Abstract:
A mechanism is provided by which the wheels of an amphibious vehicle are simultaneously retracted or protracted for switching between a land mode and a marine mode of operation. Each of a transversely spaced pair of wheels is supported by a horizontally arranged spring and damper combination acting on at least one suspension link. Each spring and damper combination is in turn supported by a movable anchor point, the position of which determines the extent of protraction or retraction of the associated wheel. The anchor points for the two wheels may be located at opposite ends of a rotatable rocker member, the rotational orientation of which is determined by an actuator taking the form of for example a hydraulic ram or electric motor.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims priority from Great Britain Application Ser. No. 0423474.6, filed Oct. 22, 2004. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a wheel suspension and retraction system, more particularly to a retraction arrangement for an amphibious vehicle capable of powered travel on land and water. The invention has particular, but not exclusive, application for an amphibious vehicle in which a user sits astride the vehicle in the manner of a motorcycle, jet ski, quad bike, or the like. 
   Amphibious vehicles capable of powered travel on both land and water are known. In a typical arrangement, a prime mover (such as an engine or electric motor) is arranged to provide drive to a marine propulsion unit to propel the vehicle on water and to one or more road wheels for travel on land. 
   Because of the need to drive a marine propulsion unit and at least one road wheel, the power train of an amphibious vehicle is often more complex than that of a conventional road going vehicle or marine-only craft. This can give rise to difficulties in designing a power train to be accommodated in the limited available space, whilst also ensuring that the weight distribution is satisfactory for both land and marine usage. This problem is particularly acute for smaller “sit-astride” type amphibious vehicles that are similar in form to motorcycles, jet-skis, or quad bikes. Such vehicles tend to have narrower, taller bodywork that makes it difficult to accommodate a conventional amphibious vehicle power train. 
   The problems associated with amphibious vehicles are not limited to the arrangement of the power train, since the limitations of space also have an impact on other aspects of amphibious vehicle design. For example, the suspension for the road wheels has to be accommodated alongside the power train. This can be a particular problem in the case of an amphibious vehicle in which the road wheels are retractable when the vehicle is being used for marine travel. 
   U.S. Pat. No. 3,903,831 (Bartlett) is one of many prior art examples of amphibious vehicle wheel retraction arrangements. It uses a single hydraulic ram to retract both of the vehicle&#39;s front wheels via a rotatable shaft and a series of linkages and chain drives. The use of a single ram to retract two wheels is attractive; but Bartlett&#39;s system has many practical drawbacks. For example, it does not disclose shock absorbers (also known as dampers), and wheel suspension is provided using leaf springs and a single rigid axle. Leaf springs are heavy, bulky, and liable to unpredictable onset of inter-leaf friction, especially as the vehicle ages and corrosion sets in. Furthermore, the live axle design used in U.S. Pat. No. 3,903,831, which is almost extinct in contemporary passenger car production, commits a vehicle builder using Bartlett&#39;s design to standards of ride and handling which would be uncompetitive against modem light road vehicles. It should be noted that Bartlett&#39;s vehicle is an amphibious motor home; while leaf sprung live axles are still common for heavy vehicles, it is imperative for a smaller vehicle to offer independent wheel springing. 
   Further examples of retractable suspensions may be seen in U.S. Pat. No. 5,755,173 (Rorabaugh), U.S. Pat. No. 5,590,617 (Aquastrada), U.S. Pat. No. 4,958,584, (Williamson), and U.S. Pat. No. 4,241,686 (Westphalen). U.S. Pat. No. 5,590,617 utilises a dual-piston hydraulic ram as an actuator for retracting a pair of wheels, whereas a lead screw is used in the other cases. In all of these designs, use of a single actuator for pairs of wheels and their associated suspension arrangements is apparently a good way to save money and weight; but it is clear from the cross-sectional views provided in each of these patents that all of these retraction arrangements take up a lot of room in terms of height and width. 
   There is a need, therefore, for an amphibious vehicle having an improved or alternative arrangement for retraction of the wheels that can be more easily accommodated in a smaller space, particularly, but not exclusively, for a sit-astride type amphibious vehicle. 
   Japanese patent application JP-63-002712A1 of Ishida describes an amphibious vehicle with retractable wheels. The retractable wheels are arranged in pairs longitudinally, fore and aft, along the vehicle; w. With the wheels of each pair connected to a common rocker member whose rotation can raise or lower the wheels. A separate spring and damper assembly is provided for each wheel. The front wheels are not connected to a common rocker arm, nor are the back wheels. This requires a mechanism which extends along the length of the vehicle, which is difficult to package. It mandates deep and wide bodyside cavities, making occupant access difficult, and would raise the vehicle&#39;s center of gravity when compared to a conventional suspension, increasing roll angles on land and on water, and with negative effects on ride motions. Ishida requires this packaging to enable easy connection of each rocker arm to a manually windable handle accessible from the vehicle cabin. The rocker arm of Ishida is a straight member and the wheels of Ishida on retraction pivot about axes running transversely, left to right, across the vehicle. This in turn dictates a suspension geometry of pure leading and trailing arms. This allows negligible camber change on corners, severely limiting roadholding on land. A single suspension arm to a front wheel in particular, may lead to considerable problems with wheel shimmy, and associated unpleasant feedback through the steering system. 
   SUMMARY OF THE INVENTION 
   In a first aspect the present invention provides a wheel suspension and retraction system for an amphibious vehicle comprising:
         a pair of wheels spaced apart transversely across the vehicle, each wheel mounted for rotation on a hub assembly;   suspension linkages for the wheels connecting the hub assemblies to a common rocker member via spring and damper assemblies, one spring and damper assembly for each wheel, with the rocker member mounted for rotation about a pivot axis and a first of the wheels being connected via an associated suspension linkage to the rocker member on one side of the pivot axis and a second of the wheels being connected via an associated suspension linkage to the rocker member on the other side of the pivot axis;   an actuator connected to the rocker member for rotating the rocker member about the pivot axis thereof, with rotation in one sense retracting the wheels to raised marine mode locations and with rotation in an opposite sense protracting the wheels to lowered land mode locations; wherein:   the suspension linkages each extend transversely across the vehicle to the rocker member which is mounted for rotation at a location lying inboard of the pair of wheels.       

   In a second aspect the present invention provides a wheel suspension and retraction system for an amphibious vehicle comprising:
         a pair of wheels each mounted for rotation on a hub assembly;   suspension linkages for the wheels connecting the hub assemblies to a common Z-shaped rocker member via spring and damper assemblies, one spring and damper assembly for each wheel, the pivot member having a central section through which passes a rocker axis of the rocker member, a first of the wheels being connected via an associated suspension linkage to one end section of the rocker member on a first side of the pivot axis and a second of the wheels being connected via an associated suspension linkage to the other end section of rocker member on the other side of the pivot axis; and   an actuator connected to the rocker member for rotating the rocker member about the pivot axis thereof, with rotation in one sense retracting the wheels to raised marine mode locations and with rotation in an opposite sense protracting the wheels to lowered land mode locations.       

   In a third aspect the present invention provides a wheel suspension and retraction system for an amphibious vehicle with a hull, the system comprising:
         a pair of wheels each mounted for rotation on a hub assembly;   suspension linkages for the wheels connecting the hub assemblies to a common rocker member via spring and damper assemblies, one spring and damper assembly for each wheel, with the rocker member mounted for rotation about a pivot axis and a first of the wheels being connected via an associated suspension linkage to the rocker member on one side of the pivot axis and a second of the wheels being connected via an associated suspension linkage to the rocker member on the other side of the pivot axis;   an actuator connected to the rocker member for rotating the rocker member about the pivot axis thereof, with rotation in one sense retracting the wheels to raised marine mode locations and with rotation in an opposite sense protracting the wheels to lowered land mode locations; wherein:   each suspension linkage comprises a suspension arm pivotally connected at a distal end, the end furthest from a vehicle centre line when the wheels are in their lowered land mode locations, to a hub assembly and attached at a proximal end to a rotatable suspension shaft for rotation with the shaft, the shaft also forming part of the suspension linkage; and   each rotatable suspension shaft extends through the hull and an annular seal provides a water resistant seal around the shaft as the shaft passes through the hull.       

   In a fourth aspect the present invention provides a wheel suspension and retraction system for an amphibious vehicle comprising:
         a pair of wheels each mounted for rotation on a hub assembly;   suspension linkages for the wheels connecting the hub assemblies to a common rocker member via spring and damper assemblies, one spring and damper assembly for each wheel, with the rocker member mounted for rotation about a pivot axis and a first of the wheels being connected via an associated suspension linkage to the rocker member on one side of the pivot axis and a second of the wheels being connected via an associated suspension linkage to the rocker member on the other side of the pivot axis; and   an actuator connected to the rocker member for rotating the rocker member about the pivot axis thereof, with rotation in one sense retracting its wheels to raised marine mode locations and with rotation in an opposite sense protracting the wheels to lowered land mode locations; wherein:   each suspension linkage comprises a suspension arm connected to a hub assembly, which suspension arm constrains the hub assembly to rotate about an axis of rotation running longitudinally fore and aft along the vehicle during wheel retraction and protraction.       

   In a fifth aspect the present invention provides a wheel suspension and retraction system for an amphibious vehicle comprising:
         a pair of wheels each mounted for rotation on a hub assembly;   suspension linkages for the wheels connecting the hub assemblies to a common rocker member via spring and damper assemblies, one spring and damper assembly for each wheel, with the rocker member mounted for rotation about a pivot axis and a first of the wheels being connected via an associated suspension linkage to the rocker member on one side of the pivot axis and a second of the wheels being connected via an associated suspension linkage to the rocker member on the other side of the pivot axis; and   an actuator connected to the rocker member for rotating the rocker member about the pivot axis thereof, with rotation in one sense retracting the wheels to raised marine mode locations and with rotation in an opposite sense protracting the wheels to lowered land mode locations; wherein:   the actuator is part of a powered mechanism for rotating the rocker member and is controlled by a driver-operated switch.       

   In a sixth aspect the present invention provides a wheel suspension and retraction system for an amphibious vehicle comprising:
         at least two wheels each mounted on a hub assembly; and   for each wheel a suspension linkage linking the associated hub assembly via a spring and damper assembly to an actuator individual to that wheel;   wherein each actuator is operable to retract the single wheel associated therewith from a lowered ground-engaging land mode position to a raised marine mode position.       

   In a seventh aspect the present invention provides a wheel suspension arrangement for an amphibious vehicle, the suspension arrangement having a wet side exposed in use to water through which the vehicle travels and a dry side shielded from exposure to the water, the suspension arrangement comprising:
         a swing arm having rotatably mounted thereon a wheel hub;   a shaft on which the swing arm is mounted, the shaft being rotatably mounted in the suspension assembly and the shaft and swing arm rotating together;   a spring and a damper both acting on the shaft; and   a barrier separating the wet side from the dry side; wherein:   the shaft extends through the barrier from the wet side to the dry side;   the swing arm is attached to a part of the shaft lying on the wet side of the barrier;   the spring and the damper are both attached to a part of the shaft lying on the dry side of the barrier; and   a seal provides a water resistant seal around the shaft as the shaft passes from the wet side to the dry side.       

   The invention is particularly applicable for sit-astride type amphibious vehicles. 
   These and other features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic view from the side of an amphibious vehicle in accordance with a preferred embodiment of the invention; 
       FIG. 2  is a schematic, part cross-sectional view from the front of part of an amphibious vehicle in accordance with another preferred embodiment of the invention, similar to that shown in  FIG. 1 , with the rear road wheels in an extended position; 
       FIG. 3  is a view similar to  FIG. 2 , showing the rear road wheels in a retracted position; 
       FIG. 4  is a schematic view from the rear of the wheel retraction arrangement for the rear axle of the amphibious vehicle shown in  FIGS. 2 and 3 , with the rear road wheels in an extended position; 
       FIG. 5  is a view similar to  FIG. 4 , showing the rear road wheels in a retracted position; 
       FIG. 6  is a schematic view from the above of the wheel retraction arrangement as shown in  FIG. 4 ; 
       FIG. 7  is a schematic view from the above of the wheel retraction arrangement as shown in  FIG. 5 ; 
       FIG. 8  is a schematic perspective view from the rear of the front axle of an amphibious vehicle in accordance with a further preferred embodiment of the invention, similar to that shown in  FIG. 1  and  FIGS. 2 to 7 ; 
       FIG. 9  is a schematic view from the front of the front axle arrangement shown in  FIG. 8 ; 
       FIG. 10  is a schematic view from above of a further embodiment of the wheel retraction arrangement according to the invention; and 
       FIG. 11  is a view similar to that of  FIG. 5 , showing a yet further embodiment of the wheel retraction arrangement according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an amphibious vehicle, in accordance with a preferred embodiment of the invention, is indicated generally at  2 . The amphibious vehicle  2  is of the “sit astride” type, of the kind similar to a motorcycle, jet-ski, quad bike or the like. 
   The amphibious vehicle  2  has a main body  14  and a hull  16 , which are formed separately and joined at a split line indicated at  18 . A driver&#39;s seat  22  is provided on the main body  14 , along with a passenger seat  24  astern of the driver&#39;s seat  22 . Although only a single passenger seat  24  is illustrated, more than one passenger seat may be provided. The axis of the seating is arranged on the longitudinal axis of the vehicle, or parallel thereto. Handlebars  26  are provided in front of the driver&#39;s seat  22 , with a windscreen  28  mounted in front of the handle bar  26 . 
   The lower surface of the hull  16  has a planing surface  20 , for marine travel. Mounted in the hull  16 , the amphibious vehicle  2  has a prime mover  4 , typically an internal combustion engine, to provide both marine drive and locomotion. The axis of the prime mover  4  may be along, or parallel to, the longitudinal axis of the vehicle  2 . 
   Marine drive is facilitated by marine drive means  6  mounted at the rear of the hull  16 , to the right as viewed in  FIG. 1 . This may be in the form of a jet-type propulsion arrangement, connected to the prime mover  4  by a marine transmission (not shown) or a propeller. The vehicle has a front pair of wheels on transversely opposite sides (i.e. the left and right sides) of the vehicle and a rear pair of wheels on transversely opposite sides of the vehicle. In the  FIG. 1  there can be seen wheels  8  and tyres  10 . Only one side of vehicle is shown in  FIG. 1 , although it will be understood that the other side is a mirror image. The rear wheels and/or the front wheels are driven by the prime mover  4  through a road transmission (not shown) mounted within the hull  16 . The wheels  8  and tyres  10  are shielded by wheel arches  12  provided on the hull  16 . 
   In use, during marine travel the vehicle  2  is configured to sit in water with the water line substantially in line with or below the split line  18  between the main body  14  and hull  16 . The vehicle  2  will sit substantially “nose up”, i.e. with the front end of the vehicle, to the left as viewed in  FIG. 1 , at a higher level than the rear end of the vehicle  2 . 
   Referring now to  FIGS. 2 and 3 , a schematic part cross-sectional view from the front of an amphibious vehicle is indicated generally at  100 . The vehicle  100  is in accordance with a preferred embodiment of the invention and is the same as or similar to that illustrated in  FIG. 1 . However, only the hull of the vehicle and associated components are illustrated, with particular emphasis on the rear axle of the vehicle. 
   The rear axle of the vehicle  100  incorporates two driven road wheels  104  supported by in-board wheel suspension arrangements indicated at  106  (described in more detail below with reference to  FIGS. 4 to 7 ). The wheel suspension arrangements are coupled to a wheel retraction mechanism, as will be described later. Hull slots  108  are formed as recesses on either side of the hull  102 , each having an internal wall  110  dividing the hull gap from the inboard side of the hull  102 . A portion of each wheel retraction arrangement  106  extends outboard from each hull slot  108 . The road wheels  104  are mounted such that they can be moved between a ground engaging, lowered, protracted position in which the wheels  104  are each aligned generally vertically, such that the hull  102  can be supported clear of the ground on the wheels  104  (as shown in  FIG. 2 ) and a retracted position in which they are elevated for use of the vehicle  100  on water and tilted relative to the vertical (as shown in  FIG. 3 ). The retractability of the wheels  104  helps the vehicle  100  to plane over water. 
   The hull  102  has a substantially V-shaped profile in cross-section over a majority of its length and has a lower surface  112 , which incorporates a planing surface for marine travel of the vehicle  100 . The lower surface  112  includes covers  114  provided on either side of the vehicle  100  for each hull slot  108 . Rods  116  connect the covers  114  to the wheel retraction arrangements, so that the covers  114  move with the wheel retraction arrangements during retraction or protraction of the road wheels  104 , as will be apparent from a comparison of  FIGS. 2 and 3 . The covers  114  provide a substantially smooth planing surface when the wheels are retracted, as shown in  FIG. 3 . In the retracted position, the covers  114  also reduce the amount of water entering the hull slots  108  during marine travel. 
   The hull  102  further includes a recess  118  which forms a water inlet for the jet drive of the vehicle, the periphery of which is also sealed for preventing the inboard passage of undesired moisture and detritus. 
   For the avoidance of doubt, references to “vertical” or “vertically” throughout this specification, including the claims, should be understood as relating to a direction from top to bottom (or vice versa) of the vehicle when it is supported by its wheels or by its hull in an orientation conventional for locomotion on land, rather than a direction that is necessarily orthogonal to the horizontal. 
   For ease of description, the wheel retraction and suspension arrangement for the rear axle arrangement shown in  FIGS. 2 and 3  shall now be described with reference to  FIGS. 4 to 7 . 
   The rear axle arrangement is indicated generally at  200  and includes a pair of suspension upright members  202 , each having a wheel hub  204  mounted thereon. Each upright member  202  and wheel hub  204  forms a wheel hub assembly. The term “upright” should be understood not to imply that the member extends vertically, but that it is the member to which upper and lower suspension arms are connected with their points of attachment spaced vertically. As can be seen in  FIGS. 4 and 5 , the suspension upright members  202  have drilled out portions, indicated at  206 , which reduce the overall weight of the rear axle  200 . An upper suspension arm  208  is pivotably connected to the upper end of each suspension upright  202  by a pivot  210 . The upper suspension arms  208  are substantially H-shaped, as can be seen most clearly in  FIG. 6 , which also shows that the upper end of the suspension uprights  202  are received between opposing limbs  212  at one end of a respective upper suspension arm  208 . The proximal end of each upper suspension arm  208 , closest to a centre line of the vehicle, is adapted to be pivotably connected to the chassis or hull of the vehicle, or to a subframe for the rear axle  200 , if provided, by means of bearings  213  formed in the other opposing limbs  215  of the upper suspension arms  208 , for receiving a pivot pin or the like. 
   A lower suspension arm  214  is pivotably connected to the lower end of each suspension upright  202  by means of a pivotal connection, as indicated at  217  in  FIGS. 4 and 5 . 
   As can be seen most clearly in  FIGS. 6 and 7 , a pair of torsion tubes (i.e. shafts which relay torsional loading)  216 ,  218  are arranged parallel to one another and to be parallel with the longitudinal axis of the vehicle. The lower suspension arms  214  are rotatably mounted about respective torsion tubes  216 ,  218 , so as to be orthogonal thereto and configured to rotate with said torsion tubes. Each torsion tube is rotatably mounted at one end in a bearing  900  externally mounted on a part of the hull surface forming the slot  108 . Each torsion tube at the other end passes through an annular seal  901  into the interior of the hull and is then supported in a bearing  902  inside the hull. 
   The arrangement ensures that each wheel suspension needs only one aperture through the hull below the waterline and this aperture can easily be sealed by an annular seal such as lip seal or a shaft seal and avoids the need for a diaphragm seal (which can be difficult to make watertight). If preferred the bearing  900  could also be mounted inboard of the hull, in which case the torsion tubes  216  and  218  would extend through two aligned apertures in the hull, with annular seals provided to seal around each aperture. 
   The annular seals could be inboard. A flange  219  is provided on each of the torsion tubes  216 ,  218 , (at the lower ends thereof as viewed in  FIGS. 6 and 7 ). These flanges  219  sealingly abut annular seals connected in respective walls of the hull gaps, in use, for preventing the passage of water etc to components inboard of the hull. Hence, in use, only a section of the torsion tubes  216 ,  218  extends across the respective hull gaps, between the two sealed connections described above. 
   Referring back to  FIGS. 4 and 5 , a dedicated spring and damper assembly, indicated generally at  224 , is provided for each road wheel. The spring and damper assemblies  224  are operatively coupled to a respective torsion tube  216 ,  218  by a link arm  220 . The lower end of each link arm  220  is fixedly coupled to the second end of a respective torsion tube  216 ,  218  (i.e. towards the bottom of  FIGS. 6 and 7 ) and the upper end of each link arm  220  is pivotably connected to a respective spring and damper assembly  224  by means of a pivotal connection  222 . 
   The two spring and damper assemblies  224  are mounted substantially parallel to one another when viewed in plan ( FIG. 6 ) and aligned substantially in the same horizontal plane when viewed from the rear of the hull ( FIG. 4 ). They extend transversely across the vehicle, i.e. from left to right. 
   The spring and damper assemblies  224  include a coil spring  226  disposed around a telescopic damper unit  228  having a push rod  230 . The components of the suspension arrangements  224  are of known construction and are not described in detail. 
   The inner ends of push-rods  230  are both pivotably connected to a common rocker member  232  by means of a pivotal connection  234 , visible in  FIGS. 6 and 7 . The pivotal connections  234  are contained in spherical bushings, known as Rose (RTM) joints. The rocker member  232  has a main body, substantially Z-shaped in plan view, as seen in  FIGS. 6 and 7 , which defines a pair of limbs or end sections  236 ,  238 . A rocker arm pivot shaft  240  extends through a central section of the rocker member  232 , orthogonal to the limbs  236 ,  238  and is aligned vertically within the vehicle in use; i.e. the rocker member  232  rotates in a substantially horizontal plane. The upper and lower ends of the shaft  240  are intended to be fixedly connected to the chassis or hull of the vehicle, or a to subframe for the rear axle  200 , if provided (for example as shown at  302  in  FIG. 8 , described below), and the main body of the rocker arm  232  is adapted to rotate about the shaft  240 . 
   The relationship between the rocker member  232  and spring and damper assemblies  224  is such that the rocker member  232  maintains the two spring and damper assemblies in a spaced apart relationship with respect to the longitudinal axis of the vehicle. 
   A hydraulic actuator indicated at  242  in  FIGS. 6 and 7 , is provided as part of a powered wheel retraction mechanism  200 . The actuator  242  has a double acting hydraulic cylinder  244  and a piston rod  246 . One end of the piston rod  246  is operatively received in the cylinder  244 , with the other end pivotably connected to the rocker member  232  by means of a pivotal connection  248  at the end of the rocker limb  236 . The distal end of the cylinder portion  244  is pivotably connected to a mounting member  250  by means of a pivot pin connection  252 , such that the cylinder portion  244  pivots through a small angle about pin connection  252  during retraction/protraction. The mounting member  250  is intended to be fixedly connected, in use, to the chassis or hull of the vehicle, or to a subframe for the rear axle  200 , if provided. However, mounting member  250  includes an internal pivot  254 , viewable in  FIG. 6 , so as to be rotatably adjustable about a vertical axis during installation, for example to set the ride height of the suspension. 
   As will be appreciated from  FIGS. 1 to 3 , the retraction means is located below the split line  18  between hull  16  and upper body  14 , and above the effective axis of rotation of the wheels for the rear axle during locomotion, such as in the protracted position shown in  FIG. 2 . The arrangement is particularly compact in the vertical plane, so as to be effectively accommodated in the space available in the hull of the amphibious vehicle. 
   The arrangement shown in  FIGS. 4 to 7 , in particular the retraction assembly  242 , is adapted for communication with a remote control means. In a preferred embodiment, an amphibious vehicle such as that shown in  FIG. 1 , or as illustrated and described with reference to  FIGS. 2 and 3 , and  FIGS. 4 to 7 , includes an on-board controller, not illustrated, which is located on the main body of the vehicle, in the vicinity of the driver&#39;s seat position. The controller is in communication with the retraction assembly  242 , for sending signals to actuate retraction or protraction of the road wheels, as required. The controller is also configured for controlling other aspects of the conversion from land mode to marine mode, or vice versa. 
   A typical operation of the preferred embodiment will now be described, with particular reference to  FIGS. 2 and 3 , and  FIGS. 4 to 7 . 
   If it is desired to move the wheels  104  from the protracted position shown in  FIG. 2  to the retracted position shown in  FIG. 3 , a control signal is communicated from the on-board controller to the wheel retraction assembly  242 , in particular to the hydraulic cylinder  244 . In response to the signal, the cylinder  244  acts to retract push rod  246  under hydraulic pressure from the extended position shown in  FIG. 6 . As the push rod  246  is retracted to the position shown in  FIG. 7 , the end of the limb  236  on the rocker member  232  is pulled in the direction of the cylinder  244 , to the right as viewed in  FIGS. 6 and 7 . As the push rod retracts, the rocker member  232  is caused to rotate about the shaft  240 , such that the ends of the limbs  236 ,  238  of the rocker arm  232  prescribe an arcuate, generally circular path. For at least part of this arcuate path, the end of limb  236 , and the various suspension components connected thereto, will move in a direction substantially parallel to the movement of actuator  244 . 
   Rotation of the rocker member  232  in this manner causes the spring and damper assemblies  224  to be pulled ‘inboard’, from the extended position shown in  FIG. 6  to the retracted position shown in  FIG. 7 . In the extended position, the spring and damper assemblies  224  are maintained by the rocker member  232  in a spaced apart relationship in both the longitudinal axis of the hull and in the transverse axis of the hull (from left to right as viewed in  FIG. 6 ). During the inboard movement referred to above, the suspension arrangements proscribe a shallow arc as they move from the transversely spaced relationship to a retracted position in which they partially overlap one another in said transverse direction (as can be seen in  FIG. 7 ). This ‘overlapping’ movement is particularly advantageous in reducing the space taken up by the suspension arrangements  224  across the body of the vehicle, resulting in a compact retracted configuration. 
   The arrangement of the components  224  and  232  defines a substantially Z-shaped mechanism, with the suspension arrangements  224  forming acute angles with the rocker member  232  when the suspension is retracted. The mechanism is hinged at its vertices so that the angle between a respective suspension arrangement  224  and the rocker member  232  becomes more acute during retraction of the actuator piston rod  246 , that is to say that the angle between the suspension arrangement  224  and the rocker member  232  decreases during retraction. The mechanism is such that the retraction of the actuator piston rod  246  causes the two spring and damper assemblies  224  to fold together, relative to the rocker member  232 , as can be seen from a comparison of  FIGS. 6 and 7 . 
   The inboard movement of the suspension arrangements  224  causes the upper ends of the link arms  220  to pivot inwardly with respect to the longitudinal axis of the vehicle, as viewed in  FIGS. 4 and 5 . Since the link arms are fixedly coupled to the torsion tubes  216 ,  218 , movement of the link arms  220  in this manner causes a simultaneous rotation of the torsion tubes  216 ,  218 . As viewed in  FIGS. 4 and 5 , the torsion tube  216  on the left hand side of the vehicle rotates in a clockwise direction, whereas the torsion tube  218  on the right hand side of the vehicle rotates in an anti-clockwise direction. 
   Rotation of the two torsion tubes  216 ,  218  is imparted to the lower suspension arms  214 , so that the outer ends move outwardly and upwardly with respect to the longitudinal axis of the vehicle, as will be apparent from a comparison of  FIGS. 4 and 5 . 
   The suspension upright members  202  are connected to the rest of the rear axle arrangement  200  through the upper and lower suspension arms  208 ,  214 , so that they move upwardly to the retracted position shown in  FIG. 5  with the movement of the lower suspension arms  214 . The upper suspension arms  208  are connected to the vehicle chassis, hull or support frame (whichever is appropriate) by pivot pins  213 , so that a first end of each support arm  208  is raised above the other respective end connected to the vehicle, to the position shown in  FIG. 5 , as the suspension upright members  202  move upwardly. 
   Since the wheel hubs  204  are connected to the suspension upright members  202 , the actuation of the retraction arrangement in the manner described above acts to move the rear wheels of the vehicle to a retracted position, as shown in  FIG. 3 . The wheel retraction arrangement is configured such that the wheels  104  arrive at their fully retracted position simultaneously with the arrival of the piston rod  246  being fully received in the hydraulic cylinder  244  of the retraction assembly  242 . With the wheels  104  in the retracted position, the vehicle is more suited to marine travel than with the wheels  104  in an extended or protracted position, due to the reduction of hydrodynamic drag as the wheels are lifted out of the water. 
   As described above, the retraction assembly  242  includes a member  232 , which is configured to rock about a pivot point  240 , with this movement being translated to the suspension upright members  202  for retracting or protracting both wheel hubs  204 , in a single operation. 
   As can be seen from  FIGS. 4 and 5 , retraction of the wheels causes the spring and damper assemblies  224  to move from an angled position relative to the horizontal to a substantially horizontal attitude, with the damper unit ends of the suspension arrangements being moved upwardly as they pivot with respect to their respective link arms  220 . Nevertheless, in the retracted position, the spring and damper arrangements  224  are maintained below the upper boundary of the hull, i.e. below the split line between the hull and body, in use. 
   If it is desired to move the wheels from the retracted position shown in  FIG. 3  to the extended position shown in  FIG. 2 , the on-board controller is operated to send a further signal to the retraction assembly  242 , to cause the push rod  246  to be extended from the cylinder  244  under hydraulic pressure. As will be understood, having regard for the above description of the wheels being retracted, actuation of the cylinder  244  in this way causes an opposite movement of the rocker member  232  to that described above, to push the two spring and damper assemblies  224  outboard, to rotate the torsion tubes  216 ,  218  in the opposite direction and thereby lower the suspension upright members  202 . The motion of each component in the rear axle arrangement  200  during protraction of the wheels in this manner will be readily apparent to the skilled addressee on the basis of the above description and is therefore not described in further detail. 
   In a further preferred embodiment of the invention, the wheels at the front axle of the amphibious vehicle, for example as shown in  FIG. 1  or described with reference to  FIGS. 2 and 3  and  4  to  7 , are also capable of being moved between retracted and extended positions. 
   A preferred front axle arrangement is shown in  FIGS. 8 and 9 , for illustrative purposes. Similar components to those described with reference to  FIGS. 2 to 7  are given the same reference numerals in  FIGS. 8 and 9  and will not be described in significant detail. 
   The front axle arrangement, indicated at  300 , is mounted on a suspension support frame or subframe structure  302 , which is configured for mounting in the hull of an amphibious vehicle, such as that shown in  FIG. 1 . As can be seen, the mounting member  250  for the retraction assembly  242  is fixedly bolted to the subframe  302  and is arranged substantially vertically, so that the actuator  244  is mounted horizontally within the subframe  302 , above the effective axis of rotation of the wheels of the vehicle. The upper suspension arms  208  are also connected to the subframe  302  for pivotable movement, by pivots  213  as shown in  FIGS. 5 and 6 . 
   A steering assembly, indicated at  304 , is also mounted on the support frame  302  in communication with the suspension upright members  202 , so as to steer the vehicle on land in a manner well known in the art. The steering assembly  304  includes a steering box input shaft  306 , rotatably mounted on the support frame  302 , for communication between the wheel hubs  204  and manually operated steering means, for example a handle bar or steering wheel type arrangement, attached to the main body of the amphibious vehicle. 
   As will be understood from  FIGS. 8 and 9 , the upper end of the suspension upright members  202  includes a trunnion mount  308 , which is used to rotatably mount upright  202  to upper arm  208 , for steering the front wheels (via hubs  204 ). Likewise, ball joints  310  are provided between the lower ends of the suspension upright members  202  and the lower suspension arms  214 , to facilitate steering of the wheel hubs  204 . 
   The general construction and operation of the wheel retraction and suspension arrangements shown in  FIGS. 8 and 9  is substantially similar to that shown in  FIGS. 2 to 7 , as will be appreciated by the skilled addressee, and therefore is not described further. 
     FIG. 10  shows an alternative arrangement of wheel retraction mechanism. In it a pair of hydraulic actuators  500  and  501  are used, one for each hub assembly  502 ,  503 . Each actuator  500  is pivotally mounted to a supporting structure by a pivotal connection  504 ,  505 . Each actuator is connected via a spring and damper assembly  506 ,  507  and a torsion tube  508 ,  509  to a lower suspension arm  510 ,  511  which rotates with the torsion tube  508 , 509 , thereby raising and lowering hub assemblies  502 ,  503  pivotally connected to the suspension arms  510 ,  511 . As described above, annular seals  512 ,  513  seal around the torsion tubes  508 ,  509  to prevent ingress of water into the interior of the vehicle. Each wheel suspension thereby has a wet side (outboard parts of the torsion tubes  508 , 509  and suspension arms e.g.  510 ,  511 ) and a dry side (inboard parts of the torsion tubes  508 , 509  and the spring and damper assemblies  506 ,  507 ). 
   Whilst above a hydraulic system of actuation is described, an electrical motor system (e.g. an arrangement of a threaded spindle engaged by a rotating nut) or a pneumatic cylinder system could be used.  FIG. 11  shows a view of an embodiment of the invention which uses an electric motor  600  instead of hydraulic actuators. The  FIG. 11  corresponds to  FIG. 5  previously described and components in common are given identical reference numerals. The electric motor  600  is mounted in the vehicle to have an output shaft  601  which forms the axis of rotation of the rocker shaft, with the rocker shaft rotating on rotation of shaft  601  by motor  600 . The motor  600  will be controlled by a driver-operated switch in the vehicle cabin. 
   It can be seen that the present invention provides compact and versatile wheel retraction and suspension arrangements for an amphibious vehicle. In the preferred embodiments, a single actuator is mounted horizontally within the vehicle, for lifting and lowering wheels on either side of the vehicle at each axle. The hydraulic wheel retraction arrangement is effectively isolated from the individual coil springs and dampers, which enables spring and damper rates to be tuned independently of the wheel retraction actuator. 
   Whereas the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention. Where three road wheels are provided, the retraction arrangements described above could be used for two transversely aligned wheels, and a separate system provided for the one remaining wheel. Accordingly, it is not intended that the invention be limited except by the appended claims.