Rear brake fade on a steep descent with a pillion
You ride a 125 cm³ A1 motorcycle down a 2 km, 12% mountain pass with a pillion passenger (55 kg) and a 10 kg top box. The combined load is near the bike's maximum. The road is damp in the shade. Your bike has a single front disc and a drum rear brake. After 1.2 km of continuous descending at 55–65 km/h, the rear brake pedal feels spongy and travels further; the front lever is still firm. The engine is in fourth gear and screams at 8,000 rpm. A sign shows a 50 km/h limit and a sharp left bend 80 m ahead. The passenger is calm but holding the grab rail loosely.
1. What is happening to the rear brake, and why is it happening?
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The rear drum is overheating and fading because you have been dragging it down a 12% grade with near-maximum load. A 12% slope adds roughly 0.12 × total mass × g; with a combined mass near 265 kg that is about 310 N pushing you downhill, so the brakes are doing a lot of work. The spongy pedal means less braking force and a risk of complete loss. Do not keep riding the rear brake. Shift down to second or third to use engine braking, use the front brake more (weight transfers forward) but gently on the damp surface, and stop at a safe pull-off to let the drum cool.
2. How should you manage speed and gears before the bend?
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Before the bend, reduce to 30–40 km/h using engine braking and both brakes early. Select second gear for the steep section; at 8,000 rpm in fourth you have little engine braking and no reserve. The top box and passenger load the rear and raise the centre of gravity, so brake in a straight line and avoid trail-braking into the wet bend. Keep a larger following gap. Do not pour water on the hot drum; let it cool naturally and check adjustment and shoe condition before continuing.
3. What should you tell the passenger, and what if the rear brake fails completely?
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Tell the passenger to sit still, hold the grab rail firmly, and lean with you; no sudden movements. If the rear brake fades completely, use the front brake and engine braking, and if necessary choose a safe uphill escape or ditch rather than entering a blind bend too fast. After stopping, check the drum adjustment and shoes; if the brake is not restored, do not continue the descent. The legal load limit and tyre pressures should have been checked before the ride.