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ANTI-LOCK BRAKES

ABS - anti-lock braking system - is a safety feature now on many cars yet few drivers understand what it can and can't do, most of those with ABS on their car will never have experienced the effects.

The emergency stop is a regular driving test feature, a hypothetical scenario in the test but reality in many accident situations, fraught with danger and difficult to practise but practise you must. The emergency stop is an essential skill best practised on very quiet roads or, better, off the road.

But it's not just hit the brakes and hope. You have to think about the road surface. If it's wet or greasy the car will slide, just enough braking effort is needed to stop without sliding and that takes practise.

Try it, driving at say 15-20 mph, clutch hard down, hard on the brakes to the point of lock up, stop, come off the brakes, keep on/off until stopped then  handbrake on, gears in neutral.     

ABS uses wheel mounted speed and slip sensors to monitor how hard it can apply the brakes, it re-creates the emergency stop but it's much more effective than a human driver. The sensors on each wheel allow braking to the point of locking then cycle between locking and braking very rapidly optimising the stopping effort. 

On a wet or icy road the electro-mechanical ABS system will enable the car to come to a halt more quickly and smoothly by rapidly applying the brakes to the point of maximum adhesion, releasing and then re-applying them so that steering control can be retained sufficiently to avoid an impact.

ABS mimics human nature in an emergency when the natural reaction is to press the brake pedal harder and harder until impact. If your car has ABS you should try it on a very quiet road or, better still, off road, with an instructor. Keep your foot HARD on the brake pedal - you will feel the vibrations of the system at work - and drive around any obstruction. ABS will optimise the braking force while keeping full steering control.

ABS is, however, less well adapted to deep snow where the "snow plough" effect of snow building up in front of the wheels can have a useful braking effect. On icy roads it needs some grip to work and maximises that grip but if there is absolutely no grip then it is ineffective.

The manual equivalent of ABS is cadence braking - as taught to police drivers for many years - which mimics the effect of the electro-mechanical aid. It is a difficult technique to master but, with practise, one can use it to avoid hitting an object in the way - a child chasing a ball perhaps - a very worthwhile skill to learn and, again, tuition on a skid pan is the only way to learn, never practise on the road.

Article © Graham Benge 2007

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