How do a vehicle's brakes work, and what is the difference between disc, drum and ABS brakes?
The hydraulic braking system, disc and drum brakes, the handbrake, ABS, and how friction converts the vehicle's kinetic energy into heat to stop it.
A CCEA GCSE Motor Vehicle and Road User Studies answer on braking: the hydraulic system, disc and drum brakes, the handbrake, ABS, and how friction turns kinetic energy into heat to stop the car.
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What this dot point is asking
CCEA wants you to explain the hydraulic braking system, the difference between disc and drum brakes, the handbrake, and ABS, and to link braking to friction turning kinetic energy into heat. Braking connects the vehicle-systems content to the road-safety topic on stopping distances.
The answer
How brakes stop a car
Braking works by friction. Pressing the brakes presses a friction material against a part that turns with the wheel, slowing it.
The hydraulic system
Car brakes are hydraulic - they use brake fluid to transmit the force.
- The driver presses the brake pedal.
- This pushes a piston in the master cylinder, putting the brake fluid under pressure.
- Because liquids cannot be compressed, the pressure passes through the brake pipes equally to all four wheels.
- At each wheel, the fluid pushes the brake pads or shoes against the disc or drum, creating friction that slows the wheel.
A leak that lets air into the system (air can be compressed) makes the brakes feel spongy and reduces braking - which is why the fluid level and condition matter.
Disc and drum brakes
- Disc brakes - a disc spins with the wheel; a calliper squeezes brake pads onto it. They are exposed to the air, so they cool well and are less likely to fade. Usually fitted to the front (and often all round).
- Drum brakes - shoes press outward against the inside of a spinning drum. They are cheaper and often fitted to the rear, but trap heat more, so can fade under heavy use.
The handbrake and ABS
- The handbrake (parking brake) is a mechanical brake (usually on the rear wheels) that holds the car when parked and can help in an emergency if the footbrake fails.
- ABS (anti-lock braking system) stops the wheels locking up under heavy braking by rapidly releasing and re-applying the brakes. This keeps the tyres gripping so the driver can still steer while braking hard, especially on slippery roads.
Worked example: braking on a wet road
Examples in context
Example 1. A long descent. Braking continuously downhill can overheat the brakes and cause fade, so drivers use a low gear to let engine braking help.
Example 2. The spongy pedal. Air in the brake fluid makes the pedal feel soft and reduces braking, so the system must be "bled" to remove the air.
Try this
Q1. Energy-wise, what happens when a car brakes? [1 mark]
- Cue. Kinetic energy is converted into heat in the brakes.
Q2. Why can brake fluid transmit the braking force but air cannot do so well? [1 mark]
- Cue. Liquids cannot be compressed, but air can, so air makes the brakes spongy.
Q3. What does ABS prevent during heavy braking? [1 mark]
- Cue. The wheels locking up (skidding), so the driver can still steer.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksExplain how a hydraulic braking system uses brake fluid to apply the brakes, and state why brakes get hot.Show worked answer →
When the driver presses the brake pedal, it pushes a piston in the master cylinder, which forces brake fluid through the brake pipes to the wheel cylinders/callipers at each wheel. Because liquids cannot be compressed, the pressure is transmitted equally and pushes the brake pads/shoes against the disc/drum, creating friction that slows the wheel.
The brakes get hot because friction converts the vehicle's kinetic energy into heat: stopping the car means getting rid of its movement energy, which appears as heat in the brakes.
Markers reward: pedal - master cylinder - fluid - wheel cylinders - pads/shoes - friction; liquid transmits the pressure; friction turns kinetic energy into heat.
CCEA style4 marksState one advantage of disc brakes over drum brakes, and explain how an anti-lock braking system (ABS) helps a driver in an emergency stop.Show worked answer →
Disc brakes are better at getting rid of heat (they are more exposed to the air), so they are less likely to fade under heavy or repeated braking than drum brakes; they also tend to give more consistent stopping. (Drum brakes are cheaper and often used on the rear.)
ABS (anti-lock braking system) stops the wheels from locking up under heavy braking by automatically releasing and re-applying the brakes very rapidly. This keeps the tyres gripping and rolling rather than skidding, so the driver can still steer while braking hard and can stop in a controlled way, especially on slippery roads.
Markers reward: disc advantage (better cooling/less fade), and ABS prevents wheel lock so the driver keeps steering control and avoids skidding.
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