Last time I did a comparison of my Powertap versus Vector. I'd had strong reason to believe the Vector was reporting low, and the test confirmed it. The maximal power curves lined up fairly nicely when I multiplied the Vector power by 0.84 (emphasis on fit to high powers). Since the Vector should measure more power than the Powertap, this indicated an error somewhat greater than this.
You'd expect the Vector to measure more power, because the Vector power includes power which doesn't make it to the rear hub and is instead lost in the drivetrain. This power loss includes losses which are proportional to load (like bending the chain under a high-tension state at the top of the run) and losses which are independent of load (like spinning the pulleys). The result is that drive train losses, as a fraction of total power, are higher at lower powers than higher powers. What I saw was consistent with this, which was a good sign.
The obvious candidate was that I'd not inserted any washers under the spindles. The Vectors rely upon the pedal spindle bending a predictable amount under a given applied moment. This requires that the interface between the spindle and the crank arm be nice and constant: contact along a single ring, independent of how much the spindle bends. If the contact is along more than just a ring, then the calibration would have been incorrect. Had it been calibrated to your specific set-up, this would have been fine, but it's calibrated at the factory, so you need to make sure the spindle-to-crank contact is as close to equivalent in your bike as it is at the factory calibration rig. This is why the spacers are necessary, and why the pedals should be installed with a torque wrench. I bought a torque wrench specifically because of the Vector, and use 300 lb-inches (25 lb-ft) as recommended.
I'd hoped my White Industries Cranks, with their relatively flat faces, wouldn't require a spacer, and so I could minimize my pedal stance, since I have narrow hips. The spacers add only a few mm to stance, but over the course of hundreds of thousands of pedal strokes, even a few mm has an accumulated effect.
But after my first test, I decided to add one spacer to each pedal. I then went for another ride: to watch the Escape from Alcatraz with Cara, then climb the Headlands. I did this ride, despite legs still feeling tired after the Memorial Day Tour, the recovery from which was somewhat retarded by a challenging SF2G last week via a mountainous coastal route which included two extended dirt sections.
Alcatraz was super-cool, watching the swimmers emerge from the 57F water to the long swim-to-bike transition run. I simply can't contemplate swimming in that water, which I've seen people do without wetsuits. Even with a wet suit, just jumping in hits you with an icy cold shock. Not for me, although the bike and run legs are highly attractive, the run including the infamous sand ladder climb which is a lot of fun.
But while I was at the race, my Powertap battery died. My Vector Batteries had started issuing low-battery warnings along the way, but that wasn't a problem, since under low-battery there's still plenty of time to finish a ride. My first-generation wireless Powertap's "low-battery warning" is that it suddenly stops transmitting.
So I was working with partial data, with none of the extended efforts I made during my three climbs of blustery, fogged-in Hawk Hill. The climb was perhaps less interesting than the descents, which included strong cross-winds. I took them relatively slowly, but I experienced none of the instability I'd felt on MDR. I think tightening the headset may have worked.
Anyway, here's a comparison of the data. Note I get alignment along the much of the curve when I multiply Powertap power by 0.91. So this is an improvement over the 0.84 I used without a spacer. To match the highest power, I'd have wanted to use a factor closer to 0.90. The lower fraction is consistent with what I noted earlier about drivetrain losses.
As a sanity check, here's the L-R balance again, adding this ride to the previous day's. Here I've adjusted power by the 0.84 and 0.91 ratios to get power more closely matched to the Powertap.
I was initially puzzled by what could cause this. Of course the obvious explanation was the crank length was set incorrectly. I can't check that on my Edge 500 or Edge 800: only the newer units, the 510, 810, and 1000 have the capability to interact that closely with the Vector. But of course I set crank length when I first set up the system.
I decided to upgrade the firmware, so after swapping the batteries, I plugged the Ant+ USB stick into my Mac, and ran the Vector updater. Sure enough, firmware was upgraded from 2.1 to 2.4. But then it provided a widget to set the crank length. Crank length was 144.5 mm, it said.
Whoa! How'd that happen? Obviously I'd not set it up correctly, or else one of the Edge units overrode the value I'd set. 144.5 mm is 85% of the correct value of 170 mm. The Vector measures cadence and force, but to get power you need an extra factor of distance, and that distance is proportional to crank length. So having 85% of the correct crank length yields 85% of the correct power.
I set the crank length to 170 mm and went for another ride. That one didn't work out so well, however, because initially my Edge 800 which I'd been using to monitor the Vector wasn't displaying power, then I tried to sync with the Vector and it complained it found two separate power meters. Then I removed my Powertap wheel and put it half-a-block down the street, in a metal box, and finally the computer found only the Vector (I'd tried progressively less drastic displacements without luck before this). But it still wasn't reading power. So I power-cycled it and then it was reading power, but I must have neglected to hit start, because when I got home, the rest of my ride was missing.
So I'll try again. Hopefully this time setting the crank length stuck.
Note that if it does stick then I would get with zero spacers the Vector reading 1% below the Powertap. This is obviously incorrect since it would imply the drivetrain losses are negligible. But with the 0.91 conversion factor the Vector would read 7% higher than the Powertap. This would imply quite a high degree of drivetrain losses. But I need to see what my next test ride shows.