Steering, Suspension, Brakes and Wheels
This domain covers the systems that keep a vehicle pointing where the driver wants and stopping when asked: steering geometry, suspension types, brake hydraulics and wheels/tyres. Expect definitions of camber, caster, toe, steering ratio and the Ackermann principle, brake fade and hydraulic operation, and tyre-size decoding.
Steering geometry
- Camber: wheel tilt from vertical seen from the front. Excess positive camber wears the outer edge of the tyre.
- Caster: tilt of the steering axis from vertical seen from the side. Positive caster gives self-centring and straight-line stability.
- King pin inclination: inboard tilt of the steering axis, giving directional stability and light steering effort.
- Toe-in or toe-out: the front of the wheels closer or further apart than the rear. Wrong toe scrubs the tread in a feathered pattern.
- Ackermann principle: the inner wheel steers a larger angle than the outer because it runs on a smaller radius.
Suspension and brakes
- Leaf springs for commercial vehicles; coil springs, McPherson strut, double wishbone and torsion bars for cars. Shock absorbers damp oscillation; they do not carry vehicle weight.
- Drum brake: self-energising and enclosed, but prone to fade. Disc brake: better cooling, self-adjusting, needs pads and caliper seals.
- Hydraulic brake: pedal, master cylinder, lines, wheel cylinder or caliper. Brake fluid is hygroscopic; old fluid boils and the pedal sinks (vapour lock). ABS prevents wheel lock during hard braking.
Worked example
Tyre marked 165/70 R14 81T. Section width = 165 mm. Sidewall = 70 percent of 165 = 115.5 mm. Rim = 14 inch = 355.6 mm. Overall diameter = 355.6 + 2(115.5) = 586.6 mm, radius 293.3 mm. Load index 81, speed rating T (190 km/h). Reasoning: the aspect ratio is a percentage of section width, not a fixed millimetre dimension.
Traps that cost marks
Swapping camber and caster. Assuming larger wheels always improve grip. Using only tread depth and ignoring tyre age. Topping up with an incompatible brake fluid.