Abstract:
The pilot suspension apparatus of the present invention is capable of biasing a damping valve in a direction for closing the valve, due to application of a pilot pressure. The suspension apparatus comprises at least one pair of hydraulic shock absorbers which are capable of controlling a damping force and which are connected by a connecting pipe. The at least one pair of hydraulic shock absorbers are provided at front and rear wheels or left and right wheels on the same side of a vehicle or are provided in a diagonally shaped relationship. A control valve is provided so as to control the pilot pressure. The control valve is adapted to be controlled by application of a differential pressure which is generated between respective cylinders of the at least one pair of shock absorbers.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a suspension apparatus for a vehicle, such as an automobile. 
     Generally, in a suspension apparatus for an automobile, comprising a hydraulic shock absorber generating damping force, a small damping force is desired while driving in a straight line, from the viewpoint of ease in absorbing vibration and obtaining good riding quality. On the other hand, a large damping force is desired during braking, acceleration or cornering, from the viewpoint of suppressing pitching and rolling of the vehicle body and obtaining good steering stability. Therefore, there has been conventionally employed a suspension control apparatus comprising: a hydraulic shock absorber capable of controlling a damping force; an actuator for switching the damping force; a controller which controls an operation of the actuator; and various sensors which detect an acceleration of the vehicle body, etc. In this suspension control apparatus, the damping force is controlled appropriately according to a road surface condition, a vehicle running condition, etc., so as to obtain good riding quality and steering stability. 
     The above-mentioned suspension control apparatus requires use of various electronic devices, such as the controller and the actuator. These electronic devices are expensive, and further require costs for ensuring reliability of the devices. As a countermeasure, Unexamined Japanese Patent Application Public Disclosure No.  10-213171  proposes a suspension apparatus in which hydraulic shock absorbers provided at left and right wheels of a vehicle are connected through a pipe to each other. In this apparatus, a spool provided in a piston rod in the hydraulic shock absorber is moved by utilizing a difference in hydraulic pressure in the left-handed and right-handed hydraulic shock absorbers, to thereby adjust a damping force of the hydraulic shock absorber automatically according to a vehicle running condition. 
     Thus, there has been an increasing demand for a suspension apparatus which is capable of adjusting a damping force of the hydraulic shock absorber mechanically and automatically according to a vehicle running condition, without using electronic devices such as a controller and an actuator. In addition, it has been required to develop a suspension apparatus which has a simple construction and which is capable of generating an appropriate damping force according to a vehicle running condition, by adjusting damping force characteristics in a wide range. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention has been made. It is an object of the present invention to provide a suspension apparatus which is capable of adjusting a damping force automatically according to a vehicle running condition and which has a simple construction and is capable of generating an appropriate damping force by adjusting damping force characteristics in a wide range. 
     According to the present invention, there is provided a pilot suspension apparatus capable of biasing a damping valve in a direction for closing the valve, due to application of a pilot pressure, the suspension apparatus comprising: 
     at least one pair of hydraulic shock absorbers capable of controlling a damping force, the at least one pair of hydraulic shock absorbers being provided at front and rear wheels or left and right wheels on the same side of a vehicle or being provided in a diagonally spaced relationship; 
     a connecting pipe for connecting the at least one pair of hydraulic shock absorbers; and 
     a control valve for controlling the pilot pressure, the control valve being adapted to be controlled by application of a differential pressure generated between respective cylinders of the at least one pair of shock absorbers. 
     The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-sectional view showing damping force control type hydraulic shock absorbers which are applied to a suspension apparatus in a first embodiment of the present invention. 
     FIG. 2 is an enlarged view of a damping force generating mechanism of the hydraulic shock absorber of FIG.  1 . 
     FIG. 3 is a vertical cross-sectional view showing damping force generating mechanisms of damping force control type hydraulic shock absorbers and a variable-volume chamber unit which are applied to a suspension apparatus in a second embodiment of the present invention. 
     FIG. 4 is a vertical cross-sectional view of a damping force control type hydraulic shock absorber which is applied to a suspension apparatus in a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, embodiments of the present invention are described in detail, referring to the accompanying drawings. 
     A first embodiment of the present invention is described, with reference to FIGS. 1 and 2. As shown in Pigs.  1  and  2 , a suspension apparatus  100  in the first embodiment of the present invention comprises two damping force control type hydraulic shock absorbers  101 . Each hydraulic shock absorber  101  comprises a dual cylinder including an inner cylinder  102  and an outer cylinder  103  provided outside the cylinder  102 . A reservoir  104  is formed between the cylinders  102  and  103 . A piston  105  is slidably provided in the cylinder  102  so as to divide an interior of the cylinder  102  into an upper cylinder chamber  102   a  and a lower cylinder chamber  102   b . A generally cylindrical piston bolt  106  extending through the piston  105  is fixed by a nut  107 . A proximal end portion of the piston bolt  106  is threadably engaged with one end portion of a piston rod  108  having a hollow structure. The piston rod  108  on a side opposite to the piston bolt  106  extends to the outside of the cylinder  102  through the upper cylinder chamber  102   a  and a rod guide  109  and an oil seal  110  provided at an upper end portion of the cylinder  102  and outer cylinder  103 . A base valve  111  for separating the lower cylinder chamber  102   b  and the reservoir  104  is provided at a lower end portion of the cylinder  102 . 
     An extension-stroke fluid passage  112  and a compression-stroke fluid passage  113  are formed in the piston  105 , so as to enable communication between the upper cylinder chamber  102   a  and the lower cylinder chamber  102   b . An extension-stroke damping force generating mechanism  114  is provided between the piston  105  and the nut  107 , so as to control flow of a hydraulic fluid in the extension-stroke fluid passage  112 . A compression-stroke damping force generating mechanism  115  is provided between the piston  105  and the proximal end portion of the piston bolt  106 , so as to control flow of the hydraulic fluid in the compression-stroke fluid passage  113 . Fluid passages  116  and  117  are provided in the base valve  111 , so as to enable communication between the lower cylinder chamber  102   b  and the reservoir  104 . A check valve  118  is provided on the base valve  111  so as to permit flow of the hydraulic fluid through the fluid passage  116  only from the reservoir  104  to the lower cylinder chamber  102   b . Further, a disk valve  119  is provided on the base valve  111 . When a pressure of the hydraulic fluid in the lower cylinder chamber  102   b  has reached a predetermined level, the disk valve  119  opens, to thereby permit flow of the hydraulic fluid from the lower cylinder chamber  102   b  through the fluid passage  117  to the reservoir  104 . The hydraulic fluid is sealably contained in the cylinder  102 . The hydraulic fluid and a gas having a predetermined pressure are sealably contained in the reservoir  104 . 
     Next, the extension-stroke damping force generating mechanism  114  is described in detail, mainly with reference to FIG. 2. A protruding annular valve seat  120  is formed on an end surface of the piston  105  on a side of the lower cylinder chamber  102   b , and a main disk valve (or a damping valve)  121  is seated over the valve seat  120 . An annular fixing member  122  is attached to the piston bolt  106  between the piston  105  and the nut  107 . A movable ring  123  is slidably fitted onto an outer circumferential surface of the fixing member  122 . A slide ring  124  made of fluororesin is provided between the fixing member  122  and the movable ring  123 . The slide ring  124  seals a space between the fixing member  122  and the movable ring  123  and enables smooth sliding movement of the movable ring  123 . The movable ring  123  abuts against the main disk valve  121  under force of a disk-like leaf spring  125  clamped between the fixing member  122  and the nut  107 , thus forming a pilot chamber  126  between the main disk valve  121  and the fixing member  122 . An internal pressure of the pilot chamber  126  is applied in a direction for closing the main disk valve  121 . The pilot chamber  126  is communicated with the extension-stroke fluid passage  112  through a fixed orifice  127  provided in the main disk valve  121 . Further, the pilot chamber  126  is communicated through fluid passages  128  and  129  provided in a side wall of the piston bolt  106  with the side of the fixing member remote from the pilot chamber  126  through an extension-stroke variable pressure control valve (or a sub damping valve)  130 , which is provided in the piston bolt  106 . The pilot chamber  126  is also communicated with the lower cylinder chamber  102   b  through a check valve (or a disk valve)  131  on the fixing member  122  and a fluid passage (a cut portion)  132  in the leaf spring  125 . 
     Next, the damping force generating mechanism  115  is described in detail. The damping force generating mechanism  115  has substantially the same structure as the extension-stroke damping force generating mechanism  114 . A protruding annular valve seat  133  is formed on an end surface of the piston  105  on a side of the upper cylinder chamber  102   a , and a main disk valve (or a damping valve)  134  is seated over the main disk valve  133 . An annular fixing member  135  is attached to the piston bolt  106  between the proximal end portion thereof and the piston  105 . A movable ring  137  is slidably fitted onto an outer circumferential surface of the fixing member  135 . A slide ring  136  is provided between the fixing member  135  and the movable ring  137 . The movable ring  137  abuts against the main disk valve  134  under force of a digk-like leaf spring  138 , thus forming a pilot chamber  139  between the main disk valve  134  and the fixing member  135 . An internal pressure of the pilot chamber  139  is applied in a direction for closing the main disk valve  134 . The pilot chamber  139  is communicated with the compression-stroke fluid passage  113  through a fixed orifice  140  provided in the main disk valve  134 . Further, the pilot chamber  139  is communicated through fluid passages  141  and  142  provided in the side wall of the piston bolt  106  with the side of the fixing member remote from the pilot chamber  139  through a compression-stroke variable pressure control valve (or a sub damping valve)  143 , which is provided in the piston bolt  106 . The pilot chamber  139  is also communicated with the upper cylinder chamber  102   a  through a check valve (or a disk valve)  144  on the fixing member  135  and a fluid passage (a cut portion)  145  in the leaf spring  138 . 
     Next, the extension-stroke and compression-stroke variable pressure control valves  130  and  143  are described in detail below. A small-diameter bore  146  to which the fluid passages  128  and  141  are open is formed at a central portion of the piston bolt  106 . Large-diameter bores  147  and  148  to which the fluid passages  129  and  142  are open are formed on opposite sides of the small-diameter bore  146 . Stepped portions are provided between the small-diameter bore  146  and the large-diameter bores  147  and  148 . These stepped portions abut against outer peripheral portions of annular sub disk valves  149  and  150 . The sub disk valves  149  and  150  are fixed by rings  151  and  152  press-fitted into the large-diameter bores  147  and  148 . 
     A cylindrical slider (a valve member)  153  is slidably provided in the small-diameter bore  146 . Small-diameter portions  154  and  155  are formed on opposite end portions of the slider  153 , and annular valve chambers  156  and  157  communicated with the fluid passages  128  and  141  are formed between the small-diameter portions  154  and  155  and the small-diameter bore  146 . End portions of the small-diameter portions  154  and  155  move to and away from the sub disk valves  149  and  150 , in accordance with sliding movement of the slider  153 , to thereby open and close the valve chamber  156  as a flow path between the fluid passages  128  and  129  and the valve chamber  157  as a flow path between the fluid passages  141  and  142 . It should be noted that when the slider  153  is located at an intermediate position, both the small-diameter portion  154  and the small-diameter portion  155  are spaced apart from the corresponding sub disk valves, i.e., the sub disk valve  149  and the sub disk valve  150 . 
     The end portion of one small-diameter portion  154  extends through the sub disk valve  149  and a spring bearing  158  is attached to the lowermost portion of the small-diameter portion  154 . The end portion of the other small-diameter portion  155  extends through the sub disk valve  150  and a pressure-receiving member  159  is attached to the uppermost portion of the small-diameter portion  155 . The pressure-receiving member  159  is slidably provided in the large-diameter bore  148  of the piston bolt  106 . An adjusting screw  160  is threadably engaged with the lowermost portion of the large-diameter bore  147  of the piston bolt  106  and is fixed by a lock nut  161 . A compression spring  162  is provided between the spring bearing  158  and the adjusting screw  160  and a compression spring  163  is provided between the pressure-receiving member  159  and the end portion of the piston rod  108  connected to the piston bolt  106 . The slider  153  is resiliently held at the intermediate position by force of these compression springs. 
     The lower cylinder chamber  102   b  is communicated with the large-diameter bore  147  through an orifice passage  164  provided in the adjusting screw  160  and is further communicated through a fluid passage  165  in the slider  153  and an orifice passage  166  in the pressure-receiving member  159  with a fluid chamber  106 A formed by the piston bolt  106  and the piston rod  108 . An internal pressure of the fluid chamber  106 A is applied to the pressure-receiving member  159 . Further, the fluid chamber  106 A is communicated with a fluid passage  167  formed in the piston rod  109 . A pipe  168  (FIG. 1) is connected to the uppermost portion of the fluid passage  167 . Thus, the fluid passage  167  in the piston rods  108  of the two hydraulic shock absorbers  101  are communicated with each other by the pipe  168 . The hydraulic shock absorbers  101  are connected to wheel-supporting portions for left and right wheels of a vehicle. 
     Hereinbelow, an operation of the suspension apparatus in the above-mentioned embodiment is described. 
     During an extension stroke of the piston rod  108 , the hydraulic fluid in the upper cylinder chamber  102   a  is pressurized in accordance with movement of the piston  105 . In this instance, before the main disk valve  121  of the extension-stroke damping force generating mechanism  114  opens (in a low speed range of the piston speed), the hydraulic fluid flows from the upper cylinder chamber  102   a  to the lower cylinder chamber  102   b  through the extension-stroke fluid passage  112 , the fixed orifice  127  of the main disk valve  121 , the pilot chamber  126 , the fluid passage  128 , the valve chamber  156 , the extension-stroke variable pressure control valve  130 , the fluid passage  129 , the check valve  131  and the fluid passage  132 . When the pressure in the upper cylinder chamber  102   a  reaches the valve opening pressure for the main disk valve  121  (a high speed range of the piston speed), the main disk valve  121  opens, to thereby permit flow of the hydraulic fluid directly from the extension-stroke fluid passage  112  to the lower cylinder chamber  102   b . It should be noted that the hydraulic fluid in a volume corresponding to that of the portion of the piston rod  108  which has escaped from the cylinder  102  flows from the reservoir  104  to the lower cylinder chamber  102   b  through the check valve  118  in the fluid passage  116  of the base valve  111 . 
     By this arrangement, before the main disk valve  121  opens (in the low speed range of the piston speed), the damping force is generated by the fixed orifice  127  and the extension-stroke variable pressure control valve  130 . In the extension-stroke variable pressure control valve  130 , when the slider  153  is located at the intermediate position, the end portion of the small-diameter portion  154  is spaced apart from the sub disk valve  149 , so that the sub disk valve  149  is open. When the slider  153  moves toward the adjusting screw  160 , the end portion of the small-diameter portion  154  abuts against the sub disk valve  149 , so that the sub disk valve  149  is closed and the valve opening pressure for the sub disk valve  149  becomes high. In this instance, the pressure in the upstream pilot chamber  126  changes in accordance with the valve opening pressure for the sub disk valve  149 . The pressure in the pilot chamber  126  is applied in a direction for closing the main disk valve  121  as a pilot pressure. Therefore, not only the valve opening pressure for the sub disk valve  149 , but also the valve opening pressure for the main disk valve  121  can be adjusted, thus making it possible to control the damping force in the high speed range of the piston speed and the damping force in the low speed range of the piston speed at the same time. 
     During a compression stroke of the piston rod  108 , the check valve  118  in the base valve  111  is closed in accordance with movement of the piston  105 , to thereby pressurize the hydraulic fluid in the lower cylinder chamber  102   b . In this instance, before the main disk valve  134  of the damping force generating mechanism  115  opens (in the low speed range of the piston speed), the hydraulic fluid flows from the lower cylinder chamber  102   b  to the upper cylinder chamber  102   a  through the compression-stroke fluid passage  113 , the fixed orifice  140  of the main disk valve  134 , the pilot chamber  139 , the fluid passage  141 , the valve chamber  157 , the compression-stroke variable pressure control valve  143 , the fluid passage  142 , the check valve  144  and the fluid passage  145 . When the pressure in the lower cylinder chamber  102   b  reaches the valve opening pressure for the main disk valve  134  (the high speed range of the piston speed), the main disk valve  134  opens, to thereby permit flow of the hydraulic fluid directly from the compression-stroke fluid passage  113  to the upper cylinder chamber  102   a . It should be noted that the hydraulic fluid in a volume corresponding to that of the portion of the piston rod  108  which has entered the cylinder  102  flows from the lower cylinder chamber  102   b  to the reservoir  104  through the disk valve  119  in the fluid passage  116  of the base valve  111 . 
     By this arrangement, before the main disk valve  134  opens (in the low speed range of the piston speed), the damping force is generated by the fixed orifice  140  and the compression-stroke variable pressure control valve  143 . In the compression-stroke variable pressure control valve  143 , when the slider  153  is located at the intermediate position, the end portion of the small-diameter portion  155  is spaced apart from the sub disk valve  150 , so that the sub disk valve  150  is open. When the slider  153  moves toward the piston rod  108 , the end portion of the small-diameter portion  155  abuts against the sub disk valve  150 , so that the sub disk valve  150  is closed and the valve opening pressure for the sub disk valve  150  becomes high. In this instance, the pressure in the upstream pilot chamber  139  changes in accordance with the valve opening pressure for the sub disk valve  15 O. The pressure in the pilot chamber  139  is applied in a direction for closing the main disk valve  134  as a pilot pressure. Therefore, not only the valve opening pressure for the sub disk valve  150 , but also the valve opening pressure for the main disk valve  134  can be adjusted, thus making it possible to control the damping force in the high speed range of the piston speed and the damping force in the low speed range of the piston speed at the same time. 
     The left-handed and right-handed hydraulic shock absorbers  101  are connected by the pipe  168 , so as to permit communication between the respective fluid chambers  106 A, each provided on one side of the slider  153 . While the vehicle is being driven in a straight line, the phase of the stroke of the piston rod  108  in one hydraulic shock absorber  101  is equal to that in the other hydraulic shock absorber  101 , relative to vertical movement of the vehicle. Therefore, the two hydraulic shock absorbers  101  become substantially equal in terms of a pressure introduced from the lower cylinder chamber  102   b  through the orifice passage  164  into the large-diameter bore  148  on the other side of the slider  153 , so that the slider  153  in each hydraulic shock absorber  101  is held at the intermediate position. Consequently, the extension-stroke and compression-stroke variable pressure control valves  130  and  143  are open, to thereby generate a small damping force during the extension stroke and the compression stroke. Thus, vibration of the vehicle body can be suppressed while maintaining good riding quality. 
     During cornering, the phase of the stroke of the piston rod  108  in one hydraulic shock absorber  101  is opposite to that in the other hydraulic absorber  101 , relative to rolling of the vehicle body. Therefore, the pressure in the lower cylinder chamber  102   b  (that is, the pressure in the large-diameter bore  147 ) in the hydraulic shock absorber during the compression stroke becomes high and the pressure in the lower cylinder chamber  102   b  in the hydraulic shock absorber during the extension stroke becomes low. Due to a diffference in pressure in the lower cylinder chamber  102   b  between the two hydraulic shock absorbers, in the hydraulic shock absorber  101  during the compression stroke, the slider  153  moves toward the piston rod  108 , so that the valve opening pressure for the compression-stroke variable pressure control valve  143  becomes high and the damping force for the compression stroke becomes large, and that the extension-stroke variable pressure control valve  130  opens and the damping force for the extension stroke becomes small. On the other hand, in the hydraulic shock absorber  101  during the extension stroke, the slider  153  moves toward the adjusting screw  160 , so that the valve opening pressure for the extension-stroke variable pressure control valve  130  becomes high and the damping force for the extension stroke becomes large, and that the compression-stroke variable pressure control valve  143  opens and the damping force for the compression stroke becomes small. Consequently, a large damping force is applied to a change in vehicle attitude, thus effectively suppressing rolling and obtaining good steering stability. 
     In the extension-stroke and compression-stroke variable pressure control valves  130  and  143 , a sharp rise in hydraulic fluid pressure can be relieved due to deflection of the sub disk valves  149  and  150 . Therefore, it is possible to absorb large vibration inputted suddenly clue to the vehicle hitting a bump on the road surface, thereby improving riding quality. Further, with respect to the direction in which the vehicle after change in attitude returns to a horizontal position, a small damping force is generated, so that the vehicle body can be smoothly returned to the horizontal position. 
     The orifice passage  164  between the lower cylinder chamber  102   b  and the large-diameter bore  147  serves as a filter for high-frequency input vibration. Therefore, relative to the vibration of the unsprung mass (high-frequency vibration), a differential pressure is unlikely to be generated and a small damping force is maintained, so that good riding quality can be maintained. Further, an appropriate damping force can be applied to the movement of the slider  153  by virtue of the orifice passage  166  in the pressure-receiving member  159 , so that malfunctioning due to self-induced vibration can be prevented. 
     Next, a second embodiment of the present invention is described, referring to FIG.  3 . The second embodiment is substantially the same as the first embodiment, except that the structure of the pressure-receiving member  159  is changed and a variable-volume chamber unit  62  is provided in the pipe  168  connecting the left-handed and right-handed damping force control type hydraulic shock absorbers  101 . Therefore, in FIG. 3, the same parts or portions as those shown in FIGS. 1 and 2 are designated by the same reference numerals and characters, and overlapping explanation is omitted, 
     As shown in FIG. 3, in the suspension apparatus in the second embodiment, the orifice passage  166  for permitting communication between an upper side and a lower side of the pressure-receiving member  159  in the hydraulic shock absorber  101  is not provided, so that there is no communication between the fluid passages  165  and  167 . Further, the variable-volume chamber unit  62  is connected to an intermediate portion of the pipe  168  connecting the left-handed and right-handed hydraulic shock absorbers  101 . 
     The variable-volume chamber unit  62  includes connection ports  63  and  64  and a cylinder bore  65 . The connection ports  63  and  64  are connected to the pipe  168  which are connected to the fluid passages  167  in the hydraulic shock absorbers  101 . A free piston  66  is slidably provided in the cylinder bore  65 . A plug  67  is attached to an open end of the cylinder bore  65  and a compression spring  68  is provided between the free piston  66  and the plug  67 . A variable-volume chamber  69  having volume elasticity is formed in the cylinder bore  65  by the free piston  66 . The connection ports  63  and  64  are communicated with each other through a fluid passage  70 . The fluid passage  70  is communicated with the variable-volume chamber  69  through a throttle passage  71 . Reference numerals  72  and  73  in FIG. 3 denote plugs. 
     The variable-volume chamber  69  is filled with the hydraulic fluid and is pressurized to a predetermined level by the compression spring  68 . Normally, in each hydraulic shock absorber  101 , the pressure in the large-diameter bore  147  balances the pressure in the fluid chamber  106 A (that is, the pressure in the variable-volume chamber  69 ), so that the slider  153  is located at a neutral position. The flow path area of the throttle passage  71  is set to a value such that relative to the cycle of vibration of the suspension apparatus in normal running condition, the throttle passage  71  serves as a filter so that substantially no flow of the hydraulic fluid occurs between the fluid passage  70  and the variable-volume chamber  69 . 
     Next, an operation of the suspension apparatus in the second embodiment is described. 
     In the cycle of vibration of the suspension apparatus in normal running condition, the throttle passage  71  serves as a filter and substantially no flow of the hydraulic fluid occurs between the fluid passage  70  and the variable-volume chamber  69 . Therefore, substantially direct transmission of the hydraulic fluid pressure in the fluid chamber  106 A above the slider  153  occurs between the left-handed and right-handed hydraulic shock absorbers  101  through the pipe  168 . 
     By this arrangement, as in the case of the first embodiment, when the phase of the stroke of the piston rod  108  in one hydraulic shock absorber  101  is equal to that in the other hydraulic shock absorber  101 , the two hydraulic shock absorbers  101  are balanced in terms of a pressure in the large-diameter bore  147 , so that the slider  153  does not move and a small damping force is maintained. On the other hand, when the phase of the stroke of the piston rod  108  in one hydraulic shock absorber  101  is opposite to that in the other hydraulic shock absorber  101 , a differential pressure is generated between the large-diameter bores  147  in the two hydraulic shock absorbers, so that the slider  153  moves in each hydraulic shock absorber and a large damping force is generated. Therefore, rolling during cornering can be effectively suppressed and good steering stability can be obtained, while maintaining good riding quality when driving in a straight line. 
     When the load carried by the vehicle becomes large, the height of the vehicle lowers and the piston rods  108  retract. Consequently, the internal pressure of each hydraulic shock absorber  101  increases by an amount corresponding to the portion of the piston rod  108  which has retracted into the cylinder  102  and reaches a constantly high level. This pressure is transmitted to the variable-volume chamber unit  62  through the pipe  168 , and introduced into the variable-volume chamber  69  through the throttle passage  71 . The compression spring  168  is compressed, to thereby increase the volume of the variable-volume chamber  69 . Consequently, the hydraulic fluid in a volume corresponding to the increase in volume of the variable-volume chamber  69  flows from the fluid chambers  106 A of the left-handed and right--handed hydraulic shock absorbers  101  into the variable-volume chamber  69 , and the slider  153  moves toward the fluid chamber  106 A in each hydraulic shock absorber, so that the valve opening pressure for the compression-stroke variable pressure control valve  143  becomes high, to thereby generate a large damping force for the compression stroke. Thus, the damping force can be increased according to an increase in the load carried by the vehicle. This avoids the problem of insufficiency of damping force for a vehicle carrying a heavy load. 
     A third embodiment of the present invention is described below, referring to FIG.  4 . 
     The suspension apparatus in the third embodiment is substantially the same as that in the first embodiment, except that a part of the damping force generating mechanism of the hydraulic shock absorber is changed. Therefore, in FIG. 4, only an essential part of the hydraulic shock absorber is shown and the same parts or portions as those in the first embodiment are designated by the same reference numerals and characters, with overlapping explanation being omitted. 
     In a damping force control type hydraulic shock absorber  169  in the suspension apparatus in the third embodiment, in extension-stroke and compression-stroke variable pressure control valves  170  and  171 , sub disk valves such as those in the first embodiment are not used. In accordance with movement of a slider  172 , valve members  173  and  174  attached to opposite ends of the slider  172  are moved to and away from valve seats (stepped portions)  175  and  176  formed between the small-diameter bore  146  and the large-diameter bores  147  and  148  of the piston bolt  106 . Thus, a pressure in the flow path between the fluid passages  128  and  129  and a pressure in the flow path between the fluid passages  141  and  142  are controlled. It should be noted that when the slider  172  is located at an intermediate position, both the valve member  173  and the valve member  174  are spaced apart from the corresponding valve seats, i.e., the valve seat  175  and the valve seat  176 . 
     One valve member  173  includes a flange portion  177  and is slidably provided in the large-diameter bore  147 . The valve member  173  forms a valve chamber  178  in the large-diameter bore  147 , which chamber communicates with the fluid passage  129 , and forms a fluid chamber  180  between the valve member  173  and a relief valve  179  attached to the lowermost portion of the large-diameter bore  147 . The other valve member  174  includes two flange portions  181  and  182  and is slidably provided in the large-diameter bore  148 . The valve member  174  forms a valve chamber  183  in the large-diameter bore  148 , which chamber communicates with the fluid passage  142 , and forms a fluid chamber  184  between the flange portions  181  and  182 . The valve member  174  also forms a fluid chamber  185  between the valve member  174  and the end portion of the piston rod  108  connected to the piston bolt  106 . 
     The fluid chamber  180  is communicated with the upper cylinder chamber  102   a  through a fluid passage  186  in the valve member  173 , a fluid passage  187  in the slider  172 , a fluid passage  188  in the valve member  174 , the fluid chamber  184  and a fluid passage  189  in a piston bolt  159 . The relief valve  179  opens when the pressure in the fluid chamber  180  has reached a predetermined level, and relieves the hydraulic fluid in the fluid chamber  180  into the lower cylinder chamber  102   b . The fluid passage  188  in the valve member  174  is communicated with the fluid chamber  185  through an orifice passage  190 . The fluid chamber  185  is communicated with the fluid passage  167  in the piston rod  108 . The respective fluid chambers  185  of the left-handed and right-handed hydraulic shock absorbers  169  are communicated with each other through the pipe  168 . A compression spring  191  is provided between the valve member  173  and the relief valve  179 , and a compression spring  192  is provided between the valve member  174  and the piston rod  108 . The slider  172  is resiliently held at the intermediate position by force of these compression springs. 
     An operation of the suspension apparatus in the third embodiment of the present invention is described below. 
     As in the case of the first embodiment, while the vehicle is being driven in a straight line, the phase of the stroke of the piston rod  108  in one hydraulic shock absorber  169  is equal to that in the other hydraulic shock absorber  169 , relative to vertical movement of the vehicle. Therefore, the two hydraulic shock absorbers  169  become substantially equal in terms of a pressure introduced from the upper cylinder chamber  102   a  through the fluid passage  189 , the fluid chamber  184 , the fluid passage  188 , the fluid passage  187  and the fluid passage  186  into the fluid chamber  180 , so that the slider  172  in each hydraulic shock absorber  169  is held at the intermediate position. Consequently, the extension-stroke and compression-stroke variable pressure control valves  170  and  171  are open, to thereby generate a small damping force during the extension stroke and the compression stroke. Thus, vibration of the vehicle body can be suppressed while maintaining good riding quality. 
     During cornering, the phase of the stroke of the piston rod  108  in one hydraulic shock absorber  169  is opposite to that in the other hydraulic absorber  169 , relative to rolling of the vehicle body. Therefore, the pressure in the upper cylinder chamber  102   a  (that is, the pressure in the fluid chamber  180 ) in the hydraulic shock absorber during the extension stroke becomes high and the pressure in the fluid chamber  180  in the hydraulic shock absorber during the compression stroke becomes low. Due to a difference in pressure in the fluid chamber  180  between the two hydraulic shock absorbers, in the hydraulic shock absorber  169  during the extension stroke, the slider  172  moves toward the piston rod  108 , so that the valve opening pressure for the extension-stroke variable pressure control valve  170  becomes high and the damping force for the extension stroke becomes large, and that the compression-stroke variable pressure control valve  171  opens and the damping force for the compression stroke becomes small. On the other hand, in the hydraulic shock absorber  169  during the compression stroke, the slider  172  moves toward the relief valve  179 , so that the valve opening pressure for the compression-stroke variable pressure control valve  171  becomes high and the damping force for the compression stroke becomes large, and that the extension-stroke variable pressure control valve  170  opens and the damping force for the extension stroke becomes small. Consequently, a large damping force is applied to a change in vehicle attitude, thus effectively suppressing rolling and obtaining good steering stability. 
     The pressure-receiving area of the piston  105  on a side of the upper cylinder chamber  102   a  is smaller than that on a side of the lower cylinder chamber  102   b . Further, the hydraulic shock absorber is generally arranged so as to generate a large damping force for the extension stroke as compared to the compression stroke. Therefore, in the upper cylinder chamber  102   a , a pressure change according to movement of the piston  105  is greater than that in the lower cylinder chamber  102   b . In this embodiment, the slider  172  is moved, based on the pressure change in the upper cylinder chamber  102   a . This ensures high responsiveness of the apparatus and enables adjustment of the damping force in a wide range. Further, when the vehicle travels over a depression in a road surface and the piston rod  108  largely extends, to thereby sharply increase the pressure in the upper cylinder chamber  102   a  (that is, the pressure in the fluid chamber  180 ), the relief valve  179  opens and the hydraulic fluid in the fluid chamber  180  is relieved into the lower cylinder chamber  102   b . Therefore, no excessive load is applied to seal portions of the hydraulic shock absorber. 
     With respect to the direction in which the vehicle after change in attitude returns to a horizontal position, a small damping force is generated, so that the vehicle body can be smoothly returned to the horizontal position. Further, an appropriate damping force can be applied to the movement of the slider  172  by virtue of the orifice passage  190  in the valve member  174 , so that malfunctioning due to self-induced vibration can be prevented. 
     In the first to third embodiments, the hydraulic shock absorbers provided at left and right wheels of a vehicle are connected so as to suppress rolling of the vehicle. However, this should not be construed as limiting the present invention. In the present invention, for suppressing pitching (for example, forward displacement during braking or backward displacement during acceleration) of a vehicle (which may be a two-wheeled vehicle), the hydraulic shock absorbers provided at front and rear wheels of a vehicle may be connected.