FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION

FMCSA Contact: Gary R. Woodford, MC-PSV, (202) 366-2978

Performance-Based Brake Testers—

Round Robin Study

Introduction

In the early 1990s, the Federal Motor Carrier Safety Administration (FMCSA), formerly

the Office of Motor Carriers (OMC) of the Federal Highway Administration, initiated

research to evaluate various types of performance-based brake testing technologies for

application to commercial vehicle inspections. The purpose of the research was to

determine if performance-based brake testers (PBBTs) could be used to evaluate 

commercial vehicle braking capability. A PBBT is a device that assesses the braking

capability of a vehicle through quantitative measure of individual wheel brake forces

or overall vehicle brake performance in a controlled test. They are widely used for

brake inspection in Europe and Australia and are beginning to emerge as both an

enforcement tool and diagnostic aid for private sector maintenance and repair shops. 

This tech brief summarizes a study based on a series of “round robin” tests (i.e., tests in

which a single standard is used to evaluate the consistency of various test apparatuses)

conducted by the OMC in July 1998 to assess the suitability of PBBTs for use in law

enforcement. The study final report is available at http://www.fmcsa.dot.gov.

Background

Assessment of large truck and bus braking capability in the United States traditionally

has been conducted using visual- and sensory-based inspection methods that require

an inspector to crawl beneath a vehicle to check indicators of potential brake 

performance. The Commercial Vehicle Safety Alliance (CVSA) provides the inspection

procedure and out-of-service criteria for braking systems on commercial motor vehicles

(CMVs) to be used by federal, state, and provincial agencies in the United States,

Mexico, and Canada. Under CVSA guidelines, if a vehicle fails to meet federal 

requirements, it may be placed out-of-service.

While the traditional sensory inspection method generally has been successful, it can

be subjective, time-consuming, labor-intensive, and difficult. PBBTs cannot replace an

inspector in finding brake defects unrelated to immediate brake performance, e.g., 

air leaks, chafed brake hoses, or thin brake pads. However, PBBTs provide a consistent,

objective measure of a vehicle’s braking performance—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). Because the two techniques measure

different factors, inspectors can use both performance-based and sensory evaluations

to assess and maintain brake performance.

Field Test Evaluations

After analyzing various PBBT technologies in its initial research, the OMC selected 

several types for further evaluation in roadside field-test inspections. Researchers 

concluded that no insurmountable performance or operational limitations existed for

roller dynamometers, flat-plate testers, or breakaway torque testers that would 

prevent those technologies from being used in the future for screening or 

enforcement. The study final report, Development, Evaluation, and Application of

Tech Brief

The goal of the Federal Motor

Carrier Safety Administration

(FMCSA) is to reduce the 

number and severity of large

truck-involved crashes

through more commercial

motor vehicle and operator

inspections and compliance

reviews, stronger enforcement

measures against violators,

expedited completion of 

rulemaking proceedings, 

scientifically sound research,

and effective CDL testing,

recordkeeping, and sanctions.

The Office of Research and

Technology manages research

and technology development

and deployment programs for

the FMCSA.

The FMCSA R&T activities

encompass a range of issues

and disciplines relating to

motor carrier safety, including

problem assessment; policy,

safety management, and 

outreach; drivers; truck 

and bus vehicle safety 

performance; and compliance,

enforcement, and operations.

Compliance, enforcement, 

and operations concentrates

on studies directed toward

improving carrier compliance

with, and/or enforcement 

of, existing Federal Motor

Carrier Safety Regulations.

Technology development and

deployment focuses on 

identifying, collecting, and

communicating information

about technological advances

to improve safety.

Office of Research and

Technology

400 Seventh Street, SW

MC-RT; Room 3107

Washington, DC 20590
Performance-Based Brake Testing Technologies, was

published in February 1999, and is available from the

National Technical Information Service (PB99-134454). 

Proposed Functional Specifications

In June 1998, the OMC issued a notice requesting

public comments on proposed functional specifications for PBBTs. The specifications applied to a range

of PBBT technologies and included requirements for

functional performance, physical characteristics, 

environmental resistance, and operator safety. The

specifications were intended to serve as a guideline

for states in determining whether a particular PBBT

would be eligible for funding under the Motor

Carrier Safety Assistance Program (MCSAP) and to

ensure a certain level of PBBT accuracy and 

performance. In August 2000, after the conclusion of

the round robin test series, the

FMCSA issued a notice of 

proposed rulemaking (NPRM)

and request for comments. The

agency also published its final

PBBT functional specifications. 

Purpose

Several PBBTs were evaluated

during the round robin test

series in order to assess their

functional performance and

potential for use in law enforcement. The purpose of the tests

was to evaluate the ability of

current-generation PBBTs to

accurately and consistently 

(1) measure the brake forces 

and wheel loads of CMVs, and

(2) then predict the vehicle’s

deceleration capability from a

32.2 km/hr (20 mph) on-road

stop.

Methodology

Researchers used two types of

commercial vehicles (1) a twoaxle flatbed straight truck, and

(2) a three-axle tractor, two-axle

flatbed semi-trailer combination.

Each vehicle was tested fully

laden and unladen and set up

with target low brake-force to

wheel-load ratios (BF/WL) on

selected wheels, keeping the

braking capability of the vehicle

consistent with the performancebased regulation then under consideration by the

OMC, i.e., the ratio of the total brake force to the

gross vehicle weight (BFTOT/GVW) = 0.4. Eight PBBTs

were used in the testing—five roller dynamometers

(two in-ground and three portable), two flat-plate

testers, and one portable breakaway torque tester.

The testing program consisted of two parts. 

Part I

In the first part of the program, several evaluations

were performed on vehicles with weak brakes.

First, to assess the accuracy and applicability of 

brake-force measurements, the PBBT per-wheel

brake-force measurements were compared to those

measured using a calibrated torque wheel, and the

PBBT total brake-force measurements (BFTOT) were

Portable roller dynamometer

In-ground roller dynamometer
compared to measurements from the 32.2 km/hr 

on-road stops. Second, in evaluating the accuracy of

wheel-load measurements, sets of cement blocks

were placed on the PBBT weighing mechanisms and

the PBBT results were compared to those known

weights. For applicability, the PBBT-measured axle

loads were compared to axle loads obtained using

traditional scales. Third, to assess the applicability of

the equivalent deceleration predicted using the PBBT

measurements (decelEQ = BFTOT/GVW), the PBBT results

were compared to decelerations achieved during 

32.2 km/hr road stops. Finally, in order to evaluate the

repeatability of the measurements (brake-force,

wheel-load and decelEQ), results from the three 

replicates were compared.

Part II

In the second part of the program, several evaluations were performed on the two-axle vehicle with

strong brakes.

First, to assess the applicability of the equivalent

deceleration (decelEQ), the equivalent deceleration

predicted by PBBT measurements was compared with

the deceleration obtained during 32.2 km/hr road

stops. Second, in evaluating the repeatability of the

brake-force measurements, PBBT-reported brake-force

measurements from three replicates were compared.

Finally, the effect of wet test surfaces on the 

PBBT-reported brake-force measurements was

assessed by comparing the maximum brake-force

measurements reported under both wet and dry 

conditions.

Findings

Researchers determined that under most test 

conditions the accuracy and repeatability of most of

the participating PBBTs, regardless of the principle of

operation, were acceptable for meeting the proposed

functional specifications and therefore for use in

roadside inspection and enforcement.

Nearly all of the PBBTs were able to accurately 

measure the vehicle brake forces. The PBBT brakeforce measurements generally agreed with the 

brake-force measurements of the torque wheel. The

roller dynamometers as a group reported slightly

higher brake-force measurements for vehicles with

weak brakes on dry pavement than the corresponding reference values derived from road stops.

Researchers suspected that this discrepancy was a

result of either geometry of the wheel/roller contact

patch or changes in brake torque output as a 

function of speed (the portable roller dynamometers

operate at less than 2 km/hr, while the road stops

were performed at 32.2 km/hr). 

In contrast, several of the PBBTs had difficulty 

reporting accurate vehicle weights. For the most 

part, however, this was related to test procedures.

Calibration checks of the PBBT weighing mechanisms

indicated that all of the PBBTs could meet the 

functional specifications. Weight measurements were

found to be affected by specific characteristics of the

vehicles or by the elevation and ramp configurations

of the portable PBBTs. Researchers recommended 

that consideration be given to additional criteria for

judging brake effectiveness in cases in which weights

are not available or cannot be measured in a 

representative manner due to vehicle configuration. 

Recommendations 

In those instances in which PBBT accuracy did not

achieve acceptable performance in the round robin

tests, the problems were identified and conveyed to

the PBBT manufacturers as recommendations for

improvement. Modifications needed to obtain 

acceptable PBBT performance for use in enforcement

were either those consistent with the PBBT functional

specifications developed for eligibility for funding

through MCSAP, or those needed to improve the

applicability of the PBBT results to the on-road 

stopping results.

The following recommendations were made to the

PBBT manufacturers to assist them in meeting the

functional specifications: 

• alter the test surface to meet the minimum 

“coefficient of friction” requirement; 

• standardize test protocols, including data analysis

and reporting problems; and 

• develop appropriate calibration procedures. 

The brake-force results of some PBBTs were not

affected by the condition of the test surfaces.

Although the coefficient of friction in wet conditions

was not part of the proposed PBBT functional 

specifications at the time of the round robin testing,

researchers said that PBBTs for which brake-force

measurements were affected by the test surface 

conditions should address this problem.

Proposed Rulemaking & Guidelines

for Functional Specifications

After several years of research (including the round

robin testing and field test evaluations) and public

meetings, in August 2000 the FMCSA issued a NPRM

and request for comments on pass-fail criteria for
Researcher

This study was performed by

Battelle Memorial Institute, 

505 King Avenue, Columbus,

OH, 43201. Contract No.

DTFH61-96-C-00077

Distribution

This Tech Brief is being distributed according to a standard

distribution. Direct distribution

is being made to the Service

Centers and Divisions.

Availability

The study final report (DOT-MC00-100) is available from the

National Technical Information

Service (PB-2000-105555) 

Key Words

brake, brake force, brake

inspection, brake performance,

breakaway torque brake tester,

commercial motor vehicle, 

flat-plate brake tester, 

performance-based brake tester,

roller dynamometer brake

tester, round robin tests

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

USDOT endorsement of the 

conclusions or recommendations. The U.S. Government

assumes no liability for its 

contents or their use.

Web Site

All FMCSA Tech and Analysis

Briefs may be accessed at:

http://www.fmcsa.dot.gov.

Please send us your comments

or suggestions about the

Tech/Analysis briefs. 

Technical Writer

Wendy Grzywacz, Avalon

Integrated Services.

PBBTs (Docket No. FMCSA-99-6266). Accompanying the proposal were final functional

specifications for PBBTs (Docket No. FMCSA 98-3611). 

In the NPRM, the FMCSA proposed to amend the Federal Motor Carrier Safety

Regulations to allow the use of PBBTs to determine compliance with existing brake 

performance criteria for CMVs. The brake performance criteria specify CMV 

requirements for minimum brake force as a percentage of actual gross vehicle weight,

and minimum deceleration from 32.2 km/hr. 

Like the specifications proposed in 1998, the final functional specifications for PBBTs

outline generic requirements, including performance, verification and documentation,

for a range of brake testing technologies. The specifications serve as a guideline for

states in determining whether a particular PBBT would be eligible for funding under

MCSAP and ensure a certain level of PBBT accuracy and performance.

The FMCSA currently is reviewing the comments received in response to the NPRM. The

next regulatory action will occur in 2001.

Future Research 

Since the round robin test program, the FMCSA has continued its research to support

the use of PBBTs in evaluating CMV braking performance for enforcement purposes.

This work includes investigating the feasibility of using PBBTs for enforcement of 

federal regulations on CMV lane stability while braking; conducting a design 

evaluation and cost-benefit analysis of current and proposed PBBT technologies; 

investigating further remaining issues in follow-up to the previous round robin testing;

providing general support to the FMCSA in promoting the use of PBBTs by states and

motor carriers; and identifying the training needs and requirements of PBBT users. 

References

Development, Evaluation, and Application of Performance-Based Brake Testing Technologies.

FHWA Tech Brief No. FHWA-MCRT-98-001. July 1998.

Shaffer, S.J. Development, Evaluation, and Application of Performance-Based Brake Testing

Technologies. FHWA Report No. FHWA-MC-98-048. Feb. 1999.

Shaffer, S.J., and Christiaen, A.-C. Performance-Based Brake Testers Round Robin Final Report.

FMCSA Report No. DOT-MC-00-100. Feb. 2000.

November 2000

Publication No. FMCSA-MCRT-01-002 

Federal Motor Carrier Safety

Administration