Friday, August 16, 2013

Applying pedal smoothness algorithm to Metrigear Vector data

Last time I proposed an algorithm for pedal smoothness. I can hardly take credit for it, it was basically the reciprical of Coggan's variability index without the 30-second smoothing. It's fairly obvious to apply it to the pedal stroke, as well.

Here's some data left over from the old Metrigear Vector blog, showing measurements taken with the Metrigear-era Speedplay Vectors. These data are at a much higher sampling rate than would be recorded by an Edge computer: they show the detailed power and cadence during just a few seconds of a longer "ride" (on the trainer):

I used Plot Digitizer to pull points off the plot (off-topic: I really like Plot Digitizer; it's replacing g3data, which I previously used). Here's a view of a subset of those data. Curiously, the left leg is going negative power, while the right leg does not.

The plot also shows total power, the sum of the L and R legs. This shows a strong oscillatory character: it goes from a maximum of near 800 watts to a low of near zero each half-pedal-stroke.

So then I did a running calculation of smoothness for each leg, smoothness for net power, and of course L-R power balance:

You can see the values oscillating with each pedal stroke. This isn't really a problem, since Vector uses "event-drive" data recording, generating numbers with each pedal stroke. Even after only 3 seconds of smoothing the amplitude of the oscillations nicely. But as I noted, as long as Vector averages over complete pedal strokes, the oscillations should be essentially eliminated.

Not surprisingly, single-leg smoothness numbers are lower than smoothness for the two legs combined. This is because the power from the two legs are anticorrelated: when one drops off, the other tends to pick up the slack. The sum is smoother than either of its parts.

Also note the left leg is scoring a lower smoothness than the right. Since the left leg is dipping into negative power much more prominently than the right leg, this seems like a reasonable result.

Another factor is whether the numbers are of the correct order of magnitude. Without any definition, if you asked someone how smooth their pedal stroke was considering both feet together, then considering each foot separately, I think 70% and 35% are plausible responses. Really, most users of a number aren't going to delve into the detailed derivation: they just look at a number and judge it at face value. 70% says to people "mostly smooth, but not perfectly smooth", while 35% says "not very smooth". These seem like the correct messages.

Metrigear Waterbottle
Metrigear Waterbottle data recorder

Ideally, it would be interesting to be able to access each of these numbers. Why not? I think left leg, right leg, and total power smoothness all provide an interesting story. But what would be most interesting is if they finally support the detailed pedal stroke curves, like these data published on that Metrigear blog. There's no reason that can't happen. Maybe it would require a pod upgrade, but with pods selling at $69 each, less than 5% the cost of the full system, that's the sort of upgrade a lot of people would be happy to make down the road. The issue is how to transfer the data to the user. The old Metrigear water bottle isn't necessary any more, with low-energy bluetooth almost universally available on mobile devices. Then we'll see Vector truly differentiate itself.

This, after all, is the brilliant aspect of the Vector design: with so many of the "brains" in the cheap, external pods, the investment made in the more expensive pedal spindles should be a lasting one.

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