Chopper's center leg isn't a footshell like R2D2 and other droids. It's just a single large wheel on a caster. The hardest part was finding a caster/wheel combination that worked. I eventually had to build a caster around a wheel that I liked. I'm testing that now. So far I've had minimal issues from the assembly.

The caster tracks properly as the droid turns. However, just moving him by hand resulted in a nasty face-plant. I'm looking at positioning the batteries to help compensate.