About one year ago I bought a smart home trainer from Tacx. It was the model Tacx Flux Smart T2900, modelyear 2017.

After starting to use it I quickly realized that something with the power output numbers of the Tacx Flux wasn’t right. I got out of breath at wattage numbers that were normally far below my limits. First I thought, that I’m not using the smart home trainer correctly. But after reading manuals and different online resource I came to the conclusion that it was not my fault.

I have to say that I use power meters on my race bikes since about four years. And I think I have a pretty good understanding of how much power I can produce under given external parameters such as season, health, fatigue, etc. I have one road bike equipped with a Rotor 2InPower and one road bike with a Shimano Dura-Ace R9100-P power meter. So, two crank-based power meters measuring real physical power output with the help of a strain-gauge.

Unfortunately, I was not able to find information from Tacx about how their smart trainers measure power output. However, from various sources in the Internet I learnt that most smart trainers do actually not measure real power output. Instead, they calculate (or better said: estimate) power output based on speed, resistance and statistical data generated for each smart trainer model. So, when we talk about the accuracy of power output data of smart trainers it all comes down whether the smart trainer’s algorithms and statistical data have been put together with care or not.

In order to see if my feeling, that the power output numbers from my Tacx Flux were not appropriate, was right, I run two tests. For the first test I installed my Rotor 2inPower power meter on my third road bike which was permanently attached to the smart trainer. Then I performed a 20 minutes interval training while recording the watt numbers from Tacx and from the Rotor power meter at the same time. The recording devices were a Garmin Edge 520 and a Garmin Edge 820. A few days later I did the exact same test procedure with the Shimano power meter installed on my third road bike. So, the test conditions were quite comparable. Of course, I also calibrated all devices before starting the tests and the Tacx Flux had the latest available firmware installed.

The results of the two tests can be seen in the following two diagrams. It’s quite disillusioning. It can clearly be seen that Tacx is reporting more or less accurate power output numbers as long as the actual power output is below 150 watt. However, the higher the applied forces the bigger the discrepancy between real power output and the estimates of Tacx.

Tacx product specification states that the measurement error of their product is at around +/- 2%. At 250 watt the acceptable error margin would therefore be at around +/- 5 watt. However, in reality in the area of 200 to 250 watt of real power output the discrepancy is -15 to -20 watt and in the area of 250 to 300 watt the discrepancy is already somewhere between -20 to -30 watt. This is completely useless. With such numbers you can forget to follow any power-based training plans.

Just recently I stumbled over the product review for ‘CycleOps H2 Smart Trainer’ from road.cc. The H2 is a very similar product to the Flux and also in a similar price range. As the review shows it’s absolutely possible for a product in this product category to estimate reasonable power output numbers. However, what Tacx Flux delivers is just a joke. A product made by amateurs. I have sold my Flux, and I will definitely never buy a Tacx product again.

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