Brake Fade on the Long Descent from Bokor
Your three-axle rigid tipper is loaded to about 18 tonnes and you are descending a 6 km continuous grade of about 7% on the road down from Bokor. You are in 4th of 8 gears at 45 km/h, holding speed with repeated applications of the footbrake. After about 2 km the pedal begins to feel long and spongy, you can smell hot friction material, and the air-pressure gauge has fallen from 7.5 bar to 5.5 bar after a series of applications. The engine brake is switched on but the retarding effect now feels weak. There is a shallow gravel run-off on the outside of the next left-hand bend, and beyond it a steep drop. Two cars are following about 40 m behind you. The nearest safe lay-by is 800 m ahead.
1. Explain what is happening to the braking system and state the immediate priority.
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Two things are happening at once, and both are caused by using the footbrake to hold speed rather than to slow the vehicle.
Brake fade. Continuous light-to-medium applications overheat the drums and linings. Once they are hot, the lining material loses friction and gives off gas that forms a cushion between lining and drum. The pedal goes long and spongy and the same pedal force produces less and less braking. Pumping harder does not fix it - it adds more heat.
Air pressure depletion. Each application uses air, and the compressor cannot recharge fast enough while you are braking repeatedly at only 45 km/h. Falling from 7.5 to 5.5 bar is a warning that you are drawing on your reserve. Below the low-pressure warning threshold the spring brakes will apply automatically, and that will happen on a bend, at speed, with cars 40 m behind.
Immediate priority: get the speed down now, using the engine, and stop at the lay-by 800 m ahead. Select the next lower gear while you still have brakes, let the engine brake and compression do the work, and use the service brake only in short, firm applications with a pause between them so the drums can cool.
2. How should the descent be driven for the remaining distance?
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The rule is simple: the gear you descend in is the gear you would climb the hill in. Choose it before the grade, not during it.
- Select a low gear early and stay in it. In a loaded tipper on 7% you should be in a gear where the engine braking alone holds a steady, slow speed without the footbrake.
- Use the service brake in short, hard, separate applications, dropping 5–10 km/h then releasing fully. Continuous light pressure is what destroys the brakes; long releases let air and heat recover.
- Watch the air gauge constantly. If it will not hold above roughly 6 bar and keep recovering, stop at the lay-by, chock the wheels, and let the system recharge. Do not resume until pressure builds and holds.
- Do not cool drums with water. Cold water on overheated drums can crack or distort them.
- Keep the exhaust brake on and use engine rpm, not road speed, to control the descent.
- Let the following cars past at the first safe straight, so they are not sitting behind a slowing heavy vehicle on a blind bend.
3. Assume the brakes fail completely on the approach to the left-hand bend. What do you do?
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Once braking is gone, the only tools left are gears, geometry and the terrain - and decisions must be made in that order, immediately.
- Warn everyone. Hazard lights, horn held on, headlights on main beam, so the cars behind and anyone ahead know something is wrong.
- Downshift through the gears one at a time, using the engine and exhaust brake to bleed off speed. Do not skip gears; a missed shift or an over-revved engine removes the only retardation you have left.
- Use the gravel run-off on the outside of the bend. That is exactly what it is for. Aim the truck into it rather than trying to force the bend, because a loaded tipper that runs wide at that point goes over the edge.
- If there is still not enough room, scrub off speed deliberately against the rock face or embankment on the inside, at a shallow angle, sacrificing the truck's bodywork rather than going over the drop. Keep the cab clear.
- Do not choose a head-on or a rollover as a way of stopping. A controlled scrape against terrain is survivable; a 20-tonne truck over a cliff edge is not.