An auxiliary suspension unit (A) for mounting parallel to a standard shock absorber comprises a cylinder (52) receiving a piston (42) which includes a piston head (46) mounted to a piston rod (44) and defining in the cylinder first and second chambers (72, 74) filled with oil (76). The first chamber (72) is on the rod side of the piston head (46). A spring structure (132) comprises a floating piston (112) and a high pressure accumulator (126) contained in a third chamber (122) with the floating piston (112) being slidable therein. The working fluid (76) from the second chamber (74) exerts pressure on the floating piston (112) and is opposed by the high-pressure accumulator (126) of the third chamber (122). The auxiliary unit (A) also comprises an actuator (20) for positioning a valve (64) provided on the piston (42) into first and second positions thereof and further comprises a control device (C) for the actuator (20). In the first position of the valve (64) and upon compression of the auxiliary unit (A), a bypass (288) allows fluid transfer from the second (274) to the first chamber (272). Alternatively, the bypass (288) can be replaced by a first orifice (86), whereby the fluid transferring from the second (74) to the first chamber (72) creates a damping force. In the second position of the valve (64), the bypass (288) or the first orifice (86) is closed. Therefore, fluid from the second chamber (74) passing through a second orifice (110) exerts pressure on the floating piston (112). The high-pressure accumulator (126) opposes a downward movement of the floating piston (112) into the third chamber (122) thereby providing a spring force to the auxiliary unit (A) along with a damping force resulting from the second orifice (110).