An anti-lock braking system (commonly known as ABS, from the German name "Antiblockiersystem" given to it by its inventors at Bosch) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is twofold: to allow the driver to maintain steering control and to shorten braking distances.
The first antilock systems were developed for automobiles by Bosch, becoming available in 1978. They first appeared in trucks and German limousines from Mercedes-Benz. Systems were later introduced on motorcycles.
The antilock brake controller is also known as the CAB (controller antilock brake).
A typical ABS is composed of a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit. The electronic unit constantly monitors the rotation speed of each wheel. When it senses that one or more wheel is rotating slower than the others (a condition that will bring it to lock), moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel.
ABS do not cause a lengthening of the braking distance on high-traction surfaces such as bitumen (even when wet): it is true that its main function is to decrease braking force on selected wheels, but it does so only to wheels that were locking, and so were not contributing effectively to the braking action. Bringing the wheel up to the real vehicle speed with the others will instead offer the optimum braking action. How much they actually reduce braking distances on bitumen is a subject of debate and depends in any case on driver skill. The recommended technique for non-expert drivers, in a typical full-braking emergency, in a straight line on a highway, is to press the brake as firmly as possible and to steer around the obstructions.
In gravel and snow, ABS without doubt increases braking distances; on these surfaces a locked wheel digs in and stops the vehicle quickly. ABS prevents this from occurring. Some ABS controllers reduce this problem by slowing the cycling time, thus letting the wheels repeatedly, briefly, to lock and then unlock again. What ABS does accomplish on these surfaces is that it allows you to maintain control of the car rather then go into a skid. With 4-wheel ABS you are able to brake and steer at the same time, in order to avoid an obstacle, and don't have to worry about entering into a skid.
When activated, the ABS causes the brake pedal to pulse significantly. As most drivers rarely or never brake hard enough to cause brake lockup, and rarely bother to read the car's manual, this is usually not discovered until an emergency. When drivers do encounter an emergency situation that causes them to brake hard and thus encounter this pulsing for the first time, many are believed to reduce pedal pressure and thus lengthen braking distances, contributing to a higher level of accidents than the superior emergency stopping capabilities of ABS promise. Some manufacturers have therefore implemented "brake assist" systems that determine the driver is attempting a crash stop and maintain braking force in this situation.
The ABS equipment may also be used to implement traction control on acceleration of the vehicle. If when accelerating, the tire loses traction with the ground, the ABS controller can detect the situation and apply the brakes to reduce the acceleration so that traction is regained. Manufacturers often offer this as a separately priced option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously, leading to what Bosch terms the "Electronic Stability Program" (ESP).
See also: car safety.