Sub-assembly for a bicycle rear wheel, comprising: a sprocket-carrying body equipped with a radially inner threaded portion at an axially outer end thereof; a cogset coaxially mounted on the sprocket-carrying body, wherein the cogset comprises a plurality of axially adjacent sprockets between a first sprocket having minimum number of teeth and a last sprocket having maximum number of teeth, wherein at least the first three sprockets (26d, 26c, 26b, 26a) are fixedly connected to one another so as to form a monolithic set with an axial through opening in which - in at least one axially inner portion thereof - a coupling profile is formed in shape engagement on a corresponding coupling profile formed on a radially peripheral surface of the sprocket-carrying body; a fixing element for axially fixing the monolithic set to the sprocket-carrying body, said fixing element comprising a ring nut, formed in two parts screwed together, an axially outer first part and an axially inner second part, wherein the first part comprises a tubular body on which a flange is formed, radially peripheral and projecting radially with respect to the tubular body, the flange being at an axially outer end of the ring nut; wherein the second part comprises a tubular body, on which a radially peripheral threaded portion is formed, at an axially inner end of the second part of the ring nut, in screwing engagement with the radially inner threaded portion of the sprocket-carrying body, wherein the flange abuts against an axially outer face of the monolithic set and the threaded portion has a larger diameter than the minimum diameter of the axial through opening of the monolithic set, characterized in that the second part of the ring nut further comprises an inner flange, projecting radially from the tubular body towards the rotation axis, the inner flange being at the axially inner end of the second part of the ring nut, the inner flange being in abutment against an axially inner face of the first part of the ring nut so that the ring nut is constrained to the monolithic set only in the axial direction and is free to rotate with respect to the monolithic set. a sprocket-carrying body equipped with a radially inner threaded portion at an axially outer end thereof; a cogset coaxially mounted on the sprocket-carrying body, wherein the cogset comprises a plurality of axially adjacent sprockets between a first sprocket having minimum number of teeth and a last sprocket having maximum number of teeth, wherein at least the first three sprockets (26d, 26c, 26b, 26a) are fixedly connected to one another so as to form a monolithic set with an axial through opening in which - in at least one axially inner portion thereof - a coupling profile is formed in shape engagement on a corresponding coupling profile formed on a radially peripheral surface of the sprocket-carrying body; a fixing element for axially fixing the monolithic set to the sprocket-carrying body, said fixing element comprising a ring nut, formed in two parts screwed together, an axially outer first part and an axially inner second part, wherein the first part comprises a tubular body on which a flange is formed, radially peripheral and projecting radially with respect to the tubular body, the flange being at an axially outer end of the ring nut; wherein the second part comprises a tubular body, on which a radially peripheral threaded portion is formed, at an axially inner end of the second part of the ring nut, in screwing engagement with the radially inner threaded portion of the sprocket-carrying body, wherein the flange abuts against an axially outer face of the monolithic set and the threaded portion has a larger diameter than the minimum diameter of the axial through opening of the monolithic set, characterized in that the second part of the ring nut further comprises an inner flange, projecting radially from the tubular body towards the rotation axis, the inner flange being at the axially inner end of the second part of the ring nut, the inner flange being in abutment against an axially inner face of the first part of the ring nut so that the ring nut is constrained to the monolithic set only in the axial direction and is free to rotate with respect to the monolithic set.