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
  • FWIW - I have been riding with powermeters for at least the last 5 years. This is more than 75.000 km clocked with a powermeter. I always train structured and according to a plan. So I do have a quite good interpretation of what I can pedal. Heck I'd even postulate; based on feel, I can say within 10W accuracy what I'm riding. This is of course in no way scientific backed. But I have done it many times while riding - guessing my wattage - just for fun part of it. And with the years, you'd be amazed how good you learn. I can do the same with my HR, but that's not the case here.

    So when I compare wattage, first thing I do is look for pure facts. Then historical data. Then reputation.


    I disagree. Watch the video below. You're not right. Actually a crank based powermeter is the best place. It's the place for the least parasitics in the measuring system. The pedalaxle, the crankarm flexes i 2-3 dimensions before the power is transmitted to the chain. This is what is parasitics and has to counted for in filtering the wattage from pedals.

    Whether this is ACTUALLY an issue; admittedly I can't tell. But I saw a comparison somewhere on the Net (unfortunately I can't remember where - I'd love to paste a link and if I find it - I'll of course post it here) where V3 came out with 90% OK signals. The other powermeters - crank based - came out with 98-99% OK signals. Add to this that V3 actually does not have the best reputation for accuracy. Afterall I'm not the first commenting on accuracy issues. Not exactly the same can be said about Quarqs Dzero (and other crank based powermetes). In fact it's the total opposite based on many reviews.

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

    The last part in this is where I compare the data to my historical data. This is where my own interpretation comes into the picture. And FWIW - the winter season 2017/2018 I ACTUALLY did add 6% to my Assioma Duo's readout to align them with my indoor trainer at that time (a Kettler Racer S). By doing this I did get comparable data inside/outside. So something is happening when riding indoors when using pedal powermeters. This is my conclusion. Is it due more parasitics, due a more steady/stable bike in the trainer? Outside your bike moves around a lot more. I don't know? Does the NEO have a reputation for being inaccurate - no it doesn't. Does the NEO align with my historical data - indeed it does. Does the pedals do that? No none of them?!

    So where does all this leave us. Well if you're happy with your Vectors and they work as expected - cool stuff - stick with them :-)

    For me - I have accuracy issues - like or not. Data does not fit inside vs. outside. I think this is due to the pedal concept - it just doesn't work - too many parasitics indoors - too much filtering that doesn't work - this is my take.

    Maybe it's just me being too sensible. I'm not to tell. But I'm happy with my current setup and most important of all; I have solid compareable data across NEO and Quarq. This matters big time too me. [/QUOTE

    Strange...i have used the vector 3 with Dzero and there was 2 watts diff on a outside ride (avg power).Also no more thsn 2-4 watts diff sgainst Elite direto and 3~4 against my p2max ngeco.So i dont know how you conclude they are bad.i can post a dozen of rides with elite direto and vector 3 (almost identical numbers).can you provide some files that you recorded with the activties between neo and vector3?
Reply
  • FWIW - I have been riding with powermeters for at least the last 5 years. This is more than 75.000 km clocked with a powermeter. I always train structured and according to a plan. So I do have a quite good interpretation of what I can pedal. Heck I'd even postulate; based on feel, I can say within 10W accuracy what I'm riding. This is of course in no way scientific backed. But I have done it many times while riding - guessing my wattage - just for fun part of it. And with the years, you'd be amazed how good you learn. I can do the same with my HR, but that's not the case here.

    So when I compare wattage, first thing I do is look for pure facts. Then historical data. Then reputation.


    I disagree. Watch the video below. You're not right. Actually a crank based powermeter is the best place. It's the place for the least parasitics in the measuring system. The pedalaxle, the crankarm flexes i 2-3 dimensions before the power is transmitted to the chain. This is what is parasitics and has to counted for in filtering the wattage from pedals.

    Whether this is ACTUALLY an issue; admittedly I can't tell. But I saw a comparison somewhere on the Net (unfortunately I can't remember where - I'd love to paste a link and if I find it - I'll of course post it here) where V3 came out with 90% OK signals. The other powermeters - crank based - came out with 98-99% OK signals. Add to this that V3 actually does not have the best reputation for accuracy. Afterall I'm not the first commenting on accuracy issues. Not exactly the same can be said about Quarqs Dzero (and other crank based powermetes). In fact it's the total opposite based on many reviews.

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

    The last part in this is where I compare the data to my historical data. This is where my own interpretation comes into the picture. And FWIW - the winter season 2017/2018 I ACTUALLY did add 6% to my Assioma Duo's readout to align them with my indoor trainer at that time (a Kettler Racer S). By doing this I did get comparable data inside/outside. So something is happening when riding indoors when using pedal powermeters. This is my conclusion. Is it due more parasitics, due a more steady/stable bike in the trainer? Outside your bike moves around a lot more. I don't know? Does the NEO have a reputation for being inaccurate - no it doesn't. Does the NEO align with my historical data - indeed it does. Does the pedals do that? No none of them?!

    So where does all this leave us. Well if you're happy with your Vectors and they work as expected - cool stuff - stick with them :-)

    For me - I have accuracy issues - like or not. Data does not fit inside vs. outside. I think this is due to the pedal concept - it just doesn't work - too many parasitics indoors - too much filtering that doesn't work - this is my take.

    Maybe it's just me being too sensible. I'm not to tell. But I'm happy with my current setup and most important of all; I have solid compareable data across NEO and Quarq. This matters big time too me. [/QUOTE

    Strange...i have used the vector 3 with Dzero and there was 2 watts diff on a outside ride (avg power).Also no more thsn 2-4 watts diff sgainst Elite direto and 3~4 against my p2max ngeco.So i dont know how you conclude they are bad.i can post a dozen of rides with elite direto and vector 3 (almost identical numbers).can you provide some files that you recorded with the activties between neo and vector3?
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