Most of the studies concerning wet skid resistance of tires concentrate on the effect of a single factor, the other variables being kept constant or assumed to be constant.
In the Netherlands, the Vehicle Research Laboratory of the Delft University of Technology, in cooperation with a working party of the Institute for Road Safety Research (SWOV) has conducted extensive series of measurements according to statistical multifactor test programs.
The wet skid resistance is rated by three coefficients: The peak value of the normalized braking force, the locked wheel value, and the maximum of the normalized side force.
Special test programs are described which aim to investigate the influence of road surface parameters, such as texture depth and SRT values (British Pendulum Tester), and of tire parameters on wet skid resistance. The tire parameters consist of tread factors, compound factors, and a carcass stiffness parameter.
Empirical formulas have been established which indicate that wet skid resistance is dependent on adhesion and hydrodynamic factors. In these two factors, both the influence of road surface parameters and of tire parameters have been quantified. The road surface parameters were studied, particularly in a special test series performed on six different test strips. The dominant tire parameters have been established in a special test program using 18 different common production tires. In another special test, the influence of tread depth of the tire has been examined. It appears to be feasible to describe this influence by means of a function depending on wear percentage, speed, and texture depth of the road surface.