power meter drift issues?

hey all,

unfortunately i've fallen into the power meter accuracy snake pit.

recently i've been seeing what looks to me like power meter drift. you
can see a pretty good example of this from my last trainerroad workout
here (done on a pre-2017 wahoo kickr snap):

https://analyze.dcrainmaker.com/#/public/ade655fb-936b-462b-6037-ef06a2775681

one power meter is a set of garmin vector 3 pedals with a claimed
accuracy of +/- 1.0%. (i've followed the service advisory notes for the
vector and verified the spindle torque. after installing the vectors i
reset the install angles and i also did a static torque test, as
described on the official garmin support pages, using a rice lake
calibrated 20 kg weight). the second power meter is a powertap g3 hub
with a claimed accuracy of +/- 1.5%. both power meters were calibrated
(zero'd) before the workout, which was done indoors at a consistent
temperature of around 70 F.

at the start of the ride, the powertap numbers are a few watts below the
vector numbers, which makes sense given some drivetrain power loss. but
by the end of the ride the power meters have reversed, with the powertap
giving numbers about 10% above the vector numbers, which seems wrong.
(aside from the drift, the difference between the two power values seems
out of spec for claimed accuracy of the devices, especially when you
consider that there should be some drivetrain power losses).

has anyone else seen something like this before? to me it seems like
one of the power meters is drifting out of spec? any ideas what else
might be going on or how to root cause the problem (short of buying
a tacx neo or a third power meter ;)?

cheers,
ed
Parents
  • Two things I am wondering about here:

    (1) I've never ridden a Neo but at least my Flux gets pretty warm when I crank up the power - so it might be, that the thing drifting here is the Neo.
    (2) I am always a bit skeptical when I see power meter comparisons on a static trainer that is used in ERG mode: At least for the Flux the recorded power in ERG mode appears to be much smoother than the one recorded with my Vector 3 - not totally unrealistic but definitely smoother. Of course a noisier signal - probably closer to reality - can produce substantially different average power numbers.

    Maybe a good way to remove ERG mode from the equation would be to repeat the test on something like Alpe du Zwift and trying to roughly aim for certain average power levels by adjusting your personal output power and e.g. the gearing.
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  • Two things I am wondering about here:

    (1) I've never ridden a Neo but at least my Flux gets pretty warm when I crank up the power - so it might be, that the thing drifting here is the Neo.
    (2) I am always a bit skeptical when I see power meter comparisons on a static trainer that is used in ERG mode: At least for the Flux the recorded power in ERG mode appears to be much smoother than the one recorded with my Vector 3 - not totally unrealistic but definitely smoother. Of course a noisier signal - probably closer to reality - can produce substantially different average power numbers.

    Maybe a good way to remove ERG mode from the equation would be to repeat the test on something like Alpe du Zwift and trying to roughly aim for certain average power levels by adjusting your personal output power and e.g. the gearing.
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