Abstract
We have shown that the cornering wet traction performance of tires, as measured with a special cornering trailer, is influenced by a number of factors and their interaction with each other. Unlike conventional low speed “spin-out” wet cornering traction testing, we have evaluated tire traction over the range 30–60 mph. Over this range there is a marked speed dependence in the rating of various tread rubbers and tread patterns. In general, tread rubbers show a wide range of performance ratings at the lower speeds (30–35 mph) and a narrower range at high speeds (55–60 mph). Various tread patterns on the contrary show similar behavior at low speeds but a wide divergence in traction level at high speeds. Higher durometer tread compounds show improved high speed traction for any given rubber. Tread hardness cannot be used as an omnibus indicator of wet traction performance, however, as each rubber has its own separate correlation line. Low coefficient pavement can have either low or high degrees of macrotexture, but the lack of microtexture or harshness (asperities in the fraction of a millimeter range) produces this type of pavement. Tires must perform safely on such pavement sections of public highways and the testing reported here was done on such test surfaces. Evaluations of four types of tread rubber show that they rank from high to low traction level in the order: SBR, Butyl, NR and BR (solution type) on smooth, low microtexture surfaces. Although BR gives low traction when used alone it is not so used in commercial tread compounds. When properly blended with SBR or NR, tread compounds containing BR give satisfactory traction performance and improved wear performance. The overall behavior of tires can be explained in terms of the concepts of hydrodynamic and boundary layer lubrication. At low speeds boundary layer lubrication predominates on all but the smoothest pavements. This accounts for the marked influence of tread rubber at low speeds. At high speeds both thick and thin film elastohydrodynamic lubrication predominate. In this speed range tread materials play a lesser role and tread pattern or geometry plays a larger role. The relative softness and deformability of tread compound, compared to pavement aggregate, accounts for the importance of elastohydrodynamic lubrication. Drawing on the work of many previous investigators and the data of this work it is postulated that the fraction of the tire contact area of a cornering tire that is in the elastohydrodynamic mode of lubrication is a linear function of speed. This accounts for the good linearity of the plots of traction as a function of speed. Test variability is discussed and steps taken to measure and control such relevant factors as water depth are outlined. The use of statistically designed testing programs with their inherent averaging character are advocated for those doing this work. In addition to their power at averaging test results, such designs uncover the strong interaction between tire and test variables that underlie all wet traction testing.
