So when doing experiments, you always want a "control case". It's better to compare the results of two similar experiments than to compare the result of one experiment with theory.
So I borrowed a wheel, one I suspect works well, and tested that. Here's the result:
It's nice having access to a second PowerTap: these things aren't cheap.
You can see from the results that this one tests better in two ways:
- the slope is closer to 1
- the intercept is substantially closer to zero
But the differences really go beyond this. Before loading the weight on the pedal, I'd zero the torque. Powertap never measures zero torque: it's designed to measure 512 lb-in when unloaded, then measures more than this when torque is applied to the hub. But this "bias" value isn't perfectly controlled, and varies with conditions. Saris considers a normal range for this bias value to be from 500 to 525 in-lb. Outside this range, and Saris wants the hub back for servicing. You can read this "raw" number in the test mode of the Powertap head unit (the Cervo, or "little yellow computer"). So to know what "zero" is, it needs to be told when it's in a zero-torque state. This is normally automatically done by the Cervo when it detects coasting (positive speed with zero cadence). During my measurements, I coast the wheel, but then I manually force a zero after the wheel has stopped but before I apply the brakes in preparation for loading the pedal spindle.
With my wheel, I'd zero the torque, apply the brake, load the spindle, find the pedal orientation which maximized the torque reading, removed the weight, and let off on the brake. So the reading should be zero again, right? Not on my wheel: it would vary, but was as high as 9 in-lb. With the borrowed wheel, it never exceeded 1 in-lb. So that was a huge difference between the wheels. This alone would explain the large difference in the intercept of the regressions done to the data from the two wheels.
But then the question is what would one expect from the test? After all, I'm applying the torque to the pedal, not the hub directly. Maybe all of the torque doesn't transfer. Indeed, when riding, drivetrain losses result in less torque being transmitted to the hub than the idealized equation predicts.
But that's riding. In this test, the rear wheel is prevented from rotating by the brake. There may still be some difference in transmitted torque from the ideal, but many of the effects which might cause power loss in dynamic pedaling aren't present in this case. I admit, it's not obvious, but the consensus is that a static test should come a lot closer to ideal than the fraction of power transmitted to the hub during normal riding.
Another plot from this test: here I show the difference between the measured and applied torque versus the rear cog. The deviation seems largest in the 39/12 and 39/13, but other than that, there was a persistent error of around one to two lb-in. Some of this error may have been an inability to nail the optimal orientation of the pedal. The display, after all, only reads to 1 lb-in precision, so errors smaller than this aren't significant. Overall, I'm quite pleased with the results of this test.
Another result: this one reported by John Meyers on BikeTechReview. He did his test a bit differently: he put the bike on a trainer suspended between two desks, used the 39/25 exclusively, and hung precision masses from the pedal. So he varied the weight, while I varied the leverage. In either case we varied applied torque.
Nice and linear! His intercept is quite close to what I got with the borrowed wheel, and his slope is amazingly close to one.
So the odd wheel out in this experiment is mine. Something's up. I then did one more test: I put the Cervo into test mode, then alternately monitored the torque for a minute and applied then removed ad-hoc torque to the hub, either by grabbing and rotating the cassette, or by applying force to the pedal.
With my wheel, it started reading 523 lb-in (very close to the upper bound of the normal range, which is from 500-525). Then I applied torque and removed it. Now it was varying from 523 to 525. Again: apply and remove torque. Now it was reading 517.
I tested the wheel a bit more later. Torque would shift when I applied and removed torque. This time, I got numbers which varied from 519 up to 525. It was generally stable when idle: it would shift mode when torque was applied then removed. Seems like something's loose in there, huh?
With the borrowed wheel, applying then removing torque didn't have much of an effect. It generally wanted to hang out in the 505 to 507 range. Sure, I'd prefer it be more stable than this: I'd prefer it be rock-solid in the generally constant conditions of the indoor test. But I'll live with that. 517 up to 525, on the other hand, and we're talking serious power errors.
I informed Saris of these results. I'll see what they say. Hopefully I can get the wheel looked over, so I can trust my power numbers again. On the other hand, it's sort of nice not worrying about power numbers, to be honest.