Tacx Flux 2 Power Issues

I have a Tacx Flux 2 that I have had for about 6 months. I also have a pair of Favero Assioma Duos.

The Tacx reads lower than the Duos. This in itself is not unusual I realize. However, it’s not the standard ~20W drive train issue. I’ve been seeing fluctuations sometimes as high as 100W. So I did some testing the other day. Here is what I found, running each power over an interval of a minute, with the Tacx reporting directly to the Tacx app and the Faveros to my Element Bolt.

At 100W - <1% variance in avg power
At 150W - 1% variance in avg power
At 225W - 8% variance in avg power
At 337W - 14% variance in avg power

This was in Erg mode. So with the TACX thinking I was steady at 337W, the Faveros were measuring 384.

I can tell you that this trend keeps going, but I didn’t feel like blowing up my legs as I’m supposed be at the end of SSB II with an FTP test pending this coming Tuesday to start my build. Obviously, I’m not going to be able to get an accurate FTP test in Erg mode.

FWIW, its not the Faveros. I’ve tested and found discrepencies with other power meters on the TACX, though I didn’t do this level of side by side comparison.

I sent the files corresponding to my test with TACX support. Their response was ridiculous.

"With 3rd party power meters there can be up to a 20% difference in power numbers.
This is due to the point where the power is measured with the flux 2. "

Here is a link to a recent test I did illustrating the issue:

Anyone seen something similar?


I have the flux 2 too but, compared to my quarq xx1 PM they are almost spot on in terms of power… slightly 3-4-5W lower than quarq.
I compared those to in a 30-40min test in zwift(sim mode) but below 300w effort.

I have the same setup, and roughly the same differences in those Wat zones.

I recalibrate my Flux 2 using Tacx Utility and calibrate my pedals via Garmin before every virtual ride and it is not going any better.


It took some persistance with Tacx support but I was able to get them to honor this under warranty.

Once I got them to agree that this was an issue with the warranty, this was further complicated because they were completely out of stock of functioning replacements. I actually bought the Flux 2 originally from an online retailer. They sent me a check to refund my original purchase, and I ended up purchasing a Neo 2T online from another retailer (since Tacx was also out of stock of the Neos). The 2T has been flawless.

Thanks for letting me know. I may report this as an issue under warranty.


I went to the forum to see if there were problems with a flux 2 in combination with TR. As i started to become sceptic of what i saw i bought myself a pair assioma’s and guess what. When you get to 300 watts or higher the discrepancy is ridiculous and exactly like described by rrlolier who started this thread. I think i 'dd better contact my supplier too. They’ll be so happy with me as i allready destroyed a flux s.

Just to clarify, my issues were with the trainer. Trainerroad was not part of the problem. I was able to replicate the same results regardless of the software I used including Tacx own software. This is what I would expect to be the case for you as well.

I know, it all points to the Flux 2 in ERG mode. If i’dd try it in zwift i would see the same. I allready contacted my supplier about this as it makes high power TR sessions impossible for me. I am done with tacx as i allready had a faulty Flux S too.


I recently bought a flux2 to replace my original kicker and I am seeing this problem. I am not 100% sure its the TR software or the Trainer, what I did find that with the Kicker I just leave it in the little ring and the middle of the block and useing the Favero’s and power match I got a decent workout.

when I go into the settings pages the favero and the flux2 weere within a few watts of each other but aside from that all was great.

i started the workout and on the little ring on the front I was getting nowhere near the power or the resistance I needed, I dropped onto the big ring and could hit the numbers and in the end I disconnected the pedals and recorded the pedals on the garmin and then the trainer on the laptop via bluetooth. Its was a fairly simple session of 90 minutes and so no peaks. tomorrow I have Dade-1 so I should get a solid kick on the peaks, I will try them in the little and the big rings but I have a feeling that this trainer is going back and I will be getting a credit note for a kicker when they get back in stock. my 2014 is now fairly inaccurate in the cold however I was going to drop in the office on the TT bike for some lunchtime spins

See below when I changed to the big ring on the front!

Hey Stuart, want to make sure I am following you here. The screenshot was a workout using the Flux 2, using the Faveros in Powermatch mode?

the faveros were in power match mode, when you see power jump up to the correct line I moved to the big ring, if you see the up and down just after the first peak thats when I removed/ disconnected the pedals

Got it. Assuming you always use powermatch?
The best way to test (and rule out any issues with TR or powermatch specifically) would be to do a workout where you use the Flux 2 in ERG mode in TR, and record the power coming off the Faveros separately via a head unit. You can then compare the average power over the duration of a single interval. This is how I completed the evaluation at the top of the thread.

Is that an option?

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Thats ultimately what I ended up doing although there was a lot of faffing around. so tomorrow it will be a dry run and see what comes out. it was light chalk and cheese on the big ring though which should happen in erg mode should it?

so i did Dade -1 and my ftp 195 according to my pedals/kickr combo.

the average power for the workout on tha pedals was 128. the flux was 135.


if you look ag TR graph

you get the feelings i am not hitting the top of the efforts. i would shoot off and hit the target, nothing would change and i drift off the required output. but i wzs fine i only really had issues with one effort where i over cookedthe first effort in the last set.

its like it try to maintain an average effort power.

here is what i mean.

the required wattage is 200watts for 1 minute

the trainer ramps up for the effort and you hit the pedals at 250.

TR Or the trainer then says right in order to meet the 200 average you need to do 150 for resistance changes.

on the way down you get another sample at 225 now you are averaging 208, 192 to offset that.

my point here is that the trainer is try to match the interval average which is skewed by the initial load.

i would mind but i had it in the legs i was spinning out the gears at 110 rpmand so the power would got down

This was in full ERG mode with the Flux connected to TR and the Faveros somewhere else, or was this using power match?

process as follows:

turn on tacx turn on laptop
load TR
associate to hrm
associate to tacx

start workout

press go on garmin watch (fenix 6X) connect to pedals over ant+ power/ cadence.

do workout

u definately think the trainer is doing average power for the interval as it seem to hit it everytime at the start the ease off:-(


I had a similar issue with my Flux 2 that was actually unrelated to the power accuracy issues which led to the replacement. This particular issue is essentially that the trainer is simply not providing enough resistance to meet the power requirements. I found that I had to constantly increase the cadence in order to hold the interval’s power.

The solution was to actually roll back the firmware with the help of Tacx support. I hope this helps.

do you know what version you needed to back to in order to resolve

My flux 2 woks the same. I don’t bother I use power match but the inaccurate issue exists. This is why I have changed it to Elite Direto X recently and the differences are only a few watts between my Stages, Giant Power Pro, Garmin Vector S and the Direto X. Tacx, Garmin didn’t help me with that at all.

I had this trouble after a firmware update around March/April last year as did lots of people. In ERG mode the signal would just continually drop out and there would be 10-20w drop offs in power even with steady pedalling speed. It’s especially obvious if something’s up, because the Flux 2 smoothes the power line like a mofo. I downloaded the firmware rollback from the Tacx Flux Facebook group, and it’s been fine ever since. I don’t even bother to look for firmware updates after that experience. If you go into the group and look at the shared files, you’ll find what you need.

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