so i was watching MithBusters and they were playing with antigravity, and to make sure that there is -or isnt at all- antigravity involved with their gadgets tests they called these guys who bring a machine that measure gravity.
the gravity measure machine works firing a laser to a falling mirror, so the faster the mirror fall the stronger gravity is, and viceversa.
they say the machine show gravity changes acording to the moon position, and, here comes the point, on weekends!, because, they concluded, their lab is located in a basement under a stadium, so when the stadium is full of people the mass of this people produce gravity over their heads.
SO! ... let me introduce this really big wheel with half of it under a really big mass which produces the "counter" gravity needed to make it run.
tadaa! :P