Best material for wheels.

"Where the rubber meets the road" was the old slogan for Firestone tires. "Where the wheel meets the carpet" will be key to this years contest. I know there was a white paper on friction somewhere. Anyone remember where it was at? It is easy to make a test rig to measure the coefficent of friction. glue a piece of the sample carpet to a board and cut a couple on equal sized squares of wood, and glue various samples of possible tread material on them. add some weight, a spring scale and measure away. Won't give away or secret material yet, but it ain't gonna be plain Skyway wheels.

If any one else has done this - I am not sure carpet is a good material to measure the friction on, since the pile deforms as more
weight is put on it - so it may not be a linear function.

Also the rule about bunching up the carpet has me confused.
If your robot is stopped, and someone pushing you causes the carpet to bunch - who is at fault?