Transmission: Clutch, Gearbox, Propeller Shaft and Differential
This domain covers the chain that carries engine power to the road wheels: clutch, gearbox, propeller shaft, final drive, differential and axles.
Clutch
A single-plate dry clutch has the flywheel, a friction-lined disc, a pressure plate, a diaphragm or coil spring and a release bearing. It connects and disconnects the engine smoothly, allows gear changing and protects the drive line from shock. Pedal free play is essential: too little and the clutch slips and the release bearing wears; too much and it does not fully disengage, so gears clash. Clutch slip shows as rising engine speed without road speed; judder points to a contaminated or warped disc.
Gearbox
A gearbox provides neutral, reverse and several ratios so that engine speed suits the load. First gear gives low speed and high torque. Sliding-mesh, constant-mesh and synchromesh designs differ in how the gears engage; synchromesh cones equalise speeds before engagement. Gear ratio equals driven teeth divided by driving teeth. Gearboxes may be manual, or automatic with epicyclic gears and a torque converter.
Propeller shaft and universal joints
Except in front-wheel-drive vehicles, a propeller shaft carries torque to the rear axle. Universal joints allow for angular changes as the axle moves, and a slip joint allows length changes. Hotchkiss drive uses the leaf springs to take the driving thrust; a torque tube encloses the shaft and takes thrust through a ball joint.
Differential and axles
The crown wheel and pinion give the final drive reduction. In the differential, the planet pinions and sun gears divide speed between the wheels, allowing the outer wheel to turn faster on a bend while both receive torque. An open differential sends equal torque to both wheels, so the wheel with less grip spins. Live axles may be semi-floating or full-floating.
Worked example
First gear is 4:1, the final drive 4.5:1, engine torque 100 N·m. Overall ratio = 4 x 4.5 = 18:1, so the torque at the axle is about 1800 N·m ignoring losses, while the speed of the wheels falls to one eighteenth of the engine speed.
Traps that cost marks
Thinking the differential makes the wheels turn at different speeds in a straight line; forgetting that an open differential gives the spinning wheel no traction; ignoring clutch free play.