Retarder
An auxiliary braking system on heavy vehicles that slows the vehicle without using the service brakes.
A retarder gives a lorry or bus a way of controlling speed on a long descent without heating the main brakes. It acts on the transmission or driveline and is usually operated by a lever or a stalk with several stages, so the driver can dial in as much retardation as the gradient needs.
The reason it exists is brake fade. Service brakes on a heavy vehicle that are used continuously down a long hill get hot and lose effectiveness, and a loaded vehicle that has lost its brakes on a descent has very few options left. The retarder keeps the service brakes cool and in reserve.
It is not a replacement for choosing the right gear before the descent, and on a slippery surface a strong retarder setting can upset the drive axle — which is why the rule of thumb is to use it with care in poor grip rather than treat it as free braking.
19 practice questions on this
Each one shows the answer, and our note on why every other option is not it.
Lorry & Bus (LGV/PCV)
- When is an articulated vehicle most at risk of jack-knifing? Braking systems
- You're driving a vehicle fitted with anti-lock brakes. What should you do when braking in an emergency? Braking systems
- What's 'brake fade'? Braking systems
- 'Brake fade' is a loss of effectiveness of the brakes when they overheat. When would 'brake fade' be most likely to happen? Braking systems
- What should you do when driving a large vehicle down a steep hill? Braking systems
- When can an endurance brake (retarder) be especially useful? Braking systems
- You're driving down a long hill. What should you use to prevent the brakes from overheating? Braking systems
- What system is used to control a vehicle's speed without using the footbrake? Braking systems
- When should an endurance brake (retarder) be used? Braking systems
- How can you help to prevent 'brake fade'? Braking systems
- What causes 'brake fade'? Braking systems
- Where must extra care be taken when using an endurance brake (retarder)? Braking systems
- How should you slow down or stop when you're driving on an icy road? The road
- You've parked on the roadside. What must you do before leaving the vehicle? Leaving the vehicle
- Which component will last longer when you use a retarder? Braking systems
- You're driving down a snow-covered hill. Why should you take care when using an independent endurance brake (retarder)? Braking systems
- What should you avoid when using an independent endurance brake (retarder) on slippery roads? Braking systems
- You're driving a three-axle double-deck bus and have the steering turned onto full lock. What should you do to avoid rear tyre scrub? Vehicle condition
- What's the most powerful brake on a bus? Braking systems