Transmission and Drivetrain
Clutches and manual gearboxes
A clutch transmits torque through friction between flywheel, disc and pressure plate, released by hydraulic or cable actuation. Symptoms map to causes: slip under load means worn disc or weak pressure plate; drag or difficult engagement points to hydraulics, release bearing or a warped disc; judder suggests contamination or uneven mating surfaces. Manual gearboxes use constant-mesh gears with synchromesh cones to match speeds.
Automatic transmissions
Torque converters multiply torque at low speed through fluid coupling and lock up at cruise for efficiency. Planetary gearsets provide ratios by holding or driving members through clutches and bands. CVTs use a belt or chain between variable pulleys, and dual-clutch units use two clutches with pre-selected gears. Fluid specification is critical: the wrong fluid changes friction characteristics and can destroy the unit.
Final drive and driveshafts
The differential allows wheels to turn at different speeds while splitting torque, and the final drive ratio sets the relationship between engine speed and road speed. Constant velocity joints at the wheel end and universal or CV joints on propshafts must be checked for play, torn boots and vibration. Four-wheel-drive systems add transfer cases, couplings and differential locks.
Rule of thumb: road speed equals engine rpm multiplied by tyre circumference, divided by the overall gear ratio (gearbox ratio times final drive), with the units converted consistently. Traps include confusing gearbox ratio with overall ratio, diagnosing a torque converter fault from a fluid colour alone, and forgetting tyre size changes speedometer calibration.