Heavy Vehicle Braking, Speed Control and Stability
What this domain tests
Braking, speed control and stability tests whether you can stop a heavy vehicle within the distance available and keep it upright and in line while you do it.
Stopping distance
Total stopping distance = thinking distance + braking distance. Braking distance rises with the square of speed: double the speed and you need four times the distance. Thinking distance rises in proportion. The arithmetic is not intuitive, and examiners test it.
Brake systems
- Air brakes: apply progressively and hold; do not pump or fan the pedal, which wastes air.
- Check pressure before every trip and watch the gauge on long descents.
- ABS stops the wheels locking; it does not shorten the distance on gravel and it does not beat physics.
Descents and fade
Brake fade comes from heat. On a long downhill, choose a low gear before the crest, use engine braking and any retarder, and use the service brake in short applications. Never coast in neutral.
Stability
- Slow down before the bend, not in it: braking in a curve transfers weight and can tip a high load.
- Keep the centre of gravity low and the load restrained; an unrestrained load shifts on the first hard brake.
- On a slippery surface an unladen truck brakes light at the rear and can jackknife; brake gently and early.
- In rain or on gravel, double the gap and cut speed by a third.
Worked example
Dry braking distance at 30 km/h is roughly 9 m. At 60 km/h it is about 35 m - the speed doubled and the distance roughly quadrupled. A bend you can pull up for at 30 is not a bend you can pull up for at 60.
Traps
Riding the brakes downhill; relying on ABS to fix a wet road; coasting in neutral to save fuel.