Transmission
What the exam tests
Transmission covers the clutch, gearbox, propeller shaft, universal joints, final drive, differential and drive layouts. Questions test power flow, gear ratios, torque multiplication and the function of each component.
Core rules and concepts
- The clutch connects and disconnects engine and gearbox. It must engage smoothly and transmit torque without slip.
- A manual gearbox provides stepped ratios. Lower gears multiply torque and reduce speed; higher gears do the opposite.
- A torque converter uses fluid coupling and a stator to multiply torque at low speed.
- The propeller shaft transmits torque across changing angles; universal joints allow angular movement and a slip joint allows length change.
- The final drive provides a fixed reduction. The differential allows inner and outer wheels to turn at different speeds during a turn.
- Overall ratio = gearbox ratio × final drive ratio.
Worked example
An engine turns at 3,000 rpm. Gearbox ratio is 3:1 and final drive is 4:1. Tyre circumference is 2.0 m. Wheel rpm = 3,000/(3 × 4) = 250 rpm. Road speed = 250 × 2.0 m/min = 500 m/min = 30 km/h. In top gear at the same engine speed, a 1:1 gearbox ratio would give 750 wheel rpm and 90 km/h, showing the trade-off between torque and speed.
Traps that cost marks
- Multiplying ratios when you should divide, or forgetting the final drive.
- Using engine rpm as wheel rpm.
- Confusing clutch slip with normal clutch take-up.
- Assuming a differential always splits torque equally; it splits torque according to traction and resistance.