after which the maneuver
concludes. The vehicle is subjected to two series of runs, one using
clockwise steering and the other using counterclockwise steering, with
three tests performed for each test series. During each test run, ESC
system activation must be confirmed. If ESC system activation does not
occur during the maneuver, then the commanded steering wheel angle is
increased by 270-degree increments up to the vehicle's maximum
allowable steering angle until ESC activation is confirmed.
From the SIS tests, the value ``A'' is determined. ``A'' is the
steering wheel angle, in degrees, that is estimated to produce a
lateral acceleration of 0.5g for that vehicle. Using linear regression
on the lateral acceleration data recorded between 0.05g and 0.3g for
each of the six valid SIS tests, a linear extrapolation is used to
calculate a steering wheel angle where the lateral acceleration would
be 0.5g. If ESC system activation occurs prior to the vehicle
experiencing lateral acceleration of 0.3g, then the data used during
the linear regression will be that data recorded between 0.05g and the
lateral acceleration measured at the time of ESC system activation. The
six values derived from the linear regression are then averaged and
rounded to the nearest 0.1 degree to produce the final quantity, ``A,''
used during the SWD maneuver.
As part of the SIS characterization test, the engine torque
reduction test is also conducted. As mentioned above, during each of
the six completed SIS maneuvers, ESC activation is confirmed by
verifying that the system automatically attempts to reduce engine
torque. To confirm ESC activation, engine torque output and driver
requested torque data are collected from the vehicle's J1939
communication data link and compared. During the initial stages of each
maneuver, the rate of change over time of engine torque output and
driver requested torque will be consistent. Upon ESC activation, the
ESC system activation causes a commanded engine torque reduction, even
though the driver requests increased torque by attempting to accelerate
the vehicle to maintain the required constant speed. Therefore, the
rate of change over time of engine torque output and driver requested
torque will diverge.
For each of the six SIS test runs, the commanded engine torque and
the driver requested torque signals must diverge at least 10 percent
1.5 seconds after the beginning of ESC system activation. This test
demonstrates that the ESC system has the capability to reduce engine
torque, as required in the functional definition.
The metric used to measure the engine torque reduction performance
is stated in terms of the difference in percent between the actual
engine torque output and driver requested torque input just after ESC
activation. The pass-fail criterion that the agency proposes for this
test is that the stability control system must be able to reduce engine
torque output by a minimum of 10 percent from the torque output
requested by the driver, which will be measured 1.5 seconds after the
time when the ESC activated. The vehicles that the agency tested were
all able to meet this proposed performance level.
2. Roll and Yaw Stability Test--SWD
The objective of the sine with dwell test is to subject a vehicle
to a maneuver that will cause both roll and yaw instabilities and to
verify that the ESC system activates to mitigate those instabilities.
The SWD test is based on a single cycle of a sinusoidal steering input.
For testing, we are proposing to use a frequency of 0.5 Hz (\1/2\ cycle
per second or 1 cycle in 2 seconds) was used with a pause or dwell of
1.0 second after completion of the third quarter-cycle of the sinusoid.
We chose a 0.5 Hz frequency because it produces the most consistently
high severity on the majority of the vehicles tested by the agency.
Hence, the total time for the steering maneuver is three seconds.
Conceptually, the steering profile of this maneuver is similar to
that expected to be used by real drivers during some crash avoidance
maneuvers. As the agency found in the
[[Page 30795]]
light vehicle ESC research program, the severity of the SWD maneuver
makes it a rigorous test while maintaining steering rates within the
capabilities of human drivers. We believe that the maneuver is severe
enough to produce rollover or vehicle loss-of-control without a
functioning ESC system on the vehicle.
For a truck tractor, the SWD test would be conducted with the truck
tractor coupled to an unbraked control trailer and loaded with ballast
directly over the kingpin. The combination vehicle would be loaded to
80 percent of the tractor's GVWR. Testing indicates that this is
sufficient load on the tractor to enable the tractor's stability
control mass estimation program to provide full tractor braking
intervention during the SWD maneuver. The ballast is placed low on the
trailer to minimize the likelihood of actual trailer rollover, and the
trailer is equipped with outriggers in case the ESC system does not
function properly to prevent the trailer from rolling over.
For a bus, the vehicle is loaded with a 68-kilogram (150-pound)
water dummy in each of the vehicle's designated seating positions,
which would bring the vehicle's weight to less than its GVWR. No
ballast is placed in the cargo hold beneath the passenger compartment
so that the desired CG height of the test load can be attained.
The SWD test would be conducted at a speed of 72 km/h (45 mph). An
automated steering machine would be used to initiate the steering
maneuver. Each vehicle is subjected to two series of test runs. One
series uses counterclockwise steering for the first half-cycle, and the
other series uses clockwise steering for the first half-cycle. The
steering amplitude for the initial run of each series is 0.3A, where A
is the steering wheel angle determined from the SIS maneuvers discussed
in section V.F.1 above. In each of the successive test runs, the
steering amplitude would be increased by increments of 0.1A until a
steering amplitude of 1.3A or 400 degrees, whichever is less, is
achieved. Upon completion of the two series of test runs, post-
processing of the yaw rate and lateral acceleration data to determine
the lateral acceleration ratio, yaw rate ratio, and lateral
displacement, as discussed below.
(a) Roll Stability Performance
The LAR is a performance metric developed to evaluate the ability
of a vehicle's ESC system to prevent rollovers. Lateral acceleration is
measured on a bus or a tractor and corrected for the vehicle's roll
angle. As a performance metric, the corrected lateral acceleration
value is normalized by dividing it by the maximum lateral acceleration
that was determined at any time between 1.0 seconds after the beginning
of steering and the completion of steering.
Conceptually, stability control system intervention will reduce
lateral acceleration of the vehicle during a crash avoidance steering
maneuver. This intervention increases the roll stability of the vehicle
by reducing the vehicle speed, which results in a reduction in the
lateral acceleration, Ay, because Ay = V\2\/R,
where V is the vehicle speed, and R is the radius of curvature of
vehicle path. However, lateral acceleration was found to be less
favorable than a ``normalized'' calculation, lateral acceleration
ratio, developed from the vehicle's lateral acceleration measured
during the maneuver because the lateral acceleration alone does not
account for different stability thresholds among different vehicles.
The agency believes that LAR has the most potential for an accurate
measure of an ESC system to prevent rollovers. From the agency's
testing, we have noted that LAR differentiates vehicles equipped with
stability control systems as well as the potential determine and
quantify roll instability. Lateral acceleration ratio is calculated by
dividing the vehicle's lateral acceleration, corrected for roll angle,
at a specified time after the completion of steer (COS) by the peak
corrected lateral acceleration experienced during the second half of
the sine maneuver (including the dwell period). The LAR at two time
intervals after completion of steer is calculated to determine the
change in lateral acceleration from the peak lateral acceleration. A
reduction or decay in the lateral acceleration ratio at specified
intervals after completion of steer is an indication that the stability
control system has intervened to reduce the likelihood of vehicle
rollover. The lateral acceleration ratio, LAR, is determined as
follows:
[GRAPHIC] [TIFF OMITTED] TP23MY12.006
Where A--y Veh (COS + 0.75 sec, + 1.5 sec,) is the
corrected for roll lateral acceleration value at the specified time
after the completion of steer, and Max Ay is the peak
corrected lateral acceleration measured during the second half of the
sine maneuver (including the dwell period), i.e., from time 1.0 second
after the beginning of steer to the completion of steer.
In developing the performance requirements for light vehicle ESC
systems, several commenters requested that the agency include a
definition for the term ``lateral acceleration'' and define a method
for determining the lateral acceleration at the vehicle's center of
gravity. In FMVSS No. 126, the agency uses the definition from SAE
J670e, Vehicle Dynamics Terminology, which states, ``Lateral
Acceleration means the component of the vector acceleration of a point
in the vehicle perpendicular to the vehicle x axis (longitudinal) and
parallel to the road plane.'' This definition was carried over,
effectively unchanged, to the more recent revision of SAE's Vehicle
Dynamics Terminology, SAE J670--200801. The agency is proposing to use
the same definition of lateral acceleration for this standard as was
used in FMVSS No. 126.
The agency's research also looked at wheel lift measurement as a
possible performance measure. Wheel lift is the most intuitive
performance measure we considered because wheel lift precedes all
rollovers. Wheel lift is considered to be lift that is two inches or
greater, which occurs for any wheel of the vehicle, including the
control trailer for the tractor during a test. One challenge with using
wheel lift is that it does not necessarily indicate that rollover is
imminent. For example, certain vehicle suspension designs are likely to
cause wheel lift during severe cornering maneuvers, and also non-
uniform test surfaces can cause brief instances of wheel lift.
Therefore, the agency proposes evaluating vehicle roll stability
performance by calculating the LAR at 0.75 seconds and at 1.5 seconds
after the completion of steer. The two performance criteria are
described below:
From data collected from each SWD maneuver executed, a
vehicle equipped with a stability control system must have a LAR of 30
percent or less 0.75 seconds after completion of steer. This
[[Page 30796]]
LAR will be calculated from the vehicle's lateral acceleration,
corrected for roll angle, at its center of gravity position.
From data collected from each SWD maneuver executed, a
vehicle equipped with stability control must have a LAR of 10 percent
or less at 1.5 seconds after completion of steer. This LAR will be
calculated from the vehicle's lateral acceleration, corrected for roll
angle, at its center of gravity position.
The performance criteria mean that 0.75 seconds after the
completion of the steering input, the corrected lateral acceleration
must not exceed 30 percent of the maximum lateral acceleration recorded
during the steering maneuver, and at 1.5 seconds after the completion
of the steering input, the lateral acceleration must not exceed 10
percent of the maximum lateral acceleration recorded during the
steering maneuver. The agency believes that these criteria represent an
appropriate stability threshold. NHTSA's research indicates that an ESC
system's ability to maintain an LAR above these criteria would provide
an acceptable probability that the vehicle would remain stable and that
a level of LAR above these criteria would result in a high probability
of the vehicle becoming unstable.
(b) Yaw Stability Performance
The yaw rate ratio is a performance metric used to evaluate the
ability of a vehicle's ESC system to prevent yaw instability. The YRR
expresses the lateral stability criteria for the sine with dwell test
to measure how quickly the vehicle stops turning, or rotating about its
vertical axis, after the steering wheel is returned to the straight-
ahead position. A vehicle that continues to turn or rotate about its
vertical axis for an extended period after the steering wheel has been
returned to a straight-ahead position is most likely experiencing
oversteer, which is what ESC is designed to prevent. The lateral
stability criterion, expressed in terms of YRR, is the percent of peak
yaw rate that is present at designated times after completion of steer.
The yaw rate ratio, YRR, is determined as follows:
[GRAPHIC] [TIFF OMITTED] TP23MY12.007
Where [Psi]Vehicle (COS + 0.75 sec, + 1.5 sec) is yaw
rate value at a specified time after the completion of steer, and Max
[Psi]Vehicle is the maximum yaw rate measured during the
second half of the sine maneuver including the dwell period from time
1.0 second after the beginning of steer until the completion of steer
during each maneuver.
This performance metric is identical to the metric used in the
light vehicle ESC system performance requirement in FMVSS No. 126. We
believe that this metric is equally applicable to truck tractors and
large buses, though it is calculated at different time intervals after
the completion of steer.
Therefore, the agency proposes to evaluate yaw stability
performance by calculating the YRR at 0.75 seconds and at 1.5 seconds
after the completion of steer. The two performance criteria are
described below:
From data collected from each 45-mph SWD maneuver
executed, a vehicle equipped with a stability control system must have
a YRR of 40 percent or less 0.75 seconds after completion of steer.
From data collected from each 45-mph SWD maneuver
executed, a vehicle equipped with stability control must have a YRR of
15 percent or less at 1.5 seconds after completion of steer.
The performance criteria mean that 0.75 seconds after the
completion of the steering, the yaw rate must not exceed 40 percent of
the peak yaw rate recorded during the second half of the sine maneuver
including the dwell period, and at 1.5 seconds after the completion of
the steering input, the yaw rate must not exceed 15 percent of the peak
yaw rate recorded. The agency believes that these criteria represent an
appropriate stability threshold. NHTSA's research indicates that an ESC
system's ability to maintain an YRR above these criteria would provide
an acceptable probability that the vehicle would remain stable and that
a level of YRR above these criteria would result in a high probability
of the vehicle becoming unstable.
(c) Lateral Displacement
Lateral displacement is a performance metric used to evaluate the
responsiveness of a vehicle, which relates to its ability to steer
around objects. Stability control intervention has the potential to
significantly increase the stability of the vehicle in which it is
installed. However, we believe that these improvements in vehicle
stability should not come at the expense of poor lateral displacement
in response to the driver's steering input.
A hypothetical way to pass a stability control performance test
would be to make either the vehicle or its stability control system
intervene simply by making the vehicle poorly responsive to the speed
and steering inputs required by the test. An extreme example of this
potential lack of responsiveness would occur if an ESC system locked
both front wheels as the driver begins a severe avoidance maneuver that
might lead to vehicle rollover. Front wheel lockup would create an
understeer condition in the vehicle, which would result in the vehicle
plowing straight ahead and colliding with an object the driver was
trying to avoid. It is very likely that front wheel lockup would reduce
the roll instability of the vehicle since the lateral acceleration
would be reduced. This is clearly, however, not a desirable compromise.
Because a vehicle that simply responds poorly to steering commands
may be able to meet the proposed stability criteria, a minimum
responsiveness criterion is also proposed for the SWD test. Using a
lateral displacement metric to measure responsiveness ensures that the
vehicle responds to an initial steering input to avoid an obstacle.
This metric was chosen because it is objective, easy to measure, has
good discriminatory capability, and has a direct relation to obstacle
avoidance.
The proposed lateral displacement criterion is that a truck tractor
equipped with stability control must have a lateral displacement of 7
feet or more at 1.5 seconds from the beginning of steer, measured
during the sine with dwell maneuver. For a bus, the proposed
performance criterion is a lateral displacement of 5 feet or more at
1.5 seconds after the beginning of steer. The lateral displacement
criteria is less for a bus because a large bus has a longer wheelbase
than a truck tractor and higher steering ratio, which makes it less
responsive than a truck tractor. The value will be calculated from the
double integral with respect to time of the measurement of the
corrected for roll lateral acceleration at the vehicle center of
gravity, as expressed by the formula:
Lateral Displacement = [int][int]AyCG dt
Where: AyCG is the corrected for roll lateral
acceleration at the center of gravity height of the vehicle
[[Page 30797]]
This is the same performance metric used in FMVSS No. 126.
Furthermore, the vehicle would be required pass this requirement during
the every execution the SWD maneuver where the steering wheel angle is
0.7A or greater.
3. Alternative Test Maneuvers Considered
We have considered other test maneuvers besides the sine with dwell
test. The SWD maneuver was tentatively selected over the other
maneuvers discussed above and below because our research demonstrates
that it has the most optimal set of characteristics, including the
severity of the test, repeatability and reproducibility of results, and
the ability to address rollover, lateral stability, and responsiveness.
The agency's research initially focused on developing the ramp
steer maneuver to evaluate the roll stability performance and the sine
with dwell maneuver to evaluate the yaw stability performance. However,
after additional testing, we were able to develop test parameters for
the sine with dwell maneuver so that both roll stability and yaw
stability could be evaluated using a single loading condition and test
maneuver. The sine with dwell maneuver has typically been used to
evaluate only the yaw instability of a vehicle. The agency has
previously used a lightly loaded vehicle weight condition for such
evaluations where the lightly loaded condition and the resulting lower
CG height were much more likely to cause vehicle directional loss-of-
control as opposed to rollover. In the light vehicle ESC standard, the
sine with dwell maneuver is used to evaluate only yaw instability, not
roll instability, with the vehicle loaded to LLVW only but not to GVWR.
Given the different dynamics of heavy vehicles when compared to light
vehicles, NHTSA evaluated several loading conditions and found that a
loading condition which equals 80 percent of the tractor's GVWR enables
us to evaluate roll instability as well as yaw instability.
The number of tests that would be needed to cover all likely
vehicle operational conditions for varying vehicle designs is
potentially large, and many tests (particularly those using low
friction surfaces) may not be sufficiently repeatable for an objective
performance requirement. Our testing indicates that the SWD maneuver is
sufficiently severe to ensure that nearly all vehicles without ESC
would not be able to comply with the proposed performance requirements.
For example, the vehicles we tested without ESC either had wheel lift
or spun out during the SWD maneuver. Hence, a vehicle that avoids loss
of control according to our objective lateral acceleration and yaw rate
decay definitions demonstrates that it has an ESC system typical of
today's technology and would have safety benefits.
In addition to our test results, the agency thoroughly evaluated
the test vehicles and test data submitted by EMA and others to the
agency. EMA provided information on one tractor that appeared to
satisfy the agency's proposed SWD performance criteria without a
stability control system. After careful review of this data, we do not
believe this fact means the test has no value.\43\ It is possible that
there are currently truck tractors or large buses sold today that are
exceptionally yaw stable, even in a severe maneuver such as a double
lane change, which the SWD maneuver is designed to simulate. When
evaluating light vehicles, the agency noted that there was a very small
number of vehicles that were stable enough without a stability control
system to pass our performance criteria without an ESC system.
Therefore, the existence of vehicles that could pass the proposed SWD
test without a stability control system simply indicates that it would
take many tests to cover all potential instability scenarios across
varying vehicle designs in order to design a perfect test regime, as
discussed earlier. Such a complex test regime would require excessive
costs to manufacturers to ensure compliance and excessive costs to the
agency to determine and enforce compliance.
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\43\ As discussed earlier, EMA's testing of Vehicle J used a
control trailer with a wider track width and a lower deck and used
ballast that resulted in a lower vehicle center of gravity than used
by NHTSA's researchers. Each of these differences caused EMA's
combination vehicle to be more stable than NHTSA's during testing.
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We recognize that manufacturers may wish to base their
certification of compliance with this proposed standard on their
vehicles' performance in NHTSA's proposed test maneuvers. If
manufacturers intend to conduct the maneuvers proposed by the agency,
they may need to make additional investments in their facilities or
have their certification testing performed at a contractor's facility.
However, we believe some manufacturers may have already made these
investments, and others would make similar investments as they develop
and validate ESC systems for their vehicles. This is based on our
understanding of the maneuvers used by the heavy-vehicle industry for
ESC system development and validation, some of which include variations
of the agency's proposed maneuver.
We also recognize that, over time, manufacturers will be able to
develop other methods for certifying compliance with the proposed
standard. For example, manufacturers can develop computer models or
simulations to demonstrate ESC system performance. However, we
recognize that these alternative methods may not be suitable for
atypical vehicles that are custom-built for customers. We seek comment
on the issues surrounding manufacturers' certification of compliance
including the assumptions made regarding manufacturers' current and
future test facilities, the methods used by manufacturers to validate
ESC system performance, the ability of manufacturers to use other
methods (such as computer modeling, simulation, or alternative test
maneuvers) to certify compliance, the cost of certification, and the
issues surrounding certification of atypical truck tractors.
Below, we discuss the alternative test maneuvers that were
considered and what we considered to be acceptable performance criteria
for each test. We also discuss why we are choosing the SWD maneuver for
compliance testing in lieu of each of these maneuvers. We invite
comment on each of these test maneuvers, including whether they should
be used instead of, or along with, the proposed compliance test
maneuvers.
(a) Characterization Maneuver
While NHTSA has conducted extensive testing using the SIS maneuver,
we believe that alternative methods may be used to determine the
steering wheel angle needed to achieve 0.5g of lateral acceleration at
30 mph. For example, a test based on the SAE J266 circle test may yield
a similar steering wheel angle without requiring the track space
necessary to conduct the SIS maneuver. The steering wheel angle that
produces 0.5g of lateral acceleration at 30 mph may be above the ESC
system's activation threshold for some vehicles, making it impractical
to conduct a direct measurement of the steering wheel angle. The agency
seeks comment on the feasibility of an alternative characterization
test based upon the SAE J266 circle test.
(b) Roll Stability Test Maneuvers
To evaluate roll instability, we have considered two alternative
roll stability test maneuvers--the J-turn and the ramp steer maneuver.
The two tests are similar in that both maneuvers require the tested
vehicle to be driven at a
[[Page 30798]]
constant speed and then the vehicle is turned in one direction for a
certain period of time. The test speed and the severity of the turn are
designed to cause a test vehicle to approach or exceed its roll
stability threshold such that, without a stability control system, the
vehicle would exhibit signs of roll instability. Both tests would be
performed with the tractor loaded to its GVWR. Furthermore, we would
not expect a vehicle that could pass one test to fail the other.
The most notable difference between the J-turn and the RSM
maneuvers is that the J-turn is a path-following maneuver. That is, it
is performed on a fixed path curve. In contrast, the RSM maneuver is a
non-path-following maneuver that is performed with a fixed steering
wheel input. For example, during the agency's and EMA's testing, the J-
turn maneuver was performed on a 150-foot radius curve. In contrast,
the RSM is performed based on a steering wheel angle derived from the
SIS test. We would expect that, with the RSM, the radius of the curve
would be close to the fixed radius used in the J-turn maneuver.
However, in the RSM, the driver would not have to make adjustments and
corrections to steering to maintain the fixed path.
When comparing the J-turn to the RSM, the agency considers the RSM
to be a preferable test maneuver because the RSM maneuver can be
performed with an automated steering wheel controller. Because the J-
turn is a path-following maneuver, a test driver must constantly make
adjustments to the steering input for the vehicle to remain in the lane
throughout the test maneuver. Moreover, driver variability could be
introduced from test to test based upon minor variations in the timing
of the initial steering input and the position of the test vehicle in
the lane.
In addition, the RSM appears to be more consistent because it
involves a fixed steering wheel angle rather than a fixed path. There
is negligible variability based on the timing of the initial steering
input because the test is designed to begin at the initiation of
steering input, rather than the vehicle's position on a track.
Moreover, an automated steering wheel controller can more precisely
maintain the required steering wheel input than a driver can.
Therefore, we tentatively conclude that the RSM is more consistent and
more repeatable than the J-turn, which is critical for agency
compliance testing purposes.
Notwithstanding the above observations, we recognize that many
manufacturers perform NHTSA's compliance tests in order to certify that
their vehicles comply with NHTSA's safety standards. We also recognize
that, over time, manufacturers are likely to use other methods such as
simulation, modeling, etc., to determine compliance with Federal Motor
Vehicle Safety Standards. In this regard, we observe that, because the
J-turn and the ramp steer maneuvers are so similar, manufacturers may
be able to determine compliance with a stability control standard by
using the J-turn maneuver even if the agency ultimately decides to use
the RSM for compliance testing. Thus, if a manufacturer sought to
certify compliance based upon performance testing, a manufacturer would
not necessarily need to perform compliance testing with an automated
steering controller.
In considering the RSM test conditions, the agency looked to its
test data and the data submitted by EMA. Data analysis indicated that
the RSM test performed from at an initial speed of 30 mph is sufficient
to demonstrate effective stability control performance for truck
tractors. At GVWR, the tested buses were observed to have different
speed thresholds at which wheel lift occurred and stability control
initially activated. Without stability control, buses were observed to
produce wheel lift between 35 and 39 mph in the RSM, compared to
tractors, which ranged from 28 to 30 mph. Large bus stability control
systems initially activated at speeds greater than 30 mph in the RSM,
which was higher than the 26 mph observed with tractors. In light of
these differences, an initial speed of 36 mph was selected for buses to
ensure an appropriate level of test severity and that stability control
would intervene.
Another issue in conducting the RSM is whether to use fixed rate
steering or to steer at a rate such that the full steering input is
reached in a fixed time. Using fixed rate steering, the steering wheel
is turned a 175 degrees per second until the desired steering wheel
angle is reached. If a vehicle with a lower steering wheel angle input,
such as a short wheelbase 4x2 tractor, is tested using this steering
method, the desired steering wheel angle would be reached relatively
quickly after the initial steering input. In contrast, for a longer
wheelbase truck or a large bus, the desired steering wheel angle would
be reached relatively slowly after the initial steering input. This
results in a more severe test for vehicles with a lower steering wheel
angle because the predicted lateral acceleration of 0.5g would be
reached more quickly than for vehicles with a higher steering wheel
angle. In an extreme case with an exceptionally large steering wheel
angle, such as a bus with a long wheelbase the system may activate
before the full steering wheel is input.
Using a fixed-time steering input, we would program the steering
wheel controller to reach the desired steering wheel angle in exactly
1.5 seconds using a constant steering rate, which was derived from the
manually steered 150-foot J-turn maneuver. Using this steering method
would prevent the RSM results from varying with steering wheel angle
input. We are requesting comment as to whether fixed-rate steering or
fixed-time steering is a preferable manner for conducting the RSM.
The RSM would use a similar, but not identical lateral acceleration
ratio performance metric to evaluate roll stability. As with the SWD
maneuver, the LAR used in the RSM would indicate that the stability
control system is applying selective braking to lower lateral
acceleration experienced during the steering maneuver. In the SWD
maneuver, the LAR is the ratio of the lateral acceleration at a fixed
point in time to the peak lateral acceleration during the period from
one second after the beginning of steer to the completion of steer. In
contrast, the LAR metric we would use for the RSM would be the ratio of
the lateral acceleration at a fixed point in time to the lateral
acceleration at the end of ramp input, which is the moment at which the
steering wheel angle reaches the target steering wheel angle for the
test. Also, in contrast to the SWD maneuver, the LAR measurements for
the RSM would be taken at a time when the steering wheel is still
turned. This means that, although the SWD maneuver is a more dynamic
steering maneuver, the LAR criteria for the RSM would be greater than
the LAR criteria for the SWD maneuver.
The performance criteria for the RSM would depend on whether fixed-
rate steering or fixed-time steering input is used. For truck tractors
and large buses using fixed-time steering input, we would expect that
the LAR would be less than 1.05 two seconds after the end of ramp input
and less than 0.8 three seconds after the end of ramp input. For truck
tractors tested using fixed-rate steering inputs, we would expect that
the LAR would be less than 1.1 two seconds after the end of ramp input
(the point in time at which the target steering wheel angle is reached)
and less than 0.9 three seconds after the end of ramp input. For buses
using fixed-rate steering, we would expect that the LAR would be less
than 1.0 two seconds after the end of ramp input and less than 0.7
three seconds after the end of ramp input. The performance criteria for
large
[[Page 30799]]
buses would be lower because, as we stated above, when using fixed-rate
steering input, the longer wheelbases of buses cause the maneuver to be
less dynamic.
In a March 2012 submission, which was revised with additional
details in April 2012, EMA suggested that NHTSA use different test
speeds and performance criteria for the J-turn maneuver.\44\ EMA
suggested that a test speed that is 30 percent greater than the minimum
speed at which the ESC system intervenes with engine, engine brake, or
service brake control. Instead of measuring LAR, EMA suggested that,
during three out of four runs, the vehicle would be required to
decelerate at a minimum deceleration rate. NHTSA has conducted testing
on variations of this EMA maneuver, and we plan to conduct further
testing. We request comments on EMA's suggested test procedure and
performance criteria for the J-turn maneuver.
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\44\ Docket No. NHTSA-2010-0034-0032; Docket No. NHTSA-2010-
0034-0040.
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Based on our testing to date, the agency tentatively concludes that
the RSM is a preferable test to the J-run to demonstrate a stability
control system's ability to prevent roll instability. However, as
discussed in greater detail below, in order to reduce the number of
compliance tests that the agency and those manufacturers who choose to
demonstrate compliance by conducting the agency's performance tests
must perform, the agency proposes using on test maneuver, the SWD, to
demonstrate both roll and yaw stability performance. Although we are
proposing to use the SWD maneuver for evaluating roll stability, we
request comment on issues related to the RSM and J-turn tests,
including test conditions, steering input method, and performance
criteria.
(c) Yaw Stability Test Maneuvers
After evaluating several maneuvers on different surfaces, the
agency was unable to develop any alterative performance-based dynamic
yaw test maneuvers that were repeatable enough for compliance testing
purposes. Bendix described two maneuvers intended to evaluate the yaw
stability of tractors.\45\ However, neither of these test maneuvers was
developed to a level that would make them suitable for the agency to
consider using as yaw performance tests.
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\45\ These tests are discussed in section IV.E.3. See Docket No.
NHTSA-2010-0034-0037 and Docket No. NHTSA-2010-0034-0038.
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In July 2009, EMA provided research information on several yaw
stability test maneuvers.\46\ One of these maneuvers was the SWD on dry
pavement that is similar to what is proposed in this notice. The second
maneuver was an SWD maneuver conducted on wet Jennite. The third
maneuver was a ramp with dwell maneuver on wet Jennite.\47\ EMA did not
provide any test data on the last two maneuvers. Thus, we considered
them to be concepts rather than fully developed maneuvers that we could
consider using for yaw stability testing.
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\46\ Docket No. NHTSA-2010-0034-0035.
\47\ This ramp with dwell maneuver is the same one identified by
Bendix referenced in the prior paragraph and in section IV.E.3.
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We received no other alternative yaw performance tests from
industry until EMA's submission of Vehicle J data in late 2010.\48\ EMA
suggested using a wet Jennite drive through test maneuver demonstrated
yaw performance in a curve on a low friction surface. The maneuver is
based upon a maneuver the agency currently conducts on heavy vehicles
to verify stability and control of antilock braking systems while
braking in a curve. As part of the test, a vehicle is driven into a
500-foot radius curve with a low-friction wet Jennite surface at
increasing speeds to determine the maximum drive-through speed at which
the driver can keep the vehicle within a 12-foot lane. As with the J-
turn, we are concerned about the repeatability of this test maneuver
because of variability in the wet Jennite test surface and the driver's
difficulty in maintaining a constant speed and steering input in the
curve.
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\48\ Docket No. NHTSA-2010-0034-0022; Docket No. NHTSA-2010-
0034-0023.
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In a March 2012 submission, which was revised with additional
details in April 2012, EMA provided information about another yaw
stability test along with additional information on the J-turn
maneuver.\49\ This maneuver would simulate a single lane change on a
wet roadway surface. It would be conducted within a 4 meter (12 foot)
wide path. The roadway condition would be a wet, low friction surface
such as wet Jennite with a peak coefficient of friction of 0.5. The
other test conditions (i.e., road conditions, burnish procedure,
liftable axle position, and initial brake temperatures) would be
similar to those proposed in this NPRM. In this maneuver, the truck
would enter the path at progressively higher speeds to establish the
minimum speed at which the ESC system intervenes and applies the
tractor's brakes. The maneuver would then be repeated four times at
that speed with the vehicle remaining within the lane at all times
during the maneuver. EMA suggests, as a performance criterion, that
during at least three of the four runs, the ESC system must provide a
minimum level (presently unspecified) of differential braking. The
agency has not had an opportunity to conduct testing of this maneuver,
but we intend to do so to determine whether this is a viable
alternative yaw stability test.
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\49\ Docket No. NHTSA-2010-0034-0032; Docket No. NHTSA-2010-
0034-0040.
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In light of the inability to develop a different performance-based
yaw stability test, the agency is proposing to use the SWD test
maneuver to evaluate yaw stability performance. Although we are
proposing to use the SWD maneuver for evaluating yaw stability, we
request comment on other yaw stability tests that could be suitable for
performance testing and possible performance criteria for any such
test. Furthermore, we specifically request comment on all aspects of
EMA's yaw stability test discussed in its March and April 2012
submissions, including the test conditions, test procedure, and
possible performance criteria that would allow the agency to test both
trucks and buses with this maneuver.
(d) Lack of an Understeer Test
The SWD maneuver is designed to induce both roll and yaw responses
from the vehicle being evaluated. However, the agency has no test to
evaluate how the ESC responds when understeer is induced. The technique
used by a stability control system for mitigating wheel lift, excessive
oversteer or understeer conditions is to apply unbalanced wheel braking
so as to generate moments (torques) to reduce lateral acceleration and
to correct excessive oversteer or understeer. However, for a vehicle
experiencing excessive understeer, if too much oversteering moment is
generated, the vehicle may oversteer and spin out with obvious negative
safety consequences. In addition, excessive understeer mitigation acts
like an anti-roll stability control where it momentarily increases the
lateral acceleration the vehicle can attain. Hence, too much understeer
mitigation can create safety problems in the form of vehicle spin out
or rollover.\50\
---------------------------------------------------------------------------
\50\ EMA's testing of Vehicle J on the 500-foot wet Jennite
curve shows understeer mitigation at maneuver entry speeds up to 34
mph, but at 35 mph, the vehicle could not overcome understeer. See
Docket No. NHTSA-2010-0034-0022; Docket No. NHTSA-2010-0034-0023. At
these low levels of lateral acceleration, no adverse effects
appeared to occur as a result of the understeer mitigation.
---------------------------------------------------------------------------
During the testing to develop FMVSS No. 126, the agency concluded
that understanding both what understeer mitigation can and cannot do is
complicated, and that there are certain
[[Page 30800]]
situations where understeer mitigation could potentially produce safety
disbenefits if not properly tuned. Therefore, the agency decided to
enforce the requirements to meet the understeer criterion included in
the ESC definition using a two-part process. First, the requirement to
meet definitional criteria ensured that all had the hardware needed to
limit vehicle understeer. Second, the agency required manufacturers to
submit engineering documentation at the request of NHTSA's Office of
Vehicle Safety Compliance to show that the system is capable of
addressing vehicle understeer.
Based on the agency's experience from the light vehicle ESC
rulemaking and the lack of a suitable test to evaluate understeer
performance, the agency is not proposing a test for understeer to
evaluate ESC system performance for truck tractors and large buses. The
agency requests comment on this NPRM's lack of a proposed understeer
test.
4. ESC Malfunction Test
During execution of a compliance test the agency proposes
simulating several malfunctions to ensure the system and corresponding
malfunction telltale provides the required warning to the vehicle
operator. Malfunctions are generally simulated by disconnecting the
power source to an ESC system component or disconnecting an electrical
connection to or between ESC system components. Examples of simulated
malfunctions might include the electrical disconnection of the sensor
measuring yaw rate, lateral acceleration, steering wheel angle sensor,
or wheel speed. When simulating an ESC system malfunction, the
electrical connections for the telltale lamp would not be disconnected.
Also, because a vehicle may require a driving phase to identify a
malfunction, the vehicle would be driven for at least two minutes
including at least one left and one right turning maneuver. A similar
drive time exists in the FMVSS No. 126 test procedure.
After a malfunction has been simulated and identified by the
system, the system is restored to normal operation. The engine is
started and the malfunction telltale is checked to ensure it has
cleared.
5. Test Instrumentation and Equipment
For the truck tractor and large bus stability control system
research program, each test vehicle was fitted with specific
instrumentation and equipment necessary to execute each test safely and
to collect necessary performance data. The compliance test program
proposed in this NPRM would use essentially the same equipment and a
subset of the instrumentation. As was done for FMVSS No. 126, the
agency proposes including in the regulatory text the basic design
parameters for the automated steering machine, outriggers, and the
control trailer because this test equipment and instrumentation can
influence test vehicle performance. However, the proposed regulatory
text does not include a list of the less critical test instrumentation
used during the compliance test. The agency's common practice has been
to provide instrumentation details, test instrumentation range,
resolution, and accuracy for all the required instrumentation in the
separate NHTSA Laboratory Test Procedure. Furthermore, the agency is
aware that manufacturers and test facilities will be interested in
knowing what instruments will be used for a compliance test program.
The following table and corresponding discussions identify the critical
equipment and instrumentation used by NHTSA's researchers and for the
most part, the same or similar is proposed for use by NHTSA's Office of
Vehicle Safety Compliance.
Table 3--Critical Test Instrumentation Used for Data Collection by NHTSA Research
--------------------------------------------------------------------------------------------------------------------------------------------------------
Vehicle test instrumentation Output/input Range Resolution Accuracy Make/model used
--------------------------------------------------------------------------------------------------------------------------------------------------------
Programmable Steering Machine with Controls Steering Max 40-60Nm (29.5-44.3 ..................... ..................... Automotive Testing
Steering Angle Encoder. Wheel Angle Input. lb-ft) torque at a Inc. (ATI) Model:
hand wheel rate up to Spirit.\3\
1200 deg/sec.
Handwheel Angle....... 800 deg... 0.25 deg............. 0.25 deg.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Multi-Axis Inertial Sensing System. Longitudinal, Lateral Accelerometers: 2 g. ug. <=0.05% of full Model: MP-1.
Acceleration. range
Roll, Yaw, and Pitch Angular rate sensors: Angular Rate Sensors: Angular Rate Sensors:
Rate. 100 deg/ <=0.004 deg/s. 0.05% of full range.
sec.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Speed Sensor....................... Vehicle Speed to DAS 0-201 km/h (0-125 mph) .014 km/h (.009 mph). 0.1 km/h full scale.. Make: RaceLogic
and Steering Machine. Model: VBox.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Infrared Distance Measuring Sensor. Left and Right Side 350-850 mm (14-35 0.3-8.0 mm (0.01-0.3 1%................... Sensor Make: Wenglor.
Vehicle Height (For inches). inches). Model: HT66MGV80.
calculated vehicle
roll angle).
--------------------------------------------------------------------------------------------------------------------------------------------------------
During research additional instrumentation was used for collecting
data outside the scope of the proposed standard and that
instrumentation is not discussed here. Furthermore, this table does not
include a discussion of non-critical instrumentation like the brake
pedal load cell used to ensure the test driver does not apply the brake
during the maneuver, or the thermocouples used to monitor brake
temperatures.
(a) Outriggers
Throughout the agency's research program, truck tractors and buses
were equipped with outrigger devices to prevent vehicle rollover.
During the program, the agency encountered many instances of wheel lift
and outrigger contact with the ground indicating that it was probable
that rollover could occur during testing. Over many years of research
of ESC systems, it has been proven that outriggers are essential to
[[Page 30801]]
ensure driver safety and to prevent vehicle and property damage during
NHTSA's compliance testing. Although NHTSA conducted some of its
testing with ESC systems disabled, thereby increasing the need for
outriggers, outriggers are still necessary as a safety measure during
testing of vehicles equipped with an ESC system in case the system
fails to activate.
The agency proposes that outriggers be used on all truck tractors
and buses tested. Nevertheless, the agency acknowledges, as it did
during the development of the light vehicle ESC system testing program,
that outriggers have the potential to influence the dynamics of a
vehicle during performance testing. For light vehicles, the agency
determined that outrigger influence could be noticeable. However, we
believe that outrigger influence on heavy vehicles is minimal because
of the higher vehicle weight and test load. The agency has invested
significant effort in outrigger designs that are both functional and
minimize the impact to the test vehicle dynamic performance. To reduce
test variability and increase the repeatability of the test results,
the agency proposes to specify a standard outrigger design for the
outriggers that will be used for compliance testing. The agency used
this same approach in FMVSS No. 126 for compliance testing of light
vehicle ESC systems. The agency also made available the detailed design
specifications by reference to a design document located in the agency
public docket.
For truck tractors, the document detailing the outrigger design to
be used in testing has been placed in a public docket.\51\ This
document provides detailed construction drawings, specifies materials
to be used, and provides installation guidance. For truck tractor
combinations, the outriggers would be mounted on the trailer. The
outriggers are mounted mid-way between the center of the kingpin and
the center of the trailer axle (in the fore and aft direction of
travel), which is generally near the geometric center of the trailer.
They will be centered geometrically from side-to-side and bolted up
under the traditional flatbed control trailer. Total weight of the
outrigger assembly, excluding the mounting bracket and fasteners
required to mount the assembly to the flatbed trailer, is approximately
1,490 pounds. The bulk of the mass, over 800 pounds, is for the
mounting bracket which is located under the trailer near the vehicle's
lateral and longitudinal center of gravity so that its inertial effects
are minimized. The width of the outrigger assembly is 269 inches and
the contact wheel to ground plane height is adjustable to allow for
various degrees of body roll. A typical installation on a flatbed type
trailer involves clamping and bolting the outrigger mounting bracket to
the main rails of the flatbed.
---------------------------------------------------------------------------
\51\ Docket No. NHTSA-2010-0034-0010.
---------------------------------------------------------------------------
For buses, the outrigger installations will not be as
straightforward as the outrigger installations on the control trailers,
and we desire comments on bus outrigger design. This is because
outriggers cannot be mounted under the flat structure, but instead must
extend through the bus. NHTSA used outriggers on the three large buses
tested during its research program and proposes using outriggers for
testing buses for compliance with this rule. The agency will use the
same outrigger arms of the standard outrigger design that it plans to
use for truck tractor testing. Therefore, the size, weight, and other
design characteristics will be similar.
The location and manner of mounting the outriggers on buses cannot
be identical to truck tractors. Nonetheless, there are a limited number
of large bus manufacturers, which results in a limited number of unique
chassis structural designs. Also, the agency understands that large bus
structural designs do not change significantly from year-to-year. We
believe that once outrigger mounts have been constructed for several
different bus designs, those mountings can be modified and reused
during subsequent testing. The agency has, in the document described
above, provided additional engineering design drawings and further
installation guidelines for installing the standard outrigger assemble
to large buses.
(b) Automated Steering Machine
As part of the heavy vehicle ESC system research programs, the
agency performed testing that compared multiple runs with test-driver-
generated steering inputs, and found that test drivers cannot provide
the same repeatable results as those obtained with an automated
steering machine. Therefore, this NPRM proposes that an automated
steering machine be used for the test maneuvers on the truck tractors
and large buses in an effort to achieve highly repeatable and
reproducible compliance test results.
An essential element of any compliance test program is for the test
being executed to be reproducible, a test that can be easily executed
the same way by different testing facilities, and repeatable, test
results from repeated tests of the same vehicle are identical. The
proposed 0.5 Hz SWD maneuver is a complex test maneuver where the
steering must follow an exact sinusoidal pattern over a three-second
time period. For the SWD maneuver, each test vehicle is subjected to as
many 22 individual test runs all requiring activation at a specific
vehicle speed, each of which will require a different peak steering
wheel angle and corresponding steering wheel turning rate. To ensure
the agency has an effective compliance program that will not vary from
one test laboratory to another, from one test driver to another, or
from one test vehicle to another, each maneuver must be repeatable and
reproducible. The agency has extensive experience with execution of
these and other steering maneuvers utilizing both human drivers and
automated steering controllers. Based upon this experience, the agency
has determined that a test driver cannot consistently execute these
kinds of dynamic maneuvers exactly as required repeatedly. We note
that, for the same reasons, the agency currently requires that
automated steering machines be used for execution of the steering
maneuvers performed under both the NCAP Rollover program and the FMVSS
No. 126 light vehicle ESC program.
(c) Anti-Jackknife Cables
The agency proposes using anti-jackknife cables when testing truck
tractors. Anti-jackknife cables would prevent the trailer from striking
the tractor during testing in the event that a jackknife event occurs
during testing. This would prevent damage to the tractor that may occur
during testing. We do not believe that the use of anti-jackknife cables
would affect test results, nor have we observed any damage to test
vehicles, including vehicle finishes, caused by anti-jackknife cables.
Nevertheless, we request comment on the necessity of the use of anti-
jackknife cables during agency compliance testing.
(d) Control Trailer
The agency proposes using a control trailer to evaluate the
performance of a tractor in its loaded condition. A control trailer
would not be used when testing buses. In FMVSS No. 121, the agency
specifies the use of an unbraked control trailer for compliance testing
purposes. An unbraked control trailer minimizes the effect of the
trailer's brakes when testing the braking performance of a tractor in
its loaded condition.
The agency has also considered using a braked control trailer in
ESC performance testing for truck tractors because the tractor-based
stability control systems have the capability to apply the trailer
brakes during stability
[[Page 30802]]
control intervention. This ability provides a slightly greater vehicle
retardation that could further help prevent an impending rollover or
reduce yaw instabilities.
As described in section IV.C above, the agency conducted numerous
vehicle research test maneuvers using six different trailers. For each
trailer, a test series was conducted collecting data for each trailer
in a braked and unbraked condition. The effects of stability control,
trailer brakes, and trailer type were analyzed using a logistical
regression model to predict if wheel lift occurred during the test. A
test was conducted to determine the effects of trailer brakes when
stability control systems were enabled. With stability control systems
enabled and trailer braking in the ``off'' position, the trailer was
found not to be a significant factor in predicting wheel lift. Hence,
the results indicate that the current FMVSS No. 121 unbraked control
trailer can be used effectively in the stability control system testing
to determine the capability of the tractor-based stability control
system.
NHTSA's compliance tests must be objective, repeatable and
reproducible. The goal of the testing program is to ensure that the ESC
system takes the necessary actions of reducing engine torque and
applying brakes to prevent yaw and roll instability. To achieve this
goal any trailer type could be used as long as that trailer type
becomes the ``standard'' trailer or ``control trailer'' used for all
tractor trailer testing. Because it is the tractor performance that is
being evaluated, the use of a standardized trailer will allow the test
to distinguish the performance differences between different ESC
systems and tractor types.
We believe that the current FMVSS No. 121 unbraked control trailer
can be used effectively in the stability control testing to determine
the capability of an ESC system. However, as discussed in section
IV.D.5.(b) earlier, NHTSA's testing of EMA's Vehicle J revealed that
the specifications for the control trailer in FMVSS No. 121 were not
sufficient to ensure test repeatability.\52\
---------------------------------------------------------------------------
\52\ The FMVSS No. 121 control trailer specifications, set forth
in S6.1.10.2 and S6.1.10.3 of FMVSS No. 121 provide that the center
of gravity of the ballast on the loaded control trailer be less than
24 inches above the top of the tractor's fifth wheel and that the
trailer have a single axle with a GAWR of 18,000 pounds and a
length, measured from the transverse centerline of the axle to the
centerline of the kingpin, of 258 6 inches.
---------------------------------------------------------------------------
There were three specifications, not set forth in FMVSS No. 121,
which could affect test performance and prevent repeatable, consistent
test results using different control trailers. First, the track width
of the control trailer is not specified. A trailer with a wider track
width would be more stable than a trailer with a narrower track width,
potentially affecting test results. Second, the center of gravity of
the control trailer is not specified in FMVSS No. 121. The center of
gravity of the trailer may be affected by the height of the load deck.
A trailer with a higher load deck height would be less stable than a
trailer with a lower load deck height. Third, the center of gravity of
the load in FMVSS No. 121 testing is only specified to be less than 24
inches above the top of the tractor's fifth wheel. However, a load with
a lower center of gravity (for example 12 inches) would be more stable
than a load with a higher center of gravity (for example 24 inches).
The performance measures specified in this proposal were based upon
NHTSA's testing using the control trailer used by VRTC researchers.
Although the track width and center of gravity of the trailer are not
specified in the proposed regulatory text and the center of gravity of
the load is specified only by an upper bound, we request comment on
possible specifications and appropriate levels of variability in
trailer track width, trailer CG height, and load CG height for a
control trailer to be used during ESC system testing.
(e) Sensors
A multi-axis inertial sensing system would be used to measure
longitudinal, lateral, and vertical linear accelerations and roll,
pitch, and yaw angular rates. The position of the multi-axis inertial
sensing system must be measured relative to the center of gravity of
the tractor when loaded. To simplify testing, the vertical center of
gravity location is assumed to be at the top of the frame rails for
tractors. For buses, the center of gravity height is assumed to be at
the height of the main interior floor of the bus. The measured lateral
acceleration and yaw rate data are required for determining the lateral
displacement, LAR and YRR performance criteria. All six of the sensing
system signals are utilized in the equations required to translate the
motion of the vehicle at the measured location to that which occurred
at the actual center of gravity to remove roll, pitch, and yaw effects.
The vehicle speed would be measured with a non-contact GPS-based
speed sensor. Accurate speed data is required to ensure that the SWD
maneuver is executed at the required 72.4 1.6 km/h (45.0
1.0 mph) test speed. Sensor outputs are available to allow
the driver to monitor vehicle speed and data are provided as input to
the automated steering machine for maneuver activation.
Infrared height sensors would be used to collect left and right
side vertical ride height or displacement data for calculating vehicle
roll angle. One sensor would be mounted on each side of the vehicle.
With these data, roll angle is calculated during post-processing using
trigonometry and would be used for correcting the measured lateral
acceleration data due to the effects caused by body roll.
6. Test Conditions
(a) Ambient Conditions
The ambient temperature range specified in other FMVSSs for outdoor
brake performance testing is 0 [deg]C to 38 [deg]C (32 [deg]F to 100
[deg]F). However, when the agency proposed a range of 0 [deg]C to 40
[deg]C (32 [deg]F to 104 [deg]F) for FMVSS No. 126, the issue of tire
performance at near freezing temperatures was raised. The agency
understood that near freezing temperatures could impact the variability
of compliance test results. As a result, the agency increased the lower
bound of the temperature range to 7 [deg]C (45 [deg]F) to minimize test
variability at lower ambient temperatures. For the same reasons, this
NPRM proposes an ambient temperature range of 7 [deg]C to 40 [deg]C (45
[deg]F to 104 [deg]F) for testing.
The agency proposes that the maximum wind speed for conducting the
compliance testing for be no greater than 5 m/s (11 mph). This is the
same value specified for testing multi-purpose passenger vehicles
(MPVs), buses, and trucks under FMVSS No. 126. This is also the same
value used for compliance testing for FMVSS No. 135, Light Vehicle
Brake Systems. For FMVSS No. 126, the agency initially proposed a
maximum wind speed of 10 m/s (22 mph) for all vehicles. However, the
agency decided to reduce the speed for MPVs, buses, and trucks because
of a concern that the higher wind speeds could impact the performance
of certain vehicle configurations (e.g., cube vans, 15 passenger vans,
vehicles built in two or more stages).\53\ Commenters to the proposed
rule had estimated that a cross wind of 22 mph could reduce lateral
displacement by 0.5 feet, compared to the same test conducted under
calm conditions. The agency agreed that wind speed could have some
impact on the lateral displacement for certain vehicle configurations
and believes that the same argument is applicable testing truck
tractors and large buses.
[[Page 30803]]
Nevertheless, the agency notes that specifying such a low maximum wind
speed can impose additional burdens on testing by restricting the
environmental conditions under which testing can be conducted.
---------------------------------------------------------------------------
\53\ See 72 FR 17286 (Apr. 6, 2007).
---------------------------------------------------------------------------
(b) Road Test Surface
The SWD maneuver executed on a high friction surface is a
dynamically challenging maneuver that evaluates the effectiveness of an
ESC system. Low friction surfaces, such as wet Jennite, are well known
for producing a high degree of braking and handling tests variability
compared to similar tests on high friction surfaces. The variability is
exacerbated by the difficulty in ensuring a consistent water depth
across the test surface. Therefore, this NPRM proposes conducting the
SWD test on a dry test surface with a PFC of 0.9, which is typical of a
dry asphalt surface or a dry concrete surface. As in other standards
where the PFC is specified, we propose that the PFC be measured using
an ASTM E1136 standard reference test tire in accordance with ASTM
Method E1337-90, at a speed of 64.4 km/h (40 mph), without water
delivery. We are proposing incorporating these ASTM provisions into the
Standard.
(c) Vehicle Test Weight
The agency proposes that the combined weight of the truck tractor
and control trailer be equal to 80 percent of the tractor's GVWR. To
achieve this load condition the tractor is loaded with the fuel tanks
filled to at least 75 percent capacity, test driver, test
instrumentation and ballasted control trailer with outriggers. Center
of gravity of all ballast on the control trailer is proposed to be
located directly above the kingpin. When possible, load distribution on
non-steer axles is in proportion to the tractor's respective axle
GAWRs. Load distribution may be adjusted by altering fifth wheel
position, if adjustable. In the case where the tractor fifth wheel
cannot be adjusted so as to avoid exceeding a GAWR, ballast is reduced
so that axle load equals specified GAWR, maintaining load proportioning
as close as possible to specified proportioning.
The agency is proposing that liftable axles be in the down position
for testing. This is because we are conducting our proposed performance
test in a loaded condition. Typically, in real world use, we believe
that a truck tractor loaded to 80% of its GVWR would operate with the
liftable axle in the down position. Consequently, we propose to conduct
compliance testing in that configuration.
For testing large buses, the agency proposes loading the vehicle to
a simulated multi-passenger configuration. For this configuration the
bus is loaded with the fuel tanks filled to at least 75 percent
capacity, test driver, test instrumentation, outriggers and simulated
occupants in each of the vehicle's designated seating positions. The
simulated occupant loads are obtained by securing a 68 kilogram (150
pound) water dummy in each of the test vehicle's designated seating
positions without exceeding the vehicle's GVWR and GAWR. The 68
kilogram (150 pound) occupant load was chosen because that is the
occupant weight specified for use by the agency for evaluating a
vehicle's load carrying capability under FMVSS Nos. 110 and 120. During
loading, if any rating is exceeded the ballast load would be reduced
until the respective rating or ratings are no longer exceeded.
(d) Tires
We propose testing the vehicles with the tires installed on the
vehicle at time of initial vehicle sale. The agency's compliance test
programs generally evaluate new vehicles with new tires. Therefore, we
are proposing as a general rule that a new test vehicle have less than
500 miles on the odometer when received for testing.
For testing, the agency proposes that tires be inflated to the
vehicle manufacturer's recommended cold tire inflation pressure(s)
specified on the vehicle's certification label or the tire inflation
pressure label. No tire changes would occur during testing unless test
vehicle tires are damaged before or during testing. We are not
proposing using inner tubes for testing because we have not seen any
tire debeading in any test.
Before executing any SIS and SWD maneuvers, the agency is proposing
to condition tires to wear away mold sheen and achieve operating
temperatures. To begin the conditioning the test vehicle would be
driven around a circle 46 meters (150 feet) in radius at a speed that
produces a lateral acceleration of approximately 0.1g for two clockwise
laps followed by two counterclockwise laps.
(e) Mass Estimation Drive Cycle
Both truck tractors and large buses experience large changes in
payload mass, which affects a vehicle's roll and yaw stability
thresholds. To adjust the activation thresholds for these changes,
stability control systems estimate the mass of the vehicle after
ignition cycles, periods of static idling, and other driving scenarios.
To estimate the mass, these systems require a period of initial
driving.
The agency proposes to include a mass estimation drive cycle as a
part of pre-test conditioning. To complete this drive cycle the test
vehicle is accelerated to a speed of 64 km/h (40 mph), and then, by
applying the vehicle brakes, decelerated at 0.3g to 0.4g to a stop.
(f) Brake Conditioning
Heavy vehicle brake performance is affected by the original
conditioning and temperatures of the brakes. We believe that
incompletely burnished brakes and excessive brake temperatures can have
an effect on ESC system test results, particularly in the rollover
performance testing, because a hard brake application may be needed for
the foundation brakes to reduce speed to prevent rollover.
FMVSS No. 126 uses a simple conditioning procedure by executing ten
stops from 35 mph followed by three stops at 45 mph. Subsequently, a
cool down period of between 90 seconds and 5 minutes is required
between each SWD maneuver allowing sufficient time for the brakes to
cool down but not so long that the brakes lose all their retained heat.
However, for heavy vehicles, brake conditioning and operating
temperatures are more critical to brake performance than for light
vehicles primarily because the vast majority of heavy vehicles use drum
brakes, which require more conditioning than disc brakes. We believe
that conditioning needs to be more extensive and a brake temperature
range is preferable to a specified cool-down period because each
vehicle may have different cooling rates based on its configuration.
The agency is proposing that the brakes be burnished before any
testing is executed. We believe that the burnish procedure specified in
S6.1.8 of FMVSS No. 121, Air Brake Systems, provides the brake
conditioning needed for the stability control system testing. The
burnish procedure is performed by conducting 500 brake snubs \54\
between 40 mph and 20 mph at a deceleration of 10 fp \2\. If the
vehicle has already completed testing to FMVSS No. 121, we are not
proposing to require the procedure be repeated. Instead, the brakes
would be conditioned for the ESC with 40 snubs. The agency proposes
that the brake temperatures be in the range of 65 [deg]C to 204 [deg]C
(150 [deg]F to 400 [deg]F) at the beginning of each test maneuver. We
also propose that the
[[Page 30804]]
brake temperature be measured by plug-type thermocouples installed on
all brakes and that the hottest brake be used for determining whether
cool-down periods are required.
---------------------------------------------------------------------------
\54\ A snub is a brake application where the vehicle is not
braked to a stop but to a lower speed.
---------------------------------------------------------------------------
After the brakes are burnished, immediately prior to executing any
SIS or SWD maneuvers, the agency would perform 40 brake application
snubs from a speed of 64 km/h (40 mph), with a target deceleration of
approximately 0.3g. At end of the 40 snubs, the hottest brake
temperature would be confirmed within the temperature range of 65
[deg]C to 204 [deg]C (150 [deg]F to 400 [deg]F). If the hottest brake
temperature is above 204 [deg]C (400 [deg]F) a cool-down period would
be provided until the hottest brake temperature is measured within that
range. If the hottest brake temperature is below 65 [deg]C (150 [deg]F)
individual brake stops would be repeated to increase any one brake
temperature to within the target temperature range before the
compliance testing can be continued.
7. Data Filtering and Post Processing
To determine if a test vehicle meets the performance requirements
of the proposed standard, data needs to be measured and processed and
ultimately used to calculate the lateral displacement, lateral
acceleration ratio and yaw rate ratio performance measures. The agency
understands that filtering and post processing methods, if not defined,
can have a significant impact on the final test results used for
determining vehicle compliance. When developing FMVSS No. 126 the
agency received several comments recommending that filtering and
processing methods be defined and included in the regulatory text. The
agency decided to add to the test procedures section of the final
rule's regulatory text a section that specified the critical test
filtering protocols and techniques to be used for test data processing.
We propose to include the same information in this standard. In
addition, the agency proposes to make available on NHTSA's Web site the
actual MATLAB code used for post-processing the critical lateral
acceleration, yaw rate and lateral displacement performance data.
During post-processing the following data signals will be filtered
and conditioned as follows:
1. Filter raw steering wheel angle data with a 12-pole phaseless
Butterworth filter and a cutoff frequency of 10 Hz. Zero the filtered
data to remove sensor offset utilizing static pretest data.
2. Filter raw yaw, pitch and roll rate data with a 12-pole
phaseless Butterworth filter and a cutoff frequency of 3 Hz. Zero the
filtered data to remove sensor offset utilizing static pretest data.
3. Filter raw lateral, longitudinal and vertical acceleration data
with a 12-pole phaseless Butterworth filter and a cutoff frequency of 3
Hz. Zero the filtered data to remove sensor offset utilizing static
pretest data.
4. Filter raw speed data with a 12-pole phaseless Butterworth
filter and a cutoff frequency of 2 Hz.
5. Filter left side and right side ride height data with a 0.1-
second running average filter. Zero the filtered data to remove sensor
offset utilizing static pretest data.
6. The J1939 torque data collected as a digital signal does not get
filtered. J1939 torque data collected as an analog signal is to be
filtered with a 0.1-second running average filter.
There are several events in the calculation of performance metrics
that require determining the time and/or level of an event, including:
Beginning of steer, 1.5 seconds after beginning of steer, completion of
steer, 0.75 second after completion of steer, and 1.50 seconds after
completion of steer. The agency proposes using interpolation \55\ for
all of these circumstances because interpolation provides more
consistent results than other approaches, such as choosing the sample
that is closest in time to the desired event.
---------------------------------------------------------------------------
\55\ Interpolation is a way of computing data values at the
exact time that any of these events occur, even though the digital
samples did not coincide with the exact event point. Rather, one
sample is collected slightly before the time of the event and a
second sample slightly after the time of the event.
---------------------------------------------------------------------------
The beginning of steer is a critical moment during the maneuver
because the lateral displacement performance measure is determined at
exactly 1.5 seconds after the beginning of steer. For compliance
purposes it is essential that the beginning of steer be determined
accurately and consistently during each maneuver and each test. The
process proposed in this NPRM to identify the beginning of steer uses
three steps. The first step identifies when the steering wheel velocity
exceeds 40 degrees per second. From this point, steering wheel velocity
must remain greater than 40 degrees per second for at least 200 ms. If
the condition is not met, the next time steering wheel velocity exceeds
40 degrees per second is identified and the 200 ms validity check is
applied. This iterative process continues until the conditions are
satisfied. In the second step, a zeroing range defined as the 1.0
second time period prior to the instant the steering wheel velocity
exceeds 40 degrees per second. In the third step, the first instance
the filtered and zeroed steering wheel angle data reaches minus 5
degrees (when the initial steering input is counterclockwise) or plus 5
degrees (when the initial steering input is clockwise) after the end of
the zeroing range is identified. The time identified is taken to be the
beginning of steer.
The agency understands that an unambiguous reference point to
define the start of steering is necessary in order to ensure
consistency when computing the performance metrics measured during
compliance testing. The practical problem is that typical ``noise'' in
the steering measurement channel causes continual small fluctuations of
the signal about the zero point, so departure from zero with very small
steering angles does not reliably indicate that the steering machine
has started the test maneuver. NHTSA's extensive evaluation of zeroing
range criteria has confirmed that the method successfully and robustly
distinguishes the initiation of the SWD steering inputs from the
inherent noise present in the steering wheel angle data channel. The
value for time at the beginning of steer used for calculating the
lateral displacement metric is interpolated.
The completion of steer is a critical moment during the maneuver
because the LAR and YRR metrics are determined at specific time
intervals after the completion of steer. The agency believes that an
unambiguous point to define the completion of steer is also necessary
for consistency in computing the required performance metrics during
compliance testing. The agency proposes considering the first
occurrence of the ``zeroed'' steering wheel angle crossing zero degrees
after the second peak of steering wheel angle during the sine maneuver
to be the completion of steer. Although signal noise results in
continual zero crossings as long the data is being sampled, the first
zero crossing after the steering wheel has begun to return to the zero
position is a logical end to the steering maneuver.
Given the potential for the accelerometers used in the measurement
and determination of lateral acceleration and lateral displacement to
drift over time, the agency uses the data one second before the start
of steering to ``zero'' the accelerometers and roll signal. Prior to
the test maneuver, the driver must orient the vehicle to the desired
heading, position the steering wheel angle to zero, and be coasting
down (i.e., not using throttle inputs) to the target test speed of 45
mph. This process, known as achieving a ``quasi steady-state,''
typically occurs a few seconds prior to initiation of the maneuver, but
can be influenced by external factors such as test track traffic,
[[Page 30805]]
differences in vehicle deceleration rates, etc. Any zeroing performed
on test data must be performed after a quasi-steady-state condition has
been satisfied, but before the maneuver is initiated. The proposed
zeroing duration of one second provides an adequate combination of
sufficient time (i.e., enough data is present so as to facilitate
accurate zeroing of the test data) and performability (i.e., the
duration is not so long that it imposes an unreasonable burden on the
driver).
The lateral acceleration data are collected from an accelerometer,
corrected for roll angle effects, and resolved to the vehicle's CG
using coordinate transformation equations. The use of accelerometers is
commonplace in the vehicle testing community, and installation is
simple and well understood. However, in most cases, it is not possible
to install a lateral acceleration sensor at the location of the
vehicle's exact center of gravity. For this reason, it is important to
provide a coordinate transformation to resolve the measured lateral
acceleration values to the vehicle's center of gravity location. The
specific equations proposed to perform this operation, as well as those
used to correct lateral acceleration data for the effect of chassis
roll angle, will be incorporated into the laboratory test procedure and
are included in the MATLAB post processing routines used by the agency.
The equations used for coordinate transformation and vehicle body
roll are as follows:
Equation 1: x''corrected = x''accel-([Theta]' \2\
+ [Psi]' \2\)xdisp + ([Theta]'[Phi]'-
[Psi]'')ydisp + ([Psi] '[Phi]' + [Theta]'')zdisp
Equation 2: y''corrected = y''accel +
([Theta]'[Phi]' + [Psi] '')xdisp-([Phi]' \2\ + [Psi] '
\2\)ydisp + ([Psi] '[Theta]'-[Phi]'')zdisp
Equation 3: z''corrected = z''accel + ([Psi]
'[Phi]'-[Theta]'')xdisp + ([Psi] '[Theta]' +
[Phi]'')ydisp-([Phi]' \2\ + [Theta]' \2\)zdisp
Where:
x''corrected, y''corrected, and
z''corrected = longitudinal, lateral, and vertical
accelerations, respectively, at the vehicle's center of gravity
x''accel, y''accel, and z''accel =
longitudinal, lateral, and vertical accelerations, respectively, at
the accelerometer location
xdisp, ydisp, and zdisp =
longitudinal, lateral, and vertical displacements, respectively, of
the center of gravity with respect to the accelerometer location
[Phi]' and [Phi]'' = roll rate and roll acceleration, respectively
[Theta]' and [Theta]'' = pitch rate and pitch acceleration,
respectively
[Psi] ' and [Psi] '' = yaw rate and yaw acceleration, respectively
If the sensors used to measure the vehicle responses are of
sufficient accuracy, and have been installed and configured correctly,
use of the analysis routines provided by NHTSA are expected to minimize
the potential for performance discrepancies among NHTSA and industry
test efforts. The equations utilized are the same equations used by the
agency for its NCAP rollover program and the FMVSS No. 126 light
vehicle ESC program, and were derived from equations of general
relative acceleration for a translating reference frame utilizing the
SAE convention for Vehicle Dynamics Coordinate Systems.
Furthermore, NHTSA does not propose using inertially stabilized
accelerometers for this test procedure. Therefore, lateral acceleration
must be corrected for vehicle roll angle during data post processing.
Non-contact displacement sensors are used to collect left and right
side vertical displacements for the purpose of calculating vehicle roll
angle. One sensor is mounted on each side of the vehicle, and is
positioned at the longitudinal CG. With these data, roll angle is
calculated during post-processing using trigonometry as follows:
Equation 4: ayc = aymcos [Phi] -
azmsin [Phi]
Where:
ayc is the corrected lateral acceleration (i.e., the
vehicle's lateral acceleration in a plane horizontal to the test
surface)
aym is the measured lateral acceleration in the vehicle
reference frame
azm is the measured vertical acceleration in the vehicle
reference frame
[Phi] is the vehicle's roll angle
Note: The z-axis sign convention is positive in the downward
direction for both the vehicle and test surface reference frames.
G. Compliance Dates and Implementation Schedule
The agency proposes that all new typical 6x4 truck tractors and all
buses covered by this proposal would be required to meet this proposed
standard effective two years after the final rule is published. The
current annual installation rate for stability control systems on new
truck tractors is approximately 18 percent. Because there are currently
only two suppliers of truck tractor and large bus stability control
systems, Bendix and Meritor WABCO, we believe that the industry will
need lead time to ensure that the necessary production stability
control systems are available to manufacturers.
For severe service tractors and tractors with four axles or more,
the agency believes that manufacturers of these atypical truck
tractors, which represent about 5 percent of annual truck tractor
sales, may need additional lead time to develop, test and equip these
vehicles with a stability control system. Therefore, we are proposing
to require that severe service tractors and other atypical tractors be
equipped with ESC systems beginning four years after the final rule is
published. We note that we made a similar distinction between typical
6x4 tractors and other tractors in specifying the lead time for
amendments to FMVSS No. 121 mandating improved stopping distance
performance.\56\
---------------------------------------------------------------------------
\56\ See 49 CFR 571.121, Table IIA.
---------------------------------------------------------------------------
However, in our stopping distance rulemaking, we allowed extra time
for two-axle tractors to comply because shorter wheelbase tractors
(i.e., two-axle tractors) showed a risk of instability resulting from
the improved stopping distance requirements. However, the increased
risk of instability in shorter wheelbase vehicles led us to the
opposite tentative conclusion in this rulemaking. Because two-axle
tractors have a particular risk of instability, we do not believe
extending lead time for two-axle tractors is warranted.
The vast majority of new truck tractors are three-axle (6x4)
vehicles, which facilitates standardization of ESC for these vehicles.
The available test data for typical three-axle (6x4) tractors with
stability control systems show that the existing ESC technology should
enable these vehicles to readily comply with stability control
requirements proposed by the agency. In addition, the agency's benefit
analysis indicates that ESC provides substantial safety benefits to
truck tractors. Hence, we believe that it is important that the
implementation date for ESC on these vehicles be as early as
practicable so that these safety benefits could be achieved.
Several manufacturers of Class 8 buses are already offering ESC as
standard equipment on their vehicles but we are not aware of any Class
7 bus that is available with ESC. We believe that the manufacturers of
Class 7 buses would need some lead time to have the ESC systems
developed, tested and installed on their vehicles. Hence, for large
buses, the agency proposes an effective date of two years after the
final rule is published, primarily to accommodate manufacturers of
Class 7 buses.
VI. Benefits and Costs
A. System Effectiveness
As discussed above, direct data that would show the effectiveness
of stability control systems is not available
[[Page 30806]]
because stability control system technology on heavy vehicles is so
new. Accordingly, NHTSA sponsored a research program with Meritor WABCO
and UMTRI to examine the potential effectiveness of stability control
systems on the fleet of truck tractors. A copy of UMTRI's report has
been placed in the docket.
However, for NHTSA to calculate the effectiveness of stability
control systems for truck tractors, two modifications were necessary.
First, the UMTRI study based its effectiveness estimates on a simple
aggregation of cases rather than weighting the likelihood of occurrence
of each case. Second, based on NHTSA's independent review of the 159
cases, two cases were incorrectly categorized as loss of control rather
than untripped rollover and the effectiveness rating of six cases were
revised downward.
The results of UMTRI's study and the agency's revised effectiveness
estimates were published in a January 2011 research note entitled
``Effectiveness of Stability Control Systems For Truck Tractors'' (DOT
HS 811 437).\57\ The effectiveness estimates from that research note
are summarized in the following table.
---------------------------------------------------------------------------
\57\ Docket No. NHTSA-2010-0034-0043.
Table 4--Effectiveness Rates for ESC and RSC by Target Crashes
[Current NHTSA estimates]
----------------------------------------------------------------------------------------------------------------
Overall effectiveness Untripped rollover Loss of control
Technology (%) effectiveness (%) effectiveness (%)
----------------------------------------------------------------------------------------------------------------
ESC.................................. 28-36 40-56 14
RSC.................................. 21-30 37-53 3
----------------------------------------------------------------------------------------------------------------
For large buses, it was not feasible to conduct a similar
statistical analysis because of limited crash data. However, NHTSA's
testing revealed that an identical set of test maneuvers could be used
to evaluate truck tractor and large bus systems' ability to prevent
rollover and loss-of-control crashes. Therefore, for the purpose of
this proposal, the effectiveness of ESC and RSC systems on large buses
was assumed to be identical to the performance of systems on truck
tractors.
B. Target Crash Population
The initial target crash population for estimating benefits
includes all crashes resulting in occupant fatalities, MAIS 1 and above
nonfatal injuries, and property damage only crashes that were the
result of either (a) first-event untripped rollover crashes and (b)
loss-of-control crashes (e.g., jackknife, cargo shift, avoiding,
swerving) that involved truck tractors or large buses and might be
prevented if the subject vehicle were equipped with a stability control
system. For this analysis, particularly in multi-vehicle crashes, the
subject vehicle is the at-fault or striking vehicle. The initial target
crash populations were retrieved from the 2006-2008 Fatality Analysis
Reporting System (FARS) and General Estimate System (GES). The FARS
data were used for evaluating fatal crashes and the GES data were used
for evaluating nonfatal crashes. The injury data were converted to MAIS
format and the following number of crashes, fatalities, injuries, and
deaths were estimated.
Table 5--Initial Target Crashes, MAIS Injuries, and Property Damage Only Vehicle Crashes by Crash Type
----------------------------------------------------------------------------------------------------------------
MAIS 1-5
Crash type Crashes Fatalities Injuries PDOVs
----------------------------------------------------------------------------------------------------------------
Rollover........................................ 5,510 111 2,217 3,297
Loss of control................................. 4,803 216 1,141 3,935
---------------------------------------------------------------
Total..................................... 10,313 327 3,358 7,332
----------------------------------------------------------------------------------------------------------------
Source: 2006-2008 FARS, 2006-2008 GES.
PDOVs: property damage only vehicles.
The 2006-2008 crash data were then adjusted to take account of ESC
and RSC system installation rates in 2006-2008 and in model year 2012.
To determine the number of crashes that could be prevented by requiring
that ESC systems be installed on new truck tractors, the agency had to
consider two subsets of the total crash population--those vehicles that
would not be equipped with stability control systems (Base 1
population) and those vehicles that would be equipped with RSC systems
(Base 2 population). The Base 1 population would benefit fully from
this proposal. However, the Base 2 population would benefit only from
the incremental increased effectiveness of ESC systems over RSC
systems.
Based upon data obtained from industry, the agency estimates that
about 1.9 percent of truck tractors in the on-road fleet in 2008 were
equipped with ESC systems and 3.3 percent were equipped with RSC
systems. Based upon manufacturer production estimates, about 26.2
percent of truck tractors manufactured in model year 2012 would be
equipped with ESC systems and 16.0 percent would be equipped with RSC
systems. Adjusting the initial target crash populations using these
estimates, the agency was able to estimate the Base 1 and Base 2
populations and the projected target crash population (Base 1 + Base 2)
expressed in the following table.
[[Page 30807]]
Table 6--Projected Crashes, MAIS Injuries, and Property Damage Only Vehicle Crashes by Crash Type, Crash
Severity, Injury Severity, and Vehicle Type for 2012 Level
----------------------------------------------------------------------------------------------------------------
MAIS 1-5
Crash type Crashes Fatalities Injuries PDOVs
----------------------------------------------------------------------------------------------------------------
Base 1
----------------------------------------------------------------------------------------------------------------
Rollover........................................ 3,263 66 1,313 1,952
Loss of Control................................. 2,786 125 662 2,283
---------------------------------------------------------------
Total....................................... 6,049 191 1,975 4,235
----------------------------------------------------------------------------------------------------------------
Base 2
----------------------------------------------------------------------------------------------------------------
Rollover........................................ 903 18 364 540
Loss of Control................................. 771 35 183 632
---------------------------------------------------------------
Total....................................... 1,674 53 547 1,172
----------------------------------------------------------------------------------------------------------------
Base 1 + Base 2 (Projected Target Population)
----------------------------------------------------------------------------------------------------------------
Rollover........................................ 4,166 84 1,677 2,492
Loss of Control................................. 3,557 160 845 2,915
---------------------------------------------------------------
Total....................................... 7,723 244 2,522 5,407
----------------------------------------------------------------------------------------------------------------
Source: 2006-2008 FARS, 2006-2008 GES.
PDOVs: property damage only vehicles.
The agency has also examined the same crash data sources for large
buses. Based upon this examination, the agency estimates that an
average of one target bus rollover and one target bus loss-of-control
crash occurs per year that would be affected by this proposal.
C. Benefits Estimate
ESC systems are crash avoidance countermeasures that would mitigate
and even prevent crashes. Preventing a crash not only would save lives
and reduce injuries, it also would alleviate crash-related travel
delays and property damage. Therefore, the estimated benefits include
both injury and non-injury components. The injury benefits are the
estimated fatalities and injuries that would be mitigated or eliminated
by ESC. The non-injury benefits include the travel delay and property
damage savings from crashes that were avoided by ESC. Savings from
reducing property-damage-only vehicle crashes also were included in the
non-injury benefits.
The benefits estimates for rollover crashes are presented in a
range in this analysis. This is the result of a range of ESC
effectiveness figures in addressing rollover crashes that were used for
the analysis. In contrast, at the publication, there is only one
effectiveness estimate for addressing loss-of-control crashes.
The benefits of this proposal were derived by multiplying the
projected target population by the corresponding effectiveness rates.
As shown in Table 7, this proposal would prevent 1,807 to 2,329 target
crashes, 49 to 60 fatalities, and 649 to 858 MAIS 1-5 injuries.
Furthermore, the proposal would eliminate 1,187 to 1,499 property-
damage-only crashes. Table 7 presents the benefits by target crash
type.
Table 7--Estimated Benefits of the Proposal
----------------------------------------------------------------------------------------------------------------
MAIS 1-5
Crash type Crashes Fatalities Injuries PDOVs
----------------------------------------------------------------------------------------------------------------
Base 1 Benefits
----------------------------------------------------------------------------------------------------------------
Rollover........................................ 1,305-1,827 26-37 526-735 781-1,093
Loss of Control................................. 390 18 93 320
---------------------------------------------------------------
Total........................................... 1,695-2,217 44-55 619-828 1,101-1,413
----------------------------------------------------------------------------------------------------------------
Base 2 Benefits
----------------------------------------------------------------------------------------------------------------
Rollover........................................ 27 1 11 16
Loss of Control................................. 85 4 19 70
---------------------------------------------------------------
Total....................................... 112 5 30 86
----------------------------------------------------------------------------------------------------------------
Benefits of the Proposal (Base 1 + Base 2)
----------------------------------------------------------------------------------------------------------------
Rollover........................................ 1,332-1,854 27-38 537-746 797-1,109
Loss of Control................................. 475 22 112 390
---------------------------------------------------------------
Total....................................... 1,807-2,329 49-60 649-858 1,187-1,499
----------------------------------------------------------------------------------------------------------------
Source: 2006-2008 FARS, 2006-2008 GES.
PDOVs: property damage only vehicles.
[[Page 30808]]
The non-injury benefits also include savings from the elimination
of crash-related travel delay and vehicle property damage. Table 8
shows the total travel delay and property damage savings from this
proposal, broken down by target crash type. These benefits were derived
by determining the unit cost of property damage and travel delay for
each level of crash severity (e.g., fatal, MAIS 1-5, or property damage
only) and multiplying that cost by the number of incidents of each type
of crash prevented. As shown in Table 8, this proposal would save
(undiscounted) $17.1 to $22.0 million from travel delays and property
damage as a result of crashes that would be prevented by this proposal.
Table 8--Total Travel Delay and Property Damage Savings
[Undiscounted 2010 $]
----------------------------------------------------------------------------------------------------------------
Property damage +
Property damage Travel delay travel delay
----------------------------------------------------------------------------------------------------------------
Rollover--Lower Bound............................... $7,713,841 $4,655,187 $12,369,028
Rollover--Upper Bound............................... 10,735,872 6,475,446 17,211,318
Loss of Control..................................... 3,006,977 1,765,804 4,772,781
-----------------------------------------------------------
Total--Lower Bound.................................. 10,720,818 6,420,991 17,141,809
-----------------------------------------------------------
Total-Upper Bound................................... 13,742,849 8,241,250 21,984,099
----------------------------------------------------------------------------------------------------------------
D. Cost Estimate
The cost of this proposal is derived from the product of the
average unit cost of an ESC system and the number of vehicles affected
by this proposal. The number of vehicles affected by this proposal
would include vehicles that would have no stability control systems and
vehicles that would be equipped with RSC systems. Therefore, when
considering vehicles equipped with RSC systems, the average cost would
be the difference between the cost of an ESC system and the cost of an
RSC system.
Based upon data received from manufacturers, the agency estimates
that the average unit cost for an ESC system is $1,160 and the average
unit cost for an RSC system is $640; therefore, the incremental cost of
installing an ESC system instead of an RSC system is $520 per vehicle.
The agency did not receive cost information from large bus
manufacturers. However, because the components used on truck tractors
and buses are nearly identical, the unit cost estimates for truck
tractors are used for buses.
The agency has estimated that 150,000 truck tractors and 2,200
buses covered by this proposal would be produced in model year 2012. As
stated earlier, the agency estimates that 26.2 percent of truck
tractors and 80 percent of buses covered by this proposal manufactured
in model year 2012 would be equipped with ESC systems. In addition,
16.5 percent of truck tractors would be equipped with RSC systems.
Accordingly, 57.8 percent of truck tractors and 20 percent of buses
would be required to be equipped with an ESC system and 16.5 percent of
truck tractors would be required to upgrade from an RSC system to an
ESC system.
Table 9 summarizes the costs of this proposal based on the
estimated unit cost of an ESC system and the number of vehicles that
would need to be equipped with ESC systems. As shown in Table 10, the
incremental cost of providing ESC systems compared to manufacturers'
planned production in model year 2012 would cost $113.1 million for
truck tractors and $0.5 million for large buses. Therefore, the total
cost of this proposal is estimated to be $113.6 million.
Table 9--Annual Total Costs for the Proposal
[2010 $]
----------------------------------------------------------------------------------------------------------------
Technology upgrade needed
-----------------------------------------------------------
None Incremental ESC ESC
----------------------------------------------------------------------------------------------------------------
Truck Tractors:
% Needing Upgrade............................... 26.2% 16.0% 57.8%
150,000 Sales Estimated......................... 39,300 24,000 86,700
Costs per Affected Vehicle...................... 0 $520 $1,160
-----------------------------------------------------------
Total Costs................................. 0 $12.5 M $100.6 M
Large Buses:
% Needing Upgrade............................... 80% 0% 20%
2,200 Sales Estimated........................... 1,760 0 440
Costs per Affected Vehicle...................... 0 $520 $1,160
-----------------------------------------------------------
Total Costs................................. 0 0 $0.5 M
----------------------------------------------------------------------------------------------------------------
M: million.
[[Page 30809]]
Table 10--Summary of Vehicle Costs
[2010 $]
------------------------------------------------------------------------
Average
vehicle Total
costs costs
------------------------------------------------------------------------
Truck Tractors.................................... $753.7 $113.1 M
Large Buses....................................... 232.0 0.5 M
---------------------
Total......................................... 746.1 113.6 M
------------------------------------------------------------------------
M: million.
We also note that manufacturers may incur costs to certify their
vehicles as compliant with the proposed standard. We have estimated the
cost to conduct the proposed test maneuvers. We believe that the
execution of the proposed SIS and SWD maneuvers would cost
approximately $15,000 per test, assuming access to test facilities,
tracks, and vehicles. Because it is not possible to anticipate how many
tests manufacturers might choose to run to certify a specific make,
model, and configuration, the agency cannot estimate the total
compliance costs for manufacturers. However, compliance costs are
implicitly included in the estimated consumer cost, which includes a
150% markup to account for fixed and overhead costs.
E. Cost Effectiveness
Safety benefits can occur at any time during the vehicle's
lifetime. Therefore, the benefits are discounted at both 3 and 7
percent to reflect their values in 2010 dollars, as reflected in Table
11. Table 11 also shows that the net cost per equivalent life saved
from this proposal ranged from $1.5 to $2.0 million at a 3 percent
discount rate and from $2.0 to $2.6 million at a 7 percent discount
rate. The net benefits of this proposal are estimated to range from
$228 to $310 million at a 3 percent discount rate and from $155 to $222
million at a 7 percent discount rate.
Table 11--Summary of Cost-Effectiveness and Net Benefits by Discount Rate
[2010 $]
----------------------------------------------------------------------------------------------------------------
3% Discount 7% Discount
-----------------------------------------------------------------------
Low High Low High
----------------------------------------------------------------------------------------------------------------
Fatal Equivalents....................... 51 63 40 50
Injury Benefits......................... $328,197,087 $405,419,931 $257,409,480 $321,761,850
Property Damage and Travel Delay Savings $13,862,581 $17,778,541 $11,006,756 $14,115,990
Vehicle Costs *......................... $113,562,400 $113,562,400 $113,562,400 $113,562,400
Net Costs............................... $99,699,819 $95,783,859 $102,555,644 $99,446,410
Net Cost Per Fatal Equivalent........... $1,954,898 $1,520,379 $2,563,891 $1,988,928
Net Benefits............................ $228,497,268 $309,636,072 $154,853,836 $222,315,440
----------------------------------------------------------------------------------------------------------------
* Vehicle costs are not discounted, since they occur when the vehicle is purchased, whereas benefits occur over
the vehicle's lifetime and are discounted back to the time of purchase.
F. Comparison of Regulatory Alternatives
The agency considered two alternatives to the proposal. The first
alternative was requiring RSC systems be installed on all newly
manufactured truck tractors and buses covered by this proposal. The
second alternative was requiring RSC systems be installed on all newly
manufactured trailers.
Regarding the first alternative, requiring RSC systems be installed
on truck tractors and large buses, our research has concluded that RSC
systems are less effective than ESC systems. Overall for the target
crash population, our research has indicated that RSC systems have a 21
to 30 percent effectiveness rate, whereas ESC systems have a 28 to 36
percent effectiveness rate. An RSC system is only slightly less
effective at preventing rollover crashes than an ESC system (37 to 53
percent versus 40 to 56 percent effective, respectively), but it is
much less effective at preventing loss of control crashes (3 percent
versus 14 percent). However, RSC systems are only estimated to cost
$640 per unit, whereas ESC systems are estimated to cost $1,160 per
unit. Furthermore, only approximately 57.8% of truck tractors would be
required to install RSC systems based on the data discussed earlier
regarding manufacturers' plans.
A summary of the cost effectiveness of RSC systems is set forth in
Table 12. When comparing this alternative to the regulatory proposal,
requiring RSC systems rather than ESC systems would be slightly more
cost effective. However, this alternative would save fewer lives and
have lower net benefits than this proposal.
Table 12--Summary of Cost-Effectiveness and Net Benefits by Discount Rate Alternative 1--Requiring Tractor-Based
RSC Systems
[2010 $]
----------------------------------------------------------------------------------------------------------------
3% Discount 7% Discount
---------------------------------------------------------------------------
Low High Low High
----------------------------------------------------------------------------------------------------------------
Fatal Equivalents................... 31 43 24 34
Injury Benefits..................... $199,492,347 $276,715,191 $154,445,688 $218,798,058
Property Damage and Travel Delay $9,714,383 $13,649,563 $7,713,126 $10,837,621
Savings............................
Vehicle Costs *..................... $55,769,600 $55,769,600 $55,769,600 $55,769,600
Net Costs........................... $46,055,217 $42,120,037 $48,056,474 $44,931,979
Net Cost Per Fatal Equivalent....... $1,485,652 $979,536 $2,002,353 $1,321,529
Net Benefits........................ $153,437,130 $234,595,154 $106,389,214 $173,866,079
----------------------------------------------------------------------------------------------------------------
* Vehicle costs are not discounted, since they occur when the vehicle is purchased, whereas benefits occur over
the vehicle's lifetime and are discounted back to the time of purchase.
[[Page 30810]]
The second alternative considered was requiring trailer-based RSC
systems to be installed on all newly manufactured trailers. Trailer-
based RSC systems would only be expected to prevent rollover crashes.
Based on 2006-2008 GES data, 98 percent of the target truck-tractor
crashes involve truck tractors with trailers attached. Therefore, the
base crash population would be 98 percent of Base 1 discussed above.
As discussed in the proposal, it became apparent during testing
that trailer-based stability control systems were less effective than
tractor-based systems because trailer-based systems could only control
the trailer's brakes. Based upon the agency's test data, it is
estimated that the effectiveness of trailer-based RSC systems in
preventing rollover crashes is 7 to 10 percent. Therefore, the benefits
of trailer-based RSC systems in preventing rollover are about 17.2
percent of tractor-based ESC systems.
The agency estimates that about 203,000 new trailers are
manufactured each year. Further, based on information from
manufacturers, the agency estimates that a trailer-based RSC system
would cost $400 per trailer. Available data indicates that less than
0.2 percent of the current annual production of trailers comes with RSC
systems installed. Assuming all new trailers would be required to
install RSC, the cost of this alternative is estimated to be $81.2
million.
Table 13 sets forth a summary of the cost effectiveness of trailer-
based RSC systems. Because the operational life of a trailer
(approximately 45 years) is much longer than that of a truck tractor,
it would take longer for trailer-based RSC systems to fully penetrate
the fleet than it would for any tractor-based system. Therefore, when
the benefits of trailer-based RSC systems are discounted at a 3 and 7
percent rate, there is a much higher discount factor. As can be seen in
Table 13, this results in this alternative having negative net benefits
and a high cost per life saved. Also, this alternative would have no
effect on buses. Accordingly, the agency does not favor this
alternative.
Table 13--Summary of Cost-Effectiveness and Net Benefits by Discount Rate Alternative 2--Requiring Trailer-Based
RSC Systems
[2010 $]
----------------------------------------------------------------------------------------------------------------
At 3% Discount At 7% Discount
---------------------------------------------------------------------------
Low High Low High
----------------------------------------------------------------------------------------------------------------
Fatal Equivalents................... 5 7 3 5
Injury Benefits..................... $30,754,672 $43,935,246 $20,700,937 $29,572,767
Property Damage and Travel Delay $1,459,169 $2,038,560 $982,165 $1,372,153
Savings............................
Vehicle Costs *..................... $81,200,000 $81,200,000 $81,200,000 $81,200,000
Net Costs........................... $79,740,831 $79,161,440 $80,217,835 $79,827,847
Net Cost Per Fatal Equivalent....... $15,948,166 $11,308,777 $26,739,278 $15,965,569
Net Benefits........................ -$48,986,159 -$35,226,194 -$59,516,898 -$50,255,080
----------------------------------------------------------------------------------------------------------------
* Vehicle costs are not discounted, since they occur when the vehicle is purchased, whereas benefits occur over
the vehicle's lifetime and are discounted back to the time of purchase.
The information in Tables 12 and 13 can be contrasted with this
proposal. A summary of the total costs and benefits and annualized
costs and benefits of this proposal appears in Table 14.
Table 14--Estimated Total Costs and Benefits of the Proposal
[In millions of 2010 dollars]
--------------------------------------------------------------------------------------------------------------------------------------------------------
Property damage Cost per
Total costs Injury benefits and travel delay equivalent live Net benefits
savings saved
--------------------------------------------------------------------------------------------------------------------------------------------------------
At 3% Discount................................................ $113.6 $328-$405 $13.9-$17.8 $1.5-$2.0 $228-$310
At 7% Discount................................................ 113.6 257-322 11.0-14.1 2.0-2.6 155-222
--------------------------------------------------------------------------------------------------------------------------------------------------------
VII. Public Participation
How do I prepare and submit comments?
Your comments must be written and in English. To ensure that your
comments are correctly filed in the Docket, please include the docket
number of this document in your comments.
Your comments must not be more than 15 pages long (49 CFR 553.21).
We established this limit to encourage you to write your primary
comments in a concise fashion. However, you may attach necessary
additional documents to your comments. There is no limit on the length
of the attachments.
Please submit two copies of your comments, including the
attachments, to Docket Management at the beginning of this document,
under ADDRESSES. You may also submit your comments electronically to
the docket following the steps outlined under ADDRESSES.
How can I be sure that my comments were received?
If you wish Docket Management to notify you upon its receipt of
your comments, enclose a self-addressed, stamped postcard in the
envelope containing your comments. Upon receiving your comments, Docket
Management will return the postcard by mail.
How do I submit confidential business information?
If you wish to submit any information under a claim of
confidentiality, you should submit the following to the NHTSA Office of
Chief Counsel (NCC-110), 1200 New Jersey Avenue SE., Washington, DC
20590: (1) A complete
[[Page 30811]]
copy of the submission; (2) a redacted copy of the submission with the
confidential information removed; and (3) either a second complete copy
or those portions of the submission containing the material for which
confidential treatment is claimed and any additional information that
you deem important to the Chief Counsel's consideration of your
confidentiality claim. A request for confidential treatment that
complies with 49 CFR Part 512 must accompany the complete submission
provided to the Chief Counsel. For further information, submitters who
plan to request confidential treatment for any portion of their
submissions are advised to review 49 CFR part 512, particularly those
sections relating to document submission requirements. Failure to
adhere to the requirements of Part 512 may result in the release of
confidential information to the public docket. In addition, you should
submit two copies from which you have deleted the claimed confidential
business information, to Docket Management at the address given at the
beginning of this document under ADDRESSES.
Will the Agency consider late comments?
We will consider all comments that Docket Management receives
before the close of business on the comment closing date indicated at
the beginning of this notice under DATES. In accordance with our
policies, to the extent possible, we will also consider comments that
Docket Management receives after the specified comment closing date. If
Docket Management receives a comment too late for us to consider in
developing the proposed rule, we will consider that comment as an
informal suggestion for future rulemaking action.
How can I read the comments submitted by other people?
You may read the comments received by Docket Management at the
address and times given near the beginning of this document under
ADDRESSES.
You may also see the comments on the Internet. To read the comments
on the Internet, go to http://www.regulations.gov and follow the on-
line instructions provided.
You may download the comments. The comments are imaged documents,
in either TIFF or PDF format. Please note that even after the comment
closing date, we will continue to file relevant information in the
Docket as it becomes available. Further, some people may submit late
comments. Accordingly, we recommend that you periodically search the
Docket for new material.
VIII. Regulatory Analyses and Notices
A. Executive Order 12866, Executive Order 13563, and DOT Regulatory
Policies and Procedures
NHTSA has considered the impact of this rulemaking action under
Executive Order 12866, Executive Order 13563, and the Department of
Transportation's regulatory policies and procedures. This rulemaking is
considered economically significant and was reviewed by the Office of
Management and Budget under E.O. 12866, ``Regulatory Planning and
Review.'' The rulemaking action has also been determined to be
significant under the Department's regulatory policies and procedures.
NHTSA has placed in the docket a Preliminary Regulatory Impact Analysis
(PRIA) describing the benefits and costs of this rulemaking action. The
benefits and costs are summarized in section VI of this preamble.
Consistent with Executive Order 13563 and to the extent permitted
under the Vehicle Safety Act, we have considered the cumulative effects
of the new regulations stemming from NHTSA's 2007 ``NHTSA's Approach to
Motorcoach Safety'' plan and DOT's 2009 Motorcoach Safety Action Plan,
and have taken steps to identify opportunities to harmonize and
streamline those regulations. By coordinating the timing and content of
the rulemakings, our goal is to expeditiously maximize the net benefits
of the regulations (by either increasing benefits or reducing costs or
a combination of the two) while simplifying requirements on the public
and ensuring that the requirements are justified. We seek to ensure
that this coordination will also simplify the implementation of
multiple requirements on a single industry.
NHTSA's Motorcoach Safety Action Plan identified four priority
areas--passenger ejection, rollover structural integrity, emergency
egress, and fire safety. There have been other initiatives on large bus
performance, such as ESC systems--an action included in the DOT plan--
and an initiative to update the large bus tire standard.\58\ In
deciding how best to initiate and coordinate rulemaking in these areas,
NHTSA examined various factors including the benefits that would be
achieved by the rulemakings, the anticipated vehicle designs and
countermeasures needed to comply with the regulations, and the extent
to which the timing and content of the rulemakings could be coordinated
to lessen the need for multiple redesign and to lower overall costs.
After this examination, we decided on a course of action that
prioritized the goal of reducing passenger ejection and increasing
frontal impact protection because many benefits could be achieved
expeditiously with countermeasures that were readily available (using
bus seats with integral seat belts, which are already available from
seat suppliers) and whose installation would not significantly impact
other vehicle designs. Similarly, we have also determined that an ESC
rulemaking would present relatively few synchronization issues with
other rules, because the vehicles at issue already have the foundation
braking systems needed for the stability control technology and the
additional equipment necessary for an ESC system are sensors that are
already available and that can be installed without significant impact
on other vehicle systems. Further, we estimate that 80 percent of the
affected buses already have ESC systems. We realize that a rollover
structural integrity rulemaking, or an emergency egress rulemaking,
could involve more redesign of vehicle structure than rules involving
systems such as seat belts, ESC, or tires.\59\ Our decision-making in
these and all the rulemakings outlined in the ``NHTSA's Approach to
Motorcoach Safety'' plan and DOT's Motorcoach Safety Action Plan will
be cognizant of the timing and content of the actions so as to simplify
requirements applicable to the public and private sectors, ensure that
requirements are justified, and increase the net benefits of the
resulting safety standards.
---------------------------------------------------------------------------
\58\ 75 FR 60037 (Sept. 29, 2010).
\59\ The initiative on fire safety is in a research phase.
Rulemaking resulting from the research will not occur in the near
term.
---------------------------------------------------------------------------
B. Regulatory Flexibility Act
Pursuant to the Regulatory Flexibility Act (5 U.S.C. 601 et seq.,
as amended by the Small Business Regulatory Enforcement Fairness Act
(SBREFA) of 1996), whenever an agency is required to publish a notice
of rulemaking for any proposed or final rule, it must prepare and make
available for public comment a regulatory flexibility analysis that
describes the effect of the rule on small entities (i.e., small
businesses, small organizations, and small governmental jurisdictions).
The Small Business Administration's regulations at 13 CFR Part 121
define a small business, in part, as a business entity ``which operates
primarily within the United States.'' (13 CFR 121.105(a)).
[[Page 30812]]
No regulatory flexibility analysis is required if the head of an agency
certifies the rule will not have a significant economic impact on a
substantial number of small entities. SBREFA amended the Regulatory
Flexibility Act to require Federal agencies to provide a statement of
the factual basis for certifying that a rule will not have a
significant economic impact on a substantial number of small entities.
NHTSA has considered the effects of this NPRM under the Regulatory
Flexibility Act. I certify that this NPRM will not have a significant
economic impact on a substantial number of small entities. This
proposed rule would directly impact manufacturers of truck-tractors,
large buses, and stability control systems for those vehicles. NHTSA
believes these entities do not qualify as small entities.
C. Executive Order 13132 (Federalism)
NHTSA has examined today's final rule pursuant to Executive Order
13132 (64 FR 43255, August 10, 1999) and concluded that no additional
consultation with States, local governments or their representatives is
mandated beyond the rulemaking process. The agency has concluded that
the rulemaking would not have sufficient federalism implications to
warrant consultation with State and local officials or the preparation
of a federalism summary impact statement. The final rule would not have
``substantial direct effects on the States, on the relationship between
the national government and the States, or on the distribution of power
and responsibilities among the various levels of government.''
NHTSA rules can preempt in two ways. First, the National Traffic
and Motor Vehicle Safety Act contains an express preemption provision:
When a motor vehicle safety standard is in effect under this chapter, a
State or a political subdivision of a State may prescribe or continue
in effect a standard applicable to the same aspect of performance of a
motor vehicle or motor vehicle equipment only if the standard is
identical to the standard prescribed under this chapter. 49 U.S.C.
30103(b)(1). It is this statutory command by Congress that preempts any
non-identical State legislative and administrative law addressing the
same aspect of performance.
The express preemption provision described above is subject to a
savings clause under which ``[c]ompliance with a motor vehicle safety
standard prescribed under this chapter does not exempt a person from
liability at common law.'' 49 U.S.C. 30103(e). Pursuant to this
provision, State common law tort causes of action against motor vehicle
manufacturers that might otherwise be preempted by the express
preemption provision are generally preserved. However, the Supreme
Court has recognized the possibility, in some instances, of implied
preemption of such State common law tort causes of action by virtue of
NHTSA's rules, even if not expressly preempted. This second way that
NHTSA rules can preempt is dependent upon there being an actual
conflict between an FMVSS and the higher standard that would
effectively be imposed on motor vehicle manufacturers if someone
obtained a State common law tort judgment against the manufacturer,
notwithstanding the manufacturer's compliance with the NHTSA standard.
Because most NHTSA standards established by an FMVSS are minimum
standards, a State common law tort cause of action that seeks to impose
a higher standard on motor vehicle manufacturers will generally not be
preempted. However, if and when such a conflict does exist--for
example, when the standard at issue is both a minimum and a maximum
standard--the State common law tort cause of action is impliedly
preempted. See Geier v. American Honda Motor Co., 529 U.S. 861 (2000).
Pursuant to Executive Order 13132 and 12988, NHTSA has considered
whether this rule could or should preempt State common law causes of
action. The agency's ability to announce its conclusion regarding the
preemptive effect of one of its rules reduces the likelihood that
preemption will be an issue in any subsequent tort litigation.
To this end, the agency has examined the nature (e.g., the language
and structure of the regulatory text) and objectives of today's rule
and finds that this rule, like many NHTSA rules, prescribes only a
minimum safety standard. As such, NHTSA does not intend that this rule
preempt state tort law that would effectively impose a higher standard
on motor vehicle manufacturers than that established by today's rule.
Establishment of a higher standard by means of State tort law would not
conflict with the minimum standard announced here. Without any
conflict, there could not be any implied preemption of a State common
law tort cause of action.
D. Executive Order 12988 (Civil Justice Reform)
With respect to the review of the promulgation of a new regulation,
section 3(b) of Executive Order 12988, ``Civil Justice Reform'' (61 FR
4729; Feb. 7, 1996), requires that Executive agencies make every
reasonable effort to ensure that the regulation: (1) Clearly specifies
the preemptive effect; (2) clearly specifies the effect on existing
Federal law or regulation; (3) provides a clear legal standard for
affected conduct, while promoting simplification and burden reduction;
(4) clearly specifies the retroactive effect, if any; (5) specifies
whether administrative proceedings are to be required before parties
file suit in court; (6) adequately defines key terms; and (7) addresses
other important issues affecting clarity and general draftsmanship
under any guidelines issued by the Attorney General. This document is
consistent with that requirement.
Pursuant to this Order, NHTSA notes as follows. The issue of
preemption is discussed above. NHTSA notes further that there is no
requirement that individuals submit a petition for reconsideration or
pursue other administrative proceedings before they may file suit in
court.
E. Protection of Children From Environmental Health and Safety Risks
Executive Order 13045, ``Protection of Children from Environmental
Health and Safety Risks'' (62 FR 19855, April 23, 1997), applies to any
rule that: (1) Is determined to be ``economically significant'' as
defined under Executive Order 12866, and (2) concerns an environmental,
health, or safety risk that the agency has reason to believe may have a
disproportionate effect on children. If the regulatory action meets
both criteria, the agency must evaluate the environmental health or
safety effects of the planned rule on children, and explain why the
planned regulation is preferable to other potentially effective and
reasonably feasible alternatives considered by the agency.
This notice is part of a rulemaking that is not expected to have a
disproportionate health or safety impact on children. Consequently, no
further analysis is required under Executive Order 13045.
F. Paperwork Reduction Act
Under the Paperwork Reduction Act of 1995 (PRA), a person is not
required to respond to a collection of information by a Federal agency
unless the collection displays a valid OMB control number. There is not
any information collection requirement associated with this NPRM.
[[Page 30813]]
G. National Technology Transfer and Advancement Act
Section 12(d) of the National Technology Transfer and Advancement
Act (NTTAA) requires NHTSA to evaluate and use existing voluntary
consensus standards in its regulatory activities unless doing so would
be inconsistent with applicable law (e.g., the statutory provisions
regarding NHTSA's vehicle safety authority) or otherwise impractical.
Voluntary consensus standards are technical standards developed or
adopted by voluntary consensus standards bodies. Technical standards
are defined by the NTTAA as ``performance-based or design-specific
technical specification and related management systems practices.''
They pertain to ``products and processes, such as size, strength, or
technical performance of a product, process or material.''
Examples of organizations generally regarded as voluntary consensus
standards bodies include ASTM International, the Society of Automotive
Engineers (SAE), and the American National Standards Institute (ANSI).
If NHTSA does not use available and potentially applicable voluntary
consensus standards, we are required by the Act to provide Congress,
through OMB, an explanation of the reasons for not using such
standards.
This NPRM proposes to require truck tractors and large buses to
have electronic stability control systems. In the proposed definitional
requirement, the agency adapted the criteria from the light vehicle ESC
rulemaking, which was based on (with minor modifications) SAE Surface
Vehicle Information Report on Automotive Stability Enhancement Systems
J2564 Rev JUN2004 that provides an industry consensus definition of an
ESC system. In addition, SAE International has a Recommended Practice
on Brake Systems Definitions--Truck and Bus, J2627 AUG2009 that has
been incorporated into the agency's definition. The agency has also
incorporated by reference two ASTM standards in order to provide
specifications for the road test surface. These are: (1) ASTM E1136-93
(Reapproved 2003), ``Standard Specification for a Radial Standard
Reference Test Tire,'' and (2) ASTM E1337-90 (Reapproved 2008),
``Standard Test Method for Determining Longitudinal Peak Braking
Coefficient of Paved Surfaces Using a Standard Reference Test Tire.''
H. Unfunded Mandates Reform Act
Section 202 of the Unfunded Mandates Reform Act of 1995 (UMRA)
requires federal agencies to prepare a written assessment of the costs,
benefits, and other effects of proposed or final rules that include a
Federal mandate likely to result in the expenditure by State, local, or
tribal governments, in the aggregate, or by the private sector, of more
than $100 million annually (adjusted for inflation with base year of
1995). Before promulgating a NHTSA rule for which a written statement
is needed, section 205 of the UMRA generally requires the agency to
identify and consider a reasonable number of regulatory alternatives
and adopt the least costly, most cost-effective, or least burdensome
alternative that achieves the objectives of the rule. The provisions of
section 205 do not apply when they are inconsistent with applicable
law. Moreover, section 205 allows the agency to adopt an alternative
other than the least costly, most cost-effective, or least burdensome
alternative if the agency publishes with the final rule an explanation
of why that alternative was not adopted.
This NPRM will not result in any expenditure by State, local, or
tribal governments or the private sector of more than $100 million,
adjusted for inflation. When $100 million is adjusted by the implicit
gross domestic product price deflator for the year 2010, the result is
$136 million. This NPRM is not subject to the requirements of sections
202 and 205 of the UMRA because it is not estimated to result in an
expenditure of more than $136 million annually by State, local, or
tribal governments or the private sector.
I. National Environmental Policy Act
NHTSA has analyzed this rulemaking action for the purposes of the
National Environmental Policy Act. The agency has determined that
implementation of this action will not have any significant impact on
the quality of the human environment.
J. Plain Language
Executive Order 12866 requires each agency to write all rules in
plain language. Application of the principles of plain language
includes consideration of the following questions:
Have we organized the material to suit the public's needs?
Are the requirements in the rule clearly stated?
Does the rule contain technical language or jargon that
isn't clear?
Would a different format (grouping and order of sections,
use of headings, paragraphing) make the rule easier to understand?
Would more (but shorter) sections be better?
Could we improve clarity by addling tables, lists, or
diagrams?
What else could we do to make the rule easier to
understand?
If you have any responses to these questions, please include them
in your comments on this proposal.
K. Regulatory Identifier Number (RIN)
The Department of Transportation assigns a regulation identifier
number (RIN) to each regulatory action listed in the Unified Agenda of
Federal Regulations. The Regulatory Information Service Center
publishes the Unified Agenda in April and October of each year. You may
use the RIN contained in the heading at the beginning of this document
to find this action in the Unified Agenda.
L. Privacy Act
Anyone is able to search the electronic form of all comments
received into any of our dockets by the name of the individual
submitting the comment (or signing the comment, if submitted on behalf
of an association, business, labor union, etc.). You may review DOT's
complete Privacy Act Statement in the Federal Register published on
April 11, 2000 (65 FR 19477-78).
List of Subjects in 49 CFR Part 571
Imports, Incorporation by reference, Motor vehicle safety, Motor
vehicles, Rubber and rubber products, and Tires.
Proposed Regulatory Text
In consideration of the foregoing, we propose to amend 49 CFR part
571 to read as follows:
PART 571--FEDERAL MOTOR VEHICLE SAFETY STANDARDS
1. The authority citation for part 571 continues to read as
follows:
Authority: 49 U.S.C. 322, 30111, 30115, 30166 and 30177;
delegation of authority at 49 CFR 1.50.
2. Revise paragraphs (d)(32) and (d)(33) of Sec. 571.5 to read as
follows:
Sec. 571.5 Matter incorporated by reference.
* * * * *
(d) * * *
(32) ASTM E1136-93 (Reapproved 2003), ``Standard Specification for
a Radial Standard Reference Test Tire,'' approved March 15, 1993, into
Sec. Sec. 571.105; 571.121; 571.126; 571.135; 571.136; 571.139;
571.500.
(33) ASTM E1337-90 (Reapproved 2008), ``Standard Test Method for
[[Page 30814]]
Determining Longitudinal Peak Braking Coefficient of Paved Surfaces
Using a Standard Reference Test Tire,'' approved June 1, 2008, into
Sec. Sec. 571.105; 571.121; 571.126; 571.135; 571.136; 571.500.
* * * * *
3. Revise the heading of Sec. 571.126 to read as follows:
Sec. 571.126 Standard No. 126; Electronic stability control systems
for light vehicles.
* * * * *
4. Add Sec. 571.136 to read as follows:
Sec. 571.136 Standard No. 136; Electronic stability control systems
for heavy vehicles.
S1. Scope. This standard establishes performance and equipment
requirements for electronic stability control (ESC) systems on heavy
vehicles.
S2. Purpose. The purpose of this standard is to reduce crashes
caused by rollover or by directional loss-of-control.
S3. Application. This standard applies to truck tractors and buses
with a gross vehicle weight rating of greater than 11,793 kilograms
(26,000 pounds). However, it does not apply to:
(a) Any truck tractor or bus equipped with an axle that has a gross
axle weight rating (GAWR) of 29,000 pounds or more;
(b) Any truck tractor or bus that has a speed attainable in 2 miles
of not more than 33 mph;
(c) Any truck tractor that has a speed attainable in 2 miles of not
more than 45 mph, an unloaded vehicle weight that is not less than 95
percent of its gross vehicle weight rating (GVWR), and no capacity to
carry occupants other than the driver and operating crew;
(d) Any bus with fewer than 16 designated seating positions
(including the driver);
(e) Any bus with fewer than 2 rows of passenger seats that are
rearward of the driver's seating position and are forward-facing or can
convert to forward-facing without the use of tools;
(f) School buses; and
(g) Any urban transit buses sold for operation as a common carrier
in urban transportation along a fixed route with frequent stops.
S4. Definitions.
Ackerman Steer Angle means the angle whose tangent is the wheelbase
divided by the radius of the turn at a very low speed.
Electronic stability control system or ESC system means a system
that has all of the following attributes:
(1) That augments vehicle directional stability by applying and
adjusting the vehicle brake torques individually at each wheel position
on at least one front and at least one rear axle of the vehicle to
induce correcting yaw moment to limit vehicle oversteer and to limit
vehicle understeer;
(2) That enhances rollover stability by applying and adjusting the
vehicle brake torques individually at each wheel position on at least
one front and at least one rear axle of the vehicle to reduce lateral
acceleration of a vehicle;
(3) That is computer-controlled with the computer using a closed-
loop algorithm to induce correcting yaw moment and enhance rollover
stability;
(4) That has a means to determine the vehicle's lateral
acceleration;
(5) That has a means to determine the vehicle's yaw rate and to
estimate its side slip or side slip derivative with respect to time;
(6) That has a means to estimate vehicle mass or, if applicable,
combination vehicle mass;
(7) That has a means to monitor driver steering inputs;
(8) That has a means to modify engine torque, as necessary, to
assist the driver in maintaining control of the vehicle and/or
combination vehicle; and
(9) That, when installed on a truck tractor, has the means to
provide brake pressure to automatically apply and modulate the brake
torques of a towed semi-trailer.
Initial brake temperature means the average temperature of the
service brakes on the hottest axle of the vehicle immediately before
any stability control system test maneuver is executed.
Lateral acceleration means the component of the vector acceleration
of a point in the vehicle perpendicular to the vehicle x axis
(longitudinal) and parallel to the road plane.
Oversteer means a condition in which the vehicle's yaw rate is
greater than the yaw rate that would occur at the vehicle's speed as
result of the Ackerman Steer Angle.
Peak friction coefficient or PFC means the ratio of the maximum
value of braking test wheel longitudinal force to the simultaneous
vertical force occurring prior to wheel lockup, as the braking torque
is progressively increased.
Sideslip or side slip angle means the arctangent of the lateral
velocity of the center of gravity of the vehicle divided by the
longitudinal velocity of the center of gravity.
Understeer means a condition in which the vehicle's yaw rate is
less than the yaw rate that would occur at the vehicle's speed as
result of the Ackerman Steer Angle.
Yaw Rate means the rate of change of the vehicle's heading angle
measure in degrees per second of rotation about a vertical axis through
the vehicle's center of gravity.
S5. Requirements. Each vehicle must be equipped with an ESC system
that meets the requirements specified in S5 under the test conditions
specified in S6 and the test procedures specified in S7 of this
standard.
S5.1 Required Equipment. Each vehicle to which this standard
applies must be equipped with an electronic stability control system,
as defined in S4.
S5.2 System Operational Capabilities.
S5.2.1 An electronic stability control system must be operational
over the full speed range of the vehicle except at vehicle speeds less
than 20 km/h (12.4 mph), when being driven in reverse, or during system
initialization.
S5.2.2 An electronic stability control system must remain capable
of activation even if the antilock brake system or traction control is
also activated.
S5.3 Performance Requirements.
S5.3.1 Slowly Increasing Steer Maneuver. During the slowly
increasing steer test maneuver performed under the test conditions of
S6 and the test procedure of S7.6, the vehicle with the ESC system
enabled must satisfy the engine torque reduction criteria of S5.3.1.1.
S5.3.1.1 The engine torque reduction when measured 1.5 seconds
after the activation of the electronic stability control system must be
at least 10 percent less than the engine torque requested by the
driver.
S5.3.2 Sine With Dwell Maneuver. During each sine with dwell
maneuver performed under the test conditions of S6 and the test
procedure of S7.10, the vehicle with the ESC system enabled must
satisfy the roll stability criteria of S5.3.2.1 and S5.3.2.2, the yaw
stability criteria of S5.3.2.3 and S5.3.2.4, and the responsiveness
criterion of S5.3.2.5 during each of those tests conducted with a
commanded steering wheel angle of 0.7A or greater, where A is the
steering wheel angle computed in S7.6.2.
S5.3.2.1 The lateral acceleration measured at 0.75 seconds after
completion of steer of the sine with dwell steering input must not
exceed 30 percent of the peak value of the lateral acceleration
recorded during the 2nd half of the sine maneuver (including the dwell
period), i.e., from time 1 second after the beginning of steer to the
completion of steer during the same test run.
S5.3.2.2 The lateral acceleration measured at 1.5 seconds after
completion of steer of the Sine With
[[Page 30815]]
Dwell steering input must not exceed 10 percent of the peak value of
the lateral acceleration recorded during the 2nd half of the sine
maneuver (including the dwell period), i.e., from time 1 second after
the BOS to the COS during the same test run.
S5.3.2.3 The yaw rate measured at 0.75 seconds after completion of
steer of the Sine With Dwell steering input must not exceed 40 percent
of the peak value of the yaw rate recorded during the 2nd half of the
sine maneuver (including the dwell period), i.e., from time 1 second
after the BOS to the COS during the same test run.
S5.3.2.4 The yaw rate measured at 1.5 seconds after completion of
steer of the Sine With Dwell steering input must not exceed 15 percent
of the peak value of the yaw rate recorded during the 2nd half of the
sine maneuver (including the dwell period), i.e., from time 1 second
after the BOS to the COS during the same test run.
S5.3.2.5 The lateral displacement of the vehicle center of gravity
with respect to its initial straight path must be at least 2.13 meters
(7 feet) for each truck tractor and at least 1.52 meters (5 feet) for
each bus when computed 1.5 seconds after the BOS.
S5.3.2.5.1 The computation of lateral displacement is performed
using double integration with respect to time of the measurement of
lateral acceleration at the vehicle center of gravity, as expressed by
the formula:
Lateral Displacement = [int][int] AyCG dt
S5.3.2.5.2 Time t = 0 for the integration operation is the instant
of steering initiation, known as the BOS.
S5.4 ESC System Malfunction Detection. Each vehicle shall be
equipped with an indicator lamp, mounted in front of and in clear view
of the driver, which is activated whenever there is a malfunction that
affects the generation or transmission of control or response signals
in the vehicle's electronic stability control system.
S5.4.1 The ESC malfunction telltale must illuminate only when a
malfunction exists and must remain continuously illuminated for as long
as the malfunction exists, whenever the ignition locking system is in
the ``On'' (``Run'') position.
S5.4.2 The ESC Malfunction telltale must be identified by the
symbol shown for ``Electronic Stability Control System Malfunction'' or
the specified words or abbreviations listed in Table 1 of Standard No.
101 (49 CFR 571.101).
S5.4.3 The ESC malfunction telltale must be activated as a check of
lamp function either when the ignition locking system is turned to the
``On'' (``Run'') position when the engine is not running, or when the
ignition locking system is in a position between the ``On'' (``Run'')
and ``Start'' that is designated by the manufacturer as a check
position.
S5.4.4 The ESC malfunction telltale need not be activated when a
starter interlock is in operation.
S5.4.5 The ESC malfunction telltale lamp must extinguish at the
next ignition cycle after the malfunction has been corrected.
S5.5 ESC System Technical Documentation. To ensure that a vehicle
is equipped with an ESC system that meets the definition of ``ESC
System'' in S4, the vehicle manufacturer must make available to the
agency, upon request, the following documentation:
S5.5.1 A system diagram that identifies all ESC system hardware.
The diagram must identify what components are used to generate brake
torques at each controlled wheel, determine vehicle lateral
acceleration and yaw rate, estimate side slip or the side slip
derivative, monitor driver steering inputs, and for a tractor, generate
the towed vehicle brake torques.
S5.5.2 A written explanation describing the ESC system basic
operational characteristics. This explanation must include a discussion
of the system's capability to apply brake torques at each wheel, how
the system estimates vehicle mass, and how the system modifies engine
torque during ESC system activation. The explanation must also identify
the vehicle speed range and the driving phases (acceleration,
deceleration, coasting, during activation of ABS or traction control)
under which the ESC system can activate.
S5.5.3 A logic diagram that supports the explanation provided in
S5.5.2.
S5.5.4 Specifically for mitigating, avoiding, and preventing
vehicle rollover, oversteer, and understeer conditions, a discussion of
the pertinent inputs to the computer or calculations within the
computer and how its algorithm uses that information and controls ESC
system hardware to limit these loss of control conditions.
S6. Test Conditions. The requirements of S5 shall be met by a
vehicle when it is tested according to the conditions set forth in the
S6. On vehicles equipped with automatic brake adjusters, the automatic
brake adjusters must remain activated at all times.
S6.1 Ambient conditions.
S6.1.1 The ambient temperature is between 7 [deg]C (45[emsp14]
[deg]F) and 40 [deg]C (104[emsp14] [deg]F).
S6.1.2 The maximum wind speed is no greater than 5 m/s (11mph).
S6.2 Road test surface.
S6.2.1 The tests are conducted on a dry, uniform, solid-paved
surface. Surfaces with irregularities and undulations, such as dips and
large cracks, are unsuitable.
S6.2.2 The road test surface produces a peak friction coefficient
(PFC) of 0.9 when measured using an American Society for Testing and
Materials (ASTM) E1136-93 (Reapproved 2003) standard reference test
tire (incorporated by reference,, in accordance with ASTM Method E
1337-90 (Reapproved 2002), at a speed of 64.4 km/h (40 mph), without
water delivery (both documents incorporated by reference, see Sec.
571.5).
S6.2.3 The test surface has a consistent slope between 0% and 1%.
S6.3 Vehicle conditions.
S6.3.1 The ESC system is enabled for all testing, except for the
ESC Malfunction test in S7.11.
S6.3.2 Test Weight.
S6.3.2.1 Truck tractors. The combined total weight of the truck
tractor and control trailer (specified in S6.3.4) is 80 percent of the
tractor GVWR. The tractor is loaded with the fuel tanks filled to at
least 75 percent capacity, test driver, test instrumentation, and a
ballasted control trailer with outriggers. Center of gravity of all
ballast on the control trailer is located directly above the kingpin.
The load distribution on non-steer axles is adjusted so that it is
proportional to the tractor's respective rear axles GAWRs by adjusting
the fifth wheel position, if adjustable. If the fifth wheel of the
truck tractor cannot be adjusted without exceeding a GAWR, ballast is
reduced so that axle load is equal to or less than the GAWR,
maintaining load proportioning as close as possible to specified
proportioning.
S6.3.2.2 Buses. A bus is loaded to a simulated multi-passenger
configuration. For this configuration the bus is loaded with the fuel
tanks filled to at least 75 percent capacity, test driver, test
instrumentation and simulated occupants in each of the vehicle's
designated seating positions. The simulated occupant loads are attained
by securing a 68-kg (150-lb) water dummy in each of the test vehicle's
designated seating positions without exceeding the vehicle's GVWR and
each axle's GAWR. If any rating is exceeded the ballast load is reduced
until the respective rating or ratings are no longer exceeded.
S6.3.3 Transmission selector position. The transmission selector
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control is in a forward gear during all maneuvers.
S6.3.4 Control Trailer.
S6.3.4.1 The control trailer is an unbraked flatbed semi-trailer
that has a single axle with a GAWR of 8,165 kilograms (18,000 pounds)
and a length of 655 + 15 cm (258 + 6 inches) when measured from the
transverse centerline of the axle to the centerline of the kingpin.
S6.3.4.2 The center of gravity height of the ballast on the loaded
control trailer is less than 61 cm (24 inches) above the top of the
tractor's fifth wheel.
S6.3.5 Tires. The vehicle is tested with the tires installed on the
vehicle at time of initial vehicle sale. The tires are inflated to the
vehicle manufacturer's recommended cold tire inflation pressure(s)
specified on the vehicle's certification label or the tire inflation
pressure label.
S6.3.6 Outrigger. An outrigger is used for testing each vehicle.
The outrigger is designed with a maximum weight of 726 kg (1,600 lb),
excluding mounting fixtures.
S6.3.7 Automated steering machine. A steering machine programmed to
execute the required steering pattern is used during the slowly
increasing steer and sine with dwell maneuvers. The steering machine is
capable of supplying steering torques between 40 to 60 Nm (29.5 to 44.3
lb-ft). The steering machine is able to apply these torques when
operating with steering wheel velocities up to 1200 degrees per second.
S6.3.8 Truck Tractor Anti-jackknife System. The truck tractor is
equipped with anti-jackknife cables that allow a minimum articulation
angle of 45 degrees between the tractor and the control trailer.
S6.3.9 Special drive conditions. A vehicle equipped with an
interlocking axle system or a front wheel drive system that is engaged
and disengaged by the driver is tested with the system disengaged.
S6.3.10 Liftable axles. A vehicle with a liftable axle is tested
with the liftable axle down.
S6.3.11 Initial brake temperature. The initial brake temperature is
not less than 65 [deg]C (150 [deg]F) and not more than 204 [deg]C (400
[deg]F).
S6.3.12 Thermocouples. The brake temperature is measured by plug-
type thermocouples installed in the approximate center of the facing
length and width of the most heavily loaded shoe or disc pad, one per
brake. A second thermocouple may be installed at the beginning of the
test sequence if the lining wear is expected to reach a point causing
the first thermocouple to contact the rubbing surface of a drum or
rotor. The second thermocouple is installed at a depth of 0.080 inch
and located within 1.0 inch circumferentially of the thermocouple
installed at 0.040 inch depth. For center-grooved shoes or pads,
thermocouples are installed within 0.125 inch to 0.250 inch of the
groove and as close to the center as possible.
S6.4 Selection of compliance options. Where manufacturer options
are specified, the manufacturer shall select the option by the time it
certifies the vehicle and may not thereafter select a different option
for the vehicle. Each manufacturer shall, upon request from the
National Highway Traffic Safety Administration, provide information
regarding which of the compliance options it has selected for a
particular vehicle or make/model.
S7. Test Procedure.
S7.1 Tire inflation. Inflate the vehicle's tires to the cold tire
inflation pressure(s) provided on the vehicle's certification label or
tire information label.
S7.2 Telltale lamp check. With the vehicle stationary and the
ignition locking system in the ``Lock'' or ``Off'' position, activate
the ignition locking system to the ``On'' (``Run'') position or, where
applicable, the appropriate position for the lamp check. The ESC system
must perform a check of lamp function for the ESC malfunction telltale,
as specified in S5.3.3.
S7.3 Mass Estimation Cycle. While driving in a straight line, one
stop is performed from a speed of 65 km/h (40 mph), with a target
longitudinal deceleration between 0.3-0.4g.
S7.4 Tire Conditioning. Condition the tires using the following
procedure to wear away mold sheen and achieve operating temperature
immediately before beginning the Brake Conditioning, SIS and SWD
maneuver test runs.
S7.4.1 The test vehicle is driven around a circle 46 meters (150
feet) in radius at a speed that produces a lateral acceleration of
approximately 0.1g for two clockwise laps followed by two
counterclockwise laps.
S7.5 Brake Conditioning. Conditioning and warm-up the vehicle
brakes must be completed before and during execution of the SIS and SWD
maneuver test runs.
S7.5.1 Prior to executing the first series of SIS maneuvers for a
test vehicle, the brakes are burnished according to the procedure in
S6.1.8 of Standard No. 121, Air brake systems.
S7.5.2 After the brakes are burnished in accordance with S7.5.1,
initiate the vehicle compliance test according to S7.6. For a vehicle
on which a full FMVSS No. 121 compliance test was performed,
immediately prior to executing any slowly increasing steer or sine with
dwell maneuvers, the brakes are burnished using 40 brake application
snubs from a speed of 64 km/h (40 mph) to a speed of 32 km/h (20 mph),
with a target deceleration of approximately 0.3g. After each brake
application, accelerate to 64 km/h (40 mph) and maintain that speed
until making the next brake application at a point 1 mile from the
initial point of the previous brake application. At end of the 40
snubs, the hottest brake temperature is confirmed to be within the
temperature range of 65 [deg]C-204 [deg]C (150 [deg]F-400 [deg]F). If
the hottest brake temperature is above 204 [deg]C (400 [deg]F) a cool
down period is performed until the hottest brake temperature is
measured within that range. If the hottest brake temperature is below
65 [deg]C (150 [deg]F) individual brake stops shall be repeated to
increase any one brake temperature to within the target temperature
range of 65 [deg]C-204[deg]C (150 [deg]F-400 [deg]F) before the subject
maneuver can be performed.
S7.6 Slowly Increasing Steer Test. The vehicle is subjected to two
series of runs of the slowly increasing steer test using a constant
vehicle speed of 48.3 1.6 km/h (30.0 1.0 mph)
and a steering pattern that increases by 13.5 degrees per second until
ESC system activation is confirmed. Three repetitions are performed for
each test series. One series uses counterclockwise steering, and the
other series uses clockwise steering. During each run ESC activation is
required for the Engine Torque Reduction test and is confirmed as
specified in S7.7.
S7.6.1 The slowly increasing steer maneuver sequence is started
using a commanded steering wheel angle of 270 degrees. If ESC
activation did not occur during the maneuver then the commanded
steering wheel angle is increased by 270 degree increments up to the
vehicle's maximum allowable steering angle or until ESC activation is
confirmed.
S7.6.2 From the slowly increasing steer tests, the quantity ``A''
is determined. ``A'' is the steering wheel angle in degrees that is
estimated to produce a lateral acceleration of 0.5g for the test
vehicle. Utilizing linear regression on the lateral acceleration data
recorded between 0.05g and 0.3g, and then linear extrapolation out to a
lateral acceleration value of 0.5g, A is calculated, to the nearest 0.1
degrees, from each of the six satisfactory slowly increasing steer
tests. If ESC activation occurs prior to the vehicle experiencing
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a lateral acceleration of 0.3g then the data used during the linear
regression will be that data recorded between 0.05g and the lateral
acceleration measured at the time of ESC activation. The absolute value
of the six A's calculated is averaged and rounded to the nearest 0.1
degrees to produce the final quantity, A, used during the sine with
dwell maneuvers below.
S7.7 Engine Torque Reduction Test. During each of the six completed
slowly increasing steer test maneuvers, ESC activation is confirmed by
comparing the engine torque output and driver requested torque data
collected from the vehicle J1939 communication data link. During the
initial stages of each maneuver the two torque signals with respect to
time will parallel each other. Upon ESC activation, the two signals
will diverge when ESC system activation causes a commanded engine
torque reduction and the driver attempts to accelerate the vehicle
maintaining the required constant test speed causing an increased
driver requested torque.
S7.7.1 During each of the six slowly increasing steer test runs,
verify the commanded engine torque and the driver requested torque
signals diverge at least 10 percent 1.5 seconds after the beginning of
ESC activation occurs as defined in S7.12.15.
S7.7.2 If ESC activation does not occur in all of the six slowly
increasing steer test maneuvers the test is terminated.
S7.8 After the quantity A has been determined in S7.6, without
replacing the tires, the tire and brake conditioning procedures
described in S7.4 and S7.5 are performed immediately prior to
conducting the sine with dwell test.
S7.9 Check that the ESC system is enabled by ensuring that the ESC
malfunction telltale is not illuminated.
S7.10 Sine With Dwell Test. The vehicle is subjected to two series
of test runs using a steering pattern of a sine wave at 0.5 Hz
frequency with a 1.0 sec delay beginning at the second peak amplitude
as shown in Figure 1 (sine with dwell maneuver). One series uses
counterclockwise steering for the first half cycle, and the other
series uses clockwise steering for the first half cycle. Before each
test run brake temperatures are monitored and the hottest brake is
confirmed to be within the temperature range of 65 [deg]C-204 [deg]C
(150 [deg]F-400 [deg]F).
[GRAPHIC] [TIFF OMITTED] TP23MY12.008
S7.10.1 For manual transmissions, the steering motion is initiated
with the vehicle coasting (dropped throttle) with the clutch disengaged
at 72.4 1.6 km/h (45.0 1.0 mph). For
automatic transmissions, the steering motion is initiated with the
vehicle coasting and the transmission in the ``drive'' selection
position.
S7.10.2 In each series of test runs, the steering amplitude is
increased from run to run, by 0.1A, provided that no such run will
result in steering amplitude greater than that of the final run
specified in S7.10.4.
S7.10.3 The steering amplitude for the initial run of each series
is 0.3A where A is the steering wheel angle determined in S7.6.
S7.10.4 The steering amplitude of the final run in each series is
the lesser of 1.3A or 400 degrees. If any 0.1A increment, up to 1.3A,
is greater than 400 degrees, the steering amplitude of the final run
shall be the 0.1A amplitude that is closest or equal to, but not
exceeding, 400 degrees.
S7.10.5 Upon completion of the two series of test runs, post
processing of the yaw rate and lateral acceleration data to determine
Lateral Acceleration Ratio (LAR), Yaw Rate Ratio (YRR) and lateral
displacement, is done as specified in S7.12.
S7.11 ESC Malfunction Detection.
S7.11.1 Simulate one or more ESC malfunction(s) by disconnecting
the power source to any ESC component, or disconnecting any electrical
connection between ESC components (with the vehicle power off). When
simulating an ESC malfunction, the electrical connections for the
telltale lamp(s) are not to be disconnected.
S7.11.2 With the vehicle initially stationary and the ignition
locking system in the ``Lock'' or ``Off'' position, activate the
ignition locking system to the ``Start'' position and start the engine.
Place the vehicle in a forward gear and obtain a vehicle speed of 48.3
8.0 km/h (30.0 5.0 mph). Drive the vehicle
for at least two minutes including at least one left and one right
turning maneuver and at least one service brake application. Verify
that within two minutes of obtaining this vehicle speed the ESC
malfunction indicator illuminates in accordance with S5.3.
S7.11.3 Stop the vehicle, deactivate the ignition locking system to
the ``Off'' or ``Lock'' position. After a five-minute period, activate
the vehicle's ignition locking system to the ``Start'' position and
start the engine. Verify that the ESC malfunction indicator again
illuminates to signal a malfunction and remains illuminated as long as
the engine is running or until the fault is corrected.
S7.11.4 Deactivate the ignition locking system to the ``Off'' or
``Lock'' position. Restore the ESC system to
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normal operation, activate the ignition system to the ``Start''
position and start the engine. Verify that the telltale has
extinguished.
S7.12 Post Data Processing--Calculations for Performance Metrics.
Engine torque reduction, lateral acceleration and yaw rate decay
calculations, and lateral responsiveness checks must be processed
utilizing the following techniques:
S7.12.1 Raw steering wheel angle data is filtered with a 12-pole
phaseless Butterworth filter and a cutoff frequency of 10Hz. The
filtered data is then zeroed to remove sensor offset utilizing static
pretest data.
S7.12.2 Raw yaw, pitch and roll rate data is filtered with a 12-
pole phaseless Butterworth filter and a cutoff frequency of 3 Hz. The
filtered data is then zeroed to remove sensor offset utilizing static
pretest data.
S7.12.3 Raw lateral acceleration data is filtered with a 12-pole
phaseless Butterworth filter and a cutoff frequency of 6Hz. The
filtered data is then zeroed to remove sensor offset utilizing static
pretest data. The lateral acceleration data at the vehicle center of
gravity is determined by removing the effects caused by vehicle body
roll and by correcting for sensor placement via use of coordinate
transformation. For data collection, the lateral accelerometer shall be
located as close as possible to the position of the vehicle's
longitudinal and lateral centers of gravity.
S7.12.4 Raw vehicle speed data is filtered with a 12-pole phaseless
Butterworth filter and a cutoff frequency of 2 Hz.
S7.12.5 Left and right side ride height data is filtered with a
0.1-second running average filter.
S7.12.6 The J1939 torque data collected as a digital signal does
not get filtered. J1939 torque collected as an analog signal is
filtered with a 0.1-second running average filter.
S7.12.7 Steering wheel velocity is determined by differentiating
the filtered steering wheel angle data. The steering wheel velocity
data is then filtered with a moving 0.1-second running average filter.
S7.12.8 Lateral acceleration, yaw rate and steering wheel angle
data channels are zeroed utilizing a defined ``zeroing range.'' The
``zeroing range'' is the 1.0-second time period prior to the instant
the steering wheel velocity exceeds 40 deg/sec. The instant the
steering wheel velocity exceeds 40 deg/sec is the instant defining the
end of the ``zeroing range.''
S7.12.9 The beginning of steer (BOS) is the first instance filtered
and zeroed steering wheel angle data reaches -5 degrees (when the
initial steering input is counterclockwise) or +5 degrees (when the
initial steering input is clockwise). The value for time at the BOS is
interpolated.
S7.12.10 The Completion of Steer for the sine with dwell maneuver
(COS) is the time the steering wheel angle returns to zero. The value
for time at the COS is interpolated.
S7.12.11 The peak lateral acceleration is the maximum lateral
acceleration measured during the second half of the sine maneuver,
including the dwell period from 1.0 second after the BOS to the COS.
The lateral accelerations at 0.75 and 1.0 seconds after COS are
determined by interpolation.
S7.12.12 The peak yaw rate is the maximum yaw rate measured during
the second half of the sine maneuver, including the dwell period from
1.0 second after the BOS to the COS. The yaw rates at 0.75 and 1.0
seconds after COS are determined by interpolation.
S7.12.13 Determine lateral velocity by integrating corrected,
filtered and zeroed lateral acceleration data. Zero lateral velocity at
BOS event. Determine lateral displacement by integrating zeroed later
velocity. Zero lateral displacement at BOS event. Lateral displacement
at 1.50 seconds from BOS event is determined by interpolation.
S7.12.14 The ESC activation point is the point where the measured
driver demanded torque and the engine torque first begin to deviate
from one another (engine torque decreases while driver requested torque
increases) during the slowly increasing steer maneuver. The torque
values are obtained directly from each vehicle's SAE J1939
communication data bus. Torque values used to determine the ESC
activation point are interpolated.
S8. Compliance Date.
S8.1 Buses. All buses manufactured on or after [date that is two
years after publication of a final rule implementing this proposal]
must comply with this standard
S8.2 Truck tractors.
S8.2.1 All two-axle and three-axle truck tractors with a front axle
that has a GAWR of (14,600 pounds) or less and with two rear drive
axles that have a combined GAWR of (45,000 pounds) or less manufactured
on or after [date that is two years after publication of a final rule
implementing this proposal] must comply with this standard.
S8.2.2 All truck tractors manufactured on or after [date that is
four years after publication of a final rule implementing this
proposal] must comply with this standard.
Issued: May 15, 2012.
Christopher J. Bonanti,
Associate Administrator for Rulemaking.
[FR Doc. 2012-12212 Filed 5-16-12; 4:15 pm]
BILLING CODE 4910-59-P