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
  • I verified that the Vectors were indeed torqued down more than 20 Nm.

    I did a static weight test after a ride and the scaling factors didn't change much (0.003%)
    Left: Pre-ride: 0.9781; Post-ride: 0.9809; Difference: -0.0028
    Right: Pre-ride: 0.9928; Post-ride 0.9905; Difference: 0.0023

    I disabled auto-zero and did a couple more indoor rides. I determined that doing a manual zero after a bit of warm up changes the values reported for both power meters. But past that I''m not able to draw any satisfying conclusions from the data and I still see my PowerTap consistently reporting notably higher values than my Vectors (well past the accuracy of both power meters added together), which just seems wrong.

    here's two rides that i've recently done and my notes about the rides.

    https://analyze.dcrainmaker.com/#/public/7bc54314-8146-480d-68b2-f1be8e3bcd65
    ---8<---
    TR PowerMatch with Vector3 was enabled. (this means that TrainerRoad would adjust the trainer power levels based on the power values are reported by the Vectors.)
    Auto-Zero was disabled for both power meters.
    I calibrated the Vectors and PowerTap before the start of the ride.

    At the start of the ride the Vectors were reading higher than the PowerTap, which makes sense. But by the first set of intervals, the PowerTap was reading higher than the Vectors, without any recalibrations.

    I recalibrated the Vectors after the 1st set (@ ~17:00:32), which resulted in an ~10 W drop in the PowerTap numbers. Since PowerMatch was enabled, this indicates that there was actually a ~10W jump in the numbers being reported by the Vectors.

    I recalibrated the PowerTap after the 2nd set (@ ~17:16:23), and that resulted in a ~27W jump in the PowerTap numbers.

    Take aways:
    - Power values at the start of the ride made sense (with Vectors reporting more than the PowerTap), but without any recalibration they both drifted to reporting equal values.
    - Recalibration increased the values reported by both power meters.
    - At the end of the ride, the PowerTap values are on average 12% higher than the Vector values, which doesn't make much sense.
    ---8<---

    https://analyze.dcrainmaker.com/#/public/d14a4585-0bfb-45a8-75c2-bb88bf77eea3
    ---8<---
    TR PowerMatch was disabled.
    Auto-Zero was disabled for both power meters.
    I calibrated the Vectors and PowerTap before the start of the ride.
    I pedaled for about 9 mins to warm up the Kickr and then at ~16:51:31 I did a Kickr spindown, I re-calibrated both power meters, and I started the TR workout.
    I re-calibrated the Vectors after the 1st set (@ ~17:19:32) and the 3rd set (@ ~17:54:59).
    I re-calibrated the PowerTap after the 2nd set (@ ~17:36:24) and the 4th set (@ ~18:09:27).

    During the warmup the Vectors were reporting higher power than the PowerTap (for example: Vectors @ 57W vs PT @ 35W), but after recalibrating both power meters and starting the actual workout (@ ~16:51:31), they power values reversed (for example: Vectors @ 92W vs PT @ 109W).

    The Vector recalibration at ~17:19:32 didn't seem to change anything. The Vector recalibration at ~17:54:59 resulted in a 3 W jump, but the PowerTap also showed a jump here, so i'm guessing that can be attributed to variance in the Kickr.

    The PowerTap recalibration at ~17:36:24 didn't seem to change anything. The PowerTap recalibration at ~18:09:27 resulted in a 5W drop, but once again the Vectors also registered a drop. So once again probably a change in the Kickr Power.

    My takeaway here is that:
    - The values from the warmup make sense, with the Vectors reporting more power than the PowerTap
    - Any subsequent re-calibration after about 10 minutes of warming up don't seem to make any difference.
    - At the end of the ride, the PowerTap values are on average 6% higher than the Vector values, which doesn't make much sense.
    ---8<---
Reply
  • I verified that the Vectors were indeed torqued down more than 20 Nm.

    I did a static weight test after a ride and the scaling factors didn't change much (0.003%)
    Left: Pre-ride: 0.9781; Post-ride: 0.9809; Difference: -0.0028
    Right: Pre-ride: 0.9928; Post-ride 0.9905; Difference: 0.0023

    I disabled auto-zero and did a couple more indoor rides. I determined that doing a manual zero after a bit of warm up changes the values reported for both power meters. But past that I''m not able to draw any satisfying conclusions from the data and I still see my PowerTap consistently reporting notably higher values than my Vectors (well past the accuracy of both power meters added together), which just seems wrong.

    here's two rides that i've recently done and my notes about the rides.

    https://analyze.dcrainmaker.com/#/public/7bc54314-8146-480d-68b2-f1be8e3bcd65
    ---8<---
    TR PowerMatch with Vector3 was enabled. (this means that TrainerRoad would adjust the trainer power levels based on the power values are reported by the Vectors.)
    Auto-Zero was disabled for both power meters.
    I calibrated the Vectors and PowerTap before the start of the ride.

    At the start of the ride the Vectors were reading higher than the PowerTap, which makes sense. But by the first set of intervals, the PowerTap was reading higher than the Vectors, without any recalibrations.

    I recalibrated the Vectors after the 1st set (@ ~17:00:32), which resulted in an ~10 W drop in the PowerTap numbers. Since PowerMatch was enabled, this indicates that there was actually a ~10W jump in the numbers being reported by the Vectors.

    I recalibrated the PowerTap after the 2nd set (@ ~17:16:23), and that resulted in a ~27W jump in the PowerTap numbers.

    Take aways:
    - Power values at the start of the ride made sense (with Vectors reporting more than the PowerTap), but without any recalibration they both drifted to reporting equal values.
    - Recalibration increased the values reported by both power meters.
    - At the end of the ride, the PowerTap values are on average 12% higher than the Vector values, which doesn't make much sense.
    ---8<---

    https://analyze.dcrainmaker.com/#/public/d14a4585-0bfb-45a8-75c2-bb88bf77eea3
    ---8<---
    TR PowerMatch was disabled.
    Auto-Zero was disabled for both power meters.
    I calibrated the Vectors and PowerTap before the start of the ride.
    I pedaled for about 9 mins to warm up the Kickr and then at ~16:51:31 I did a Kickr spindown, I re-calibrated both power meters, and I started the TR workout.
    I re-calibrated the Vectors after the 1st set (@ ~17:19:32) and the 3rd set (@ ~17:54:59).
    I re-calibrated the PowerTap after the 2nd set (@ ~17:36:24) and the 4th set (@ ~18:09:27).

    During the warmup the Vectors were reporting higher power than the PowerTap (for example: Vectors @ 57W vs PT @ 35W), but after recalibrating both power meters and starting the actual workout (@ ~16:51:31), they power values reversed (for example: Vectors @ 92W vs PT @ 109W).

    The Vector recalibration at ~17:19:32 didn't seem to change anything. The Vector recalibration at ~17:54:59 resulted in a 3 W jump, but the PowerTap also showed a jump here, so i'm guessing that can be attributed to variance in the Kickr.

    The PowerTap recalibration at ~17:36:24 didn't seem to change anything. The PowerTap recalibration at ~18:09:27 resulted in a 5W drop, but once again the Vectors also registered a drop. So once again probably a change in the Kickr Power.

    My takeaway here is that:
    - The values from the warmup make sense, with the Vectors reporting more power than the PowerTap
    - Any subsequent re-calibration after about 10 minutes of warming up don't seem to make any difference.
    - At the end of the ride, the PowerTap values are on average 6% higher than the Vector values, which doesn't make much sense.
    ---8<---
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