Anti-lock Braking System  |  Discussion forums Glossary of motoring terms


Anti-lock Braking System (ABS) 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 under heavy braking and, in most situations, to shorten braking distances (by allowing the driver to hit the brake fully without the fear of skidding or loss of control). Disadvantages of the system include increased braking distances under certain conditions and the creation of a "false sense of security" among drivers who do not understand the operation and limitations of ABS.

Since it came into widespread use in production cars (with "version 2" in 1978), ABS has made considerable progress. Recent versions not only handle the ABS function itself (i.e. preventing wheel locking) but also Traction Control, Brake assist, and Electronic Stability Control, amongst others. Not only that, but from history at Bosch its version 8.0 system now weighs less than 1.5 kilograms, compared with 6.3 kg of version 2.0 in 1978.


Anti-lock braking systems were first developed for aircraft. An early system was Dunlop's Maxaret system, introduced in the 1950s and still in use on some aircraft models. This was a fully mechanical system. It saw limited automobile use in the 1960s in the Ferguson P99 racing car, the Jensen FF and the experimental all wheel drive Ford Zodiac, but saw no further use; the system proved expensive and, in automobile use, somewhat unreliable. However, a limited form of anti-lock braking, utilising a valve which could adjust front to rear brakeforce distribution when a wheel locked, was fitted to the 1964 Austin 1800. The first car to have ABS fitted as standard was the Ford Granada Mk 3 (of 1985).

The American firm Kelsey Hayes (now part of TRW Automotive), developed the first electronic ABS system in the late sixties and introduced on the 1970 Lincoln Town Car. It was a rear wheel system. The German firm Bosch had been developing anti-lock braking technology since the 1930s, but the first production cars using Bosch's electronic system became available in 1978. They first appeared in trucks and the Mercedes-Benz S-Class. BMW started using ABS at the time, making the technology standard on all vehicles in 1986. ABS Systems were later introduced on motorcycles.

General Motor's Cadillac division employed a simplified version of anti-lock brakes called Track Master. This system worked solely on the rear brakes and was introduced in 1970 on the Eldorado coupe and made an option for the entire line in the following year.


The anti-lock brake controller is also known as the CAB (Controller Anti-lock 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 any number of wheels are rotating considerably slower than the others (a condition that will bring it to lock it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel. Wheel(s) then turn faster and when they turn too fast, the force is reapplied. This process is repeated continuously, and this causes the characteristic pulsing feel through the brake pedal.

The sensors can become contaminated with metallic dust and fail to detect wheel slip; this is not always picked up by the internal ABS controller diagnostic.

In modern ESP systems, two more sensors are added to help the system work: these are a wheel angle sensor, and a gyroscopic sensor. The theory of operation is simple: when the gyroscopic sensor detects that the direction taken by the car doesn't agree with what the wheel sensor says, the ABS software will brake the necessary wheel(s) (up to three with ABS 8.1 or greater) so that the car goes the way the driver intends. The wheel sensor also helps in the operation of CBC, since this will tell the ABS that wheels on the outside of the curve should brake more than wheels on the inside, and by how much.

^ The electronic unit needs to determine when some of the wheels turn considerably slower than any of the others because when the car is turning the two wheels towards the center of the curve inherently move slightly slower than the other two which is the reason why a differential is used in virtually all commercial cars.

On high-traction surfaces such as bitumen, whether wet or dry, most ABS-equipped cars are able to attain braking distances shorter than those that would be easily possible without the benefit of ABS. An alert skilled driver without ABS should be able, through the use of techniques like threshold braking, to match or improve on the performance of a typical driver with an ABS-equipped vehicle. However, for a majority of drivers, in most conditions, in typical states of alertness, ABS will reduce their chances of crashing, and/or the severity of impact. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control.

In gravel and snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on detecting individual wheels skidding). Availability of ABS should not deter drivers from learning to master threshold braking.

Braking distance from 80-0 km/h: locked wheels ABS
dry pavement 45 m 32 m
snow 53 m 64 m
ice 255 m 404 m

Note, however, that this somewhat simplistic test compares ABS with locked wheels. A good driver with a car with a decently designed braking system, designed to minimize the chances of accidentally locking the brakes during a "panic stop", would fare better under these conditions. A June 1999 NHTSA study found that ABS increased stopping distances on loose gravel by an average of 22 percent.

In contrast to the above, glare ice is another situation where the driver must consider their own opinion on the use of ABS or cadence braking. Independent tests shows results that differ from those above when braking on ice. An independent test, with a 1989 Dodge Omni, a small economy car, and a 1995 Pontiac Grand Am equipped with ABS (Mid Sized family Vehicle) The Pontiac matched or had shorter stopping distances on the glare ice, despite being heavier.

As noted above, maximum braking effect is achieved with the wheels on the limit of friction, whereas ABS works by releasing the brakes as the wheels break traction, so a skilled driver should be able to exceed the braking performance of an ABS system.

When activated, the ABS causes the brake pedal to pulse noticeably. As most drivers rarely or never brake hard enough to cause brake lockup, and a significant number rarely bother to read the car's manual, this may not be discovered until an emergency. When drivers do encounter an emergency 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 would otherwise promise. Some manufacturers have therefore implemented "brake assist" systems that determine the driver is attempting a "panic stop" and maintain braking force in this situation. Nevertheless, ABS can significantly improve safety and control for drivers in on-road situations if they know not to release the brakes when they feel the pulsing of ABS.

Sports cars with highly-developed braking systems without ABS have been shown to outbrake vehicles equipped with ABS. For example, the British car magazines "Evo" and "Autocar" conduct periodic tests of sports cars and compares their ability to accelerate from a standing start to 100mph and then brake to a stop the so-called "0-100-0" test. They find that cars without ABS outperform comparable cars equipped with ABS.

Traction control

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 take suitable action 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.

Risk compensation

ABS brakes are the subject of some widely-cited experiments in support of risk compensation theory, which support the view that drivers adapt to the safety benefit of ABS by driving more aggressively. The two major examples are from Munich and Oslo. In both cases taxi drivers in mixed fleets were found to exhibit greater risk-taking when driving cars equipped with ABS, with the result that collision rates between ABS and non ABS cars were not significantly different.

Above text originally from  This text is available under GPL.

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