Browse Publications Technical Papers 2020-01-1640
2020-10-05

Variability Analysis of FMVSS-121 Air Brake Systems: 60-mi/hr Service Brake System Performance Data for Truck Tractors 2020-01-1640

In support of the Federal Motor Carrier Safety Administration’s (FMCSA) ongoing interest in truck platooning, this report summarizes analyses conducted to measure variability in stopping distance tests conducted on commercial truck tractors. The data used were retrieved from tests performed under the controlled conditions specified for FMVSS 121 air brake system compliance testing. The report explores factors affecting the variability of the service brake stopping distance as defined by 49 CFR 571.121, S5.3.1 Stopping Distance—trucks and buses stopping distance. Variables examined in this analysis include brake type, weight, wheelbase, and tractor antilock braking system (ABS). This analysis uses existing test data collected between 2010 and 2019. Several of the examined parameters affected both tractor stopping distance and stopping distance variability. First, the average stopping distance for disc/disc brakes was shorter than either drum/drum or disc/drum brakes, but brake type did not have a statistically significant effect on stopping distance variability. Second, tractors with a gross vehicle weight rating (GVWR) of 45,000–50,000 lbs had shorter stopping distances than any other examined weight category. Weight was found to have a significant effect on stopping distance variability; the 50,000–55,000-lb GVWR range had more variability than both the next lower (45,000–50,000 lb) and next higher (55,000–60,000 lb) range. Third, the 151–200-in. wheelbase category of vehicles had the longest average stopping distance, and vehicles with a 251–300-in. wheelbase had a lower stopping distance variability than those with a 151–200-in. wheelbase. Finally, tractors with the 6S6M ABS system had stopping distances shorter than those with either the 4S4M or the 6S4M ABS systems, and the 6S6M ABS system had a lower stopping distance variability than the 4S4M ABS system.

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