Abstract: The invention relates to a metal torsion bar for vehicle suspension, including a straight central part which has a circular cross-section, the central part extending longitudinally between two fixing heads suitable for being rigidly fixed to other mechanical members, the torsion bar being work-hardened by pretwisting and at least the central part of the torsion bar having an annular cross-section. According to the invention, the cross-section of the central part has a radially outside plasticized zone, which has been deformed plastically by the pretwisting, and a radially inside elastic zone, which has been deformed elastically by the pretwisting, the cross-section of the central part of the torsion bar having a total area S and the plasticized zone having an area Sp such that their ratio Sp/S is at least equal to 0.5 and at most equal to 0.6.