Wednesday, June 4, 2014

Vector vs Powertap ride #3: crank length problem solved?

The plot thickens...

Another day, another ride. This time I made sure to do a static zero test before the ride. I further checked that I had the latest firmware in my Edge units (Mac WebUpdater says I did). I was full of hope that the crank length fix had corrected things. So I set off on a ride to fetch new contact lenses, but where I took "scenic detours" up five significantly steep, significantly painful climbs in San Francisco. For the record: 17th from Market to Twin Peaks (and on to Twin Peaks summit), Sanchez from 17th to 18th, Church from 18th to 21st, 21st from Church to Sanchez (these two are contiguous, but I took a break in between, not initially planning to ride 21st, which is daunting), and finally, just because I felt like I could, the ever-painful 22nd from Vermont to Carolina.

No luck. My suspicion now is that the head unit is overriding the crank length setting. Issue is I don't know how to change the crank length in the head units. In the 510 or 810: no problem. There's a menu for that. But I can't find it for my 800 or 500.

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Here's the data. First I show the maximal power curve. Note this time I get a decent match with an 86% scale factor. This is back to the first of my three rides, the one without the spacer, where I used a ratio of 84%. The second ride had a ratio of 91%. That one may have been an anomaly. It was my first ride after reinstalling the pedals. There seems to be a settling in time with Vector, which has been observed before.

I check my LR balance for sanity. This ride was a bit different than the previous two, but same general pattern.

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I then added a new plot: with the maximal power curve comparison, instead of plotting power versus duration, I plot the ratio of Vector to Powertap powers for each duration versus Powertap power (duration hidden). This is a remarkable plot: the scatter in data is within 1%. Of course the units claim that this should be the case, but I don't trust such claims, which I expect are based on riding in a controlled environment at fixed cadence on a stationary trainer. There's two trends of note. One is the ratio initially decreases with increasing Powertap power. This makes sense considering the expected trend in drivetrain efficiency, which increases with increasing power. But then there's a precipitous drop. I suspect this is due to the way Powertap calculates power over an integral number of wheel rotations rather than over an integral number of pedal rotations. Integral pedal rotations, used by pedal and crank-based power meters, is preferred. But this plots my "by eye" determination of 86%, which applies over much of the power range.

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So what's the problem? By far the most likely explanation to why I've tried twice to set the crank length to 170 mm and failed twice is that the head unit is overriding my selection. The philosophy is if a user sets a crank length for his different bikes, transferring the Vector from one to the other, he will select different profiles on his head unit, and shouldn't need to reprogram his Vector. This makes sense.

But why have I seen no crank length option on my Edge 800? Apparently the options only show up when the Vector is "detected and active" (according to support). I probably checked only when this wasn't the case. But there's another way: the file Garmin/Settings/settings.fit on the Vector has the length encoded within. I can check this with the Perl module Garmin::FIT which comes included with a very useful "fitdump" utility. I ran this on the Edge 500 and I get 0xfe, which corresponds to "auto". On the Edge 800, on the other hand, I get:

> fitdump /Volumes/GARMIN/Garmin/Settings/Settings.fit | grep -i length
crank_length (19-1-UINT8): 144.5mm (69)
crank_length (19-1-UINT8): 144.5mm (69)
crank_length (19-1-UINT8): 144.5mm (69)
crank_length (19-1-UINT8): 144.5mm (69)
crank_length (19-1-UINT8): 144.5mm (69)

144.5 mm! On one hand, this seems silly for a default. On the other hand, had it been 172.5 mm, the problem would have been much tougher to identify since the error would have been only 1.5%, consistent with drive train losses. But I'm not sure why it wasn't "auto". Note it's 144.5 mm for all of my bike profiles, not just the "Vector" one. So this wasn't a problem of me mis-setting it.

I then paired the 800 again with the Vector then went through the menus to my bike entry for Vector power. I initially looked for crank length under the "ANT+ Power" submenu and didn't find it. But then there it was under the main list of bike parameters. Indeed, it was 144.5 mm, overriding the value I set with the Vector Updater. I set it back up to 170 mm. I should now be ready to go.

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