Wednesday, March 20, 2013

Calibration–On the bike–Shows Powertap the Door


I just attempted to recalibrate the meter but this time with it installed in the bike. It confirms my original calibration, the one that is less than that of the Powertap G3 with calibrations around 26.5/170.1 compared to the original of 26.3/171.3 and the Powertap Corrected of 23.4/154.3.



N-M R Bits/N-m L Bits/N-M
7.410253 -25.2131 171.9632
14.90343 -26.9906 168.91
22.87888 26.49627 -170.11

What is going on?

Once on the bike the left measurements bending moment issue goes away – Completely! 100% no more issue. Apparently having it resting in a BB on a table doesn’t count because the cups are not rigidly held. Once on the bike in a relatively stiff frame the bearings are held more rigidly and the bearings remove the bending moment like I theorized.


However that still leaves the mismatch to the Powertap G3.

If my meter is accurately reproducing the torque on the crank pedals then how does the Powertap G3 read higher with drivetrain losses.

Two answers, I’m calculating cadence wrong or the Powertap G3 is out of wack.

Since my data lines up perfectly with the speed cadence sensor and my math looks correct, that only leaves the Powertap G3 is incorrectly calibrated.

I’ll rent the Powertap again in a few weeks potentially to check. Apparently the Powertap will update the torque value without spinning and can be used to check calibration.

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