Transmission, Clutch, Gearbox and Driveline
This domain tests power flow from the engine to the wheels: clutch operation, manual gearbox layout, synchronisers, final drive, differential, CV joints and driveline service. Numerical questions on gear ratio and road speed are common.
Core rules
- Clutch: flywheel, friction disc, pressure plate and release bearing. Free play must allow the release bearing to clear the pressure plate. A slipping clutch shows rising engine rpm without matching road speed.
- Manual gearbox: input shaft, countershaft and output shaft. Synchroniser rings match shaft speeds before engagement; do not force a gear.
- Gear oil must meet the specified grade and GL rating. GL-5 is not automatically a substitute for GL-4 in a gearbox with yellow-metal synchronisers.
- Differential: crown wheel and pinion provide the final drive ratio. Check backlash and preload after repair. Limited-slip differentials may need a friction modifier.
- CV joints need intact boots and the correct grease. Check propeller-shaft phasing and driveline angles after joint or centre-bearing work.
Worked example
A ring gear has 43 teeth and the pinion has 11; final drive ratio = 43 ÷ 11 = 3.91:1. At 3000 rpm in a 1:1 gear, wheel speed = 3000 ÷ 3.91 = 767 rpm. With a tyre circumference of 1.95 m, road speed = 767 × 1.95 × 60 ÷ 1000 = 89.8 km/h.
Traps
- Resting a foot on the clutch pedal and causing release-bearing wear.
- Filling a GL-4 gearbox with GL-5 oil and damaging synchronisers.
- Ignoring a split CV boot until the joint fails.
- Diagnosing slip as a worn disc without checking free play or oil contamination.