Transmission, Clutch, Gearbox, Differential and Axles
Transmission, Clutch, Gearbox, Differential and Axles
This domain covers how engine torque is matched to road speed and how drive reaches the wheels while allowing them to turn at different speeds. The exam tests clutch operation, gear ratio calculation, final drive, differential action and axle layout.
Core rules
- The clutch connects and disconnects engine and gearbox smoothly; a single-plate dry clutch uses a pressure plate, friction disc and release bearing. Free play must be correct: too little causes slip and wear, too much causes incomplete release and gear clash.
- A gearbox provides torque multiplication and speed reduction. Gear ratio = driven gear teeth ÷ driving gear teeth; a reduction ratio greater than 1 increases torque and reduces output speed.
- Overall ratio = gearbox ratio × final drive ratio. The differential allows inner and outer wheels to rotate at different speeds during a turn while still delivering drive.
- Live axles drive the wheels; dead axles only support them. Front-wheel-drive layouts commonly use a transaxle.
Worked example
An engine turns at 3000 rpm. The selected gearbox ratio is 2:1 reduction and the final drive is 4:1. Overall ratio = 2 × 4 = 8:1. Wheel speed = 3000 ÷ 8 = 375 rpm. Ideal torque at the wheels is 8 times engine torque, ignoring losses. Reasoning: each reduction stage divides speed and multiplies torque, so the effects compound.
Traps
- Inverting the gear ratio and predicting a speed increase where a reduction occurs.
- Saying the differential makes both wheels turn at the same speed; it permits different speeds.
- Confusing clutch slip with brake drag.
- Forgetting that tyre rolling radius also affects road speed for a given wheel rpm.