There are a few posts on this subject, this one related to KickR.

My real question is: if spindown is a measure in time of how long your trainer coasts on its own, and that is somehow related to it’s natural resistance of flywheel, gears, internals, pressure in the back tire (if it’s a wheel on trainer)… what does Zwift (algorithm) do with this info? Does a large spindown time say 15seconds mean your avatar coasts longer? But Zwift will counter-act this to brake your avatar more?

I’d like to know the theory, and math, behind the spindown calibration.

Zwift does not do anything with the data. It is only a calibration of the trainer. The Trainer use the data to adjust the Zero offset before sending Data to Zwift.

Does the “zero offset” have something to do with the power? I still don’t understand why a spindown is measured in seconds. There’s probably a calculation in the trainer CPU that figures out how many watts it takes to turn over the wheel, so when it reads power, it’s got to upward adjust that power to include the force it took to turn the gears. I know this is esoteric, I’m just curious. Thanks.

I am not going to pretend to know the ins and outs of indoor trainers. My thinking is that the spindown measure the time it take for the trainer to coast down while applying the minimum resistance, if the spin down is to long and it fail it will apply less resistance and the opposite for a short spin down.

These trainers does not have strain gauge power meters they have a set formula for the position of the resistance magnets and the speed of the drum, so it is very important for the system to know the Zero point of the power curve.