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Antilock Braking Systems

Antilock Braking Systems (ABS)

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Think fast!

As you drive along dog runs out close in front of you, you hit the brakes - hard - and try to steer round it, right?

But what if you brake TOO hard (highly likely with modern brakes without an anti-lock braking system). You could easily lock up (skid) one or more wheels and lose steering control and lengthen your stopping distance.

Threshold & cadence braking

Without ABS, the recognised braking methods are:
  • Threshold braking
    is applying enough braking force to give maximum speed loss without actually locking up any road wheels (i.e. to achieve optimum braking effort). Most effective but very difficult to do unless well practiced. It is likely an experienced driver or one who does not know the car's braking performance at the limit, will cause unintentional wheel lock up - which leads us to:

  • Cadence braking.
    If wheel lock up is detected by the driver, the brake pedal is briefly released and re-applied to allow the wheel to commence turning and get a better "grip", this can be repeated several times in quick succession. Hopefully steering control will be regained, and perhaps the stopping distance decreased. It needs very quick thinking on part of the driver.
Even after practicing either of the above, who knows if the driver will actually do it correctly while suffering the stress of an actual emergency situation?

Anti-lock braking systems

To put it briefly, sensors at each of the road wheels detect an imminent wheel skid situation and feed the information to a controller which will briefly release and then re-apply the brakes - this can happen several times a second, much faster than a human driver can do it. A measure of steering control is maintained and stopping distance is often (but not always) reduced *.
* Reduced compared with what? Well, reduced stopping distance compared with locking the wheels up and skidding, or poorly executed threshold braking. But NOT necessarily reduced stopping distance compared with properly executed threshold braking.
  • With ABS: brake hard and allow the ABS to do the cadence braking for you.
  • Without ABS: brace yourself with your left foot, avoid locking the wheels up (easier said than done, especially in an emergency!)
DON'T assume you have "perfect brakes" just because your car is equipped with ABS.
ABS is NOT magic, it cannot give your tyres grip that isn't potentially there in the first place - in fact it will lengthen the stopping distance in extremely slippery conditions - although some measure of steering control might be retained.
DO NOT use ABS as an excuse to drive fast or carelessly in slippery conditions!

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