Abstract: Embodiments are directed to bicycle drive trains, as well as methods and systems for changing gear ratios within the bicycle drive trains. A bicycle drive train includes a crank assembly, where the crank assembly is configured to rotate about an axis. The crank assembly defines a plane generally transverse to the axis. The crank assembly includes at least one driving gear in the plane that is of a first gear size. The crank assembly is also configured to selectively vary the size of the driving gear in the plane, so that the driving gear changes size while remaining positioned within the plane. The crank assembly may also be configured to include hinged gear segments that, when moved about their hinge, allow gear shifts along the same linear plane.