Calculating power output on a spin bike

[SpinnerDude]

So...I have a home spin bike which unfortunately lacks the sensors of some of the more expensive models. What I'm trying to do is work out if I can dynamically calculate my power output.

The spin bike itself has:
- An 18kg flywheel of radius 30cm
- Direct drive between the crank and the flywheel (one turn of the crank = six turns of the flywheel)
- Resistance is provided by a magnet that is lowered onto the flywheel rim
- An additional cadence sensor on one of the crank arms

It feels like I should be able to map the resistance settings on the bike, such that the power output can be calculated based on the cadence alone.

My first thoughts are around pedalling at a set cadence and seeing how long the flywheel takes to stop, and from there working out the resistance produced by the magnets (which I'm currently assuming to have a linear relationship with the angular velocity of the flywheel)

Any advice/comments?

 

[berkeman]

My first thoughts are around pedalling at a set cadence and seeing how long the flywheel takes to stop, and from there working out the resistance produced by the magnets (which I'm currently assuming to have a linear relationship with the angular velocity of the flywheel)

That sounds like a good plan to me. You will need to accurately calculate the Moment of Inertia of the flywheel (is it a perfect disk shape, or does it have a flange like lifting weights?), and you should remove the magnet temporarily so you can see how long it takes to spin down with no explicit damping/drag (to be able to separate the friction terms of the damping).