Brake fade on the Dschang escarpment
At 16:20 a six-axle tipper carrying 33 t of sand is descending the Dschang escarpment on the N5 toward Bafoussam. The road is wet after a storm, with a series of tight bends and a 9% gradient. The truck has a working engine brake and retarder, and the driver selected 4th gear at the top. Halfway down the driver notices the service pedal travels almost to the floor, a burning smell, and the speed has crept from 40 to 65 km/h despite remaining in the same gear. The retarder has begun to fade. There is a loaded fuel tanker about 80 m behind and a line of cars ahead. The driver's instinct is to hold the brakes harder and stay in 4th. There is no escape lane on this section; the only run-off is a rocky shoulder.
1. Diagnose what is happening and explain why holding the brakes harder is the wrong response.
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The combination is in brake fade. The long, soft pedal, burning smell and rising speed in the same gear show that the drums and linings are overheating. As lining temperature rises, friction falls, so the same pedal effort produces less retardation. The pedal travel may also be lengthening because the slack adjusters have ratcheted out to compensate for the heat and wear.
Holding the brakes harder makes the problem worse. Continuous application keeps the linings pressed against already-hot drums, adding heat faster than the airflow can remove it. That can glaze the linings, crack or distort the drums, boil off any moisture in the system and, in the worst case, leave almost no braking force. It also removes the cooling intervals that would let the drums recover.
The speed creeping from 40 to 65 km/h is the critical signal: the current gear and braking effort are no longer controlling the descent. The driver needs to reduce the energy input immediately, not add more heat.
2. Set out the sequence of actions for the next 30 seconds and the next 2 minutes.
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Next 30 seconds:
- Shift down immediately, 4th to 3rd and then to 2nd as soon as the speed allows. Do not wait for the speed to fall first; the lower gear is what creates engine braking.
- Put the retarder and engine brake on their highest usable setting, respecting the manufacturer's limit for the road speed.
- Apply the service brake in firm, short stabs and release between them, rather than riding the pedal. This gives the drums brief cooling intervals.
- Switch on hazard lights and sound the horn to warn the cars ahead and the tanker behind. Keep to the right and do not attempt to overtake.
- Look ahead for the rocky shoulder and any flatter section, and plan a controlled use of it if speed continues to rise.
Next 2 minutes:
If the speed is still climbing after the downshift and stabs, use the rocky shoulder deliberately: steer onto it at a shallow angle, keep the wheels straight, and let the rough surface and gradient scrub speed. Avoid the tanker's path and warn it early. Once stopped, set the parking brake, chock the wheels, switch off and let the brakes cool. Do not pour water on hot drums. Inspect for cracked drums, glazed linings and air leaks before moving again; if in doubt, call for assistance.
3. What should the driver have done before starting the descent, and what should be checked after the truck has stopped?
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Before the descent the driver should have stopped at the crest, carried out a brake test, and checked pushrod travel and air pressure. The correct gear should have been selected before the gradient, not after the speed had built: for a 33 t load on a wet 9% descent, 3rd or even 2nd may be appropriate, with the retarder and engine brake already on. The driver should have noted the absence of an escape lane, set a maximum descent speed in mind, and kept well back from the vehicle in front so there was room to use the shoulder if needed.
After stopping, check the following:
- Drums and linings: look for cracking, blueing, glazing and oil contamination. Do not touch them until cool.
- Air system: pressure, audible leaks, and pushrod travel on every chamber. A long stroke after a hot descent can indicate a brake out of adjustment.
- Wheels and tyres: heat, tyre damage and any sign of bearing trouble.
- Load: the sand may have shifted; check the body and tailgate.
If any drum is cracked, a lining is glazed or the pushrod travel is over limit, the truck must not continue. The descent should be resumed only after repair, in a lower gear and at a lower speed.