Winter Tyres, Ice and a Steep Descent
It is early morning in January and the temperature is −6 °C. You are descending a long, steep hill on a rural road. The surface looks black and wet, but the spray from the car ahead has stopped and the road shines slightly. Your car has winter tyres with about 3 mm of tread on the front and 5 mm on the rear, and the tyre pressures were last checked in autumn. Your car has ABS and stability control. A sign at the top of the hill warns of a steep descent, and there is a lay-by about 200 m down. A lorry is following you, not closely, but it is heavy and will need a long distance to stop.
1. What does the appearance of the road tell you, and what should you do before the descent gets steeper?
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Black, shiny surface at −6 °C with no spray from the car ahead is a strong indication of black ice - frozen moisture that looks like wet tarmac. The absence of spray is the clue: water being thrown up would suggest liquid, and there is none.
Before the gradient steepens:
- Reduce speed gently and early, while you still have grip and while the lorry behind has time to react.
- Use the lay-by at 200 m if you need to check conditions, fit anything, or simply let the lorry pass. Stopping on the descent itself is far more dangerous than stopping in a lay-by.
- Avoid sudden steering, braking or acceleration. On ice, smoothness is grip.
- Select a lower gear so that engine braking helps hold your speed, reducing reliance on the brakes.
- Increase your following distance to many seconds, not two.
The judgement is that the road is telling you something the thermometer alone does not.
2. Your front tyres have 3 mm of tread and the rear 5 mm. Does that matter here, and what is the risk?
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It matters. On a wet or icy surface, tread depth governs how quickly water or slush can be cleared from under the tyre. Three millimetres on the front - the tyres that steer and do most of the braking - gives noticeably less grip than five millimetres at the rear.
On a descent in these conditions that imbalance has two consequences. First, the front can lose grip first under braking, so the car may not steer where you point it. Second, if the rear has more grip than the front, the car is more stable in a straight line but the front will wash out in a turn - understeer - which on a downhill bend means running wide.
ABS and stability control help, but they cannot create grip that the tyre cannot provide; they work by releasing and reapplying brake force, and on ice there may be very little force to modulate. The practical answer is to reduce speed further, brake earlier and more gently, and plan to replace the front tyres. Do not rely on electronics to compensate for worn rubber.
3. Partway down, the rear of the car begins to slide. What do you do, and what must you not do?
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If the rear steps out, the correction is to look and steer where you want the car to go - steer gently into the slide - and to ease off the accelerator or brake smoothly rather than stamping on either. Abrupt inputs on ice provoke a bigger slide.
Do:
- Ease off the throttle progressively.
- Steer smoothly in the direction of the skid to bring the rear back in line.
- Keep the steering inputs small; large corrections cause a swing the other way.
Do not:
- Brake hard. That transfers weight forward, unloads the rear further and can lock or trigger ABS in a way that reduces steering control.
- Accelerate out of it. On ice that will spin the rear more.
- Make a sudden steering correction in the opposite direction.
After the car is straight, reduce speed further, keep the descent slow, and consider stopping at the first safe lay-by. The lorry behind is a reason to keep your speed predictable rather than to hurry.