Lateral force/Tyre slip

The diagram shows lateral force as a function of tyre slip.

Fig 1: Lateral Force And Tyre Slip Diagram

Courtesy of SAAB-SCANIA OF AMERICA, INC.

1. Lateral force 
2. Stable position 
3. Unstable position 
4. Tyre slip 

The maximum braking force is reached at a point known as the limit of optimum slip. The section of the curve between 0% slip and the limit of optimum slip is called the stable braking zone, and the section of curve between the limit of optimum slip and 100% slip is called the unstable braking zone, as stable braking cannot be achieved within this zone. This is because the wheel quickly becomes locked after the limit of optimum slip has been reached, unless the braking force is immediately reduced.

Slip also occurs when the tyre is called upon to transmit a lateral force, e.g. during cornering. Diagram 2 shows how the lateral force falls away sharply with increasing slip. At 100% slip, i.e. when the wheels have locked up, no lateral force remains for steering and the driver will no longer be able to control the vehicle.

The diagram shows both curves with ABS modulation range above.

Fig 2: Identifying ABS Modulation Range Graph

Courtesy of SAAB-SCANIA OF AMERICA, INC.

1. Braking/lateral force 
2. Stable position 
3. Unstable position 
4. ABS operating range 
5. Limit of optimum slip 
6. Tyre slip 

During braking, the braking force is allowed to increase to a point near the limit of optimum slip when ABS prevents further braking force. Hydraulic pressure is then adjusted so that the braking force is kept as close to the optimum level (limit of optimum slip) as possible, irrespective of how hard the driver depresses the brake pedal.

Thus, because the ABS system prevents the degree of slip from exceeding the limit of optimum slip, the car never enters the unstable braking zone. At the same time, some lateral force is preserved to ensure that steering control can be maintained (curve II).