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I’m lucky enough to have a few power meters and I’ve noticed when using one I seem to put out less power. So with a spare hour on my hands I tested 4 of them back to back against my tacx neo. All 4 are left sides cranks, 3 are stages, one is a 4ii.
At the same power on the turbo (around 240 watts) sure enough the meter that I had concerns about consistently ran 10-15 watts lower than the other 3, all of which gave out almost identical numbers on numerous different runs
Now I appreciate it’s not the end of the world however in my mind if power meters aren’t accurate they are pointless, especially if switching between them on different bikes. And that appears to be outside of the specified accuracy
It’s still in warranty. Reckon I should send it back or will they laugh at me?
The minute you start changing between meters and brands Nd even measurement methods (crank Vs turbo) the numbers are increasingly meaningless especially as cadences and powers get higher
So long as the readings are consistent then there isn't really a problem. You should also only be using the figures for the same bike/power meter combination so you shouldn't be testing on one bike and using that FTP value on another. Even indoors vs outdoors makes a difference.
The only issue is if you use something like Training Peaks/WKO then it's likely to see any workouts done with the PM that tracks low as not being as hard as those workouts done with the others.
If you've got that many power meters I'd just leave the one that reports low values on whatever bike you use on the turbo.
The minute you start changing between meters and brands Nd even measurement methods (crank Vs turbo) the numbers are increasingly meaningless especially as cadences and powers get higher
Whilst I'd agree with this if I was measuring power at different points on drive train, or both left and right sides, left arm power meters should all be very very close. I'd say the fact 3 of them are practically identical is evidence of this as I can swap between all of them no bother.
Did you read the link I posted at all ?
Also. Be wary of a test at one specified power.ita not always a linear calibration Across the range.
So long as the readings are consistent then there isn’t really a problem.
Surely this is the answer.
In reality it's not like the actually number matter, just is it comparable, and if you're really bothered - test a load and buy the one that registers the highest numbers 🙂
You really do need to read the link @trail_rat posted.
These can be even further our - I have a wattbike at work that sits me at threshold HR doing what it reports as my 'Z2/3' power.
In fact, my first advice would be to use heart rate to manage this and advise you.
Secondly, i would find tracking software that allows you to offset files.
In fact, my first advice would be to use heart rate to manage this and advise you.
I tend to watch my HR as well, but fatigue and terrain varies that a lot. On Sundays Road Z2 ride I was in Z2 HR on the flats and close to race HR on the climbs despite the same power output. But I am at the end of a particularly fatiguing training block.
So reading that I would not be glad I recently bought a stages PM, but then again as I bought it for MTB which is very hard to read over short durations e.g. 5s and I only bought it for post race analysis / a rough guide in the main I'm ok with that. FWIW I'm using 5s & 20s power to maintain a Z2 MTB ride appreciating that terrain makes it approximate.
It's always going to be a loosing battle trying to get numbers from various different power meters to match. I was similar in that I had 4 different stages at one point but decided to sell them all and buy a pair of Assioma Duo's. It only takes a minute to swap them between bikes and only slightly longer to swap the pedal body to SPD's for mtb use. Now I just have one set of numbers to train with. Doesn't matter if they're accurate or not as now they're consistent.
My L/R balance also varies from day to day and at different power levels. My left leg does more work at lower Z2 stuff and my right leg does more at threshold and above. Kind of highlights the downfalls of a single sided power meter.
+-3% accuracy on most PMs so 240 watt equals somewhere between 232.8 and 247.2 so seeing a 10 watt difference between PMs sounds fairly plausible (without taking into account the +-1% on the Neo too)
I presume you've updated the firmware on all the PMs?
There's no standard shared across manufacturers when it comes to accuracy. They can all claim whatever % accuracy they want and still be very different from one another. It's not ideal but that's just something we have to live with for now. Accuracy is not important unless you want to compare yourself against others (or have multiple power meters). Consistency is key and you can't get more consistent than just using one power meter for all bikes.
My L/R balance also varies from day to day and at different power levels. My left leg does more work at lower Z2 stuff and my right leg does more at threshold and above. Kind of highlights the downfalls of a single sided power meter.
Playing devil's advocate slightly..
What can you actually do with this info? Everyone is assymetric and will have one leg stronger than the other. Unless its a massive difference it doesn't matter and very unlikely you can change it.
It throws the training zones out a fair bit. My FTP would have been set using efforts that favoured my right leg and lower Z2 stuff would therefore be way be too easy. Random example (not using my values) -
Threshold efforts at say 47/53
Zone 2 efforts at say 53/47
Actual FTP 300w (measured L & R output)
Stages would give FTP of 282w due to weaker left leg.
Zone 2 work at say 75% should equate to 225w but would be 212w based on the left leg FTP.
Now take into account a favoured left leg of 53/47 at Zone 2 and i'd actually be doing 198w but it would be report as 211w when I should really be doing 225w (based on both leg output).
So if your L/R balance swings by 3% in each direction your zone 2 range could be 27w out. That's quite a difference over something like a 4 hour ride.
I think my maths is right but don't hold me to it..!
Consistency is the key. If 3 out 4 are close I’d certainly be asking the vendor to perform a calibration test to verify the crank is operating correctly as they would at point of manufacture.
You’ll always get differences where the power is being read at different points e.g. crank vs wheel on / off trainer but shouldn’t if read from same method. I’ve got a dZero and that’s advertised as +-1.5%.
I wouldn't get too hung up on it, I have one that matches my Kickr almost perfectly and another that tracks about 10w high.
Would be nice if they all played nicely together but eventaully I accepted that there are many variables, as long as they're 'reasonably' close I'm happy.