Monday, July 14, 2008

15 Question : Is it Easier/Difficult to Ride in The Rain?

After cycling close to 50 miles yesterday in pouring rain, some people in the group were throwing an interesting question around at the coffee table.

Is it easier or more difficult to ride in the rain? The question was more in terms of rolling resistance but what might make riding in the rain a more easier or difficult experience?

There's all kinds of directions you can go with this. I've got some thoughts so bear with me.

1) I have noticed that in short term rain, with a very fine layer of moisture on the road, it makes it somewhat easier to go fast. This is not just because of the fact that its raining, but because of what is happening at the road surface itself. Water mixes with dirt and other crap and creates a fine emulsion that acts like a lubricant. You can take some sand, add water and just feel it between your fingers yourself. [Read Lennard Zinn's take on a film of water reducing rolling resistance]

2) A lot of relentless rainfall cleans the road of all this dirt and increases the friction component once again. So the friction component is time dependant on rain. [See Road Surface Properties]

3) The lesser friction you have, the lower your skidding resistance. A decent amount of friction is good!

Picture Courtesy : Graham Watson

4) Think about road condition. Suppose there are potholes everywhere and they are filled with water, that's not going to be fun to ride in. There is an interruption of friction coefficient. I guess surface tension might also play a tiny role there.

5) Okay, what about surface tension? I don't know about surface tension, but I've heard a few things or two that it makes all the gravel and other grit on the road 'stick' to your tires making the chances of a puncture higher. [See Surface Tension]

6) Regarding rolling resistance again, a nice thin film may be nice to cut friction by so many units, but if you have too much water and cross a threshold, you're literally trying to ride in water. After a point, all the buoyant forces kick in and you've essentially made a sail bike, with you as the sail...and the clown.

7) A cool, wet day is constantly spraying moisture on the skin so it decreases the body temperature during exertion and could perhaps increase performance (like being able to ride harder for a longer time interval for instance).

8) But a rainy day also makes jerseys and shorts soggy so there you are with some more added weight to pull with you. So if you're a fatter person wearing more area of clothing, you suck in more water.

Well, that sucks!

9) Racing bicycle tires are so thin and high pressured (100-120 psi) anyway so it doesn't take a lot to keep water away from the point of contact. What I'm saying is that there's not much chance that bicycle tires can hydroplane [See Hydroplaning]. The contact shape in very thin and almost single plane (isn't it) as compared to a car tyre that's almost square or rectangular in shape.

Leaving rolling resistance aside, what about drag?

10) Some might say that moist air is less denser than dry air which is true isn't it? So lesser density of air allows you to go through it easier than the same quantity of dry air, huh?

11) But wait, there are pellets of raindrops falling from the sky so the bicyclist is literally trying to shove his way across this confusion. The interaction of wind, and rain is complex that I'm wondering how they affect bicycling performance. My idea was that someone should use computers to CFD simulate such conditions so we can see through the model what is actually happening. [See CFD Analysis]

12) Finally, this might be a trivial point but in an industry that talks so much about light weight wheels and lesser moment of inertia, think about your wheels running on water. Rain water is being picked up by the tire and thrown off a tangent (which is why if you're directly behind another rider who's riding in the rain, its not such a good experience)..

Plus, if you have a tall rim depth, more water and grit can stick onto that surface essentially increasing the weight on the rims by so much amount.

Moreover, if you have carbon aero wheels with drain holes in them, the water collects in the wheels through these holes and there's some addition to weight there as well. Water can also seep through frame tubing and collect in the bike itself, increasing weight.

So what is it finally? From a rolling resistance standpoint, can we purely state that it makes bicycling easier? There's a lot of other factors to consider that can take away from this little gain, if gained at all, in bad visibility, poor traction, and the feeling of being wet and dirty which robs from the general pleasure of cycling CLEAN.

In the world of post EPO, if someone asks you if you're riding clean, they generally mean - "ARE YOU DRY?" Or are you dry clean. Or dry cleaned... whatever...

Whats your theory? Maybe we can round up all theories and arrive at an average. I guess the average would be "Man are you crazy, could you stay at home when it rains??!!!"


  1. Anonymous7:09 PM

    Although I don't like riding in the rain, I ride better in it. I have bad allergies that trigger asthma. When it's raining I can breathe better.

  2. Thats pretty interesting Rob. So you're saying that rain wipes out pollen and other crap and gives you freedom to breathe without irritation?

  3. I have never even asked myself that question because my mind is always so full of HATRED for the rain when I am riding in it that there is no room for other thought processes.

  4. I believe that IS what ROb is saying. Further I'd agree with him. However, I have exercise induced asthma which is brought on by the air being too cool and too moist, causing my warmer moist lungs to go into a spasm. So if it rains and is warm I get the benefits of the rain "cleaning the air" of pollen and other allergens that normally irritate me, but if it's too cold I have a (once almost fatal) asthma attack.

  5. Reading your post gave me one thought "my asthma", then I read the comments! I only get it during the hayfever season, though I no longer get hayfever itself. I struggle mostly when it's raining and warm, or just after rain when it's drying up quickly i.e. it's warm and humid. As an aside I have been using a PowerBreathe for 4 months and it seems to have made a real difference this year.

  6. That's some serious thinking Ron... but you're good at that! I'm like Chris, I don't really like riding in the rain. I haven't experienced any breathing problems so I tend to think more about the slippery road factor and the chill from wet jersey, etc. Unless it's really hot and it helps me cool off.

  7. Anonymous10:13 AM

    My chain seems to be quieter when it gets wet so maybe the rain also decreases drivetrain friction?

  8. I love riding in the rain... last year with a powertap and very consistent training/commute sections I was generating between 10-20% higher wattage for a given perceived exertion in rain vs similar temps and wind/road conditions.

    Most likely due to the cooling effect on the legs.

    FWIW i like to think i'm much better suited to racing cyclocross at 30-40°F in long sleeve skinsuit than in 85-95°F temps in a short sleeve skinsuit.

    The only ONLY down side is when it rains I typically prefer to be at the front unless everyone's got nice full coverage fenders...

    (rolf vector pros fill up quite well with water - great as flywheels, not so good for acceleration)

    I'm all Pro Rain. No theories other than it works for me.

  9. Interesting points in your post (as always!).

    Personally, I always enjoy riding/racing in the rain. Somehow, I feel like I do perform better. Again, that can be a result of going slower (less effort) than in a dry day.

    One more point to address is breaking. It becomes more technical as we have to remember to dry the rims before applying the breaks as they won't work with that thin film of water in between them and the pads. Have you used carbon rims in the rain? Extra adrenaline!!!!

  10. Personally, I think the rolling resistance issue is so small that it could likely (possibly) be measured in a lab, and at any speed above a walk the other resistances (air, mechanical friction of bearings and drivetrain) would overwhelm the tiny changes...

    Physiological differences affecting the engine (you) are MUCH more significant. The cooling aspect is huge, and if you get cooled too much - that's big too. Any change in breathing is likewise huge... for better or worse.

    Of course, bad visibility, braking, or cornering could lead to a rather abrupt decrease in performance...

  11. i am pretty sure it rolls better when its wet...everything is lubricated..!!it makes me hungry really fast though...

  12. Very interesting comments, both pro rain and against rain.

    Gewilli, yours was especially interesting since you mentioned a 10-20% increase in power wattage.

    Perhaps it'll be nice to design apparel, maybe just a prototype for now, with little tubes of cool water somehow integrated into them and test them out on a subject in hot weather to see how much superficial skin cooling plays an effect in power output and overall performance?

  13. That's correct about the air being clearer in the rain. A lot of pro photographers know that the best time to take a picture is right after a rain the dirt is taken out of the air.


  14. Thanks for the insight blue. Yeah I didn't know photographers did that, I guess they have their tricks to the trade.

  15. Ha!
    Just last night I was reading Bicycling Science by DG Wilson so I could pull out some boring and random facts on fellow cyclists (and my poor wife) so I can look clever and now is my chance!!!!
    The rolling resistance coefficient on a wet and dry road has been measured at 0.014; having said that this is for a radial tyre and would only be for a certain size making this information virtually useless!
    Hope I've been of some help.....


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