I created a macro to help calculate the density/mass moment of inertia for complicated parts like this:
https://github.com/mdiehr/FCMacros/blob ... fo.FCMacro
I used some portions of the great FCInfo macro to create this.
How it works, is that it looks through all of the objects in the document:
Then, for each part, it pulls the density out of the part label (it has to be exactly at the end of the label), displays the mass of each part, and the overall mass and moment of inertia for the assembled yoyo:
Storing the density in the label is not the best way to do this, but it was easy for me to figure out how to store information there, it's highly visible, and it allows an easy way to include/omit parts that shouldn't be part of the calculations.