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  1. #1
    Senior Member Snicklefritz's Avatar
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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 10:24 AM.

  2. #2
    Huh?
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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.

  3. #3
    Isaias NoRacer's Avatar
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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 10:46 AM.
    2009 mileage = 14,738 miles; 2010 mileage = 15,234 miles; 2011 mileage = 17,344 miles; 2012 mileage = 11,414 miles; 2013 = 12,169

  4. #4
    Senior Member
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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.

  5. #5
    Carpe Diem bdcheung's Avatar
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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.
    ???????
    "When you are chewing the bars at the business end of a 90 mile road race you really dont care what gear you have hanging from your bike so long as it works."
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  6. #6
    Huh?
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    Quote 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)

  7. #7
    Banned. El Diablo Rojo's Avatar
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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.

  8. #8
    Royal Grand Exalted Pooba smoke's Avatar
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    Quote Originally Posted by El Diablo Rojo
    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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  9. #9
    semifreddo amartuerer 'nother's Avatar
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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?
    Can you pass the test?
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  10. #10
    Banned. El Diablo Rojo's Avatar
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    Thanks!

  11. #11
    Carpe Diem bdcheung's Avatar
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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.
    "When you are chewing the bars at the business end of a 90 mile road race you really dont care what gear you have hanging from your bike so long as it works."
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  12. #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.

  13. #13
    semifreddo amartuerer 'nother's Avatar
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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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  14. #14
    Senior Member Duke of Kent's Avatar
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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.
    "If a non personal post makes you feel as if you've been attacked, maybe the problem IS you."

  15. #15
    Isaias NoRacer's Avatar
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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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  16. #16
    Not obese just overweight ratebeer's Avatar
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    Two words:

    COAST DOWN
    Joe

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  17. #17
    Senior Member Duke of Kent's Avatar
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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.
    "If a non personal post makes you feel as if you've been attacked, maybe the problem IS you."

  18. #18
    Not obese just overweight ratebeer's Avatar
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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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  19. #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.

  20. #20
    Not obese just overweight ratebeer's Avatar
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    Quote Originally Posted by tfro
    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.
    Joe

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  21. #21
    this portrait of karma transplant's Avatar
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    ditto to the velodrome

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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

  23. #23
    this portrait of karma transplant's Avatar
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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.

  24. #24
    Making a kilometer blurry waterrockets's Avatar
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    PowerTap = poor man's anything =

  25. #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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