How do I tell if my torque wasn't zero'd from my PT data download?

Discussion in 'Power Training' started by shawndoggy, Apr 29, 2007.

  1. shawndoggy

    shawndoggy New Member

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    I had an anomalously high power reading on Saturday's time trial. How do I tell whether my torque was out of wack?
     
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  2. Woofer

    Woofer New Member

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    Did you coast before or afterwards? One would see a non zero torque reading during those times if there was the specific issue you think you had. I assume you meant overall just too high and not just a one time spike.
     
  3. shawndoggy

    shawndoggy New Member

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    like I'd be seeing a torque reading at zero watts or zero torque with a watts reading? There do seem to be a couple of instances where torque is "--" but there is a watts reading (like 8 or 22 watts... barely anything).
     
  4. rmur17

    rmur17 New Member

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    non-zero torque and watts while COASTING (i.e. not pedalling) is the key point. Very easy to detect on a PT....
     
  5. shawndoggy

    shawndoggy New Member

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    Well, I'm apparently still too brain dead to be able to tell... can you guys make sense of this file?
     
  6. peterpen

    peterpen New Member

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    To my uneducated eye, it looks easiest to spot in interval #5 - there are periods with 0 rpm cadence but watts and torque readings. You can also see this at each of the turns.

    Not sure when your TT was, but for mine at 10:30am, it looked like it was a rare case of going hardest from the gun producing the best times. We had a strong headwind on the first section which turned into a cross-head after the first turn. Tailwind was very welcome after turn #2. :p
     
  7. shawndoggy

    shawndoggy New Member

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    Argh. As suspected. Is there a way to glean "corrected" data?

    Ours was exactly opposite... wind out of the NW, so tailwind out, xwind on second leg, headwind on third, and dead on into the wind on the last leg.
     
  8. rmur17

    rmur17 New Member

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    yes if the offset was constant or reasonably so. i don't have access to wko here but if you dumped a .csv I could take a look for me. I've corrected a couple of my own files over the years.

    This assumes your speed sensor was working correctly. With rear wheel speed, reported torque and a constant torque offset, one can easily correct torque and power on a per-record basis in .csv format. If your offset torque is all over the place, all bets are off.
     
  9. shawndoggy

    shawndoggy New Member

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    Cool! I'll resave as csv and post-up when I get home tonight.
     
  10. rmur17

    rmur17 New Member

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    http://www.midweekclub.ca/powerFAQ.htm#Q46

    here's a link if you want to do it yourself. Not that much to it really. then re-import the new .csv and get rid of the old 'inflated' file.
     
  11. shawndoggy

    shawndoggy New Member

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    Wow, I read that and I understand the concept, but I have no idea how to make that happen in excel. The csv file is attached. It looks like it is reading "high" by 1.6 nm. If someone could fix, I'd be eternally grateful!
     
  12. jbvcoaching

    jbvcoaching New Member

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    I wrote these out (more detailed instructions) on the wattage list (formerly on Topica, now on google) in 2004:

    1) Open the PT file in Excel.

    2) Sort it by torque (ascending) so you find the lowest torque value (or, as

    chris says, just look for periods of coasting and observe the torque there).

    3) Click Edit -> Undo to put the file back in it's original order (or

    re-sort by time).

    4) Create a column that is a formula, you want to subtract the lowest

    observed torque value from the torque column. Column should be as long as

    the file itself.

    5) Calculate power using the new torque using the instructions Rick Sladkey

    gives in the FAQ. That is, power = 1746*recalculated torque*(speed/2093).

    Adjust the 2093 number (which is wheel circumference) if using other than a

    700x23 wheel/tire.

    6) Copy the new power cells and paste -> special (values) over the original

    power column. Do the same with the new torque values.

    7) Delete the now extraneous recalculated columns, save the file as a .csv

    again, and now import into CPSoft.

    8) Lobby Hunter & company to add the torque-zero-fix to CPSoft (along with

    an SRM slope fix). :)
     
  13. shawndoggy

    shawndoggy New Member

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    THIS is the part I don't know how to do... I'm an excel doofus.
     
  14. bliksimpie

    bliksimpie New Member

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  15. rmur17

    rmur17 New Member

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    here you go Tom :) You should be able to import this file straight in - I named it workout 'b' for the day. If you're happy with it - just delete the original workout.

    BTW, the lowest torque was 1.5N.m so I used that instead of 1.6 --- but it still does make quite a difference in the TT power.

    Huck
     
  16. woodgab

    woodgab New Member

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    I was told by PowerTap folks that there is no correcting a non-zeroed file. Believe me, I want to save my data if I could. When asked if the mistake was linear to the power readings I was simply told it is not. So, you can not take the 0 cadence torque/watts and deduct that constant from all readings via Excel. In my case, watts at 0 CAD ranged with speed, from ~15-40.

    Having said this, I bet doing it is better than not and believe its the only way to save the data for ATL/CTL/TSS... use.
     
  17. rmur17

    rmur17 New Member

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    ah smarter folks than I have figured this out. No doubt watts aren't linearly affected by the offset but torque has been shown to.

    Now that assumes a 'well-behaved' or quasi-constant offset. If it's simply bouncing around all over the place (hub problem other than simple failure to zero), then I agree the case would be hopeless.
     
  18. djconnel

    djconnel New Member

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    If the unit is reading high, if you plot torque versus time, you should see a minimum reported torque, which was reported during coasting periods. Obviously, on a trainer, there is likely no coasting until the end of the workout, when you may see it.

    If it's reading low, unfortunately powertap reports negative torque as zero, so I'm not sure the best way to observe this: you'll still get zero torque during coasting periods.

    Power = torque * rate of wheel rotation (radius/second).
    rate of wheel rotation = 2 pi * speed (kph) / wheel calibration number

    So you'll get a speed-proportional error in power (similar to rolling resistance).

    Powertap 2.4, at least, has a "auto-zeroing" mode where it resets the torque offset during coasting periods. I've been riding without this, but I just today set it, due to experiences of forgetting to reset the torque manually.

    Dan
     
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