Pre-twisted metal torsion bar and method of making same

Method of manufacturing a metal torsion bar, including a straight central part which has a circular cross-section, the said central part extending longitudinally between two fixing heads suitable for being rigidly fixed to other mechanical members, the said torsion bar being work-hardened by pretwisting, at least the central part of the torsion bar having an annular cross-section and thus having an inside surface, this method comprising

• subjecting a tubular torsion bar to a pretorque whose value is chosen so that the cross-section of the central part of the torsion bar has a radially outside plasticized zone, deformed plastically by the pretwisting, and a radially inside elastic zone, deformed elastically by the pretwisting, the annular 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 the ratio Sp/S is at least equal to 0.5 and at most equal to 0.6, and
• working the inside surface of at least the central part of the torsion bar to create residual compressive stresses to a radial thickness of at least 0.3 mm from the said inside surface.