Seven Kilometres of Seven Percent
You are driving a 40-foot coach with 38 passengers and their luggage down a 7 km descent averaging 7 percent. It is 4 °C and drizzling, and water is running across the pavement in the shaded sections. The posted limit is 80 km/h. At the top you select a gear and start down at 50 km/h with the engine retarder on its highest setting. Two kilometres in, your speed has crept to 72 km/h, the retarder is still engaged, and the brake pedal now travels noticeably further before it bites. A loaded pickup is 60 m ahead and has just braked hard for a curve. You can smell hot brake lining.
1. Your speed climbed from 50 to 72 km/h despite the retarder. Explain what is happening and state exactly what you do in the next 15 seconds.
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The retarder is absorbing only part of the energy that a 7 percent grade adds at this weight. Below a certain speed it holds the load; once you are rolling faster it cannot, and the service brakes have quietly been making up the difference. That is the trap on a descent: the retarder still feels like it is working while the speed climbs.
Next 15 seconds:
- Slow down deliberately. One firm application down to about 40 to 45 km/h, hard enough that you feel it, then release. Light sustained pressure is what glazes linings and cooks drums.
- Get into a lower gear that holds that speed without brakes. Use the manual hold on an automatic; the correct gear is the one that requires no pedal to maintain the target speed.
- Leave the retarder at maximum and stop trusting it as the primary control.
- Open the gap from the pickup and identify the next pullout now, before you need it.
Rule of thumb: you should be able to descend the whole grade without the drums getting hotter than you can hold your hand near.
2. Longer pedal travel plus the smell of hot lining, with 5 km of grade left. Do you continue, and what is the procedure?
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That pair of symptoms is brake fade developing, not a nuisance. Fade means the brakes are converting less pedal force into stopping force exactly when you need more of it. Continuing at speed with 38 passengers and 5 km to go is the wrong risk.
In order:
- Do not pump the pedal. Downshift immediately; the engine and retarder are now your primary braking.
- Take the first pullout, chain-up area or wide shoulder you reach, even if it is earlier than planned.
- Stop, park brake on, wheels chocked, and let the drums cool in air. Never pour water on hot drums - warping and cracking.
- Walk around. Check for smoke, a dragging wheel, or one drum far hotter than the others; look for a seized caliper or a slack adjuster that has backed off.
- Call dispatch and report the condition. Do not resume until the brakes are inspected and the cause is known - 'it cooled down' is not a diagnosis.
If no pullout arrives before the fade worsens, slow hard and early and use the shoulder. A controlled stop against a barrier beats an uncontrolled one at the curve.
3. The pickup is 60 m ahead at 72 km/h. Assess that gap and set a following distance for the rest of the descent.
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At 72 km/h you cover 20 m every second, so 60 m is 3 seconds - under the 4-second minimum for a bus in good conditions, and these are not good conditions and not level ground.
The factors stack:
- Wet pavement at 4 °C - longer stopping distance, and possible ice where runoff crosses the road.
- Downgrade - gravity adds to stopping distance.
- Load - 38 passengers and luggage add mass the brakes must dissipate.
- No warning - the pickup's brake lights are your only notice before a curve.
Set 6 seconds or more: 20 m/s × 6 s = 120 m. That gap lets you brake once, firmly, instead of lightly for a long time, which is what keeps the drums cool. If traffic keeps cutting into the space, drop your speed rather than surrender the gap - at 50 km/h the same 6 seconds is only 83 m, which you can actually hold on this road.