To elaborate on butterfly drive as Team Neutrino has used it in the past two years:
Is this drive train each corner has a small traction wheel and a larger omni or traction wheel. By using pneumatic cylinders, a module can be moved to allow the robot to drive on either all the small traction wheels (slower, high pushing force, low maneuverability) or on the larger wheels (faster, lower force, high maneuverability).
Here is an older blog post on the modules used last year:
http://www.teamneutrino.org/seasons/...bot/butterfly/