Abstract: This crank-gear includes cranks centered on a first axis and respectively mounted on shafts journalled into one another in the bicycle crank-gear case. Each shaft bears a guide cooperating with sliders which are fixed to rotate with the sprocket plates actuating the chain with an orbital movement around a second axis. In view of the offset between the first and second axes and the relative arrangement of the cranks and guides the cranks are aligned with one another only when horizontal. Furthermore, when one of the cranks is at bottom dead center the other crank is forwardly offset by a certain distance relative to top dead center. An improved pedalling efficiency is thereby obtained.