I'm not a fitness fanatic or anything, but I do like to crunch data on myself to see areas I may be making an improvement with my fitness. I considered getting the cadence and distance sensor pair from Garmin, however I wouldn't get much use of them on my real bike as I'm using GPS to measure distance and heart rate to gauge effort involved, so it would only be worth while putting on my spin bike, which I've looked up, can be a bit fiddly but perfectly doable.
But then I wondered if there's any real benefit to doing so, as far as can tell the distance/cadence sensors vaguely work by comparing the wheel rpm to the crank rpm and extrapolating that into a usable metric roughly based on estimating what gear you would be in on your bike, that loosely signifies how much effort you are putting in at any given time (I'm aware this wouldn't give you a numerical value of your effort but hopefully you understand what I mean here.)
However spin cycles don't have gears, the resistance is induced via magnets on mine so the crank to wheel turn ratio will never change and therefore the only thing of use the sensors will be able to determine how many rotations I've done overall, something Sunderland to Rhu by car which would mean very little considering the resistance could be really high or really low and the sensors can't pick it up, only my heart rate sensor will show any difference between the two scenarios which is no improvement over my current setup.
So have I applied the simplified physics incorrectly when thinking about this, or would I be correct in saying that a cadence/distance sensor would be of no practical benefit on a spin cycle as a fitness metric.