BMW Cars 1999-06

Description & Operation


The basic principle of the Antilock Braking System (ABS) is to:

Limit wheel slippage (skidding) to ensure stability while braking
Allow steerability (directional control) while braking
Provide the minimum (optimum) braking distance without loss of control

The control of the brakes by the ABS system consists of three phases:

Building pressure
Holding pressure
Releasing pressure

When applying the brake while driving in conditions where traction is marginal, the tires could loose traction (skid) reducing the directional stability of the vehicle, especially in a panic situation. Reduced traction conditions include unavoidable environmental situations such as driving in the rain, snow, and road conditions such as loose gravel, sand, painted lines, or fluid spills on the roadway (found especially at intersections).

The ABS control unit monitors the rotational speed of each wheels and compares them to one another. If the speed of 1 or more wheels drops drastically below that of the others during braking, the ABS control unit will reduce hydraulic pressure to that wheel until it is rotating at the same speed as the others. This will provide the minimum (optimum) braking distance without loss of control. The ABS system responds so quickly, it can actually apply and release each brake at a rate of 15 times a second.

In the case of the 325iX four wheel drive vehicle, a deceleration sensor is needed to sense the relative motion of the car. Since the possibility exists that 1 or more wheels can be spinning due to acceleration, not braking, a sensor independent of the wheel speed sensors is needed.

An example of the difference between an ABS equipped vehicle and a non-equipped ABS vehicle can be observed easily in a safe, restricted area covered with fresh snow. If, while maintaining control in a turn the brake is abruptly applied on a non-ABS equipped vehicle, the brake hydraulic pressure is applied evenly to each wheel. Because the snow covered conditions offer reduced traction, the wheels will tend to lock up (skid). The vehicle may begin to reduce its speed, but the lock up typically causes the vehicle to begin to slide straight (in the direction of its momentum) and no longer follow the turn regardless of the steering wheel's position. Under the same conditions, an ABS equipped vehicle will regulate the brake pressures individually for each wheel to allow the vehicle to follow the turn while reducing the vehicle's speed. When this occurs, the ABS hydraulic unit is activated by the ABS control unit and the brake pedal will exhibit some feedback caused by the activation of the ABS hydraulic unit. If a driver has never experienced an ABS activation condition before, it can be quite startling when it occurs for the first time.

ABS can not perform miracles. Safe driving principles should always be adhered to and not compromised just because a vehicle is equipped with ABS.