Transmission, Clutch, Gearbox and Driveline
This domain tests how engine torque is engaged, multiplied and delivered to the wheels. Expect questions on clutch operation and free play, manual gearbox layout, gear ratios, final drive, differential action and driveline components.
Clutch and gearbox
- The clutch connects and disconnects engine and transmission. It has a flywheel, friction disc, pressure plate and release bearing. Correct pedal free play is typically about 20-30 mm, but always follow the manufacturer's figure.
- Slipping clutch symptoms include engine speed rising without matching road speed, a burning smell and poor acceleration under load.
- A constant-mesh gearbox keeps gears in mesh; synchromesh cones match shaft speeds before engagement. Gear ratio = driven gear teeth ÷ driving gear teeth.
- The final drive provides a permanent reduction. The differential allows inner and outer wheels to turn at different speeds during a turn.
Driveline
- A rear-wheel-drive vehicle uses a propeller shaft with universal joints; a front-wheel-drive transaxle uses constant-velocity (CV) joints. Check boots for splits and grease loss.
Worked example
An engine turns at 3000 rpm in first gear with a gearbox ratio of 3.5:1 and a final drive of 4.0:1. Tyre circumference is 2.0 m. Reasoning: total reduction = 3.5 × 4.0 = 14.0. Wheel speed = 3000 ÷ 14 = 214.3 rpm. Road speed = 214.3 × 2.0 m/min = 428.6 m/min, which is 428.6 × 60 ÷ 1000 = 25.7 km/h.
Traps
Inverting the gear ratio, measuring clutch free play at the wrong point, riding the clutch, using engine oil in a manual gearbox that requires gear oil, and ignoring a split CV boot are common mistakes. A differential is not a limited-slip device unless specified.