Aerodynamics Question

bernithebiker

So I think we all know that you ride faster in warm air, because the air density is lower and the drag force against you is therefore lower. (Some estimates give as much as 2km/h difference between 0 and 30'C, at approx 30km/h).

But here's the question; supposing you have a tailwind that is greater than your speed, eg. you're climbing at 20km/h and the wind is right behind you at 40km/h. In this case, would it be that the colder the temperature, the more there is to gain? As the higher density air (wind) will have a greater force in your favour.

Garry H

Wouldn't it just be offset (To some degree at least) by the density of the cold air in front of you??

bernithebiker

Garry H wrote:

Wouldn't it just be offset (To some degree at least) by the density of the cold air in front of you??

To some degree, yes, but if your airspeed is negative (i.e. you are moving slower than the wind) then overall the following wind is more important than the air in front of you.

mfin

bernithebiker wrote:

So I think we all know that you ride faster in warm air, because the air density is lower and the drag force against you is therefore lower. (Some estimates give as much as 2km/h difference between 0 and 30'C, at approx 30km/h).

But here's the question; supposing you have a tailwind that is greater than your speed, eg. you're climbing at 20km/h and the wind is right behind you at 40km/h. In this case, would it be that the colder the temperature, the more there is to gain? As the higher density air (wind) will have a greater force in your favour.

If the first bit's true, I would have thought logically that the second bit is. I can't really see the point in knowing either though as a cyclist.

drlodge

When it gets hot (say 25 deg C or more) I physically can't put out as much power as my body gets too hot...there must be sweet temperature spot somewhere for me. Muscles don't work as well when they're cold and you wear less clothing (and hence weigh less) when its warmer...lots of factors to consider!

homers_double

Aren't we overthinking riding a bike here?

bernithebiker

homers double wrote:

Aren't we overthinking riding a bike here?

Depends!

If you're looking to set a PR up your favourite climb, it's interesting (to me anyway!) to know the variables that will affect it. Wind, temperature, weight, etc.

It seems that many of the best Strava times are set in the winter, which is counter-intuitive. It could be that the winds are simply stronger then, but I wonder if a cold tailwind is worth more than a warm one.

Smokin Joe

When I first started racing the old hands used to claim that you rode faster if it had rained overnight as there was more oxygen in the air.

crikey

http://en.wikipedia.org/wiki/Density_of_air

Look here for a table regarding the speed of sound in air at different temperatures.
It's an interesting thought experiment, but only for a given value of interesting; a lady with not many clothes on would be more than sufficiently interesting to make me not think about this.

I suspect that wind speed, road surface, rolling resistance (using clinchers would be better than tubs*), tyre pressure, gearing, cadence, what you had for breakfast, how long ago that was and so on, will all have far more effect.

*https://janheine.wordpress.com/ for some attempts to look at tubs, clinchers and pressures in an experimental way.

crikey

...and look here for attempts to model climbing with various variables; I'm not sure if you can change the air density in the model because a lady walked past wearing not enough clothes...

http://www.analyticcycling.com/WheelsClimb_Page.html

...and don't forget that air density also varies with altitude as well as temperature....

keezx

Pure theoretical there might be an advantage with a cold tailwind, but not enough to set records, it's a matter of waiting for the strongest wind in the direction somebody wants.
That's why I don't care about something like Strava.
What's the fun of competing with guys who wait for a storm in the wanted direction or wait for a moped....
Last time I looked at Strave I saw a KOM from a guy I know personally.
It's a 30 km segment on the "Afsluitdijk", the dyke that closes our inner sea, completely flat, good tarmac.
His average is 60 Km/h over there, while I know he cannot TT > 45 km/h without wind, so he either had a storm in the back or had a friend on a moped before him.....