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
  • My Edge 1030 reports torque values during calibration for the PowerTap,
    so i did a static weight test for that unit as well. I used the same
    procedure as with the Vectors. (But you have to divide the result by
    the gearing ratio.) The measured results were 1.5% below the
    calculated. Unfortunately there is no way for me to set the slope on
    the PowerTap unit. When i did the test the unit was cold though, so
    I'll try doing another test after a workout to see if the accuracy
    changes.

    My kickr snap is a pre-2017 model, so the accuracy is supposed to be +/-
    5%, way lower than either of the power meters. so generally I don't
    trust the numbers from that unit. But once it's warmed up, calibrated,
    set to a specific power, and i'm pedaling smoothly at that power, i
    generally expect it not to drift from that power. Looking around online
    it seems that most folks say that the reported kickr power values run
    high.

    The .fit files are downloadable via the dc rainmaker analyzer links.
    Just look for "Download Set Files", this has has a .zip with all the
    .fit files, and then look for a file with a -fenix5x.fit suffix. (I
    always just my Fenix 5x to record the Vector data.) If you'd prefer
    that I email them just lemme know.

    When installing pedals I normally remove any old grease from threads and
    the cranks and then add fresh grease. I normally don't use a torque
    wrench to install them, but I do use the Park Tool PW-4 pedal wrench
    (the largest one they have), which allows me to crank them down pretty
    good. (And I did verify with a torque wrench they are installed with >
    20 Nm force.) For the Vectors I also always reset the install angle
    (via a head unit) after installing them.
Reply
  • My Edge 1030 reports torque values during calibration for the PowerTap,
    so i did a static weight test for that unit as well. I used the same
    procedure as with the Vectors. (But you have to divide the result by
    the gearing ratio.) The measured results were 1.5% below the
    calculated. Unfortunately there is no way for me to set the slope on
    the PowerTap unit. When i did the test the unit was cold though, so
    I'll try doing another test after a workout to see if the accuracy
    changes.

    My kickr snap is a pre-2017 model, so the accuracy is supposed to be +/-
    5%, way lower than either of the power meters. so generally I don't
    trust the numbers from that unit. But once it's warmed up, calibrated,
    set to a specific power, and i'm pedaling smoothly at that power, i
    generally expect it not to drift from that power. Looking around online
    it seems that most folks say that the reported kickr power values run
    high.

    The .fit files are downloadable via the dc rainmaker analyzer links.
    Just look for "Download Set Files", this has has a .zip with all the
    .fit files, and then look for a file with a -fenix5x.fit suffix. (I
    always just my Fenix 5x to record the Vector data.) If you'd prefer
    that I email them just lemme know.

    When installing pedals I normally remove any old grease from threads and
    the cranks and then add fresh grease. I normally don't use a torque
    wrench to install them, but I do use the Park Tool PW-4 pedal wrench
    (the largest one they have), which allows me to crank them down pretty
    good. (And I did verify with a torque wrench they are installed with >
    20 Nm force.) For the Vectors I also always reset the install angle
    (via a head unit) after installing them.
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