The Effects of Foundation Brake Configuration on Class-8 Tractor Dry Stopping Performance

Abstract

This study compares dry 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 brake 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. Analysis of variance tests indicate statistically different stopping distance means between all foundation brake configurations, whether the results for both weight configurations were combined or analyzed separately. Combining the results for both tractors, an all disc brake configuration could yield a 20% improvement in stopping distance at GVWR over the standard all S-cam brake configuration on dry pavement, and a 16% improvement at LLVW. With hybrid disc brakes, the improvements were 12% for GVWR and 19% for LLVW. For hybrid drum brakes, the improvements were 10% for both GVWR and LLVW. Margins of compliance for the minimum stopping distances (versus a 30% reduction in current standards) are shown for each brake configuration.