When a vehicle is braked, the load on the front axle increases while the rear axle is relieved of load (dynamic axle load shift). This is due to the inertia of the vehicle body. The resultant torque about the vehicle’s transverse axis increases the load on the front axle. In practice this effect can be observed in the dipping of the vehicle body at the front axle during a braking manoeuvre (brake dive). Due to the increase in front axle load, the maximum amount of brake force transmissible to the front axle also increases. The reduction in rear axle load has the opposite effect – the maximum amount of brake force transmissible to the rear axle decreases.

In order to prevent instability due to “overbraking” of the rear axle, only the brake pressures which produce the maximum transmissible amount of brake force may be applied to the rear axle. If the vehicle is braked by applying the same brake pressure to all four wheels, valuable stopping distance will be “wasted”. This is due to the fact that much less brake force is produced at the front wheels than can theoretically be transmitted to these wheels.

The EBD control system prevents overbraking of the rear axle. The required brake pressure is calculated and set by the ABS control unit depending on the brake slip values of the rear wheels. The full brake pressure applied by the driver continues to act on the front axle.

This provides the maximum physically possible amount of braking power. The vehicle’s stopping distance is reduced to a minimum. Chronologically, the EBD system intervenes before the ABS system, i.e. at much lower brake slip values.

Previously, use was made of hydraulic pressure reducing valves, which limited the brake pressure applied to the rear axle. Since the introduction of ABS, this function can be performed by the EBD system. When the rear wheels show signs of beginning to lock up, the brake pressure at the corresponding wheel brake is
reduced. The “brake pressure holding” function, which works by closing the intake valves, is primarily used for this purpose.

If necessary, brake pressure is also reduced by opening the exhaust valves and closing the intake valves. To reduce the brake pressure, brake fluid flows into the internal storage chambers. Once the storage chambers have been filled to a preset degree, the brake fluid is pumped back into the master brake cylinder against the
pressure the pedal by the recirculating pump.

Without EBD:
If equal brake force is applied to all four wheels, the rear wheels may lock up due to reduced load (caused by the weight shifting forward). Locked rear wheels could make the car skid or even spin out, leading to a loss of control.

With EBD:
The EBD system detects that the front wheels can handle more braking force due to the increased load and applies higher brake pressure to the front wheels. Simultaneously, it reduces the brake pressure on the rear wheels to prevent them from locking up.

This optimized distribution ensures the car stops in the shortest distance possible while maintaining stability and preventing skidding.

Real-world Scenario:

You are driving an Audi Q7 down a steep hill. The “Downhill Assist” feature activates, and EBD adjusts the brake pressure dynamically.

• While reversing downhill, EBD applies less brake pressure to the front axle to avoid overbraking, which could destabilize the car.
• This ensures a safe and controlled descent, even on rough or uneven terrain.

In both cases, EBD prevents accidents by maintaining vehicle stability and optimizing braking performance.