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
  • thanks for the reply.

    the last time i installed the vector pedals was about three months ago, and i normally ride at least three times a week, so i'm assuming they've settled in by now. i also made sure to put a few hundred miles on the powertap before i started paying attention to the numbers it's generating.

    i do have auto-zero enabled for both power meters. for the vectors, i assume they won't be able auto-zero themselves during a workouts unless i stop and unclip (which i normally don't do). but i do sometimes backpedal during recovery intervals, and i know the powertap can auto-zero itself when coasting, so that may have been a factor here. i'll be sure to disable auto-zero for both devices to make sure that isn't impacting my results. (i always do a manual zero-offset for each power meter at the start of every ride and i'll continue to do that.)

    during my next workout i can try doing the manual zero offset or different devices during recovery intervals to see how that affects the results.

    of course looking at the data from this last workout it seems like there was drift within the first 11 minutes of the workout (before my first backpedal). the powertap numbers started out below the vector numbers, but by 11 minutes in they were both reporting matching numbers.

    that said, do you think there's any downside to just doing a static weight test at the end of my next ride (regardless of the effects of the manual zero offsets)? i recall reading some youtube comments where someone recommended putting power meter components through a few stress cycles before doing a weight test, so presumably doing a static weight test immediately after a workout couldn't hurt?
Reply
  • thanks for the reply.

    the last time i installed the vector pedals was about three months ago, and i normally ride at least three times a week, so i'm assuming they've settled in by now. i also made sure to put a few hundred miles on the powertap before i started paying attention to the numbers it's generating.

    i do have auto-zero enabled for both power meters. for the vectors, i assume they won't be able auto-zero themselves during a workouts unless i stop and unclip (which i normally don't do). but i do sometimes backpedal during recovery intervals, and i know the powertap can auto-zero itself when coasting, so that may have been a factor here. i'll be sure to disable auto-zero for both devices to make sure that isn't impacting my results. (i always do a manual zero-offset for each power meter at the start of every ride and i'll continue to do that.)

    during my next workout i can try doing the manual zero offset or different devices during recovery intervals to see how that affects the results.

    of course looking at the data from this last workout it seems like there was drift within the first 11 minutes of the workout (before my first backpedal). the powertap numbers started out below the vector numbers, but by 11 minutes in they were both reporting matching numbers.

    that said, do you think there's any downside to just doing a static weight test at the end of my next ride (regardless of the effects of the manual zero offsets)? i recall reading some youtube comments where someone recommended putting power meter components through a few stress cycles before doing a weight test, so presumably doing a static weight test immediately after a workout couldn't hurt?
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