Summer rain turns hot tarmac into a slippery slope in the blink of an eye. If you understand the physics and adjust your speed, riding technique and braking, you’ll stay safely in the saddle.
Conclusion
A wet road surface halves grip and doubles braking distance – this calls for maximum concentration when cycling. To avoid falls in summer rain, it is crucial to ride with foresight, slightly reduce tyre pressure and strictly separate braking from steering. With correct weight transfer and forward-thinking cornering technique, you’ll always stay safe and in control, even in wet conditions.
When the first heavy raindrops hit the hot summer tarmac, the risk of a fall increases immediately. What was just moments ago providing reliable grip turns, when combined with dust and road grit, into a highly dangerous film of slush. For cyclists, this means a fundamental shift in mindset: Otherwise, what was just complete control can turn into a total loss of control. To ride safely, confidently and dynamically even on wet roads, it’s not enough simply to ride a little more slowly. Those who understand the physics involved and adapt their riding technique accordingly will remain in full control of their bike, even in heavy rain.
Wet grip: What physics tells us
The contact between the tyres and the road is surprisingly minimal. Essentially, the bike only touches the road surface over an area barely larger than two 50-cent coins – depending on tyre width and tyre pressure. On dry tarmac, the microscopic irregularities in the rubber interlock perfectly with the rough surface of the road. The rubber literally clings to the pores of the tarmac.
As soon as water comes into play, a layer of liquid forces its way into these tiny gaps. The water acts as a lubricant: the so-called coefficient of static friction – that is, the measure of how much force the tyre can transmit before it slips – is roughly halved on a wet road – ‘roughly’, as not all wet surfaces are the same.
The braking distance doubles
In practical terms, this means a very simple but drastic principle: a halving of the grip value directly results in the braking distance doubling. To be more precise: as braking deceleration depends directly on grip, when grip is halved, the tyre must travel twice the distance to dissipate the same kinetic energy. So, if you come to a standstill after about three metres at 25 km/h on a dry road, in the rain – all other things being equal – you will only come to a halt after more than six metres. If you factor in a reaction time of about one second, you will have travelled just under seven metres without braking during that split second of panic. The total stopping distance therefore increases from just under ten metres to over 13 metres – a huge difference if an obstacle suddenly appears.
Cornering: Why leaning into a bend can be dangerous
Physics is just as unforgiving when it comes to cornering. Here, the tyre must absorb the centrifugal force that pushes the cyclist outwards. The faster you ride, or the tighter the bend, the greater this force becomes. This means that the maximum cornering speed depends on the square root of the coefficient of friction. If the coefficient of friction is halved, the maximum rideable speed does not fall by half, but by around 30 per cent. A bend that can be negotiated without difficulty at 30 km/h in dry conditions becomes an accident hazard in wet conditions at speeds as low as just under 21 km/h. Anyone who enters such a bend at full speed or leans the bike too far is exceeding the physical limit – the front wheel will lose grip before you even have a chance to react.
Expert tips: How to drive safely on wet roads
The basics of braking in the rain
- Reducing speed in anticipation: Adjust your speed before you reach any dangerous sections. The doubled braking distance leaves no room for late reactions.
- Using both brakes together: Apply both brakes in a controlled manner. Although the front wheel provides most of the deceleration, the rear wheel helps to stabilise the direction of travel.
- You must avoid locking the front wheel at all costs: It is often still possible to recover from a rear wheel locking up – but if the front wheel slips, a fall is usually unavoidable.
- Optimising tyres and tyre pressure: Use tyres that are as wide as possible and reduce the tyre pressure by 0.3 to 0.5 bar. This significantly increases the contact area with the ground.
- Braking without the brake pads: With rim brakes, you must first wipe the water off the rim flange. Before approaching bends or junctions, apply the brake pads lightly for a moment (‘dry braking’).
Confident handling through wet bends
- Read the road carefully: Avoid road markings, patches of tarmac that have been repaired with bitumen, cobblestones and metal manhole covers. They become as slippery as glass when wet. Cobblestones are often unavoidable – bear in mind that your braking distance here is significantly longer than on tarmac. You must also adopt a much more defensive approach to cornering speeds on wet cobblestones.
- Maximise the curve radius: Start the bend from the very outside, move inwards towards the apex (without crossing the centre line) and let the bike carry you back outwards at the exit. The larger the radius, the less lean angle is required, and the safer the cornering.
Riding technique: pay attention to your posture
The correct braking technique
When braking hard, your weight shifts forwards dynamically. To prevent the rear wheel from skidding out or the front wheel from sliding, you should shift your centre of gravity backwards and downwards. Slide back a little in the saddle and bend your arms slightly.
Build up the braking pressure gradually, rather than all at once: start with light pressure to allow the tyre to deform and grip the road, and only then increase the pressure. If the wheel does start to slip, release the brake immediately for a fraction of a second to regain grip.
The correct technique when cornering
When cornering on a wet track, the golden rule is: keep braking and steering strictly separate! All braking manoeuvres must be completed before you lean the bike into the bend.
When cornering, use the ‘pushing’ technique: keep your upper body more upright than the bike itself and gently push the bike beneath you into the bend. As you do so, shift your full body weight onto the pedal on the outside of the bend (6 o’clock position on the crank). This ensures the tyre presses down onto the road with maximum pressure. Keep your shoulders relaxed and guide the bike through the exit of the bend by looking well ahead – because the bike always goes where your eyes are looking.
Josh Welz
Editor-in-Chief