aerodynamics of an inch

Discussion in 'Cycling Training' started by Old Junker, Jun 19, 2006.

  1. Old Junker

    Old Junker New Member

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    i have 1 inch spacers under my aerobar, if i remove these asuuming all things constant ie,power,comfort etc how much time is it worth at 40kph on a 40k itt???
    i cant find any info/data or website on this :confused:
     
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  2. Dietmar

    Dietmar New Member

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    Simple answer: There is no way to tell, without performing a numerical simulation, or wind tunnel tests, each of which will cost you very serious money. Note also that it is not at all clear whether such a modification will decrease, or actually increase aerodynamic drag.
     
  3. Old Junker

    Old Junker New Member

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    ok now for a real response
     
  4. Woofer

    Woofer New Member

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    Do a field test with a powermeter and see if you get differences in speed for a given constant power - people who use suitable roads and testing methodology can find differences that are repeatable and have been verified by others for changes in equipment.

    If you don't have a powermeter you can do a slowdown test from a given speed.

    All of this requires a non-windy or very low wind day as a small change in this variable throws everything off.

    Look up Allan Lim and aerodynamics for some instructions or read Training and Racing with a Power meter by Hunter/Coggan.

    This of course assumes no losses/gains due to adaption to one position or the other.

    The accuracy and precision of this method depends on the exactness of the experimenter, hence it's easier to use the first *real* answer.
     
  5. Dietmar

    Dietmar New Member

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    What do you mean by a "real response"?
    I have given you an accurate response. The aerodynamics of the configuration you are interested in is not trivial. It is no accident that you could not find the information you were looking for anywhere: That information does not exist.
     
  6. Dietmar

    Dietmar New Member

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    Correct. In this particular case, the one thing that is clear is that the effect of the proposed modification will be below the experimental noise of the methods you suggested. Not to be misunderstood, those methods are perfectly fine, in principle, but they are not sensitive enough to decide this question.
     
  7. Woofer

    Woofer New Member

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    It could be, dunno as I have never tried it for this particular question, and I am too lazy to look up the results for people that I know do this sort of testing and don't feel like arguing this one particular fine point. :)

    Another method which people actually pay money for and can also do on their own is to take a head on image of the rider in the aero position. Get the exact pixel size of the profile. Change position. Take another image and get the exact pixel size of the profile. The smaller sized profile is assumed to be the most aerodynamic position, all other things equal. All sorts of assumptions in there...
     
  8. Dietmar

    Dietmar New Member

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    Really? People actually pay money for this kind of nonsense? How much? We could perform much more sophisticated analyses in our group, for a comparably reasonable amount of money, so I am genuinely interested in whether there is a market for such capabilities.

    P.S.: The way I understood this poster, he was going to eliminate the spacers, but then raise the stem such that everything else, including rider position, remains equal.
     
  9. Woofer

    Woofer New Member

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    http://www.slowtwitch.com/mainheadings/features/fitspecialist.html

    Never used this, but this sounds like it.
    http://www.pkracing.com/afa.html

    p.s. Yeah, no worries, most posts manage to leave out some detail.
     
  10. Dietmar

    Dietmar New Member

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  11. esandman

    esandman New Member

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    As mentioned earlier...

    One way to go about this without going to a wind tunnel is to take a frontal (head on) picture of yourself in the two positions and convert it to a silloutte. From the two sillouttes you can approximate the frontal area (based on total silloutte area) differences between the two positions. One thing to consider though is that even though one position may be more aerodynamic (or have less frontal area) you may produce less power in it and be slower overall. Another is that it may take time to adapt to the new position to get the most out of it, if the TT is getting close I wouldn't alter your position.

    Check out

    http://biketechreview.com/v-web/bulletin/bb/index.php?sid=9f1a714eb92b3d58f090b2c81cd432b9

    for more information.

    Erik
     
  12. DannoXYZ

    DannoXYZ New Member

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    In addition to Ca (frontal-area), total drag also depends upon Cd-coefficient of drag. It's hard enough to figure out frontal-area, I really like the counting-pixels approach, it's even harder to determine coefficient of drag. And it's really not possible to use the equation with a revised Ca before & after because any change in frontal-area will change Cd as well.

    One example is the change from the praying-mantis position to the current model of flat-forearms in the past 15-years. Looking from the front, this change does nothing to decrease front-area because the air that flows past the forearms will be still hit by the hips as well. With equal front-area, why is the flat forearms faster than praying-mantis? Because Cd of flat-forearms is lower, the entire forearm "drafts" behind the fists. So even with the rider's upper-body in the exact same position blocking the exact same amount of air, the flat-forearms disturbs that air less than praying-mantis.

    Fluid-simulations is a highly complex mathematical model and only recently has computing power increased enough to allow 'affordable' CFD software to come to the engineering workstation. Here's some links to CFD software:
    http://www.cd-adapco.com
    http://www.fluent.com/software/index.htm
    http://www.cfdesign.com
    http://www.cfd.ru

    Keep in mind though that most of this software works on a solid static model, a biker with flailing legs and flapping jersey's gonna need to be modeled with the legs in different positions as well. The PK-racing site's got good intentions, but misses out on the other half of the equation, Cd-coefficient of drag. I'm with Dietmar, you really need a wind-tunnel to figure out the quantitative changes in order to find a number that shows performance differences in those two positions.
     
  13. alfeng

    alfeng Well-Known Member

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    I think we all know the question you are asking, however the way you posed the question you seem to have "fixed" the speed at 40kph over the length of the distance traversed which would mean that there would be NO time differential; but, you will have hopefully expended less energy in doing so!

    Having said that, as noted, rather than a wind tunnel test, you could do a "field test" of sorts ... if you have either a RELATIVELY LONG descent OR LONGER false flat you can simply change your body position and see how much more slowly you COAST when you are catching the additional bit of air than when you are 1" lower ...

    As one very imprecise benchmark, from the tops to the drops on a road bar is between 2-to-3 MPH as I recall from making the CASUAL observation whether coasting on a particular descent (~35 MPH vs. ~38 MPH ... :eek: that's slow in a car, but for some people it is fast on a bike) or coasting on a particular false flat (~15 MPH vs. ~18 MPH) ... move to the tops, the speed would drop by about 3 MPH, move down, and the speed would resume toward the higher end of the particular range. Your results may (make that will) vary, and it was probably more coincidental than not (or, just my memory) that the differential in both cases was about 3 MPH since one would think that there would be more wind resistance as the speed increases ... you may coast at 5 MPH faster when in the drops because your body position is more aggressive OR possibly only 2 MPH because your higher body position is more efficient than mine.

    There is obviously more than a 1" difference in body position when moving from the tops to the drops with a road bar (and, not all road bars have the same drop) than the 1" adjustment you plan to make; AND similarly, we don't know how efficient your current body position is ...

    So, before doing your field test, I would be greatly surprised if your gain wasn't well less than 1 KPH -- is that the type of answer you were looking for? Well, seconds ARE seconds in any time trial ...

    FWIW. A possible consideration is how much less efficient you may or may not be pedaling-in-and-pedaling-out-of some turns if you are in a lowered position ... an empirical observation would probably help you more than theoretical calculations in this portion of your analysis.
     
  14. frenchyge

    frenchyge New Member

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    Hmmm.... that wouldn't make much sense, but now I see why you said there was a chance that drag could increase. I read that he was wanting to lower his torso by 1".

    Either way, your answer was correct -- there's no way at all to know. In addition to the uncertainty of the aerodynamics (which should improve, although it's impossible to guess by how much) there's also the question of whether he would be able to generate as much power in the lower, more cramped riding position (again, impossible to know based on the post).

    I guess there just aren't any easy answers. :)
     
  15. wilmar13

    wilmar13 New Member

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    I hate it when people respond to my posts and don't tell me what I want to hear too!

    In order to get you a real response I sent the info you gave in your original post to my company's team of highly trained engineers in Mumbai (they are cheap and underutilized), they processed all the variables and came up with a time savings of 14 seconds for the 40K TT with a standard deviation of 2.67 seconds for calm conditions and a 40kph avg. They tell me most of the savings is due to the weight reduction of eliminating the spacers rather than the intuitive aero increase.
     
  16. Old Junker

    Old Junker New Member

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    no i would not be raising the stem
     
  17. Old Junker

    Old Junker New Member

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    you read it right 1 inch torso lower and i beleive its worth about 2 minutes in a 40k at same power/weather/course
     
  18. Old Junker

    Old Junker New Member

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    i will try a feild test thanks alfeng
     
  19. Dietmar

    Dietmar New Member

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    Alright, my fault, but I was confused by your reference to other things being equal.

    Well, obviously that depends on your position before you lowered your bar, and you need to take into account that the efficiency of your power output is affected by your position, too. But, yes, with the scenario as I understand it now, your best affordable bet is to do test runs as described by several people above.
     
  20. artmichalek

    artmichalek New Member

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    Depending on how flexible the OP is, this could have a much bigger effect than the aerodynamic savings. You can spend all you want on complicated testing and analysis, but the only way to really find out if it works is to get out on the road and try it.
     
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