So I finally tested my Powertap, using the following procedure:
- I filled a bucket, first part way, then later further, with water, in each case measuring the mass (actually, the weight) with my Ultimate hanging scale before and after the test, and averaging. Mass values: first fill line = 9.765 kg, filled further = 12.56 kg, further still = 13.955 kg.
- I hung my bike from my Park stand. I shimmed the stand so the frame measured vertical with a plumb bob which hung vertical from the top tube. I wanted the string to just brush the down tube. My bike (Ritchey Breakway) has round steel tubes so this works.
- For each gear combination tested:
- shift into gear of interest, spinning the rear wheel
- bring rear wheel to a stop
- zero torque (hold right button on PT)
- hang bucket from pedal while holding down rear brake to keep wheel from rotating.
- find a pedal orientation which maximizes torque reading (make sure bucket doesn't rub against chain or chainring). My estimate is I was generally able to come within 1 lb-in of the optimal value.
- record torque
- remove bucket and check torque reading
It was often the case the torque read a significant positive value after removing the bucket. In some cases, I remeasured, but did not discard the previous values. Since the PT doesn't read the magnitude of negative readings, it would bias the results to discard points based on large positive offsets only.
My procedure was a bit sloppier on earlier measurements (the two lighter masses) than on the latest measurement (the heaviest mass). However, this didn't significantly affect the results, so I combined the results for analysis.
I calculate the theoretical torque as follows:
torque = M g L Nr / Nf,
where M g = weight of bucket with water, L = crank length, Nf = teeth on front ring, and Nr = teeth on rear ring.
I plot the theoretical value versus the reported value and fit:
You can see from the plot two things: First, the Powertap is considerably underreporting the torque. Torque doesn't have much meaning to me, power does. So on the plot I convert torque to the equivalent Old La Honda power. You can see the Powertap underreports power by around 10 watts. Worse, the scatter in the data are huge.
The scatter is highlighted in a plot of the residuals:
Points are indicated by the gear ratio used to generate them, color coded by the mass load. No obvious pattern: just scatter.
On the last measurements I did, the heaviest load, I recorded the torque the powertap read after I removed the weight. This should have been zero, obviously, as I'd zero'ed it out under the same unloaded condition before putting the weight on the pedal spindle. But it was all over the place. To quote Andrew Coggan, perhaps the expert in the field, who responded to my post on Wattage: your PowerTap isn't 100% "healthy". Here's the residual versus this post-measurement torque offset. There is a clear correlation.
So back to Saris for torque tube recalibration: it appears the strain gauges have fatigued. My guess is I'm seeing a hysteresis behavior: the powertap isn't just reporting the present torque, but some residual of a previous torque measurement. In test mode, it reports 519 lb-in as the internal offset, which is higher than the nominal 512 lb-in.
So what do I conclude with my power data, going back to 2007? Not much. It's not reliable. Bummer for someone who loves doing data analysis.
The moral of the story: if you're depressed about the numbers your power meter is telling you, it might not be your legs, it could be the meter. So check those things. Don't believe everything you're told, even if its from computer controlled strain gauges. And test those PowerTaps!
Future post: I check Cara's Powertap wheel. It seems to behave much better in informal stomp testing. I look forward to seeing how it responds to the water bucket.