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Of the several types of performance-based brake testers (PBBTs), roller dynamometers (RDs) have been used for more than 20 years in Europe to judge the braking capabilities of commercial vehicles and for enforcement of minimum brake performance requirements. These RDs, however, have been exclusively of the in-ground design. In recent years, both in Australia as well as in the United States, the use of portable RDs for assessing commercial vehicle braking performance has been increasing. This paper describes some of the differences between the two types of RDs. Analyses of the effect of the roll geometry and coefficient of friction on maximum brake force measurements are made. It is concluded that careful considerations must be taken with the use of such measurements for predicting stopping distance and braking stability using the results from both in-ground as well as portable RDs. Recommendations are made based on the results of the analyses.

The ability of performance-based brake testers (PBBTsa) to accurately determine the braking capability of commercial vehicles was investigated through a field study of over 2,800 trucks and buses. Under certain conditions, good agreement was found between the observation of brake-related defects by visual inspection and the measurement of weak brake forces by a PBBT. It was determined that the PBBTs' assessment was an independent measure of a vehicle's as-is braking capability, and should not be expected to correlate well with a visual inspection under any condition. It was also determined that predictions of stopping capability should be possible combining the PBBT results of the brake force and axle load measurements with certain assumptions regarding brake application time and road/tire coefficient of friction.