Braking Systems and Vehicle Control
This domain tests the hydraulic braking circuit, the difference between disc and drum brakes, the role of ABS and the parking brake, and how speed, reaction time and friction decide stopping distance.
Hydraulic system
Pedal → master cylinder → pipes and hoses → wheel cylinders or calipers. By Pascal's law, pressure is equal throughout, so a larger wheel-cylinder piston gives more force. Brake fluid DOT 3 or DOT 4 is glycol-based and hygroscopic; absorbed water lowers the boiling point and causes fade. Bleed the system to remove air, because air compresses. Discs cool better and self-adjust; drums self-energise more but fade sooner.
Control aids
ABS modulates pressure so wheels do not lock, keeping steering control, but it does not always shorten stopping distance on loose gravel. The parking brake is cable-operated and must have the specified lever travel. A load-sensing valve reduces rear braking when the vehicle is lightly laden.
Worked example
At 40 km/h (11.1 m/s) with a 1 s reaction time and μ = 0.7: thinking distance = 11.1 m; braking distance = v² ÷ (2μg) = 123.4 ÷ 13.7 ≈ 9 m; total ≈ 20 m. Stopping distance is thinking plus braking, and braking distance rises with the square of speed - double the speed, four times the distance.
Traps
Topping up with mineral or engine oil, which destroys the seals; checking fluid level without checking pad thickness; assuming ABS always shortens the stop; ignoring pedal free play; road-testing a car with a spongy pedal, which signals air in the system.