This picture is in response to the number of people who came up to our pits to look at our drivetrain and ask, “How did you push two robots sideways at the same time?”
The answer is this folks: These are 6" elevator guide wheels with a medium rubber bonded to the rim. We made custom hubs and voila, there we go. the bevel on the rubber works so that the more the wheel spins, the smaller the wheel gets: this gives us more torque. It also means that we have more rubber touching the the carpet for more traction. the more the wheels spin, the more our robot adapts to stop the wheels from spinning.
Sort of like a built-in CVT…cool! 
I dont get it. How do they get smaller? Does the rubber bend?
After awhile of running on the wheels the rubber begins to wear off and the wheels diameter shinks because of the loss of rubber. The wheels don’t bend, they just wear out like anything else does after a long use.