Shock absorber for bicycles or motorcycles comprising a hydraulic cartridge provided with a hydraulic cylinder filled with a viscous fluid and connected to a first external attachment point a hydraulic piston connected by a stem to a second attachment point and slidable in said hydraulic cylinder a fluid passage for passing a quantity of viscous fluid from a first section to a second section of the hydraulic cylinder in response and opposing resistance to a reciprocal movement of said attachment points (4, 6). the shock absorber further comprising a pneumatic spring for vehicle suspensions including, at least one compression chamber within a pneumatic spring cylinder provided with said first external attachment point, a sliding pneumatic piston with a pressure seal in the chamber and connected to a stem provided with said second attachment point outside the cylinder, wherein said pneumatic spring comprises a closed-cell elastomer body arranged inside the compression chamber, said piston being able to move between a resting configuration of the spring wherein the internal pressure of the chamber has a first value and at least one compressed spring configuration corresponding to an approach of the first and second attachment points (4, 6) with respect to the resting configuration, wherein the internal pressure of the chamber takes a second value, greater than said first value, wherein in response to said second value said elastomer body assumes an elastically deformed contracted configuration, and returns to the resting configuration as the pressure decreases, showing a substantially elastic deformation associated with a viscous behaviour, characterised in that said elastomer body is free to move inside the compression chamber. a hydraulic cylinder filled with a viscous fluid and connected to a first external attachment point a hydraulic piston connected by a stem to a second attachment point and slidable in said hydraulic cylinder a fluid passage for passing a quantity of viscous fluid from a first section to a second section of the hydraulic cylinder in response and opposing resistance to a reciprocal movement of said attachment points (4, 6). the shock absorber further comprising a pneumatic spring for vehicle suspensions including, at least one compression chamber within a pneumatic spring cylinder provided with said first external attachment point, a sliding pneumatic piston with a pressure seal in the chamber and connected to a stem provided with said second attachment point outside the cylinder, wherein said pneumatic spring comprises a closed-cell elastomer body arranged inside the compression chamber, said piston being able to move between a resting configuration of the spring wherein the internal pressure of the chamber has a first value and at least one compressed spring configuration corresponding to an approach of the first and second attachment points (4, 6) with respect to the resting configuration, wherein the internal pressure of the chamber takes a second value, greater than said first value, wherein in response to said second value said elastomer body assumes an elastically deformed contracted configuration, and returns to the resting configuration as the pressure decreases, showing a substantially elastic deformation associated with a viscous behaviour, characterised in that said elastomer body is free to move inside the compression chamber.