Monday, March 22, 2010

Powertap torque test: revisited

Prior to sending my Powertap back to Saris for calibration, I decided to do one more torque test (previous posts: my wheel, comparison with other wheels), this time bypassing the Powertap internal zeroing, and go directly to the raw torque numbers, which are available in torque mode on the Cervo head unit.

My procedure:
  1. weigh test load (a bucket of fresh kitty litter) with my Ultimate hanging scale = 14.63 kg
  2. go into test mode
  3. for each gear tested:
    1. shift into gear of interest: 36/12-13-14-15-17-19-21-23-26 on my wheel, 36/13-14-15-17-19-21-23-25-28 on C's wheel.
    2. record torque
    3. load pedal with mass
    4. find orientation which maximizes torque reading
    5. record torque
    6. remove mass
    7. record torque
    8. subtract avg of unloaded torque readings from loaded torque reading

I plot theoretical torque delivered to the hub on the x-axis, with the y-axis showing the measured value, for each hub.

Powertap torque test
It looks good for C's wheel, but as before, my wheel is off. Curiously, each hub reported a slightly larger number than trend when in the largest cog. I'm not sure if there's any significance there....

Anyway, a similar test was reported on the Wattage list by Doug. I plotted his data. His procedure differed, and his test mass was weighed with a Tomita body weight scale, which is much less accurate than the Ultimate hanging scale. Still, the comparison between wheels is telling, and the weight load is the same in each case.

Doug's torque test data
Black seems fine within the accuracy of his scale. Blue on the other hand seems substantially lower. Of course, I can't vouch for his procedure. It may be his results are a measurement artifact. But certainly these results should have you questioning whether your Powertap hub is reporting accurately.

Okay -- time to ship that wheel back to Saris...


gregclimbs said...


You're killing me....

Weights in KG, torque in lb-in, and extra scales (w@18minOLH??)

That said, I think it would be interesting to plot the error bounds of torque that would still keep the ptap in spec (+/-1.5%) and see if you hub is outside those bounds...


djconnel said...

Yeah -- I can't help the lb-in. That's what Saris uses, so since their reported number is to such low precision, changing units from what they report would be unwise.

"W@18min OLH" is watts at the speed it takes to climb Old La Honda Road in 18 minutes. Old La Honda time is my canonical metric of cycling power.

djconnel said...

Oh, yeah -- WRT the weight in kg.... Newton would be better, I admit, but it's the metric unit reported by the Ultimate.

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