Transmission, Drivetrain and Differential Systems
Transmission, Drivetrain and Differential Systems
This domain tests how engine torque reaches the wheels through the clutch, gearbox, propeller shaft, final drive and differential, including ratios and common faults. Expect clutch operation, manual and automatic gearboxes, CV joints, four-wheel drive and differential action.
Core rules and concepts
- The clutch connects and disconnects the engine and gearbox. Slipping under load indicates a worn disc or weak pressure plate; dragging causes difficult gear engagement.
- A manual gearbox uses input, countershaft and main shafts with synchronisers. A synchroniser matches speeds before engagement; do not force a gear.
- An automatic uses a torque converter, planetary gears and hydraulic control. Check fluid level according to the maker's procedure, with the correct temperature and range.
- A differential lets wheels turn at different speeds in a turn. Final drive ratio equals ring-gear teeth divided by pinion teeth.
- The final drive multiplies torque and reduces speed. Propeller shafts need correct universal-joint phasing; independent suspension uses CV joints.
- On four-wheel drive, mismatched tyre circumferences cause driveline wind-up and damage.
Worked example
A ring gear has 41 teeth and the pinion has 10, so the final drive ratio is 4.1:1. With 200 Nm engine torque and a 3.5:1 first gear, ignoring losses, axle torque = 200 × 3.5 × 4.1 = 2870 Nm. With a 0.30 m tyre radius, tractive force = 2870 ÷ 0.30 = about 9567 N.
Traps that cost marks
Fitting different tyre sizes on a 4WD; not marking a propeller shaft before removal; holding a car on the clutch; checking automatic fluid in the wrong range; and overtightening a pinion nut without measuring preload.
Key points
- Clutch slip shows under load; drag causes hard gear selection.
- Final drive ratio = ring teeth ÷ pinion teeth.
- Differential allows different wheel speeds in turns.
- Match tyre sizes on 4WD to avoid wind-up.
- Mark shaft flanges before removal.