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Old 07-02-07, 09:08 AM   #1
Snicklefritz
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powertap as a poor man's wind tunnel?

So I've heard it said one can use a powermeter to do some primitive aero tests to
optimize your position on a TT bike. How would you do this?

I'm thinking about all the places where I ride and it's hard to find a course that
doesn't have some kind of wind on it.

Any good articles out there on this stuff? Has anyone here actually tried this
with their powermeter?

edit: by the way, I have a powertap (not ibike)

Last edited by Snicklefritz; 07-02-07 at 09:24 AM.
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Old 07-02-07, 09:16 AM   #2
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I myself haven't tried or even thought about this. With that being said, my first thought would be to put your bike on a trainer with a large fan in front of it. This would give a lab setting type feel rather than a field test. Then, I guess, you could change positions and rate your level of discomfort for each position.
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Old 07-02-07, 09:18 AM   #3
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Quote:
Originally Posted by Snicklefritz
So I've heard it said one can use a powermeter to do some primitive aero tests to
optimize your position on a TT bike. How would you do this?

I'm thinking about all the places where I ride and it's hard to find a course that
doesn't have some kind of wind on it.

Any good articles out there on this stuff? Has anyone here actually tried this
with their powermeter?
That is said about the iBike since it derives a number that represents an aero drag. Through the use of multiple Coast Down calibrations, theoretically, you should be able to see the effect of changes in position or equipment.

Coggan has been to the iBike Topica website on several occasions and has given many reasons why he believes that the users of the iBike are deluding themselves about using the iBike as a "poor man's wind tunnel".

I suppose with a strain guage based PM, you could use speed as your yardstick for changes in aero drag.

Last edited by NoRacer; 07-02-07 at 09:46 AM.
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Old 07-02-07, 09:30 AM   #4
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Quote:
Originally Posted by Snicklefritz
Any good articles out there on this stuff? Has anyone here actually tried this
with their powermeter?
As with most power related topics, this has been discussed at length on the wattage list at Google.
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Old 07-02-07, 09:30 AM   #5
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Quote:
Originally Posted by runtimmyc
I myself haven't tried or even thought about this. With that being said, my first thought would be to put your bike on a trainer with a large fan in front of it. This would give a lab setting type feel rather than a field test. Then, I guess, you could change positions and rate your level of discomfort for each position.
???????
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Old 07-02-07, 09:34 AM   #6
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Originally Posted by bdcheung
???????
Yeah, it might not make a whole lot of sense. Probably much easier to do it on a stretch of road.

(and replace the word discomfort with exertion, I used the wrong word)
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Old 07-02-07, 10:50 AM   #7
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Quote:
Originally Posted by asgelle
As with most power related topics, this has been discussed at length on the wattage list at Google.
Could you give me a link to the wattage list on google please. Thanks in advance.
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Old 07-02-07, 10:53 AM   #8
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Could you give me a link to the wattage list on google please. Thanks in advance.
http://groups.google.com/group/wattage?hl=en
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Old 07-02-07, 11:02 AM   #9
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It seems like the debate over whether the iBike is a poor man's wind tunnel is kind of irrelevant. If your goal is to improve your TTing, shouldn't you experiment with different positions and use the one that allows you to produce the most power over the TT period, regardless of aerodynamics?? Seems like some positions may indeed be more aero, but may cause you to lose power — probably not the goal of such an exercise. Anyone want to set me straight on this?
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Old 07-02-07, 11:06 AM   #10
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Originally Posted by smoke
Thanks!
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Old 07-02-07, 11:09 AM   #11
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Quote:
Originally Posted by 'nother
It seems like the debate over whether the iBike is a poor man's wind tunnel is kind of irrelevant. If your goal is to improve your TTing, shouldn't you experiment with different positions and use the one that allows you to produce the most power over the TT period, regardless of aerodynamics?? Seems like some positions may indeed be more aero, but may cause you to lose power probably not the goal of such an exercise. Anyone want to set me straight on this?
It's a balance between power and aerodynamics. Producing the most power isn't always the best because you have such a high CdA that your power is being wasted. Lowering your CdA to its absolute minimum is also not the best strategy as you will sacrifice significant amounts of power.
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Old 07-02-07, 11:10 AM   #12
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Quote:
Originally Posted by 'nother
It seems like the debate over whether the iBike is a poor man's wind tunnel is kind of irrelevant. If your goal is to improve your TTing, shouldn't you experiment with different positions and use the one that allows you to produce the most power over the TT period, regardless of aerodynamics?? Seems like some positions may indeed be more aero, but may cause you to lose power probably not the goal of such an exercise. Anyone want to set me straight on this?
First, the goal is to find the position that produces the lowest time over the course. Everything else is irrelevant. That said, it's been found that while there may be a drop in power at first with a new position, after adopting to the new position, most riders recover this loss. Therefore, it makes sense to find the position with the lowest drag, fully adapt to it, then decide if some modification is beneficial.
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Old 07-02-07, 11:13 AM   #13
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Quote:
Originally Posted by asgelle
First, the goal is to find the position that produces the lowest time over the course. Everything else is irrelevant.
Ah, yes, this is the truth!

Thanks for the additional clarifications. I guess I get too wrapped up in hill climbing thinking, where the aerodynamics are practically irrelevant. Need to do more TT stuff I guess.
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Old 07-02-07, 11:15 AM   #14
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Quote:
Originally Posted by 'nother
It seems like the debate over whether the iBike is a poor man's wind tunnel is kind of irrelevant. If your goal is to improve your TTing, shouldn't you experiment with different positions and use the one that allows you to produce the most power over the TT period, regardless of aerodynamics?? Seems like some positions may indeed be more aero, but may cause you to lose power probably not the goal of such an exercise. Anyone want to set me straight on this?
Dave Zabriskie can produce more power at LT with his hands on the hoods or on the drops than he does in his TT position. However, he reduces his CdA so much in his TT position that the power loss is overcome.

I think, if you have a powermeter, you'd find that you are capable of producing the most power with your hands on the hoods or tops, but you'll go faster with less power in the drops.

@ Snizzle Frizzle - Hit your local velodrome. They probably have some off-peak hours that they'd open up the track for you, I'd imagine. Go 10 laps or so in each position, gradually narrowing it down.
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Old 07-02-07, 11:28 AM   #15
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Quote:
Originally Posted by Duke of Kent
Snizzle Frizzle - Hit your local velodrome. They probably have some off-peak hours that they'd open up the track for you, I'd imagine. Go 10 laps or so in each position, gradually narrowing it down.
Apparently, this is one of Coggan's latest experiments--using the velodrome.
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Old 07-02-07, 02:39 PM   #16
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Two words:

COAST DOWN
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Old 07-02-07, 02:51 PM   #17
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Quote:
Originally Posted by ratebeer
Two words:

COAST DOWN
The problem with that is that you aren't generating any power to do that. I can put myself into all sorts of positions doing a coast down test that are much, much faster than anything I can feasibly use on the flats.

In order to be the fastest on the flats, you need to be in a position that yields you both a low CdA and a high amount of power. One or the other will not work; you have to have both.
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Old 07-02-07, 03:05 PM   #18
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Quote:
Originally Posted by Duke of Kent
The problem with that is that you aren't generating any power to do that. I can put myself into all sorts of positions doing a coast down test that are much, much faster than anything I can feasibly use on the flats.

In order to be the fastest on the flats, you need to be in a position that yields you both a low CdA and a high amount of power. One or the other will not work; you have to have both.
temporal independence
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Old 07-02-07, 03:20 PM   #19
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Quote:
Originally Posted by ratebeer
temporal independence
I thought the same thing (although not in those exact words).

Coast Down for position for Cd, trainer for position for power. Test a couple positions on each, and go from there.
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Old 07-02-07, 04:20 PM   #20
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Quote:
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I thought the same thing (although not in those exact words).

Coast Down for position for Cd, trainer for position for power. Test a couple positions on each, and go from there.
Yep. And if you find a long enough hill and have enough patience, you can check wheelsets, water bottle placement, jerseys, gloves, helmets, etc.

Rotate your pedals x times or at x cadence (below delivering any power) as you coast down so that your foot position and related turbulence is more natural.
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Old 07-02-07, 04:59 PM   #21
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ditto to the velodrome
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Old 07-02-07, 05:58 PM   #22
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Quote:
Originally Posted by transplant
ditto to the velodrome
The data says otherwise.

"After this past week, I can now say that I've been there, done that,
and have gotten the figurative t-shirt. I'm still digesting the data,
but here are a few preliminary conclusions:
"1) the precision of such testing is no better (but also no worse) than
testing outdoors on a very still day, and as such isn't quite as good
as testing in a wind tunnel. To be specific, in my hands at least the
imprecision in estimating CdA using the field test approach (indoors
or out) is ~2%, vs. the ~1% variability that we got in the Texas A&M
wind tunnel a few weeks ago."

http://groups.google.com/group/watta...fd75baeb60b361
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Old 07-02-07, 06:03 PM   #23
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and the next sentence

"Nonetheless, if you have ready access and
you're primarily interested in changes in CdA at zero/low yaw angles,
testing on the track can be a very powerful tool (and it certainly
beats having to get up at 5 a.m. to drive to some isolated stretch of
road out in the country)."

so basically there's very little difference in data accuracy between testing outdoors vs. an indoor velodrome. but a velodrome does give you the benefit of repeatability, absence of traffic, etc. that was my point (albeit unstated) in suggesting a velodrome.
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Old 07-02-07, 06:14 PM   #24
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PowerTap = poor man's anything =
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Old 07-02-07, 06:19 PM   #25
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Quote:
Originally Posted by transplant
so basically there's very little difference in data accuracy between testing outdoors vs. an indoor velodrome. but a velodrome does give you the benefit of repeatability, absence of traffic, etc. that was my point (albeit unstated) in suggesting a velodrome.
Except that there is exactly one indoor velodrome in the U.S. So for the vast majority of people it is not a reasonable option. To direct them to it over the equally accurate roads they have access to is misleading.
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