Abstract: A variable strength spring includes a tubular body, at whose inner surface is coaxially provided a concave contact seat, a ring made of elastomeric material rigidly engaged in the tubular body and a connection tang rigidly engaged in the elastic ring. The elastic ring has a radial groove that defines, on the longitudinal length of the ring, a first portion rigidly engaged inside the tubular body opposite the contact seat, and a second portion slidably engaged in the tubular body. On the outer circumferential end of the second portion is situated a rigid rabbet edge positioned opposite to a stopping seat inside the tubular body.