Confirmed My Suspicions - Tacx Vortex Under Reporting Power

My Assioma Uno power pedals were delivered yesterday and I had my first ride this morning. For the past month or so it’s felt like something changed with my vortex trainer, which was purchased in Dec 18. The effort seemed much harder each workout while my average power stayed the same or in some cases would decline resulting in failed workouts. I ramp tested a few weeks ago and went from 256 to 239. I had been increasing my run mileage so I figured my legs were just a bit spent. However, my outdoor rides were getting stronger, so I wasn’t convinced I had lost bike fitness. In come the assioma pedals. While riding through Pioneer this morning I kept an eye on the trainer power output as compared to the pedals. The trainer was always reporting 20-30 watts less. I thought maybe it would level out after 10-15 minutes as the trainer warmed up but it never did. So for the past few weeks I’ve been training at an FTP roughly 10-12% higher than what’s actually stated. No wonder VO2 has been so damn hard. Here’s my effort on Pioneer today compared to a similar power effort a few weeks ago that shows the huge decrease in HR with the same effort.


I just figured I’d post this in case someone else is running into a similar issue and is searching the forums for confirmation.

With a left side only powermeter you are assuming that your left-right balance is 50:50. If your actual left:right was 55:45 you’d see the Assioma reporting 10% higher than actual. So, it is possible that both of your power sources are correct. I know that my left:right is about 47:53 at FTP and my Assioma Duo and Tacx Neo agree to about 1.5% which is well within the combined error margins. If I was using an Uno it would appear to read 7% low compared to my Neo. So, I don’t think that you have enough data to say which reading is correct. I could only do the comparison because I have two Duos and two Neos and I could compare all four readings.

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As someone with Assioma Duo and a Vortex. the Vortex returns lower watts at lower wattages than the pedals but tracks pretty well at FTP and above. The issue with any wheel on trainer is the consistency of the setup which is why a power meter will always make sense with this trainer.

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You’re right in that I can’t 100% confirm anything, but seeing a 20bpm difference in heart rate between similar power number readings from the trainer and pedals is pretty telling. I’m also right leg dominant so if anything the power discrepancy could be even worse.

The most important aspect of this topic to share with others is this:

Whenever you get a different power measuring device, you MUST retest your FTP with that new device!

  • Differences can and do exist between nearly every trainer and power meter on the market.

    • Sometimes they are small, sometimes they are large.
    • Sometimes they are higher, sometimes they are lower.
  • They are often different enough that it will have an impact on your relative training zones via a difference in measured FTP.

    • The reason behind these differences vary and are largely irrelevant for this discussion.
  • What matters is recognizing that all of these “tape measures” have the potential to measure power a bit different.

    • That is best handled via testing to reset FTP for proper training zone designation.

I have a Tacx Vortex too and a stages 3rd gen. My Vortex is ~25w lower than my stages

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The issue I’m facing is out of the box the vortex seemed to do be accurate, but I had nothing to compare it to. Only a month or two ago something has changed with the vortex. Feels like there’s extra resistance but it’s not reflected in the watt output. I’m pretty sure the firmware is all up to date. I can’t pinpoint the problem.

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Without before / after data it’s hard to know.

But running through the normal wheel-on list to make sure everything is consistent (unchanged):

  • Same tire?
  • Same tire pressure?
  • Same trainer roller pressure?
  • Were you using the Tacx calibration app and procedure?
  • Same room conditions?

And any other possible equipment or environmental changes could also impact the results. These are tricky devices and can be altered by seemingly small changes.

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Yeah I’ve run through that list and I’ll just chalk it up to the unknown. End of the day, my fitness has improved and my rides outside are much more enjoyable. So I’m happy!


I had the same issue with my Tacx Bushido and Favero Assioma. I did find an article online that refers to the Bushido as a “Watts vampire”.

I’ve found the difference isn’t consistent across the watts profile. I can match PM and turbo up for sweetspot work but when I hit VO2 the trainer is reporting 20-30 watts less.

I now just dial down the % of my TR workouts when I’m doing VO2 max stuff.

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@ChrisL I have a bushido and vector 3 pedals, the difference between the two can be as high as 50W with mine. I use powermatch now and found the V02 work more manageable.

I had this problem with the bushido too, i now follow this method to calibate my trainer (i badly need a proper power meter though)


Another thing to remember when you measure power at different points on the bike…Don’t forget drivetrain losses.
If it’s 2.5% at 200w you just dropped 5 watts on your trainer. Then you have to double that figure if you’re using a single sided unit that doubles the power from one side. That’s 10 watts gone.
I’d hazard a guess that a wheel on trainer will also have extra losses for rolling resistance that will also be doubled on a single sided power meter. That would explain 20 watts quite easily.

As much as I’d like to see power meters/trainers reporting accurately, I’d be a happier man if I could easily make adjustments in the various apps that I use to review performance over time.
Multiple bikes and different power sources are a pain in the rear.

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This is similar to my experiences. FTP is pretty close between my Vortex and Assioma duos. However, as you get away from FTP (both high and low) the gap widens. This meant before I got my PM, VO2 max intervals were much too hard, and recovery intervals were also being done at higher power than they should be. A consistent offset would be fine, but the lack of consistency was a bigger issue. To be fair the TR guys have always said on the podcast that cheaper smart trainers account for a huge portion of their support tickets!

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Interesting video, thanks. I’m going to give this a try as I’m sure the Vortex suffers from the same issues.

Think I had something like 50W difference when I did a ramp test. Probably one reason to do 8 or 20 minute tests - more accurate in the mid watts…