CdA in various positions

Discussion in 'Road Cycling' started by quenya, Nov 3, 2011.

  1. quenya

    quenya New Member

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    After a fair amount of playing and practicing I've finally done a few serious attempts at quantifying my aero drag in various positions.

    On my TT rig, with aero helmet (but not aero wheels, no skin suit, nor shoe covers) I measured .2236

    on my road bike :
    on the hoods, .426
    in the drops, with elbows ~straight .4076
    and in the drops with elbows at ~90, .3817

    Is it just me or am I a bit of a parachute on my road bike?
     
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  2. alienator

    alienator Well-Known Member

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    It's hard to say. What did you use to calculate your CdA? It's also not so simple as just having the CdA. Your body is the largest source of drag on the bike, but your data can't quantify how much your body position is changing the CdA because you're not using the same bike. Moreover, body CdA, bike CdA, wheel CdA, and so on affect each other so while you can get a measure, using the same bike, of how your body position is affecting CdA, you cannot pull out a CdA for your body independent of the bike and parts, helmet, and etc.

    You also have to remember that there is uncertainty in every measurement. How much uncertainty exists is a function of the experimental setup and instrument(s) used to to the measurements, as well as the calibration done on the instrument(s). If you're using an iBike power meter, the uncertainty can be relatively high.
     
  3. quenya

    quenya New Member

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    Alienator, I used a powertap SL+ on each bike and did many laps of roughly 1/4 mile in a parking lot with a little elevation change, that is pretty well sheltered from the wind. I did this early (it was still dark) before any wind picked up anyway.

    The thing is I know that the numbers are probably not perfect but the three road bike numbers seem to have a difference of about the right amount, and my tt CdA was pretty close to an estimate I got from using some equations for Cd and a based on physical measurements Dr. Coggan posted a while ago. So if the estimate is low then i'm a barn door and if my estimates are high then I am a purpose built TTr?
     
  4. alienator

    alienator Well-Known Member

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    How exactly did you calculate your CdA? How exactly did you do each lap? Did you vary the speed on each lap? Did you do a coast down at all? A very rough guess at the CdA for an average sized rider on a regular ol' road bike is on the order of 0.5 m^2. Did I emphasize "very rough guess?" Given that very off the cuff, first blush, first order guesstimate, you sound roughly average. I'd really know the details of your test(s) and how you got the values you did (i.e., how you did calculations).
     
  5. quenya

    quenya New Member

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    Evidently I was sleepy, I used GC's virtual elevation, aero lab feature.
     
  6. alienator

    alienator Well-Known Member

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    I'd consider that a guesstimate. How many laps did you do? Straight courses? Turns? Pavement changes?

    From their explanation, GC is trying to do what iBike does, without having a barometer, a pitot tube, and accelerometers. Even with those types of instruments/sensors, iBike's measurements are a function of its very finicky calibration, wind changes during calibration, body position changes, road surface changes, and etc.

    You might want to do a lot more of repetitions of GC's method at different locations. The more test runs you do, the more the "noise" in the measurements go down.
     
  7. daveryanwyoming

    daveryanwyoming Well-Known Member

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    Quenya,
    As I'm sure you know both classic regression testing and Chungs VE testing (which is what GC offers via Aerolab) are very sensitive to any wind including things like passing cars and of course both are sensitive to air density changes which creates problems when doing many tests as air density changes with temperature and humidity changes and can change quite a bit from day to day which makes it tough to compare many different positions or equipment combos. A portable weather station with recording feature can help but field results can still be quirky. So demonstrating repeatable results for the same position and same equipment tested on different days or at the beginning and end of the same session (bookending your other trials) can help you increase confidence or at least identify changing conditions or potential measurement errors.

    If you have confidence that your measurements are good (repeatable) then yeah, I'd say your road bike CdA seems quite high especially for someone that has a TT bike CdA in the 0.22 region.

    FWIW, across many testing days and candidate positions my TT bike CdA (full TT rig, skinsuit, rear disc, deep front, helmet, etc.) varies in the range of roughly 0.22 to 0.25 (surprisingly I've gotten repeatable lower numbers in a comfortable and UCI legal high and long position vs a more conventional low and tight with shoulder angle square position and race results agree with these tests) and my road bike drops position comes in around 0.3 to 0.33 which rises to the 0.35 to 0.37 range when up on the hoods. I haven't tested up on the bar tops.

    -Dave
     
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