Transmission, Clutch, Gearbox and Driveline
This domain tests how engine torque reaches the road: clutch, gearbox, propeller shaft, final drive, differential and half shafts. Expect clutch construction and adjustment, gearbox types, ratio calculations, differential purpose, and driveline layouts for front-wheel drive, rear-wheel drive and 4WD.
Clutch
Single dry plate: flywheel face, friction plate with torsional springs, pressure plate, diaphragm spring, release bearing and fork. Torque passes by friction; pressing the pedal disengages it. Pedal free play (typically 20-30 mm) must exist, or the clutch slips and the release bearing wears. Symptoms: slip (revs rise but speed does not), drag (gears crunch), judder on take-up.
Gearbox and driveline
- Sliding mesh: gears slide into mesh, noisy but simple; constant mesh: gears always in mesh, dog clutches select.
- Synchromesh: a baulk ring matches speeds before the dog engages, preventing crunch.
- Overdrive: a ratio below 1:1, so the output shaft turns faster than the engine, used for cruising economy.
- Differential: lets the outer wheel turn faster in a corner while still delivering torque to both wheels.
- Universal joints and CV joints handle shaft angle and plunge.
Worked example
Engine speed 3000 rpm, gearbox ratio 3.5:1, final drive 4.1:1, tyre rolling circumference 2.0 m. Overall ratio = 3.5 x 4.1 = 14.35. Wheel rpm = 3000 / 14.35 = 208.9. Road speed = 208.9 x 2.0 = 417.8 m/min = 25.1 km/h. Reasoning: ratios multiply along the driveline; a numerically larger overall ratio gives more torque at the wheels and less road speed.
Traps that cost marks
Inverting the ratio (engine speed divided by wheel speed, not the reverse). Believing a differential makes both wheels turn at the same speed. Ignoring clutch pedal free play. Calling a transfer case a differential, or forgetting to convert m/min into km/h.