Without ABS System

General
The braking system is divided into two separate diagonal brake circuits. One circuit comprises the right-hand front wheel and left-hand rear wheel (primary circuit), while the other comprises the left hand front wheel and right-hand rear wheel (secondary circuit).

This means that if one of the brake circuits fails, e.g. as a result of a leak, 50%of the total braking effect is always retained.





Legend
1 Direction Of Travel
2 Braking Force
3 Lateral Force

Illustration Note: All the forces taken up by the tire are either lateral forces or braking forces.

To understand how the Antilock Brake System (ABS) system provides optimum braking without any loss of directional stability, we need to examine the forces acting on a wheel during braking.





Legend
1 Braking Force
2 Stable Braking Zone
3 Unstable Braking Zone
4 Slip

On the diagram in the illustration above, curve (1) shows the relationship between braking force (expressed as a coefficient) and tire slip (expressed as a percentage).

The braking force is equivalent to the coefficient of adhesion, i.e. the friction between the tire and the road surface. Each application of a braking force gives rise to a certain degree of slip, ranging from 0%when the wheel is rolling freely to 100% when the wheel is locked.

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





Legend
1 Lateral force
2 Stable braking zone
3 Unstable braking zone
4 Slip

The maximum braking force (coefficient of adhesion) 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 known as the stable braking zone. The section of curve between the limit of optimum slip and 100%slip is known as 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 lateral force is applied on the tire (e.g. on cornering).

Curve II on the diagram shows lateral force as a function of slip. As can be seen, 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.

Brake System





When the brake pedal is depressed, pedal force is boosted by the brake servo unit and pressure builds up in the master cylinder. The pressure on the pistons in the master cylinder is propagated in the brake system and acts on the brake piston in each caliper. The brake piston presses the brake pads against the brake disc.

When the brake pedal is released, the pistons in the master cylinder return to their original positions and the return passage is opened. Pressure is relieved and the brake pistons are retracted to the rest (brakes off) position by means of the piston sealing ring in the brake cylinder.