Stages Power Meter Compared (my best DC Rainmaker impersonation)

Discuss light weight issues concerning road bikes & parts.
youngs_modulus
Posts: 512
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Location: Madison, WI USA

by youngs_modulus

Maybe my last post wasn't detailed enough for you. ;)

You have two knobs to turn if you're trying to ensure consistency: offset and slope. Offset is easily adjusted before each ride and some meters automatically adjust it anytime you're stopped and not loading the meter. Slope rarely needs to be adjusted, but that can be done by loading the pedal with a known mass (and a known crank length).

This all assumes a linear structural response from both the power meter and the hub/crank/whatever. That's a fundamentally valid assumption. If you think you're getting a nonlinear response from your crank, then you're essentially suggesting that your crank is made of Silly Putty. Maybe it is, but if so, the burden of proof is on you: extraordinary claims require extraordinary evidence.

In short, Ray Maker does a pretty good job of showing consistency between meters; when he's out of his depth, he's good about checking with others who know their stuff. Beyond that, any meter that produced unreliable readings would be skewered on forums like this one and Slowtwitch.

Unless you're talking to people who have their own labs, they can't individually provide better information than Maker does. But in aggregate, posts by people like Savechief would root out bad power meters. The upshot is that as long as you're using a decent powermeter and you're careful about setting your offset, it's safe to assume that you're getting consistent readings.

Does that answer your question?

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TonyM
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by TonyM

@ youngs_modulus and all: :thumbup: :thumbup:
Thanks for the discussion! It is extremely interesting and it is nice to learn so many things on the power meters (that the manufacturers are sometimes not mentioning) 8)

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jekyll man
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by jekyll man

Just curious savechief, when you are doing your rides, what (or which) do you actually look at, and is either of them on a rolling power average?
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savechief
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by savechief

That's the thing with the Elite Drivo...I don't *need* to look at any power numbers. I just program in my intervals, load up the session in Trainerroad and the trainer takes care of keeping me where I need to be power-wise based on my cadence.

For rides on the road, I have my Edge 520 display 10 second average power.
Time VXRS Ulteam (7.16 kg)
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pantelones
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Joined: Mon Jan 31, 2011 2:24 am

by pantelones

youngs_modulus wrote:Maybe my last post wasn't detailed enough for you. ;)


Maybe you need to coach 10 athletes who have consistent only power meters who are not technically savy. You'll understand why accuracy matters and consistent only isn't good enough.

Again, if you are willing to accept a consistent only power meter where the numbers have no solid basis to physically reality then you should use a speedo and HR monitor.

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

Easy there, tiger. I was making fun of myself and my logorrheic post, not you.

I'm going to gently suggest that you're not following the terminology. You didn't understand Bilwit's use of "normalized," and instead went on a tangent about normalized power, which isn't remotely what he was talking about. When kulivontot suggested that you misunderstood Bilwit's post, you didn't take the hint. In a technical context, "accuracy" doesn't mean what you think it means.

A power meter that reads 100 watts high on one interval and 100 watts low on the next is a very accurate power meter, but not a precise one. A meter that reads 11 watts low on both intervals is a very precise meter but not an accurate one. I'd take the precise-but-not-accurate meter, myself, but what do I know?

I defined "consistent" as clearly as I could in that long post:
youngs_modulus wrote:I'm defining "consistency" as "pretty precise" and "somewhat accurate." A "pretty precise" meter will give essentially the same output under the same conditions every time. It may be off by a bit [...] but it's off by the same amount every time. A "somewhat accurate" meter will be off from the "true" value, but maybe only 7-10 watts at your FTP.

I'm not sure why you think a power meter that meets that definition has "no solid basis to physical reality." I am beginning to understand, however, your point about how frustrating it can be to discuss these things with people who aren't technically savvy.

I'll try to put it another way: Imagine you're doing sprint workouts with an SRM Science meter and you do a sprint at exactly 746 watts. You've also got a Powertap hub and Garmin pedals. You trust the SRM because it has the best accuracy spec, but you're dismayed to see the that the Powertap is reporting 550 for that sprint and aghast to see that the pedals are reporting 1.000 for the same sprint. Such inaccurate numbers from the Powertap and the pedals means they're wildly miscalibrated, yes?

No. All of these values are exactly one horsepower. The SRM is reporting it in watts, the Powertap is reporting it in ft*lbf/s and the pedals are reporting it in horsepower.

Two months later, you do the same workout with the same three meters. This time, the SRM reads 821 watts for the same sprint, a 10% improvement. Not bad! The Powertap is up to 605 ft*lbf/s and the pedals are showing 1.1 horsepower. These are all 10% improvements.

This is what Bilwit was trying to explain to you: powermeters set to different units are exactly like consistent but inaccurate powermeters: they all show the same improvement with respect to their baselines. Reporting changes with respect to the appropriate baselines is what normalized data are.

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