I’ve just got a Tacx Flux 2 and gave it a whirl this afternoon for the first time. I’ve also got a 4iiii crank power meter and comparing the Flux power reading to my 4iiii via my Garmin there was a consistent 20 – 40 watt under-read of the Tax vs. the 4iiii/Garmin. Is this normal and a function of the fact the 4iiii is measuring power at the crank and the Tacx measuring it at the wheel? a bit like cars on rolling roads where you get a flywheel power reading or an at the wheels power reading? or should I expect the reading to be the same?

Ta.

I’d expect closer than that, only a few watts between my Kickr and P2M.

feed the data in here,

Gap sounds pretty big, id tend to believe the tacx data,

My neo & quarq are within 5w of each other, which is about ehat you would expect for drivetrain loss and general accuracy

At the end of the day, it doesnt matter how “out” they are you just need repeatable data for training

http://www.dcrainmaker.com/analyzer/amp

If its anything like my Tac X Vortex its very temperature dependent. I find it over reads in warm weather and under reads in cold weather

My flux and stages are significantly closer than that.

As ypu say..There will be a few watts lost as the reading is at a different part of bike. You may have a imbalance between left and right leg..ie stronger left leg and then this power reading is doubled which could account for a few watts also. Ie..

If you had a 52/48 imbalance at 250 watts that’s a 10 watt difference right there.

Did you calibrate (something like spindown) the trainer and the power meter ?

If the trainers not calibrated they can be a bit erratic on power readings.

Ah, ok thanks. Sounds like I should expect some discrepancy then. I’ve got my Garmin set to sample the 4iiii power over 3 seconds to smooth out the power fluctuations and can’t seem to set the Tacx output in the same way so the tacx fluctuates more, but still getting a sizeable discrepancy with both 4iiii and Tacx freshly calibrated. I’ll look into it.

My Tacx Flux would read about 15-20w lower than my Stages.

For the first few months I just did my FTP tests and TrainerRoad stuff using the Flux. It meant that my FTP wasn’t ‘right’ for outdoor riding, but I wasn’t doing structures intervals outside so it didn’t really matter. Now I use the PowerMatch feature in TR to control the flux from my Stages.

What are you using the trainer for?

I have quite a discrepency on mine – bepro pedal reads something like 8% higher than the kickr core. Seems consistent so it’s not a hindrance for training (as said), but it does give me two different power regimes when I’d prefer to just have one.

I’ve got my Garmin set to sample the 4iiii power over 3 seconds to smooth out the power fluctuations and can’t seem to set the Tacx output in the same way so the tacx fluctuates more

You’re taking an average over time, surely?