DFPM (Stages) Now What Do I do with the Data?

[hawkesworthm]

I now have a Stages and connected it to my Newton+. All the data collection appears to be working, but I"m struggling to determine what am I supposed to do with the comparison data. Am I supposed to tweak Newton settings based on what I'm seeing? Here are screen shots of today's ride.

Matt

[Velocomp]

This is a tough one, because your DFPM is a one-leg power meter.

One-leg power meters measure half of your actual effort (left leg in the case of Stages), then multiply the measurement by 2.

Where there are differences between the Newton and a one-leg DFPM, is it because a change of ride position, because of a difference in PM calibration, or because your two legs are not working equally?

Note, for example, the huge DFPM surge around mile 29. If your left leg is stronger than your right leg, then the one-leg meter is going to give a false reading, by a lot.

With a one-leg power meter it is impossible to know what's what. In our testing we've seen one-leg power errors as much as +51% and -22%.

The Newton measures the power of both your legs, so the one-leg problem does not apply.

When you're comparing power measurements it's important to be sure you are comparing the same kind of measurement. Comparing one-leg and both-leg measurements is intrinsically difficult.

[hawkesworthm]

Thank you John. That surge at mile 29 was a rapid acceleration of me leading someone out to sprint. So this leads me to think that the Newton doesn't equate the acceleration as requiring as much power as the Stages because there is a significant difference in that jump or that my initial kick may have been with my left leg prompting a higher reading. What I'm trying to understand is that on other posts I've read, people are trying to understand their difference in seconds and meters but when I look at my data, I see the difference in minutes and miles.

[Velocomp]

Yes, it is the initial kick with your left leg that is causing the Stages to report too-high power (it looks like it over reported by 350W!). Newton measures acceleration 800 times per second, from both legs, so Newton is not susceptible to the left-leg over-reporting problem. By the way: if you started your surge with your right leg, your one-leg power would have been underreported.

Regarding minutes and miles: one-leg and two-leg data can be compared only if you assume that the one-leg left/right split is truly 50%/50% and does not change as you ride. You can, for example, adjust the Newton CdA so that it provides the same on-the-flats measurement as the one-leg measurement, but is that adjustment because of an actual correction to your Newton profile, or because the 50/50 assumption of the one-leg measurement is not correct? Or, when power deviates between the Newton and the one-leg measurement, is that because your left/right leg output has changed (your body may respond differently to stresses) or because you've changed ride position (from your ride position on the flats to your ride position on hills, for example)?

My point is, it's impossible to tell. Most certainly you can try to adjust your Newton profile so it more closely matches the one-leg numbers, but I don't know that this means anything.

[hawkesworthm]

Thank you and very valuable information as always. I think I'll do some sprint or hard effort intervals and purposefully start with each leg to see if the trend is consistent over time.