Power-speed question
 

I have always thought that given wind, weight, etc., power determines speed. But I was fooling around in a group ride today and noticed that going a steady speed (behind some guy), shifting between gears altered the power (and yes I did let the power meter settle down). I believe it was the case that easier gears (and thus higher cadence) led to higher power. Again, that's maintaining the same speed.

Physics nerds, what gives?

And if I'm right (and I saw it with my own eyes), is it possible to overvalue power?

That's not exactly a controlled environment for a test. Firstly you don't know if the rider in front of you was putting out constant power. Secondly power reported by a meter is quite stochastic so you'd need to carefully average over a reasonable period of time to get meaningful numbers to do a comparison.

There can be small changes in drivetrain efficiency between different gears which might be measureable with a crank-based powermeter but as I said above the conditions for your test are too variable to measure the small differences that exist.

Suspect you were just seeing coincidental changes in power.

Continue to think that.

Which power meter?

For your own safety and that of others, please look up rather than stare at your computer.

I have a Stages Power meter.

I did it several times and it showed the same effect. Why don't you try it the next time you can?

It is not necessary to stare at the power meter in order to get the idea.

The power needed is determined by the product of the total drag force (incl. aerodynamic, rolling, gravitational, and mechanical losses) and your speed. But there could be small changes in the drag force depending on the gear ratio you choose - aero drag could change with cadence as your legs move faster or your body positioning changes slightly, mechanical losses can change with a different chainline and chain tension. However, I'd expect the changes to be pretty small.

Another cause for what you saw would be inaccuracies in the power meter. It has sensors to measure your pressure on the crank and therefore the effective torque and also determines your cadence in order to estimate your power input to the bike. If those sensors aren't perfectly linear then you'd get a different reading in a lower gear with decreased torque but higher cadence even if the actual power didn't change.

The white horse was 1/8 inch shorter than the black one. OP is using a Stages.

Why? Staring at computer works for that Froome guy, from what I can tell.

How much of a power change are you talking about here?

It's entirely possible your power balance changes with cadence. If you follow at a constant pace your power output will be constant but the number reported by your meter will vary depending on the balance between your left and right legs. Tough to do accurate measurements with stages unfortunately.

I agree. This is why I usually have my Garmin display a 10s avg for more reasonable smoothing.

Ah. So you may be undervaluing power.

? In the context of the right leg potentially averaging more power than the left?

Keith

In the context of using the Stages to raise and answer questions about power. People often buy the Stages for its (relatively) low price, but subsequently are puzzled by some aspect of the data they collect. They're undervaluing good quality power data. Many power meter users, not just Stages owners, claim that consistency is what matters, not accuracy or precision. The OP appears to have noticed a consistent pattern of varying behavior in the power reported at a constant speed while varying gear ratios, which challenges physics.

Not everyone needs high quality data from their power meter but there's danger in using lower quality data to try to overturn physics. If you're trying to overturn physics, you need accuracy and precision, too. That's why when someone presents a power conundrum you can save a lot of time if the first question is "which power meter are you using?"