A rear derailleur of a bicycle is disclosed. The rear derailleur includes a four-bar linkage. A fixing pin is received in a blind inner cavity of a base body and a fixing screw is constrained to a frame. A spring loading insert is arranged in the blind inner cavity. The base body is rotatable with respect to the spring loading insert. Connection members are inserted in seats formed on the fixing screw and on the spring loading insert to axially constrain the fixing screw and the spring loading insert. A toothed sector is mounted coaxially to the spring loading insert and is active on a sprocket which is active on the base body. A torsion spring is active between the spring loading insert and the toothed sector and is configured to constrain the spring loading insert and the toothed sector in rotation during controlled movements of the chain guide.