Page 6 - Tailwind and speed difference

20-03.-2008, 07:57 AM

#76

Fday

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Join Date: Dec 2005

Location: California

Posts: 683

Re: Tailwind and speed difference

Quote:

Originally Posted by Fday

Well, I put in a starting speed close to the average speed, trying to account for that, but even if I didn't do that it still wouldn't be a bad number for such a race as most people almost come to a stop at those turn arounds so there is one acceleration into the wind and one with.

I did make some other changes to see if it might be possible for a thinking person to improve the situation.

decreasing the frontal area from 0.5 to 0.4 and decreasing the power from 250 to 240 to simulate what one might do into the wind and then increasing the frontal area to 0.6 and increasing the power to 270 (to average about 250 watts for the ride) resulted in an approximate 5 second improvement over the 4 k out and back course. I am sure one could do better than that as those were just guesses as to what is possible.

So, it would appear that we can minimize the damage of wind, just as we can do thing to minimize the impact of hills on a rolling course.

05-04.-2008, 03:40 PM	#77
velomanct Registered User Join Date: Dec 2003 Location: Connecticut Posts: 956	Re: Tailwind and speed difference I didn't read all 6 pages but was wondering if this was mentioned. Did any of you stop think that perhaps there is never a complete sustained unidirectional tail/wind? Even when you swear you just a sustained headwind, it is garranteed that it wasn't a constant speed, in a constant direction. This would account for the reason that a "4m/s tailwind" won't make you 4m/s faster on flat or negative gradients. Same logic for headwinds. Wind is very much so not uniform, constantly shifting and changing velocities, even when it appears uniform. Even cycling through a long wind tunnel wouldn't have completely uniform wind. Also, I believe rolling resistence and mechanical drag should have no impact on the equation, since they are essentially equal at all speeds (with a few exceptions with gear crossovers). __________________ "friendship, family, religion. These are the three demons you must slay if you wish to succeed in business!" -Mr. Burns The faster you go, the fewer passing cars

05-04.-2008, 07:02 PM

#78

Alex Simmons

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Join Date: Mar 2006

Location: Sydney, Australia

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Re: Tailwind and speed difference

Quote:

Originally Posted by velomanct

Also, I believe rolling resistence and mechanical drag should have no impact on the equation, since they are essentially equal at all speeds (with a few exceptions with gear crossovers).

You believe incorrectly though. Otherwise riding faster on rollers would not require more power.

Rolling resistance essentially increases linearly with speed (relative to road surface). Resistance of bearings is nearly linear with speed.

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06-04.-2008, 02:27 AM	#79
velomanct Registered User Join Date: Dec 2003 Location: Connecticut Posts: 956	Re: Tailwind and speed difference I thought about it, and it makes sense. At higher speeds, the tire is being compressed more often and at a faster rate than at lower speeds. __________________ "friendship, family, religion. These are the three demons you must slay if you wish to succeed in business!" -Mr. Burns The faster you go, the fewer passing cars

06-04.-2008, 06:15 AM

#80

dhk2

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Join Date: Aug 2006

Location: Huntsville, AL, USA

Posts: 717

Re: Tailwind and speed difference

Quote:

Originally Posted by velomanct

I thought about it, and it makes sense. At higher speeds, the tire is being compressed more often and at a faster rate than at lower speeds.

You're both right in a way, depending on whether you're comparing force or power required. In the equations I've seen, rolling resistance (force) is usually considered as a constant factor x weight, independent of speed. But the power needed to "overcome" rolling resistance is linear with speed, because power is always force x velocity.

EG, at a typical coefficient of rolling resistance (CRR) of .006, and a total weight (bike plus rider) of 90 kg, drag force due to tires would be .006 x 90 x 9.8 = 5.3 newtons Since power is just force x velocity, at a speed of say 10 m/sec, power needed for RR is 53 n-m/sec, or 53 watts. Halving the speed would of course halve the watts.

If you think about gearing, it's easy to see why the power needed to overcome a constant drag doubles when the speed doubles. Even though the force needed at the rear wheel doesn't change, we have to either pedal twice as fast at the same force, or use "twice" the gear at the same cadence, meaning we have to push twice as hard on the pedals. Sadly, either way, power output of the rider has to double

06-04.-2008, 06:36 AM

#81

TheDarkLord

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Join Date: Dec 2007

Posts: 3,846

Re: Tailwind and speed difference

Quote:

Originally Posted by dhk2

Yes, but is CRR itself a function of velocity? According to this website, at least for motor vehicles, that seems to be the case. If correct, then it is probably true for bicycle tires as well.

06-04.-2008, 07:32 AM

#82

Alex Simmons

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Location: Sydney, Australia

Posts: 1,736

Re: Tailwind and speed difference

Quote:

Originally Posted by dhk2

You're both right in a way, depending on whether you're comparing force or power required.

Yes, my bad - I meant power required to overcome resistance, not the resistance value itself.

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06-04.-2008, 07:39 AM

#83

Alex Simmons

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Re: Tailwind and speed difference

Quote:

Originally Posted by TheDarkLord

Yes, but is CRR itself a function of velocity? According to this website, at least for motor vehicles, that seems to be the case. If correct, then it is probably true for bicycle tires as well.

I was aware it can change with temperature, although increased speed would alter the tyre temperature and resistance accordingly.

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07-04.-2008, 12:49 PM

#84

dhk2

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Join Date: Aug 2006

Location: Huntsville, AL, USA

Posts: 717

Re: Tailwind and speed difference

Quote:

Originally Posted by TheDarkLord

Yes, but is CRR itself a function of velocity? According to this website, at least for motor vehicles, that seems to be the case. If correct, then it is probably true for bicycle tires as well.

The equation in the link which has CRR varying with speed was new to me....but then a lot of things continue to be

www.biketechreview.com published results last year of a "home" test to determine CRR of many common race tires. The data indicate a big variation in CRR and power requirements. However, the tests were done on a set of small (79 mm dia) rollers, which I think might exaggerate differences vs riding on the road.

It's noted on the test results that CRR may increase by 50-100% on actual road surfaces. The final note states that tire temps rose 20-40* F during testing vs an estimated 4-8 *F on a flat road surface.

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