In (automotive) vehicle dynamics, slip is the relative motion between a tire and the road surface it is moving on.
This slip can be generated either by the tire's rotational speed being greater or less than the free-rolling speed (usually described as percent slip), or by the tire's plane of rotation being at an angle to its direction of motion (referred to as slip angle).
It is distinguished from the local sliding velocity of surface particles of wheel and rail, which is called micro-slip.
The longitudinal slip is generally given as a percentage of the difference between the surface speed of the wheel compared to the speed between axle and road surface, as: where
is the longitudinal component of the rotational speed of the wheel,
is vehicle speed in the plane of the tire.
This can occur, for instance, in cornering, and is enabled by deformation in the tire carcass and tread.
Despite the name, no actual sliding is necessary for small slip angles.
Sliding may occur, starting at the rear of the contact patch, as slip angle increases.