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 I bought a Quarq Dfour91 (it's Quarq's Dzero for Shimano 9100 series). I'm so glad I did - here's why...

    On todays ride, the first one with Quarq, the first part (50 min) of it was an ERG-mode workout with 4x10s 1kW surges.

    - NEO....... = avg. 213 max 1085
    - Quarq..... = avg. 211 max 1034

    The remaining part (55 min) was free-ride in Zwift with cadence drills:

    - NEO....... = avg. 178 max 362
    - Quarq..... = avg. 182 max 369

    This within +/- 2% error margin (I don't pay much attention to the 10s surges - since it's afterall only 10s intervals)

    In addition, I had a 10s rolling avg. power on my Edge 1030. The Quarq was always within +/- 2-4 W when riding steady. This is SO MUCH more in line with what I'd expect.

    So are my Vector 3 wrong? I did a solid static weight test with great result. So did Assioma. But then what is wrong here? I don't know. But I do think this has something to do with this video. Pedals are just more prone parasitics in power measurement. Some may see it more than others. I think this is what I did.

    https://www.youtube.com/watch?v=zZ9fh6srmnk

    So where does this leave me. Well as I see it; I learned a lesson. Pedals are just no good placement for precise measurement of power. At least not if you want super reliable, super trustable power each and everytime. For this, you need a crankbased powermeter. This is my take on this.

    With this, fairwell Vector 3. I will never buy a pedalbased powermeter again. I'm just too sensible to the marginal errors that I think I see.
Reply
  • So I bought a Quarq Dfour91 (it's Quarq's Dzero for Shimano 9100 series). I'm so glad I did - here's why...

    On todays ride, the first one with Quarq, the first part (50 min) of it was an ERG-mode workout with 4x10s 1kW surges.

    - NEO....... = avg. 213 max 1085
    - Quarq..... = avg. 211 max 1034

    The remaining part (55 min) was free-ride in Zwift with cadence drills:

    - NEO....... = avg. 178 max 362
    - Quarq..... = avg. 182 max 369

    This within +/- 2% error margin (I don't pay much attention to the 10s surges - since it's afterall only 10s intervals)

    In addition, I had a 10s rolling avg. power on my Edge 1030. The Quarq was always within +/- 2-4 W when riding steady. This is SO MUCH more in line with what I'd expect.

    So are my Vector 3 wrong? I did a solid static weight test with great result. So did Assioma. But then what is wrong here? I don't know. But I do think this has something to do with this video. Pedals are just more prone parasitics in power measurement. Some may see it more than others. I think this is what I did.

    https://www.youtube.com/watch?v=zZ9fh6srmnk

    So where does this leave me. Well as I see it; I learned a lesson. Pedals are just no good placement for precise measurement of power. At least not if you want super reliable, super trustable power each and everytime. For this, you need a crankbased powermeter. This is my take on this.

    With this, fairwell Vector 3. I will never buy a pedalbased powermeter again. I'm just too sensible to the marginal errors that I think I see.
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