Seven Percent Grade, Hot Linings and a Falling Gauge
You are driving a 2016 tractor pulling a loaded 53-foot van, grossing about 36,000 kg. Westbound on a two-lane mountain highway, the sign at the summit reads '7% grade, 9 km, winding.' You crest the top at 90 km/h in 10th gear, high range, engine brake on high. Two kilometres down, the trailer is pushing and you are holding 85-90 km/h with light, steady pressure on the treadle valve. At kilometre 4 you smell hot lining. The pedal now travels almost to the floor before you get any braking, the gauge reads 75 psi and the low-air light flickers each time you release the pedal. There is traffic behind you and a runaway lane 300 m ahead on the right.
1. Explain what the light steady pressure and the road speed are doing to the brakes, and why the pedal travel has grown.
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Light, continuous pressure is the worst possible way to use air brakes on a grade. It holds the shoes against the drum almost all the time, so the drums never get a chance to cool. Energy from a 36,000 kg combination descending 7 percent has to go somewhere, and continuous light drag puts all of it into the drums as heat, continuously, for kilometres.
The pedal travel then grows for a chain of reasons, all driven by that heat:
- Drums expand away from the shoes, so the shoes must travel further to make contact.
- Linings fade - as they overheat, the friction coefficient drops and some linings outgas a gas layer between lining and drum.
- More stroke is used before force is produced, so the pushrods extend further and the chambers approach the end of their travel.
So the long pedal is not 'air in the system.' It is a heat signature. The truck is using up its reserve of stroke at the same time as it is using up its reserve of friction.
Holding 85-90 km/h in 10th means the engine is nowhere near governed rpm, so the engine brake is contributing very little and the service brakes are doing essentially all the work.
2. What should have been done at the summit sign? Be specific about gear choice and braking technique.
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At the sign, before the descent starts:
- Read the sign and set the gear for the grade, not for the crest speed. A 7 percent grade with 36,000 kg requires a gear low enough that engine braking alone (plus the engine brake on high) holds a safe speed without service brakes. In practice that is a gear or two below what feels natural - the test is that you should not need the treadle to hold speed.
- Slow down at the top. Entering at 90 km/h already spends the brakes before the grade begins.
- Use snub braking, not drag braking. Apply firmly to shed roughly 10-15 km/h, then release completely and let the drums cool while the engine brake and gear hold the speed. Repeat. The release phase is not optional - it is the cooling phase.
- Keep engine rpm up where the engine brake and fan are effective, and watch the air gauge as a running instrument, not just at the bottom.
- Check the trailer brake operation at low speed before the grade, so you know the trailer is contributing.
Short version: the gear decides the descent; the brakes should be an occasional tool, not a continuous load.
3. At kilometre 4, with hot brakes, a long pedal, 75 psi and a runaway lane 300 m ahead, what do you do - and what do you not do?
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Do now:
- Aim for the runaway lane. It is 300 m ahead and it is engineered for exactly this. A truck with fading brakes and a falling gauge does not get a second chance at it. Use hazard lights and horn to warn traffic.
- Downshift progressively - one gear at a time as speed allows, getting the engine brake into a useful rpm band. Do not attempt a big jump that would over-rev.
- Keep whatever braking you have, applied smoothly and held - do not pump the pedal. Each release dumps air and each re-apply spends more, and the gauge is already down to 75 psi.
Do not:
- Do not pull the parking brake / tractor protection to 'stop it.' That applies the spring brakes at speed on a curve-swept grade - locked wheels, jackknife.
- Do not ride it out to the bottom hoping it recovers. Fade does not reverse while the drums are still being loaded.
- Do not ignore the gauge. From 75 psi you are roughly 15 psi above the low-pressure warning and about 30 psi above a spring brake application. If that happens on its own while you are at 80 km/h in a curve, you have lost control of the timing. That is why the runaway lane decision is made now, at 75 psi, not at 45 psi.