Do they look like stains, or is it like "rust" in the finish? You might want to try some kind of a heavy duty degreaser like simple green or something of that nature. If that doesn't work, try getting some kind of a wheel/metal polish and see if that'll work. If that is to no avail, you might have to get it either professionally cleaned, or the surface of the wheels has to be redone (either remachined or repolished).