Transmission
Domain focus
Transmission tests how engine torque reaches the wheels and how speed and torque change. Expect clutch, manual gearbox, propeller shaft, differential, final drive, and CV joint questions.
Core rules
- The clutch connects and disconnects engine and gearbox. The pressure plate clamps the friction disc to the flywheel; the release bearing acts on the diaphragm when the pedal is pressed.
- Gear ratio = driven gear teeth / driving gear teeth. A higher numerical ratio multiplies torque and reduces output speed.
- Synchronisers match gear speeds before engagement. The differential lets driven wheels turn at different speeds in a turn while still transmitting torque.
- CV joints allow drive through changing angles on independent suspension. Never hold a vehicle on the clutch on a slope; use the handbrake.
Worked example
A driving gear has 20 teeth and the driven gear 40 teeth. Ratio = 40/20 = 2:1. If input torque is 100 Nm, ideal output torque is 200 Nm, while output speed is half. The reasoning is conservation of power: torque rises as speed falls, ignoring losses.
Traps
Inverting the ratio, confusing clutch slip with dragging, assuming a differential is only a speed reducer, and ignoring lubricant level in the gearbox are common errors.