Long descent from Velebit with a braked trailer
From the Velebit pass down to the coast, a driver descends about 12 km of continuous 8% gradient with a 1,200 kg braked trailer behind a 1,650 kg diesel car. The road is wet and the general speed limit for a car with trailer is 80 km/h; a sign at the top prohibits overtaking for vehicles with trailers. The driver selects fifth gear to save fuel and keeps light pressure on the footbrake almost continuously, because the car speeds up whenever he releases it. About halfway down, the brake pedal feels soft and travels further, and there is a hot, acrid smell from the trailer wheels. The trailer has an overrun brake, and its handbrake is fully released. Traffic behind is close.
1. What is happening to the braking system, and why is the driver’s technique wrong?
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The car and trailer are converting a large amount of potential energy into heat. A descent of about 960 m with roughly 2,850 kg moving down the grade gives millions of joules that the brakes must dissipate. Continuous light braking keeps the pads and discs hot for minutes at a time, so the friction material can fade and the brake fluid can approach boiling. A soft, longer pedal is a warning of exactly that. The trailer’s overrun brake is also working continuously and overheating, hence the acrid smell.
Fifth gear is the core mistake: it gives almost no engine braking, so the footbrake has to do all the work. The correct technique on a long descent is a low gear, using engine braking to hold speed, with short, firm brake applications rather than riding the pedal. The wet road reduces tyre grip and cooling is not a substitute for correct gear selection.
2. What should the driver do now, and what should he have done at the top of the descent?
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He should stop as soon as there is a safe place, using the brakes as little as possible, switch on hazard lights and let the brakes cool. He should not pour cold water on hot discs or drums. A soft pedal means the car should not continue until the brake system has been checked; if the pedal does not recover, call for assistance. The trailer brakes and wheel temperatures should also be checked.
At the top he should have selected a low gear before the gradient, checked the trailer brake adjustment, tyre pressures and load, and kept well below 80 km/h. He should have used the engine to control speed and applied the brakes in short, firm bursts. The no-overtaking sign also required him to stay behind slower vehicles rather than use speed to pass.
3. How should the sign and the wet conditions change his plan for the rest of the descent?
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The no-overtaking sign applies to vehicles with trailers, so he must not overtake even if a slower vehicle is holding him up. He should keep a large gap, stay right and accept a lower speed. On a wet road, grip is reduced and the trailer’s overrun brake can lock a wheel more easily, so braking must be smoother and steering inputs gentler. The 80 km/h limit is a maximum, not a target; on an 8% wet descent, a much lower speed in a low gear is appropriate. If the brakes are suspect, the only safe plan is to stop and recover the outfit rather than continue to the coast.