Shock absorber having controlled damping force characteristics for use in a suspension system of a vehicle

A suspension system of a vehicle comprising:

• a shock absorber having variable damping force characteristic provided between a sprung member and an unsprung member of said vehicle;
• a sprung member absolute speed detecting means for detecting absolute speed of said sprung member;
• relative speed detecting means for detecting relative speed between said sprung member and said unsprung member;
• relative displacement detecting means for detecting a relative displacement between said sprung member and said unsprung member;
• control means for controlling said damping force characteristic of said shock absorber by receiving signals from detecting means and calculating a product which is the absolute speed of said sprung member multiplied by the relative speed between said sprung member and said unsprung member wherein, when the product is more than a predetermined value (Ki), said damping force characteristic of said shock absorber is set high and when the product is less than said predetermined value (Ki), the damping force characteristic of said shock absorber is set low;
• and
• an insensible range of the absolute speed of said sprung member and an insensible range of said relative displacement between said sprung member and said unsprung member setting means for setting, with respect to control of said control means, insensible range which restrict changing of said damping force characteristic when an absolute value of the absolute speed of said sprung member and an absolute value of the relative displacement between said sprung member and said unsprung member are less than predetermined values (.delta.Zi) and (.delta.ri), respectively, wherein said damping force characteristic of said shock absorber is maintained soft when the absolute speed of said sprung member and the relative displacement between said sprung member and said unsprung member are within said respective insensible ranges so that an oscillation frequency of said sprung member is in a high oscillation frequency region, and said damping force characteristic of said shock absorber is maintained hard when the absolute speed of said sprung member is out of said respective insensible ranges and the relative displacement between said sprung member and said unsprung member is within said insensible range so that an oscillation frequency of said sprung member is in a low oscillation frequency region.