Driving with Air Brakes: Stopping, Brake Fade and Downhill Control
Stopping distance has extra pieces
Total stopping distance = perception distance + reaction distance + brake lag distance + braking distance. Air brakes have a measurable lag (roughly half a second or more) while air travels from the treadle to the chambers, so an air-braked vehicle needs more room than a similar hydraulic-braked one. Read the road further ahead and start braking earlier.
Brake fade
Brakes convert motion into heat. Repeated or continuous application overheats drums and linings, and friction drops - that is fade. It is caused by use, not by a broken part, so the cure is prevention: correct speed and correct gear.
Downhill technique
- Check brakes and air pressure before the descent.
- Select a gear low enough that engine braking (and a retarder if fitted) holds the speed without service brakes - choose the gear before the hill.
- Use snub braking: a firm application to pull speed down, then release fully for several seconds to let the brakes cool. Repeat.
- Never ride the brakes continuously and never coast in neutral.
- If brakes fade, use a runaway lane rather than continuing.
Traps
- Using the trailer hand valve for downhill control is wrong and can cause a jackknife.
- 'Fanning' lightly and constantly is not the same as snub braking; it adds heat without cooling.
- The parking brake is not a downhill brake.
Rule of thumb: if you are using the service brake more than a few seconds at a time on a long descent, you are in too high a gear - stop, shift down and start again.