Automotive Transmission, Steering, and Suspension Systems
What this domain tests
This section examines the path of power from the engine to the road wheels and the systems that keep the vehicle stable and controllable. Expect questions on clutch operation, gear ratios, final drive, differential action, steering geometry and the main suspension types.
Transmission rules
- A clutch transmits torque through friction and must slip when starting, then lock fully; free play at the pedal is essential to prevent release-bearing wear.
- Gear ratio = driven gear teeth ÷ driving gear teeth. A lower numerical ratio gives higher speed and lower torque.
- Final drive ratio multiplies gearbox ratio; the differential lets wheels turn at different speeds while splitting torque.
- In a manual gearbox, the input shaft carries the clutch plate, the countershaft carries the fixed gears, and the output shaft carries the sliding or synchromesh gears.
Steering and suspension
- Steering ratio = steering wheel angle ÷ road-wheel angle; a higher ratio reduces effort but slows response.
- Camber is the inward or outward tilt viewed from the front; caster is the steering-axis tilt viewed from the side; toe is the difference in front and rear track of the wheels.
- MacPherson strut, double wishbone, torsion beam and leaf spring are the common layouts; shock absorbers control spring oscillation and do not carry the vehicle's weight.
Worked example
First gear is 3.5:1, final drive 4.0:1, tyre rolling radius 0.30 m, engine torque 150 N·m. Wheel torque = 150 × 3.5 × 4.0 = 2,100 N·m. Tractive force = torque ÷ radius = 2,100 ÷ 0.30 = 7,000 N. The reasoning is simply to multiply every reduction in series, then divide by the lever arm at the tyre.
Traps that cost marks
Mixing camber with caster, giving toe only in millimetres when the specification is in degrees, and forgetting that the differential allows speed difference but not torque difference under normal driving.