Power accuracy is REALLY BAD!

I am really dissatisfied with the power measurements on the Flux S.
I wanted to know how my FTP stack upp between mu differnt Power meters since i use different ones on my 3 bikes. The power meter on my day to day bike is a Rotor inPower (left only),  tri bike has a Quarq DZero and the bad weather bike has a 4iiii (left only).
The current setup on the trainer is I have the tri bike on it (with the Quarq cranks paired to Zwift). In order to validate the power accuracy I checked so both Quarq and Flux had the latest SW. Did a new calibration even though Quarq has autoZero/magicZero och did a spin down calibration on the Flux according to instructions.
Pairing: Zwift + Flux
Garmin 1030 + Quarq
I did a few rides and Flux show consistently 40-50W lower than the Quarq, average!  Sprints can diff more than 100W! There also seems to be some kind of smoothing going on on the Flux which might explain the it missing out on shorter sprints.
So in order to dig deeper into this I borrowed a pair of PowerTap P1 pedals from a friend. and threw them into the mix. Worth mentioning the his pedelas stack up perfectly with both his Quarq  and his trainer wattage. I paired the pedals with my Garmin Tactix Charlie and did another test.
Pairing: Zwift + Flux
Garmin 1030 + Quarq
Garmin Tactix + Powertap pedals
The results in the graph speek for themselves after a 10 min ride with both sprints, steady state and a ramp at the end. Data is aligned with HRM which is paired to all 3.
Flux avg: 279W
Powertap avg: 309W
Quarq avg: 317W
This is NOT ok. It is not <3% acuracy, it is more like 10-15% accuracy. How to fix this?
  • Ever get any progress on this.  Did similar check over the weekend and flux s vs bepro pedals and flux s vs 4iii crank got very similar results to you.  Interestingly bepro and 4iii are within 1 or 2 watts of each other.  Anyone else have the same issues

  • Hi Adrian, I was wondering which version do you have ? And the resistance is normal or also higher as usual ??

  • Tested the Flux 2 against my Vectors over a 35 min ride on Zwift. 

    Zwift average wattage was 30 watts lower than my Vectors 

  • I have the Flux S. Yes, since the trainer shows too low power values I need to push slightly harder for the same watts. 

  • I have contacted Garmin about this and also did a few tests they wanted me to do for comparison. The values did show did show a significant separation of power values the higher watts I was pushing (when I looked att the values in a similar graph as above) but when I got my response back from Garmin I they said the opposite; That the powers meter and the trainer was more on par with each other the higher power I had. This is REALLY strange. I will contact them again to make sure they have compared the correct files. 

    Any progress from your side? 

  • I also do have the Flux S and after the update firmaware last sunday, I do have much more resistance than usual. Normally I can cycle 200 watts (towards Treshold) for an hour or so and after the update I bearly reach 150 watts (average). I contacted Garmin (Holland) and they told me there was ONE identical case like me (because of the update) and the only thing I can do is exchange mine for a repair. So my issue is like yours (Tacx do not show the watts like it should be) but also a resistance problem. I have to ride a much higher gear to reach the same watts.

    I downgrade the firmware to 3.3.40/1.1.12 (I did have 3.3.40/1.1.16) because of the problems with the resistance I could hardly cycle but unfortuneately it made not a big change. 

    But I am certain the output Tacx Flux S gives is NOT the rigt one. I can feel my legs and they don't lie.

  • I have a similar problem, writing to see if there are any solutions. Did a similar comparison:

    Edge 530+vector 3

    Zwift + tacx flux S

    Tacx Flux  S was consistently reading 30-50 watts lower on a average 200 watt effort. I know my pedals are close as I have compared them with Kickr Core and Tacx (wheel on) trainers. 

    Is this software or physical issue? Any solutions yet? Pretty poor for $1,000 trainer.

  • After mailing back and forth with Garmin support and sending several power comparison activities with several power meters they agreed to replace my unit. Smiley

  • Great that you are getting a replacement.  Would be brilliant if you could update us all if the new one is any better.  Seems a number of us have similar problems and wonder whether this us related to all flux  or just a few

  • Hi Adrian I send you a message by Strava. Please let me no anything !