Robert - here's a photo of one of the wheels on the stand and another taken off.
Note the little aluminum block. It goes under the setscrew inside the rectangular cutout in the wheel's mounting assembly. The setscrew in the mounting assembly bears on the aluminum block, pushing it tight it to the leg. Also, the aluminum block has a little round projection that locks into a hole in the leg. The combination is extremely solid when assembled.
The wheels themselves have rubber tires and are mounted on rather sturdy ball-bearing assemblies. The wheels are approximately 3-1/8" diameter. The stand's legs are approximately .880" diameter.

