Wheel circumference - speed/distance adjustments / Tacx Flux S

Hey guys,

yes, i know that speed is not relevant for indoor training, but its interesting for me in some cases, also the resulting distance at the end of the training. :wink:

I have owned a Tacx Flux S, which is connected to my iPad via bluetooth.
My idea was to adjust the speed (=distance) with the wheel circumference option in the app…

So if i edit the wheel circumference with a higher or lower value, it should have a effect on the speed showed and the distance if i download the gpx-File after the training - correct?

But this didn’t work, whatever value I insert = the speed is still the same… it seems that its completely irrelevant! If i insert 100 or 3000 - it is the same speed…!.

Do i have a thinking error?

Thanks and best regards!

Hey Marcus,

I suspect this issue relates to the fact that many of the Tacx trainers use something they call VirtualSpeed. Most trainers calculate your speed by knowing the RPM of the rear wheel, and calculating what your outdoor speed would be by using the circumference of your rear wheel. What VirtualSpeed does is estimate your speed using an algorithm instead. You can learn more about VirtualSpeed here.

What this means is that the circumference likely does not factor into the speed reading from the Flux, since the Flux performs its own separate calculation to determine speed :confused:

To test this, select a workout with a steady warmup using ERG mode. Then,

  1. Pick an easy gear and spin a contant cadence for a minute.
  2. Shift up a couple gears and continue at the same cadence
  3. Shift up a few more gears and keep the same cadence

In other words, your power output and cadence remain the same, while your gear changes.

  • If your speed is calculated based on wheel speed, your speed should increase each time you shift.
  • If it is based on VirtualSpeed, it should remain the same throughout.

You can analyze the file after it has been exported to a third-party app such as Strava.

Let me know what you find!

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