US Patent 7694785 Controllable damping force hydraulic shock absorber

A controllable damping force hydraulic shock absorber including a pilot type damping valve having a back-pressure chamber for each of an extension stroke and a compression stroke. A piston connected to a piston rod is fitted into a sealed cylinder in which a hydraulic fluid is contained. During an extension stroke of the piston rod, the hydraulic fluid in an upper cylinder chamber flows to a lower cylinder chamber through an extension-side hydraulic fluid passage, an extension-side orifice hydraulic fluid passage, an extension-side back-pressure chamber, an axial hydraulic fluid passage, a radial hydraulic fluid passage, a compression-side back-pressure chamber, an extension-side check valve and a compression-side hydraulic fluid passage. During a compression stroke, the hydraulic fluid in the lower cylinder chamber flows to the upper cylinder chamber through the compression-side hydraulic fluid passage, the compression-side orifice passage, the compression-side back-pressure chamber, the radial hydraulic fluid passage, the axial hydraulic fluid passage, the extension-side back-pressure chamber, the compression-side check valve and the extension-side hydraulic passage. The hydraulic fluid passage for an extension stroke and the hydraulic fluid passage for a compression stroke have some elements in common, thus simplifying the structure of the controllable damping force hydraulic shock absorber.