When making a rover with the Versalino http://www.versalino.com we had problems attaching the wheel to our motor.The solution was simple a 3D printed shaft that matched the inside of my hobby tire, and had a hole down the center for the motor itself.We had to ream out the inside a bit, and snip the overhang on the motor belt driver on the motor shaft to get a really nice snug fit.Just print, ream, snip, squish it on, and push the wheel in place.Sounds like a lot of things, but it was pretty easy once we made the model.This is a part for the Versalino Rover which is a work in progress using a Rapid Prototyping board we designed http://www.versalino.com