Though it might not look like it, this fidget spinner maintains a center of mass at the center bearing. Try it out and discover that it still spins smoothly!
Uses 608 (Skateboard/Scooter) bearings. A higher quality ceramic 608 bearing is recommended for the center.
I use these great bearing caps by hodginsa: http://www.thingiverse.com/thing:1815244
Version 2 has a slightly tighter bearing fit, fillets instead of chamfers, and a higher quality .STL