Braking force/tire slip

WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.

The braking force is equivalent to the coefficient of adhesion or the friction between the tire and the road surface. Each application of braking force gives rise to a certain degree of slip. The slip of a freely rotating wheel is expressed as 0% and of a locked wheel as 100%.

When the brake is first applied at 0% slip, braking force increases sharply but the degree of slip increases only gradually up to a certain limit. Beyond that point, braking force decreases with increasing slip.

Information on the rotational speed of each wheel comes from the wheel sensors. The TC/ABS control module measures the reference speed, normally on the fastest-rotating rear wheel. The reference speed is subsequently used to determine whether any wheel is exceeding the slip threshold.

Diagram 2: Lateral force as a function of slip.

Vertical axis = Lateral force

Horizontal axis = Slip

Lateral force
Stable braking zone
Unstable braking zone
Slip