Bicycle gear change

A derailleur bicycle gear change comprising a chain guiding device having a pivoted yoke carrying the pivots of a chain guide roller and a chain tension roller and a displacement device including a first part arranged to be fixed to the frame of the bicycle and a second part which acts as a support for the guiding device, said second part being arranged to be movable in relation to the first part in order to cause the guiding device to move transversely in relation to the rear wheel of the bicycle gear change along sprockets associated with the wheel, the guiding device incorporating a guiding member in the form of a guiding lug mounted so as to pivot in relation to said yoke in a plane perpendicular to the axis of the sprockets and common to that of a cheek of the yoke, and the gear change further comprising a mechanical linkage device connected to said displacement device and arranged to cause the guiding lug to pivot when said guiding device moves transversely so as to bring the free extremity of the guiding lug adjacent the circumference of the sprocket whose position corresponds to that of the guiding device.