. . . and, if your sides of the run/tractor whatever are more than an inch off the ground just hope that there are no predators who can sneak in a small space like that.
I tried using 8-inch wheels I got at the hardware store. Rigged up a way to raise the wheels when not in use so the sides of the tractor were on the ground (with an extra two inches of hardware cloth as an 'apron'. Problem was that the axle was so long that the wheels wobbled and created too many problems.
Soooo, I took those wheels off and put on two large rear wheels from a lawnmower. They have a lever that can be adjusted to raise or lower the wheel assembly. You just have to figure out the correct height before you do your final installation of the wheel assembly. Now the tractor works perfectly, sits on the ground, and can be raised quickly and pushed wherever it needs to go.
If you figure out how to do it with bicycle tires, please post it on here so we can see how you do it.
Thanks.