Brake Fade on a Six Per Cent Descent
You are driving a 39,000 kg five-axle combination loaded with a 53 ft van down a 6% grade on Highway 17. The descent runs 8 km, with a posted truck speed of 60 km/h, dry pavement, +4 °C, and light rain. You entered the grade at 95 km/h in 13th gear with the engine brake on high, to make up time. Halfway down you notice the speed creeping from 60 to 75 km/h even with the engine brake on and the service brake applied lightly and continuously. The pedal feels firm but the truck is not slowing. You can smell hot brake lining. Three hundred metres ahead, a pilot truck for an oversize load is crawling at 30 km/h in your lane, and the oncoming lane has traffic. The runaway lane is 600 m ahead on the right.
1. Explain what is happening to the brakes and identify the driving decisions that produced it.
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This is brake fade, and the firm pedal is the clue. There is air and there is no leak - the problem is heat. Continuous light application at 95 km/h down a 6% grade converts the combination's potential energy into heat faster than the drums can shed it. Once drums and linings are hot enough, the lining's coefficient of friction drops and the brakes stop converting motion into heat effectively. The hot-lining smell confirms it, and at the extreme the lining glazes or the drum cracks.
The driving decisions:
- Entering at 95 km/h. Brake capacity is rated at a speed, and the energy to be shed goes with the square of that speed. The whole descent should have started at the posted 60 km/h.
- 13th gear. At 95 km/h the engine is turning too slowly for the engine brake to hold anything. The gear must be low enough that the retarder alone holds the truck at 60 km/h - often three or four gears lower than cruise.
- Riding the service brake. Light continuous application never lets the drums cool and is the textbook way to cook a set of brakes.
The correct technique: slow to the posted speed before the crest, select the gear that holds it on the retarder alone, and use the service brake only in short, firm applications to shed about 10 km/h at a time, then release and let air cool the drums.
2. You are at 75 km/h and still accelerating, 300 m behind a 30 km/h pilot truck, with the runaway lane 600 m ahead. Decide and justify.
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Commit to the runaway lane now.
Reasoning:
- The service brake cannot do the job. Faded brakes will not produce the deceleration needed to go from 75+ km/h to 30 km/h in 300 m on a downgrade. A firm application now will feel like nothing, and you will discover that too late.
- Downshifting will not save you. At 75 km/h a downshift can over-rev the engine or damage the driveline, and the retarder is already at maximum. Engine braking capacity falls as speed rises.
- The ramp only works if you arrive in control. An escape ramp is an ascending gravel bed designed to use gravity and rolling resistance to stop a heavy vehicle. It needs you straight, upright and deliberate - not swerving around a pilot truck at 75 km/h into oncoming traffic, which turns a brake problem into a head-on.
Actions: hazards on, horn, both hands on the wheel, aim for the centre of the ramp, keep the truck straight, and stay off the service brake once you are in deep gravel - braking there can pull the tractor sideways or trip the unit. Do not shut the engine off; you would lose the retarder and the steering assist.
3. After you have stopped in the gravel, what do you do - and what must you not do?
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Do not:
- Set the parking brakes immediately. Clamping red-hot drums can crack them and the heat can damage seals and springs. Leave the transmission in gear, engine off, and chock the wheels.
- Try to nurse the truck to the next terminal. A fade event means the brake system has been overheated; the drums may be heat-cracked and the lining glazed.
- Walk out into the ramp casually. It is a live traffic facility and another truck may follow you in.
Do:
- Watch for fire. Hot drums ignite grease and debris; keep the extinguisher out and check the wheel ends.
- Chock and secure the unit, then call dispatch and maintenance.
- Have the brakes inspected: drums for heat cracks and blueing, linings for glazing and thickness, slack adjusters and pushrods, then a complete air brake test before the vehicle moves.
- Complete the inspection record and the incident report, including the conditions and the decisions that led to it. The fix is the speed at the top of the grade and the gear, not a set of new brakes.