BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a suspension system for use with automobiles and other vehicles. 
     2. Description of the Prior Art 
     Various types of suspension systems incorporating shock absorbers have been developed to absorb the shocks vehicles encounter on the road. 
     The drawback with these conventional suspension systems is to sacrifice either riding comfort or steering stability. 
     To provide a comfortable ride, suspensions must be elastic (or &#34;soft&#34;). But soft suspensions inevitably lower handling stability. 
     If a vehicle with a soft suspension runs over an upward step (including a bulge) or a downward step (including a depression) on the road, the car body may pitch or bounce uncomfortably, and further the riding comfort is lowered and some parts of the vehicle might be damaged with the suspension either over-compressed or over-stretched. 
     Among the prior art, a Japanese Patent Application whose early publication number is 53-26021, discloses a suspension system wherein the spring constant of a suspension is adapted to be increased when the stroke amount (=the moving distance of a wheel with respect to a car body) exceeds a preset value. This suspension system can prevent the car-height from lowering through hard buffering action in the case of the spring constant increased. On the other hand, however, this suspension system aggravates the upward displacement of the car-height. And further it has such a drawback that the car body continues to bounce uncomfortably for a rather long time after the car passed a step (e.g., a depression or bulge) on the road. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to eliminate the aforementioned shortcomings by providing a new type of suspension system for vehicles that provides a high degree of riding comfort and steering stability by varying the damping capacity of the shock absorber and the spring constant of the air-spring chamber depending upon the road conditions while keeping the car height at a level high enough to leave a desired road clearance, reduces the pitching and bouncing caused by uneven road surface, and prevents the over-compression or over-stretching of the suspension. 
     And another object of this invention is to provide a suspension system which is equipped with a main air-spring chamber and an auxiliary air-spring chamber and a switch valve connecting and disconnecting the two air-spring chambers thereby permitting the change of air volumes in the air-spring chambers which in turn results in a change in the spring constant of the suspension, with the auxiliary air-spring chamber and the switch valve provided quite compactly in the system. 
     And further, another object of this invention is to provide a suspension system having said two air-spring chambers wherein only one actuator can actuate at the same time both a spring-constant switching mechanism and a damping capacity switching mechanism in a shock absorber. 
     And moreover, another object of this invention is to provide a suspension system with said two air-spring chambers wherein the bottom of the auxiliary air-spring chamber serves as also a spring bearing supporting the top end of a coil spring placed in parallel with an air-spring which helps constructing the entirety of the suspension system in a very compact form with the number of parts reduced. 
     In order to achieve the above objects, the suspension system for vehicles according to this invention has a shock absorber equipped with a damping capacity switching mechanism and an air-spring chamber equipped with a spring constant switching mechanism, plus a car-height sensor to detect the compressed and stretched conditions of a suspension, and control means feeding a control signal to said damping-capacity switching mechanism and spring-constant switching mechanism in order to change the damping capacity of the shock absorber and the spring constant of the air-spring chamber in accordance with a signal supplied from said car-height sensor, said control means being adapted to supply said control signal for increasing both the damping capacity of the shock absorber and the spring constant of the air-spring chamber to said damping-capacity switching mechanism and said spring-constant switching mechanism when the car height detected by said car-height sensor differs from a standard car-height by such a value as to exceed a preset one. 
     Accordingly, the suspension system for vehicles according to this invention provides the following desirable effects or advantages: 
     (1) Riding comfort and safety can be increased by keeping the car&#39;s wheels in firm constant contact with the road and enhancing steering stablility depending upon the acceleration and other factors working on the car body while keeping the car height at a given level through the variation of the damping capacity of the shock absorber and the spring constant of the air-spring chamber. 
     (2) The bouncing frequency fn of the car body is decreased (with a resulting increase in riding comfort) by lowering the damping capacity and spring constant and distributing the damping capacity and spring constant appropriately between the front and rear wheels. In addition, such a pitching frequency fn&#39; and a bouncing frequency fn that provide the most comfortable ride can be choosen as desired. 
     (3) Detecting the force working on the car body in the pitching direction, the car-height sensor initiates a signal to put the suspension into a &#34;hard&#34; state, thereby reducing the tilting or pitching of the car body and preventing the over-compression or over-stretching of the suspension. 
     (4) Provision can be made to detect the car height when it sinks with a gradient steeper than the preset one. Based upon the result of detection, the suspension can be turned &#34;hard&#34; before the car height becomes too low to insure safety. 
     (5) Provision can be made to detect the car height when it rises with a gradient steeper than the preset one. Based upon the result of detection, the suspension can be turned &#34;hard&#34; before the car height becomes too high to insure safety. 
     (6) The spring constant can be changed easily by changing the air volumes of the air-spring chambers consisting of the main and auxiliary air-spring chambers depending on the connecting and disconnecting action of the switch valve between said two air-spring chambers. 
     (7) The auxiliary air-spring chamber and the switch valve can be provided in the suspension system quite compactly. 
     (8) One actuator can actuate at the same time both the spring-constant switching mechanism and the damping capacity switching mechanism. 
     (9) It is possible to make the system still more compact through the use of common parts such as the bottom of the auxiliary air-spring chamber which serves also as a spring bearing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate a suspension system for vehicles embodying the principle of this invention. Sketches (a) and (b) of 
     FIG. 1 show the side elevation of a car equipped with the suspension system of this invention. 
     FIG. 2 is an overall view showing the construction of the suspension system. 
     FIG. 3 is a block diagram of the same suspension system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A suspension system for automobiles embodying the principle of this invention will be described hereunder by reference to the accompanying drawings. 
     As shown at (a) and (b) of FIG. 1 and in FIG. 2, this suspension system incorporates a strut-type damping-capacity switchable shock absorber 4. The shock absorber 4 comprises a cylinder 1a attached to the side of a front wheel or a rear wheel and a piston 19 that is slidably fitted in the cylinder 1a. 
     The piston 19 has an orifice passage which is divided into intercommunicating sections 19a, 19b and 19c. The orifice passage permits connecting and disconnecting a first chamber 1b and a second chamber 1c that are partitioned by the piston 19. 
     The chambers 1b and 1c are filled with hydraulic oil. 
     To the piston 19 is connected a piston rod 5 which extends upward, passing through the first chamber 1b in a fluid-tight manner. The upper end of the piston rod 5 is supported by a body frame 9 through a bearing 6 and rubber mount 7. The support is provided by means of several bolts or other fastening means. 
     While up-and-down motion of the piston rod 5 is limited by nuts or other means, a bearing 6 is provided to permit the rotation thereof. 
     The piston rod 5 contains a drive pin 15 as a control rod that extends along the longitudinal axis of the piston rod 5 in such a manner as to be slidable with respect thereto. 
     The lower end of the drive pin 15 extends into a space that forms a part of the orifice passage in the piston 19. To the lower end of the drive pin 15 is attached a shutter 15a serving as a control valve to open and close the orifice passages 19a, 19a. 
     The upper end of the drive pin 15 projects above the upper end of the piston rod 5, with an actuating solenoid mechanism 13 serving as a first and second actuator to actuate the shiutter 15a through the drive pin 15 coupled thereto. 
     A projection at the upper end of the drive pin 15 is fitted in a notch in the arm of the actuating solenoid mechanism 13 so that the drive pin 15 is rotated by the tension exerted by a suspension softening solenoid or a suspension hardening solenoid. 
     The solenoid mechanism 13 softens a hard suspension by actuating the softening solenoid to turn the drive pin 15, for example, counterclockwise to a given position as viewed from the direction indicated by the arrow II in FIG. 2. In this condition, the opening in the shutter 15a mates or registers with the opening in the piston 19 to establish (or open) a passage through the orifice section 19a. With the hydraulic oil thus flowing over a larger effective area through the orifice sections 19a, 19b and 19c, the shock absorber 4 becomes &#34;softened.&#34; 
     The solenoid mechanism 13 hardens a soft suspension by actuating the hardening solenoid to turn the drive pin 15, for example, clockwise to a given position as viewed from the direction indicated by the arrow II in FIG. 2. In this condition, the opening in the shutter 15a gets out of mating or registry with the opening in the piston 19 to cut off (or close) a passage through the orifice section 19a. With the hydraulic oil thus flowing over a smaller effective area through the orifice sections 19b and 19c, the shock absorber 4 becomes &#34;hardened.&#34; 
     These actions of the solenoid mechanism 13 permit switching the damping capacity of the shock absorber 4. Namely, a damping capacity switching mechanism D (FIG. 3) is composed of the solenoid mechanism 13, drive pin 15, shutter 15a, and so on. 
     The damping-capacity switchable shock absorber 4 effectively absorbs the road shocks with the damping capacity thereof adjusted to the position of the shutter 15a that changes with the up-and-down motion of the cylinder 1a with respect to the piston rod 5 which occurs as the car runs over irregularities on the road. 
     There is provided a main air-spring chamber 2 above the shock absorber 4 coaxially with the piston rod 5. 
     An auxiliary air-spring chamber 10 rests directly above the main air-spring chamber 2 coaxially with the piston rod 5. 
     The two air-spring chambers 2 and 10 are interconnected by a communicating passage 11 extending through the drive pin 15 and piston rod 5. The communicating passage 11 is provided with a switch valve 12 to open and close the passage. 
     The switch valve 12 consists of a first valve section 12a and a second valve section 12b. 
     The first valve section 12a opens or closes the valve by matching a passage running through the piston rod 5 and auxiliary air-spring chamber 10 with a passage running through the drive pin 15 and communicating passage 11 or unmatching the two passages by rotating the drive pin 15, thereby connecting or disconnecting the auxiliary air-spring chamber 10 and communicating passage 11. 
     The second valve section 12b controls the opening of the valve by fully or partly matching a passage running through the piston rod 5 and main air-spring chamber 2 with the passage running through the drive pin 15 and communicating passage 11 by similarly rotating the drive pin 15, thereby regulating the opening of the passage between the main air-spring chamber 2 and communicating passage 11. 
     This permits making the spring constant small (or softening the suspension) by connecting the main air-spring chamber 2 with the auxiliary air-spring chamber 10 when the valve section 12a is in the &#34;open&#34; mode, and making the spring constant large (or hardening the suspension) by disconnecting the main air-spring chamber 2 with the auxiliary air-spring chamber 10 when the valve section 12a is in the &#34;close&#34; mode. 
     Rotating the drive pin 15 opens and closes the switch valve 12 and thereby regulates the air volumes in the two air-spring chambers. The switch valve 12, passage 11, and so on comprise a spring-constant switching mechanism E (FIG. 3). 
     The change in the air volumes in the two air-spring chambers results in a change in the spring constant of the suspension. 
     In FIGS. 1(a) and (b) and 2, reference character 8 designates a bump stopper which protects the walls of the main air-spring chamber 2 from a damage that might result from the relative ascent of the cylinder 1a of the shock absorber 4 on rough roads, 3 a coil spring, 16a and 16b spring bearings to support the coil spring 3, 20 a bellows forming a part of the main air-spring chamber 2, A the longitudinal direction of the car body, and P and P&#39; the directions in which the car body B tilts as a result of pitching. 
     Compressed air for car-height control is generated in a compressor and supplied to each suspension unit S through a drier, joint, rear and front solenoid valves, connecting piping 1, and the communicating passage 11 through the partly tubular drive pin. 
     The suspension unit S is attached to each wheel of the car body B. FIGS. 1(a) and (b) show only suspension units SF and SR on the left-hand front and rear wheels, and the identical units on the right-hand side are not shown. 
     The compressor compresses atmospheric air supplied from an air cleaner and feeds the compressed air to the drier. The compressed air dried by silica gel or other dehydrating agent in the drier is then supplied to each suspension unit S. 
     The drier is connected to a reserve tank from which part of the compressed air is supplied through an intake solenoid valve to each suspension unit S. 
     At the right-hand front of the automobile, a front car-height sensor 17F to detect the height of the car front is attached to a lower arm. The front car-height sensor 17F comprises a higher car-height sensor 17H that detects a state in which the suspension is fully stretched and a lower car-height sensor 17L that detects a state in which the suspension is compressed, with the upper end of the shock absorber 4, for example, contacting the bump stopper 8. 
     The car-height sensors 17H and 17L feed a signal representing the detected front car-height to a control unit (microcomputer) 14 (see FIG. 3) as a control means. 
     At the left-hand rear of the automobile, a rear car-height sensor 17R to detect the height of the car rear is attached to a lateral rod. The rear car-height sensor 17R has a structure similar to that of the front car-height sensor 17F and feeds a signal representing the detected rear car-height to the control unit 14. 
     The higher and lower car-height sensors 17H and 17L detect higher and lower car-heights by means of a hall effect IC device and a magnet that are attached to the wheel and car body, respectively, or vice versa. 
     The speedometer contains a car-speed sensor 18 that detects the car-speed and feeds a corresponding signal to the control unit 14. The car-speed sensor 18 is of the lead-switch type when used with a mechanical speedometer and of the transistor open-collector output tupe when used with an electronic speedometer. 
     The accelerator pedal is equipped with an accelerator opening sensor 21 that feeds a signal indicating the opening of the accelerator to the control unit 14. 
     There is also provided a lateral acceleration sensor 22 that detects the acceleration, and its change, in the direction in which the car body rolls. Detecting a rolling-induced change in the position of the car body on springs, the lateral acceleration sensor 22 feeds a corresponding signal to the control unit 14. 
     Based on the signals supplied from the individual sensors, the control unit 14 feeds a control signal through a solenoid actuation circuit 13a to a damping-capacity switching mechanism D and a spring-constant switching mechanism E of the suspension unit S, thereby hardening or softening the suspension unit S. 
     When the engine is started, the car-height is set to a normal level by supplying compressed air from the compressor through the piping 1 to the air-spring chambers 2 and 10 so that the suspension system is held in a normal state. 
     With the switch valve 12a kept open, the pressure inside the auxiliary air-spring chamber 10 is maintained at the same level as that in the main air-spring chamber 2. 
     The spring-constant and damping-capacity switching functions of the suspension system according to this invention are as follows. 
     The spring constant and damping capacity of the front and rear wheels can be switched simultaneously from the &#34;soft&#34; state to the &#34;hard&#34; state and vice versa. This switching is effected by the automatic actuation of the solenoid 13 by the control unit 14. 
     To be more specific, the suspension unit S, which is normally kept in a &#34;soft&#34; state, gets hardened when any one of the signals from the sensors 17F, 17R, 18, 21 and 22 satisfies the hardening conditions described later and actuates the hardening solenoid. 
     First, the manner in which the lower car-height sensor 17L detects the relative lowering of the car-height that occurs when the car encounters a bulge or upward step on the road will be described. As the suspension unit SF moves from a flat portion 23 of the road to over a bulge or upward step 23a, the lower car-height sensor 17L detects a lowered car-height and outputs a corresponding signal. 
     This signal indicating the lowered car-height is fed to the control unit 14 that turns the damping-capacity switching mechanism D of the shock absorber 4 and the spring-constant switching mechanism E of the air-spring chambers 2 and 10 into a &#34;hard&#34; state through the solenoid actuation circuit 13a. 
     Thus, when the suspension unit SF on each front wheel receives a sudden up-thrust from the bulge or upward step 23a, the lower car-height sensor 17L thereon detects the resulting lowering of the car-height and outputs a corresponding signal that turns the front-wheel suspension unit SF into a &#34;hard&#34; state. 
     The front-wheel suspension unit SF thus hardened eases the tilting of the car body B due to pitching (see reference character P in FIG. 1(a)), without getting over-compressed. 
     When the lower car-height sensor 17L on the rear-wheel suspension unit SR detects the relative lowering of the car-height following the detection made by the lower car-height sensor 17L on the front-wheel suspension unit SF, the suspension is hardened in substantially the same manner as with the front-wheel suspension unit SF just described. 
     Provision may be made to turn the suspension units SF and SR on all of the four wheels into a &#34;hard&#34; state when any one of the lower car-height sensors 17L thereon detects the lowering of the car-height. 
     Next, the manner in which the higher car-height sensor 17H detects the relative rising of the car-height that occurs when the car encounters a depression or downward step on the road will be described. As the suspension unit SF moves from a flat portion 23 of the road to over a depression or downward step 23b, the higher car-height sensor 17H detects a raised car-height and outputs a corresponding signal. 
     This signal indicating the raised car-height is fed to the control unit 14 that turns the damping-capacity switching mechanism D of the shock absorber 4 and the spring-constant switching mechanism E of the air-spring chambers 2 and 10 into a &#34;hard&#34; state through the solenoid actuation circuit 13a. 
     Thus, when the suspension unit SF on each front wheel receives a sudden down-thrust from the depression or downward step 23b, the higher car-height sensor 17H thereon detects the resulting rising of the car-height and outputs a corresponding signal that turns the front-wheel suspension unit SF into a &#34;hard&#34; state. 
     The front-wheel suspension unit SF thus hardened eases the tilting of the car body B due to pitching (see reference character P&#39; in FIG. 1(b)), without getting over-stretched. 
     When the higher car-height sensor 17H on the rear-wheel suspension unit SR detects the relative rising of the car-height following the detection made by the higher car-height sensor 17H on the front-wheel suspension unit SF, the suspension is hardened in substantially the same manner as with the front-wheel suspension unit SF just described. 
     Provision may be made to turn the suspension units SF and SR on all of the four wheels into a &#34;hard&#34;  state when any one of the higher car-height sensors 17H thereon detects the rising of the car-height. 
     The higher and lower car-height sensors 17H and 17L are appropriately combined so that the suspensions are hardened when they detect the rising or lowering of the car-height. 
     Provision may be made so that the control unit 14 gives a suspension hardening instruction based on a combination of the car-height signals from the car-height sensors 17F and 17R and the car-speed signals from the car-speed sensor 18, rather than on the former alone. 
     Also, a differentiator may be positioned between the car-height sensors 17F and 17R and the control unit 14 so that the car-height can be detected when the gradient of its change exceeds a given limit. This permits predicting the car-height before it becomes too high or too low, thereby allowing the suspensions to be hardened beforehand and assuring higher safety. 
     When the differentiator is provided, provision may also be made to feed a &#34;hardening&#34; signal to the switching mechanisms D and E of the suspension unit S  if the car-height lowers below a given level and/or with a gradient steeper than the preset one. 
     Provision may also be made to feed a &#34;hardening&#34; signal to the switching mechanisms D and E of the suspension unit S if the car-height rises above a given level and/or with a gradient steeper than the preset one. 
     Switching from a &#34;hard&#34; state to a &#34;soft&#34; state is accomplished by the softening solenoid that is actuated a few seconds after all of the hardening conditions have been released. This time-lag of a few seconds is conducive to the prevention of chattering that might occur when switching is effected. 
     The spring-constant and damping-capacity switching ratios between the &#34;soft&#34; and &#34;hard&#34; states are set at appropriate values. 
     The spring constant and damping capacity are distributed differently between the front and rear wheels. That is, the spring constant and damping capacity of the individual suspension units S are set so that the rear suspension becomes harder than the front suspension in the &#34;soft&#34; state, and vice versa. 
     By thus changing the hardness of the suspension units S on the front and rear wheels, the center of elasticity and the understeer/oversteer characteristics (US/OS characteristics) can be controlled appropriately. 
     Only one of the right-hand and left-hand suspension units S, such as the one on which greater component of vertical force works, may be hardened. 
     Adequate steering stability is secured even in the &#34;soft&#34; state. Besides, the switching function is designed to stop, even in the case of failure, only after the suspension has been turned to the &#34;hard&#34; state. 
     The function to switch the spring constant and dumping capacity of the suspension unit S can be actuated independently of a car-height adjusting function described later, whereby one of the two functions can work even when the other fails. 
     The hardening conditions are set on the basis of the signals fed from the sensors 17F, 17R, 18, 21 and 22. 
     The lower and higher car-height sensors 17L and 17H set the hardening conditions as described previously. 
     Setting is made so that the suspension unit S gets hardened when the lateral acceleration sensor 22 detects a lateral acceleration. 
     Setting is made so that the suspension unit S is maintained in the &#34;soft&#34; state when the car speed remains below a given limit. 
     Also setting is made so that the suspension unit S gets hardened when the opening or closing rate (m/sec) of the accelerator, as detected by the accelerator opening sensor 21, and the car speed both exceed given limits. 
     The suspension system according to this invention is also capable of adjusting (or maintaining) the car-height by regulating the pressure of the compressed air supplied from the compressor to the main air-spring chamber 2. The front and rear car-height sensors 17F and 17R detect the car height and feed a corresponding signal to the control unit 14 which, in turn, supplies a control signal to the compressor for car-height regulation. 
     When the car height is lower than a preset level, that is, when the car body sinks under the increased weight of passengers or other load for several seconds or when the control unit 14 judges, based on the signals from the car-height sensors 17H and 17R, that the car height is lower than the preset level, the reserve tank begins to supply air to the main air-spring chamber 2 shown in FIG. 2, and also the the auxiliary air-spring chamber 10 if the suspension is in the &#34;soft&#34;  state. 
     When the car body lifts to the preset level, the control unit 14 initiates a control signal based on the signals received from the car-height sensors 17F and 17R, thereby stopping the air supply. 
     As described above, the damping capacity and spring constant can be appropriately and automatically switched depending upon the running conditions of the car (i.e., depending upon whether the car is running under normal conditions, or with the brakes jammed suddenly, or travelling in a sharp curve or over a rough road). This assures a high degree of riding comfort and steering stability under different running conditions. 
     Placing the auxiliary air-spring chamber 10 and main air-spring chamber 2, one over the other, on top of the shock absorber 4 helps constructing the entirety of the suspension unit S in a very compact form. With the upper end of the coil spring 3 supported by the spring bearing 16b provided at the bottom of the auxiliary air-spring chamber 10, it is possible to make the unit still more compact through the use of common parts. 
     Even when used on the front wheels, the suspension system of this invention provides a simple structure because the piping 1 can rotate as the coil spring 3 and the auxiliary air-spring chamber 10 turn with the steering of the front wheels. 
     Separate actuators may be used as the first and second actuators, in which case the damping capacity and spring-constant can be controlled independently.
When an automobile or other vehicle travels over a bump with a suspension in its switchable suspension system kept in a "soft" state, the car body pitches violently. This pitching is reduced by turning the suspension into a "hard" state by increasing the damping capacity of its shock absorber and the spring constant of its air-spring chambers in accordance with a signal supplied from a car-height sensor detecting the compressed or stretched condition of the suspension. The change of the spring constant is executed with the following construction: an auxiliary air-spring chamber resting above a main air-spring chamber surrounding a piston rod, and an air passage within the piston rod for intercommunicating the main and auxiliary air-spring chambers are provided in the system, and the connection and disconnection between the two chambers through the air passage are controlled by the piston rod and a control rod extending in the piston rod.