Anti-lock braking systems: what is ABS and how does it work?

Anti-lock braking systems (ABS) have been with us for longer than you might think. 

Developed in 1929 for use on aircraft, motorists first experienced the benefits of ABS in the Jensen Ferguson Formula, a four-wheel-drive car unveiled in 1966.

Further development was slow and most motorists had to wait until the mid-1980s to benefit from ABS, most notably when an anti-lock brake system was fitted as standard to the Ford Scorpio. 

ABS is now fitted to just about every new car and is used to help stability while cornering and as a crude aid to traction too as engineers start to think laterally about its benefits in situations other than braking.

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How does the anti lock braking system work?

Maximum braking effort, such as that required in an emergency stop, is developed at the point at which the wheel has just started to lock up and skid. 

Knowing this, it is a relatively simple matter to use an anti-lock braking sensor on each wheel to detect the moment a wheel starts to stop moving, or lock: at this point the retardation is now much lower than it was and the car cannot be steered. The driver is now effectively a passenger and has no control over the car whatsoever.

However, if the brake pressure – which is being applied through a hydraulic fluid that is pumped throughout the system as a result of the driver pressing on the brake pedal – is momentarily released, the brakes are freed and the car’s wheels and tyres can revolve again.

This intervention from the anti lock brake system happens hundreds of times a second. By releasing and reapplying the brakes, the car’s anti-lock braking system prevents it going into a brake-induced skid, enabling the driver to continue to steer.

So essentially, the anti-lock braking system does exactly what it says on the tin - having ABS in your car should prevent the wheels locking as you brake, giving you more control and allowing you to steer.

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Does my anti lock braking system help me to stop faster?

No, it doesn’t, not generally. 

What your anti braking system does do is to allow the driver to maintain better control of the car under hard braking in two ways: it stops the car from going into a skid with all four wheels locked, and allows you to continue to steer the car, helping to avoid hitting the very object that caused you to brake so hard in the first place.

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Anti lock brakes can help you to corner more quickly

Car manufacturers are now using ABS to help their cars to corner more quickly and more safely. 

They do this by using the car’s anti lock brake system as a traction control and chassis stability aid; essentially the car detects when the car is in danger of losing control on a corner and brakes one or more wheels gently to control it.

The effect can be subtle or dramatic. 

I recently drove a Suzuki Vitara through a series of cones on a runway at about 25mph with the stability control turned off. I only managed to steer through three or four before the car got progressively out of shape and I hit a cone. 

No damage was done, but if I’d been driving at motorway speeds and had taken emergency action to avoid a piece of debris in the carriageway or a broken-down car, the results could have been catastrophic.

With stability control (aka the anti lock braking system) turned on, I could feel the left-hand rear brake grabbing to help me to steer into the left-hand turn more easily. 

The effect is exactly the same as when we used to do a skid on our bikes as a kid: do you remember pulling on the rear brake to lock the wheel so you could swing the back end of the bicycle round in a very satisfactory fashion? 

(I used to pretend that I was one of the police drivers in The Sweeney, skidding to a halt in front of a bank that was being robbed.)

Going back to my slalom, with the stability control turned back on, I could sail through the first few cones at 30mph, although a corollary of this was that the car was being slowed down all the time by the very same action. 

It was a very neat and very impressive demonstration of the value of the anti lock braking system.

Incidentally, some manufacturers also use the same principle more subtly to help a car turn into a bend more quickly. Sports cars, like the McLaren range, use this same principle to lightly brake the inside rear wheel to help kill understeer and provide a sharper initial turn-in.

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ABS as traction control

Of course, the other situation in which traction control can be applied is when your car is struggling to gain traction in a low-friction environment. 

The classic case is that of the modern Range Rover, which uses the in-built anti lock braking system to gently brake a slipping wheel to help restore traction and grip.

Written like that, it sounds simple and low-tech (and it is a bit more complicated than I describe) and barely effective, but the reality is anything but. I took a new Range Rover along a favourite green lane a couple of years ago, not knowing that it had deteriorated significantly since I’d last driven it. 

Like a fool, I was travelling alone and, to make things worse, there wasn’t the space to turn around.

I kept driving and the Range Rover completed the whole ten-mile stretch with aplomb on road tyres, much to the amusement of a pair of fully kitted-out Land Rovers – complete with raised suspension, winches, and knobbly tyres – that had got stuck at the other end.

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ABS in side winds

Modern cars are all but immune to the insidious effects of cross winds on motorways and other fast roads. 

The same can not be said of lorries, vans and other high-sided vehicles whose bulk acts as a very effective sail in high winds; in extreme circumstances they can be blown over, but even a moderate breeze can blow them across a carriageway with predictably serious consequences.

Mercedes-Benz uses the ant lock braking system fitted to some of its vans to control this movement by (yes, you’ve guessed it) judiciously applying the brakes to a wheel to help steady it, reducing lateral movement.

I drove a prototype on a test-track in Germany and was astonished at how serious the problem was – and how effective such a simple intervention as ABS could be.  

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Self-braking using ABS

Finally, the number of cars with a self-braking system is rising. 

The system links a forward-facing camera or radar to the car’s braking system. When it detects a slower moving vehicle or a stationary object in front of the car, it will alert the driver. If the driver doesn’t act, it will initiate braking automatically.

What is astonishing is not how well it works but how quickly a modern, ABS-equipped car can come to a halt. 

I’ve tested various systems and they’ve all been capable of stopping in about a third of the distance I would have guessed would have been needed.

I guess this demonstrates that no matter how dire the situation, hitting the brakes as hard as you can and trying to steer around the problem is always a viable option, thanks to clever engineering. 

And finally...

Why is the ABS light on in my car?

We've all been there - driving along, minding your own business, when suddenly a light glowing on the dashboard catches your eye.

If it's the anti-lock braking light that's demanding attention, it unsurprisingly indicates a problem with your car's anti-lock brake system. 

Firstly, don't panic - unless the brake light is also illuminated, your car will still be able to brake, though not with the same immediacy you've grown used to in your ABS-enabled car, so be mindful of this and drive with even more caution than usual. 

If you'd like to see if you can solve the problem yourself, check your fluid levels to ensure the brake fluid is filled to the line on the reservoir, but if you're still concerned, pop into a garage and have them check the car over - better safe than sorry. 

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