What Are Anti-Lock Brakes?

Ellie Dyer-Brown, 3 months ago

5 min read

  • Car ownership
  • Brakes
  • How it works
Spinning car wheel.

WhoCanFixMyCar explains everything you need to know about the Anti-Lock Brake System (ABS).

Modern cars have numerous features that operate in the background to keep you safe as you drive. One of these is the Anti-Lock Brake System (ABS), which helps to maintain traction and prevent the wheels from locking up. 

All drivers, whether learner or experienced, should take the time to familiarise themselves with what the ABS is and what it does. This guide covers everything you need to know.

Contents

What are anti-lock brakes?

How do anti-lock brakes work?

What is anti-lock braking for?

How is ABS different from normal brakes?

Is ABS better than traditional brakes?

What's the difference between ABS and rear-wheel-only ABS?

How should you use anti-lock brakes in an emergency?

When are anti-lock brakes most effective?

Benefits of ABS

How do anti-lock brakes go wrong?

What to do if your car doesn't have ABS


What are anti lock brakes?

Anti-Lock Brake Systems are an important safety feature present in most modern cars. When you apply your brakes too hard or too suddenly, the ABS kicks in to prevent the wheels from locking up, which allows the vehicle to maintain stability even when braking at high speeds.

The ABS consists of:

  • Computer - monitors the sensors and controls the valves.

  • Speed sensors - warn the computer when a wheel is going to lock-up.

  • Valves - control the pressure in each brake line.

  • Pump - if a valve reduces pressure in a brake line, the pump can increase it again.


How do anti lock brakes work?

The ABS uses sensors at each wheel to monitor the rotation speed. If the wheel is rotating too quickly, the system automatically applies and releases the brakes in a fast but controlled way. When this happens, it may feel like the pedal is vibrating. Rest assured, the ABS is ensuring you maintain control of your vehicle.

Close up of a car wheel travelling at speed

The speed of each wheel is monitored individually, and the correct amount of brake pressure is applied. When one wheel starts to slow down faster than the others, it means the wheel is going to lock up. 

To counter this, the ABS adjusts the brake pressure on the locking wheel, allowing it to regain traction. This process is repeated for each wheel until, finally, the car stops.

The ABS is often part of the Electronic Stability Control (ESC) system. All cars with an ESC must have anti-lock brakes, but not all vehicles with ABS have an ESC. You should be able to find out which systems your car has in your vehicle handbook.


What is anti lock braking for?

The purpose of anti-lock braking is to prevent the brakes from locking up, which can cause you to lose control of the car as it starts to skid. Thus, the ABS provides extra steering in an emergency and is most helpful when driving in heavy rain or through floods.

What does it mean when a wheel locks up?

When you apply your brakes, the calliper pushes against the wheel to slow it down. However, if too much pressure is applied and it grips the wheel too forcefully, it could cause the wheel to lock up, which is when it stops spinning even though the car is still in motion. As a result, the car skids, and you lose control.


How is ABS different from normal brakes?

Regular brakes and anti-lock brakes are two different braking systems in modern cars. Both rely on brake callipers (which act as clamps) to apply pressure on the wheels, bringing the vehicle to a stop. However, they work in different ways.

Brake disc diagram

As you know, the regular braking system relies on you, the driver, to control when the brakes are activated. You apply the correct pressure to achieve your desired speed. Find out more about how brakes work here.

On the other hand, the ABS is controlled by a computer, which monitors wheel speed and applies the correct braking pressure based on this data.


Is ABS better than traditional brakes?

In one sense, ABS is better because it is more advanced - it provides better control and safety in an emergency. The system is more complex, requiring regular checks and calibration, so it costs more to maintain. 

Pushing the brake pedal.

Traditional brakes give the driver total control and use a more straightforward system, but they also leave room for human error. 

Ultimately, when it comes to car brakes, it isn’t an either/or situation; both systems are valuable and present in most vehicles.


What’s the difference between ABS and rear wheel only ABS?

Four-wheel ABS aids steering and is found on most modern cars. Rear-wheel-only ABS is usually found on older vehicles (mainly pick-up trucks, vans and SUVs) and is only designed to help those vehicles avoid skidding on slippery surfaces.


How should you use anti lock brakes in an emergency?

As the driver, you don’t have to think about using the Anti-Lock Brake System, even in an emergency, because it kicks in automatically. A computer constantly monitors wheel speed and triggers the system when needed. So, when you press the brake suddenly in an emergency, the ABS will engage and adjust the braking pressure to give you more control.

You can aid this process by:


When are anti lock brakes most effective?

Anti-lock brakes don’t kick in under usual driving conditions. The system is only activated when you brake sharply (for instance, if you perform an emergency stop).

The ABS works best on dry, solid road surfaces, allowing the driver to brake as quickly as possible while maintaining control. However, it can increase stopping distances because it pulses the brakes on and off.

A black 2018 BMW 5 Series in the rain.

Slippery conditions - such as rain, snow and ice - make the ABS slightly less effective, though these are the conditions when it is often activated because the wheels are more likely to lock up.


Benefits of ABS

The benefits of ABS can’t be overstated and have been proved in testing. Cars fitted with ABS braking will stop at a significantly shorter distance on the tarmac than an identical car without ABS, even if that car is driven by someone who is an expert in cadence braking.

Likewise, ABS allows the steering to work even with maximum brake pressure. In a car with locked wheels, the forward motion overcomes the grip and steering lock applied by the driver, whereas in an ABS-equipped vehicle, the wheels keep rotating, allowing the car to follow the direction of the steered wheels.


How do anti lock brakes go wrong?

The ABS can malfunction in several ways.

1. Loss of brake sensitivity

A common problem with the ABS is loss of brake sensitivity, which can happen due to worn brake pads or contamination from oil, dirt or water. As a result, the brakes are less responsive, making it difficult to bring the vehicle to a stop.

Here are five brake problems you shouldn’t ignore.

You can help prevent this by following a regular car servicing schedule. Ideally, you should book a full service once a year or every 12,000 miles.

2. Poor calibration

When the system isn’t calibrated correctly, it can fail to kick in when required, leading to a delayed reaction. You’ll need to visit a brake technician to have the ABS calibrated.

3. Stuck ‘on’

Sometimes, the ABS can get stuck in the ‘on’ position, which causes the brakes to become overly sensitive. As a result, they may lock up when you push the pedal, leading to an unexpected stop.

If the ABS develops a problem, the dashboard warning light is triggered to inform you that the system isn’t active. This may affect other systems, such as traction and stability control (if your car has them). 

ABS warning light

Getting the ABS fixed as soon as possible is of paramount importance. Your car will automatically fail its MOT if the warning light is on - and, more importantly, it makes driving less safe.

You can read more about problems with the ABS and why the warning light might come on in this guide.


What to do if your car doesn’t have ABS

You can find out whether your car has ABS in your vehicle handbook. If it doesn’t, there are ways to simulate the effects of anti-lock braking manually.

A car skidding in rain.

When you feel your car start to skid, repeatedly pump the brake pedal, allowing it to come back up again before pressing it down. Braking this way should help unlock the wheels, giving you more control. 

You should only pump your brake pedal if your car doesn’t have ABS. If it does, the best thing you can do is firmly hold the brake and allow the system to do its job.


Having trouble with your car's ABS? You're in the right place. Our trusted garage network has plenty of mechanics who can help.

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