What is ABS?

Written By Andrew Brady

What are ABS brakes and who do they work?
Find out why ABS was invented
Does your car have ABS fitted?

If you’ve ever had to slam on the brakes and make an emergency stop in wet weather, you’ll know how frightening it can be. But at least you can jump on the brake pedal as hard as possible and there’s every chance the car will stop without going into a scary skid. That’s because your car will probably be fitted with ABS - an Anti-lock Braking System. This is designed to help you stop more safely in the case of an emergency.

Why were ABS brakes invented?

Before ABS brakes, when drivers had to do an emergency stop, there was a very good chance the wheels would stop but the car wouldn’t. This was caused by the brakes slowing the wheels but the tyres losing grip on the road surface, causing the car to skid.

At this point, the driver would be sliding towards the scene of the accident. In the wet or the depths of winter, this was even more likely, as tyres would have even less grip on the slippery road.

To combat this, drivers were taught a technique called cadence braking. This is when they released the brake pedal the instant the wheels started to skid then immediately braked heavily again, repeating the action until the car stopped.

Pumping the brake like this prevented the tyre losing grip with the road surface. This helped the driver to keep control and even steer around objects. Trouble is, it was very difficult to do. ABS brakes were designed to copy the effect of cadence braking without the driver doing anything other than braking.

How does ABS work?

ABS cars use sensors attached to the wheel. By monitoring wheel speed versus the speed of the car, these can tell the sensors that the car’s brakes are on, but it's tyres aren’t gripping. The computer can then release the brakes to enable the tyres to regain grip. If the driver is still braking, the brakes come on again until the wheels skid again. And the process is repeated.

This on-off pulsing of the brakes happens within fractions of a second, much faster than any driver can manage it. And it occurs differently on each wheel depending on how much grip they have. This enables each tyre to have as much grip on the road as possible.

What’s the effect of ABS?

If you’re in an ABS car, you might feel a pulsing through the brake pedal. This is the brakes quickly releasing and engaging again.

If you think back to when you rode a bike and jammed on the back brake to do skids, you didn’t really have much control. Most of the time you could only go in a straight line leaving those satisfying black marks on the pavement. It’s the same when a car’s skidding.

But with ABS, because the wheels are still slowly turning, the driver keeps control. This lets you drive around obstacles if needed and, hopefully, avoid a serious crash.

When are anti-lock brakes most effective?

Anti-lock brakes are of most use to drivers on a wet road. This is because they can help to maximise braking power on a slippery surface where there is some light grip to help them bring the car to a stop.

On a dry surface, there isn’t that much difference in stopping distance between an ABS car and one without it. However, the ABS car can steer around obstacles while braking heavily. A non-ABS car is likely to just plough straight on.

But it’s worth remembering that as amazing as ABS is, it can’t defy the laws of physics. On snow, an ABS car will struggle to stop just as much as one without it. As soon as the brakes are applied, the tyres will skid. And they’ll keep skidding when the ABS releases the brakes because there is no grip for the tyres to find.

Do all cars have ABS?

ABS was originally developed for the aircraft industry in the 1920s. It wasn’t first used in cars until the early 1970s. The first motor to use a system similar to those on our cars today was the 1978 Mercedes-Benz S-Class.

Since 1987, all Mercedes cars have had ABS as standard. And from 2004, all new passenger cars sold in the European Union had to be fitted with ABS.

Does ABS stop crashes?

Research by the EU says: “ABS gives a relatively small, but statistically significant reduction in the number of crashes.”

Should your car have ABS?

If you’re buying a car and it was registered new after 2004, it must have working ABS. When the ABS fails, a dashboard warning light will come on.

Some people have been known to disable the warning light without fixing the ABS. It’s well worth having a mechanical inspection to be sure features such as the ABS are working as they should.