Last time I compared Powertap to Vector on my Winter Allaban, I had failed to properly set the crank length on the Garmin 800. I'd set it on the Vector, but the menu item on the Garmin is a bit tricky to find, as it only appears when it is in communication with a Vector, so if you scan the menus without activating the Vector and pairing the Edge to the Vector you'll never see it, as I did not. I actually first confirmed my Edge was asserting the wrong cranklength by running the settings.fit file through fitdump. But this is a digression.
I basically repeated my first calibration ride, heading steep 14th Street to Buena Vista in San Francisco and doing one-legged climbing on a more gradual grade, then riding home and doing a sprint.
I previously did three comparisons where the maximal power curves of the Powertap and Vector were remarkably coincident after I scaled one of the powers by an empirically determined factor close to the 85% value which I later discovered was the ratio of the programmed (144.5 mm) to actual (170 mm) crank lengths. These ratios were 84% (first ride, no washers), 91% (after adding one washer), and 86% (2nd ride with one washer). Since Vector measures power upstream from the Powertap I'd have expected a number of around 87%, considering drivetrain losses, so to me this seems consistent with the washer improving things slightly but requiring a settling in period following re-insallation. So I fully expected on this fourth test, with no crank length adjustment, to be as impressive except without any power scaling.
It was still good, but maybe my expectations were unreasonably high. Here's the maximal power curve. One expects the Vector to Powertap ratio to be higher at lower powers, since drivetrain losses fail to scale fully proportionally with power, and so the fractional losses are greater at low power, less at high power. But I wouldn't expect, except perhaps in sprints (<10 seconds), for the Vector to actually measure less than the Powertap. I plot two curves here for the Vector: one the power measured by Vector, the other the power which I'd expect to reach the Powertap, around 97% of the total (although the actual fraction would depend on power).
The ride was divided into parts. First I rode to Buena Vista, which included several blocks of steep climbing. These data I time-synchronized (despite hitting "start" at the same time on the Edge 800 and Edge 500, the data in the two are time-shifted, perhaps by auto-pause which I was too lazy to shut off). The main reference in time shifting is the cadence, since the Powertap and Vector both generally agree closely on cadernce. Then I take only time points for which both meters register at least 10 watts. This filters out marginal time points, for example where I might be freewheeling and registering power on the Vector but not the Powertap. Then I plot the Vector power versus the Powertap power.
The scatter in points don't mean much. The reason is that the two aren't measuring power at the same time interval. The Vector measues it once per pedal stroke, the Powertap once per wheel revolution. Even without this difference, they're not synchonized. So a lot of the power difference is actually time difference. If I take the average value of the Vector divided by the average value of the Powertap I get 98.4%. If I take the root-mean-squared, which puts a higher weight on high values, I get 98.7%. The slope in the plot is 97.0%, but that's a bit harder to interpret. In any case, the Vector is reading lower than the Powertap, slightly.
Then I did one-legged sprints. First I did right leg, then left leg, then on the same climb I did both legs. Here's right leg:
Then left leg:
So far so good: the Vector is reading a bit higher than the Powertap, as I'd expect for drivetrain losses. But then I plot both legs.
This isn't so good. What was up with that?
Here's some quick plots I did with xgraph, looking at cadence. First, the cadence on the steep climbs up to Buena Vista. Vector is green, Powertap in red. The two agree very well:
The Powertap drops cadence for a few samples here, but generally the two are in close agreement.
The following is the right and then the left leg pedaling. The Powertap is choppy. It gets confused by the blocky pedal stroke of a single leg. The Vector, however, uses the accelerometer, and produces nice smooth cadence. The Vector needs cadence to determine power, while the Powertap does not, so the Powertap power estimate is not affected by this choppy cadence estimate.
Now comes the 2-legged sprint. Here the Powertap is smooth cadence again. For some reason the Vector cadence estimate drops a bit on two occasions, at just before 2180 seconds on the time axis, and another time a bit after.
If I look at power during this bit, the Powertap and the Vector agree fairly closely, except at these dips where the Vector produces power dips. Since it uses cadence to determine power from force, this dip in power was inevitable.
The dips are dragging down the averages from the maximal power curve.
Why did this happen here? I'm not sure. The cadence is determined from the accelerometers tracking the direction of gravity's pull (or so I assume). Could the fact I'd recently clipped in affect this? I can see it affecting the torque measurement, but not the cadence. So I'm slightly puzzled. Was it something about my pedal stroke? If so, I might expect some anomaly in L-R power, but L-R is spot on 50%:
Overall, the test confirmed that I'd fixed the problem with the crank length. Additionally, the two power meters were in very respectable agreement, well within the stated accuracies of the units until you factor in the drivetrain losses. I definitely want to do another run.