Braking on a Wet Downhill Approach
You drive a 26-tonne rigid lorry (4 axles, load roughly evenly distributed, ABS fitted, tyres at 4.5 mm tread on the drive axle and 2.1 mm on the front axle) on a secondary road in the Ardennes. It has been raining for two hours; the asphalt is wet, with standing water in the wheel ruts. You approach a long descent of about 900 m at 6% leading to a T-junction where you must give way. Your speed at the crest is 70 km/h; the signposted limit is 70 km/h and visibility is good. A car is following you at a short gap. Halfway down, the car ahead of you brakes sharply for a cyclist.
1. Explain how your stopping distance changes on this descent compared with dry, level road, and identify the factors in this setup that most increase your risk.
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Three factors stack up here.
Grip. Wet asphalt with standing water in the ruts reduces the friction coefficient substantially compared with dry road. Braking distance grows roughly with the inverse of the friction available, so a wet surface can lengthen the pure braking phase by a large multiple even before the gradient is counted.
Gradient. On a 6% descent, gravity adds a component that works against you: the lorry decelerates more slowly for the same brake force, and the brakes must also absorb the potential energy of 26 tonnes dropping 900 m × 6% ≈ 54 m. That is roughly 26 000 kg × 9.81 × 54 ≈ 13.8 MJ of energy to dissipate as heat - a serious thermal load on the foundation brakes.
Tyres. The front axle at 2.1 mm is legal for a lorry but close to the limit; wet grip falls off sharply as tread depth drops. The drive axle at 4.5 mm is better but the front axle does most of the initial weight transfer work under braking.
Practical consequence: increase the gap, reduce speed before the crest rather than on the slope, and use engine braking and a retarder to keep foundation brakes cool.
2. The car ahead brakes hard. Describe, step by step, what you do in the first seconds and why.
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- Release the accelerator immediately and let engine braking begin - this starts deceleration without adding heat to the foundation brakes.
- Brake progressively, not abruptly. Squeeze the pedal to build force; a sudden full application on wet ruts risks locking a wheel and losing steering, and ABS will cycle but stopping distance on standing water can still be long.
- Steer within the available gap. Because ABS preserves steering, look for the escape route - the shoulder or the lane edge - while braking, not after stopping.
- Do not swerve into the oncoming lane on a blind descent; the risk of a head-on collision outweighs a low-speed rear-end impact.
- Warn the follower. Once you are committed to braking, a brief hazard-light flash tells the tailgating car you are decelerating hard, reducing the chance of being hit from behind.
- After stopping, if the brakes are hot, do not hold the lorry stationary on the pedal alone on a slope; use the parking brake and let the drums cool before continuing.
3. What should you have done differently at the crest, and what does this imply for brake temperature management on the rest of the descent?
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At the crest you should already have been at a speed from which you could stop within the distance you could see to be clear - on a wet 6% descent that is well below 70 km/h, and certainly below the speed at which the car ahead was following closely. You should also have selected a lower gear before the slope so that engine braking and the retarder carry most of the deceleration, and increased the gap to the car ahead so that a sharp brake in front does not become an emergency.
For the rest of the descent: use a steady low gear and retarder, apply the foundation brakes in short firm applications rather than riding them, and monitor for fade - a pedal that travels further or a lorry that decelerates less for the same pedal force means the drums are overheating. If fade appears, stop in a safe place and let them cool; do not continue relying on hot brakes on a gradient.