Ergomo Pro or Power Tap SL 2.4?

TamMan

On that note - does anybody have any experiences with the SRM Amateur? I was wondering whether the 5% accuracy is an issue? I would assume that between the PT and the SRM Amateur, the PT would be a much better choice (apart from the financial aspect).

Bruce Diesel

If the Ergomo is extremely sensitive to bb position and torque, then I suspect that the change in temperature would have a significant impact. The different expansion coefficients of the bb shell and the bb would cause a change in the overall torque experienced by the ergomo bb. It would be interesting to test this in an airconditioned room where the ambient temperature could be changed significantly.

Also, since the ergomo uses perforated light disks to measure the torque being applied, the resolution of light is extremely high and small changes would be detected. Maybe the use of light disks and their associated resolution is actually counting against the ergomo here.

cPritch67

Tom
Just so I'm clear, the older the chain causes Watts to increase (or decrease)?

Thanks
Chris

Tom Anhalt

According to that data and the linear fits, the old(er) chain exhibited higher drivetrain losses than the new chain...with the difference getting smaller with higher powers. Just plug some typical power levels (200W, 250W, 300W, etc.) into the 2 equations and you'll see what I mean.

acoggan

I think you're misunderstanding how I generated the data that I provided: using one, and only one, gear, I pedal my bike while it is locked in the Velodyne for 3 min at each power output, tossing the first ~30 s worth of data and averaging the rest. As the data show, when comparing crank vs. hub or wheel measurements of power, the difference appears primarily in the form of a significant intercept, not a difference in slope from unity. (IOW, efficiency increases with increasing power output.)



IIRC, it was apparent from Robert Chung's analysis of your data that even your installation shows evidence of a chainline effect.

Tom Anhalt

No...I fully understand how you are taking the data. What I'm asking is that given the data that shows the drivetrain efficiency at a given power level can vary significantly by just switching to a different gear, how can you confidently apply a model from a test where you only used ONE gear to analyze files that used multiple gears?




Yes...but then the question becomes, given the known variation of drivetrain efficiency with gear selection, which of the 2 PMs in the "test" are exhibiting what you call the "chainline effect"?

acoggan

That's easy: I don't.



You'd have to refresh my memory re. your test conditions before I could answer this question.

Tom Anhalt

I would submit that you actually DO...if you assume that your "one gear" analysis result (i.e. the fixed offset value) is applicable to multi-gear power files (i.e. non-single gear files taken "in the field"). Are you saying that you don't assume that when looking at a PT file?




The original testing that Robert analyzed and showed a "chainline" effect was a constant wheel speed power test ON A TRAINER, which for a couple reasons is not surprising for the Polar.

Just recently, he analyzed some of my PT vs. Polar crit data and it "appeared" there may be some "chainline" effect on there...but, that's where it becomes unclear which device is contributing that appearance...with the added uncertainties involved in determining the power vs. gear data due to the differences in the recording algorithms of the 2 devices. In short, that exercise was inconclusive IMHO....