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
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
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
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
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
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
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.
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
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.