What is Cd^2

ddr
this is about SR in practice...

imagine a Cd with inner radius r, outter radius R and middle radius d. the Cd starts to spin from 0 to some V spatial speed with respect to the radius d. as the speed increases all the perimeters shrink, but the outter one shrinks far beyound the inner because it has greater spatial velocity cause it corresponds to bigger radius. so when the speed of d-circle becomes some V (critical) the Cd will (in a SR kinda sense) turn into square ie. the outter and the inner parameters will equalize.
am I right?