Damping force adjustable shock absorber

A damping force adjustable shock absorber, comprising:

• a cylinder sealingly enclosing a working liquid;
• a reservoir sealingly enclosing the working liquid and a gas;
• a piston, which is slidably fitted into the cylinder, and separates a space inside the cylinder into a first chamber and a second chamber;
• a piston rod having one end coupled to the piston and another end extending to an outside through the first chamber;
• a base valve separating the second chamber and the reservoir from each other;
• a first low speed valve, which is provided in the piston, and is configured to allow a flow of the working liquid from the second chamber toward the first chamber;
• a second low speed valve, which is provided in the base valve, and is configured to allow a flow of the working liquid from the reservoir toward the second chamber;
• a first passage connecting the first chamber and the reservoir to each other;
• a damping force adjustment mechanism capable of adjusting a damping force from a soft characteristic corresponding to a low damping force to a hard characteristic corresponding to a high damping force in accordance with an external command by controlling a flow of the working liquid in the first passage;
• a second passage connecting the reservoir and the second chamber;
• a third passage arranged in parallel or in series to the first low speed valve;
• and
• a third low speed valve, which is provided in the third passage, and is configured to allow a flow of the working liquid from the first chamber to the second chamber, the third low speed valve being opened at a pressure lower than a pressure at which the first low speed valve is opened,
• wherein the damping force adjustment mechanism includes:
• a main valve to be opened under a pressure of the working liquid;
• a pilot chamber configured to exert an internal pressure on the main valve in a valve-closing direction;
• an introduction orifice configured to introduce the working liquid into the pilot chamber;
• a pilot passage configured to allow communication between a downstream side of the introduction orifice and the pilot chamber and a downstream side of the main valve;
• and
• a control valve provided in the pilot passage;
• and
• a fail-safe valve provided on a downstream side of the control valve,
• wherein the control valve, the fail-safe valve, and a fourth low speed valve, which are configured to allow a flow of the working liquid from the first chamber to the reservoir, is provided in the first passage, and
• wherein a piston speed at which the third low speed valve is opened is different from a piston speed at which the fourth low speed valve is opened.