Rear derailleur of a bicycle

A rear derailleur of a bicycle comprising:

• a four-bar linkage having a base body and a movable body and a pair of connecting rods articulated to the base body and to the movable body along articulation axes at four pin elements;
• said movable body being connected to a chain guide at a connection pin;
• a fixing pin received in an inner cavity that is formed by one closed end of the base body and comprising a fixing screw directly or indirectly constrained to a frame of the bicycle and a spring loading insert arranged in said blind inner cavity, said base body being rotatable with respect to said spring loading insert;
• connection members inserted in seats formed on said fixing screw and said spring loading insert to axially constrain said fixing screw and said spring loading insert to each other;
• a toothed sector and a sprocket parts of an adaptation linkage configured to change a relative angular position between said base body and said frame as a function of a controlled movement of the chain guide, said toothed sector being mounted coaxially to the spring loading insert and being active on said sprocket, said sprocket being directly or indirectly active on the base body;
• and
• a torsion spring active between said spring loading insert and said toothed sector configured to constrain said spring loading insert and said toothed sector in rotation during controlled movements of the chain guide.