Class 8 Straight Truck and Class 7 School Bus, Brake Performance Improvement Study

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Class 8 Straight Truck and Class 7 School Bus, Brake Performance Improvement Study

S. B. Zagorski / R. L. Hoover / A. L. Al Dunn

A Class 7 school bus and Class 8 straight truck were tested by NHTSA VRTC to evaluate relative braking performance and stability levels of various high-output foundation brake configurations. An abbreviated FMVSS No. 121 test sequence was conducted, with additional tests being performed for research purposes. Each vehicle was tested with three brake configurations: (1) standard S-cam drums (baseline) on all wheel positions; (2) hybrid-Air-disc brakes on the steer axle and S-cam drums on the drive axles; and (3) air disc brakes on all wheel positions. The vehicles were tested at two load conditions, a Lightly Loaded Vehicle Weight (LLVW) and Gross Vehicle Weight Rating (GVWR). Each vehicle-brake-load combination met the current FMVSS No. 121 standard service brake stops on a dry surface from 60 mph. At LLVW, both vehicles exhibited reductions in stopping distance when the S-cam brakes were replaced with either hybrid or all disc configuration brakes, but the difference between the two change types was negligible at this load. However at GVWR, the benefits of disc brakes at all wheel positions were clearly seen. The disc brake configuration showed shorter stopping distances of 20 and 22 percent for the straight truck and the bus, respectively, where the hybrid brake configurations only produced stopping distances that were 10 percent shorter than S-cam brakes for both trucks.