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
  • Further to my post yesterday, I went on again last night and started with no new calibration i.e. exactly where I left off yesterday morning and the Auto Zero turned off. I did a 20 minute warm up with a few sprint hard efforts in then stopped and calibrated. The data for the first 20 minutes reflects where I left off yesterday morning. Average power from the Neo was 220 watts with the Vectors reading 208 watts a 5.5% difference. I then did a one hour event after calibrating and noticed my readings seemed to be tracking reasonably well for the duration of the event with no drift. This is borne out by the results which showed the Neo reading 226 watts average and the Vectors reading 229 watts. Normalised readings were 231watts and 233 watts so within +/-1% accuracy of both devices and given the Vectors are at the front of the drivetrain I'd expect these to read a bit higher than the Neo which is my past experience of using both.

    I'm not entirely sure what to conclude. Perhaps they needed some time to bed back in again after servicing? Perhaps I need to leave a longer period of time from the start before calibrating. Ordinarily this wouldn't be a problem if I was leaving them on the same bike but I change them between bikes regularly. I will continue to monitor and see how they go.
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  • Further to my post yesterday, I went on again last night and started with no new calibration i.e. exactly where I left off yesterday morning and the Auto Zero turned off. I did a 20 minute warm up with a few sprint hard efforts in then stopped and calibrated. The data for the first 20 minutes reflects where I left off yesterday morning. Average power from the Neo was 220 watts with the Vectors reading 208 watts a 5.5% difference. I then did a one hour event after calibrating and noticed my readings seemed to be tracking reasonably well for the duration of the event with no drift. This is borne out by the results which showed the Neo reading 226 watts average and the Vectors reading 229 watts. Normalised readings were 231watts and 233 watts so within +/-1% accuracy of both devices and given the Vectors are at the front of the drivetrain I'd expect these to read a bit higher than the Neo which is my past experience of using both.

    I'm not entirely sure what to conclude. Perhaps they needed some time to bed back in again after servicing? Perhaps I need to leave a longer period of time from the start before calibrating. Ordinarily this wouldn't be a problem if I was leaving them on the same bike but I change them between bikes regularly. I will continue to monitor and see how they go.
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