Virtually all modern cars are fitted with microprocessor-controlled anti-lock braking systems (ABS). These can react very quickly to the wheels locking, interrupting and reapplying the brakes up to 25 times a second to ensure the vehicle doesn’t skid.
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
This is a way of maintaining control in very slippery conditions. The
technique requires that the driver quickly and repeatedly releases and
reapplies the brakes. The brakes should be released just before the wheels
lock up but it is almost as effective to release them just after lock
The technique of cadence braking has to be done in perfect timing with
the car’s natural pitching motion, otherwise it may not be of any
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 pressure to the brake line after
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.
A car tyre 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 hydraulic pressure at
the brake can be released if the deceleration 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 sensors that detect when a wheel is slowing