Polo eBay finds/marketplace

Bullshit.

For I, refer to this:

which is the form by which you calculate the total moment for a body, based on the basic I = mr² formula.

Non disc vs disc:

Treating rims as if all the weight was at the eyelet, and abstact a 160mm two-piece rotor to a single point at 60mm radius rather than calculating the distribution.....

XC717 disc is 25g lighter than non-disc, eyelets at 270mm
RT86 rotor 115g at 100mm

disc inertia penalty = (115 x 60 x 60) - (25 x 270 x 270)= 414000 - 1822500 = -1408500 g.mm^2

So the braking surface has 4.5 times the moment of intertia of the rotor.
that's good to know em