Found this yesterday on YouTube:
Is technical and detailed, but makes an interesting watch and is probably right up @gpl’s street. Essentially (my take) the asymmetric shape of a crank arm prevents them from being accurate due to how the positioning of the strain gauges measure relative to the cross section of the arm.
Classical engineering demonstration: the strain gauge install cannot differentiate between relevant and irrelevant forces. Ok, good. But then - how important is it? 20%? 0.001%? First, the meter is reliable for a given force distribution - it will always give the same results for the same force applied the same way. Second, the potential variability of the force distribution (in various conditions for a given cyclist, and between cyclists) and its impact on power measurement is unquantified. Third, side-by-side comparisons do demonstrate repeatability and accuracy over a large set of use conditions - which makes one think that the impact is not significant. Fourth, the conclusion is that “your Zwift data is wrong”? Really?
@gpl I knew you’d find it an interesting video. I think his comments on the current testing being done by you and Ray are unnecessary - you are providing robust testing which has identified accurate units as well as those with their own selection of rabbit accommodation problems. Your testing is a great example of repeatable tests that a typical user could perform (and the level of accuracy and consistency they should expect).
There is certainly quite a lot of commentary about power meter/trainer accuracy with regards to Zwift and e-sports. Is it even plausible to have an international race with verifiably accurate data; and how accurate does it need to be?
Does make me think I might have hastily sold a Kickr which consistently over-read by 50w.