wet bike speed

Why does it seem to me (and to others I’ve asked) that it’s easier to pedal a bicycle when the road is wet? I’d think the opposite would be true – at least the added weight of water being displaced by the tires would slow you down – but it seems to much easier to climb hills or gain speed when it’s wet out. Try it out!

Could the difference between static friction and kinetic friction have anything to do with it?



When I’ve been caught in the rain on my bike, I haven’t noticed whether it’s easier to ride. I have been too busy trying to see through my wet glasses, grumbling about the spray from cars, and worrying about trying to stop with wet brakes. I suppose, then, I’ve wasted your time. This post offers no answers whatever. Just a silly galoot, muttering in the rain, wetly.

I never notice anything like that, and I’ve ridden in a lot of rain lately. My experiences match AskNott’s. (I also notice that I have to do a lot of cleaning of my bike after such a ride.)

Could it be the kind of bike you ride? I ride a road bike, but perhaps this is only noticeable on a mountain bike. And what kind of tires do you have?

I haven’t noticed this either on my bikes (recumbent road bikes). Could it be that your bike, and especially the chain, is not well lubricated? If so, maybe the water acts as a lubricant or washes away some of the grime and makes the movement smoother.

If your chain is well maintained, then I can’t think of a good explanation. As I recall, the major cause of rolling resistance is the deformation of the tire, and water would not change that. Coefficient of friction is smaller on wet roads, but since we usually don’t come to slipping, I don’t see why that would make the bike faster or slower. Perhaps the tire pushes water off to the side as it rolls, pushing dirt and small stones out of the way?

I’ve definately noticed the effect. I’ve always chalked it up to what scr4 mentioned, namely that the water on the chain acts as a lubricant.

I agree with the group that says it’s NOT easier to ride on wet pavement. However, I wonder if what you’re experiencing is evaporative cooling. If you’re only slightly wet, you may feel like you’re going faster (or not working as hard) because of the increased cooling effect.

I haven’t been a serious bike rider for awhile, but I have a few ideas:

-The rain provides a point of reference. If there’s water blowing by you, you may feel like you’re going more quickly.

-Most drivers will drive more slowly when it is raining hard. If you judge your speed by how quickly cars pass you, it might feel like you’re going faster than normal.

-You can’t corner as well in the rain. If you cruise at the same speed regardless of weather, you’ll have to slow down more to go around a corner in the rain.

Rain reduces adhesion between tyre and road surface thus reducing rolling resistance, so your observation is spot on.

I’ve seen one or two time trial course records broken just minutes after a good bout of summer rain, I think the moisture in the air helps breathing too which furhter aids the rider.

Humidity also reduces density, which reduces drag.

OK, that was totally not helpful. Humidity reduces the density of the air, which reduces wind resistance.

Since mountain bikes have larger tires and thus more rolling resistance, then I would expect that this effect will be more noticeable for riders of those things. Also, while rain is falling, I would expect that this effect is offset to some degree by the higher resistance of running into raindrops. Perhaps for road cyclists, these offset one another, so we don’t notice any difference, while mountain bikers notice a change for the better.

As far as I’m concerned it’s in the rolling resistance between the tire and the road. Roadies want the thinest tires to minimize the rolling resistance as much as possible. The rain lessens the resistance as well, aiding in your speed and how much energy you have to expend per stroke. Now if you’re already a roadie I don’t think you’d notice much of a difference, but if you are on a moutain bike, the tires are fairly thick so you’d be more likely to notice a difference. I actually enjoy riding in the rain though.

wouldn’t water on the pavement increase resistance by helping to “glue” the tire to the pavement?

douglips - What am I missing? How does added humidity reduce the density of air?

Humidity is a measure of the water content in the air.

Water is more dense than the collection of gasses we call “air.”

Ergo, the higher the humidity, the greater the concentration of water in the air, the more dense the mix becomes.

I think douglips is right about humid air being less dense. Water vapor is lighter than air. Air is mostly N[sub]2[/sub], which has a molecular weight of 142=28 while water is H[sub]2[/sub]O, whose molecular weight is 12+16=18. Of course, if there are actual water droplets in the air, the air gets denser. But the regular “humid air” just has water vapor that displaces the other gases.

Well, I’ll be darned. I never would have reached that conclusion. douglips, you are a gentleman and a scholar.

Here’s a site that gives an equation for density of humid air:


I plugged in numbers for my city right now (It’s pea soup out my window, MSNBC says the humidity is 96%, this is an excellent test of my hypothesis.)

It’s 45 F here, which I convert to 280.35 K (absolute temp.)

Barometric pressure is 30.11 inch Hg, which I convert to 762 mm Hg. (multiply 1 inch Hg by 25.4 to get mm Hg.)

For vapor pressure of air, I used the chart at:

At my temperature, 7.2 C, I get vapor pressure of 1.0021 kPa which I convert to 7.52 mm Hg.

Entering all of this in, my air density appears to be 1.25 Kg/m cubed right now. This is ever so slightly LESS than the given density for dry air at sea level. Note that the average altitude is I think 566 feet above sea level here. This would mean that dry air here is less dense than the given value. So the difference is even less than what I’ve shown here.

Getting back to the OP, could a bicyclist detect or benefit from this small change?

I have noticed this effect, but mostly when on smoothly paved roads (ie. dark pavement). I have two guesses, first, water reduces rolling resistance therefor lowering friction and traction which reduces the effort required to maintain the same speed. The other would be that when it’s dry the pavement is hotter and therefor the bicycle’s tires grip the road better and thus slow it down, and when it’s cooler (ie. raining) the road is cooler and thus doesn’t grip the tires as well. I would imagine both of these effects wouldn’t as noticeable on more “porous” surfaces such as sidewalks because there isn’t as much area in contact with the tire thus there is less difference when it rains, and also because the temperature doesn’t change as much relative to a black road (which are usually the smoothest ones) when it goes from sunny to rainy or vice versa.

FWIW I think that the density of the air would have a very small role to play in this effect.

Aw, shucks. Your reaction was exactly mine when I first thought about this, but of course it is true - unless you increase the air pressure, water molecules (at a slim 18 AMU or so) displace N[sub]2[/sub] and O[sub]2[/sub] at 28-32 AMU or so.

I don’t understand what you mean when you say this. You did all your calculations as if you were at sea level, so why would your actual altitude come into the equation?

Humidity is a vital component of calculating density altitude, a life-and-death operation for pilots (especially in the mountains).

This Canadian pilot page has a calculator for density altitude. At 60 degrees F and 30 inches Hg, the difference between 10% humidity and 90% humidity at sea level is about 180 feet. That is, at 90% humidity you have as much air under your wings at sea level as you would have at 180 feet when there is only 10% humidity. Not a huge difference, but worth paying attention to if your life depends on it.

A-ha! A question right up my alley, as a cyclist and geek. Unfortunately, it has not crossed my motivation level to calculate actual numbers by hand. Fortunately, I don’t have to, I can rely on other people as they are always 100% accurate.

This page contains a calculator for times & speeds on a bicycle, taking all the relevant factors I can think of into account - except humidity. Fortunately, from my density altitude calculations, I know the effect! Plugging in identical numbers except one at 0 altitude and one at 180 feet altitude, I get a difference of about 0.25% in speed/time. For a one-hour ride, that’s a difference of 9 seconds. For a 30 minute ride, that’s a difference of 4.5 seconds.

This is definitely a factor, albeit a small one. From this year’s time-trial world championships - from the Results page:

  [gold]     1.   3 HONCHAR Serhiy   UKR 56:21.75
  [silver]   2.  13 RICH Michael     GER 56:31.95  + 10.19
  [bronze]   3.   8 BODROGI Laszlo   HUN 56:45.77  + 24.01
             4.   5 BOARDMAN Chris   GBR 57:38.08  + 1:16.32
             5.   1 OLANO Abraham    ESP 57:50.13  + 1:28.37

Note that the top 3 were separated by 10-15 seconds each. Granted 90% to 10% humidity is something that is not likely to happen between a morning time-trial and an afternoon time-trial, but it could be a small factor. Note that even the slightest headwind would be a bigger factor.

Does anyone have any numbers for rolling resistance? It could certainly be a bigger factor, but I just don’t know. Any takers?

Only one I can find is from Bicycling Science by Whitt and Wilson, which quotes the following numbers:

                coefficient of    coefficient of
surface         adhesion          rolling

wet concrete    0.8-0.9           0.0014
dry concrete    0.4-0.7           0.0014

If I understand this table correctly, there is no difference in rolling resistance! But these numbers are for motor car wheels which has some differences.

No charts, or calculations, just two cents worth of layman’s observations here.

As I’ve already posted, I can attest to the fact that I have certainly noticed the wetness = ease of pedalling effect.

I think, however, you’re barking up the wrong tree attributing it to lessened rolling resistance; I put my money on the water-as-lubrication-on-the-chain school.

The basis for my contention is that I notice the effect in the rain (where the chain gets wet), and hardly at all when I’m merely biking over wet ground (where the chain stays dry).

Any science/biker geeks want to help me out here?