Browse Publications Technical Papers 2005-01-3614

Air Disc and S-Cam Brake Stopping Performance Comparison for Class 8 Tractor and Trailer Combinations 2005-01-3614

In an effort to reduce the dry stopping distance required for heavy trucks, it is imperative to increase the effectiveness of the foundation brake systems. Where brakes are torque limited, increasing the brake output can be obtained by increasing brake size, chamber size, slack length, and friction of the braking materials.
Looking just at the aspect of foundation brakes, the majority of current tractor and trailer brakes are of the S-Cam and Drum type. Two commercially available alternatives that produce higher output are Air Disc brakes and larger sized S-Cam brakes. Using one type, or a combination of these brakes (discs and drums on different axles) warrants a comparative study. The goal is to improve the effectiveness of the brake system, while maintaining or improving upon vehicle stability during braking.
NHTSA's Vehicle Research and Test Center recently completed a brake test study of the effectiveness and stability characteristics of tractor and trailer combinations. Two power units (tractors) were each tested with 4 combinations of foundation brakes. In a matrix of tractor-semitrailer combinations, each tractor was coupled to one semitrailer at a time, and recombined to one additional trailer in the matrix for a set of brake tests. Each trailer was fitted with standard S-Cam brakes, and then changed to Air Disc brakes for a repeat test, for each combination tested.
Three basic tests were performed for each data set: 1) stops from 60 mph on a high coefficient of friction surface, 2) stops from 25 to 30 mph on a low coefficient of friction surface (BIC - Brake-In-a-Curve), and 3) stops from 30 mph on a laterally split coefficient of friction surface.
This report highlights the results for 60 mph stopping tests, alone. It explores the effect of the semitrailer brake on dry surface stopping performance and stability of combination tractor-semitrailer rigs. The data analyses include the effect of semitrailer and brake type on the rankings of full combination braking tests.


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