ibike Power meter - Page 4

SD, or CV?

This doesn't surprise me.

To the engineer in me: They all are estimates and what matters is how well each model's implimentation solves the individual's needs. - TF

Edit: In other words, the part of my job that did pay the bills was, with a reasonable knowledge of the theory, to find the most cost effective solution to each individual problem regardless of the model or technology involved. - TF

I could as easily say - "Ultimately", how fast you go or how soon you get there depends only upon the power generated against the opposing forces.

I'm not saying that torque (wherever it's measured) is not a valid metric for most of our requirements. I am saying that:

- The measurement of torque is no more "real" nor, necessarily, any less of an "estimate" than any other.

- "..for many of us..." doesn't apply to all of us.

TF

That's exactly what I was about to post about, but I'm glad you said it first, since I'm no engineer.

I guess it all boils down to how much you are willing to have other "stuff" affect your "estimate" and affect the consistency of that "estimate" then, doesn't it?

But having a reasonable level of accuracy is in everyone's interest. I leave it up to the user to define "reasonable".

..and you're making corrections for kinetic energy issues, right?

What would be the "right" course? One that looked like a sine wave and you started and ended at the same "phase" of the sine function?

Thanks for clarifying. The website left me wth that impression too.

Sure. Of course.

Yup.

Simpler than that. I used a little tree-lined street that crossed a creek in the middle of the block. It had about a 20 foot drop from the ends of the block to the culvert, so I could do many loops at various speeds in only a few minutes without using my brakes or changing position. Basically, I could roll down, or accelerate down, or accelerate up as long as I let gravity slow me down just enough to make the U-turn at the ends of the block without using my brakes. Then I "bootstrapped" each loop to get an idea of the variability.

I did essentially the same thing with Dede Demet's Montreal World Cup data here, but of course she used her brakes so that was simply an illustration.

Interesting. I like your conclusion:

SD. To elaborate a bit, the 2%/6% figures came from a two-step test. I did 10 coastdowns, and after each coastdown I did a second test modelled on the procedure in the iBike manual for verifying if a coastdown produced "good" or "bad" values. This is where you watch the unit's unfiltered wattage screen (accessible in one of the setup screens) and verify that it stays within +/-20 as you coast. To make this second test more selective, I chose to do it at a higher speed than the initial coastdown test, from 30 to 20mph vs. the 20 to 8mph of the normal coastdown procedure. Only 4 of the 10 coastdowns passed this second test. For reference, SD for all 10 coastdowns wasn't nearly as good. I don't have my data in front of me, but I believe it was around 7-10%.

Yeah, I was expecting it too. Since the accelerometer is reading the sum of aero and friction during the coastdown, it makes sense that shorting one would lead to overestimating the other.