A very very complex topic. One I have explored for a long time as someone who loves getting lost in data!
As most have said consistency is the main thing. That includes the calibration protocol.
I’ve previously had Garmin vector 2 and now Assioma Duo pedals. I also have the Tacx Neo. I remember a long time ago this started with setting up a custom workout in zwift with the trainer in erg mode holding 180/240/280w for long periods and overlaying the garmin data. My initial readings were in the 1-1.5% accuracy range. I actually ended up applying a scaling factor to the vectors to get them to align to my trainer…something I now know to be wrong.
Then I got my Assioma duos. 2 things were important to me:
- how close were the left and right pedals to each other for the same loading
- What were they reporting Vs the vector Vs the neo…ie were they more accurate?
Question 1 was answered with the static loading test. I went up to an 18kg mass test. the pedals were within 30g (~0.15%) of each other. happy with that.
Then question 2. I initially followed the 180/240/280w test, getting similar results as the vectors, then I discovered this article:
What this basically means is the flywheel speed (dictated by the rear sprocket selection) can actually affect the accuracy of the trainer to hold a power figure whilst in erg mode. This blew my mind as I just assumed I could do whatever I liked in terms of moving up and down the cassette and the trainer just automatically compensated and the accuracy was constant…very wrong!!!
If I say set erg mode to 250w, largest sprocket on cassette, pedals will read around that figure, but as you step down through the cassette to the smaller cogs, what you find is the neo things its holding 250w put the pedals are saying well in excess of that figure…it can be a long way out. fortunately I run a 1x drivechain, otherwise this would get even more complex.
To cut an even longer story short, I toyed with idea of scaling factors etc (as SUF doesn’t YET have powermatch), but what I now do is do all of my workouts is to do all of my workouts in the 6th sprocket on the back. This keeps the erg target and the pedals around the sweet spot. I also only use the pedals as my power source. Never neo. I test using the pedals, so whilst the neo sets the resistance the assioma pedals provide the quantative data used in my training.
This is a very complex issue. All I suggest is be consistent in how you use your devices. Don’t get hung up with the absolute numbers, but focus on repeatability.
hope that helps!