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Usually the advice for cycling is to try to be as aerodynamic as possible (like, reduce the surface exposed to the air in front of you) in order to decrease drag as it will cost less effort to maintain a given speed. Especially so with headwinds. However I think there might be a point when riding with tailwind where it would actually be beneficial to increase your surface so the tailwind has more effect than the resistance of the air in front of the cyclist and hence actually helps 'pushing' one forward. Is there such a point and is it possible to estimate at roughly what tailwind speed it occurs (let's say on a flat road and other factors all equal)?

Or another way to think of it, in practice: say I have two jackets, one pretty tight and a looser one which tends to form a balloon, would it ever be beneficial to choose the looser one because it increases total surface so the force of tailwind applied to it is larger than the force in the opposite direction created by the resistance to the air in front of me?

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    Other than "when the wind is going faster than you are", you mean? In my experience cycle touring, even when I think I'm going to be travelling in a straight line downwind I inevitably end up with a headwind some of the time. Also, faired bikes generally go faster with a side wind than head or tailwind, at least until the side wind is strong enough to blow them over (at which point they're unrideable pretty much regardless of wind direction) – Móż Jun 4 '16 at 12:42
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    I'm pretty sure that, within the proverbial "small delta", the tailwind would have to exceed your forward speed to make it worthwhile to widen your profile. – Daniel R Hicks Jun 4 '16 at 17:53
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First thing to understand is that the drag force you experience is a function of relative air speed. If air speed reverses (tail wind stronger than speed of movement), drag becomes negative meaning the force will try to accelerate you rather than decelerate you.

Second thing is, this drag force function is not a linear function but actually dependent on relative wind speed squared. What this means is the difference in drag force is felt much more strongly when wind speed is high. In practice when wind speed is below 10-20km/h you hardly feel it, and therefore if you experience negative wind speed in the same range it will make only a marginal difference for you.

For example, let's say you are riding your bike with power that would propel you at 35km/h and you have a 25km/h tail wind, your drag force is reduced by 91% - a massive difference for sure. However, stronger winds will have relatively smaller impact - a 45km/tail wind that would result in you having a negative 10km/h windspeed would give you a 109% decrease in force (resulting in a negative drag force at a magnitude only 9% of the drag you experienced while traveling at 35km/h in no wind).

If you thought that difference between a 25 and 45km/h tailwind is significant, get this - when you have a 25km/h headwind, you are looking at a drag force 294% stronger than what you were facing in no wind, and if that headwind was 45km/h, 522% stronger. This is why you should probably be much more concerned with minimizing your profile in a headwind than maximizing it in the rare occurrence of a tailwind faster than your road speed.

You can find more about the drag formula at https://en.wikipedia.org/wiki/Drag_equation and if you are a more hands-on type of guy, you can use an online bike speed/power/drag calculator to play with some numbers and understand the impact to your speed - here is one example: http://www.kreuzotter.de/english/espeed.htm

Third thing is, in the real world wind is often attacking you at an angle and this makes everything more interesting. Think of how a ship is able to go up against the wind. In deed, a disc wheel or very deep rim can provide slight negative drag force in headwind attacking at an angle. This wasn't part of your question, but theoretically if you had a way of avoiding getting blown off the road (this tends to be a problem with disc wheels), setting a sail could provide you a benefit in a sidewind.

  • This sounds like theory without real-world experience: 10-20km/h you hardly feel it. Most cyclists I know can feel the difference between 0, 5, and 10 kph winds, let alone 20 kph! – andy256 Jun 6 '16 at 7:24
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A tailwind reduces your relative airspeed and therefor the drag you generate. You can go faster relative to the ground with the same power output. The only time increasing your profile might be beneficial is if the wind is traveling faster than you are, relative to the ground .... Unless you cycle slowly or the wind is very fast, it's usually not the case. This is cycling, so aerodynamic is mostly about reducing drag, unlike sailing where maximising lift is important.

  • Yes, and since winds of 30kph and higher tend to be gusty and turbulent, those conditions are somewhat dangerous. – andy256 Jun 4 '16 at 12:51
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Seeming, the tailwind will give us a strength to go faster. The Headwind will make a bigger drag. And actually it's true. While we need to consider the direction of wind. There little wind is from 0 angle degree. So it will reduce the effort of tailwind. So we need tight jacket. And shave the legs and arms is necessary too.

  • This doesn't really answer the question, sorry. Please try using EDIT to make the reply more relevant. – Criggie Jun 6 '16 at 9:05

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