chamber 4 and the uninterrupted chamber 5 the rod is driven outwards and the shock absorber extends until it returns to the position shown in fig1 in the invention the shock absorber is capable of overtravel in the event of a particularly hard landing for this purpose the rod 2 includes a hollow orifice opening out to the side of the piston 3 and receiving an abutment piece 50 in the bore in the rod 2 so as to project from the rod 2 the abutment piece 50 is held to the rod by a pin 51 the abutment piece 50 has an end wall and a sealing gasket 52 is disposed between the rod 2 and the abutment piece 50 thus enabling the bore of the rod to be isolated from the uninterrupted chamber 5 when the shock absorber is used for retracting the undercarriage the abutment piece 50 comes into abutment against the end wall 53 of the main cylinder 1 thereby defining the retracted position of the shock absorber when the shock absorber is used for damping impacts on landing the abutment piece 50 normally does not engage the end wall 53 of the main cylinder 1 however during certain hard landings not only can the abutment piece 50 come into contact with the end wall 53 but the energy that needs to be absorbed can be such that this contact does not suffice to stop the shock absorber being shortened in the invention the pin 51 is then designed to shear beyond a predetermined force threshold for the purposes firstly of absorbing a fraction of the surplus landing energy by rupturing the pin 51 and also of allowing the rod 2 to perform overtravel in the main cylinder 1 thereby.