How does ABS (Anti-lock Braking System) work?

ABS is a system which prevents the wheels from locking, or stopping, in order to avoid a skid while braking. It doesn’t stop the vehicle from sliding sideways but, assuming that it is stopping in roughly a straight line, will decrease the stopping distance by avoiding a skid.

In this article, we’ll explain how ABS works, what ABS feels like when it’s activated, and when ABS doesn’t work.

What happens when a vehicle skids?

Two things happen: imagine pushing a box along some carpet. It takes a bit of effort to get the box to move, but once it’s moving it takes less effort to keep it moving. Additionally if a tyre locks, it begins to suffer intense friction on the road’s surface. This causes a layer of rubber to melt, and that reduces the grip even more. This molten rubber is deposited on the road as a black line, or skid mark.

Therefore to reduce stopping distances the wheels must be prevented from reaching the point where they lock.

How does ABS stop a wheel from locking?

Each wheel has a wheel speed sensor connected to a central controller in the vehicle. These sensors measure the change in wheel speed, and the controller determines the current speed in relation to other wheels and the overall speed of the vehicle to try to determine if a wheel has locked.

If a skid is detected, a series of hydraulic valves reduce the brake pressure on that wheel allowing it to turn again, while a hydraulic pump primes the brakes ready for maximum braking power again. This system is modulated in a fraction of a second so that at the maximum braking power the wheel is constantly at the point of locking as this is where the best braking performance can occur.

What advantages does ABS have?

Before anti-lock brakes, drivers had to try to feel where the maximum braking force pressure was. The problem is that almost all drivers rarely have to explore this limit, so either don’t apply enough braking power, or apply too much and cause a skid.

During braking the road surface can change and, along with it, the level of friction, meaning that the maximum amount of braking potential on the road surface changes, along with the amount of brake pressure applied to cause a wheel to lock. Road surface friction can be different for each wheel. For example, one side of the car may be on a white line or loose gravel while the other side is on good tarmac. Or, one tyre could have a lot less tread than others, something that would be much more noticeable in wet weather where tyres at the tread depth limit perform much worse than tyres with a full 8mm tread.

The very first ABS systems did not have independent ABS on each wheel which, in many cases, could cause worse braking performance than standard brakes. This is because if one or more wheels had less friction and started skidding, brake pressure would be reduced on all the wheels equally.

Newer systems that distribute braking power effectively between all the wheels are called EBD or emergency brakeforce distribution. There are also systems that will detect when a driver should be applying full braking power and will assist if the driver hasn’t applied enough pressure to the brakes.

Research in 2004 showed that ABS reduces the risk of multi-vehicle collisions by 18% and accidents where the vehicle runs off the road by 35%.

ABS has been required on all new cars sold in the EU since 2004.

When doesn’t ABS work

ABS can be ineffective on ice, deep snow, very loose gravel and sand, and if you aquaplane/hydroplane, although modern systems are much better than older systems.

ABS for trailers

Vehicles that pull trailers will often have a separate brake controller which controls the trailer’s ABS. Anti-lock brakes will take effect under a number of conditions, for example:

• When all wheels begin to skid then the deceleration has exceeded a preset limit
• When one or more wheels lock but others are turning, the system detects that the locked wheel should be rotating at a similar rate to the other wheels.

ABS for motorbikes

ABS is becoming much more popular for motorbikes which are inherently much less stable than cars. It allows for more assured evasive manoeuvres while braking, reducing the risk of a front wheel ‘washout’ (where the front wheel slips from under the bike).
This video shows the difference between motorcycle braking with ABS and without ABS

Limitations of tyres

A tyre only has a certain amount of grip on any given surface. The maximum braking performance will be achieved in a straight line when all the tyre is required to do is slow the vehicle down. If the driver tries to corner as well, a certain percentage of the tyre’s grip has to be used to change the direction of the vehicle, and that means less grip is available for braking.

What does ABS feel like?

If you have never experienced ABS, it’s an excellent idea to familiarise yourself with it before you need it otherwise your reaction to it could be one of fright and taking your foot off the brake pedal!

First, check that you can push the brake pedal down as far as it will go. If you can’t, read our guide to setting your seat position. Now go to a deserted car park or a straight B road.  To feel it you’ll need to be travelling at around 20mph. Push the brake pedal as hard as possible. You may find that the brake pedal will go a long way down, you will almost certainly feel a vibration through your foot and you will hear the hydraulic valves make a buzzing noise as they open and close rapidly.

You can hear this noise in this video:

Don’t release the pressure on the brake pedal until you’ve reached your desired speed (e.g. stopped).

Warning lights

If the ABS warning light comes on, consult your vehicle handbook. If it can’t be resolved, take it to a garage as the brakes could fail.