Transmission
This domain tests how torque passes from the engine to the road wheels, and the purpose of the clutch, gearbox, propeller shaft, differential and final drive.
Clutch and gearbox
- The clutch transmits torque through friction, allows smooth take-up and disconnects the engine for gear changes.
- A single dry-plate clutch has a flywheel, friction plate, pressure plate, release bearing and release fork.
- Pedal free play is typically 15–25 mm; too little causes slip, too much causes incomplete disengagement.
- Gear ratio = driven gear teeth ÷ driving gear teeth. A low gear has a high numerical ratio, giving more torque and less speed.
- Synchromesh rings equalise shaft speeds before engagement.
Shafts, differential and axles
- The propeller shaft needs universal joints to allow angle changes and a slip joint to allow length changes.
- The differential lets the outer wheel turn faster on a bend.
- The crown wheel and pinion form the final drive, reducing speed and multiplying torque again.
- A live axle drives the wheels; a dead axle only supports them.
Worked example
A first gear ratio is 3.5:1 and the final drive is 4.0:1. Total ratio = 3.5 × 4.0 = 14:1. With 120 Nm of engine torque, wheel torque is 120 × 14 = 1680 Nm, ignoring losses. Wheel speed is one-fourteenth of engine speed. The reasoning is that each reduction stage multiplies torque and divides speed.
Traps
- Inverting the gear ratio by dividing driving teeth by driven teeth.
- Claiming the differential creates extra torque; it only allows different wheel speeds.
- Ignoring clutch free play.
- Forgetting the final drive when calculating total ratio.