The Effects of Foundation Brake Configuration on Class-8 Tractor Wet Stopping Performance and Stability

Abstract

This study compares wet stopping performance of various foundation brake systems on Class VIII truck tractors. Four configurations of foundation brakes were fitted to two modern 6x4 conventional truck tractors without modification to the control, application or antilock brake systems. The foundation brakes configurations included: standard S-cam drum brakes on all six positions, high output S-cam drum and then air disc brakes on the steer axles, and air disc brakes on all six brake positions. The stopping distances from 60 mph were analyzed for all test conditions. The truck tractors were tested in two weight configurations: LLVW (i.e., bobtail) and GVWR (50,000 lb total axle weight) using an unbraked control semitrailer. Analytical analyses of wet brake-in-curve testing indicate that the hybrid brake systems (employing higher-torque brakes on the steer axle only) might degrade brake-in-curve performance. This disadvantage appeared to exist for both load conditions. Techniques are offered to normalize maximum brake-in-curve speeds evaluated over a long period of time. Analyses of variance indicate significant effects from each brake configuration change on stopping distance on a wet surface. Both truck tractors experienced a 3-8% improvement in stopping performance with the all-disc brake configuration, regardless of load. These results lead to the conclusion that mechanical properties of the air disc brake assemblies might have inherent advantages over the traditional S-cam brake in terms of cycling efficiency during ABS-assisted stops.