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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 03-24-08, 08:24 AM   #1
nathbdp
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Wind and speed

Anyone that can speak to the question?

How much does wind affect momentum and speed? For instance, If I have a 7mph tailwind, will that make me 7mph faster?
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Old 03-24-08, 08:37 AM   #2
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It does, but I don't know the algorithm to use. Runners have one that they use to compute this. I want to say that a 5 mph sustained headwind running causes you to burn 10% more energy. Don't hold me to these numbers though. Search Runners Headwind Algorithm or something like that and you may find something usuable for bikers.
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Old 03-24-08, 08:51 AM   #3
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assuming you're going faster than 7mph, a tailwind of that speed will only reduce the amount of wind resistance you have to overcome-- it's not pushing you anymore.
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Old 03-24-08, 09:39 AM   #4
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Don't forget that a cross wind will have an effect proportional to cos[angle] where the angle is measured from your line of motion to the direction of the wind. For example, a wind coming at you from a 45 angle to the left/right will impart a force on you of ~71% of what it would be if you rode straight into it.
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Old 03-24-08, 12:36 PM   #5
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we use formulas like this all the time in airplanes because obviously, the plane goes where the wind goes.

For cycling though. I can't imagine that there is a formula or an algorhythm. There are simply too many variables.

I do have charts that will enable you to calculate wind components for every 10 degrees though if anyone is interested in that?

It basically would say that if you were riding on a heading of north (360) and the wind was blowing from a heading of 300@10kts, it will tell you what your headwind and direct (90) crosswind components are.

Then you could do simple math and get some form of a percentage like pinkrobe has found or come up with or estimated.
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Old 03-24-08, 12:54 PM   #6
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Yeah but we don't care about crosswinds since we don't have problems with drifting off course. But to answer the original question, there's no real way of calculating the difference in your speed other than testing to measure the amount of power it takes to move X bike, X miles per hour on X slope with X head/tailwind with a certain rider at a certain altitude and temperature etc etc etc. Unfortunately, it's not as simple as 7mph tail wind ads 7mph to your velocity. Now if you were in a submarine underwater within a laminar flow of 7mph, you would net 7mph more (in theory). In those conditions, everything is relative.

With a bike, if you subtract wind from the opposing forces, you would be able to go quite fast. But, you still have a fairly constant rolling resistance and you also have increasing resistance in turning the wheels as well as the drivetrain and, of course the obvious running out of gear.

For me, a 20mph tailwind could get me about 5mph faster while exerting roughly the same amount of effort in general. Once you exceed the tailwind speed, you start fighting the wind resistance all over again. This happened on my ride home once last week. I was doing about 20mph on flat ground and I suddenly felt no wind as a gust hit my back. Wind wise, it was like I wasn't even moving. I very quickly picked up speed to around 24mph before the gust ended and I was right back down to 19-20.
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Old 03-24-08, 01:20 PM   #7
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Quote:
Originally Posted by Slabshaft View Post
Yeah but we don't care about crosswinds since we don't have problems with drifting off course. But to answer the original question, there's no real way of calculating the difference in your speed other than testing to measure the amount of power it takes to move X bike, X miles per hour on X slope with X head/tailwind with a certain rider at a certain altitude and temperature etc etc etc. Unfortunately, it's not as simple as 7mph tail wind ads 7mph to your velocity. Now if you were in a submarine underwater within a laminar flow of 7mph, you would net 7mph more (in theory). In those conditions, everything is relative.

With a bike, if you subtract wind from the opposing forces, you would be able to go quite fast. But, you still have a fairly constant rolling resistance and you also have increasing resistance in turning the wheels as well as the drivetrain and, of course the obvious running out of gear.

For me, a 20mph tailwind could get me about 5mph faster while exerting roughly the same amount of effort in general. Once you exceed the tailwind speed, you start fighting the wind resistance all over again. This happened on my ride home once last week. I was doing about 20mph on flat ground and I suddenly felt no wind as a gust hit my back. Wind wise, it was like I wasn't even moving. I very quickly picked up speed to around 24mph before the gust ended and I was right back down to 19-20.
Dude... obv we don't care about drifting. But with the charts you could easily determine what the headwind component of a crosswind is and therefore, have some form of a number to actually crunch.
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Old 03-24-08, 01:40 PM   #8
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Last time I caught a big tailwind, I got to 35mph cruising speed. The 45mph traffic on my route that normally unnerves the crap out of me felt very tame. It was awesome.
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Old 03-24-08, 01:51 PM   #9
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whats your average speed on that stretch?
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Old 03-24-08, 01:57 PM   #10
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Usually it's a head wind around 5-10. Probably 16/17 on avg. I'd guess the tailwind was 35+ mph. Hardly ever blows any harder than that here.
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Old 03-24-08, 02:52 PM   #11
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http://www.kreuzotter.de/english/espeed.htm

Not exactly for ss/fg since it uses a constant cadence, but it will give you some idea of how your speed changes according to wind with a constant power output.
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Old 03-24-08, 03:15 PM   #12
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All of this depends on how aero you are from the start. You're more susceptible to cross winds, etc if you have a mess bag vs riding in a skin suit.
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Old 03-24-08, 06:54 PM   #13
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When I fart I go like, a billion miles an hour.
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