Zumo - software needs wheel circumference - WHY?!?!

[johnson.andrew]

As per the title. The Elite zumo is a direct drive smart trainer and i am struggling to understand why the calibration software requires a wheel circumference when there is no wheel involved. How does the value of the circumference of a non-existent wheel play into the calibration of the trainer? Can someone help me understand? I assumed all these trainers really care about is power produced divided by your weight in order to determine movement speed?

Just curious - i mean i stick the circumference of the wheel that would otherwise be on the bike anyway but i dont see why its needed

Thanks
Andrew