This is a stepping stone for a larger project I'm working on. Each wheel is powered independently, and has it's own steering. The outer wheel rides on 6 bearings and is powered by a Pololu 25D 6V motor. I used their drive shaft adapter to attach the outer wheel to the motor. The outer tire is made from Sainsmart TPU. The metal hub on the servo was purchased from Servocity. I have a printable version, but I would recommend the aluminum version that Servocity sells. The servo mount is downloaded from Servocity's website. I made some modifications to facilitate printing and beefed it up a bit. If you have the means, I would highly recommend using their aluminum servo mounts. I expect that I will be purchasing them soon.
You could probably get away with not using the inner set of bearings. My goal with this was to make it low maintenance, and to function well under stress. So far it has performed great. I think you could get away with using printed bearings if you didn't want to spend the money. If someone decides to build this, let me know and I'll get a parts list together.