Following a gravel lorry in rain
You are riding a 650 cc naked bike on a two-lane rural road with an 80 km/h limit, in steady rain. Visibility is about 150 m. Ahead of you a lorry loaded with gravel is travelling at 65 km/h, throwing up spray and the occasional small stone. You can see its rear lights but not its wheels or the road beneath it. You are 18 m behind it, which at your speed of 68 km/h is roughly one second. Your front tyre has 2.2 mm of tread left, the rear 3.5 mm, and the bike has ABS. A car is following you at about 10 m. The asphalt is worn, with polished wheel tracks and standing water in the ruts near the verge. Your visor is beading despite the coating, and after three hours in the saddle your hands are cold and your grip is tight.
1. Is 18 m an acceptable following distance here? Justify your answer with numbers and say what you would change.
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No - it is roughly half the absolute minimum.
The arithmetic. At 68 km/h you cover about 19 m every second, so 18 m is under one second. In the dry the two-second rule would already put you at ~38 m; in rain you double it, so 55–75 m (3–4 seconds) is the realistic target.
Why it is worse than the gap alone suggests. Wet braking on a polished surface gives perhaps 5 m/s². From 68 km/h the braking distance alone is v²/(2a) = 18.9²/(2 × 5) ≈ 36 m, plus ~19 m of reaction time - about 55 m, before you count the lorry's own stopping distance. At one second you are relying on the lorry's brake lights rather than on a view of the road, and the spray means you cannot see a hazard developing under or beside it.
What to do. Ease off until you have at least three seconds, and more if the spray still blocks your view. Use the left edge of the lane or the verge line to see around the lorry's flank. A one-second gap here is not a following distance, it is a reaction trap.
2. The lorry brakes hard. You brake too, but the car behind you is only 10 m back. Describe what you do in the next few seconds and why.
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Braking. Squeeze the front brake progressively and add the rear - do not snatch. Weight transfers forward, the front tyre loads, and ABS will intervene if the wheel locks. Keep the bike straight and upright: braking and swerving at the same time uses up the same grip.
Where you stop. Leave a gap in front of you if you can. That gap is your escape route if the car behind does not stop - you can pull forward into it rather than being pushed into the lorry.
The car behind. The instinct to release the front brake to 'give them room' is exactly what causes rear-end collisions into bikes, because you then run into the lorry yourself. Instead flash the brake light early, keep the lever pulled, and watch the mirrors once you are nearly stopped.
Do not aim for the standing water at the verge; at that speed it gives you no grip and no ABS benefit. Rear-ending by a tailgater is survivable; riding under a gravel lorry is not.
3. You also have to cross a stretch where water stands in the polished wheel ruts near the verge. How does that, and the state of your front tyre, change your line and your inputs?
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Risk combination. Standing water + polished, low-texture ruts + a front tyre at 2.2 mm is the classic aquaplaning recipe. Tread does not create grip on asphalt, but it does clear water; as depth falls the tyre's ability to shift water drops sharply, and the front is the wheel that keeps you upright and steers.
Line. Get out of the ruts. Ride on the higher, coarser part of the lane between the wheel tracks, where the surface is less polished and holds less water. Move there before the water, not in it.
Inputs. Cross the standing water upright, with a steady throttle and no braking or steering changes while in it - a bike that aquaplanes needs time and a straight line to recover. Reduce speed before the water, not in it.
Afterwards. 2.2 mm is legal in most countries but well into the zone where wet performance collapses. Plan a front tyre replacement before the next sustained wet ride, and check the rear too.