Here is a close view of a wheel in the wall. The center part of the wheel is 6" diameter and 2" wide. A 1/2" thick 8" diameter flange is on the outside layer of the wheel making the overall thickness of the wheel 3". A piece of rubber belting is attached to the inside for the pipe to roll on. The guide wheels were added to the design after the roof was constructed. The flanges on the wheels are not needed. Commercial rubber-tired wheels could have been used.