A lot of thoughts had crossed my mind for the wheels. Lawnmower wheels? Wagon wheels? (Note for American readers: Not the delicious snack food.) Wheels for some other kinda wheeled toy? All of these seemed to heavy. The lightest option would be to exclude them entirely. I really thought about that but if the Optimus Prime Costume has fenders, the guy needs wheels. In true spirit of this costume, rather than going with something overly expensive (that would actually look good), I decided to make them myself. I started with four 5x5 blocks of foam. I then rounded off the corners on one of them.
Realizing this wasn't too hard, and that it kinda looked like a wheel, I did the other three.
Next I glued the pieces together in pairs. Since I used leftover 1 inch thick foam, that gave me two wheels with a depth of 2 inches. Once the glue was dry, I bevelled the edge that would be facing outward.
Next the wheels got a coat of black spraypaint. (Again, the Krylon H2O stuff. Almost anything else will eat styrofoam.)
When the wheels were dry, I printed out a wheel rim pattern. I just guessed at the size, they came out at around 2.5 inches. So Optimus Prime has low profile tires. Ah well. I cut out the rims and glued them to the wheels. Are you ready for this? It actually looks alright. Seriously.