FMCSA Contact: Deborah M. Freund, MC-PSV, (202) 366-5541 : TTY 800-877-8339
The U.S. Department of Transportation has established a goal to reduce the highway fatality rate to no more than 1.0 fatalities per 100 million vehicle miles traveled by the end of 2008. Each of the Department's highway safety agencies have individual reduction targets that will contribute to this 2008 target. Federal Motor Carrier Safety Administration's (FMCSA) aim is to reduce the large truck fatality rate by 41 percent from 1996 to 2008, to 1.65 fatalities in truck crashes per 100 million miles of truck travel.
The Office of Bus and Truck Standards and Operations, which produced this Tech Brief,
develops and promotes national motor carrier safety program goals, priorities, and initiatives. It provides technical expertise and advice in the development and deployment of motor carrier
safety programs, including the development of regulations. The Office determines national motor
carrier safety operational program requirements, standards, and procedures for vehicle and roadside operations, driver and carrier operations, and bus safety.
U.S. Department of Transportation
Federal Motor Carrier Safety Administration
Office of Bus and Truck Standards
1200 New Jersey Avenue SE
Washington, DC 20590
Under Section 5117 of the Transportation Equity Act for the 21st Century of 1998 (TEA-21), Congress required the U.S. Department of Transportation to "conduct research on the deployment of a system of advanced sensors and signal processors in trucks and tractor-trailers to determine axle and wheel alignment, monitor collision alarm, check tire pressure and tire balance conditions, measure and detect load distribution in the vehicle, and adjust automatic braking systems."
As a result of a comprehensive technology scan, as well as numerous
interviews with key industry stakeholders such as truck manufacturers, fleet
operators, suppliers, and regulators, a variety of research areas were
identified, including the design, functionality, and performance of tire
pressure monitoring and inflation systems for commercial motor vehicle
The significant number of trucks operating on the highway with brake defects is a
situation that has plagued the industry and law enforcement for years, despite
attempts by many different groups to address the problem. Commercial vehicle
inspection data shows that about 19% of all inspected vehicles ? nearly one in five ?
were found to have one or more brake defects.* During Roadcheck 2002, a safety
strike force activity combining the resources of FMCSA, the States, and Canadian
Provinces, enforcement officials conducted a 72-hour intensive inspection of 49,032
vehicles in Canada, Mexico, and the United States from June 4-6, 2002. Enforcement
officials placed 22.1% of the vehicles inspected out of service due to various defects
and violations. Brake-related issues accounted for 53.3% of these vehicles being
placed out of service.
It is understood that commercial vehicle braking system design and operation are
directly linked to stopping distance, handling and, therefore, overall safety. Properly
maintaining and performing brakes are clearly the driver's best ally in preventing and
mitigating crash situations. Although vehicle defects on large trucks can rarely be
pinpointed as the causative factor(s) in crashes, when defects do occur, faulty brakes
tend to be at fault.
Optimally adjusted braking systems could help prevent or mitigate crashes even when
the braking system itself was not the initial cause of the crash. Eliminating or
mitigating key mechanical problems, including brake-related issues, would likely yield
a significant reduction in the number and seriousness of injuries sustained in
commercial vehicle-related crashes.
The overall objective of this research study was to document the performance and
operational characteristics of leading-edge technological approaches to monitoring
commercial vehicle braking systems. The study focused on the ability of the various
sensors to detect abnormalities, defects, and/or misadjustments of the brake system.
The brake sensing technologies examined in this study included:
- Anchor strain measurement to determine brake force at each wheel,
- Air chamber stroke measurement to assess brake adjustment at each wheel,
- Wheel slip measurement (using wheel speed sensor data) to determine brake force at each wheel,
- Deceleration measurement to determine total vehicle braking force (limited results due to system
software issues), and
- Temperature measurement to determine brake "work" or energy balance.
There are safety benefits associated with having a sensor or "sensor package" on-board the commercial
vehicle that would objectively and accurately measure the stopping potential of the vehicle, and
dynamically and continuously measure the actual braking force at each wheel. Such a system would
potentially have three primary applications or benefits:
- 1. Warning the driver and/or maintenance personnel if braking ability degraded to an unsafe level - and help with diagnosis of the specific problem.
- 2. Providing information to enforcement personnel for use during roadside inspections.
- 3. Integrating brake performance-sensing technologies with an electronically controlled brake system
(ECBS) in a "closed-loop" fashion. The brake force information might be used to balance the braking action at each wheel to improve service life, and/or provide an additional input for controlling braking
action at each wheel during emergency situations.
Overview of Project Approach
The various sensor systems were installed on a conventional tractor-trailer combination vehicle and tested
under controlled braking maneuvers on a test track. (All work was conducted at the Transportation
Research Center (TRC) in Columbus, OH by Radlinski and Associates, Inc.) The output of the various brake
sensor systems could then be compared on the same vehicle under identical testing conditions. This
approach facilitated objective, accurate comparison of the sensors, and eliminated problems associated
with test procedure repeatability when comparing different systems.
In addition, numerous industry stakeholders were contacted and interviewed during the study, including
suppliers of the various technologies examined. The companies and individuals were extremely helpful in
compiling the information contained in this report.
Summary of Results
Anchor Pin Strain Gauges
Pre-production instrumented anchor pins (interchangeable with conventional S-cam brake anchor pins)
fitted with strain gauges are capable of measuring the shear stresses applied to the anchor pins of the
drum brake assemblies used on heavy-duty S-cam trucks and buses. Each anchor pin is fitted with two
strain gauges orientated 90 degrees apart, in the "X" and "Y" direction. The test vehicle was equipped
with four instrumented anchor pins, two on each of the
intermediate axle brake assemblies (one on the
upper/secondary and one on the lower/primary brake
- Track testing shows a highly predictable relationship
between force data generated by instrumented (straingauged)
anchor pins and the vehicle's deceleration rate.
Instrumented anchor pin force is therefore an accurate
measure of a vehicle's braking performance.
- Instrumented anchor pins can accurately detect brake
deficiencies in specific (individual) wheel assemblies,
including out-of-adjustment, disconnected, and/or oilsoaked
shoe linings. They can also measure the effect of an out-of-adjustment brake on the other
(properly adjusted) brakes on a vehicle. This capability lends itself for application to advanced brake
balancing control schemes that may be possible with ECBS.
- Instrumented anchor pins can accurately detect even low brake forces. By resolving the resultant force
into the "X" (friction force) and "Y" (normal force) directions, the instrument anchor pins can
differentiate between an out-of-adjustment brake and a brake with oil-soaked shoe linings. With an oilsoaked
lining, less force is generated in the "X" direction when compared to an oil-free lining. This
capability could likely be leveraged to improve diagnostic efficiency and overall brake maintenance
- Instrumented anchor pins performed reliably throughout the testing.
The test truck was equipped with two commercially available stroke sensor packages and two laboratorygrade linear potentiometers mounted on the intermediate drive axle to measure stroke. Key observations and conclusions on the commercial brake stroke sensor packages, and on the utility of monitoring stroke in general, are as follows:
- Commercial brake chamber stroke sensor packages can detect brake deficiencies. Their accuracy varies
depending on the load, deceleration rate, and type of brake deficiency. Both commercial systems tested had the most difficulty detecting brake deficiencies with the trailer unloaded and at low deceleration
rates; however, both manufacturers state
that these systems are intended to
detect overstroke conditions during hard
- In-cab displays featuring indicator lights
for all ten brakes provide the driver with
valuable real-time data on the overall
condition of the vehicle's braking
- Unlike the instrumented anchor pins, brake stroke monitoring cannot differentiate between out-ofadjustment
brakes and oil-soaked shoe linings. For example, with oil-soaked shoe linings, the linear
potentiometers recorded an overstroke condition.
- The resolution and accuracy of stroke sensors make them well-suited for use in detecting brake maintenance needs and potential brake safety issues, but they are probably not appropriate for use in brake balancing systems.
Wheel-speed sensors are a standard component of anti-lock braking systems (ABS) used on heavy-duty trucks and buses. ABS wheel-speed sensors can be used to measure an individual wheel slip by comparing the calculated speed of each wheel to the calculated average for all wheels or to some other "actual" speed reference, such as a transmission signal or a contactless fifth wheel that measures ground speed.
- In general, ABS wheel-speed sensors are highly accurate and track closely with "actual" vehicle speed as
measured by an instrumented fifth wheel.
- Wheel-speed sensors are sufficiently accurate to detect grossly out-of-adjustment and disconnected
brakes. Wheel-speed sensors do not provide sufficient accuracy to detect brakes that are 1/8" or less
beyond the readjustment limit.
- Wheel-speed sensors are sufficiently accurate to detect a problem due to oil-soaked brake linings.
However, unlike instrumented anchor pins, wheel-speed sensors cannot differentiate between out-ofadjustment
brakes and oil-soaked linings.
- Wheel-speed data broadcast on the J1939 network was significantly
less accurate than data from actual ABS wheel-speed sensors.
- Although the wheel-speed sensor data broadcast over the J1939
network was less accurate than data from actual sensors, it was
sufficient for detecting grossly out-of-adjustment, disconnected, and
poorly performing brakes.
Brake Shoe Thermocouples
Standard Type J thermocouples were installed and tested as part of this program.
These tests had two objectives: (1) evaluate the thermocouples to determine whether
they could reliably be used to detect brake defects, and (2) use the thermocouples to
assist in evaluating the other sensor "packages". Thermocouples were mounted at
varying depths within the shoe lining to test their sensitivity for determining brake
- Response time of thermocouples in general is not sufficient to detect brake
problems during singular, discrete braking events.
- Because of the unpredictable variations in initial brake temperature, the
comparatively slow response time of thermocouples, and the general inaccuracies
inherent with thermocouples, their ability to detect and differentiate brake
deficiencies during discrete braking events was found to be very limited.
- During the simulated mountain testing, temperature patterns were detected and
used to identify various brake deficiencies.
Potential Sensor Applications
Several applications for the sensor technologies were identified during the study, and
described in this section.
Brake Balance Systems
The instrumented anchor pins were proven to accurately detect brake deficiencies and
provided sufficiently accurate data to measure the increase in work done by the
remaining brakes on a vehicle. This makes them ideal for use in brake balance
applications with advanced "brake-by-wire" technologies. In this application, brake
pressure could be tailored to individual brakes based on brake force output readings.
The benefits include increased brake life due to improved brake lining wear and the
ability to perform minor brake adjustments in real time.
Wireless Transfer of Brake Data
Companies in the transportation industry market products capable of wirelessly
transferring maintenance data from the vehicle to a central data processing computer
in a maintenance yard. These systems are currently configured to wirelessly transfer
engine and transmission fault codes, for example, from the vehicle's network. The
information generated from the commercial stroke sensor packages and instrumented
anchor pins could be broadcast to the vehicle's network and similarly transferred to
the maintenance yard. The data could assist in improving vehicle brake safety,
scheduling brake work, and tailoring brake rebuild schedules.
Improving Regenerative Braking in Hybrid Applications
Many hybrid propulsion manufacturers currently use an open-loop approach to
combining regenerative braking and friction braking. The initial application of the
brake treadle valve is regenerative. Exceeding a preset limit energizes the friction
brakes. This open-loop control methodology results in an arbitrary amount of
regenerative braking force being applied, and less-than-optimal energy being
captured during a braking event. Instrumented anchor pins can measure the
beginning of a friction braking application and its applied force. By factoring in this
measurement data, regenerative braking algorithms can be "closed-loop" in nature.
A closed-loop regenerative braking system, although still isolated from the service
brakes, can optimize the braking energy recovered as well as reduce emissions,
improve brake wear, and improve fuel economy.
This study was performed by
McLean, VA, and Radlinski and
Associates, Inc., Columbus, OH,
by Robert M. Kreeb, Brian T.
Nicosia, Douglas Skorupski, and
Richard Radlinski. Contract No.
The study final report (FMCSAPSV-
04-001), is available from
the U.S. Department of
Transportation and the National
Technical Information Service,
Advanced sensor, brake, braking
system, sensor study, signal
This Tech Brief is disseminated
under the sponsorship of the
Department of Transportation in
the interest of information
exchange. The Tech Brief
provides a synopsis of the
study's final publication. The
Tech Brief does not establish
policies or regulations, nor does
it imply USDOT endorsement of
the conclusions or
recommendations. The U.S.
Government assumes no liability
for its contents or their use.
FMCSA Tech and Analysis Briefs may be accessed at: www.fmcsa.dot.gov.
Michael Lang, C2 Technologies, Inc.
U.S. Department of Transportation
Federal Motor Carrier Safety Administration
Publication No. FMCSA-PSV-05-001