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Directional performance characteristics of heavy truck combinations are reviewed with respect to the influences of multiple axles and articulation points. The performance characteristics considered include steady turning, directional stability, and forced responses in obstacle avoidance maneuvers. The review provides useful insights to engineers interested in the handling and safety qualities of these types of vehicles.

The implications of restricting axle loads to preserve pavements while at the same time allowing gross combination weights over 80,000 pounds are examined with respect to the design qualities of the types of heavy trucks that might be developed. The proposed vehicles would have more axles than current designs thereby achieving higher gross combination weights with smaller axle loads. Design factors influencing mobility, productivity, preservation of the highway infrastructure, and performance in safety-related maneuvers are discussed.

A mobile dynamometer for measuring the longitudinal force acting at the fifth wheel connection between a tractor and a semitrailer has been developed. The use of this dynamometer for testing retarders installed in tractors is described and example results are presented. Computational methods for predicting total retardation of various vehicles equipped with retarders are discussed.

A new over-the-road brake dynamometer has recently been developed to measure commercial vehicle brake torque at nearly constant speed. In this paper, results are presented from the initial test program of this vehicle. The program included tests to assess the influence of initial drum temperature and rubbing velocity on brake torque, as well as the occurrence and nature of brake fade and hysteresis effects. Results of these tests indicated that the rate of energy flow into the brake system had a more significant effect on brake torque than initial temperature. Greater hysteresis was found in the S-Cam brakes than in the dual-wedge brakes.