on a wet road, the tyre has to break through the water layer. In order to displace the water, so-called baffle forces have to be superseded. The water layer and the section of the running strip in contact with the road for simplification may be subdivided into three zones (the approaching zone, the transition zone, and the contact zone) (Fig. 1-20). In the approaching zone, the principal part of the water is displaced in the running-in-area of the tyre without coming into contact with the road. In the transition zone the tyre is already deformed and partially comes into contact with the road. The contact zone describes the actual tyre contact area, the direct contact surface between running surface and road that makes the energy transition possible. Only a negligible amount of water is displaced outwards by the tyre profile.
As indicated by tests, the baffle resistance almost exclusively depends on the volume of water displaced in unit time, hence for a water level h, the tyre width b, and the rolling velocity vR. Tyre design, air pressure, or wheel load have almost no or a very small influence.
In order to calculate the baffle resistance FR baff, the following empirical relation can be used:

Eq. 1-12

where: FR baff = baffle resistance in N
w = tyre width in cm
vR = velocity in km/h