Nhtsa heavy duty vehicle brake research program report no. 1--stopping capability of air braked vehicles volume 1--technical report

TLDR: In this article, a number of different types of heavy duty air-braked vehicles including buses, trucks, truck tractors and trailers were tested to determine their stopping capability in straight line and turning maneuvers on various types of road surfaces including ice.

Abstract: A number of different types of heavy duty air braked vehicles including buses, trucks, truck tractors and trailers were tested to determine their stopping capability in straight line and turning maneuvers on various types of road surfaces including ice. In addition, the braking force distributions of the vehicles were experimentally determined, several different brake proportioning systems were evaluated and the effect of initial brake temperature on stopping capability was investigated. Results of the testing indicate that stable stopping capability is primarily determined by brake force distribution. If brake force distribution is close to the normal force distribution on the axles of a vehicle, its stopping capability will be optimum; however, if brake force distribution does not match normal force distribution, premature wheel lockup and loss of control will occur before the vehicle is able to achieve full utilization of the friction forces available at the tire/road interface. Brake force distribution on most heavy duty vehicles is fixed at a level that favors the loaded condition and therefore they do not perform as well in the empty condition. In addition, many heavy duty vehicles are "underbraked" on their front steering axles under all operating conditions and would benefit even in the loaded mode if front brake force level was increased. Devices that adjust braking distribution as a vehicle's load changes appear to provide very significant gains in braking performance not only in the straight line stopping situation but also in braking and turning maneuvers.