Descents are the cyclist's reward for climbing hills. You can travel effortlessly fast. You can relax your legs, heart, and lungs – but not your brain. It's never more important to be alert. Even a modest descent can see a cyclist's speed build to 40km/h (25mph) or beyond, and on a long steep road it can reach twice that. There's less time to react, for you and other road users, and the consequences of getting things wrong can be grave.
Limit your speed
If you feel out of control, you are. Even if you do feel in control of the bike's speed, you might not be. You need to be able to stop within the distance that you can see ahead. Bicycles cannot stop as quickly as cars. The brakes are seldom as effective and the tyres will lose traction sooner. Even in ideal conditions, you can add half as much again to the stopping distances you learned for your driving test – so about 18 metres at 32kmph and 35 metres at 48kmph. If it's raining or you're tired, stopping distances will be much greater.
How far you can see ahead will depend on bends, buildings, and vegetation – and your own vision. While there's no compulsory eye test for cyclists, the onus is on you to ensure you can see well enough for the speed you cycle at. At night, you can't see further than the beam of your front light; if you want to descend at speed in the dark, get a good one! During the day – and at night if the lenses are clear – cycling glasses will keep wind, insects, and debris out of your eyes.
Scan the road environment ahead, everything from the road surface to drivers at junctions and pedestrians on pavements. You do this anyway, but when you're travelling faster, you must begin reacting to things sooner. If things are happening too close for you to react to in time, then you're travelling too fast regardless of whether you have right of way. Slow down so that you're descending at a speed that suits the worst-case scenario. A suitable speed will vary enormously, depending on the situation. On a straight country road, you might be able to descend as fast as you like. That hill down into town with the roundabout at the bottom? Proceed with caution!
Cover your brakes for any descent where you might have to use them. That means resting one or two fingers on the lever. This reduces your reaction time: you can start to brake sooner. It also puts your hands in the right position to 'feather' the brakes.
Feathering the brakes means lightly squeezing them on for a few moments, releasing them, squeezing them on again, etc. This moderates your speed so that you shouldn't have to jam the brakes on suddenly – a situation that's best avoided, as it may cause you to skid and fall. On a long descent, feathering the brakes is better than dragging them on the whole way down. Your hands are less likely to cramp, as they get chance to relax. You're also less likely to make the brakes (or braking surfaces) overheat, which can cause the brakes to fade.
Do the bulk of your braking before you attempt any manoeuvres rather than during. You're less likely to skid or become unbalanced if you brake while you're travelling in a straight line. So brake harder before that sharp corner and only lightly – or not at all – as you go around the corner.
'Road' is the operative word. If you prefer to use off-carriageway cycle tracks, you will need to seriously limit your speed while descending. That way you will be able to react in time to Give Way markings where the cycle track meets a side road, or where the cycle track dives back onto the road alongside without warning. If it's a shared-use track, you'll need to keep your speed down in any case so that you can mix comfortably with pedestrians.
On road, your speed while descending will more closely match that of cars. This makes it easier to take the lane when you need to – and it also makes it more important to do so. If you're travelling fast in the gutter, you're at more risk of being cut up/left-hooked by an overtaking/left-turning driver who didn't appreciate your speed. Taking the lane is a reminder that you're not just traffic but fast-moving traffic, and it gives you room for manoeuvre around car doors, potholes, and more.
Bikes can occasionally become unstable while descending. Known by motorcyclists as a 'tank slapper', a shimmy or speed wobble is an unnerving oscillation in which the frame and fork twist from side to side and the front wheel jerks the steering left and right. If it happens, your instinct is to grab the bars firmly and brake hard. Both of these things can make the speed wobble worse!
The bikes that most often suffer from speed wobble tend to be road bikes, audax bikes, and small-wheeled bikes such as folders – especially if they're lightweight, torsionally flexible, and ridden by heavier cyclists. Solidly-built bikes with stable steering and fatter tyres don't tend to shimmy.
You may never suffer from a shimmying bike. But if you do, the following advice could prevent a crash. Stand up on the pedals and, if the frame has a top tube, grip it between your thighs. This damps down the vibration. You can then brake gradually to a stop.
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