I'm also working on a spinner . I personally would add more bearings than the four you have as when you collide with something (whether it be another bot or the arena itself) your the spinner will put a huge amount of stress on the bearings, possibly shattering them. A good example of this was Mr Speed Squared where the disc shattered the bearings after a few impacts. You might be able to solve this by using many more bearings around the circumference. Remember to leave space for all your electronics inside though! Batteries, speed controllers, receiver and all the wiring between it all is very easy to forget when in CAD. Looking good sofar!