Dreww10

I think most cyclists are aware of how the air density in cold temperatures effects aerodynamics and in turn their speed for a given effort, but what I haven't seen is discussion of whether this causes the wind to have a multiplied effect on a rider. I've often felt that below 60 degrees F or thereabouts, a given wind speed becomes noticeably more difficult to ride into, independent of the diminished speed already caused by the increased air density. For example, when it's in the 30's, an 8-10 mph headwind seems to have the slowing effect of a 20 mph wind in the summer, but still has the normal effect of a 10 mph wind when at your back and your side.

79pmooney

You've got me interested. I see two different (but not entirely unrelated) factors. Air density which plays directly into form drag and viscosity which governs skin friction. Now viscosity, speed and object length dictate the characteristics of the flow over an object which can cause big differences in both form drag and skin friction.

Density changes linearly with temperature. I'm guessing viscosity does too but I never thought about it, just looked it up. Viscosity changes could alter the Reynolds number, a dimensionless number that describes fluid flow. The effects of viscosity and its effect on the overall air flow will be the same for any two objects with the same Reynolds number. (You can test both ship's rudders and airplane wings using 10" models in an aquarium sized test area with up to 100 mph water flow through it.)

Might have to pull out my old text books or go online to see if I can see a chart of air viscosity vs temp.

Remember also that your surface area in 30 degrees is a lot higher than it was last summer. All those clothes. Also that wind speed is exactly the same as wind resistance from your speed. All that matters is the "relative wind": ie what a wind indicator on our helmet would read. So going 10 miles per hour into a 20 mph headwind is exactly like ridng at 30 mph in no wind. (Well not really. The road and other objects will be slowing the wind above it down - that same skin friction mentioned above and other factors. There will also be turbulence which either help or hurt you.

Fluid dynamics - the more you know, the more you know you don't know.

Ben

markjenn

Colder air is more dense and aero drag is directly proportional to density. So aero drag increases proportionally with a drop in temperature. But the effect isn't huge - for example, you'll get about 12% more aero drag bicycling at 32-deg (F) than you would at 75-deg. If you're also bicycling into a headwind then a temperature drop will amplify the affect of the headwind - for example, if the the headwind causes X drag at 75, then it will cause 1.12X drag at 32. I don't know if this is the "multiplied effect" you refer to.

- Mark

Abu Mahendra

What effects, if any, does air humidity play in this? Colder air will be drier. Anything here?

pdlamb

After you've dealt with the physical, fluid dynamics questions (aerodynamic drag), and the effects of clothing (flapping jacket), then you get down to the psychological questions.


How much does your face hurt when you're cycling into a headwind at freezing? How bad do your feet and hands feel? How hard is it to turn the cranks when you've got knee warmers wrapping around cold knees? Are your tires more stiff/less forgiving in cold weather?


It's probably not exponential, except perhaps converging to how you feel with every bit of winter gear you own on as you crank away in the coldest weather you've every cycled in.

TKJava

This was an interesting question, this article has an interesting explanation regarding air density and also to some degree cold muscles.
Technical FAQ: Why is riding in the cold so hard? | VeloNews.com

Champlaincycler

You guys are a lot smarter then me. Cold air being denser explains a lot. Now, we've been having an extended season here in northern NY, temps in the 40's by 10 am, rising to low 50's by noon all week. I've been telling my wife that 45-50 degrees feels so much colder in late fall then it does in late April/early May. Don't know why it is but I'm wearing lots more clothing now then I would be in the spring. Sun is lower, leaves and greenery gone is all I can come up with. Is this real, or imaginary?

Camilo

The effects I see at cold temps on my real road bike are.... none of them are earth shattering, but I feel them ad they're significant at least in a practical riding sense, not necessarily an engineering sense.

Wearing more clothes. Like it or not, you will tend to have more friction, binding, resistance from wearing more clothes, even properly fitting, high quality ones.

The wind chill effect - makes you less likely to hammer your speed up going downhill especially, coast more. Also tightens you up in general, even if you're warmed up.

TIRES!!. your tires are harder, less supple. They ride rougher and are therefore less efficient (in mho). They are less comfortable exacerbating the general lessened fun and comfort compared to warm weather cycling. Less comfortable, harsher = less speed, less endurance, less motivation to push it.

I can see the air density issue, but not being an engineer, that factor doesn't really cross my mind. What does cross my mind are the above reasons why, although I ride down to about 20F, I just don't go as fast or feel as fast as when it's above 60F.

caloso

Not necessarily. Depends on the climate. Here in the Sacramento Valley we have very hot, dry summers. In the fall and winter we get tule fog, which occurs when the damp ground warms up during the day, releasing water vapor into the air. At night, the air cools, the water vapor condenses into a very thick, low lying fog. A typical fall ride could be low 40sF (5-6C) and 100%RH.

wphamilton

not exponential, linear with respect to air density.

Less humid, counter-intuitively is more dense and harder to push through.

Cold air affects me more than the higher density should. I just have a hard time warming up, harder time breathing, and I suspect my circulation isn't as efficient.

caloso

Here's some anecdata. My training partner and I have a standard out-and-back 2x30' tempo ride we do often. It's on a bike trail that parallels a river so it can be quite cool and damp in the winter. I looked at a ride from February and compared it with a ride in June.

2/9/16
46F
266w avg. power
19.7mph

6/6/16
67F
263w avg. power
21.1mph

So, 21 degrees warmer meant that for the same power, I went 7% or nearly a mile and a half per hour faster.

andr0id

Water vapor molecules are spaced farther apart than the mostly N2 O2 components of air.

Also keep in mind when you do any air density, gas law equations, temperature is Kelvin where room temperature is a cozy 293K and freezing is 273K.

wphamilton

Yep. Converting @caloso to kelvin gets his 19.7 mph to about 20.6 due to air temperature alone. The other .5 mph, maybe atmospheric pressure was different, or perhaps the tires harder at 46° as someone mentioned

caloso

Clothing, probably.

asgelle

Where do you get ideas like this?

jorglueke

Great link. I knew it was harder to bike in the cold I just didn't really know why

BlazingPedals

The sun is warmer in early May than it is in November because May is just past the vernal equinox, whereas November is halfway to the winter solstice, on the far side of the autumnal equinox. (The sun is higher and has more warming power in May than it is in November.)

andr0id

Let's suppose we have a bucket of a given size into which you can place molecules.

Suppose for our experiment H20 weighs 18. O2 weighs 32, and N2 weighs 28.

It starts out full of only O2 and N2. Now take some of those out and put in some H2Os. The bucket is lighter and therefore less dense. I guess spread out may be imprecise, but a water molecule is less dense for its 3 atoms than 2 atoms of either of the other gases.

wphamilton

Close enough I'd think in terms of partial pressures, but the gist is that water vapor makes it less dense. Which can be confusing because of course liquid water is more dense than air.

asgelle

In other words, by your own example, the distance between molecules is exactly the same. It's good to know you believe in the ideal gas law.

wphamilton

Yeh I forgot about that. Even just some extra layers seems to be costly so I can see a half mph or more from that.

cyccommute 

Water decreases the density of air.

You are on the right track but have gone off the rails just a little and made it more complicated than it needs to be. It all has to do with the molar mass of the water and the air. On average, dry air has a molar mass of about 29 g/mol. Water has a molar mass of 18 g/mol. At a given temperature and pressure, the number of molecules in a volume of gas is constant. If you add water to a mass of dry air, the number of dry air molecules decreases to keep the pressure from increasing and the density of the mass of air decreases.

In layman's terms, some of the oxygen and nitrogen has to skooch over to make room for the water. The distance between the molecules doesn't change.

cyccommute 

While aerodynamics plays a bit of a role in the speed reduction, I would say that is its minor compared to other factors listed in the VeloNews article above. Above about 60°C, we tend to wear less clothing and have fewer restrictions. Our heart doesn't have to work against the cooling of the skin...if anything the heart works more efficiently at higher temperatures because the skin is shedding heat as rapidly as possible so the capillaries are wide open.

When it is warmer, we are using more energy to power the vehicle than to simply keep the engine warm (so to speak). Simply taking in air requires a significant amount of energy to warm it.

There is also a psychological component to riding in the cold. Frankly we are little monkeys from a very warm part of the world. We don't really like cold all that much and would rather be somewhere warm.

But we are also stubborn...and not a little dumb... so we just keep pushing through the cold when all the smart animals are sleeping the winter away. Way back in the back of your head there is a voice that is saying "The bears have it right. What the hell am I doing out here?" But we just keep suffering. That's not usually good for performance.

FBinNY 

The best predictor of air drag is the barometer reading. This correctly measures air density, factoring temperature, humidity (which lowers density, which is why a low barometer is a predictor of rain), and altitude.

So winter or summer, we live in a world where the normal barometer range is between 29 and 31" of mercury (at sea level), and can see swings in drag in the 10% range.

Once the increased drag from higher density is factored, that percentage difference remains the same at all speeds. However, a 10% difference in drag becomes more meaningful with increased speed because air drag is a larger percentage of total drag.

caloso

Interesting. Power meter data, like what I posted above, only measures the energy that is transmitted to the cranks (or hubs or pedals), so it doesn't take that into account.