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 09-28-09, 05:53 PM #1 kokomojuggler Junior Member Thread Starter   Join Date: Aug 2008 Bikes: Posts: 15 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Physics question Suppose I can ride at 20 MPH w/o any wind. How fast should I be able to ride into a 20 MPH headwind? With a 20 MPH tailwind? Thanks!
 09-28-09, 05:55 PM #2 10 Wheels Galveston County Texas     Join Date: Nov 2007 Location: In The Wind Bikes: 2010 Expedition, 03 GTO Posts: 31,002 Mentioned: 14 Post(s) Tagged: 0 Thread(s) Quoted: 708 Post(s) How much do you weight and what bike are you on? __________________ Fred "The Real Fred"
 09-28-09, 07:02 PM #4 eshvanu Bicycle n00B   Join Date: Oct 2006 Location: Denver, CO Bikes: None yet Posts: 383 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) It's a trick question. The answer is, of course, not fast enough.
09-28-09, 07:13 PM   #5
chinarider
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Quote:
 Originally Posted by Andy_K So, if you ride at 1 MPH into a 20 MPH headwind, the force you must overcome from air resistance is the same as the force of riding 21 MPH with no wind, .
I can't tell you why, but that doesn't sound right. I can tell you its harder to ride 20mph with no wind than 10 mph into a 10mph headwind.

 09-28-09, 07:51 PM #6 jdon Senior Member   Join Date: May 2009 Bikes: Posts: 4,239 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 20 MPH minus whatever your profile (parasitic) drag increase is for the added wind.
 09-28-09, 10:32 PM #7 StephenH Uber Goober     Join Date: Sep 2007 Location: Dallas area, Texas Bikes: Posts: 11,473 Mentioned: 2 Post(s) Tagged: 0 Thread(s) Quoted: 85 Post(s) There are some online power calculators. Hunt up one of them, check your power output in the first situation, then see what speed that gives you in the other situations. __________________ "be careful this rando stuff is addictive and dan's the 'pusher'."
09-28-09, 11:10 PM   #8
Shimagnolo
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Quote:
 Originally Posted by kokomojuggler Suppose I can ride at 20 MPH w/o any wind. How fast should I be able to ride into a 20 MPH headwind? With a 20 MPH tailwind? Thanks!
http://bikecalculator.com/veloUS.html

09-29-09, 09:05 AM   #9
chinarider
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Quote:
 Originally Posted by Shimagnolo http://bikecalculator.com/veloUS.html
I tried a couple of examples and it seems to give way to much effect to the headwind. I have nothing scientific to base this on, just my opinion based on experience.

09-29-09, 11:33 AM   #10
Pat
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Quote:
 Originally Posted by chinarider I can't tell you why, but that doesn't sound right. I can tell you its harder to ride 20mph with no wind than 10 mph into a 10mph headwind.
The physics are the same.

If the air is still, when you ride 20 mph, you are going to have an effective 20 mph headwind.

However, if you are riding into a 10 mph wind at a constant speed of 10 mph, the head wind you experience varies. If you get in the lee of a large object, the head wind will drop. Hills can shelter you from the wind. Also as you approach ground level, the effective wind speed will drop because it will be blocked by even relatively small objects like tall grass, fences, parked cars and so on. So when you are riding into a constant 10 mph headwind at 10 mph, the very biggest effective headwind you can get is 20 mph.

That is probably the effect you are noticing.

 09-29-09, 03:22 PM #11 Retro Grouch  Senior Member     Join Date: Feb 2004 Location: St Peters, Missouri Bikes: Catrike 559 I own some others but they don't get ridden very much. Posts: 27,906 Mentioned: 5 Post(s) Tagged: 0 Thread(s) Quoted: 627 Post(s) Here's what I know for sure: With a barely perceptable tailwind I can ride out, away from my car, and convince myself that I'm riding fast because I'm in better shape than I thought. Eventually I have to turn around to ride back.... You know the rest.
09-29-09, 04:06 PM   #12
zzyzx_xyzzy

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Quote:
 Originally Posted by chinarider I can't tell you why, but that doesn't sound right. I can tell you its harder to ride 20mph with no wind than 10 mph into a 10mph headwind.
Wind speeds are by convention measured 10m above the ground. The closer you get to the ground the slower the wind gets. So what the weatherman says is a 10mph wind isn't really, at the height of the bike.

Also, spokes do a good bit of air churning no matter what the wind speed.