Computrainer Powertap Wattage Offset

Discussion in 'Power Training' started by sglasgow, Feb 13, 2009.

  1. sglasgow

    sglasgow New Member

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    Was wondering if anyone has a sense of the wattage difference between PT2.4 and a Computrainer e.g. if the CT reads 200W the PT would read XXXWatts. I assume its fairly consistent but would PT Watts equal CT Watts + XX Watts or PT Watts equals CT Watts x 1 +X%. Appreciate any help trying to compare a previous ramp test I did on a climb with the powertap.

    Kind regards,

    Sean
     
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  2. rmur17

    rmur17 New Member

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    Could you add some info regarding zeroing, calibration of the CT, age of the CT?

    If attention was paid to detail and the CT is fairly new, I'd reckon <5% error would be typical.

    Are these your own devices or did you test at a center?

    My own CT and trusty ole PT Pro (circa 2004) have been adjusted to align within 1% of each other. I check 'em twice yearly and haven't had to touch a thing since 2005.
     
  3. sglasgow

    sglasgow New Member

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    Hey Thanks,

    Just did the rebuild on the PT originally bought mid-2005. The Computrainer is brand new but not a lab model. I guess there is the issue of turbo vs. road as well. When you say have been adjusted to within 1%. I know the PT zeroing procedure. Is there a trick with setting RRCs or Drag to get the same reading. I guess once they are calibrated you have the issue of 3 different things: turbo, real riding on a flat and real riding on a climb. Do you come up with different training zones for the computrainer vs. road? Seems testing is easiest on the CT but having the zones right for the road is most important as ideally that's where I would like to ride but I'm in drizzly or worse England.

    Kind regards,
     
  4. Meek One

    Meek One New Member

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    Sorry for the hijack, but did you do the rebuild yourself? Ceramic bearings?
     
  5. rmur17

    rmur17 New Member

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    So both the CT and PT are yours? The very simplest way to check them out would be to:
    1. Ensure stable conditions
    2. Use the PT on your CT bike ... make sure it's zeroed
    3. Warmup the CT and obtain a stable RRC. Despite how some folks read the manual, if you stick to ergo mode, flattish courses -- an RRC of ~2.00 is perfectly fine.
    4. Use the CT in manual ergo mode (check the manual!), to run a simple ramp test. I'd suggest 3-5min in each step to obtain good averages. Power-wise, maybe start around 150W and go up as high as you can in 25W steps. On the top end, you can shorten the steps if the power is getting to you. You should be able to exceed FTP reasonably easy ..
    5. Download the PT and plot the results, I normally put CT ergo power on the X-axis and PT 'actual' power on Y.
    6. Perform a linear regression of PT vs. CT power. That'll give you Y-intercept (difference in zero offset) and slope (difference in gain/slope). Ideally y-intercept would be quite low (<< 5W) and the slope would be 1.00
    That should tell you pretty darned well how the two compare.

    Re the other bits: watts are watts as long as the two devices are quite close.
     
  6. sglasgow

    sglasgow New Member

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    No sent to Saris. I had a wired unit they returned wireless ANT2.4 with the firmware upgrade. Don't think the bearings are ceramic. Just contact Saris for the details I forget the $cost.
     
  7. sglasgow

    sglasgow New Member

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    Brilliant, Thank You


     
  8. Porkyboy

    Porkyboy New Member

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    Hi Sean

    May or may not be of interest, these postings on my blog document my experience. Note particularly the results you get if the CompuTrainer is not calibrated properly, I tested that first! :)

    http://pedalbiker.blogspot.com/search/label/Power%20Meter%20Tests

    Cheers,

    Q
     
  9. liversedge

    liversedge New Member

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    I did a few tests with SRM cranks vs a computrainer and got almost pathalogical about calibration of the CT. In almost all cases my CT reads between 10 and 20 watts higher than the SRMs and is not as sensitive to spikes in data.

    I considered adjusting the screw on the CT lab pro to get them aligned but didn't want to fubar my precious CT!
     
  10. liversedge

    liversedge New Member

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    oops double post
     
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