Downhill Descent With a Loaded Trailer
You are descending a long mountain pass of about 8 km with a gradient of 9%. You tow a braked trailer of 1,200 kg behind a diesel car. The car has a manual gearbox and the trailer has an overrun brake. Halfway down, you notice a faint burning smell and the brake pedal feels slightly longer than at the start. Your speed has crept up despite steady braking. The engine temperature gauge is normal. There is a lay-by 300 m ahead.
1. What is happening and what should you do immediately?
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The brakes are overheating - the burning smell and the longer pedal travel are classic signs of brake fade, where heat reduces friction and can boil brake fluid. Steady use of the footbrake on a long 9% descent is the cause.
Immediate action: stop at the lay-by, select a low gear and use engine braking, and let the brakes cool. Do not pour water on hot discs or drums - thermal shock can warp or crack them. Do not continue riding the brakes. Once stopped, let them cool, check for fluid leaks, and only continue when the pedal feels firm again, using a low gear to hold speed and braking in short, firm applications rather than continuously.
2. How does the trailer's overrun brake behave on this descent, and what does that mean for your technique?
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An overrun brake works on the push of the trailer against the car. On a descent, gravity pushes the trailer forward, so the overrun brake can apply by itself and stay partly applied, adding heat and drag. If you brake hard and continuously, the trailer brake works hard too and can overheat alongside the car's brakes.
Technique: choose a low gear before the descent so the engine holds the combination back, keep speed low and steady, and brake in short applications rather than dragging. Avoid sudden sharp braking that slams the overrun brake on. Keep a longer gap so you are not forced into heavy braking, and watch for the trailer starting to push the car - if it does, slow further and use a lower gear.