PT Problem (another one)

Discussion in 'Power Training' started by gggossel, Jan 11, 2007.

  1. gggossel

    gggossel New Member

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    My first time posting here... been reading up.
    I tried searching for this, but no dice. Tonight I was doing 2X20's and noticed that the power display is erratic. I was supposed to be in zone 3, 229-248W, got my cadence and power up in the zone, started the interval and noticed that the power display is erratic... I get to where I know I'm in the zone, and the power drops down 15-20W, then back up, then down, etc. Approximately every two seconds (I have it set on 1 sec rec/ refresh) It's doing this in all zones, and I know my pedaling isn't that erratic.
    I'd really appreciate any suggestions!:confused:
     
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  2. nrhorwitz

    nrhorwitz New Member

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    Not 100% sure what it could be but few questions:


    1. This is done outdoors on the trainer
    2. How long have you had the PT? (could be battery related issue)
    3. The PT is tight in the cradle?
    4. The wires look ok to you? no nicks or anything?
    5. Is your recording interval set at 1.26 seconds? (I usually see my power jump over the place a bit with 1.26 seconds recording intervals but 15-20 - I'm not sure if that's the primary reason).

    Some other PT owners may have better suggestions/thoughts.

    Robin
     
  3. sugaken

    sugaken New Member

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    Sounds like the "precession" that is unique to PowerTap. What was your cadence at the time? The effect will disappear when you are pedaling at ~95rpm of cadence.

    It's explained in the TaRwPM book, but the chapter comparing four major bicycle power measuring devices is available online here:
    http://www.peakscoachinggroup.com/freeinfo/Power_Tools.html

    I strongly recommend that you get a copy of the book if you haven't yet.

    Hope this helps,

    Ken
     
  4. jws

    jws New Member

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    if you mean that it drops by 15W, that's the nature of power output.

    if you mean it drops to 15W, there's something wrong.
     
  5. gggossel

    gggossel New Member

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    Sorry, I should have been more specific.
    Going to sleep on it because I was too po'ed to even finish tonight. I'll try it on the road tomorrow to see if it still does it.
    Thanks for everyone's suggestions.
     
  6. nrhorwitz

    nrhorwitz New Member

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    My first reaction would be this: It's normal seeing that kind of fluctation. There are some "dead spots" around your pedal stroke which allows for some changes in the watts. If you try this, 40 rpm on a friendly grade, you should see some kind of smoothness with your watts output. If that does work, I'd be inclined to believe your powertap is in perfect working condition.
     
  7. acoggan

    acoggan Member

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    To be a bit more precise, when using a PowerTap your power will appear smoothest when pedaling 60/1.26 = 47.6 rpm, or any multiple thereof (e.g., 95.2 rpm).

    (Nine times out of 10, I can tell whether someone was using a PowerTap or some other brand of powermeter just by looking at the flucuations in power when plotted in, e.g., WKO+.)
     
  8. acoggan

    acoggan Member

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    Isn't the default refresh frequency every 3 s (i.e., the display shows a rolling average of the last three samples)? If so, changing it to every 2 s would cause the displayed values to be "jumpier".
     
  9. nrhorwitz

    nrhorwitz New Member

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    Thanks, Dr. Coggan.
     
  10. tomdavis80

    tomdavis80 New Member

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    Not necessarily jumpier from how I'm thinking. Think abou it, you might have wattage going from 185-200 in 3 sec refresh but with two or one sec refresh, you'd see 185-191-200 and so forth.
     
  11. daveryanwyoming

    daveryanwyoming Well-Known Member

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    It's a rolling average, not just a different display rate. IOW take 4 instantaneous 1 second readings of 185, 188, 199, 200... You'd see 191 after the third second and 196 after second 4 and so on. You wouldn't get instantaneous jumps to whatever the power was at that particular moment. Longer averaging smooths the data.
     
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