Transmission, clutch, gearbox, final drive and differential
Transmission, clutch, gearbox, final drive and differential covers how engine torque is matched to road speed and how drive is engaged, interrupted and distributed to the wheels.
Clutch and gearbox
A single-plate clutch uses a diaphragm spring to clamp the friction disc between flywheel and pressure plate. The release bearing acts on the spring when the pedal is pressed. Correct pedal free play prevents slip and release-bearing wear. Manual gearboxes use constant-mesh gears with synchroniser rings; automatic gearboxes use planetary gearsets and a torque converter.
Final drive and differential
The crown wheel and pinion give a fixed reduction; the differential allows inner and outer wheels to turn at different speeds in a corner while splitting torque.
Worked example. A gearbox ratio of 3.5:1 and final drive of 4.1:1 give an overall ratio of 3.5 × 4.1 = 14.35:1. With 200 Nm at the engine, wheel torque = 200 × 14.35 = 2,870 Nm, ignoring transmission losses. Speed is reduced by the same factor, so the wheels turn 14.35 times slower than the engine.
Diagnosis pointers
Clutch slip shows as rising engine speed without road speed and is tested by applying the handbrake in a high gear. Judder suggests oil contamination or a warped flywheel. Gear whine in one ratio points to that gear pair; whine in all ratios points to bearings or the final drive.
Traps
Riding the clutch. Fitting a clutch disc backwards. Mixing gearbox oil with automatic transmission fluid. Ignoring a collapsing release bearing. Forgetting to check the pilot bearing.