Calibration support against DFPM

[albafar]

Today I thought I would try and see how close the PP was to my Powertap G3 Ant+. Turns out it is pretty close. But I can't seem to dial it in more precisely. If I focus on a flat section and reduce my CDA in Edit Profile it messes things up in other parts of the cal ride. I am confident John will be able to do some wizardry - please also explain how you go about it.

[Velocomp]

Your ride file shows that your average wind speed is a bit higher than your average bike speed.

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This would cause PP watts to be slightly higher

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The way to fix this is to manually adjust the wind scaling factor in VA, such that wind speed equals bike speed. A wind scaling factor of 1.0 (determined by experiment) does the trick. DFPM and PP are within 1 w of each other:

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Corrected ride file an profile attached.

[albafar]

Thanks a lot John. That makes a lot of sense. The average power gets dialed in. But how about the Max numbers - how come they are still so much apart?

[Velocomp]

In looking at your ride file I see that you had, one time only, a 4 second interval where your DFPM power was a bit higher than your PP power. Two points:

1) Assessing cycling performance (or power meter performance) on the basis of a single 4-second burst of power is not terribly meaningful

2) At extremely short intervals, HOW the power measurement device measures stress becomes important. It's possible your DFPM over reported the maximum stress it experienced; it's possible your frame flexed a little bit, causing PP to underreport watts. It's possible, even likely, that there was a combination of these factors, and perhaps others.

Who knows?

After looking at ride files for 20 years, the only thing I know for sure is that no measurement device is perfect, and that the benefits of power measurement are most reasonably assessed over the course of an entire ride.