Transmission Systems (Clutch, Gearbox, Propeller Shaft, Differential)
Power flow and ratios
Transmission systems transfer engine torque to the road at useful speed and torque. Candidates must understand clutch operation (flywheel, friction plate, pressure plate, release bearing), manual gearbox shafts and synchronisers, propeller shafts with universal joints, final drive, and differential action. The clutch transmits torque by friction and must slip smoothly during engagement; a worn plate slips under load. Gear ratios multiply torque and reduce speed: overall ratio is the product of gearbox, transfer case and final drive ratios. The differential allows outer wheels to turn faster during a turn while delivering torque to both wheels.
Traps in questions
A frequent error is adding ratios instead of multiplying them. Another is confusing engine speed with road speed: lower numerical final drive gives higher road speed for the same engine speed but less torque at the wheels. Questions may ask why a vehicle judders when pulling away: a contaminated or worn clutch, notnecessarily the gearbox. Do not overlook propeller shaft phasing and universal joint wear as causes of vibration. Limited-slip differentials may require special lubricant; using ordinary oil can cause chatter.
Rule of thumb
For ratio calculations, multiply every stage: gearbox ratio × transfer ratio × final drive ratio. Torque rises by the same overall ratio while speed falls.
Worked example: a gearbox ratio of 3:1, transfer ratio 2:1 and final drive 4:1 give an overall ratio of 3 × 2 × 4 = 24:1. Engine torque of 200 N·m becomes 4,800 N·m at the wheels, ignoring losses.