Evaluation of Braking Strategies on Downgrades

In this study, experimental methods were used to compare the consequences of employing snubbing versus dragging strategies to control the speeds of trucks on downgrades. Vehicle tests were performed on a long steep grade. A mobile dynamometer was used to study cooling rates and hot spotting. The basic findings of the study are: (1) the average temperature per pound (kilogram) of brake drum is practically equivalent whether light dragging or snubbing is used; (2) the hottest brakes will be cooler if snubbing is used; and(3) on short downhill descents, the dragging strategy will cause hot spots to develop to a greater extent.

For many years there has been controversy between those that recommend dragging brakes versus those that recommend snubbing (pulsing) to control vehicle speed during downhill descents. Recently, interest in commercial driver licensing (CDL) has stimulated discussions of the merits of these two braking strategies. Specifically, the model manual used in creating CDL manuals favors the dragging technique and it states that the on-and-off method builds up more heat than a light, steady braking method does [1]. However, experimental evidence supporting or refuting this position has not been generally available. Furthermore, theoretical considerations indicate that either method should result in nearly the same average temperature across all brakes as long as the same average speed is maintained. Consequently, the tests and experiments described in this paper were performed to aid in advising the commercial vehicle community on downhill braking strategy [2].