Transmission, Clutch, Gearbox and Differential
This domain covers the path of power from the engine to the wheels: clutch, gearbox, propeller shaft, final drive, and differential. Questions test gear ratios, clutch operation, gearbox types, and why the differential is needed on turns.
Core concepts
- The clutch connects and disconnects engine and gearbox. Free play is the small pedal movement before the release bearing acts; too little causes slip or drag, too much causes incomplete disengagement.
- A manual gearbox provides several gear ratios. Lower gears multiply torque; higher gears allow speed with lower engine speed.
- Overall gear ratio = gearbox ratio × final-drive ratio. Wheel torque = engine torque × overall ratio, ignoring losses.
- The differential allows inner and outer wheels to turn at different speeds while transmitting torque. A limited-slip differential reduces wheelspin.
- Automatic transmissions use planetary gears, clutches, and hydraulic or electronic control; CVTs use a belt and variable pulleys.
Worked example
An engine produces 120 N·m. First gear is 3.2:1, the final drive is 4.0:1, and the tyre rolling radius is 0.30 m. Overall ratio = 3.2 × 4.0 = 12.8. Wheel torque = 120 × 12.8 = 1,536 N·m. Tractive force = 1,536 ÷ 0.30 = 5,120 N. The reasoning is that torque is multiplied by both reductions, then divided by the lever arm at the tyre.
Traps that cost marks
- Multiplying only the gearbox ratio and forgetting the final drive.
- Adjusting clutch free play at the wrong end of the cable or linkage.
- Claiming a differential makes both wheels turn at the same speed; it does the opposite on turns.
- Confusing engine torque with wheel torque.
- Ignoring transmission losses when a question asks for approximate road force.