ABS: All you need to know

ABS or Antilock Braking Systems were originally designed and developed for aircraft, allowing the pilot to stop them safely in all conditions. ABS is designed to stop the wheels from locking up and skidding when braking hard or on wet or slippery surfaces. This is a significant safety feature that has prevented thousands of accidents since it was first introduced onto cars in the early 1990’s.

ABS doesn't always reduce your stopping distance. In fact your stopping distance can actually increase on dry roads. The real advantage is where the road is wet, ABS can reduce the stopping distance by more than 25%.

Remember the only contact that your car has with the road surface are it’s tyres. When you brake hard there are tremendous forces of friction from the road surface and the rubber compound of the tyre's. It's these friction forces that stop the car.
A tyre produces it’s maximum braking effort just before it skids. Once the wheel locks the friction is decreased and the stopping distance increases.

The only exception to this rule is when a car is on loose snow. A locked tyre on loose snow allows a wedge of snow to build up just in front of it which helps it to stop in a shorter distance than a rolling tyre. It is for this reason that some ABS system manufacturers include a snow switch that turns the system off normally at speeds below 10mph.

Also remember that when your front wheels are locked turning the steering wheel will have no effect at all. ABS allows you to brake really hard and take avoiding action. ABS only comes into play when traction is reduced or during emergency stops. Most of the time, it has no effect on your normal driving or braking.

ABS systems are pretty bomb proof however if a problem occurs, most systems will deactivate themselves and switch on the ABS warning light.. The vehicle should still have normal braking control. The vehicle will be safe to drive, but without ABS. Bear in mind however that a vehicle will fail it’s Mot if the ABS light is on, and there may be problems with standard braking system. So get it checked.

How ABS Works

The wheels are fitted with speed sensors. These sensors constantly monitor each wheel and send signals to a control unit. When a wheel starts to lock the control unit overrides your braking and decreases the pressure to the locking wheel. So if one wheel starts to slow down quicker than the others, or at a faster rate than is programmed the control module takes over.

Electrical solenoid valves and an ABS pump are used to release and reapply the pressure to the brakes. This produces a rhythmic ‘pulsating' effect, which is felt through the brake pedal. On some cars, the pedal sinks towards the floor and very little pressure is felt by the driver as the system takes over completely. The driver may also hear a ‘banging' noise from the ABS hydraulic unit.

This rapid applying and releasing of brake pressure in the brake circuit reduces the load on the wheel and allows it to maintain traction, this is the same as pumping the brakes very quickly with your foot (cadence braking), except that the ABS system does automatically, and at speeds that would be completely impossible for a driver to imitate. Many systems can ‘pump the brakes’ 40 times per second.

The pressure in the braking system is released by high speed valves. After the pressure has been released, the control unit needs to reapply it. It does so by means of an ABS pump (pictured above). It then opens the valve to release the pressure, then activates the pump to reapply it, and the whole cycle goes on and on at unbelievably fast rates.

Once the braking for all the wheels returns to normal and the danger of a lockup is avoided, the car reverts to normal braking and antilock reverts to a passive mode.





ABS is now also fitted as standard to some motorcycles, On bikes, there's usually a sensor and an ABS control module fitted to both wheels. If there's only one it's always fitted to the front wheel.


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