The shock absorber has two cylinders, one inside and one outside. The piston moves in the inner cylinder, and due to the entry and extraction of the piston rod, the volume of oil in the inner cylinder increases and contracts accordingly. Therefore, it is necessary to maintain the balance of oil in the inner cylinder by exchanging with the outer cylinder. So there should be four valves in the double tube shock absorber, namely, in addition to the piston valve and compression valve mentioned above, there are also flow valves and compensation valves installed between the inner and outer tubes to complete the exchange function.