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It's a well known fact that due to air resistance increase of speed on a bike takes non linear increase of power, so the huge marginal increase of power needed to go faster while riding at around 40-45km/h has made average top speeds of the peloton stay in this range for many years. I was wondering if the air drag plays such a major role why doesn't wind affect the average speeds more?

I'd imagine that given at these speeds air resistance is a major factor (it's almost like a wall if you look at the power curve) the cyclist relative 'airspeed' in the wind should also be a major factor but it doesn't look so judging from the single day race results like Milan-San Remo (which has constant almost straight route so should be strongly affected by the wind). You can see results here: http://www.bikeraceinfo.com/classics/Milan-San%20Remo/milan-san-remo-index.html. If I were to guess I'd expect that wind should make the differences on range of at least -/+10 km/h but clearly this is not the case (it's more like a 37-45km/h range). Why is that so?

(There was an incredibly interesting question here about the increase (or lack thereof) of avg speed of TdF winners over the years: Why aren't Tour de France riders going any faster? I understand that in multiday event the wind effects and tactics will cancel out but what about single stage races?)

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  • Maybe the wind speed and direction are quite predictable from year to year at the dates selected, especially if the prevailing wind isn't a headwind.
    – Chris H
    Commented Jul 10, 2017 at 18:58
  • That's also possible. I have very little actual data other than random averages from races and my own experience that depending on the wind you can easily feel exactly the same riding 23 and 37 km/h. I may be totally wrong and the speeds may vary much more than I assume they do, that's why I ask. :-)
    – konrad
    Commented Jul 11, 2017 at 7:11
  • The prevailing wind in Europe is broadly westerly or south-westerly. For the Milan-San Remo route this means it can be a headwind but is more likely to be from the riders' front right. The east coast of Italy should be quite sheltered from those winds by the Appenines. I suspect historic weather data would show less variation for that race than you think, but without such data I can't answer.
    – Chris H
    Commented Jul 11, 2017 at 8:40
  • Here's a good summary of race where really strong winds had a large effect on the peleton: youtube.com/watch?v=KvBXtVc29c8. They were mostly cross winds, but a lot of wind in general that day.
    – Paul H
    Commented Jul 11, 2017 at 14:58
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    Also the Paris-Roubaix race is timed such that the bunch crosses railroad tracks without any trains coming by. On windy days, the group will be so much slower (or faster) that the timing is off and they have wait for trains to pass.
    – Paul H
    Commented Jul 11, 2017 at 15:01

2 Answers 2

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One thing worth noting is that battling a headwind of, say, 10 km/h will not entail an increase the effort as large as would be incurred in increasing you ground speed by the same amount.

The basic equation for the power required to overcome air resistance is:

P ~ 1/2*ACdV^3

where A is frontal area, Cd de coefficient of drag and V the ground speed = air speed. If now we add a headwind v:

P ~ 1/2*ACdV*(V+W)^2 = 1/2*ACd(V^3+Vv^2+2vV^2)

The net power increase due to the headwind is therefore 1/2*ACd(V*v^2+2*v*V^2).

If instead of a headwind we increase the ground speed by the same amount v, we have:

P ~ 1/2*ACd(V+v)^3 = 1/2*ACd(V^3+3*V*v^2+3*v^2*V+v^3)

The power increase in this case is 1/2*ACd(3*V*v^2+3*v^2*V+v^3). It is obviously larger than in the headwind case, by 1/2*ACd(V*v^2+v^2*V+v^3).

This is a factor in explaining why winds do not affect average speeds as much as you expected.

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  • OMG, that's the sort of kind of answer I expected :-) Thank you, I'll just need to untangle the notation a bit so it's more readable.
    – konrad
    Commented Jul 21, 2017 at 19:52
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Head wind can only affect the head of the peloton, while the body is not affected. Constant rotation of the position makes the effort bearable and helps keeping high the average speed.

See what happens when a single cyclist try to do a solitary run against the peloton with head wind: he will hardly be able to maintain a decent advantage and will be taken back.

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  • That's true. I assumed that the 'peloton factor' (i.e. changes at the head) is a constant.
    – konrad
    Commented Jul 12, 2017 at 3:44

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