The Effect of Application Air Pressure on Brake Stroke Measurements from 70 to 125 psi

Brake chamber construction allows for a finite stroke for pushrods during brake application. As such, the Federal Motor Carrier Safety Regulations (FMCSRs) mandate maximum allowable strokes for the various chamber types and sizing. Brake strokes are often measured during compliance inspections and post-accident investigations in order to assess vehicle braking performance and/or capability. A number of studies have been performed, and their results published, regarding the effect of brake stroke and function on braking force and heavy truck stopping performance [1] through [4]. All of the studies have relied on a brake supply pressure of 100 pounds per square inch (psi). When brake strokes are measured in the field, following the Commercial Vehicle Safety Alliance (CVSA) procedure, the application pressure is prescribed to be maintained between 90 and 100 psi. However, circumstances that are difficult for investigators to control in the field often result in brake stroke measurements being taken with the application pressure outside of the CVSA-prescribed range. Although opinions have been offered regarding suitable correction factors, the authors could not locate any published test data on the topic. The authors conducted an experiment with an in-service vehicle to quantify a relationship between measured brake stroke and system air pressure in the range after the linings have made complete contact with the drums (70 to 125 psi). This paper describes the experiment, results, and theoretical implications, including effects of the varying stiffness of different brake components.