The problem with one material is printing the bearing pockets. TPU bonds to itself so well that support is pretty much impossible to use with it (unless you have a multi material printer and can use a dedicated support material – which I don’t have and therefore can’t.)
As I was falling asleep I actually had the same idea Ryan suggests though 🙂 But even printed solid I don’t think the inner parts would do well out of TPU.
I’m going to open up the inner hole so the inner race of the bearings won’t rub and it will be possible to install a spacer like a normal wheel. (If anyone with real wheels could give me a measurement of the ID and the full width face to face that would be great.) but I’m also going to try printing the Tire with 6 perimeters to see if that stiffens it up enough since the current 3 perimeter tire really is a bit soft. I may bump the infill a bit too…but with 6 perimeters it will be pretty close to a solid tire.