WO1998047730A1

WO1998047730A1 - Improvements to vehicle suspension systems

Info

Publication number: WO1998047730A1
Application number: PCT/AU1998/000268
Authority: WO
Inventors: Christopher Brian Heyring; Richard Monk
Original assignee: Kinetic Limited
Priority date: 1997-04-17
Filing date: 1998-04-17
Publication date: 1998-10-29
Also published as: AUPO625897A0; JP2001520602A; EP0975478A1; CA2286103A1

Abstract

A suspension system for a vehicle having a body (1) supported on at least one forward pair of laterally spaced wheels and at least one rear pair of laterally spaced wheels, including suspension means for supporting the vehicle body above each said pair of wheels, said suspension means including front and rear fluid support means (4, 5) for supporting at least a portion of the load on the suspension means, at least one of the vehicle support means (4, 5) providing substantially zero roll stiffness for the vehicle, roll moment reaction means (8) for providing location of the body (1) about a substantially level roll attitude, and pitch control means (12) interconnecting the front and rear fluid support means (4, 5), wherein changes in fluid pressure within at least one of said fluid support means are transferable by the pitch control means to the other said fluid support means.

Description

IMPROVEMENTS TO VEHICLE SUSPENSION SYSTEMS

The present invention is generally directed to vehicle suspension systems, and is in particular directed to such systems which incorporate pitch control for the vehicle. It is preferable to provide some form of pitch control in a vehicle suspension system to improve the control of the pitch attitude of the vehicle body when the vehicle is travelling over bumps and uneven surfaces. For example, such pitch control is preferred when the vehicle must travel over speed humps on the road, or while travelling over sine wave profile surfaces, especially those of large amplitude and low frequency.

In the Applicant's co-pending international patent application No. PCT/AU97/00870, there is described a number of functionally related vehicle suspension systems. Each of the described suspension systems include support means for supporting the vehicle body while providing substantially zero roll and warp stiffness for the vehicle body. A separate roll attitude control mechanism having negligible warp stiffness provides the roll control for the vehicle body. The support means may be in the form of fluid actuators such as the airbag arrangement shown in Figure 4 of the above noted application. The fluid actuators could also be in the form of hydraulic rams. The suspension systems as described have no pitch coupling mechanism, and it would be advantageous to provide a greater degree of pitch control in such vehicle suspension systems.

It is therefore an object of the present invention to provide a vehicle suspension system of the type incorporating fluid support means including means for controlling the pitch attitude of the vehicle.

With this in mind, the present invention provides a suspension system for a vehicle having a body supported on at least one forward pair of laterally spaced wheels and at least one rear pair of laterally spaced wheels, including a suspension means for supporting the vehicle body above each said pair of wheels, said suspension means including front and rear fluid support means for supporting at least a portion of the load on the suspension means, at least one of the vehicle support means providing substantially zero roll stiffness for the vehicle, roll moment reaction means for providing location of the body about a substantially level roll attitude, and pitch control means interconnecting the front and rear fluid support means, wherein changes in fluid pressure within at least one of said fluid support means are transferable by the pitch control means to the other said fluid support means.

According to one preferred arrangement, the fluid support means may be in the form of a pair of laterally spaced fluid actuators, each having a fluid chamber therein. The fluid actuator may for example be an airbag or hydraulic ram arrangement. Each fluid actuator may be connected between a respective carrier member and the vehicle body. Fluid communication means may interconnect the fluid chambers of the laterally spaced pair of fluid actuators. The fluid communication means interconnecting each pair of fluid actuators may be in the form of a conduit means connecting the fluid actuators to allow fluid flow therebetween. The conduit means may include a manifold type arrangement to interconnect the fluid actuators and the front and rear support means through the pitch control means. If the fluid actuators are hydraulic, hydropneumatic accumulators may be provided in fluid communication with the fluid conduits or manifold to afford a variable amount of resilience to the support means.

According to another preferred arrangement, the fluid support means may be in the form of a single fluid actuator mechanically interconnected with a said laterally spaced pair of carrier members. The fluid actuator may be in the form of a single-acting hydraulic ram and may include an outer body, and a rod slidably accommodated within the body such that the fluid chamber formed varies in volume with relative displacement between the rod and the outer body. Alternatively, a piston may be provided on the rod and slidably accommodated within the body to thereby define a fluid chamber therein. Hydropneumatic accumulators may be provided to afford resilience to the support means. Alternatively, the fluid actuator may be in the form of an airbag arrangement with the fluid chamber provided by a flexible bag. A lever arm may rigidly extend substantially vertically from each carrier member, the lever arm swinging towards or away from the vehicle chassis as the carrier member is pivotally displaced. The piston of the fluid actuator may be mechanically connected to the lever arm of one carrier member, and the body of the fluid actuator may be mechanically connected with the lever arm of the other laterally spaced carrier member. The mechanical connection may be in the form of a rod extending from the piston and the actuator body which may be respectively pivotally connected to one of the lever arms. Such an arrangement is particularly useful where there are space restrictions under the vehicle, in part because only one fluid actuator is required for each pair of laterally spaced wheels and because the height requirement of this arrangement is relatively low.

The pitch control means may include a pitch controller in fluid communication with the front and rear fluid support means. In the arrangement where the fluid support means includes a pair of fluid actuators, the fluid actuators being interconnected by fluid communication means, the pitch control means may be in fluid communication with the fluid chamber of both of the fluid actuators, preferably via the fluid communication means. In the arrangement where the fluid support means for at least one end of the vehicle includes a single fluid actuator mechanically interconnected to laterally spaced carrier members, the pitch control means may be in direct fluid communication with the fluid chamber of the single fluid actuator.

The pitch controller may include at least one pair of fluid chambers, each chamber being respectively in fluid communication with one of the fluid support means. Pressure transfer means may be provided to transfer changes in fluid pressure within one fluid chamber of a pair of fluid chambers to the other fluid chambers of the said pair of fluid chambers within the pitch controller.

According to one preferred arrangement, the pitch controller may include a pair of fluid containers, each container being in fluid communication with the front and rear fluid support means respectively. The fluid containers may be in the form of flexible fluid bags housed in a series arrangement within a housing. A free piston may be provided between each of the fluid bags. Resilient means may substantially centre the free piston within the housing. The resilient means may typically be in the form of coil springs located on either side of the free piston within the housing thereby providing pitch support and stiffness.

The housing of the pitch controller may be supported on the chassis of the vehicle such that forces applied to the pitch controller are resolved to the vehicle chassis. Alternatively, the forces can be resolved into the housing or into a separate structure such as a subframe. The pitch controller may alternatively comprise a cylinder, divided into two major chambers by a dividing wall, each major chamber being further divided into two minor chambers by a movable piston. The respective movable pistons are interconnected by a rod forming a piston assembly. Two of the minor chambers which function inversely proportionally to each other are system chambers and may be connected to the support means, one to the front and one to the rear. The other two minor chambers are bump chambers and may contain pressurised fluid (such as air to provide resilience or hydraulic fluid which is displaced into hydropneumatic accumulators) or mechanical springs (such as coil springs or Belleville washers) to provide pitch support and stiffness. A number of other functionally equivalent embodiments are possible.

If all four of the minor chambers of the pitch controller are filled with hydraulic fluid, the pressure in one of the bump chambers may be regulated in dependence on the static pressure in the associated system chamber. This gives variable pitch stiffness related to vehicle load. The fluid volume in the other bump chamber may be adjusted by supplying and draining fluid using an additional valve with the supply arrangement provided for the bounce support means.

According to a further arrangement, the pitch controller may include a housing separated into two chambers by means of a flexible diaphragm. Each fluid chamber may be in fluid communication with a said fluid support means. Resilient means may be provided on either side of the flexible diaphragm to at least substantially centre the diaphragm within the housing when the pressure on either side of the diaphragm is substantially equal.

Valve means may be provided to control the fluid flow between the pitch controller and the fluid support means. In the preferred arrangement where the fluid support means includes a pair of fluid actuators connected by fluid communication means, the fluid communication means may also include valve means for controlling the fluid flow between the associated pair of fluid actuators. The valve means may be in the form of separate valves for controlling the fluid flow between each fluid actuator and the pitch controller. Alternatively, the valve means may be in the form of a single three way valve.

The pitch controller operates to transfer changes in fluid pressure within one fluid support means to the other fluid support means. For example, in the case of an automobile travelling on a road, when the front wheels of the vehicle hit a speed hump, both of the wheels will move upwards simultaneously relative to the body. This results in compression of the front actuator or pair of fluid actuators resulting in an increase in fluid pressure therein. This fluid pressure increase is transferred to the pitch controller which then transfers the fluid pressure increase to the rear actuator or pair of fluid actuators. The net result is that the rear wheels move downwardly thereby restricting the pitch of the vehicle as it travels over the speed hump.

It is however possible to fine control the pitch response by means of the valve means controlling the fluid flow between the fluid actuator or actuators and the pitch controller. For example, closing off the valve means to the rear actuator or actuators can be used to delay the fluid flow to the rear actuator or actuators. The timing of the downward movement of the rear wheels can therefore be controlled by means of the valve means controlling the fluid flow to the rear actuator or actuators. The pitch response of the vehicle suspension system will preferably be made to react more slowly when the vehicle is travelling slowly than in the case when the vehicle is travelling relatively faster. The above arrangement allows a slowing of the pitch response of the vehicle suspension system in these situations. This arrangement therefore allows the pitch response to be controlled as a function of the vehicle speed. The valve means may alternatively or additionally be used to provide damping of the pitch motion of the vehicle by providing a multistep or variable restriction depending on wheel velocities and/or on fluid velocities through the system. The increased fluid pressure can also be retained within the pitch controller by closing off the valve means to the pitch controller. The release of fluid from the pitch controller can then be controlled by opening the valve means to thereby allow the pitch attitude of the vehicle to be varied at any time. This can be used for example, to effectively damp pitch motions by capturing a volume of pressurised fluid during a pitch motion using the valve means and releasing said pressurised fluid out of phase with the pitch motions. Changes in fluid pressure generated by the suspension system resiliently deflecting due to pitch motions of the wheels with respect to the vehicle body can thereby be stored, then used to react against such pitch motions.

In order to sense pitch motions of the vehicle body, two longitudinally spaced vertical accelerometers may be used. Alternatively, if roll motions are also required to be sensed as described in the previously noted international patent application No. PCT/AU97/00870, pitch may be sensed through two vertically spaced longitudinal accelerometers, placed in similar positions to the vertically spaced lateral roll accelerometers. Three vertical accelerometers could be substituted at three corners of the vehicle, to sense roll and pitch. Additional accelerometers and wheel position inputs may be used to differentiate between different types of pitch to enable the controller to respond in a more appropriate way, for example, to squat and dive compared to pitch due to varying ground surface inputs.

In the simplest form of squat and dive control, not requiring accelerometers, a switch on the throttle and one on the brake can be used to close valves in the lines to the pitch controller for limiting squat and dive, with passive dampers provided in the lines to the pitch controller to damp other pitch motions. In this case, for example during braking, two modes may be provided. In the first, to limit the jacking of the rear of the vehicle, the conduit between the rear support means and the pitch controller may be blocked. In pneumatic systems, this may still leave the front support means at a softer spring rate than desired, so a second mode may be desirable to further limit brake dive wherein the conduit between the front support means and the pitch controller may also be blocked. Similarly two modes of squat control due to acceleration may be provided. Pressure transducers sensing fluid pressures, in the front and rear support means for example, may be used in place of or in addition to pitch sensing accelerometers.

The pitch control may be further tailored to vehicle type, for example to take into account typical vehicle uses to achieve the best bias towards the type of terrain the vehicle is usually operated over.

More physical characteristics may be used to time the responses of the valves correctly or change the pitch damping by modifying the restriction provided by the valves. These characteristics could for example include the wheelbase of the vehicle, its centre of mass and even its payload, if pressure transducers are provided. The control system may be tailored to the physical characteristics of the unsprung parts of the vehicle, such as component masses and tyre stiffness to reduce vibrations at the natural frequencies of the respective wheel assemblies. The sensing system may be developed to enable analysis of the frequencies of unsprung mass oscillations about their known natural frequencies, and the valves operated to reduce effects such as "wheel hop". Similarly, whole body oscillations may be analysed and controlled.

The pitch attitude control can be modified dependent on the mode of use, either as sensed by the control system or as selected by a driver operated switch such as a ride height selector.

The present invention has a number of benefits as follows:

1. The pitch resilience of the vehicle can be increased without compromising the roll control and handling of the vehicle;

2. Large amplitude pitch motions can be damped with little detriment to the vehicle ride quality;

3. The pitch attitude of the vehicle can be adjusted without affecting cross axle articulation of the vehicle wheels; and

4. A pitch moment caused by the front wheels being displaced by a bump, is countered reactively by the front to rear coupling transferring the force change on the front wheels to the rear wheels, thereby cancelling out the pitch moment.

It will be convenient to further describe the invention by reference to the accompanying drawings which illustrate a possible arrangement of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. Figure 1 is a perspective view of a first preferred arrangement of a vehicle suspension system according to the present invention;

Figure 2 is a perspective view of a second preferred arrangement of a vehicle suspension system according to the present invention;

Figure 3 is a cross-section view through a second pitch controller; Figure 4 is a perspective view of a third preferred arrangement of the vehicle suspension system according to the present invention;

Figure 5 is a cross-section view through a third pitch controller; and

Figure 6 is a schematic view of a fourth preferred arrangement of the fluid support means for a vehicle suspension system according to the present invention.

Referring initially to Figure 1 , there is shown a vehicle chassis 1 with a pair of laterally spaced front carrier members 2 and a pair of laterally spaced rear carrier members 3 hingedly supported on either side of the vehicle chassis 1. Only the carrier members 2, 3 on the left hand side of the vehicle chassis 1 are clearly visible in Figure 1. Wheels (not shown) can be supported on the carrier members 2, 3.

The vehicle suspension system includes a front fluid support means 4 and a rear fluid support means 5. Each fluid support means 4, 5 includes a pair of fluid actuators 6, which are in the form of an airbag arrangement in Figure 1 , and a fluid conduit 7 interconnecting each pair of laterally spaced fluid actuators 6. Each fluid actuator 6 includes a fluid chamber (not shown), and the fluid conduit 7 provides fluid communication between the fluid chambers of the fluid actuators 6.

A roll control arrangement 8 is also provided in the vehicle suspension system. This roll control arrangement 8 includes a pair of stabiliser bars 9, a pair of double acting hydraulic rams 10, the fluid chambers of each double acting hydraulic ram 10 being in fluid communication by means of a pair of secondary fluid conduits 1 1. The operation of the roll control arrangement 8 is described in the Applicant's co-pending international patent application No. PCT/AU97/00870, and details are incorporated herein by reference. Other free warp, roll control arrangements detailed in the previously noted patent application No. PCT/AU 97/00870 may alternatively be used.

Similarly, separate individual support means providing roll stiffness may be used in combination with the fluid support means having substantially zero roll stiffness as disclosed in the previously noted international patent application. For example coil springs may be used in parallel with the air bags (or other fluid support means).

The vehicle suspension system according to the present invention includes a pitch control means 12. In the arrangement shown in Figure 1 , the pitch control means 12 includes a pitch controller 13 having a front fluid bag 14 and a rear fluid bag 15. The front fluid bag 14 is in fluid communication with the front fluid support means 4 by means of a front conduit 16 providing fluid communication between the front fluid bag 14 and the fluid conduit 7 interconnecting the front fluid actuators 6. The rear fluid bag 15 is similarly in fluid communication with the rear fluid support means 5 via a rear conduit 17 connected to the fluid conduit 7 of the rear fluid support means 5. A free piston 18 is located between the front fluid bag 14 and the rear fluid bag 15. Coil springs 19 are provided on either side of the free piston 18 to substantially centre the free piston 18. The pitch controller 12 is supported on the chassis by means of brackets 20 therefore, the forces applied to the pitch controller 12 are resolved to the vehicle chassis 1. Valve means may also be provided to control the fluid flow to and from the pitch controller 12 and between the fluid actuators 6. The valve means may be in the form shown with respect to the rear fluid support means 5 including a valve 21 located on the rear conduit 17 connecting the pitch controller 12 to the fluid conduit 7 of the rear fluid support means 5. This valve 21 controls the fluid flow to and from the pitch controller 13 to the fluid support means 5. Further valves 22 may be provided between each fluid actuator 6 and the pitch controller conduit 17 on the fluid conduit 7 to control the fluid flow between the fluid actuators 6 and between the fluid actuators 6 and the pitch controller 12. These valves may include a fixed restriction, multi-stage damper, variable restriction and/or a lockout valve so that the pitch operation of the suspension system may be damped or controlled. Alternatively, as shown with respect to the front fluid support means 4, the valve means may be in the form of a single three way valve 23 located at the intersection between the front conduit 16 of the pitch control means 12 and the fluid conduit 7 of the front fluid support means 4.

Figure 2 shows an alternative arrangement of a vehicle suspension system according to the present invention, and components corresponding to components shown in the vehicle suspension system of Figure 1 are designated with the same reference numeral for clarity purposes. The principal differences are that the fluid actuators are in the form of hydraulic rams and the pitch controller 13 is now in the form of an enclosed housing 25 separated into a front chamber and rear chamber by a flexible diaphragm (not shown). The front chamber is in fluid communication with the front fluid support means 4 by means of the front conduit 16 whereas the rear chamber is in fluid communication with the rear fluid support means 5 by means of the rear conduit 17. The resilience for the front and rear fluid support means is provided by hydropneumatic accumulators 24. Only one accumulator is illustrated for each fluid support means, however it may be desirable to provide more. For example, it can be desirable to provide a heavily damped accumulator near each actuator, and an underdamped accumulator in the centre of the lateral conduit which may be locked off during certain manoeuvres. Alternatively, two accumulators may be provided in the centre of each conduit, one of which may be locked off as required.

Figure 3 is a cross-section through the housing of the pitch controller shown in Figure 2. The fluid from the front and rear support means is separated by a flexible diaphragm 26, with springs 27 and 28 resiliently centring the diaphragm in the housing. The springs 27 and 28 provide a controlled amount of resilience in the pitch direction and tend to maintain the level attitude of the vehicle in the pitch direction. Figure 4 shows a variation of the vehicle suspension system of Figure 2, with the rear fluid support means 5 being replaced by a single hydraulic ram 29 having a cylinder head 30 and a barrel 31 enclosing a piston (not shown) for defining a fluid chamber (not shown) therein. Each of the rear carrier members 3 further include a lever arm 32 extending from the carrier member 3. Pivotal movement of the carrier member 3 results in movement of the lever arm 32 towards or away from the vehicle chassis 1. A piston rod 33 extends from the piston, the piston rod 33 being hingedly connected with the lever arm 32 of one of the carrier members 3. A further rod 34 extends from the head 30 of the hydraulic ram 29, this further rod 34 being hingedly connected to the lever arm 32 of the opposing laterally spaced carrier member 3. Pin joints 35 can therefore be provided between the piston rod 33 and the associated lever arm 32 and between the further rod 34 and the associated lever arm 32. As can be clearly seen from Figure 4, the arrangement of the rear fluid support means 5 uses less space than the arrangement of the front fluid support means 4. This arrangement is therefore advantageous where there are packaging limitations within the vehicle.

An alternative pitch controller 37 is also shown in Figure 4. A cross- section through this pitch controller 37 is shown in Figure 5. Its operation is similar to that of the diaphragm-type pitch controller of Figure 3, with a free piston 38 being substituted for the diaphragm. The piston runs inside a cylinder 39 and preferably includes a sealing arrangement (not shown) to substantially prevent transfer of fluid from the front to the rear fluid support means.

Figure 6 is a schematic representation of a further possible form of pitch controller incorporating hydropneumatic pitch resilience allowing increased control of the pitch stiffness of the vehicle, which may for example vary with load. The fluid actuators 6 are hydraulic cylinders, laterally interconnected by fluid conduits 7, as in Figure 2. As in Figure 2 the actuators may be used as the sole form of bounce support for the vehicle body, or used in series or more preferably in parallel with independent support means for each wheel (which provide a degree of roll stiffness). Similarly, hydropneumatic accumulators 24 provide resilience for the front and rear fluid support means. The number, position and control of these accumulators may be as previously discussed for Figure 2.

The pitch controller is interposed between the front and rear fluid support means and is in fluid communication with both via conduits 16 and 17 respectively. A valve block 30 is located between the pitch controller and one of the front or rear fluid support means. The valve block may include a fixed, restriction, multi-stage damper, variable restriction and/or a lockout valve so that the pitch operation of the suspension system may be damped or controlled.

The pitch controller comprises two axially aligned major chambers 40 and 41 , each divided respectively into two minor chambers 42, 43 and 44, 45 by respective pistons 46 and 47. The pistons are interconnected by a rod forming a piston assembly. Two of the minor chambers which vary in volume in opposite senses with motion of the piston assembly are system chambers and are in fluid communication with the front and rear fluid support means respectively. For example they could be 42 and 43, or 43 and 44, but in Figure 6 they are shown as 42 and 45. Chamber 42 is the front system chamber and 45, the rear system chamber. The remaining two minor chambers are connected to accumulators 50 and 51 via conduits 48 and 49 thereby providing resilience for the motion of the piston assembly and centre the assembly within the pitch controller under static conditions. Chamber 43 (the front bump chamber) is compressed when the front wheels of the vehicle encounter a bump, the resilience of the associated accumulator 50 determining the force transfer front to rear. Similarly, the chamber 44 is the rear bump chamber. The valve 30 may be located in either of conduits 48 and 49 instead of in conduits 16 or 17.

The relative design volumes of the minor chambers when the piston assembly centralised may be varied by changing the position of the system and bump chambers and varying the diameter of the piston rod and of the bores of the two major chambers independently of one another.

To compensate at least in part for load change on the vehicle, the pressure in one of the bump chambers may be related to the pressure in the at least one of the fluid support means. For example, there may be a very low speed bleed orifice between the rear bump chamber 44 and the rear system chamber 45, thereby increasing the pressure in the rear bump chamber with increased load on the rear of the vehicle. The piston assembly may be centralised by adjusting the fluid volume in the other bump chamber, using either electrical or mechanical control. For example, the piston assembly may actuate a spool valve (which may be an integral part of one of the chambers) to communicate the desired bump chamber with either a source of pressurised fluid or a fluid drain as required. This communication may be highly restricted to prevent significant fluid changes during normal dynamic operation of the suspension system.

During operation of the vehicle suspension system, fluid flows between the front fluid support means 4 and the pitch controller 13 and between the rear fluid support means 5 and the pitch controller 13. The pitch controller 13 acts to transfer changes in fluid pressure within one of the fluid support means 4, 5 to the other of the fluid support means 4, 5. This arrangement therefore provides for improved pitch control for the vehicle.

Claims

CLAIMS:

1. A suspension system for a vehicle having a body supported on at least one forward pair of laterally spaced wheels and at least one rear pair of laterally spaced wheels, including suspension means for supporting the vehicle body above each said pair of wheels, said suspension means including front and rear fluid support means for supporting at least a portion of the load on the suspension means, at least one of the vehicle support means providing substantially zero roll stiffness for the vehicle, roll moment reaction means for providing location of the body about a substantially level roll attitude, and pitch control means interconnecting the front and rear fluid support means, wherein changes in fluid pressure within at least one of said fluid support means are transferable by the pitch control means to the other said fluid support means.

2. A suspension system according to claim 1 , wherein the pitch control means includes a pitch controller supported on the vehicle body and having at least one pair of fluid chambers, each fluid chamber being in fluid communication with one of the fluid support means, and a pressure transfer means for transferring changes in fluid pressure within one said fluid chamber to the other said fluid chamber.

3. A suspension system according to claim 2 , wherein each fluid chamber is provided by a flexible fluid container respectively mounted at one end thereof to the vehicle body by mounting means, and the pressure transfer means includes a free piston located between the fluid containers and supported by resilient means on opposing sides thereof for at least substantially centring the free piston between the mounting means when the vehicle is in a static position.

4. A suspension system according to claim 2, wherein the pitch controller includes a housing separated into two said fluid chambers by a flexible diaphragm located therein, resilient means being provided on opposing sides of the diaphragm for at least substantially centring the diaphragm within the housing when the vehicle is in a static condition.

5. A suspension system according to claim 2, wherein the pitch controller includes a housing separated into two said fluid chambers by a free piston, with resilient means being provided on opposing sides of the free piston for at least substantially centring the free piston within the housing when the vehicle is in a static condition.

6. A suspension system according to claim 2, wherein the pitch controller includes a housing divided into the two said fluid chambers by a dividing wall, a movable piston being located within each fluid chamber to divide said fluid chamber into two minor chambers, two of the minor chambers being connected to the fluid support means, the pistons being rigidly interconnected to thereby move in unison.

7. A suspension system according to claim 6, wherein resilient means are provided in the other two said minor chambers for providing pitch support and stiffness.

8. A suspension system according to claim 6, further including accumulator means in fluid communication with each said fluid chamber.

9. A suspension system according to any one of the preceding claims, wherein each wheel is supported on the body of the vehicle by a carrier member, at least one of the fluid support means including a pair of laterally spaced single acting fluid actuators, each fluid actuator being located between a said carrier member and the vehicle body, and a conduit means for providing fluid communication between the fluid actuators.

10. A suspension system according to claim 9, wherein the pitch controller is in fluid communication with the conduit means of the fluid support means.

11. A suspension system according to claim 10, further including valve means for controlling the fluid flow between the pitch controller and the conduit means.