One of the most unnerving things that can happen in motoring is that you
brake and one or more of the wheels locks up. This has two possible effects. It
can make the car slew to one side or, if the car happens to skid in a straight
line, the steering becomes useless and you lose all directional control.
The best way to prevent skidding is to apply a form of braking called
cadence braking. A driver who is skilled at this can usually avoid wheel
lockup, but an anti-lock braking system does the job automatically and usually
more efficiently. More and more cars are now being fitted with such a
How it works
An anti-lock system automatically applies a form of cadence braking by
detecting when a wheel is about to lock, releasing the brake at that wheel and
then immediately reapplying it. The system, therefore, needs three main parts:
a means of telling when a wheel is about to lock; a means of releasing its
brake; and a means of restoring the
The third feature is necessary because the anti-lock system has to work
without the driver releasing and reapplying pressure on the brake pedal, and
without the pedal sinking to the floor.
provides its best grip just before it gives up altogether and
slides. Any method of detecting a wheel about to lock must therefore allow for
its speed falling slightly below the free-running speed - the system must not
react too eagerly, but must still work quickly once the point of best grip has
In practice, there are two ways of detecting that a wheel is about to lock.
Its speed can be compared with that of the other wheels, or the rate at which
it is slowing down can be measured. In either case the
the brake can be released if the
is too great.
Computer-based electronic systems work by speed-checking the wheels against
each other, but usually run a double-check by keeping track of the rate of
deceleration too. These systems are complex and relatively expensive. Some
anti-lock systems use mechanical
that detect when a wheel is slowing
Hydraulic pressure to the brakes is cut by electronic
The hydraulic unit also contains a
pressure to reapply the brakes.
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