Braking of Commercial Vehicles Equipped with Air-Disc Brakes from High Speed - Effects on Stopping Distance

Abstract

Due to increased speed limits at the state level, NHTSA has pursued additional testing of heavy trucks at higher test maneuver entry speeds. Test results from three vehicles, a Class 7 school bus, a Class 8 truck tractor and a Class 8 straight-truck are presented here. Results are discussed for full treadle straight-ahead stops from 60, 70 and 75 mph. Each vehicle was tested with two different brake configurations. As expected, higher entry speeds resulted in increased stopping distances. Causes for increased stopping distances are briefly discussed. Comparisons show that vehicles in the hybrid configuration (air-disc brakes on steer axle and S-cam brakes on drive axle(s)) had superior stopping performance to the vehicles equipped with traditional S-cam brakes. The vehicles in the hybrid configuration were less susceptible to increased stopping distances from higher entry speeds. A linear regression of stopping distance versus speed reinforced the benefit of placing air-disc brakes on the vehicles. In terms of reduced deceleration levels, only the S-cam configured vehicles at LLVW were adversely affected due to increased entry speeds, whereas at GVWR both S-cam and hybrid configurations were. The truck tractor, when equipped with all S-cam brakes, consistently performed the worst of the three vehicles. Finally, it was demonstrated that the vehicles exhibited an increase, not a decrease, in braking performance throughout the stop.