Patent Publication Number: US-2016229254-A1

Title: Shock absorber

Description:
TECHNICAL FIELD 
     The present invention relates to a shock absorber. 
     BACKGROUND ART 
     There is known a damping force adjustable shock absorber including a cylinder, a piston slidably inserted into the cylinder, a piston rod movably inserted into the cylinder and connected to the piston, an expansion-side chamber and a contraction-side chamber partitioned by the piston inside the cylinder, an intermediate tube provided to envelop the cylinder to form a discharge passage in conjunction with the cylinder, an outer tube provided to envelop the intermediate tube to form a reservoir for storing hydraulic oil in conjunction with the intermediate tube, a charge passage that allows only a flow of hydraulic oil directed from the reservoir to the contraction-side chamber, a rectification passage provided in the piston to allow only a flow of hydraulic oil directed from the contraction-side chamber to the expansion-side chamber, and a damping force variable valve provided between the discharge passage and the reservoir. 
     In the shock absorber described above, the hydraulic oil flows from the cylinder to the reservoir through the discharge passage due to functions of the rectification passage and the charge passage in both the expanding and contracting motions. In addition, a damping force exerted by the shock absorber can be adjusted by controlling the resistance to the flow of hydraulic oil by using the damping force variable valve (for example, see JP 2009-222136 A). 
     In this manner, a damping force can be adjusted in the shock absorber described above. Therefore, it is possible to improve a vehicle ride quality by exert a damping force optimized to a vehicle vibration. In addition, in the shock absorber described above, the damping force variable valve is provided outside the cylinder. Therefore, it is very advantageous in that it is not required to sacrifice a stroke length of the shock absorber and harm loadability of a vehicle, compared to other types of shock absorbers in which the damping force variable valve is provided in the piston. 
     SUMMARY OF INVENTION 
     In the shock absorber described above, the resistance applied by the damping force variable valve to the flow of hydraulic oil is adjusted by controlling a thrust applied from a solenoid to a pilot valve body that controls a valve opening pressure of the damping force variable valve. 
     In order to generate a damping force optimized to suppress a vehicle vibration by using the shock absorber described above, an optimum damping force is obtained by using an electronic control unit (ECU) based on vibration information of a vehicle chassis detected by various sensors, and a control command is issued to a driver of the solenoid to exert the optimum damping force. 
     Therefore, an upper limit of a chassis vibration frequency that can be damped by the shock absorber by adjusting the damping force is restricted to several hertzs (Hz) depending on responsiveness of the damping force variable valve and a processing speed of the ECU. For this reason, it is difficult to suppress vibrations over this frequency level. 
     However, a chassis vibration frequency significantly affecting the vehicle ride quality is higher than the aforementioned dampable frequency level. Such a high frequency vibration is not suppressed by the shock absorber described above. Therefore, it is desired to further improve the vehicle ride quality. 
     In view of the aforementioned problems, it is therefore an object of the present invention to provide a shock absorber capable of improving a vehicle ride quality. 
     According to one aspect of the present invention, a shock absorber includes a cylinder, a piston slidably inserted into the cylinder, the piston partitioning the cylinder into an expansion-side chamber and a contraction-side chamber, a piston rod movably inserted into the cylinder, the piston rod being connected to the piston, a reservoir that stores a hydraulic fluid, a charge passage configured to allow only a flow of hydraulic fluid directed from the reservoir to the contraction-side chamber, a rectification passage configured to allow only a flow of hydraulic fluid directed from the contraction-side chamber to the expansion-side chamber, a damping force adjuster configured to allow only a flow of hydraulic fluid directed from the expansion-side chamber to the reservoir and change resistance to the flow of hydraulic fluid, and at least one of an expansion-side sensitive unit operated in an expanding motion of the shock absorber and a contraction-side sensitive unit operated in a contracting motion of the shock absorber, wherein the expansion-side sensitive unit has an expansion-side actuating chamber that communicates with the expansion-side chamber and the contraction-side chamber, and an expansion-side free piston slidably inserted into the expansion-side actuating chamber, the expansion-side free piston partitioning the expansion-side actuating chamber into a first expansion-side pressure chamber communicating with the expansion-side chamber and a second expansion-side pressure chamber communicating with the contraction-side chamber, and the contraction-side sensitive unit has a contraction-side actuating chamber that communicates with the contraction-side chamber and the reservoir, and a contraction-side free piston slidably inserted into the contraction-side actuating chamber, the contraction-side free piston partitioning the contraction-side actuating chamber into a first contraction-side pressure chamber communicating with the contraction-side chamber and a second contraction-side pressure chamber communicating with the reservoir. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a shock absorber according to a first embodiment of the present invention; 
         FIG. 2  is a characteristic diagram illustrating a damping force characteristic of the shock absorber according to the first embodiment of the present invention; 
         FIG. 3  is a cross-sectional view illustrating an expansion-side sensitive mechanism according to the first embodiment of the present invention; 
         FIG. 4  is a cross-sectional view illustrating a contraction-side sensitive mechanism according to the first embodiment of the present invention; 
         FIG. 5  is a cross-sectional view illustrating a shock absorber according to a second embodiment of the present invention; 
         FIG. 6  is a cross-sectional view illustrating an expansion-side sensitive mechanism according to the second embodiment of the present invention; 
         FIG. 7  is a cross-sectional view illustrating a contraction-side sensitive mechanism according to the second embodiment of the present invention; 
         FIG. 8  is a cross-sectional view illustrating an expansion-side sensitive mechanism provided with an expansion-side cushioning mechanism; 
         FIG. 9  is a cross-sectional view illustrating an expansion-side sensitive mechanism provided with an expansion-side liquid pressure cushioning mechanism; 
         FIG. 10  is a cross-sectional view illustrating a contraction-side sensitive mechanism provided with a contraction-side liquid pressure cushioning mechanism; 
         FIG. 11  is a cross-sectional view illustrating an expansion-side sensitive mechanism configured by using an expansion-side valve; 
         FIG. 12  is a cross-sectional view illustrating a contraction-side sensitive mechanism configured by using a contraction-side valve; 
         FIG. 13  is a cross-sectional view illustrating an expansion-side sensitive mechanism provided with an expansion-side valve in a first expansion-side passage; and 
         FIG. 14  is cross-sectional view illustrating a contraction-side sensitive mechanism provided with a contraction-side cushioning mechanism. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A description will now be made for a shock absorber  51  according to a first embodiment of the present invention with reference to the accompanying drawings. 
     Referring to  FIG. 1 , the shock absorber  51  includes a cylinder  1 , a piston  2  slidably inserted into the cylinder  1  to partition the cylinder  1  into an expansion-side chamber R 1  and a contraction-side chamber R 2 , a piston rod  14  movably inserted into the cylinder  1  and connected to the piston  2 , a reservoir R that stores hydraulic oil as hydraulic fluid, a charge passage  3  that allows only a flow of hydraulic oil directed from the reservoir R to the contraction-side chamber R 2 , a rectification passage  4  that allows only a flow of hydraulic oil directed from the contraction-side chamber R 2  to the expansion-side chamber R 1 , and a damping force variable valve V as a damping force adjuster that allows only a flow of hydraulic oil directed from the expansion-side chamber R 1  to the reservoir R and capable of changing resistance applied to the flow of hydraulic oil. 
     In addition, the shock absorber S 1  includes: an expansion-side sensitive mechanism RME acting as an expansion-side sensitive unit and having an expansion-side actuating chamber E communicating with the expansion-side chamber R 1  and the contraction-side chamber R 2 , and an expansion-side free piston  15  slidably inserted into the expansion-side actuating chamber E to partition the expansion-side actuating chamber E into a first expansion-side pressure chamber E 1  communicating with the expansion-side chamber R 1  and a second expansion-side pressure chamber E 2  communicating with the contraction-side chamber R 2 ; and a contraction-side sensitive mechanism RMC acting as a contraction-side sensitive unit and having a contraction-side actuating chamber C communicating with the contraction-side chamber R 2  and the reservoir R, and a contraction-side free piston  24  slidably inserted into the contraction-side actuating chamber C to partition the contraction-side actuating chamber C into a first contraction-side pressure chamber C 1  communicating with the contraction-side chamber R 2  and a second contraction-side pressure chamber C 2  communicating with the reservoir R. 
     Furthermore, the shock absorber S 1  includes an intermediate tube  9  provided to envelop the cylinder  1  to form a discharge passage  7  that causes the expansion-side chamber R 1  and the reservoir R to communicate with each other in conjunction with the cylinder  1 , and a bottomed cylindrical outer tube  10  provided to envelop the intermediate tube  9  and form the reservoir R in conjunction with the intermediate tube  9 . The damping force variable valve V is provided between the discharge passage  7  and the reservoir R. 
     The piston rod  14  has one end  14   a  connected to the piston  2  and the other end protruding outward while its shaft is slidably supported by an annular rod guide  8  that seals the cylinder  1 . 
     The shock absorber S 1  is interposed between a chassis and a traveling wheel, for example, by mounting an upper end of the piston rod  14  in  FIG. 1  to a chassis of a vehicle and mounting a lower end of the outer tube  10  in  FIG. 1  to an axle that supports the traveling wheel so that a damping force is exerted to suppress a vibration between the chassis and the traveling wheel. It is noted that the piston rod  14  may also be mounted to an axle of a vehicle, and the outer tube  10  may also be mounted to a chassis of a vehicle. 
     According to this embodiment, the expansion-side actuating chamber E is provided in the piston  2  connected to the piston rod  14 . However, the expansion-side actuating chamber E may be provided in the piston rod  14 . Alternatively, the expansion-side actuating chamber E may not be directly built in the piston  2  and the piston rod  14 . Instead, it may be provided in a separate member connected to the piston rod  14 . Furthermore, the expansion-side actuating chamber E may be provided in a part other than the cylinder  1 . 
     The lower ends of the cylinder  1  and the intermediate tube  9  in  FIG. 1  are sealed by the valve casing  11 . The valve casing  11  is provided with the contraction-side actuating chamber C and the charge passage  3 . The contraction-side actuating chamber C may not be directly built in the valve casing  11 . Instead, the contraction-side actuating chamber C may be provided in a separate member connected to the valve casing  11 . Alternatively, the contraction-side actuating chamber C may be provided in a part other than the cylinder  1 . 
     The expansion-side chamber R 1 , the contraction-side chamber R 2 , the expansion-side actuating chamber E, and the contraction-side actuating chamber C are filled with hydraulic oil. In addition, the reservoir R is filled with gas in addition to the hydraulic oil. It is noted that, for example, a liquid such as water or an aqueous solution other than the hydraulic oil may also be used as the hydraulic fluid. 
     A description will now be made for each part of the shock absorber S 1  in more detail. 
     The piston  2  is connected to one end  14   a  of the piston rod  14 . A gap between the piston rod  14  and the rod guide  8  is sealed with a seal member  12 , so that the inside of the cylinder  1  is maintained in a liquid tight state. 
     The rod guide  8  has an outer diameter increasing stepwise, and its outer circumference is fitted to the intermediate tube  9  and the outer tube  10 . As a result, the rod guide  8  blocks the upper ends of the cylinder  1 , the intermediate tube  9 , and the outer tube  10  in  FIG. 1 . 
     The valve casing  11  is fitted to the lower end of the cylinder  1  in  FIG. 1 . The valve casing  11  has a small diameter portion  11   a  inserted into the cylinder  1 , a middle diameter portion  11   b  having an outer diameter larger than that of the small diameter portion  11   a  so as to be fitted to the inside of the cylinder  1 , a large diameter portion  11   c  provided in the lower end side of the middle diameter portion  11   b  in  FIG. 1  so as to be fitted to the inside of the intermediate tube  9  having an outer diameter larger than that of the middle diameter portion  11   b , a tubular portion  11   d  provided in the lower end side of the large diameter portion  11   c  in  FIG. 1 , and a plurality of notches lie provided in the tubular portion  11   d.    
     The valve casing  11 , the cylinder  1 , the intermediate tube  9 , the rod guide  8 , and the seal member  12  are housed in the outer tube  10 . If the upper end of the outer tube  10  in  FIG. 1  is caulked, the valve casing  11 , the cylinder  1 , the intermediate tube  9 , and the rod guide  8  are fixed to the outer tube  10  while they are held between the caulking portion boa of the outer tube  10  and the bottom portion  10   b  of the outer tube  10 . 
     It is noted that, instead of caulking of the opening end of the outer tube  10 , a cap may be screwed to the opening end of the outer tube  10  to hold the valve casing  11 , the cylinder  1 , the intermediate tube  9 , and the rod guide  8  between the cap and the bottom portion  10   b.    
     Specifically, the charge passage  3  includes an inlet port  3   a  provided in the valve casing  11  to cause the reservoir R and the contraction-side chamber R 2  to communicate with each other and a check valve  3   b  provided in the inlet port  3   a.    
     The inlet port  3   a  is opened to the upper end of the middle diameter portion  11   b  of the valve casing  11  in  FIG. 1  and the lower end of the large diameter portion  11   c  in  FIG. 1 . In addition, the inlet port  3   a  communicates with the reservoir R through the notch lie. The check valve  3   b  is opened only when the hydraulic oil flows from the reservoir R to the contraction-side chamber R 2 . That is, the check valve  3   b  is set as a one-way passage to allow only a flow of hydraulic oil directed from the reservoir R to the contraction-side chamber R 2  and suppress a reverse flow. In this manner, the inlet port  3   a  and the check valve  3   b  constitute the charge passage  3 . 
     The piston  2  is provided with the rectification passage  4  that allows only a flow of hydraulic oil directed from the contraction-side chamber R 2  to the expansion-side chamber R 1 . Specifically, the rectification passage  4  includes a passage  4   a  provided in the piston  2  to cause the contraction-side chamber R 2  and the expansion-side chamber R 1  to communicate with each other and a check valve  4   b  provided in the passage  4   a.    
     The check valve  4   b  is opened only when the hydraulic oil flows along the passage  4   a  from the contraction-side chamber R 2  to the expansion-side chamber R 1 . That is, the check valve  4   b  is set as a one-way passage that allows only a flow of hydraulic oil directed from the contraction-side chamber R 2  to the expansion-side chamber R 1  and suppresses a reverse flow. In this manner, the passage  4   a  and the check valve  4   b  constitute the rectification passage  4 . 
     A through-hole  1   a  connected to the expansion-side chamber R 1  is provided in the vicinity of the upper end of the cylinder  1  in  FIG. 1 . As a result, the expansion-side chamber R 1  and an annular gap formed between the cylinder  1  and the intermediate tube  9  communicate with each other. The annular gap between the cylinder  1  and the intermediate tube  9  constitute the discharge passage  7  that causes the expansion-side chamber R 1  and the reservoir R to communicate with each other. 
     The damping force variable valve V is provided in a valve block  13  fixed to extend across the outer tube  10  and the intermediate tube  9 . The damping force variable valve V includes a flow passage  13   a  that connects the discharge passage  7  of the intermediate tube  9  to the reservoir R, a valve body  13   b  provided in the middle of the flow passage  13   a , a pilot passage  13   c  used to apply a pressure of the expansion-side chamber R 1  in the upstream side of the valve body  13   b  to the valve body  13   b  to be compressed in a valve opening direction, and a compressor unit  13   d  that exerts a compressing force for compressing the valve body  13   b  in a valve close direction in a variable manner. 
     According to this embodiment, the compressor unit  13   d  controls the pressure for compressing the valve body  13   b  in a valve close direction by using a solenoid as illustrated in  FIG. 1 . For this reason, the compressor unit  13   d  can change the pressure for compressing the valve body  13   b  in the valve close direction depending on a current supply amount supplied to the solenoid from the outside. 
     Alternatively, the compressor unit  13   d  may compress the valve body  13   b  only by an actuator such as a solenoid. Alternatively, the compressing force may be changed depending on a current amount or a voltage of the supplied current. 
     When the hydraulic fluid is a magnetic viscous fluid, instead of the damping force variable valve V, the damping force adjuster may change the resistance applied to the flow of the magnetic viscous fluid passing through the flow passage by applying a magnetic field to the flow passage that causes the discharge passage  7  and the reservoir R to communicate with each other, for example, by adjusting an intensity of the magnetic field by using a coil and the like based on the current amount supplied from the outside. 
     When the hydraulic fluid is an electroviscous fluid, the damping force adjuster may apply an electric field to the flow passage that causes the discharge passage  7  and the reservoir R to communicate with each other to change the resistance applied to the electroviscous fluid flowing through the flow passage by adjusting an intensity of the electric field based on an external voltage. 
     When the shock absorber S 1  makes a contracting motion, the contraction-side chamber R 2  is compressed by moving the piston  2  downward in  FIG. 1 , so that the hydraulic oil inside the contraction-side chamber R 2  moves to the expansion-side chamber R 1  through the rectification passage  4 . In addition, during the contracting motion, as the piston rod  14  intrudes the inside of the cylinder  1 , the hydraulic oil overflows as much as the intrusion volume of the rod inside the cylinder  1 , and the surplus hydraulic oil is extracted from the cylinder  1  and is discharged to the reservoir R through the discharge passage  7 . The shock absorber S 1  raises the pressure inside the cylinder  1  by generating resistance to the flow of hydraulic oil moving to the reservoir R through the discharge passage  7  by using the damping force variable valve V to exert the contraction-side damping force. 
     When the shock absorber S 1  makes an expanding motion, the expansion-side chamber R 1  is compressed by moving the piston  2  upward in  FIG. 1 , and the hydraulic oil inside the expansion-side chamber R 1  moves to the reservoir R through the discharge passage  7 . In addition, during the expanding motion, the volume of the contraction-side chamber R 2  increases by moving the piston  2  upward in  FIG. 1 , and the hydraulic oil corresponding to the increase amount is supplied from the reservoir R through the charge passage  3 . The shock absorber S 1  raises the pressure inside the expansion-side chamber R 1  by generating resistance to the flow of hydraulic oil moving to the reservoir R through the discharge passage  7  by using the damping force variable valve V to exert an expansion-side damping force. 
     In this manner, if the shock absorber S 1  makes an expanding or contracting motion, the hydraulic oil is necessarily discharged to the reservoir R from the inside of the cylinder  1  through the discharge passage  7 . Therefore, the shock absorber S 1  is considered as a uni-flow type shock absorber because the hydraulic oil flows sequentially in the order of the contraction-side chamber R 2 , the expansion-side chamber R 1 , and the reservoir R in a one-way passing manner. As a result, the shock absorber S 1  generates both the expansion-side and contraction-side damping forces by using the single damping force variable valve V. 
     In the shock absorber S 1 , by setting a cross-sectional area of the piston rod  21  to a half of that of the piston  2 , it is possible to set the amount of the hydraulic oil discharged from the cylinder  1  equally between the expanding and contracting motions if the amplitude is equal therebetween. Therefore, by setting the resistance generated by the damping force variable valve V equally, it is possible to set the expansion-side and contraction-side damping forces to the same value. 
     The expansion-side sensitive mechanism RME includes an expansion-side actuating chamber E communicating with the expansion-side chamber R 1  and the contraction-side chamber R 2 , and an expansion-side free piston  15  slidably inserted into the expansion-side actuating chamber E to partition the expansion-side actuating chamber E into a first expansion-side pressure chamber E 1  communicating with the expansion-side chamber R 1  and a second expansion-side pressure chamber E 2  communicating with the contraction-side chamber R 2 . 
     According to this embodiment, the expansion-side actuating chamber E is formed by a cavity portion provided in the piston  2 . The expansion-side actuating chamber E communicates with the expansion-side chamber R 1  through the first expansion-side passage  17  and communicates with the contraction-side chamber R 2  through the second expansion-side passage  18 . 
     The expansion-side free piston  15  is slidably inserted into the expansion-side actuating chamber E. The expansion-side free piston  15  partitions the expansion-side actuating chamber E into the first expansion-side pressure chamber E 1  and the second expansion-side pressure chamber E 2 . Therefore, as the expansion-side free piston  15  moves inside the expansion-side actuating chamber E, any one of the first expansion-side pressure chamber E 1  and the second expansion-side pressure chamber E 2  expands while the other one contracts. 
     The first expansion-side pressure chamber E 1  communicates with the expansion-side chamber R 1  through the first expansion-side passage  17 , and the second expansion-side pressure chamber E 2  communicates with the contraction-side chamber R 2  through the second expansion-side passage  18 . The first expansion-side pressure chamber E 1  and the second expansion-side pressure chamber E 2  do not directly communicate with each other because they are separated by the expansion-side free piston  15 . However, as the expansion-side free piston  15  moves inside the expansion-side actuating chamber E, one of the volumes of the first expansion-side pressure chamber E 1  and the second expansion-side pressure chamber E 2  increases while the other volume is reduced in proportion. Therefore, apparently, the first expansion-side passage  17 , the expansion-side actuating chamber E, and the second expansion-side passage  18  act as a passage that causes the expansion-side chamber R 1  and the contraction-side chamber R 2  to communicate with each other. 
     According to this embodiment, an expansion-side valve element  19  is provided in the middle of the second expansion-side passage  18  to generate resistance to the flow of hydraulic oil passing through the second expansion-side passage  18 . The expansion-side valve element  19  is formed by a throttle such as an orifice or a chalk. The expansion-side valve element  19  allows a flow of hydraulic oil directed from the second expansion-side pressure chamber E 2  to the contraction-side chamber R 2  and a flow of hydraulic oil directed from the contraction-side chamber R 2  to the second expansion-side pressure chamber E 2  and generates resistance to the flow of hydraulic oil. It is noted that the expansion-side valve element  19  may be provided in the first expansion-side passage  17  instead of or in addition to the second expansion-side passage  18 . 
     The contraction-side sensitive mechanism RMC includes a contraction-side actuating chamber C communicating with the contraction-side chamber R 2  and the reservoir R, and a contraction-side free piston  24  slidably inserted into the contraction-side actuating chamber C to partition the contraction-side actuating chamber C into a first contraction-side pressure chamber C 1  communicating with the contraction-side chamber R 2  and a second contraction-side pressure chamber C 2  communicating with the reservoir R. 
     According to this embodiment, the contraction-side actuating chamber C is formed by a cavity portion provided in the valve casing  11 . The contraction-side actuating chamber C communicates with the contraction-side chamber R 2  through the first contraction-side passage  26  and communicates with the reservoir R through the second contraction-side passage  27 . 
     The contraction-side free piston  24  is slidably inserted into the contraction-side actuating chamber C. The contraction-side free piston  24  partitions the contraction-side actuating chamber C into the first contraction-side pressure chamber C 1  and the second contraction-side pressure chamber C 2 . Therefore, as the contraction-side free piston  24  moves inside the contraction-side actuating chamber C, any one of the first and second contraction-side pressure chambers C 1  and C 2  expands while the other one contracts. 
     The first contraction-side pressure chamber C 1  communicates with the contraction-side chamber R 2  though the first contraction-side passage  26 , and the second contraction-side pressure chamber C 2  communicates with the reservoir R through the second contraction-side passage  27 . The first and second contraction-side pressure chambers C 1  and C 2  are partitioned by the contraction-side free piston  24 , and thus, they do not directly communicate with each other. However, as the contraction-side free piston  24  moves inside the contraction-side actuating chamber C, one of the volumes of the first contraction-side pressure chamber C 1  and the second contraction-side pressure chamber C 2  expands while the other volume is reduced in proportion. Therefore, apparently, the first contraction-side passage  26 , the contraction-side actuating chamber C, and the second contraction-side passage  27  act as a passage that causes the contraction-side chamber R 2  and the reservoir R to communicate with each other. 
     According to this embodiment, a contraction-side valve element  28  that generates resistance to the flow of hydraulic oil passing through the first contraction-side passage  26  is provided in the middle of the first contraction-side passage  26 . The contraction-side valve element  28  is formed by a throttle such as an orifice or a chalk. The contraction-side valve element  28  allows a flow of hydraulic oil directed from the first contraction-side pressure chamber C 1  to the contraction-side chamber R 2  and a flow of hydraulic oil directed from the contraction-side chamber R 2  to the first contraction-side pressure chamber C 1  and generates resistance to these flows of the hydraulic oil. It is noted that the contraction-side valve element  28  may be provided in the second contraction-side passage  27  instead of or in addition to the first contraction-side passage  26 . 
     Since the shock absorber S 1  is configured as described above, the piston  2  moves upward in  FIG. 1  when the shock absorber S 1  makes an expanding motion. For this reason, the hydraulic oil is discharged from the compressed expansion-side chamber R 1  to the reservoir R through the damping force variable valve V. In addition, the hydraulic oil is supplied from the reservoir R to the expanding contraction-side chamber R 2  through the charge passage  3 . Therefore, while the pressure of the expansion-side chamber R 1  increases, the pressure of the contraction-side chamber R 2  is nearly equalized with the pressure of the reservoir R. 
     Since the first expansion-side pressure chamber E 1  of the expansion-side actuating chamber E communicates with the expansion-side chamber R 1  through the first expansion-side passage  17 , the pressure of the first expansion-side pressure chamber E 1  is equalized with the pressure of the expansion-side chamber R 1  during the expanding motion of the shock absorber S 1 . In addition, since the second expansion-side pressure chamber E 2  communicates with the contraction-side chamber R 2  through the second expansion-side passage  18 , the pressure of the second expansion-side pressure chamber E 2  is reduced under the pressure of the first expansion-side pressure chamber E 1 . Therefore, the expansion-side free piston  15  moves downward in  FIG. 1 . As a result, the first expansion-side pressure chamber E 1  expands while the second expansion-side pressure chamber E 2  contracts. It is noted that, in this case, since the expansion-side valve element  19  generates resistance to the flow of hydraulic oil passing through the second expansion-side passage  18 , abrupt displacement of the expansion-side free piston  15  is suppressed. 
     The first contraction-side pressure chamber C 1  of the contraction-side actuating chamber C communicates with the contraction-side chamber R 2  through the first contraction-side passage  26 , and the second contraction-side pressure chamber C 2  communicates with the reservoir R through the second contraction-side passage  27 . In addition, the pressure of the contraction-side chamber R 2  is nearly equalized with the pressure of the reservoir R during the expanding motion of the shock absorber S 1 . For this reason, during the expanding motion of the shock absorber S 1 , the pressure of the first contraction-side pressure chamber C 1  and the pressure of the second contraction-side pressure chamber C 2  are nearly equalized with the pressure of the reservoir R, so that the contraction-side free piston  24  does not move. Therefore, during the expanding motion of the shock absorber S 1 , the contraction-side free piston  24  is not operated. 
     Therefore, when the shock absorber S 1  makes an expanding motion, the contraction-side sensitive mechanism RMC is not operated, and only the expansion-side sensitive mechanism RME is operated, so that the expansion-side actuating chamber E acts as an apparent flow passage depending on the movement amount of the expansion-side free piston  15 . As a result, the hydraulic oil moves from the expansion-side chamber R 1  to the contraction-side chamber R 2  by detouring the damping force variable valve V. 
     When the shock absorber S 1  makes a contracting motion, the piston  2  moves downward in  FIG. 1 . Therefore, the contracting contraction-side chamber R 2  and the expanding expansion-side chamber R 1  communicate with each other through the rectification passage  4 , and the hydraulic oil is discharged from the cylinder  1  to the reservoir R through the damping force variable valve V. Accordingly, both the pressures of the expansion-side chamber R 1  and the contraction-side chamber R 2  increase to be equalized with each other. 
     Since the first contraction-side pressure chamber C 1  of the contraction-side actuating chamber C communicates with the contraction-side chamber R 2  through the first contraction-side passage  26 , the pressure of the first contraction-side pressure chamber C 1  is equalized with the pressure of the contraction-side chamber R 2  during the contracting motion of the shock absorber S 1 . In addition, since the second contraction-side pressure chamber C 2  communicates with the reservoir R through the second contraction-side passage  27 , the pressure of the second contraction-side pressure chamber C 2  becomes lower than the pressure of the first contraction-side pressure chamber C 1 . Therefore, the contraction-side free piston  24  moves downward in  FIG. 1 . As a result, the first contraction-side pressure chamber C 1  expands, while the second contraction-side pressure chamber C 2  contracts. It is noted that, in this case, since the contraction-side valve element  28  generates resistance to the flow of hydraulic oil passing through the first contraction-side passage  26 , abrupt displacement of the contraction-side free piston  24  is suppressed. 
     The first expansion-side pressure chamber E 1  of the expansion-side actuating chamber E communicates with the expansion-side chamber R 1  through the first expansion-side passage  17 , and the second expansion-side pressure chamber E 2  communicates with the contraction-side chamber R 2  through the second expansion-side passage  18 . In addition, during the contracting motion of the shock absorber S 1 , the pressure of the expansion-side chamber R 1  is nearly equalized with the pressure of the contraction-side chamber R 2 . For this reason, during the contracting motion of the shock absorber S 1 , the pressure of the first expansion-side pressure chamber E 1  is nearly equalized with the pressure of the second expansion-side pressure chamber E 2 . Therefore, the expansion-side free piston  15  does not move. Therefore, during the contracting motion of the shock absorber S 1 , the expansion-side free piston  15  is not operated. 
     Therefore, in a contracting motion of the shock absorber S 1 , the expansion-side sensitive mechanism RME is not operated, and only the contraction-side sensitive mechanism RMC is operated, so that contraction-side actuating chamber C acts as an apparent flow passage depending on a movement amount of the contraction-side free piston  24 . As a result, the hydraulic oil moves from the cylinder  1  to the reservoir R by detouring the damping force variable valve V. 
     Here, it is assumed that the piston speed is equal for both high and low vibration frequencies input to the shock absorber S 1 . 
     If the vibration frequency input to the shock absorber S 1  is low, an amplitude of the input vibration is large. For this reason, during the expanding motion, an amplitude of the expansion-side free piston  15  increases. In addition, if the expansion-side free piston  15  is displaced until the second expansion-side pressure chamber E 2  is fully compressed, the expansion-side free piston  15  is not allowed to make displacement to compress the second expansion-side pressure chamber E 2  any more. For this reason, the expansion-side actuating chamber E is not allowed to act as an apparent flow passage, and the hydraulic oil directed from the expansion-side chamber R 1  to the contraction-side chamber R 2  entirely passes through the damping force variable valve V. Therefore, the shock absorber S 1  exerts a strong damping force. 
     If the vibration frequency input to the shock absorber S 1  is low, an amplitude of the contraction-side free piston  24  is large, during the contracting motion. In addition, if the contraction-side free piston  24  is displaced until the second contraction-side pressure chamber C 2  is fully compressed, the contraction-side free piston  24  is not allowed to make displacement to compress the second contraction-side pressure chamber C 2  any more. For this reason, the contraction-side actuating chamber C is not allowed to act as an apparent flow passage, and the hydraulic oil directed from the cylinder  1  to the reservoir R entirely passes through the damping force variable valve V. Therefore, the shock absorber S 1  exerts a strong damping force. 
     That is, the shock absorber S 1  exerts a strong damping force when it expands and contracts at a low vibration frequency. 
     When the input frequency of the shock absorber S 1  is high, the amplitude of the input vibration is reduced. Therefore, the amplitude of the piston  2  is reduced, and the flow rate of the hydraulic oil discharged from the cylinder  1  to the reservoir R is also reduced. 
     In this case, during the expanding motion of the shock absorber S 1 , the amplitude of the expansion-side free piston  15  is reduced. Therefore, the expansion-side free piston  15  is not displaced until the second expansion-side pressure chamber E 2  is fully compressed. As a result, since the displacement of the expansion-side free piston  15  is not interfered, the expansion-side actuating chamber E acts as an apparent flow passage, and a part or all of the hydraulic oil directed from the expansion-side chamber R 1  to the contraction-side chamber R 2  detours the damping force variable valve V. Therefore, the damping force generated by the shock absorber S 1  is reduced. 
     In comparison, during the contracting motion of the shock absorber S 1 , the amplitude of the contraction-side free piston  24  is reduced. Therefore, contraction-side free piston  24  is not displaced until the second contraction-side pressure chamber C 2  is fully compressed. As a result, since the displacement of the contraction-side free piston  24  is not interfered, the contraction-side actuating chamber C acts as an apparent flow passage, and a part or all of the hydraulic oil directed from the cylinder  1  to the reservoir R detours the damping force variable valve V. Therefore, the damping force generated by the shock absorber S 1  is reduced. 
     It is noted that, if the expansion/contraction speed of the shock absorber S 1  increases over a certain level, the expansion-side valve element  19  and the contraction-side valve element  28  generate significant resistance to the flow of hydraulic oil. In this case, it is difficult to move the expansion-side free piston  15  and the contraction-side free piston  24 . Therefore, the damping force attenuation effect is nearly not generated. Accordingly, a characteristic of the damping force of the shock absorber S 1  is plotted as illustrated in  FIG. 2 . 
     The solid lines of  FIG. 2  indicate damping force characteristics when the expansion-side and contraction-side damping forces of the shock absorber S 1  are set to SOFT, MEDIUM, and HARD by using the damping force variable valve V as a damping force adjuster. The dotted lines indicate damping force characteristics when the damping force is set to SOFT, MEDIUM, and HARD, assuming that a high frequency vibration is input to the shock absorber S 1 , and the damping force is reduced. 
     As illustrated in  FIG. 2 , in the shock absorber S 1 , it is possible to cause a change of the damping force to depend on the input vibration amplitude, that is, the frequency. As a result, for a low frequency vibration input of a sprung mass resonant frequency level of a vehicle, having a large vibration amplitude, a strong damping force is generated, so that it is possible to stabilize a posture of a chassis (sprung mass member) and prevent an uncomfortable feeling of a passenger during turns. In addition, for a high frequency vibration input of an unsprung mass resonant frequency level of a vehicle, having a small vibration amplitude, a weak damping force is generated, and a vibration of the traveling wheel side (unsprung mass member side) is not transmitted to the chassis side (sprung mass member side). Therefore, it is possible to provide an excellent vehicle ride quality. 
     As described above, in the shock absorber S 1 , the damping force can be adjusted by controlling resistance generated by the damping force variable valve V and applied to the flow of hydraulic oil. That is, using the shock absorber S 1 , it is possible to reduce a damping force for a high frequency vibration with a small amplitude while the damping force of the damping force variable valve V is adjusted. 
     Therefore, in the shock absorber S 1 , for a relatively low frequency vibration, it is possible to damp a vibration of a chassis by adjusting the damping force by controlling the damping force variable valve V. In addition, for a high frequency vibration which is difficult to suppress by controlling the damping force variable valve V, a weak damping force can be exerted mechanically, so that it is possible to effectively suppress a vehicle vibration by blocking a vibration from the traveling wheel side. Therefore, it is possible to remarkably improve a vehicle ride quality. 
     By providing at least one of the expansion-side sensitive mechanism RME and the contraction-side sensitive mechanism RMC, it is possible to exert a weak damping force for a high frequency vibration, which is difficult to suppress by controlling the damping force variable valve V, even in a uni-flow type shock absorber S 1 . 
     By providing both the expansion-side sensitive mechanism RME and the contraction-side sensitive mechanism RMC, it is possible to set the characteristic of the damping force attenuation effect individually for expanding and contracting motions of the shock absorber S 1 . The expansion-side sensitive mechanism RME generates a damping force attenuation effect when a high frequency vibration is input to the shock absorber S 1  to make an expanding motion. The contraction-side sensitive mechanism RMC generates a damping force attenuation effect when a high frequency vibration is input to the shock absorber S 1  to make a contracting motion. Therefore, if a damping force attenuation effect is desired only in an expanding motion, only the expansion-side sensitive mechanism RME may be provided. Similarly, if the damping force attenuation effect is desired only in a contracting motion, only the contraction-side sensitive mechanism RMC may be provided. 
     It is noted that, in order to return the expansion-side free piston  15  to a position where the first expansion-side pressure chamber E 1  is fully compressed during the contracting motion of the shock absorber S 1 , the pressure of the contraction-side chamber R 2  may be set to be slightly higher than the pressure of the expansion-side chamber R 1  by generating a pressure loss in the check valve  4   b  when the hydraulic oil passes through the rectification passage  4 . 
     By returning the expansion-side free piston  15  to a position where the first expansion-side pressure chamber E 1  is fully compressed during the contracting motion of the shock absorber S 1 , it is possible to guarantee a stroke margin of the expansion-side free piston  15  during the expanding motion. 
     In order to return the contraction-side free piston  24  to a position where the first contraction-side pressure chamber C 1  is fully compressed during the expanding motion of the shock absorber S 1 , the pressure of the reservoir R may be set to be slightly higher than the pressure of the contraction-side chamber R 2  by generating a pressure loss in the check valve  3   b  when the hydraulic oil passes through the charge passage  3 . 
     By returning the contraction-side free piston  24  to a position where the first contraction-side pressure chamber C 1  is fully compressed during the expanding motion of the shock absorber S 1 , it is possible to guarantee a stroke margin of the contraction-side free piston  24  during the contracting motion. 
     It is noted that, in order to return the expansion-side free piston  15  to a position where the first expansion-side pressure chamber E 1  is fully compressed during the contracting motion of the shock absorber S 1 , a spring may be provided. The spring may have a very weak biasing force for biasing the expansion-side free piston  15  to compress the first expansion-side pressure chamber E 1 . 
     In order to return the contraction-side free piston  24  to a position where the first contraction-side pressure chamber C 1  is fully compressed during the expanding motion of the shock absorber S 1 , a spring may be provided. The spring may have a very weak biasing force for biasing the contraction-side free piston  24  to compress the first contraction-side pressure chamber C 1 . 
     Since the expansion-side sensitive mechanism RME and the contraction-side sensitive mechanism RMC are provided separately, it is possible to widen displaceable ranges of the expansion-side free piston  15  and the contraction-side free piston  24 . Therefore, even when a flow rate of the hydraulic oil flowing to the expansion-side actuating chamber E and the contraction-side actuating chamber C increases, it is possible to continuously obtain the damping force attenuation effect. 
     The expansion-side actuating chamber E of the expansion-side sensitive mechanism RME may be provided, for example, in the expansion-side housing  31  installed to the piston rod  30  as illustrated in  FIG. 3 . The expansion-side housing  31  is installed to one end  30   a  of the piston rod  30  to fix the piston  32  to the piston rod  30 . 
     The piston  32  having an annular shape is mounted to the outer circumference of one end  30   a  of the piston rod  30  and is provided with a port  32   a  that causes the contraction-side chamber R 2  and the expansion-side chamber R 1  to communicate with each other. The port  32   a  is stacked above the piston  32  in  FIG. 3  and is opened or closed by an annular check valve  33  mounted to the outer circumference of one end  30   a  of the piston rod  30 . 
     While the check valve  33  is fixed to the piston rod  30 , its outer circumference side can be flexed. The check valve  33  opens the port  32   a  for a flow of hydraulic oil directed from the contraction-side chamber R 2  to the expansion-side chamber R 1  to allow a passage of the hydraulic oil and closes the port  32   a  for a flow of hydraulic oil directed from the expansion-side chamber R 1  to the contraction-side chamber R 2  to inhibit a passage of the hydraulic oil. 
     The expansion-side housing  31  includes a tubular casing member  35  that receives the expansion-side free piston  34  inserted into the inside and a lid member  36  that blocks an opening end of the casing member  35 , which is the lower end in  FIG. 3 . 
     The casing member  35  includes a thread portion  35   a  having a small diameter in the upper side in  FIG. 3  so as to be screwed to the outer circumference of the lower end of one end  30   a  of the piston rod  30 , and a free piston housing portion  35   b  having a diameter larger than that of the thread portion  35   a  so as to slidably house the expansion-side free piston  34 . In addition, the lower end of the casing member  35  is blocked by the lid member  36  to form the expansion-side actuating chamber E. 
     The lid member  36  is provided with an orifice  36   a . As a result, the expansion-side actuating chamber E and the contraction-side chamber R 2  communicate with each other. In addition, the orifice  36   a  acts as both the expansion-side valve element  19  and the second expansion-side passage  18 . The piston rod  30  is provided with a first expansion-side passage  30   b  that is opened from the lower edge of the one end  30   a  and communicates with the expansion-side chamber R 1 . As a result, the expansion-side actuating chamber E and the expansion-side chamber R 1  communicate with each other. 
     The expansion-side free piston  34  has a bottomed cylindrical shape. The outer circumference of the expansion-side free piston  34  makes sliding contact with the inner circumference of the free piston housing portion  35   b  of the casing member  35 . The expansion-side free piston  34  partitions the expansion-side housing  31  into the first expansion-side pressure chamber E 1  communicating with the expansion-side chamber R 1  through the first expansion-side passage  30   b  and the second expansion-side pressure chamber E 2  communicating with the contraction-side chamber R 2  through the orifice  36   a.    
     By configuring the expansion-side sensitive mechanism RME in this manner, it is possible to assemble the expansion-side sensitive mechanism RME to the shock absorber S 1  without any especial difficulty and specifically implement the shock absorber S 1 . 
     The contraction-side actuating chamber C of the contraction-side sensitive mechanism RMC may be provided, for example, in the contraction-side housing  41  installed to the valve casing  40  as illustrated in  FIG. 4 . The valve casing  40  is fitted to the lower end of the cylinder  1  in  FIG. 1 . The contraction-side housing  41  is installed to the leading edge of the center rod  42  where the valve casing  40  is assembled, so that the check valve  44  stacked on the valve casing  40  is fixed to the center rod  42 . 
     The valve casing  40  having a bottomed cylindrical shape includes a small diameter portion  40   a  provided in its outer circumference and fitted to the lower end of the cylinder  1 , a middle diameter portion  40   b  that is fitted to the intermediate tube  9  and has an outer diameter larger than that of the small diameter portion  40   a , and a large diameter portion  40   c  that is provided in the lower end side of the middle diameter portion  40   b  in  FIG. 4  and has an outer diameter larger than that of the middle diameter portion  40   b . An insertion hole  40   d  that receives the inserted center rod  42  is provided in the bottom portion of the valve casing  40 . A plurality of notches  40   e  is provided in the lower end of the large diameter portion  40   c  in  FIG. 4 . The valve casing  40  is housed in the outer tube  10  while being nipped between the outer tube  10  and the cylinder  1 . 
     The center rod  42  includes a shaft portion  42   a  having a thread portion in its leading edge and a head portion  42   b  provided in a basal end of the shaft portion  42   a . The valve casing  40  may be assembled to the center rod  42  by inserting the shaft portion  42   a  of the center rod  42  to the insertion hole  40   d  from the downside of the valve casing  40 . 
     An inlet port  40   f  causing the contraction-side chamber R 2  and the reservoir R to communicate with each other is provided in the bottom portion of the valve casing  40 . The inlet port  40   f  is opened or closed by an annular check valve  44  stacked above the valve casing  40  in  FIG. 4  and mounted to the outer circumference of the center rod  42 . 
     While the check valve  44  is fixed to the center rod  42 , its outer circumference side can be flexed. The check valve  44  is operated to opens the inlet port  40   f  for a flow of hydraulic oil directed from the reservoir R to the contraction-side chamber R 2  to allow a passage of the hydraulic oil and closes the inlet port  40   f  for a flow of hydraulic oil directed from the contraction-side chamber R 2  to the reservoir R to inhibit a passage of the hydraulic oil. 
     The contraction-side housing  41  includes a tubular casing member  46  provided inward to receive the inserted contraction-side free piston  45  and a lid member  47  that blocks an opening end of the upper edge of the casing member  46  in  FIG. 4 . 
     The lower side of the casing member  46  in  FIG. 4  has a smaller diameter. The casing member  46  includes a thread portion  46   a  screwed to the outer circumference provided in the upper end of the center rod  42  and a free piston housing portion  46   b  that has a diameter larger than that of the thread portion  46   a  and slidably houses the contraction-side free piston  45 . In addition, the upper end of the casing member  46  is blocked by the lid member  47  to form the contraction-side actuating chamber C. 
     The lid member  47  is provided with an orifice  47   a . As a result, the contraction-side actuating chamber C and the contraction-side chamber R 2  communicate with each other. In addition, the orifice  47   a  acts as both the contraction-side valve element  28  and the first contraction-side passage  26 . The center rod  42  is provided with a second contraction-side passage  42   c  that is opened from the leading edge of the shaft portion  42   a  and communicates with the lower end of the head portion  42   b . As a result, the contraction-side actuating chamber C and the reservoir R communicate with each other. 
     The contraction-side free piston  45  has a bottomed cylindrical shape, and its outer circumference makes sliding contact with the inner circumference of the free piston housing portion  46   b  of the casing member  46 . The contraction-side free piston  45  partitions the contraction-side housing  41  into the first contraction-side pressure chamber C 1  communicating with the contraction-side chamber R 2  through the orifice  47   a  and the second contraction-side pressure chamber C 2  communicating with the reservoir R through the second contraction-side passage  42   c.    
     By configuring the contraction-side sensitive mechanism RMC in this manner, it is possible to assemble the contraction-side sensitive mechanism RMC to the shock absorber S 1  without any especial difficulty and specifically implement the shock absorber S 1 . 
     Second Embodiment 
     Next, a description will now be made for a shock absorber S 2  according to a second embodiment of the present invention. 
     Referring to  FIG. 5 , the shock absorber S 2  is provided with a check valve  50  that allows only a flow of hydraulic oil directed from the contraction-side chamber R 2  to the expansion-side chamber R 1 . The check valve  50  is arranged in parallel with the expansion-side valve element  19 . 
     According to this embodiment, the expansion-side valve element  19  is provided in the second expansion-side passage  18 . Therefore, the check valve  50  is preferably set to allow only a flow of hydraulic oil directed from the contraction-side chamber R 2  to the second expansion-side pressure chamber E 2  corresponding to a flow of hydraulic oil directed from the contraction-side chamber R 2  to the expansion-side chamber R 1 . 
     As a result, when the shock absorber S 2  makes an expanding motion so that the expansion-side free piston  15  moves to compress the second expansion-side pressure chamber E 2  by virtue of the pressure from the expansion-side chamber R 1 , and then, the shock absorber S 2  makes a contracting motion, the check valve  50  is opened. Therefore, it is possible to rapidly release the highly compressed pressure of the first expansion-side pressure chamber E 1  to follow the decompressed pressure of the expansion-side chamber R 1 . Therefore, the expansion-side free piston  15  can be pushed back promptly to compress the first expansion-side pressure chamber E 1 . 
     As a result, it is possible to prevent reduction of a displacement margin of the expansion-side free piston  15  for compressing the second expansion-side pressure chamber E 2 , that may be generated when vibrations are continuously input to the shock absorber S 2 , and a residual pressure of the first expansion-side pressure chamber E 1  forces the expansion-side free piston  15  to be deviated to the second expansion-side pressure chamber E 2  side. 
     In this manner, using the shock absorber S 2 , it is possible to prevent the expansion-side free piston  15  from being deviated to the second expansion-side pressure chamber E 2  side. Therefore, it is possible to prevent reduction of a displacement margin of the expansion-side free piston  15  during the expanding motion and a failure in obtaining the damping force attenuation effect. 
     When the expansion-side valve element  19  is provided in the first expansion-side passage  17 , the check valve  50  may be provided in parallel with the expansion-side valve element  19  to allow only a flow of hydraulic oil directed from the first expansion-side pressure chamber E 1  to the expansion-side chamber R 1 . 
     In addition, the shock absorber S 2  may be provided with a check valve  51  in parallel with the contraction-side valve element  28  to allow only a flow of hydraulic oil directed from the reservoir R to the contraction-side chamber R 2 . 
     According to this embodiment, the contraction-side valve element  28  is provided in the first contraction-side passage  26 . Therefore, the check valve  51  is preferably set to allow only a flow of hydraulic oil directed from the first contraction-side pressure chamber C 1  to the contraction-side chamber R 2  corresponding to a flow of hydraulic oil directed from the reservoir R to the contraction-side chamber R 2 . 
     As a result, when the shock absorber S 2  makes a contracting motion so that the contraction-side free piston  24  moves to compress the second contraction-side pressure chamber C 2  by virtue of the pressure from the contraction-side chamber R 2 , and then, the shock absorber S 2  makes an expanding motion, the check valve  51  is opened. Therefore, it is possible to rapidly release the highly compressed pressure of the first contraction-side pressure chamber C 1  to follow the decompressed pressure of the contraction-side chamber R 2 . Therefore, the contraction-side free piston  24  can be pushed back promptly to compress the first contraction-side pressure chamber C 1 . 
     As a result, it is possible to prevent reduction of a displacement margin of the contraction-side free piston  24  for compressing the second contraction-side pressure chamber C 2 , that may be generated when vibrations are continuously input to the shock absorber S 2 , and a residual pressure of the first contraction-side pressure chamber C 1  forces the contraction-side free piston  24  to be deviated to the second contraction-side pressure chamber C 2  side. 
     In this manner, using the shock absorber S 2 , it is possible to prevent the contraction-side free piston  24  from being deviated to the second contraction-side pressure chamber C 2  side. Therefore, it is possible to prevent reduction of a displacement margin of the contraction-side free piston  24  during the contracting motion and a failure in obtaining the damping force attenuation effect. 
     When the contraction-side valve element  28  is provided in the second contraction-side passage  27 , the check valve  51  may be provided in parallel with the contraction-side valve element  28  to allow only a flow of hydraulic oil directed from the reservoir R to the second contraction-side pressure chamber C 2 . 
     When the check valve  50  is specifically applied to the shock absorber S 2 , for example, the lid member  36  of the expansion-side housing  31  of the shock absorber S 1  of  FIG. 3  may be modified as illustrated in  FIG. 6 . 
     The lid member  52  of the shock absorber S 2  of  FIG. 6  blocks the opening end of the casing member  35  and has a port  52   a  that causes the second expansion-side pressure chamber E 2  and the contraction-side chamber R 2  to communicate with each other. In addition, a disc-like check valve  50  that blocks the opening end of the second expansion-side pressure chamber E 2  side of the port  52   a  is stacked on the lid member  52 . The check valve  50  is mounted to the outer circumference of the center rod  53  penetrating through the lid member  52 . The center rod  53  fixes the check valve  50  to the lid member  52  in conjunction with a ring  54  caulked to the leading edge. The check valve  50  can be flexed toward the outer circumference side while the inner circumference side is fixed to the lid member  52  by the center rod  53 . 
     The check valve  50  is flexed for a flow of hydraulic oil directed from the contraction-side chamber R 2  to the second expansion-side pressure chamber E 2  to open the port  52   a . For a reverse flow, the check valve  50  closes the port  52   a  to inhibit the reverse flow. 
     The check valve  50  is provided with an orifice  55  formed by notching. When the check valve  50  is closed, and the port  52   a  is closed, the orifice  55  acts as the second expansion-side passage  18  and the expansion-side valve element  19  for generating resistance to a flow of hydraulic oil directed from the second expansion-side pressure chamber E 2  to the contraction-side chamber R 2 . 
     As a result, it is possible to provide the shock absorber S 2  with the check valve  50  and the orifice  55  acting as the expansion-side valve element  19  and the second expansion-side passage  18  without any especial difficulty by saving space. 
     When the check valve  51  is specifically applied to the shock absorber S 2 , for example, the lid member  47  of the contraction-side housing  41  of the shock absorber S 1  of  FIG. 4  may be modified as illustrated in  FIG. 7 . 
     The lid member  56  of the shock absorber S 2  of  FIG. 7  blocks the opening end of the casing member  46  and has a port  56   a  that causes the first contraction-side pressure chamber C 1  and the contraction-side chamber R 2  to communicate with each other. In addition, a disc-like check valve  51  that blocks the opening end of the contraction-side chamber R 2  side of the port  56   a  is stacked on the lid member  56 . The check valve  51  is mounted to the outer circumference of the center rod  57  penetrating through the lid member  56 . The center rod  57  fixes the check valve  51  to the lid member  56  in conjunction with a ring  58  caulked and fixed to the leading edge. The outer circumference side of the check valve  51  can be flexed while its inner circumference side is fixed to the lid member  56  by the center rod  57 . 
     The check valve  51  is flexed for a flow of hydraulic oil directed from the second contraction-side pressure chamber C 2  to the contraction-side chamber R 2  to open the port  56   a . For a reverse flow, the port  56   a  is closed to inhibit the reverse flow. 
     In addition, the check valve  51  is provided with an orifice  59  formed by notching. When the check valve  51  is closed, and the port  56   a  is closed, the orifice  59  acts as the first contraction-side passage  26  and the contraction-side valve element  28  for generating resistance to a flow of hydraulic oil directed from the contraction-side chamber R 2  to the first contraction-side pressure chamber C 1 . 
     As a result, it is possible to provide the shock absorber S 2  with the check valve  51  and the orifice  59  acting as the contraction-side valve element  28  and the first contraction-side passage  26  without any especial difficulty by saving space. 
     In the shock absorber S 1  of  FIG. 3 , when the expansion-side free piston  34  fully compresses the second expansion-side pressure chamber E 2 , the expansion-side free piston  34  abuts on the lid member  36 , so that further movement of the expansion-side free piston  34  for compressing the second expansion-side pressure chamber E 2  is restricted. In addition, when the expansion-side free piston  34  returns to the position where the first expansion-side pressure chamber E 1  is fully compressed, the expansion-side free piston  34  abuts on the casing member  35 , so that further movement of the expansion-side free piston  34  for compressing the first expansion-side pressure chamber E 1  is restricted. 
     In this case, if the expansion-side free piston  34  and the expansion-side housing  31  collide violently, a striking sound is generated and is recognized by a passenger in a vehicle as a noise. 
     In this regard, in order to alleviate the striking sound level, it is preferable to provide an expansion-side cushioning mechanism as an expansion-side cushioning portion as illustrated in  FIG. 8 , including a cushion  60  that brings in contact with the expansion-side free piston  34  to inhibit collision between the expansion-side free piston  34  and the lid member  36  when the expansion-side free piston  34  is displaced up to the stroke end, and a cushion  61  that brings in contact with the expansion-side free piston  34  to inhibit collision between the expansion-side free piston  34  and the casing member  35  when the expansion-side free piston  34  returns to the full compressing position of the first expansion-side pressure chamber E 1 . 
     The cushions  60  and  61  may have an annular shape. In addition, a plurality of cushions  60  and  61  may be provided in places of the lid member  36  and the casing member  35  where the expansion-side free piston collides. Alternatively, a cushion may be provided in the expansion-side free piston  34  to bring into contact with the lid member  36  and the casing member  35 . The cushion may be formed of an elastic body such as rubber or synthetic resin or may include a waved washer or a disc spring. 
     Naturally, the cushion may be applied to the contraction-side sensitive mechanism RMC. For example, if a cushion is provided in the contraction-side sensitive mechanism RMC of  FIG. 4 , cushions  71  and  72  may be installed to the casing member  46  and the lid member  47  to act as a contraction-side cushioning mechanism as a contraction-side cushioning portion as illustrated in  FIG. 14  to inhibit direct collision between the contraction-side free piston  45  and the contraction-side housing  41 . Alternatively, a cushion may also be provided in the contraction-side free piston  45 . 
     As a result, it is possible to alleviate a striking sound level generated when the expansion-side free piston  34  collides with the expansion-side housing  31  and a striking sound level generated when the contraction-side free piston  45  collides with the contraction-side housing  41 . Therefore, it is possible to prevent a vehicle passenger from having an uncomfortable or uneasy feeling. Naturally, the expansion-side cushioning mechanism and the contraction-side cushioning mechanism may also be applied to the shock absorber S 2 . 
     In order to prevent the expansion-side free piston  34  from violently colliding with the lid member  36 , as illustrated in the shock absorber S 3  of  FIG. 9 , the structure of the shock absorber S 1  may be additionally provided with an expansion-side liquid pressure cushioning mechanism as an expansion-side liquid pressure cushioning portion for preventing the expansion-side free piston  34  from violently colliding with the expansion-side housing  31  by reducing a flow passage area of the second expansion-side passage  18  as the expansion-side free piston  34  is displaced up to the stroke end. 
     The expansion-side liquid pressure cushioning mechanism has an orifice  65  that is opened from the outside of the free piston housing portion  35   b  of the casing member  35  and communicates with the inside, an annular groove  66  formed along a circumferential direction of the outer circumference of the tubular portion of the expansion-side free piston  34 , and a passage  67  provided in the expansion-side free piston  34  to cause the second expansion-side pressure chamber E 2  to communicate with the annular groove  66 . 
     While the expansion-side free piston  34  is positioned to fully compress the first expansion-side pressure chamber E 1 , the annular groove  66  faces the orifice  65 . In this state, the contraction-side chamber R 2  and the second expansion-side pressure chamber E 2  communicate with each other through the orifice  65 , the annular groove  66 , and the passage  67 . In addition, the second expansion-side pressure chamber E 2  also communicates with the contraction-side chamber R 2  through the orifice  36   a  provided in the lid member  36 . Therefore, the orifice  65 , the annular groove  66 , and the passage  67  constitute the second expansion-side passage  18  in conjunction with the orifice  36   a.    
     As the expansion-side free piston  34  is displaced to compress the second expansion-side pressure chamber E 2 , the orifice  65  does not face the annular groove  66  until the expansion-side free piston  34  reaches its stroke end. In addition, the orifice  65  is slowly blocked by the outer circumference of the expansion-side free piston  34 , so that the flow passage area of the second expansion-side passage  18  is reduced up to the cross-sectional area of the orifice  36   a.    
     In this manner, as the expansion-side free piston  34  is displaced to compress the second expansion-side pressure chamber E 2  up to the vicinity of the stroke end, the flow passage area of the second expansion-side passage  18  is slowly reduced. In addition, the pressure inside the second expansion-side pressure chamber E 2  increases so as to restrain movement of the expansion-side free piston  34 . As a result, it is possible to decelerate the expansion-side free piston  34 . 
     Therefore, it is possible to prevent the expansion-side free piston  34  from violently colliding with the expansion-side housing  31 . As a result, it is possible to reduce a striking sound level generated when both components make contact and prevent a vehicle passenger from having an uncomfortable or uneasy feeling. 
     It is noted that an alternative structure may be employed, in which the flow passage area of the first expansion-side passage  17  is reduced by displacing the expansion-side free piston  34  to compress the second expansion-side pressure chamber E 2 . Alternatively, a liquid pressure lock chamber may be employed, which is locked by displacement of the expansion-side free piston  34  for compressing the second expansion-side pressure chamber E 2  so as to stop the movement of the expansion-side free piston  34  by virtue of an internal pressure. 
     Naturally, the liquid pressure cushioning mechanism may also be applied to the contraction-side sensitive mechanism RMC. If the liquid pressure cushioning mechanism is provided in the contraction-side sensitive mechanism RMC, for example, as illustrated in the shock absorber S 4  of  FIG. 10 , a contraction-side liquid pressure cushioning mechanism as a contraction-side liquid pressure cushioning portion may be built by providing an orifice  68  in the casing member  46  and providing an annular groove  69  and a passage  70  that causes the annular groove  69  to communicate with the first contraction-side pressure chamber C 1  in the contraction-side free piston  45 . 
     In this case, while the contraction-side free piston  45  is positioned to fully compress the first contraction-side pressure chamber C 1 , the annular groove  69  faces the orifice  68 . In this state, the contraction-side chamber R 2  and the first contraction-side pressure chamber C 1  communicate with each other through the orifice  68 , the annular groove  69 , and the passage  70 . In addition, the first contraction-side pressure chamber C 1  also communicates with the contraction-side chamber R 2  through the orifice  47   a  of the lid member  47 . Therefore, the orifice  68 , the annular groove  69 , and the passage  70  constitute the first contraction-side passage  26  in conjunction with the orifice  47   a.    
     As the contraction-side free piston  45  is displaced to compress the second contraction-side pressure chamber C 2 , the orifice  68  does not face the annular groove  69  until the contraction-side free piston  45  reaches its stroke end. In addition, the orifice  68  is slowly blocked by the outer circumference of the contraction-side free piston  45 , so that the flow passage area of the first contraction-side passage  26  is reduced up to the cross-sectional area of the orifice  47   a.    
     In this manner, as the contraction-side free piston  45  is displaced up to the vicinity of the stroke end so as to compress the second contraction-side pressure chamber C 2 , the flow passage area of the first contraction-side passage  26  is slowly reduced. In addition, the pressure inside of the first contraction-side pressure chamber C 1  is suppressed from increasing so as to restrain the movement of the contraction-side free piston  45 . As a result, it is possible to decelerate the contraction-side free piston  45 . 
     Therefore, it is possible to prevent the contraction-side free piston  45  from violently colliding with the contraction-side housing  41 . Therefore, it is possible to reduce a striking sound level generated when both components collide with each other and prevent a vehicle passenger from having an uncomfortable or uneasy feeling. 
     It is noted that the contraction-side liquid pressure cushioning mechanism may be provided with a structure for reducing the flow passage area of the second contraction-side passage  27  by displacing the contraction-side free piston  24  to compress the second contraction-side pressure chamber C 2 . Alternatively, a liquid pressure lock chamber may be employed, which is locked by displacement of the contraction-side free piston  45  for compressing the second contraction-side pressure chamber C 2  so as to stop the movement of the contraction-side free piston  45  by virtue of an internal pressure. 
     As illustrated in  FIG. 11 , the expansion-side sensitive mechanism RME of the shock absorber S 5  may be provided with a valve having a valve body as the expansion-side valve element  19  instead of the orifice or the chalk. 
     The shock absorber S 5  is obtained by applying an expansion-side valve  80  to the structure of the shock absorber S 1  of  FIG. 3 . Specifically, as illustrated in  FIG. 11 , the expansion-side valve  80  is stacked on a lid member  81  that blocks the opening end of the casing member  35  of the expansion-side housing  31 , so that the port  81   a  of the lid member  81  is opened or closed by the expansion-side valve  80 . 
     The lid member  81  is provided with ports  81   a  and  8113  that cause the second expansion-side pressure chamber E 2  and the contraction-side chamber R 2  to communicate with each other. The expansion-side valve  80  is a disc-like leaf valve. The expansion-side valve  80  is stacked on the contraction-side chamber R 2  side of the lid member  81  and is mounted to the outer circumference of the center rod  82  penetrating through the lid member  81 , while its inner circumferential side is fixed to the lid member  81 . 
     As the outer circumference of the expansion-side valve  80  is flexed by virtue of the pressure of the second expansion-side pressure chamber E 2 , the port  81   a  is opened so that the hydraulic oil is allowed to flow from the second expansion-side pressure chamber E 2  to the contraction-side chamber R 2  while resistance is applied to the flow of hydraulic oil. Conversely, for the flow of hydraulic oil directed from the contraction-side chamber R 2  to the second expansion-side pressure chamber E 2 , the port  81   a  is closed so as to act as a check valve for suppressing the flow of hydraulic oil. 
     The port  81   b  is opened or closed by the disc-like check valve  84 . The check valve  84  is stacked on the second expansion-side pressure chamber E 2  side of the lid member  81  and is mounted to the outer circumference of the center rod  82 . 
     As the outer circumference of the check valve  84  is flexed by virtue of the pressure of the contraction-side chamber R 2 , the port  81   b  is opened so that the hydraulic oil is allowed to flow from the contraction-side chamber R 2  to the second expansion-side pressure chamber E 2 . Conversely, for the flow of hydraulic oil directed from the second expansion-side pressure chamber E 2  to the contraction-side chamber R 2 , the port  81   b  is closed to suppress the flow of hydraulic oil. 
     In the shock absorber S 5  configured as described above, similar to the shock absorber S 1 , it is possible to damp a vehicle vibration by adjusting the damping force by controlling the damping force variable valve V for a relatively low frequency vibration. In addition, for a high frequency vibration difficult to suppress by controlling the damping force variable valve V, a weak damping force can be exerted mechanically, so that it is possible to effectively suppress a vehicle vibration by blocking a vibration from the traveling wheel side. Therefore, it is possible to remarkably improve a vehicle ride quality. 
     In the shock absorber S 5 , as the expansion speed increases, and the flow rate of the hydraulic oil directed from the second expansion-side pressure chamber E 2  to the contraction-side chamber R 2  increases, the expansion-side valve  80  fully opens the port  81   a  depending on the flow rate. For this reason, even when the shock absorber S 5  expands at a high speed range, it is possible to exert the damping force attenuation effect without fail. 
     In the shock absorber S 5 , the check valve  84  is arranged in parallel with the expansion-side valve  80 . For this reason, as the shock absorber S 5  starts to make a contracting motion after the expansion-side free piston  34  moves to compress the second expansion-side pressure chamber E 2  by virtue of the pressure from the expansion-side chamber R 1 , the check valve  84  is opened. Therefore, it is possible to rapidly release the pressure of the highly compressed first expansion-side pressure chamber E 1  to follow the decompressed expansion-side chamber R 1 . Therefore, the expansion-side free piston  34  can be pushed back to compress the first expansion-side pressure chamber E 1 . 
     As a result, it is possible to prevent reduction of a displacement margin of the expansion-side free piston  34  for compressing the second expansion-side pressure chamber E 2 , that may be generated when vibrations are continuously input to the shock absorber S 5 , and a residual pressure of the first expansion-side pressure chamber E 1  forces the expansion-side free piston  34  to be deviated to the second expansion-side pressure chamber E 2  side. 
     In this manner, using the shock absorber S 5 , it is possible to prevent the expansion-side free piston  34  from being deviated to the second expansion-side pressure chamber E 2  side. Therefore, it is possible to prevent reduction of a displacement margin of the expansion-side free piston  34  during an expanding motion and a failure in obtaining the damping force attenuation effect. 
     As illustrated in  FIG. 12 , the contraction-side sensitive mechanism RMC of the shock absorber S 6  may be provided with a valve having a valve body as the contraction-side valve element  28  instead of the orifice or the chalk. 
     The shock absorber S 6  is obtained by applying a contraction-side valve  86  to the structure of the shock absorber S 1  of  FIG. 4 . Specifically, as illustrated in  FIG. 12 , the contraction-side valve  86  is stacked on a lid member  87  that opens or closes the opening end of the casing member  46  of the contraction-side housing  41 , so that the port  87   a  of the lid member  87  is opened or closed by the contraction-side valve  86 . 
     The lid member  87  is provided with ports  87   a  and  87   b  that cause the first contraction-side pressure chamber C 1  and the contraction-side chamber R 2  to communicate with each other. The contraction-side valve  86  is a disc-like leaf valve. The contraction-side valve  86  is stacked on the first contraction-side pressure chamber C 1  side of the lid member  87  and is mounted to the outer circumference of the center rod  88  penetrating through the lid member  87 , while its inner circumference side is fixed to the lid member  87 . 
     As the outer circumference of the contraction-side valve  86  is flexed by virtue of the pressure of the contraction-side chamber R 2 , the port  87   a  is opened, so that the hydraulic oil is allowed to flow from the contraction-side chamber R 2  to the first contraction-side pressure chamber C 1  while resistance is generated in the flow of hydraulic oil. Conversely, for a flow of hydraulic oil directed from the first contraction-side pressure chamber C 1  to the contraction-side chamber R 2 , the port  87   a  is closed, so that the contraction-side valve  86  acts as a check valve for suppressing the flow of hydraulic oil. 
     The port  87   b  is opened or closed by the disc-like check valve  8   g . The check valve  8   g  is stacked on the contraction-side chamber R 2  side of the lid member  87  and is mounted to the outer circumference of the center rod  88 . 
     As the outer circumference of the check valve  8   g  is flexed by virtue of the pressure of the first contraction-side pressure chamber C 1 , the port  87   b  is opened, so that the hydraulic oil is allowed to flow from the first contraction-side pressure chamber C 1  to the contraction-side chamber R 2 . Conversely, for a flow of hydraulic oil directed from the contraction-side chamber R 2  to the first contraction-side pressure chamber C 1 , the port  87   b  is closed so as to suppress the flow of hydraulic oil. 
     Using the shock absorber S 6  configured as described above, similar to the shock absorber S 1 , it is possible to damp a vehicle vibration by adjusting the damping force by controlling the damping force variable valve V for a relatively low frequency vibration. In addition, for a high frequency vibration difficult to suppress by controlling the damping force variable valve V, a weak damping force can be exerted mechanically, so that it is possible to effectively suppress a vehicle vibration by blocking a vibration from the traveling wheel side. Therefore, it is possible to remarkably improve a vehicle ride quality. 
     In the shock absorber S 6 , as the contraction speed increases, and the flow rate of the hydraulic oil directed from the contraction-side chamber R 2  to the first contraction-side pressure chamber C 1  increases, the contraction-side valve  86  fully opens the port  87   a  depending on the flow rate. For this reason, even when the shock absorber S 7  contracts at a high speed range, it is possible to exert the damping force attenuation effect without fail. 
     In the shock absorber S 6 , the check valve  8   g  is arranged in parallel with the contraction-side valve  86 . For this reason, as the shock absorber S 6  starts to make an expanding motion after the contraction-side free piston  45  moves to compress the second contraction-side pressure chamber C 2  by virtue of the pressure from the contraction-side chamber R 2 , the check valve  8   g  is opened. Therefore, it is possible to rapidly release the pressure of the highly compressed first contraction-side pressure chamber C 1  to follow the decompressed contraction-side chamber R 2 . Therefore, the contraction-side free piston  45  can be pushed back promptly to compress the first contraction-side pressure chamber C 1 . 
     As a result, it is possible to prevent reduction of a displacement margin of the contraction-side free piston  45  for compressing the second contraction-side pressure chamber C 2 , that may be generated when vibrations are continuously input to the shock absorber S 6 , and a residual pressure of the first contraction-side pressure chamber C 1  forces the contraction-side free piston  45  to be deviated to the second contraction-side pressure chamber C 2  side. 
     In this manner, using the shock absorber S 6 , it is possible to prevent the contraction-side free piston  45  from being deviated to the second contraction-side pressure chamber C 2  side. Therefore, it is possible to prevent reduction of a displacement margin of the contraction-side free piston  45  during the contracting motion and a failure in obtaining the damping force attenuation effect. 
     Naturally, both the expansion-side sensitive mechanism RME of the shock absorber S 5  and the contraction-side sensitive mechanism RMC of the shock absorber S 6  may also be employed at the same time. 
     Although the shock absorber S 5  is provided with the expansion-side valve  80  in the second expansion-side passage  18 , an expansion-side valve  90  may be provided in the first expansion-side passage  17  as illustrated in the shock absorber S 7  of  FIG. 13 . 
     In addition to the configuration of the shock absorber S 1  of  FIG. 3 , the shock absorber S 7  includes a valve disc  91  provided closer to the expansion-side chamber R 1  side of the outer circumference of the piston rod  30  relatively to the piston  32 , a cap  92  mounted to the outer circumference of the piston rod  30  and fitted to the outer circumference of the valve disc  91 , a tubular spacer  93  interposed between the valve disc  91  and the cap  92 , an expansion-side valve  90  stacked beneath the valve disc  91  in  FIG. 13 , and a disc-like check valve  94  stacked above the valve disc  91  in  FIG. 13 . 
     The valve disc  91  having an annular shape is mounted to the outer circumference of the piston rod  30 . The valve disc  91  is provided with ports  91   a  and  91   b  extending from the upper end to the lower end. 
     The cap  92  having a bottomed cylindrical shape is provided with a hole  92   a  in the bottom portion to receive the inserted piston rod  30 . The cap  92  is mounted to the outer circumference of the piston rod  30  by using the bottom portion. In addition, the tubular portion is fitted to the outer circumference of the valve disc  91  to partition a room A inside the expansion-side chamber R 1  in conjunction with the valve disc  91 . 
     The spacer  93  having a tubular shape is interposed between the bottom portion of the cap  92  and the valve disc  91  and is installed to the outer circumference of the piston rod  30 . The piston rod  30  is provided with a first expansion-side passage  30   b  communicating with the first expansion-side pressure chamber E 1 . The first expansion-side passage  30   b  is opened in a part of the outer circumference of the piston rod  30  facing the spacer  93 . 
     The spacer  93  is provided with a notch  93   a . The spacer  93  causes the first expansion-side passage  30   b  to communicate with the room A through the notch  93   a . The room A communicates with the expansion-side chamber R 1  through the ports  91   a  and  91   b . Therefore, the first expansion-side pressure chamber E 1  communicates with the expansion-side chamber R 1  through the room A and the ports  91   a  and  91   b.    
     The expansion-side valve  90  is an annular leaf valve. The expansion-side valve  90  is stacked beneath the valve disc  91  in  FIG. 13  and is mounted to the outer circumference of the piston rod  30 . The outer circumference of the expansion-side valve  90  can be flexed to open or close the lower end of the port  91   a.    
     Therefore, in the expansion-side valve  90 , the port  91   a  is opened for a flow of hydraulic oil directed from the expansion-side chamber R 1  to the first expansion-side pressure chamber E 1 , and resistance is generated to the flow of hydraulic oil. For a reverse flow, the port  91   a  is closed to suppress a passage of the hydraulic oil. 
     The check valve  94  having a disc shape is stacked above the valve disc  91  in  FIG. 13  and is mounted to the outer circumference of the piston rod  30 . The outer circumference of the check valve  94  can be flexed to open or close the upper end of the port  91   b.    
     Therefore, for a flow of hydraulic oil directed from the first expansion-side pressure chamber E 1  to the expansion-side chamber R 1 , the check valve  94  opens the port  91   b  to allow a passage of the hydraulic oil. For a reverse flow, the check valve  94  closes the port  91   b  to suppress a passage of the hydraulic oil. 
     It is noted that the expansion-side valve  90  is designed not to block the lower end of the port  91   b , and the check valve  94  is designed not to block the upper end of the port  91   a.    
     In this manner, the expansion-side valve  90 , the valve disc  91 , the cap  92 , the spacer  93 , and the check valve  94  are arranged in the expansion-side chamber R 1  side, which is a dead space in the structure of the shock absorber, rather than the piston  32 . Therefore, it is possible to shorten a total length of the expansion-side housing  31  provided in the lower side in  FIG. 13  relatively to the piston  32 . Therefore, it is possible to provide the expansion-side valve  90  without sacrificing the stroke length. 
     In the shock absorber S 7  configured as described above, similar to the shock absorber S 1 , it is possible to damp a vehicle vibration by adjusting the damping force by controlling the damping force variable valve V for a relatively low frequency vibration. In addition, for a high frequency vibration difficult to suppress by controlling the damping force variable valve V, a weak damping force can be exerted mechanically, so that it is possible to effectively suppress a vehicle vibration by blocking a vibration from the traveling wheel side. Therefore, it is possible to remarkably improve a vehicle ride quality. 
     In the shock absorber S 7 , as the expansion speed increases, and the flow rate of the hydraulic oil directed from the expansion-side chamber R 1  to the first expansion-side pressure chamber E 1  increases, the expansion-side valve  90  fully opens the port  91   a  depending on the flow rate. For this reason, even when the shock absorber S 7  expands at a high speed range, it is possible to exert the damping force attenuation effect without fail. 
     In the shock absorber S 7 , the check valve  94  is arranged in parallel with the expansion-side valve  90 . For this reason, as the shock absorber S 7  starts to make a contracting motion after the expansion-side free piston  34  moves to compress the second expansion-side pressure chamber E 2  by virtue of the pressure from the expansion-side chamber R 1 , the check valve  94  is opened. Therefore, it is possible to rapidly release the pressure of the highly compressed first expansion-side pressure chamber E 1  to follow the decompressed expansion-side chamber R 1 . Therefore, the expansion-side free piston  34  can be pushed back promptly to compress the first expansion-side pressure chamber E 1 . 
     As a result, it is possible to prevent reduction of a displacement margin of the expansion-side free piston  34  for compressing the second expansion-side pressure chamber E 2 , that may be generated when vibrations are continuously input to the shock absorber S 7 , and a residual pressure of the first expansion-side pressure chamber E 1  forces the expansion-side free piston  34  to be deviated to the second expansion-side pressure chamber E 2  side. 
     In this manner, using the shock absorber S 7 , it is possible to prevent the expansion-side free piston  34  from being deviated to the second expansion-side pressure chamber E 2  side. Therefore, it is possible to prevent reduction of a displacement margin of the expansion-side free piston  34  during the expanding motion and a failure in obtaining the damping force attenuation effect. 
     Embodiments of the present invention were described above, but the above embodiments are merely examples of applications of the present invention, and the technical scope of the present invention is not limited to the specific constitutions of the above embodiments. 
     With respect to the above description, the contents of application No. 2013-194870, with a filing date of Sep. 20, 2013 in Japan, are incorporated herein by reference.