Braking, Steering, and Suspension Systems
Domain 4 tests hydraulic braking, steering geometry and suspension types - the three systems that decide whether a vehicle stops and steers safely.
Brakes
Hydraulic brakes obey Pascal's law: pressure is equal throughout, so force = pressure × area. A larger caliper piston area multiplies clamping force; a smaller master-cylinder bore and a higher pedal ratio raise line pressure. Bleeding removes air, which is compressible and gives a spongy pedal. Brake fade is friction loss from overheating; ABS modulates pressure to stop wheel lock.
Steering and suspension
Rack-and-pinion is standard on cars; recirculating-ball on heavier vehicles. Steering ratio = steering-wheel angle ÷ road-wheel angle. Geometry: camber (wheel tilt seen from the front), caster (steering-axis tilt seen from the side, gives self-centring), toe-in (front edges closer together), kingpin inclination. Suspension may be leaf spring, coil spring, torsion bar, MacPherson strut or double wishbone; the shock absorber damps movement - it does not carry load.
Worked example
Pedal force 200 N, pedal ratio 5 : 1, master-cylinder area 5 cm², each front caliper piston area 20 cm². Pushrod force = 200 × 5 = 1000 N. Line pressure = 1000 ÷ 5 = 200 N/cm². Force at one caliper = 200 × 20 = 4000 N. So the system multiplies driver effort twenty times at each front caliper.
Traps that cost marks
Confusing camber with caster; quoting toe-in in degrees when the specification is in millimetres; saying a shock absorber supports weight; mixing DOT 3/4 glycol fluid with DOT 5 silicone; forgetting to pump the pedal after fitting new pads.