Brake Performance Testing and Truck Runaway Analysis 2003-01-3396
Commercial vehicle air brake systems are associated with more heat problems than hydraulic brake systems on passenger vehicles. A typical commercial vehicle runaway involves a driver who has lost the ability to control the speed of his vehicle and runs off the road because of too much speed in a curve or collides with slower moving vehicles. In some cases the driver has the ability to control speed to some extent and is unaware that he has overused the brakes until faced with a situation requiring a heavy brake application.
The loss of braking capacity on long steep downgrades is due to the combined effects of heating, brake system design, friction material properties, maintenance, and brake adjustment. Brake drums expand as temperatures increase causing the strokes of the pushrods to increase. Brakes that are out of adjustment or at the limit of adjustment before heating can lose effectiveness with less heating than properly adjusted brakes. As trucks have been improved to reduce aerodynamic drag and parasitic horsepower losses, the burden to develop more retarding horsepower capability has increased. The rate of brake heating is a function of usage and is affected by truck speed and load, gear selection, use of engine retard and drive line retarders.
A methodology is presented for on-road data acquisition of brake temperatures, chamber pressures and pushrod travel. Data can be acquired on a grade or on flat ground such as a test track. New brake performance test data applicable to the understanding of runaway events is provided. An analytical computer utility, Brake Designer™, (1, 2) is used to demonstrate the effects of heating and adjustment on brake effectiveness.