Anti-Lock Brakes
Anti-lock brakes were first developed in the 1940’s to allow aircraft
to stop without skidding on slippery runways. Their inclusion in
automotive applications though, goes back to about 1969 when they
appeared as optional equipment on a few expensive luxury models.
These days, ABS or ALB (the two common acronyms for anti-lock brakes) is offered
as standard equipment on the majority of new cars.
How it works
To understand how ABS works we need to look more closely at how an ordinary
(non-ABS) brake system stops a moving car.
When the driver pushes the brake pedal, brake fluid is pushed from the master
cylinder through the brake lines to hydraulic cylinders at the wheels. These cylinders
then push the brake pads into contact with the brake discs, which are turning
with the wheels. The friction produced when the pads contact the discs slows the
rotational speed of the wheels. In turn, as a result of the friction between the tyre
and the road, the vehicle is able to slow down too.
However, when braking under adverse conditions, such as emergency stops or
stopping on slippery road surfaces, the level of grip (friction) between the tyre and
road surface may be exceeded, allowing wheel lock-up and skidding. When this
occurs, braking efficiency is very much reduced, the car may spin out of control and
the driver is likely to lose the ability to steer the car accurately.
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A vehicle equipped with ABS has a conventional hydraulic brake system, with some
additional components, including wheel speed sensors, an electronic control unit
(computer) and a hydraulic modulator unit.
The wheel speed sensors constantly send information to the computer, enabling it
to detect impending lock-up of any or all of the wheels during braking. When lockup is imminent, the computer instructs the hydraulic modulator to control the brake
system pressure, by “modulating” the brakes on and off (applying and releasing the
brakes on the wheel/wheels that are locking up), up to 15 times per second.
This prevents wheel lock-up while maintaining maximum available braking effect, and
happens regardless of the pressure applied to the pedal by the driver. The system has
now entered ABS braking mode. ABS braking will continue until the brake pedal is
released or the conditions allowing wheel lock-up have passed.
In this way the braking efficiency is maximized and the car’s directional stability is
maintained. The driver retains the ability to steer the car out of trouble in potential
collision situations.
At all other times the brakes work exactly the same as a normal brake system – the
system only enters ABS braking mode when necessary, with vehicle speeds above
approximately 6kph.
Brake pedal kick back
During ABS brake operation the driver may notice the brake pedal pulsing or “kicking
back” under the foot, thumping noises from the pedal area, and vibration in the
vehicle body and steering. These are all normal characteristics and do not indicate
malfunctioning, though many drivers may find these sensations a little alarming the
first time they experience them.
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Under ABS braking conditions it is very important that the driver maintains pressure
on the brake pedal, allowing the system to take control and modulate braking
pressures. The driver needs to concentrate on steering clear of hazards.
Brake Assist and other enhancements
Many ABS systems also have a function known as Brake Assist. This detects a rapid
or emergency brake application and applies maximum brake force, regardless of the
actual pedal pressure applied by the driver. In many vehicles, additional safety features
such as Traction Control Systems, (TCS), Dynamic Stability Control System (DSC),
and Hill Descent Control (HDC) are integrated with and use the ABS system.
Stopping distances
In general, ABS equipped cars will have similar or slightly shorter braking distances to
non-ABS cars. However this distance will vary according to several factors, including
the type of road surface. On some roads, especially those covered in snow or loose
gravel, stopping distances may actually be longer than with a non-ABS car. In these
conditions, wheel lock-up on the non-ABS car may result in the tyres biting through
the loose surface to a road surface that offers more grip. The wedge of loose material
pushed up in front of the locked wheels also helps slow the car. The primary function
of ABS is to ensure the car’s directional stability under adverse braking conditions.
ABS can’t prevent an accident caused by following too closely or “tailgating”.
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When it stops working
The system incorporates a dash warning light to advise the driver when a system
fault has been detected. If the lamp illuminates while driving, the ABS system will
be disabled and the system will return to standard non-ABS brake operation.
With the standard brakes operating, the car can still be driven, though it should be
taken promptly to a repairer for attention to the ABS system (Consult the owner’s
handbook for further detail).
Can ABS be retro-fitted?
Attempting to fit a true ABS system to a non-ABS vehicle is unlikely to be cost
effective. They are best installed by the carmaker at the time of vehicle manufacture.
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