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
  • So a bit more testing today....

    Did an FTP test(!) I'd say I know my body quite good, since I have done quite a few FTP tests. I'm usually around 330'ish W in off season which typically goes up to 350-365 in "hot" season. Current shape is quite good - building up for La Marmotte in France this summer.

    So in a test right now, I'd expect around 350-360'ish W.

    I also found a video on how to reset the NEO 2 to factory standards. So I did that. I then did the test according to Zwifts protocol (the long one, 20 min).

    So NEO 2 came out with.......= 350 W
    Vector 3 came out with......... = 340 W

    Hmmmm.... Throughout the warmup phase, with 10 s rolling average, V3 was consistently 5-15 W lower than NEO. The higher the wattage, the higher the offset.... Hmmm that's too much for me!

    I have not done another static weight - I might do that this week. But....

    But based on my own feeling and X numbers of FTP tests, I don't believe V3 is right here.

    Edit
    I did reset installation angles and zero'ed twice after 10 min burn in on V3.

    It's not that I have concluded I'm done with V3. As I have written in another post, I really like that pedals. But I am considering a Quarq Dzero. I am tending toward more belief in a crank based system...

    Edit 2
    Just saying
    https://support.tacx.com/hc/en-us/articles/360000455705-Calibration-and-power-measurement
Reply
  • So a bit more testing today....

    Did an FTP test(!) I'd say I know my body quite good, since I have done quite a few FTP tests. I'm usually around 330'ish W in off season which typically goes up to 350-365 in "hot" season. Current shape is quite good - building up for La Marmotte in France this summer.

    So in a test right now, I'd expect around 350-360'ish W.

    I also found a video on how to reset the NEO 2 to factory standards. So I did that. I then did the test according to Zwifts protocol (the long one, 20 min).

    So NEO 2 came out with.......= 350 W
    Vector 3 came out with......... = 340 W

    Hmmmm.... Throughout the warmup phase, with 10 s rolling average, V3 was consistently 5-15 W lower than NEO. The higher the wattage, the higher the offset.... Hmmm that's too much for me!

    I have not done another static weight - I might do that this week. But....

    But based on my own feeling and X numbers of FTP tests, I don't believe V3 is right here.

    Edit
    I did reset installation angles and zero'ed twice after 10 min burn in on V3.

    It's not that I have concluded I'm done with V3. As I have written in another post, I really like that pedals. But I am considering a Quarq Dzero. I am tending toward more belief in a crank based system...

    Edit 2
    Just saying
    https://support.tacx.com/hc/en-us/articles/360000455705-Calibration-and-power-measurement
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