Driver Knowledge Tests

How does anti-lock braking (ABS) work?

What is ABS?

ABS is an electro-mechanical system which uses a system of sensors and valves to stop a wheel from locking, which would cause a skid.

ABS does not stop a vehicle from skidding sideways, e.g. through taking a corner too fast, but if the tyres maintain forward momentum under braking then ASB will prevent a skid and decrease the overall stopping distance.

What is a skid?

A skid occurs when the direction of a vehicle is different to the rotational ability of the tyre.  The tyres ability to rotate is governed by either the brakes wanting to stop the rotation and the vehicle’s axle, i.e. a tyre can’t rotate sideways.

The noise a skid makes depends on the surface the tyre is skidding on. A skid on black ice makes almost no sound, whereas a skid on abrasive tarmac will make a loud squealing noise as the rubber initially melts due to the friction and is deposited on the road in a black line (skid mark).

It’s this melting that reduces the friction and causes a skidding vehicle to take longer to stop than one that isn’t skidding, and that’s why ABS exists.

ABS keeps the wheel rotating just at the point of it locking, ensuring the maximum braking power is obtained.

Before ABS, a driver had to try to feel where this point of skidding was. In an emergency when you need to apply all braking power immediately, the average driver is not capable of getting the maximum braking performance out of the vehicle and would tend to either skid, or not brake hard enough.

ABS allows any driver to hit the brakes with full force without skidding.

How does ABS stop a skid?

A central controller monitors a sensor attached to each wheel. These sensors monitor the speed of the wheel and when a change is detected that is outside of the usual parameters or abilities of the wheel, or is different to the other wheels, the controller will determine whether this means that the vehicle is skidding.

For example if the driver is braking, two wheels are turning but two wheels have stopped, the controller perceives this wheel to be skidding and will open a series of hydraulic valves to reduce the brake pressure on those wheels, allowing them to turn again. While this is happening (which is a fraction of a second) a hydraulic pump primes the brakes ready to apply brake pressure again, depending on how hard the brake pedal is pressed. This brake/release sequence is repeated many times a second until the brake is released or the vehicle stops.

What are the advantages of anti-lock brakes?

Only racing drivers and drivers trained in performance driving are experienced enough to consistently feel the point at which the wheels are about to lock. Even then, they can’t compensate for the fact that almost always one or two wheels will lock before the others due to the relative weight balance of the vehicle, differing tyre pressures or tread depth, or differing surfaces (e.g. two wheels are on a white line which has less grip).

ABS allows the maximum braking force to be applied through all the wheels by modulating (varying) the amount of brake force applied so that it is consistently at the limit.

The driver remains able to steer while braking; if the front wheels were locked, there would be no ability to change direction, and if the rear wheels were locked, changing direction will cause the vehicle to spin.

Initial systems didn’t operate each wheel independently which (in some circumstances) meant that brake performance could be worse than without ABS. With newer systems, the braking is always more superior than a human braking without ABS. Adding other systems such as electronic stability control (ESC) and emergency brake force distribution has further improved the effectiveness of ABS.

ABS has been required on all new cars sold in the EU since 2004. A study in 2004 by Monash University in Victoria showed that it reduces the risk of a vehicle having an accident through running off the road by 35% and having an multi-vehicle collision by 18%.

What are the disadvantages of ABS?

It is one more thing to go wrong on a vehicle (although it’s not something that does frequently go wrong).

ABS struggles to work as effectively in deep snow, very loose gravel and on ice. However, coupled with ESC, the driver still retains a lot of control in these circumstances.

Limitations of ABS

ABS cannot extract any more grip out of the tyre than is physically governed by the friction forces between the road and the tyre.

The maximum braking performance can only ever happen in a straight line because if the vehicle is turning, some of the grip is required to change the vehicle’s direction as opposed to slow it down.

In some limited circumstances it is better for the wheels to lock (e.g. deep gravel).

How does ABS feel when braking?

The first time you activate ABS in your vehicle it can give you a fright. The brake pedal vibrates and you will hear the hydraulic valves buzz as they open and close rapidly.

To test how your ABS works, find a straight piece of deserted road. You’ll need to be travelling at more than 30kph because many ABS systems shut off as the vehicle gets below around 10-20kph and you’ll need enough speed to feel it working. 40-50kph is enough. Drive in a straight line. Give yourself some margin on the left, then brake as hard as you can. Keep your foot on the brake until you stop.

ABS on motorbikes

Motorbike riders are more at risk in a skid because the bike will skid from under the rider. ABS also allows the rider to continue to steer while braking. Many new models are coming as standard with ABS, but bikes lag behind cars considerably.

This video shows a side-by-side test of a bike with ABS vs a bike without.

ABS on trucks

ABS on trucks is very common and, when you’re driving something that can weigh more than 20 tonnes, very welcome.

Here’s what every truck driver needs to know about ABS

If the front wheels lock, you can’t steer. If the drive wheels lock then you’re likely to either jack-knife if you’re pulling a trailer, or spin if you’re in a rigid vehicle. If the trailer wheels lock then the trailer can slide into an adjacent lane if you’re turning.

ABS might be on either the front wheels, the drive wheels, the trailer or all wheels.


Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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