How is the engine's power passed to the driving wheels, and what do the clutch and gearbox do?
The transmission system - clutch, gearbox, propeller/drive shaft and differential - and why a vehicle needs a clutch and a range of gears.
A CCEA GCSE Motor Vehicle and Road User Studies answer on the transmission system: the clutch, gearbox, drive shaft and differential, and why a vehicle needs a clutch and different gears.
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What this dot point is asking
CCEA wants you to know the transmission system that carries the engine's power to the driving wheels - the clutch, gearbox, drive shaft and differential - and to explain why a car needs a clutch and a range of gears. The specimen paper tests the clutch and the differential by name, so learn what each part does.
The answer
What the transmission does
The engine produces rotation, but it cannot drive the wheels directly at all speeds. The transmission carries the drive from the engine to the wheels and lets the driver control it.
The clutch
The gearbox
The gearbox provides different gear ratios so the engine works efficiently at any speed:
- Low gears give more turning force (torque) for moving off and going uphill, but at low road speed.
- High gears let the car cruise at speed with the engine turning more slowly, saving fuel.
- It also provides neutral (no drive) and reverse.
An automatic gearbox changes gear by itself; a manual gearbox is changed by the driver using the clutch and gear lever.
Carrying the drive to the wheels
In a rear-wheel-drive car, the drive passes from the gearbox along the propeller (drive) shaft to the final drive and differential, then through the half shafts to the rear wheels. (In a front-wheel-drive car the gearbox, final drive and differential are combined near the front wheels.)
Without a differential, the driven wheels would be forced to turn at the same speed, causing tyre scrub and difficult cornering.
Worked example: choosing a gear
Examples in context
Example 1. Moving off. The driver selects first gear, releases the clutch smoothly while adding power, and the car pulls away without stalling or jerking.
Example 2. Cornering smoothly. As the car turns, the differential lets the outer driven wheel spin faster than the inner one, so the tyres do not scrub.
Try this
Q1. What does the clutch allow the driver to do? [1 mark]
- Cue. Connect and disconnect the engine from the gearbox (to move off and change gear).
Q2. Which gears give the most turning force for pulling away? [1 mark]
- Cue. Low gears.
Q3. What is the main job of the differential? [2 marks]
- Cue. To let the two driven wheels turn at different speeds when cornering.
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 the purpose of (a) the clutch and (b) the gearbox in a vehicle's transmission system.Show worked answer →
(a) The clutch lets the driver connect and disconnect the engine from the gearbox smoothly. Pressing the clutch pedal disengages the drive (so the car can stop with the engine still running, and gears can be changed); releasing it engages the drive smoothly so the car can move off without stalling or jerking.
(b) The gearbox provides a range of gear ratios so the engine can drive the wheels efficiently at different speeds and loads: low gears give more turning force (torque) for pulling away and going uphill; high gears let the car cruise at speed with the engine turning more slowly. It also provides reverse and neutral.
Markers reward: clutch connects/disconnects the engine from the gearbox (for moving off and changing gear); gearbox gives different gear ratios (low for force, high for speed) plus reverse/neutral.
CCEA style4 marksName the parts that carry the drive from the gearbox to the driving wheels in a rear-wheel-drive car, and explain the job of the differential.Show worked answer →
In a rear-wheel-drive car the drive passes from the gearbox along the propeller (drive) shaft to the final drive/differential at the rear axle, then through the half shafts to the driving wheels.
The differential divides the drive between the two driven wheels and allows them to turn at different speeds when the car goes round a corner (the outer wheel travels further than the inner wheel), preventing tyre scrub and allowing smooth cornering. It also provides the final gear reduction.
Markers reward: propeller shaft to differential to half shafts to wheels, and that the differential lets the driven wheels turn at different speeds on corners.
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