Derailleur mechanism for a rear wheel derailleur of a bicycle, comprising: - a base component which is configured to be immovably fastened to the bicycle relative to a bicycle frame during its intended gear shift operation, - a movement coupling device and - a chain guide arrangement which is displaceably coupled to the base component by means of the movement coupling device in a translatory manner relative to the base component, - a cable pull securing formation which is configured for supporting a cable sheath and for passing through a cable of a gear shift cable pull portion running from a gear shift actuating device to the derailleur mechanism, - a cable pull transmission device which is rotatably arranged about a rotational axis on the base component, wherein the cable pull transmission device has a gear shift deflection contour which surrounds the rotational axis with a larger radial spacing and has a displacement deflection contour which surrounds the rotational axis with a smaller radial spacing (r), wherein the gear shift deflection contour and the displacement deflection contour are connected together for common rotational movement about the rotational axis, wherein the gear shift deflection contour is configured to receive the gear shift cable pull portion so as to bear along a gear shift winding region, and wherein the displacement deflection contour is configured to receive a displacement cable pull portion running between the displacement deflection contour and the movement coupling device so as to bear along a displacement winding region, characterized in that the larger radial spacing or/and the smaller radial spacing (r) has a non-uniform value over the respective winding region (76, 86), so that the cable pull transmission device, over its rotational angular range about the rotational axis which is effective during operation, provides a variable transmission ratio as a function of the rotational position of the cable pull transmission device relative to the base component. - a base component which is configured to be immovably fastened to the bicycle relative to a bicycle frame during its intended gear shift operation, - a movement coupling device and - a chain guide arrangement which is displaceably coupled to the base component by means of the movement coupling device in a translatory manner relative to the base component, - a cable pull securing formation which is configured for supporting a cable sheath and for passing through a cable of a gear shift cable pull portion running from a gear shift actuating device to the derailleur mechanism, - a cable pull transmission device which is rotatably arranged about a rotational axis on the base component, wherein the cable pull transmission device has a gear shift deflection contour which surrounds the rotational axis with a larger radial spacing and has a displacement deflection contour which surrounds the rotational axis with a smaller radial spacing (r), wherein the gear shift deflection contour and the displacement deflection contour are connected together for common rotational movement about the rotational axis, wherein the gear shift deflection contour is configured to receive the gear shift cable pull portion so as to bear along a gear shift winding region,