1. Field of the Invention
The present invention relates generally to a variable damping force shock absorber for an automotive suspension system. More specifically, the invention relates to a variable damping force shock absorber which automatically adjust the damping mode depending upon the mode of vibration input to a vehicular body.
2. Description of the Background Art
A typical construction of variable damping force shock absorber has been disclosed in Japanese Utility Model Second (Examined) Publication (Jikko) Showa 63-11401. The discloses variable damping force shock absorber has a lower fluid chamber communicating with an annular reservoir chamber defined between inner and outer cylinder tubes. Fluid communication between the lower fluid chamber and the reservoir chamber is established by means of a compression fluid path and a compression by-pass path. A damping valve is provided in the compression fluid path. On the other hand, a variable orifice is provided in the compression by-pass path. A valve spool is associated with the variable orifice for adjusting fluid flow path area in the variable orifice according to the position thereof.
With such a construction, the damping valve and the variable orifice are active during the piston compression or bounding stroke to restrict fluid flow through the compression fluid path and the compression by-pass path from the lower fluid chamber to the reservior chambee and whereby generator damping force which suppresses relative displacement between the vehicular body as a sprung mass and a suspension member, such as a suspension arm, suspension link and so forth, which serves as an unsprung mass together with a road wheel. Where vibration is induced due to the action of the vehicular body, such as pitching, rolling and so forth, the vibration frequency is relatively low. The mode of such vibration caused by vehicular body action will be herafter referred to as "lower frequency mode of vibration" or "low frequency mode vibration". In the disclosed shock absorber, the valve spool in the variable orifice is responsive to such lower frequency mode vibration to shut or block fluid flow through the variable orifice. As a result, the fluid communication is established only through the compression fluid path. Therefore, high damping force is generated by means of the damping valve because of high flow restrictrion rate. Therefore, in such case, the shock absorber can operates in HARD mode.
On the other hand, when vibration is induced by vibration energy input from the road wheel due to undulation or uneven on the road surface, vibration frequency is relatively high. Such mode of vibration with the hereafter referred to as "high frequency mode of vibration" or "high frequency mode vibration" In such a case, the valve spool in the compression by-pass path is placed in a position to establish fluid communication therethrough. Therefore, fluid can flow from the lower fluid chamber to the reservoir chamber through both of the compression fluid path and the compression by-path. As a result, fluid flow amount is split between the compression fluid path and the compression by-pass path so as to generate smaller damping force for hydrodynamically absorbing vibration energy. Therefore, the shock absorber operates in SOFT mode.
On the other hand, in the case of the variable orifice, the fluid flow path area at the open position is maintained constant. Therefore, as is well known, the damping force to be generated becomes proportional to square of stroke speed of a piston in the shock absorber. Therefore, at relatively low piston speed range, damping force tends to become too small to obtain satisfactory stability of the vehicle. Of course, it is possible to obtain satisfactory damping force at the variable orifice by adjusting the path area of the orifice to be small enough to provide higher damping force. However, in such case, the damping force at relatively high piston speed range becomes unacceptably high. Therefore, it is difficult to achieve satisfactorily high damping force at relatively low frequency range with eliminating excess damping force at relatively high piston speed range.
On the other hand, while the variable orifice is shut off, dmaping force is solely generated by the damping valve which comprises a resiliently deformable valve disc which is seated on a valve seat at the normal position and is deformed to be shifted away from the valve seat to form the variable path area orifice for permitting fluid flow therethrough. In such case, the damping force to be generated by the damping valve is proportional to 2/3 power of the piston stroke speed. Therefore, at relatively low piston speed range, the damping force variation rate is relatively large to cause excess damping force.
As can be appreciated, in order to optimize performance of the automotive suspension system, it is desirable to provide linear variation characteristics of the damping force relative to the piston stroke speed. Particularly, it is desirably to provide linear damping force variation characteristics at substantially low piston stroke speed range, such as less than or equal to 0.1 m/sec.