Are you faster when it rains?

Wet conditions might be the last thing you want when you plan your ride, but if ultimate speed is your goal, maybe it’s time to take a raincheck

Rain image

There are plenty of sensible reasons to avoid riding when the forecast says rain. It’s treacherous, can be just a little bit miserable and your bike gets covered in road filth. But if hitting top speed during a sportive or setting a time-trial PB features on your agenda, you might want to plan your assault for precisely the time when the clouds are at their most threatening.

There are various theories about which combination of atmospheric conditions allow you to pedal your machine at its very fastest – time-triallists call it ‘float’ – and there’s evidence that when it’s wet, or about to be wet, is prime time for maximum speeds. But never ones to just sit back and placidly accept it, Cyclist decided it was time to get some scientific verification to add to the conjecture.

Inevitably where velocity on a bicycle is concerned, things hinge on aerodynamics, and in this case how atmospheric conditions affect how easily you can slice through the air.

Andy Ruina, professor of mechanical and aerospace engineering at Cornell University, explains: ‘We know that air drag force is the major drag on a bicycle rider. It is roughly proportional to the density of air and the speed squared. The density of air is less at lower pressure [hence why human-powered speed records are done at high altitude] and at higher humidity and at higher temperature. Airplane lift has similar scaling so airplanes need longer runways on hot humid days.’

Without some serious landscaping, there’s little you can do about raising the elevation of your private TT circuit by a few thousand metres, but barometric pressure, humidity and temperature do fluctuate, and if you’re looking for the optimum combination of low pressure, high humidity and high temperature – they coincide when there’s a storm in the air.

Chris Yu, aerodynamics and R&D engineer at Specialized, takes up the story: ‘The drag force on the rider will be lower with increased humidity and low barometric pressure, but the effects are small. In extreme conditions, however, like after a storm, they may be big enough in total to show a noticeable difference.’ The question is: how much?

The science

Dig out your old school books and you may find this equation: air density (rho) = pressure / (gas constant x temperature). In other words, air density is proportional to air pressure and inversely proportional to temperature. So to benefit from low air density (and minimum drag), you want low pressure and high temperatures. In addition, high levels of humidity (water vapour) in the air reduces its density because water molecules (made of hydrogen and oxygen) are lighter than the oxygen and nitrogen molecules that constitute most of air’s volume. (The previous equation still applies for humid air, with the gas constant being bigger – reducing air density.)

To calculate the speed (v) for a rider pedalling with constant power (P), with a drag constant of c, in air with density rho, the equation is: v = 3√(P/(c x rho)). Because pressure is on the bottom of the fraction, if you decrease it, speed increases, as we’d expect. But what does that mean on the road?

Ruina says, ‘Roughly, if you decrease rho [air pressure] by 10% you can increase average speed by about 3%. This neglects, of course, that a rider might not have available the same power (P) at lower pressure, higher temperature and higher humidity. In addition to this, rolling resistance might be affected by the wetness of the pavement.’

Indeed. Low barometric pressures often coincide with unsettled or stormy weather and bring the added complication of water. While aerodynamic drag makes up 80-90% of the resistance to a fast-travelling rider, rolling resistance caused by the passage of the bike over the ground also saps energy and speed. Intuitively one might assume that water creates more friction and slows you down. Not so says Wolf vorm Walde from Continental tyres.

Humid summer evenings tend to be quicker for riding, when you can almost smell the water in the air

‘If the surface is just impregnated with a thin water film – as in the water is not rising above the peaks of the asphalt granulate – the rolling resistance should be reduced,’ he says. ‘Rolling resistance is mostly energy loss due to material deformation – the energy required to squash the tyre as it rolls over the ground.

‘However there is also rolling resistance due to adhesion, tackiness of the rubber on the surface,’ he adds. ‘The adhesive forces are much smaller than the loss from deformation. Still, rubber and road bond on a molecular level in the contact patch. The bonding is stronger the longer the dwell time – this means the slower one rides, the stronger the adhesion.This part of the rolling resistance is reduced if there is a release agent [water] between tyre and road. A wet tire is less tacky. The water obstructs the bonding in the contact patch.’

So it seems a damp road creates less rolling resistance, making you faster. But wait a minute…

‘There’s another effect of water on the road,’ says vorm Walde. ‘All the above is only true for the same temperatures. In practice, water cools the tyre and the road down. A cooler tyre has higher rolling resistance. This counters the effect of less rolling resistance due to less adhesion. There’s another factor to consider – if the water film is thick enough to cover the asphalt, the tyre has to displace the water. In this case the resistance is higher.’

Finding the sweet spot

Out on the road, one famous time-triallist is in no doubt about the best conditions. Graeme Obree was twice individual pursuit world champion and two-time holder of the one-hour distance record, and is a man known for his analytical approach. ‘When you’re waiting for a downpour and it’s calm – that’s the sweet spot,’ he says. ‘The three perfect conditions are high temperature, high humidity and low atmospheric pressure. Humid summer evenings tend to be quicker, when you can almost smell the water in the air. You don’t get those conditions very often, and when they come together it’s like “wow”, you get your best form of the season. That ’s the night when you should use your fastest wheels.’

Of course, no one will blame you if a stormy forecast keeps you off your bike, but then again, it might be worth going for a quick ride after all.