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
  • Last night I removed the pedals, degreased threads, cleaned faces, re-greased and installed with a good tighten.
    This morning I calibrated before the start, reset install angles and did a ten minute warm up with a few sprints and also turneed off the auto-zero function.
    I then re-calibrated the pedals and began a Step test, starting at 150watts increasing 25 watts every 2mins 30 seconds using the Tacx Neo ERG

    The results for each interval were of were as follows:
    Step 1 - Neo: 174watts / Vectors: 175watts +0.5%
    Step 2 - Neo: 200watts / Vectors 200watts 0%
    Step 3 - Neo: 225watts / Vectors: 222watts -1.3%
    Step 4 - Neo: 248watts / Vectors: 243watts -2.0%
    Step 5 - Neo: 274watts / Vectors: 267watts -2.6%
    Step 6 - Neo:300watts / Vectors: 289watts -3.7%
    Step 7 - Neo: 326watts / Vectors: 313watts -4.0%
    Step 8 - Neo: 347watts / Vectors: 323watts -6.9%

    I failed the next interval so killed off the ERG and decided to spin out the session at about 150 watts based on the Vectors thinking that this would coincide with the Neo once the power dropped off however, the last two remaining intervals which are 2:30 long and then 5:00 long were as follows:

    Neo 162 watts / Vectors 151 watts -6.8%
    Neo 160 watts / Vectors 150 watts -6.3%

    This would suggest that it is not necessarily the higher power outputs which the drift i occurring at as it seems to drift with time. Could this be somehow heat related within the pedal and the higher outputs are generating more heat which is throwing it out as it seems that dropping back to lower output started to reduce the difference? If I'd had more time I should have calibrated again at this point and continued to see what happened. The files from the Vectors is attached below.

    3207280854.zip

    i have the vector 3 for about 2 months.while in the first month only outside they were ok,when switching to indoor i have same problems with them drifting(to low values)compared to p2max negeco.also later in the ride . I have also noticed that when using them on rotor 3d24 cranks the numbers are more consistent from ride to ride.but when mounting them on dura ace 9100 the values are 6-10 watts lower.On the rotor they sit flusher on the crank arm and they get outside the crankarm 1-2mm more than on dura ace.
Reply
  • Last night I removed the pedals, degreased threads, cleaned faces, re-greased and installed with a good tighten.
    This morning I calibrated before the start, reset install angles and did a ten minute warm up with a few sprints and also turneed off the auto-zero function.
    I then re-calibrated the pedals and began a Step test, starting at 150watts increasing 25 watts every 2mins 30 seconds using the Tacx Neo ERG

    The results for each interval were of were as follows:
    Step 1 - Neo: 174watts / Vectors: 175watts +0.5%
    Step 2 - Neo: 200watts / Vectors 200watts 0%
    Step 3 - Neo: 225watts / Vectors: 222watts -1.3%
    Step 4 - Neo: 248watts / Vectors: 243watts -2.0%
    Step 5 - Neo: 274watts / Vectors: 267watts -2.6%
    Step 6 - Neo:300watts / Vectors: 289watts -3.7%
    Step 7 - Neo: 326watts / Vectors: 313watts -4.0%
    Step 8 - Neo: 347watts / Vectors: 323watts -6.9%

    I failed the next interval so killed off the ERG and decided to spin out the session at about 150 watts based on the Vectors thinking that this would coincide with the Neo once the power dropped off however, the last two remaining intervals which are 2:30 long and then 5:00 long were as follows:

    Neo 162 watts / Vectors 151 watts -6.8%
    Neo 160 watts / Vectors 150 watts -6.3%

    This would suggest that it is not necessarily the higher power outputs which the drift i occurring at as it seems to drift with time. Could this be somehow heat related within the pedal and the higher outputs are generating more heat which is throwing it out as it seems that dropping back to lower output started to reduce the difference? If I'd had more time I should have calibrated again at this point and continued to see what happened. The files from the Vectors is attached below.

    3207280854.zip

    i have the vector 3 for about 2 months.while in the first month only outside they were ok,when switching to indoor i have same problems with them drifting(to low values)compared to p2max negeco.also later in the ride . I have also noticed that when using them on rotor 3d24 cranks the numbers are more consistent from ride to ride.but when mounting them on dura ace 9100 the values are 6-10 watts lower.On the rotor they sit flusher on the crank arm and they get outside the crankarm 1-2mm more than on dura ace.
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