Abstract:
A hydraulic suspension strut ( 30 ) has a first on-off valve ( 58 ) connecting the first and second chambers ( 42  and  44 ), and a second on-off valve ( 62 ) connected to chamber ( 42 ) only. Suspension arm ( 3 ) of wheel ( 1 ) can be retracted beyond normal road travel, or protracted to road travel mode, by use of pump ( 9 ) and valves ( 58  and  62 ). The space in the strut above port ( 56 ) may be used as a hydraulic bump stop. An accumulator ( 50 ) may be included, as a hydraulic spring. Alternatively, strut ( 70 ) (FIG.  6 ; note alternative accumulator position) allows suspension to be raised above normal road travel. Strut ( 100 ) (FIG.  8 ) may be operated in roadgoing, raised, or retracted mode. These struts may be applied to reconfigurable suspensions; particularly for amphibious vehicles, which may require wheels to be withdrawn above the hull water line to reduce drag on water, particularly during cornering.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improved hydraulic suspension strut for use in a wheeled vehicle. 
     Known hydraulic suspension struts comprise a piston movable within a cylinder to achieve a damper effect, allowing the wheel mounted on the strut to move in the vertical direction. The damping effect is achieved by restricting the flow of hydraulic fluid from one side of the piston to the other. Often the hydraulic strut will be connected to a gas charged hydraulic accumulator to achieve a combined damper and spring effect in which the spring effect is achieved by compression of hydraulic fluid in the cylinder and fluid in the gas charged hydraulic accumulator. It is also known to provide a hydraulic bump stop in a hydraulic suspension system of this type, in which a volume of hydraulic fluid is contained within the cylinder and limits movement of the piston and hence the wheel, to the distance normal for road travel. 
     It is also known for road vehicles to use hydraulic suspensions comprising various valves and cross-connections between individual wheel suspensions, for example the BMC/British Leyland Hydrolastic and Hydragas systems. Such interconnected suspensions are disclosed in GB 2,144,378 (Alfa Romeo), GB 1,260,719 (IDCE), and U.S. Pat. No. 5,584,498 (Yamaha). These systems are designed to reduce undesirable effects such as pitch and roll; as are active and semi-active systems, which are known, for example, from U.S. Pat. No. 4,779,895 (Robert Bosch), EP 0,183,059 (Robert Bosch), and U.S. Pat. No. 5,678,846 (Lotus). All of these systems are designed to improve on-road performance, by extending the capabilities of a road car suspension. 
     It is further known, particularly on Citröen cars, for a road vehicle to have hydraulic suspension which can be raised above normal running height to traverse uneven ground. On the other hand, for amphibious vehicles, it has been found helpful to move the wheels in the opposite direction, retracting them well above road height to tuck them above the vehicle water line. This reduces drag on water, particularly during cornering. Suspensions allowing such reconfiguration are disclosed in EP 0,742,761 (Roycroft), U.S. Pat. No. 4,958,584 (Williamson), and U.S. Pat. No. 4,241,686 (Westphalen). Roycroft uses a combined hydraulic and mechanical system; whereas both Williamson and Westphalen use mechanical systems. 
     SUMMARY OF THE INVENTION 
     It is considered that the mechanical retraction systems listed above are bulky, and liable to corrosion in a salt water environment. Through development of prior art hydraulic systems, and introducing dual use of components wherever practicable, a hydraulic suspension system may be designed which not only performs well on roads, but also allows the vehicle to be reconfigured to an alternative use, particularly as an amphibious vehicle. 
     It is an object of the present invention to provide a hydraulic suspension strut which can be used in normal road travel and can also be used to retract a wheel by a greater distance than is appropriate for road travel. 
     It is a further object of the present invention to provide a hydraulic suspension strut which can be used in normal road travel and can also be used to extend a wheel by a greater distance than is normal for road travel. 
     It is a further object of the present invention to provide a hydraulic suspension strut which can be used in normal road travel and can also be used either to retract or extend a wheel by a greater distance than is normal for road travel. 
     The present invention provides a vehicle hydraulic suspension strut which comprises
         a cylinder containing hydraulic fluid   a piston movable within the cylinder and defining a first chamber and a second chamber within the cylinder   linkage means for connecting the piston to a load characterised in that   a first on/off valve is provided to control flow of fluid between the first and second chambers over at least part of the stroke of the piston and the strut further comprises a second on/off valve which is in communication with only one of the first and the second chambers.       

     In a particularly preferred embodiment, the strut also comprises a gas charged hydraulic accumulator which is in communication with at least one of the said first and second chambers. 
     The present invention further provides a vehicle hydraulic suspension strut which comprises a cylinder containing hydraulic fluid
         a piston movable within the cylinder and defining a first chamber and a second chamber within the cylinder   linkage means for connecting the piston to a load   and a gas charged hydraulic accumulator in communication with said second chamber characterised in that   a first on/off valve is provided to control the flow of fluid between the first and second chambers over at least part of the stroke of the piston and the strut further comprises a further on/off valve which is in communication with the first chamber only.       

     The present invention further provides a vehicle hydraulic suspension strut which comprises
         a cylinder containing hydraulic fluid   a piston movable within the cylinder and defining a first chamber and a second chamber within the cylinder   linkage means for connecting the piston to a load   and a gas charged hydraulic accumulator in communication with one of said first and second chambers characterised in that   a first on/off valve is provided to control the flow of fluid between the first and second chambers over at least part of the stroke of the piston and the strut further comprises a further on/off valve which is in communication with the second chamber only and the gas charged hydraulic accumulator is in communication with the first chamber.       

     The present invention further provides a vehicle hydraulic suspension strut which comprises
         a cylinder containing hydraulic fluid   a piston movable within the piston   linkage means for connecting the piston to a load characterised in that   the piston is movable within the cylinder between a first extreme position defining a first pair of first and second chambers within the cylinder and a second extreme position defining second pair of first and second chambers within the cylinder; the strut further comprising an on/off valve controlling flow of fluid between the first and second chambers in one of the extreme positions of the piston, a further on/off valve controlling flow of fluid between the first and second chambers in the other of the extreme positions of the piston, and third and fourth on/off valves, each of which is in communication only to one of the first and second chambers respectively.       

     In a preferred embodiment of a hydraulic suspension strut according to the present invention, the on/off valve or valves allowing flow of fluid between the first and second chambers, is located in a hydraulic transfer line externally of the cylinder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Several embodiments of the invention will now be described with reference to the accompanying drawings in which 
         FIG. 1  is a view of a known hydraulic suspension strut; 
         FIG. 2  is a view of a first embodiment of a hydraulic suspension strut which can be used in a first mode, for normal travel, and a second mode for wheel retraction, shown in the road travel mode; 
         FIG. 3  is a view of the hydraulic suspension strut of  FIG. 2  in the mode for wheel retraction; 
         FIG. 4  is a view showing a modification to the hydraulic suspension strut of  FIG. 2 , with the accumulator mounted in an alternative position; 
         FIG. 5  is a schematic diagram of a vehicle wheel suspension installation using a strut as shown in  FIG. 3 ; 
         FIG. 6  is a view of a second embodiment of a hydraulic suspension strut which can be used in a first mode, for normal travel, and a second mode for wheel extension, shown in the road travel mode; 
         FIG. 7  is a view of the hydraulic suspension strut of  FIG. 6  in the mode for wheel extension; 
         FIG. 8  is a view of a third embodiment of a hydraulic suspension strut which can be used in a first mode, for normal travel, a second mode for wheel retraction, and a third mode for wheel extension, shown in a mode where compression travel is limited; 
         FIG. 9  is a view of the hydraulic suspension strut of  FIG. 8  in a mode where extension travel is limited; 
         FIG. 10  is a view of the hydraulic suspension strut of  FIG. 8  in a mode where travel is not limited; and 
         FIG. 11  is a view of the hydraulic suspension strut of  FIG. 8  where the position of the strut can be controlled to either extend or retract the strut. 
     
    
    
     As can be seen from  FIG. 1 , a known hydraulic suspension strut shown generally at  10  comprises a cylinder  2  and a piston  4  movable axially within the cylinder. A linkage rod  6  is attached to one face of the piston  4  and extends through a sealed aperture  8  in the base of the cylinder  2 . The linkage rod  6  terminates at the end  12  remote from the piston  4  in a mount  14 . 
     The piston  4  defines first and second chambers  16 ,  18  in the cylinder  2 , and a restricter/damper valve  20  is provided in the piston  4 . A seal  22  is provided on the circumference of the piston  4 . The first and second chambers  16 ,  18  are filled with hydraulic fluid. 
     A hydraulic line  24  leaves the cylinder  2  from the side wall thereof close to the base of the cylinder  2  and terminates in a gas charged hydraulic accumulator  26 . 
     In use, a wheel (not shown) is linked to the linkage rod  6  via the mount  14  and the cylinder  2  is attached to the vehicle body (not shown). 
     As the wheel encounters irregularities in the road surface on which the vehicle provided with the suspension strut is being driven, the piston  4  moves axially within the cylinder  2 . 
     As the vehicle moves over, in particular, rough terrain, excessive oscillation of the vehicle is obviated because of the damping effect of the valve  20  and gas charged hydraulic accumulator  26 . 
     In the hydraulic suspension strut shown in  FIG. 1 , the total amount of travel of the piston  4 , and hence the wheel, in the vertical direction, is limited by engagement of the upper face  150  of the piston  4  with the inner surface of the upper end wall  152  of the cylinder  2  and by engagement of the lower face  154  of the piston  4  with the inner surface  156  of the base of the cylinder  2 . A bump stop (not shown) may be provided to limit the total amount of travel of the piston  4 . 
     A first embodiment of an hydraulic suspension strut in accordance with the invention is shown generally at  30  in FIG.  2 . The strut  30  comprises a cylinder  28  and a piston  32  moveable axially within the cylinder. A linkage rod  34  is attached to one face of the piston  32  and extends through a sealed aperture  36  in the base of the cylinder  28 . The linkage rod  34  terminates at the end  38  remote from the piston  32  in a mount  40 . 
     The piston  32  defines first and second chambers  42 ,  44  in the cylinder  28 . A seal  46  is provided on the circumference of the piston  32 . The first and second chambers  42 ,  44  are filled with hydraulic fluid. 
     A line  48  leaves the cylinder  28  from a port  158  in the side wall of the cylinder  28  close to the base thereof and connects to a gas charged hydraulic accumulator  50  and to the hydraulic system of the vehicle, which system includes a source of hydraulic pressure and a hydraulic fluid reservoir (not shown). 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A hydraulic transfer line  52  is provided external to the cylinder  28  and connects first and second ports  54 ,  56  in the side wall of the cylinder. The first port  54  is located in the side wall close to the base of the cylinder  28 , whilst the second port  56  is located in the side wall at approximately the mid-point of the cylinder  28 . A first on/off valve  58  is provided in the transfer line  52 . 
     In an alternative embodiment, the first port  54  could be provided in the end wall at the base of the cylinder. 
     A further hydraulic line  60  extends from a port  160  in the upper wall of the cylinder  28  and is controlled by a second on/off valve  62 . The further line  60  is connected to the hydraulic system of the vehicle. 
     In use, a wheel (not shown) is linked to the linkage rod  34  via the mount  40  and the cylinder  28  is attached to the vehicle body (not shown). 
     In road travel mode, as the wheel encounters irregularities in the road surface on which the vehicle provided with the suspension strut is being driven, the piston  32  moves axially within the cylinder  28 . 
     As the vehicle moves over, in particular, rough terrain, excessive oscillation of the vehicle is obviated because of the damping effect of the strut and the gas charged hydraulic accumulator  50 . 
     The hydraulic suspension strut shown in  FIG. 2  is shown in road travel mode. The first on/off valve  58  in the transfer line  52  is open allowing a restricted flow of fluid between the first and second chambers and the second on/off valve  62  in the top line  60  is closed. The total amount of travel of the piston  32 , and hence the wheel, in the vertically upwards direction, is shown in chain dotting. When the piston  32  is in the position shown in chain dot, the port  56  is closed by the piston  32  and further movement of the piston upwards in the cylinder is prevented by the volume of hydraulic fluid in the first chamber  42  thus providing a hydraulic stop. Movement of the piston  32  in the vertically downwards direction is limited by engagement of the lower face  162  of the piston  32  with the inner surface of the base  164  of the cylinder  28 . 
     In  FIG. 3 , the strut is shown in wheel retraction mode. The first on/off valve  58  in the transfer line  52  is closed and the second on/off valve  62  in the top line  60  is open. In this condition, the strut can now act as a hydraulic actuator and may be used to retract the wheel beyond its normal travel by fluid being pumped into the lower chamber  44  and exhausted through the upper port  160 . 
     To return the wheel to its normal operating condition, the strut is actuated in reverse with fluid being pumped into the upper chamber  42  through the upper port  160  and exhausted from the lower chamber. Once the piston  32  has moved to a position below the port  56 , the valve  62  can be closed and the valve  58  opened so that the strut can again provide suspension and damping for the wheel. 
     It should be noted that the term ‘on/off valve’ is used herein to denote a valve that when open allows flow through the valve in either direction and when closed prevents fluid from flowing through the valve in either direction. In a preferred embodiment, the on/off valves  58 ,  65  are spool valves which may be operated hydraulically, electronically or manually. However, any suitable type of valve may be used. 
       FIG. 4  shows a modification to the strut  30  of FIG.  2 . In the modified strut  30  of  FIG. 4 , the hydraulic accumulator is in communication with one of the branches of the transfer line  52  between the first and second chambers  42 ,  44 . In all other respects, the hydraulic strut  30  of  FIG. 4  is the same as the strut  30  as shown in FIG.  2  and can be operated in the same manner in both road travel and wheel retraction modes. 
       FIG. 5  shows a diagrammatic view of a vehicle installation of a hydraulic strut according to FIG.  4 . Wheel  1  is mounted on a suspension arm  3  which is attached at mounting point  5  to a chassis member  7 . It will be appreciated that a similar chassis mounting will be required for the top of the suspension strut; this is however not shown, to enable the hydraulic circuits to be clearly shown. The piston linkage rod  34  is flexibly attached to the suspension arm  3  by a mount  40 , which may comprise a bush (not shown). 
     Hydraulic lines  48  and  160  (through valve  62 ) are both connected to a pump  9 , driven by shaft  1 . This pump may be reversible, as shown; or unidirectional; and may be driven directly by the vehicle engine, or by an electric motor. If a unidirectional pump is used, valves (not shown) must be used to reverse direction of fluid flow as required. 
     Thus it can be seen that in a road travel mode, movement of the suspension arm  3 , caused as the wheel  11  goes over irregularities on the road surface, will be transferred to the piston  32  through the mount  40  and the linkage rod  34 . This movement will be damped by the strut  30  in a conventional manner. When the strut  30  is operated in the wheel retraction mode, the pump  9  can be selectively connected to one of the chambers  42 ,  44  causing the piston to move within the cylinder. This movement is translated into pivotal movement of the suspension arm  3 , and so into movement of the wheel  11 , through the linkage rod  34  and the mount  40 . 
     As can be seen from  FIG. 6 , an alternative embodiment of a hydraulic suspension strut shown generally at  70  comprises a cylinder  64  and a piston  66  moveable axially within the cylinder  64 . A linkage rod  68  is attached to one face of the piston  66  and extends through a sealed aperture  72  in the base of the cylinder  64 . The linkage rod  68  terminates at the end  74  remote from the piston  66  in a mount  76 . 
     The piston  66  defines first and second chambers  78 ,  80  in the cylinder  64 . A seal  82  is provided on the circumference of the piston  66 . The first and second chambers  78 ,  80  are filled with hydraulic fluid. 
     A hydraulic line  84  leaves the cylinder  64  from a port  164  in the side wall of the cylinder  64  close to the top thereof and connects to a gas charged hydraulic accumulator  86  and the hydraulic system of the vehicle (not shown). In an alternative arrangement, the port  164  could be provided in the end wall at the top of the cylinder. 
     A hydraulic transfer line  88  is provided external to the cylinder  64  and connects first and second ports  90 ,  92  in the walls of the cylinder. The first port  90  is located in the side wall of the cylinder at approximately the mid-point of the cylinder  64 , whilst the second port  92  is located close to the top of the cylinder  64 . A first on/off valve  94  is provided in the transfer line  88 . 
     In an alternative arrangement, the second port  92  could be provided in an end wall at the top of the cylinder. 
     A further hydraulic line  96  extends from a port  166  in the side wall close to the base of the cylinder  64  and is controlled by a second on/off valve  98 . The further hydraulic line  96  is connected to the hydraulic system of the vehicle (not shown). 
     In use, a wheel (not shown) is mounted on the linkage rod  68  via the mount  76 . 
     As the wheel encounters irregularities in the road surface on which the vehicle provided with the suspension strut is being driven, the piston  66  moves axially within the cylinder  64 . 
     As the vehicle moves over, in particular, rough terrain, excessive oscillation of the vehicle is obviated because of the damping effect of the strut and the gas charged hydraulic accumulator  86 . 
     The hydraulic suspension strut shown in  FIG. 6  is shown in road travel mode. The first on/off valve  94  in the transfer line  88  is open and the second on/off valve  98  in the lower line  96  is closed. The total amount of travel of the piston  66 , and hence the wheel, in the vertically upwards direction, is limited by engagement of the upper face  168  of the piston  66  with the inner surface of the upper end wall  170  of the cylinder  64 . The limit of travel in the vertically downwards direction is shown in chain dot and is limited by the volume of hydraulic fluid in the second chamber  80  when the piston  66  closes off the port  90 , thus providing a hydraulic stop. 
     It should be noted that the accumulator  86  shown in this Figure may be mounted on a branch of the transfer line  88 , in a fashion similar to that shown in FIG.  4 . 
     In the view shown in  FIG. 7 , the strut is shown in wheel extension mode. The first on/off valve  94  in the hydraulic transfer line  88  is closed and the second on/off valve  98  in the lower line  96  is open. In this condition, the strut now acts as a hydraulic actuator and may be used to extend the wheel beyond its normal travel by fluid being pumped into the upper chamber  78  and exhausted through the lower port  166 . Vertically downwards movement of the piston  66  being limited by engagement of its lower face  172  with the inner surface of the base  174  of the cylinder  64 . 
     To return the wheel to its normal operating condition, actuation of the strut is reversed, with fluid being pumped into the lower chamber  80  and exhausted from the upper chamber  78 . 
     Once the piston has moved above the port  90 , the valve  98  can be closed and the valve  94  opened so that the strut will again operate to provide suspension and damping for the wheel. 
     As can be seen from  FIG. 8 , a further embodiment of a hydraulic suspension strut, shown generally at  100  comprises a cylinder  102  and a piston  104  moveable axially within the cylinder. A linkage rod  106  is attached to one face of the piston  104  and extends through a sealed aperture  108  in the base of the cylinder  102 . The linkage rod  106  terminates at the end  112  remote from the piston  104  in a mount  114 . 
     The piston  104  defines first and second chambers  116 ,  118  in the cylinder  102 . A seal  122  is provided on the circumference of the piston  104 . The first and second chambers  116 ,  118  are filled with hydraulic fluid. 
     An hydraulic transfer line  132  is provided external to the cylinder  102  and is attached at a port  176  close to the top of the cylinder  102 , aport  178  at about the mid-point of the cylinder  102  and a port  180  close to the base thereof. A first on/off valve  136  is provided in the hydraulic line between the ports  178  and  180  and a further on/off valve, known as a third on/off valve,  134  is provided in the hydraulic line between the ports  176  and  178 . 
     A second hydraulic line  138  branches from the line  132  between the first and third on/off valves  136 ,  134  and terminates in a gas charged hydraulic accumulator  140 . 
     A third hydraulic line  124  leaves the cylinder  102  from the side wall thereof close to the base of the cylinder  102  and connects to an on/off valve  126 , known as a fourth on/off valve, and to the hydraulic system of the vehicle (not shown). 
     A fourth hydraulic line  128  leaves the cylinder  102  from the side wall thereof close to the top of the cylinder  102  and connects to an on/off valve  130 , known as a second on/off valve, and the hydraulic system of the vehicle (not shown). 
     In use, a wheel (not shown) is linked to the linkage rod  106  via the mount  114  and the cylinder  102  is attached to the vehicle body (not shown). 
     As the wheel encounters irregularities in the road surface on which the vehicle provided with the suspension strut is being driven, the piston  104  moves axially within the cylinder  102 . 
     As the vehicle moves over, in particular, rough terrain, excessive oscillation of the vehicle is obviated because of the damping effect of the strut and the gas charged hydraulic accumulator  140 . 
     The hydraulic suspension strut shown in  FIG. 8  is shown in a mode where compression travel is limited, for example to prevent grounding over rough terrain due to excessive wheel travel. The fourth on/off valve  126  in the hydraulic line  124  is closed and the first on/off valve  136  in the hydraulic line  132  is open. The second on/off valve  130  in the hydraulic line  128  and the third on/off valve  134  in the hydraulic line  132  are closed. The total amount of travel of the piston  104 , and hence the wheel, in the vertically upwards direction, is shown in chain dotting and is limited to the level of the port  178  by the volume of hydraulic fluid trapped in the first chamber  116  when the piston  104  covers the port  178 . The total amount of travel of the piston  104  in the vertically downwards direction is shown in chain dotting and is limited by engagement with the base of the cylinder  102 . 
     If it is intended to use the embodiment of the hydraulic suspension strut according to  FIG. 8  in a mode where extension travel is limited, for example on a road-going vehicle which only uses extra ground clearance at low speed, then the strut should be operated as shown in FIG.  9 . In this arrangement, the fourth on/off valve  126  in the line  124  and the first on/off valve  136  in the line  132  are closed. The second on/off valve  130  in the line  128  is closed and the third on/off valve  134  in the line  132  is open. 
     The total amount of travel of the piston  104 , and hence the wheel, in the vertically upwards direction, is shown in chain dotting and is limited by engagement of the upper face of the piston with the inner surface of the upper end wall  179  of the cylinder  102 . The total amount of travel of the piston  104  in the vertically downwards direction is shown in chain dotting and is limited to the level of the port  178  by the volume of hydraulic fluid trapped in the second chamber  118  when the piston covers the port  178 . 
     If it is intended to use the embodiment of the hydraulic suspension strut according to  FIG. 8  in a mode where travel is not hydraulically limited, then the strut should be operated as shown in FIG.  10 . In this arrangement, the on/off valve  126  in the line  124  is closed and the on/off valve  136  in the line  132  is open. The on/off valve  130  in the line  128  is closed and the on/off valve  134  in the line  132  is open. The total amount of travel of the piston  104  is limited by engagement with the inner surface of the upper end wall  179  of the cylinder  102  in the vertically upwards direction and by engagement with the base of the cylinder  102  in the vertically downwards direction. 
     If it is intended to use the embodiment of the hydraulic suspension strut according to  FIG. 8  in a mode where the position of the strut can be controlled to either extend or contract the strut and hence raise or lower the vehicle, for example so that it kneels, then the strut should be operated as shown in FIG.  11 . In this arrangement, the fourth on/off valve  126  in the line  124  is open and the first on/off valve  136  in the line  132  is closed. The second on/off valve  130  in the line  128  is open and the third on/off valve  134  in the line  132  is closed. By causing fluid to enter along line  124  and exit via line  128  the strut will be made to contract. Similarly, by causing fluid to enter along line  128  and exit via line  124  the strut will be made to extend. 
     In an alternative arrangement of the hydraulic suspension strut, the vehicle body (not shown) may be linked to the linkage rod via the mount and the wheel (not shown) may be attached to the cylinder. 
     All hydraulic valves described above may be, for example, spool or poppet valves. They may be operated by solenoids or motors, or manually. If the valves are operated manually, it may be convenient to use pilot operated valves. Furthermore, the various hydraulic lines may be connected to the chambers within the cylinder by means of ports located in side wall or the end walls of the cylinder as appropriate. 
     An hydraulic suspension strut in accordance with the invention can have particular application in an amphibious vehicle. It is known for amphibious vehicles to have wheels mounted so that they can be moved from a protracted position in which they are arranged to support the vehicle for use on land, in a manner similar to a wheel on a conventional motor vehicle, to a retracted position in which the wheels are elevated relative to the body of the vehicle for use on water. A hydraulic suspension strut in accordance with the present invention can be connected to a wheel of such an amphibious vehicle such that with the strut in road travel mode it provides damping and/or suspension for the wheel when the vehicle is used on land. However, when the vehicle is used in water, the strut can be used in retraction or extension mode to move the wheel between its protracted and retracted positions. Moreover, the control system for retraction and extension of the strut may be linked to other control mechanisms for conversion of the vehicle between road mode and marine mode. 
     It should be noted that different wheels or different axles of an amphibian vehicle may have different suspension retraction requirements. For example Westphalen, in U.S. Pat. No. 4,241,686 referenced above, retracts front wheels into closed pod areas; but simply retracts rear wheels into an open, recessed area above the propeller level. Also, U.S. Pat. No. 4,008,679 (Bozzano) discloses an amphibious vehicle using a combination of caterpillar tracks and individual wheels. Hence, it will be understood that the hydraulic suspension strut of the invention may be applied to a single wheel of a three-wheeled vehicle; or to a single axle of a four-wheeled vehicle; or to all wheels as required. Although described with reference to an amphibious vehicle, the suspension strut could have other applications to reconfigurable vehicles. 
     Although in all the embodiments shown the strut is connected to a gas charged hydraulic accumulator, this need not be the case and alternative means of providing a spring effect can be used. For example the strut could be hydraulically connected to a piston which is acted upon by a resilient member, such as a spring. Alternatively the strut could be used in combination with a conventional mechanical spring arrangement for a vehicle.