I found that the ball transfer units tended to fill up with dirt and then just stick, it basically became a metal wear pad. You could flush the dirt out but it was a bit of a pain. Thats why this year Im using nylon skids on my robot.

Another option is nylon wheels pointing forwards but they can slide sideways fairly easily. This can help with making your robot go in a straight line.

Richard Wenman
Team Mayhem