Comparison of tire/road coefficient of friction testing methods

Abstract

Three different tire/road coefficient of friction measuring methods are investigated and compared for their use in the field of vehicular accident reconstruction. Locked-wheel skid tests, considered to be the most accurate method, were performed with an instrumented test vehicle to establish base values that could be compared to values measured using a tow cable method and a portable drag sled method. A set of high performance radial tires and a set of standard production radial tires were each tested on two different surfaces to broaden the range of friction values measured. In a series of tow cable tests, the test vehicle was pulled, with wheels locked, over each surface at low speeds. The tow cable coefficient of friction decreased with increasing sliding speed. Portable drag sleds were constructed from a tire in each tire set. Weights were added to each drag sled to study the effect of increasing load on the coefficient of friction. Tests were performed with drag sleds weighing 26 pounds, 51 pounds, and 76 pounds. The drag sled coefficient of friction was independent of sliding speed. The investigation showed a correlation did not exist between the coefficient of friction values determined by the drag sled and tow cable methods when compared to the values determined by the locked-wheel skid test method. Depending on the tire/surface combination tested, the tow cable tests overestimated the skid test coefficient of friction by 28 to 81.4 percent. Similarly, the drag sleds overestimated the skid test coefficient of friction by 31.4 percent to as much as 114.5 percent