FEDERAL HIGHWAY ADMINISTRATION • OFFICE OF MOTOR CARRIERS

FHWA Contact: Kate Hartman, HSA-20, (202) 366-2742

Development, Evaluation, and

Application of Performance-Based

Brake Testing Technologies

Introduction

A performance-based brake tester (PBBT) is a device that can assess the braking 

capability of a vehicle through a quantitative measure of both individual brake and

overall vehicle performance in a controlled test. The primary benefit of PBBTs to both

the enforcement and the motor carrier communities is that they provide an objective, 

consistent, and standard measure of the as-is braking performance of a vehicle. PBBTs

can be used to assess braking capability irrespective of brake type (disk or drum), 

energy supply (air, hydraulic, electric, or spring), or application method (s-cam, wedge,

piston, spring, or lever and cable). This Tech Brief presents a summary to date of a

Federal Highway Administration-sponsored project that is fully documented in a 

separate report.

Purpose

The inspection procedure and pass-fail criteria for braking systems on commercial

motor vehicles in North America are provided by the Commercial Vehicle Safety

Alliance (CVSA). The inspection procedure and criteria are used by federal, state, and

provincial agencies in the United States, Mexico, and Canada for commercial motor

vehicle inspection and enforcement. The procedure requires an inspector to get 

underneath a vehicle in order to check visual, aural, and tactile indicators of potential

brake performance. These sensory inspections can be subjective, time consuming, labor

intensive, and difficult. 

PBBT evaluations are intended to be an alternative method for brake evaluation based

upon actual brake performance, rather than visual inspection, and can be conducted

without crawling underneath the vehicle. PBBTs can do an excellent job of rapidly 

identifying weak brakes or unsafe vehicles, but will not replace the current CVSA 

procedure for inspecting brakes and their components. Because the two techniques

measure different factors, inspectors can use both performance-based and visual 

evaluations to assess and maintain brake performance.

The purpose of this study was to determine, through field test data collection and

additional analytical and experimental studies, if PBBTs could improve commercial

motor vehicle safety and assist with timely throughput at inspection sites, while

improving the accuracy of currently employed sensory brake inspection techniques. 

Two specific goals were to:

• Develop performance-based criteria for identifying weak or defective brakes.

• Determine if any limitations exist that might prevent the future use of PBBTs for

assessing brake performance.

Tech Brief

The vision of the Office of

Motor Carriers is to help

move people, goods, and

commercial motor vehicles

on our Nation’s highways in

the most efficient, economical, and crash-free manner

possible. The OMC research

and technology program

focuses on improving safety

in interstate commercial

motor vehicle operations and

serves a trucking and motor

coach industry that carries

more than 40 percent of all

intercity freight.

Studies are conducted in the

following areas: commercial

driver human factors, health,

and performance needs; new

and emerging driver and

vehicle technologies; safetyrelated data collection and

analysis needs; and performance-based changes to the

Federal Motor Carrier Safety

Regulations.

The OMC’s technology

research promotes safety by

identifying, collecting, and

communicating information

about technological

advances.

Office of Motor Carrier Research

and Standards

400 Seventh Street, SW

HCS-30; Room 3107

Washington, DC 20590
Research Methodology

Researchers evaluated several first and second generation prototype PBBTs. CVSA certified inspectors 

tested roller dynamometers, flat plate testers, and

breakaway torque testers for at least 1 year in the

field. Two additional brake assessment methods,

infrared brake drum temperature measurement and

an on-board decelerometer, were also investigated,

though less extensively. Ten states participated in the

program, each assessing a particular PBBT.

During the field testing, inspectors evaluated the

brake performance of almost 3,000 commercial 

vehicles. These joint inspections consisted of a CVSA

Level 4 inspection (comprised of the driver, brake, and

tire portion of a Level 1 full inspection) and a PBBT

test. Both inspections were conducted to compare

brake defect rates between visual inspection and 

performance-based brake test techniques.

Findings

Information available from the various PBBTs includes:

brake force, brake force versus air pressure, axle or

individual wheel weight, dynamic axle or wheel

weight, parking brake force, threshold brake 

application pressure, and brake timing. In addition to

correlations derived from the results of joint CVSA

brake inspections and performance-based machine

tests, researchers collected data pertaining to

machine utility, such as vehicle throughput, machine

maintenance requirements, inspector skill level

requirements, and cost benefit analyses. This study

predicts inspectors can screen at least 30, and as many

as 80, 5-axle vehicles per 8-hour day for CVSA inspection using one of the PBBT technologies.

Performance-Based Criteria

Based on analysis of the field test data, recommended

PBBT criteria for identifying weak brakes are 

measured in terms of forces and weights, such as:

• A minimum force at a given air pressure for 

pneumatically braked vehicles, using recommendations previously developed by the National Highway

Traffic Safety Administration’s Vehicle Research 

Test Center.

• A minimum ratio of brake force balance across an

axle of 0.65 or better for any vehicle or brake type. 

• A minimum brake force (BF) as a function of wheel

load (WL) for any vehicle or brake type. A minimum

BF/WL ratio of 0.25 is recommended for steer axle

brakes. A minimum 0.35 ratio is recommended for

non-steer axle brakes.

This study also examined recommendations for 

vehicle out-of-service (OOS) criteria. Inspectors would

place a vehicle OOS if its braking capability falls

below a certain threshold and it is an imminent 

hazard. Two approaches are proposed:

• A vehicle may be placed OOS if 20 percent or more

of its brakes are found to be defective using one of

the above PBBT criteria.

• A vehicle may be placed OOS if it cannot meet a

minimum equivalent deceleration criterion (e.g., if

the deceleration is < 0.4g, where g is the acceleration due to gravity [9.8 m/sec2 or 32.2 ft/sec2]). The

equivalent deceleration can be computed from

brake force and weight measurements obtained

with a PBBT. Additionally, a stopping distance can 

be predicted (e.g., 12.2 meters from 32.2 km/hr 

[40 feet from 20 mph] or 105.2 meters from 97

km/hr [345 feet from 60 mph]) from this equivalent

deceleration.

A third type of vehicle OOS criterion based on the

results of a PBBT considers the brake force and load

distributions. This adds the consideration of braking

Flat plate brake tester stability to minimum stopping distance. Braking 
stability refers to the ability of a vehicle to maintain

stable travel in its lane during a stop. The data

required for development of these criteria were not

available from the field testing, but future work is

planned.

The overall agreement for individual weak or defective brakes identified by CVSA inspection and those

identified using a PBBT range from 52 to 88 percent,

depending upon the particular state and the type of

PBBT used. These levels of agreement are reasonable

considering that the different techniques assess 

different factors.

The study found that many of the vehicles placed

OOS using CVSA criteria had sufficient stopping 

capability when their brakes were evaluated in terms

of the proposed PBBT criteria. Of 2,865 trucks inspected using both methods, inspectors placed 396 OOS

under the CVSA standards. Out of these 396, only 215

would have been placed OOS due to the failure of 20

percent or more of their brakes in a PBBT. Only 179

had an insufficient predicted overall vehicle deceleration (< 0.4g), as measured by a PBBT. This means that

approximately 50 percent of the vehicles placed OOS

by the CVSA criteria had adequate braking capability

as judged by PBBTs.

However, the total number of vehicles that would

have been placed OOS using the proposed PBBT 

criteria was considerably larger. Of the same 2,865

vehicles, 559 would be placed OOS using the PBBT 20

percent criteria, and 1,124 vehicles would be placed

OOS using the minimum 0.4g deceleration criteria.

While there is some concern

that PBBT results may be

overly conservative with good

brakes, PBBT identification of

weak brakes and vehicles

with inadequate stopping

capability was clearly 

demonstrated.

Since the CVSA visual 

inspection is qualitative and

subjective, and the PBBT

assessment is quantitative and

objective, the degree of

agreement between the two

techniques was found to be

of minimum significance. The

research suggests that both

techniques could be used

simultaneously to help 

drivers, inspectors, and maintenance personnel maintain

and operate a safer vehicle.

Limitations of PBBTs

No insurmountable performance or operational 

limitations exist specifically for roller dynamometers,

flat plate testers, or breakaway torque testers that

would prevent these technologies from being used 

in the future for screening or enforcement.

Researchers did find that the applicability of the

infrared temperature measurement system was 

limited to the detection of inoperative brakes or

brakes with stroke measurements greater than 12.7

mm (0.5 inch) beyond the recommended adjustment

limit. This was due to the wide range of temperatures

found on brakes that appeared to be in proper working order. The on-board decelerometer, while valid

for screening or enforcement, showed limited applicability because of the logistics of test “runway” space

limitations, strong dependance on driver skills, and

potential damage to deceleration-sensitive cargo.

Further investigation of these techniques is likely.

Several limitations exist for performance-based 

technologies as a whole. PBBTs cannot replace the

inspector for finding a number of defects, such as

chafed hoses and thin brake pads, that do not directly

affect brake force. In addition, none of the PBBT

technologies currently can predict braking capabilities

for vehicles with overheated brakes based on tests

performed on cold brakes. 

Another concern is that certain low-ground clearance

vehicle configurations could not be tested on

portable PBBT units without the use of special ramps

or platforms. Research found portable units to have a

Roller dynamometer
Researcher

This study was performed by

Battelle Memorial Institute, 505

King Avenue, Columbus, OH

43201-2693, Telephone: (614)

424-4960. Contract No.

DTFH61-93-C-00055.

Distribution

This Tech Brief is being distributed according to a standard

distribution. Direct distribution

is being made to the Resource

Centers and Divisions.

Availability

The draft final report is under

review and will be available

upon completion.

Key Words

brake, performance-based

brake tester, commercial motor

vehicle, brake performance,

commercial motor vehicle 

safety, brake inspection, 

defective brake, brake force.

Notice

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 FHWA endorsement of the conclusions or 

recommendations. The U.S.

Government assumes no 

liability for its contents or 

their use.

higher initial cost and higher expected maintenance costs, along with a lower expected

reliability than the same units mounted in-ground. 

Future Uses

Through this study, a plan has been developed for using PBBTs for enforcement and 

for incorporation with Intelligent Transportation Systems technology. In the future, a

vehicle will be automatically identified as it approaches a roadside or fixed-site 

inspection facility. An identifier will be transmitted electronically to a computer 

network (Commercial Vehicle Information Systems and Networks [CVISN]) for access to

credentials and safety information that would be carrier, vehicle, and potentially driver

specific. Once downloaded, this information could be utilized to make informed 

decisions at inspection sites on the selection priority of carriers, vehicles, and drivers.

If directed into the inspection facility, vehicles would undergo a PBBT test (including

brake forces, wheel loads, and axle spacings), to be completed in just a few minutes.

Safe stopping capability and in-lane braking stability would also be calculated. The test

results would be automatically transmitted to the roadside inspector’s computer and

written into the vehicle inspection software. If the vehicle fails the minimum deceleration or stability standard, it would immediately be placed OOS. If passed, it would be

directed back into mainline traffic. In both cases, the information obtained from the

PBBT would be recorded and uploaded to a vehicle transponder, an on-board computer

system, and/or an electronic data mailbox. The inspection record would also be

uploaded to various components of the CVISN and stored, contributing to a historical

performance record of the carrier and vehicle. It may also be used to provide an

advanced safety screening of the vehicle. 

Additionally, the results of the PBBT would provide performance data to allow the

vehicle’s on-board computer to calculate a safe following distance for the current 

conditions. The results would inform a driver of predicted braking capability or 

instability problems that might arise from changes in loading conditions, or tire/road

frictional problems arising from changing weather or road conditions. The driver would

receive information indicating the current following distance and the safe following

distance, and a warning system would alert the driver if safe following distance was

compromised or violated.

PBBT technology can be used as both an enforcement tool and a diagnostic aid for

maintenance and repair. While PBBTs will not replace the CVSA visual inspection

process, they will provide maintenance and inspection personnel with an additional

tool for assessing the braking capability of a vehicle.

July 1998

Publication No. FHWA-MCRT-98-001