Originally Posted by
coco1854
1) And how does it know the circumference ? I assume it takes the diameter of a standard wheel -- 26" -- and multiplies it by 3.14. That's how you get the circumference of any circle.
Originally Posted by
gthomson
Well that's not totally true, based on how bike computers used to calculate distance before GPS existed. .
The trainer knows how far the trainer went. It doesn't care about the circumference of the bike wheel at all. It cares about the circumference of the part of the trainer that the bike wheel rides on and the number of rotations that part has made.
I agree that the distance on a trainer isn't particularly important, but in this case there is an actual distance traveled and it's measured directly by quantities known by the trainer designer. You could drive it with a penny farthing and it wouldn't matter what the wheel diameter of the bicycle was.
I'm not sure if there is anything wrong with the OP's calculation. Maybe there is a lot of slippage. But the distance given by the trainer is the circumference of the friction wheel times the number of rotations that it has made. Simple as that. No bike wheel size needs to be involved at all to know that.