Abstract:
A roll control system for a vehicle suspension system and a method for controlling said control system, the suspension system including support means, the roll control system including: wheel cylinders each including first and second chambers; and first and second fluid circuits, each said fluid circuit providing fluid communication between the said first chambers on one side of the vehicle and the second chambers on the opposite side of the vehicle by fluid conduits to thereby provide roll support decoupled from a warp mode of the vehicle suspension system by providing a roll stiffness about a level roll attitude whilst simultaneously providing substantially zero warp stiffness; and the method including bypassing fluid flow from at least a substantial portion of the conduits during predetermined wheel inputs to the control system to thereby minimize line damping and/or fluid inertia effects on the damping of the control system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention is generally directed to vehicle suspension systems, and in particular to vehicle suspension systems incorporating improved passive ride control. 
   BACKGROUND 
   The desire for improved ride control in motor vehicles has: lead to the development of “active” vehicle suspension systems. Such systems typically use sensors to sense the various ride characteristics of the vehicle, the sensors providing signals to an Electronic Control Unit (ECU). The sensors sense any excessive roll, pitch, four wheel bounce and warp motions of the vehicle and its wheels, and the ECU seeks to actively compensate for this motion by controlling the supply of high pressure fluid from a fluid pump to different actuators acting within the vehicle suspension system, or by controlling the return of high pressure fluid from the actuators to a fluid reservoir. (The warp mode of a suspension system, also known as cross axle articulation, is defined as when one pair of diagonally spaced wheels together move in the opposite vertical direction to the other pair of diagonally spaced wheels with respect to the vehicle body). Active suspension systems which attempt to control all the above-noted ride characteristics are very expensive and complicated and have therefore not proven to be commercially viable. Simpler active systems which only seek to actively control excessive roll motions of the vehicle have therefore also been developed. Similarly, adaptive damping systems are becoming popular as they can be used to influence vehicle motions such as roll, pitch and whole body bounce by changing the damping rates at each wheel without the need for a pump. 
   All the known active suspension systems however have a number of problems which have prevented commercial acceptance of such systems except in luxury vehicles. The number of components required for such systems have lead to packaging difficulties, with the limited space available for such systems under existing motor vehicles. The complexity of active suspension systems and the high stresses applied to certain components of the system lead to ongoing reliability issues. Furthermore, active systems typically require a large number of components, some of which are specially produced components that can handle high mechanical stresses leading to high manufacturing costs. Also, expensive high pressure and high speed components are typically used in such systems, resulting in relatively higher manufacturing and running costs for active systems when compared with conventional suspension systems. Another disadvantage of active systems is the poor response times generally associated with production feasible versions of such systems. Valves are generally used to control the fluid flow in the system. There is always a certain delay before a valve can be actuated to allow or prevent fluid flow. This delay, together with other delays caused by inadequately defined algorithms controlling the system, can lead to unacceptably poor response times for the active suspension system. Active roll control systems typically respond too slowly when undergoing a quick slalom test for example, the control system being unable to provide adequate control under large changes of inertia. 
   The Applicant has developed a number of different vehicle system systems which seek to avoid at least some of the problems associated with active suspension systems while providing substantial improvements in the ride of a vehicle. These systems are “passive” and do not require sensors, ECUs or fluid pumps to operate. Such systems are described in Australian Patents 670034, 694762, 671592 and 699388 and International Application No. PCT/AU97/00870, details of which are incorporated herein by reference. These systems do however generally rely on components adapted to handle high pressure fluid. 
   Adaptive damping systems have been developed specifically to improve the damping function of a vehicle suspension system. These damping systems only require relatively low pressure components when compared with those required in the previously described systems, but provide substantially no roll stiffness. They generally have electrically variable or switchable orifices and preloads which are controlled to provide more appropriate damper forces in a range of predefined conditions to avoid the compromises of a single setting to suit all conditions. 
   In U.S. Pat. Nos. 5,486,018 and 5,584,498 (Yamaha), there are described interconnected damper systems where the top chamber of at least one pair of laterally or longitudinally adjacent dampers, commonly known as Oshock absorbers&#39; are connected by a conduit. A number of arrangements are disclosed, providing a range of damping effects. However, none of the arrangements are designed to provide a roll stiffness for the suspension. 
   In U.S. Pat. No. 4,606,551 (Alfa), there is described an arrangement having dampers, each having an upper and lower chamber. At least one pair of laterally or longitudinally adjacent dampers are connected by conduits respectively connecting the upper chamber of one damper with the lower chamber of the other chamber. A number of damper valves are provided in the connecting conduits to provide various damping effects. No electronic control is required, nor can the arrangement provide a roll stiffness for the suspension. 
   Although each of the above described adaptive and interconnected damping systems provide an improved damping function over conventional damper arrangements, they do not provide any or only provide minimal control of other ride characteristics of the vehicle. For example, none of the above adaptive or interconnected damping systems provide roll support for the vehicle as they do not have any roll stiffness to enable a degree of roll control for the vehicle, only roll damping. These systems can therefore not be used to provide roll control for the vehicle. 
   SUMMARY 
   It is therefore an object of the present invention to provide a damping and roll control system which achieves improved ride control for the vehicle while avoiding at least one of the problems associated with prior art systems. 
   With this in mind, the present invention provides a damping and roll control system for a vehicle suspension system, the vehicle having at least one pair of laterally spaced front wheel assemblies and at least one pair of laterally spaced rear wheel assemblies, each wheel assembly including a wheel and a wheel mounting locating the wheel to permit movement of the wheel in a generally vertical direction relative to a body of the vehicle, and vehicle support means for providing at least substantially a major portion of the support for the vehicle; the roll control system including: 
   wheel cylinders respectively locatable between each wheel mounting and the body of the vehicle, each wheel cylinder including an inner volume separated into first and second chambers by a piston supported within the wheel cylinder; first and second fluid circuits respectively providing fluid communication between the wheel cylinders by fluid conduits, each said fluid circuit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle and the second chambers of the wheel cylinders on the opposite side of the vehicle to thereby provide roll support decoupled from the warp mode of the vehicle suspension system by providing a roll stiffness about a level roll attitude whilst simultaneously providing substantially zero warp stiffness; 
   each fluid circuit including one or more fluid accumulators for providing roll resilience; 
   damper means for controlling the rate of fluid flow out of or into at least one chamber of each wheel cylinder; and 
   selection means for selectively providing fluid communication between the first and second fluid circuit; 
   the damping and roll control system thereby providing substantially all of the damping of the vehicle suspension system. 
   The vehicle support means may in certain embodiments of the present invention provide at least substantially all of the support for the vehicle. 
   The damping and roll control system therefore provides damping for the vehicle suspension and provides a roll stiffness without introducing a corresponding warp stiffness. 
   Each fluid circuit may in one preferred embodiment include a first fluid conduit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle; and a second fluid conduit providing fluid communication between the second chambers of the wheel cylinders on the opposite side of the vehicle; the first and second fluid conduits being in fluid communication. 
   According to another preferred embodiment, each fluid circuit may include first and second diagonal fluid conduits, each respectively providing fluid communication between the first chamber of one wheel cylinder on one side of the vehicle and the second chamber of the diagonally opposite wheel cylinder on the other side of the vehicle; the first diagonal fluid conduit between one pair of diagonally opposite wheel cylinders being in fluid communication with the second diagonal fluid conduit between the other pair of diagonally opposite wheel cylinders. 
   According to yet another preferred embodiment, each fluid circuit may include a front fluid conduit providing fluid communication between the wheel cylinders of the front wheel assemblies, and a rear fluid conduit providing fluid communication between the wheel cylinders of the rear wheel assemblies, with the front and rear conduits respectively providing fluid communication between the first chamber of the wheel cylinder at one side of the vehicle with the second chamber of the wheel cylinder at the opposite side of the vehicle, the front and rear conduits being in fluid communication. 
   It is to be appreciated that other connection arrangements are also envisaged. It is also to be appreciated that the same principles may be applied to vehicles with more than four wheels. For example, to apply the system to a six wheeled vehicle, the additional left hand wheel cylinder will have its first chamber connected to the conduit connecting the first chambers of the other two left hand wheel cylinders, and its second chamber connected to the conduit connecting the second chambers of the other two left hand wheel cylinders. The connection of the other cylinder to the right hand side of the vehicle similarly communicates first chambers together and second chambers together. 
   The damper means may be located at or in the wheel cylinders, in the conduits, and/or in a manifold block. The manifold block may be centrally located in the vehicle and may provide the required fluid communication between the first and second conduits to form the first and second fluid circuits. The damper means may be a bidirectional valve (i.e. provide controlled flow restriction in both directions), in which case each wheel cylinder requires only one damper valve for one of the first or second chambers. In this case, the associated chamber may try to suck a vacuum if the damper valve is not supplying fluid at the same rate at it is being demanded. This can lead to aeration of the fluid and potential loss of ride control by the system. To avoid this effect, a single direction damper valve may be used to ensure that the wheel cylinder chambers only act through a damper valve when expelling fluid, thereby preventing fluid aeration in the cylinder chambers. Alternatively, the single direction damper valve may be used in parallel with a non-return valve. Alternatively, to provide large damping forces with reliable, compact damper vase means, a bidirectional damper means may be provided for each of the first and second chambers of at least one pair of laterally spaced wheel cylinders. 
   Each said fluid circuit includes at least a first fluid accumulator to allow for changes in the fluid volume of each circuit to thereby provide roll resilience. Also, if a wheel cylinder with differing effective piston areas between the first and second chambers is used (for example a piston having a rod extending from one side only, as in a conventional damper cylinder assembly), the accumulator needs to be able to accommodate the rod volume changes within the system during bounce motions of the suspension. In this case, in roll, the accumulator absorbs a much greater change of fluid volume per unit displacement of the wheel cylinders than it absorbs in bounce as both the effective areas of a first chamber side and a second chamber side are working to displace fluid into the accumulator giving a correspondingly higher stiffness for roll motions of the roll control system than for bounce motions. 
   Each fluid circuit may include at least one second fluid accumulator to provide increased roll resilience. Between each second accumulator and the respective fluid circuit there may be a roll resilience switching valve. When the vehicle is travelling in a straight line, the valve may be held open to allow the second accumulators to communicate with the associated fluid circuits to provide additional roll resilience, thereby further improving ride comfort. When turning of the vehicle is detected, the roll resilience switching valve is closed to provide a desirable increase in roll stiffness during cornering. The detection of vehicle cornering may be performed in any known manner, using inputs for conditions such as steering rate of change, steering angle, lateral acceleration and vehicle speed. Any or all of these sensors and/or others not cited may be used. 
   The accumulators may be of the gas or mechanically sprung piston type or the diaphragm type and either or both can be beneficial in increasing the time to maintenance of the system by replenishing fluid lost from the system through leaks past rod seals and out of fittings. Any fluid loss should be minimal, therefore the effect on the operating pressure of the system may be negligible. 
   At least one of the accumulators in each fluid circuit may have a damper means to control the rate of fluid flow into and/or out of the accumulator. Due to the higher rate of fluid flow into and out of the accumulators in roll when compared to bounce (as discussed earlier), the effect of the accumulator dampers is greater in roll than in bounce giving a desirable high roll damping to bounce damping ratio. If the accumulators are not damped, the roll damping is determined by the bounce damping, as is the case when using conventional dampers. 
   Damping the accumulators can also have a detrimental effect to single wheel input harshness as single wheel inputs are also heavily damped by accumulator dampers. To increase comfort in straight line running, it can therefore be advantageous to provide a bypass passage around the accumulator damper valve to permit fluid to bypass the damper for at least one accumulator. The bypass passage includes a valve to open or close the passage. During turning, the valve is in the closed position and the accumulator damper valves are providing high roil damping. In straight line running, the valve is open to reduce the roll and single wheel input damping forces in the system. The roll control system may have a pressure precharge to allow the accumulators to function and supply fluid in rebound motions of the wheels (where they fall away from the vehicle body). This precharge is preferably about 20 bar for the roll control system with the vehicle at standard unladen ride height. 
   It may be preferable to use a wheel cylinder design with a rod protruding from one side of the piston through only one chamber. This allows for a simple and cheap cylinder design, but any system precharge pressure acting over the unequal effective piston areas in the first and second chambers produces a net cylinder force. This force may provide some support of the vehicle body although the proportion of vehicle load supported by the roll control system is usually very small and is similar to the degree of support provided by a conventional precharged damper cylinder assembly. The exact amount is determined by the cylinder rod and bore dimensions, system precharge pressure and cylinder to wheel hub lever ratio. 
   For example, in the case where the first chamber of each wheel cylinder is in compression as the wheels move upwardly with respect to the vehicle body, and the effective area of the piston on the first chamber side is larger than the effective area of said piston on the second chamber side, thereby providing a degree of support of the vehicle body. 
   If accumulators with a non-linear spring function (ie. a hydropneumatic accumulator which has an increasing stiffness in compression and a decreasing stiffness in rebound) are used and the roll control system provides a degree of vehicle support (as outlined above), then as the vehicle rolls due to lateral acceleration, the total volume of fluid in the accumulators can decrease overall, increasing the fluid volume in the roll control system and causing an overall increase in vehicle height (known as “roll jacking”). The degree of vehicle support provided by the roll control system influences the degree of roll jacking. 
   It may be desirable to produce the inverse of the roll jacking effect such that the average height of the vehicle is lowered during cornering. This effect can be produced in the case where the first chamber of each wheel cylinder is in compression as the wheels move upwardly with respect to the vehicle body, and the effective area of the piston on the second chamber side is larger than the effective area of said piston on the first chamber side, thereby providing a degree of additional load on the vehicle support means, tending to push the vehicle down towards the ground. 
   Preferably, a simpler arrangement may be used with the cheaper cylinder design which provides vehicle support (discussed above). The resilient means in the first accumulator may include one or more mechanical springs such that the spring rate in the compression direction from the normal static position is lower than the spring rate in the rebound direction from the normal static position, to thereby give the reverse effect of a conventional hydropneumatic accumulator and lower the average height of the vehicle during cornering. Additionally or alternatively, the rebound damping rate of the accumulators may be higher than the compression damping rate to provide a similar vehicle lowering effect and better response to steering Inputs during initial cornering (turn-in). Indeed, only rebound damping may be provided for the accumulators, with a non-return valve allowing virtually unrestricted flow in the compression direction. 
   Ideally, the roll control system should not provide any vertical support of the vehicle. Therefore, in another, alternative preferred arrangement of the present invention, the effective piston areas in the first and second chambers of each cylinder may be similar, the roll control system thereby supporting substantially zero vehicle load. As the amount of vehicle load support provided by the roll control system is one of the main factors controlling the amount of roll lacking inherent in the system, using wheel cylinders with similar effective piston areas in the first and second chambers and which therefore do not provide any vehicle support provides the roll control system with zero roll jacking. 
   However, in some applications, the use of a cylinder having piston rods extending from both ends thereof can lead to packaging difficulties because of the need to provide clearance for the upwardly extending piston rod. Therefore, according to another preferred arrangement, a piston rod may extend from one side of the piston, the piston rod having as small a diameter as physically possible to minimise the vehicle support provided by the damping and roll control system. In another possible arrangement, a hollow piston rod may extend from one side of the piston, and an inner rod may be supported within the inner volume of the cylinder, the inner rod being at least partially accommodated within the hollow piston rod, the hollow piston rod moving together with the piston relative to the inner rod. This arrangement may be used to minimise the difference in area of the opposing piston faces to minimise the vehicle support provided by the damping and roll control system. 
   According to an alternative preferred embodiment, the hollow piston rod arrangement of the wheel cylinder may be adapted to also provide a vertical support function for the vehicle. The piston supported in the wheel cylinder may provide an upper and lower chamber. The inner rod when supported within the hollow piston rod defines a rod chamber. This rod chamber may be used as part of a fluid circuit of the roll control system. To this end, the area of the peripheral end of the inner rod may be at least substantially identical to the area of the piston facing the lower chamber. Alternatively, it can be preferable to use a larger lower chamber area than the rod chamber area to induce lowering of the vehicle in roll with increasing roll moment when hydropneumatic accumulators are used in the system. 
   The upper chamber may be sealed to provide a bounce chamber to provide resilient support for the vehicle. The rod chamber may be vented and, together with the lower chamber, form a respective part of a fluid circuit of the roll control chamber. 
   It should be noted that the roll moment distribution for the roll control system is determined by the ratio between the effective piston areas of the front wheel cylinders compared to the effective piston areas of the rear wheel cylinders. Ideally, in most applications, each wheel cylinder should have a constant ratio between the effective piston area on the first chamber side compared to the second chamber side. 
   One advantage of using cylinders where the piston rod is only provided extending from the one piston face is that the degree of support provided by the cylinders can be varied by varying the support height of the vehicle. As the vehicle is lowered the support provided by the roll control system increases leading to higher roll stiffness. This is an affect of having an increased volume of piston rod introduced into the roll control system. 
   The support means for at least one pair of laterally spaced wheel assemblies may include first support means which are independent for each wheel assembly, thereby contributing an additional roll stiffness to the suspension system. Both the vehicle support means and the roll control system can together provide the roll stiffness for the vehicle in this arrangement. 
   Additionally or alternatively, the support means for at least one pair of laterally spaced wheels may include second support means which are interconnected between each wheel thereby contributing substantially zero roll stiffness to the suspension system. This and other vehicle support arrangements that provide little to no roll support and combinations of support arrangements are described in the Applicants&#39; International Application No. PCT/AU97/00870 referred to previously. In such an arrangement, the damping and roll control system can provide substantially all of the roll control for the vehicle, Furthermore, if the support means have substantially zero roll stiffness, the damping and roll control system can provide substantially all of the roll control for the vehicle. In this case, neither the support means or roll control system provides significant warp stiffness. This allows for substantially free warp motion of the vehicle wheel assemblies, improving comfort, reactions to single wheel inputs and providing substantially constant wheel loads (and therefore improved traction) in low speed or non-dynamic warp motions when traversing uneven terrain such as in off-road situations. 
   According to the present invention, the first and second fluid circuits may be in fluid communication such that fluid may be transferred therebetween. To this end, at least one bridge passage may interconnect the first and second fluid circuits to provide for said fluid communication. The bridge passage may be provided by a bridge conduit. Alternatively, the bridge passage may be provided within a connector body to which the conduits of the first and second circuits are connected. At least one flow control valve may be provided for controlling the flow through the bridge passage. 
   One or more accumulators may optionally also be provided for the bridge passage. The flow control valve and accumulator may be provided on a said bridge conduit. According to another possible arrangement, the control valve and/or accumulator may be supported on the connector body. It is also possible for all the damper valves and accumulators previously referred to be located on a common said connector body to simplify the packaging of the system within a vehicle. 
   The flow control valve may be opened, for example when there is little demand on the roll control system when the vehicle is travelling on a straight road. When the flow control valve is opened, this leads to a “short-circuiting” of the system such that the first and second chambers of each cylinder are allowed to communicate directly. This controlled interconnection of the first and second fluid circuits by the controlled opening of the flow control valve provide a number of operational advantages that lead to improved comfort for the passengers of the vehicle:
     a) The damping and roll control system provides no roll stiffness, the only roll stiffness being provided by the vehicle support means.   b) The damping and roll control system no longer effects the roll split of the vehicle, the roll split only being provided by the vehicle support means. If the roll split provided by the vehicle support means is between approximately 40 and 60%, this (in combination with the low roil stiffness) acts to reduce vehicle motions leading to ahead toss.   c) As there is little resistance to the fluid flow between the chambers of each cylinder except for that provided by the wheel damper valve, the single wheel stiffness is reduced.   d) Because the accumulator damper valves are bypassed, they do not influence the damping function of the damping and roll control system, and the roll damping is the same as the bounce damping.   e) The single wheel damping is (for the same reason) the same as the bounce damping.   f) The bounce damping however remains unchanged when the flow control valve is opened.   

   The operation of the flow control valve may be controlled by an Electronic Control Unit on the basis of operational parameters such as the lateral acceleration, speed and steering rate of the vehicle. 
   It is also possible for a plurality of bridge passages to be provided interconnecting the first and second fluid circuits. Each bridge passage may be provided with a said flow control valve. 
   It is also possible that the wheel cylinder include an integral flow control valve and/or damper valve therein. The piston of the wheel cylinder may include a flow control valve and/or damper valve controlling the flow of fluid between the first and second chambers. 
   The use of a plurality of bridge passages having flow control valves or wheel cylinders having built-in flow control valves facilitates fluid flow between the first and second chambers of the wheel cylinders. This can lead to a reduction in the inertia forces due to fluid flow through the system resulting in improved isolation of high frequency inputs and sharp edge inputs to the vehicle wheels. The effect of inertia forces within the roll control system will be subsequently described in more detail. 
   As the damping and roll control system can be switched to provide substantially zero roll stiffness, the use of zero roll stiffness support means for all wheels is not viable. However, zero roll stiffness support means may still be used in combination with independent support means providing some roll stiffness. Therefore, the support means for at least one pair of laterally spaced wheels may include first support means for supporting at (east a portion of the load on the associated wheel assemblies, said first support means providing independent resilience for each respective wheel and thereby providing a roll stiffness. 
   Additionally, the support means for at least one pair of laterally spaced wheels may include second support means for supporting at least a portion of the load on the associated wheel assemblies, said second support means providing combined resilience for each associated wheel assembly and thereby providing substantially zero roll stiffness. 
   It is to be appreciated that the conduit size may be selected to provide a degree of the damping required by the damping and roll control system. Depending on the level of ride comfort required in an application, the conduit size may be selected based on a variety of factors such as fluid inertia, fluid friction due to viscosity through range of operating temperatures, etc. 
   The vehicle support means preferably provides most if not all the vertical support for the vehicle. The damping and roll control system however preferably provides little to no vertical support for the vehicle such that the operating fluid pressure within the damping and roll control system can therefore be relatively low when compared with active roll control systems and the Applicants&#39; earlier suspension systems. Theoretically, if the roll control system provides no vertical support for the vehicle, the operating pressure may be only atmospheric pressure, ie. the system has no precharge pressure. 
   The damping and roll control system of the suspension system according to the present invention can therefore use low pressure components. The wheel cylinders can be constructed using standard vehicle damper and sealing technology. This leads to substantial manufacturing cost savings when compared to higher pressure systems. Also, comfort and NVH problems associated with higher pressure systems such as “stiction” between components are minimised in low pressure systems, the stiction levels being similar to that present in a conventional damper cylinder assembly. 
   Such a damping and roll control system can be installed in existing vehicle suspension systems, the dampers used in such systems being replaced or adapted for use as the wheel cylinders of the roll control system according to the present invention. The existing vehicle support means supporting the vehicle such as conventional steel or pneumatic springs can be retained. Alternatively, the vehicle support means may be replaced by support means that provide little to no roll support as described previously. This is possible because the damping and roll control system also provides a roll stiffness for the vehicle suspension system. 
   According to a further aspect of the present invention, there is provided a method of controlling the roll damping and roll stiffness of a damping and roll control system for a vehicle suspension system, the damping and roll control system including: 
   wheel cylinders respectively locatable at wheel assemblies of the vehicle, each wheel cylinder including an inner volume separated into first and second chambers by a piston supported within the wheel cylinder; and 
   first and second fluid circuits respectively providing fluid communication between the wheel cylinders by fluid conduits, each said fluid circuit providing fluid communication between the first chambers of the wheel cylinders on one side of the vehicle and the second chambers of the wheel cylinders on the opposite side of the vehicle to thereby provide roll support decoupled from the warp mode of the vehicle suspension system by providing a roll stiffness about a level roll attitude whilst simultaneously providing substantially zero warp stiffness; 
   damper means for controlling the rate of fluid flow into and out of at least one chamber of each wheel cylinder; 
   the method including opening the selection means to provide fluid communication between the first and second fluid circuits when the damping and oil system is required to provide a relatively low level of roll stiffness and roll damping; and 
   closing the selection means to prevent fluid communication between the first and second fluid circuits when the damping and roll system is required to provide a relatively high level of roll stiffness and roll damping. 
   The fluid flow may be bypassed from at least a substantial portion of the fluid conduits by opening the selection means when there is a single wheel input or two wheel parallel bump input to the damping and roll control system. The line damping and fluid inertia effects on the damping of the control system can therefore be minimised at such wheel inputs. 
   It is also envisaged that the entire fluid flow be bypassed from the fluid conduits at the predetermined wheel inputs. This can for example be achieved by providing a control valve within the wheel cylinder as hereinbefore described. 
   Damping means such as single and bi-directional damper valves may be provided through which the bypassed fluid flow passes, these damping means clearly controlling the damping of the control system during this operational mods. 
   It will be convenient to further describe the present invention with respect to the accompanying drawings which illustrate preferred embodiment of the invention. Other embodiments 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. 

   
     DRAWINGS 
     In the drawings: 
       FIG. 1  is a partially schematic view of a first preferred embodiment of a roll control system according to the present invention mounted on wheel assemblies of a vehicle; 
       FIG. 2  is a schematic view of second preferred embodiment of a roll control system according to the present invention; 
       FIG. 3  is a detailed view of a preferred embodiment of a wheal cylinder and wheel damper valve arrangement according to the present invention; 
       FIG. 4  is a schematic view of another preferred embodiment of a wheel cylinder and wheel damper valve according to the present invention; 
       FIG. 5  is a schematic view of yet another preferred embodiment of a wheel cylinder according to the present invention; 
       FIGS. 6   a  to  6   j  are schematic views showing the fluid flow within the damping and roll control system according to the prevent invention under different wheel inputs to the vehicle; 
       FIG. 7   a  is a schematic view of a third possible arrangement of a roll control system according to the present invention; 
       FIG. 8   a  is a schematic view of a fourth possible arrangement of a roll control system according to the present invention; 
       FIG. 8   b  is a figure similar to  FIG. 8   a  but without the bridge bypass  20  and the bridge valve  26 ; 
       FIG. 9  is a schematic view of a fifth possible arrangement of a roll control system according to the present invention; 
       FIG. 10   a  is a schematic view of a sixth possible arrangement of a roll control system according to the present invention; 
       FIGS. 10   b  and  10   c  is a schematic cross-sectional view of the wheel cylinder piston with an internal flow control valve and damper valve for the arrangement shown in  FIG. 10   a;    
       FIG. 11  is a schematic view of a seventh possible arrangement of a roll control system according to the present invention; and 
       FIG. 12  is a schematic view of an eighth possible arrangement of a roll control system according to the present invention. 
   

   DESCRIPTION 
   Referring in initially to  FIG. 1 , there is shown the front wheel assemblies  2  and rear wheel assemblies  3  of a vehicle, the body of the vehicle not being shown for clarity reasons. Each front wheel assembly  2  includes a wheel mounting  5  in the form of a wish-bone link contributing to the location of a respective wheel  4  (a second wishbone may be used but is omitted for clarity, other types of wheel locating links may be used). The rear wheel assemblies  3  have a common solid axle  6  to which each wheel  4  is mounted. The vehicle support means  17   a ,  17   b  for supporting the vehicle are shown fixed to the front wishbones  5  and adjacent the rear wheel axle  6  and include independent torsion bars  22  and a pair of air springs  23  interconnected by a conduit  21 . The independent form of front vehicle support means  17   a  shown as torsion bars provide a roll stiffness and the interconnected form of rear vehicle support means provides practically no roll stiffness because fluid is allowed to flow between the air springs  23  via the conduit  21 . Alternative vehicle support means can also be used, such as any known independent support means or low roll stiffness support means, or any combination different support means. For example, the vehicle may be supported entirely by independent coil springs. Alternatively, it may be supported by a combination of independent coil springs and interconnected air springs at one or both ends of the vehicle. Any combination of independent, combined or zero roll stiffness support means may be used on the front and rear of the vehicle. Many variations are shown and described in the Applicants International Application No. PCT/AU97/00870 and incorporated herein by reference. 
   A damping and roll control system  1  interconnects the front and rear wheel assemblies  2 ,  3  and includes a wheel cylinder  8  respectively provided for each front wheel assembly  2  and rear wheel assembly  3 , and a pair of fluid circuits  7 . 
   The configuration of the damping and roll control system  1  can be more readily understood by referring to  FIG. 2 . (Alternative possible arrangements of the damping and roll control system  1  are discussed later and shown in  FIG. 5  onwards) of this arrangement and of subsequent arrangements. It should be noted that corresponding features are designated with the same reference numeral for clarity reasons. Each wheel cylinder  8  has an inner volume  50  separated into an upper chamber  51  and a lower chamber  52  by a piston  63 . Piston rods  54 ,  55  extend from both sides of the piston  53  in the wheel cylinder  8  shown in  FIGS. 2 and 3 . Each fluid circuit  7  further includes an upper conduit  9  connecting the upper chambers  51  of one pair of longitudinally adjacent wheel cylinders  8 , and a lower conduit  10  interconnecting the lower chambers  52  of the opposing pair of longitudinally adjacent wheel cylinders  8 . As best shown in  FIG. 1 , each fluid circuit  7  may further include a cross conduit  11  which connects the lower conduit  10  with the upper conduit  9 . The two cross conduits  11  are themselves connected by a bridge passage  20 . 
   Wheel damper valves  18  can be provided on the lower conduit  10 , a respective wheel damper valve  18  being provided for the lower chamber  52  of each wheel cylinder  8 . Wheel damper valves  15  can also be provided on the upper conduit  9 , a respective upper wheel damper valve  15  being provided for each upper chamber  51  of each wheel cylinder  8 . 
   An accumulator  16  can also be provided for each fluid circuit  7 . In the arrangement shown in  FIGS. 1 and 2 , each accumulator  16  is provided at the junction between the lower conduit  10  and cross conduit  11 . An accumulator damper valve  19  is provided at the mouth of each accumulator  16 . 
   A flow control valve  26  is provided on the bridge passage  20  for controlling the flow of fluid through the bridge passage  20 . The flow control valve  26  is controlled by an electronic control unit (ECU)  27  which controls the valve  26  as a function of different operational parameters.  FIG. 2  shows the ECU  27  receiving signals from a steering input sensor  35  located on a steering wheel  40  of the vehicle, a lateral acceleration sensor  36  and a speed sensor  37 . As the wheal cylinders  8  shown in  FIG. 2  include piston rods  54 ,  55  extending from both sides of the piston  53  such a wheel cylinder  8  provides no support for the vehicle. The support is therefore substantially entirely provided by the vehicle support means  17   a ,  17   b  which are schematically shown as toll springs in  FIG. 2 . 
     FIG. 3  is a detailed view of the wheel cylinder  8  of  FIG. 2  and its associated wheel damper valves  15 ,  18 . The lower wheel damper valve  18 , which is schematically shown in  FIG. 3 , provides a restriction of fluid flow to the lower chamber  52  while allowing relatively unimpeded flow of fluid from that lower chamber  52 . By comparison, the upper damper valve  15 , also shown schematically in  FIG. 3 , restricts the flow of fluid from the upper chamber  51  while at the same time providing relatively unimpeding flow of fluid to the upper chamber  51 . This arrangement allows a positive pressure to be maintained in the upper and lower chambers  61 ,  52  and the upper and lower conduits  9 ,  10  to thereby prevent a vacuum being formed therein. This which can result in aeration of the fluid which can cause the damping and roll control system  1  to not operate properly. Part of a “gimbal” style mount for this “through rod” cylinder design is shown at  49 . 
     FIG. 4  shows an alternative possible arrangement of the wheel cylinder  8  according to the present invention. This wheel cylinder  8  includes a “dummy” rod  61  extending internally through the inner volume  50  of the wheel cylinder  8 . The dummy rod  61  is slidably accommodated within a hollow rod  62  which is itself supported on the piston  60 . The piston  60  and hollow rod  62  which can therefore slide over the dummy rod  60 . This arrangement minimises the difference in area between the upper face  60   a  and the lower face  60   b  of the piston  60 . The wheel cylinder  8  according to this arrangement will therefore provide minimal support for the vehicle. 
   The wheel cylinder shown in  FIG. 4  could also be adapted to provide a support function for the vehicle as well as provide for roll control as shown in  FIG. 5 . The dummy rod  61  when located within the hollow rod  62  defines a rod chamber  63 . The dummy rod  61  has an area  51  a at its peripheral end. The diameter of the dummy rod  62 , and therefore the end area  61  a may be sized such the area of the lower face  60   b  of the piston is at least substantially the same as the end area  61   a  of the dummy rod. By sealing the upper chamber  51  and venting the rod chamber  63  along a vent passage  84  provided through the dummy rod  61  so that it becomes part of the roll control system, this allows the wheel cylinder to also function as a support for the vehicle. The sealed upper chamber  51  will in this configuration act as a bounce chamber to provide resilient support for the vehicle such that the need for other support means such as coil springs can be eliminated. The lower chamber  52  and rod chamber  63  can then respectively form part of the fluid circuit of the roll control system. 
     FIGS. 6   a  to  8   j  schematically shows the fluid flows through the damping and roll control system  1  during different wheel inputs and vehicle motions. The arrow designated with the letter D represents the magnitude and direction of the wheel input into the wheel cylinder  8  immediately adjacent the arrow. The remaining arrows represent the direction and magnitude of the fluid flows within the damping and roll control system. In all of the following Figures, the front of the vehicle is located at the top left hand corner of each Figure. 
   Single Wheel Input 
     FIGS. 6   a  to  6 C shows the fluid flows in response to a single wheel input. It should be noted that the wheel cylinders  8  are shown having a piston  70  with a single piston rod  71  extending from the bottom face of the piston  70 . Such a wheel cylinder  8  provides a small degree of support for the vehicle due to the difference in the areas of the upper and lower piston faces of the piston  70 . The degree of support provided by the wheel cylinder  8  can however be minimised by having the diameter of the piston rod  71  as narrow as physically possible. 
     FIGS. 6   a  and  6   b  show the fluid flow when the flow control valve  26  in the bridge passage  20  is closed. In  FIG. 5   a , a wheel input D is provided to the left rear wheel cylinder  8 . This results in an upward movement of the piston  70  therein which reduces the volume of the upper chamber  72  of that wheel cylinder  8 . Because the fluid is incompressible, some fluid is transferred along the upper conduit  9  to the accumulator  16 . Because of the increase in volume in the lower chamber  52  of the rear left wheel cylinder  8 , fluid must be drawn from another part of the damping and roll control system  1 . To this end, fluid can be drawn from the accumulator  16  located on the top conduit  9  on the tight hand side of the vehicle, through the cross conduit  11  to the lower conduit  10  on the left hand side of the vehicle. No fluid is therefore drawn from of directed to the other wheel cylinders  8  and there is therefore no displacement of the piston rod  71  of the other wheel cylinders  8 . It should be noted that the lower wheel damper valve  18  associated with the left rear wheel cylinder  8  and the accumulator damper valves  19  control damping of the vehicle motion. 
     FIG. 6   b  shows the effect of a single wheel input D into the left front wheel cylinder  8 . In comparison with  FIG. 6   a , a greater magnitude of fluid flow occurs within the damping and roll control system  1 , the fluid forced from the upper chamber  72  of the left front cylinder  8  being directed to the accumulator  16  on the left hand side of the vehicle, with further fluid being drawn from the accumulator  16  of the right hand side of the vehicle to the lower chamber  73  of the left front cylinder  8 . There is again no displacement of the piston rod  71  of the remaining wheel cylinders  8 . In this situation, the magnitude of flow to and from the accumulators are significantly higher than when the single wheel input is to one of the rear wheel cylinders  8 . The damping of the vehicle motion is therefore largely controlled by the accumulators. 
   In  FIG. 6   c , the fluid flow valve  26  is open allowing flow through the bridge passage  20 . This valve  26  is opened when the vehicle is not undergoing any motion that would place a demand on the damping and roll control system  1 . The same wheel input D into the front left wheel cylinder  8  simply results in fluid being delivered from the upper chamber  72  thereof along the upper conduit  9 , through the cross conduit  11 , the bridge passage  20 , the other cross conduit  11 , the lower conduit  10 , back to the lower chamber  73  of the left front wheel cylinder  8 . The fluid. In other words flows from the upper chamber  72  to the lower chamber  73  of the wheel cylinder  8  with little to no fluid flow to and from the accumulator  16  on each fluid circuit  7 . The damping is therefore entirely controlled by the lower wheel damper valve  18  associated with the left front wheel cylinder  8 . The single wheel damping in this situation is therefore the same as the bounce damping of the system. 
   Two Wheel Bounce 
     FIGS. 6   d  and  6   e  show the fluid flows in the damping and roll control system  1  when two wheel bounce is experienced. In both figures, the flow control valve  26  remains closed.  FIG. 8   d  shows a wheel input D being applied to the two rear wheel cylinders  8 . The reduction in the volume of the upper chamber  72  of each of the rear wheel cylinders  8  results in fluid being pushed through the top conduits  9  along the cross conduits  11  to the lower chambers  73  of the adjacent rear wheel cylinder  8 . There is no fluid flow to or from the front wheel cylinders  8  or the accumulators  19  and the damping is controlled by the lower wheel damper valves  18  of each of the said cylinders  8 . 
   In  FIG. 6   e , there is shown a wheel input D to the two font wheel cylinders  8 . This results in a corresponding fluid flow of fluid from the upper chamber  72  of the front wheel cylinder  8  to the lower chamber  73  of the adjacent front wheel cylinder B. The damping is again controlled by the lower wheel damping valves  18 , with little to no fluid flow to the accumulators  16 . 
   Four Wheel Bounce 
     FIG. 6   f  shows the fluid flow in the right control system  1  when a wheel input D is provided to all four wheel cylinders  8 , with the flow control valve  26  remaining closed. The fluid displaced from the upper chambers  72  of the wheel cylinders  8  on one side of the vehicle is displaced through the cross conduit  11  and the lower conduit  10  to the lower chambers  73  of the wheel cylinders  8  of the opposing side of the vehicle. There is little to no flow to and from the accumulators  16  and the damping is controlled by the lower wheel damper valves  18 . 
   Roll 
     FIGS. 6   g  and  6   h  show the fluid flow control valve  26  is closed in  FIG. 6   g  and is opened in  FIG. 6   h . The roll motion of the vehicle results in a wheel input D being provided to the wheel cylinders  8  on the left hand side of the vehicle in an upward direction, the wheel input to the wheel cylinders  8  of the right hand side of the vehicle being in a downward direction. The next result of the fluid flaw is that a substantial amount of fluid must be drawn from the accumulator  16  of one fluid circuit  7 , while the accumulator of the other fluid circuit  7  must accommodate a substantial amount of fluid. The accumulators  16  and their associated damper valves  13  therefore have a substantial effect of the damping and roll stiffness of the roll control system  1  when the flow control valve  26  is closed. 
   By comparison, in  FIG. 6   h , because the flow control valve  26  is open, the fluid flow is “short circuited” such that fluid is simply transferred between the upper and lower chambers  72 ,  73  of each wheel cylinder  8  with little to no fluid being drawn or supplied to each of the accumulators  16 . In this arrangement, the accumulator  16  have no influence of the roll stiffness of the damping and roll control system  1 . 
   Articulation 
     FIGS. 6   i  and  6   j  shows the fluid flows within the damping and roll control system  1  during articulation motion of the vehicle wheels.  FIG. 6   i  shows the fluid flows when the flow control valve  26  is closed,  FIG. 6   j  showing the fluid flow with the fluid control valve  26  open. 
   Referring to  FIG. 6   i , the wheel input D due to the articulation motion of the wheels simply result in the transfer of fluid between the upper chambers  72  and the lower chambers  73  of each pair of wheel cylinders  8  in each fluid circuit  7  with no transfer of fluid between the fluid circuits  7 , by comparison, in  FIG. 6   j , the opening of the flow control valve  26  again results in “short circuiting” of the fluid flow such that there is simply a transfer of fluid between the upper and lower chambers  72 ,  73  of each wheel cylinder  8 . 
   Any suspension system which includes an arrangement of interconnected fluid cylinders (such as the present invention) responds to inputs by producing forces which can be placed into four categories. The first is spring forces produced by compression or wind up of the fluid and/or mechanical springs in the system (and other sources of resilience such as hose expansion), this spring force being a function of the displacement of one or more of the fluid cylinders. The effect of the spring force is most noticeable at low frequencies. 
   The second category of forces is static friction forces which occur when wheel cylinder motion is initiated, or when the direction of motion is reversed. These static friction forces are often referred to as “stiction” forces or “breakout friction” forces and are due to the friction between the rod and piston seals and the respective rod and bore surfaces. 
   The third category of forces is damping forces which are a function of velocity. Primarily these damping forces are regulated by orifices, shims and springs in the damper valves  15 ,  18 ,  19 . A component of the total system damping is generally provided by “line damping”, ie. the flow of fluid along the conduits interconnecting the wheel cylinders in the system. The cross sectional areas of the wheel cylinders and the fluid conduits and the lengths of the fluid conduits should be designed to ensure that the level of line damping provided is of an acceptably low level for the different flows possible due to the motions of the suspension in the modes discussed above. 
   The fourth category of forces is the inertia forces, due primarily to the acceleration of the fluid through the system. Therefore, the inertia effect is most noticeable at high frequencies and may provide reduced isolation of high frequency inputs and sharp edge inputs resulting in body vibration and noise. Consider a theoretical system consisting of a cylinder with a piston area Ap connected to a line of length L and area Al and an incompressible fluid of density The cylinder piston is given an acceleration ρ. The resulting force, due the inertia of the fluid in the line, F is 
   
     
       
         
           F 
           = 
           
             
               
                 A 
                 p 
                 2 
               
               
                 A 
                 l 
               
             
             × 
             L 
             × 
             ρ 
             × 
             
               α 
               . 
             
           
         
       
     
   
   It can be seen that the inertia force is sensitive to fluid density (generally fixed for hydraulic fluids), line length and, line area and very sensitive to piston area. Any reduction in line length and increase in line area will reduce the fluid inertia effects. It is in practice more convenient to reduce the line length rather than increasing the line area by increasing the diameter of the fluid conduits. The latter change can lead to packaging difficulties under the vehicle because of the limited space available for installing the fluid conduits. Another beneficial change which can reduce fluid inertia effects is to increase the mechanical advantage (or lever ratio) from the wheels to the fluid cylinders. This can lead to higher peak pressures, but lower fluid accelerations. 
   The modes likely to be influenced by high frequency inputs are single wheel input and two wheel parallel bump input in the roll control system layout shown in  FIGS. 1 to 6   j , for a two wheel parallel bump input, fluid is required to travel from the front left upper chamber  51  to the front right lower chamber  52  along with fluid travel from the front right upper chamber  51  to the front left lower chamber  52 . There is a minor flow into the accumulator  16  (see  FIGS. 6   d  and  6   e ). 
   For a single wheel input fluid must travel from the cylinder chambers directly to the accumulator (see  FIG. 5   a  to  6   c ). 
   With the fluid control valve  26  open, the two wheel parallel bump input flows are unchanged. For a single wheel input the flow now passes through the fluid control valve  26  with little flow to the accumulators  16 . 
   In the above situations in the above-described roll control system layout, the fluid must travel down lines of a reasonable length and reasonable diameter. This provides a significant inertia effect. 
   As the system provides roll support and must provide a suitable roll moment distribution the front cylinders are generally a larger diameter than the rear. Due to the sensitivity to piston area the front is more likely to show fluid inertia effects than the rear. 
   The piston areas are further fixed by maximum required operating pressures. 
   Another possible arrangement of the roll control system is shown in  FIGS. 7   a  and  7   b . The layout of the fluid circuits  7  is varied to provide for the shortest route from one front cylinder  8  to the other reducing line length and hence fluid inertia effects. 
   In particular, the upper chamber  81  of each adjacent pair of front wheel cylinders  8  are connected by a respective fluid conduit  70  to the lower chamber  52  of the adjacent front wheel cylinder  8 . These fluid conduits  70  are connected to the corresponding fluid conduits  70  of the rear wheel cylinders by longitudinal fluid conduits  71  to provide a pair of fluid conduits of the system. During single wheel and two wheel bounce, much of the fluid is transferred between the fluid chambers  51  and  52  of each pair of front wheel cylinders  8  and/or between the fluid chambers  51  and  52  of each pair of rear wheel cylinders B. 
   Only a relatively small amount of fluid need pass through the longitudinal fluid conduits  71  where inertia effects are likely to be more pronounced. The opening of the flow control valve  26 , as noted previously, results in short circuiting of the fluid circuits such that fluid is caused to flow between the upper and lower chambers  61  and  52  of the wheel cylinders  8 . 
   Because the fluid inertia effects are likely to be more pronounced at the front as noted previously, accumulators may be provided on each of the fluid conduits  70  connecting the front wheel cylinder chambers  51  and  52 . The accumulators  16  act to accommodate a large fluid flow resulting from a single wheel input when the flow control valve  26  is closed. 
     FIGS. 8   a  and  8   b  show a variation of the arrangement shown in  FIG. 6 , with further accumulators  16  being provided on each of the fluid conduits  70  connecting the rear fluid cylinder chambers  51  and  52 . 
   Added accumulators  16  at the rear fluid conduits  70  allow fluid to generally bypass the longitudinal fluid conduit  70  resulting in less effective line length and reduced inertia effects. 
   The preferred embodiment of the roll control system shown in  FIG. 9  is similar to the arrangement shown in  FIGS. 1 and 2  except that the single bridge passage  20  and flow control valve  26  is replaced with a respective bridge passage  20  and flow control valve  26  for each wheel cylinder  8 . This allows the fluid flow for each wheel cylinder  8  to be independently short circuited to allow for relatively direct flow between the upper and lower chambers  51  and  52  of the wheel cylinders  8 . 
   Providing four separate bridge passages  20  and flow control valves located at each of the cylinders  8  therefore allows for a direct short circuit of the system. Most of the flow is bypassed directly around each cylinder  8  through a short and reasonable area line for all inputs with the flow control valve  26  open. This provides a significant reduction in the fluid inertia effects. The damping is however still maintained as the dampers  15 , 18  are in this fluid loop. Operation with the four flow control valves  28  closed will not however offer any fluid inertia improvements. 
   The flow control valves  26  could be digital (on or off only), multi-position or proportional depending on the level of damping control required. The valve  26  must seal when fully closed. 
     FIGS. 10   a  to  10   c  together illustrate another preferred embodiment of the roll control system similar to the embodiment shown in  FIG. 1 , but where the bridge passage  20  and roll control valve  26  is omitted (see  FIG. 10   c ). Each wheel cylinder  8  is however adapted to include a fluid flow control assembly which allows for direct fluid flow between the upper and lower chambers  51  and  52  of each wheel cylinder  8 . 
   The pistons  80  of each wheel cylinder have a control valve  81  inserted therein (see  FIGS. 10   b  and  10   c ). The control valve  81  controls the rate of fluid through a piston passage  82  providing for fluid communication between the upper and lower chambers  51 ,  52  of the wheel cylinder  8 . A rotary valve  81  is shown but any design is applicable. This rotary valve  811   e  rotatable by a shaft  83  passing through the piston rod  84  between an open position ( FIG. 10   b ) and a closed position ( FIG. 10   c ). Fluid flow between the upper and lower chambers  51 ,  52  is allowed when the valve  81  is open. This valve  81  directly connecting the upper and lower chamber of each cylinder thereby provides a short fluid path. Again fluid inertia effects are significantly reduced. An in-line damper  85  is required to damp the fluid flow through the piston passage  82  to thereby damp the wheel movement, as the dampers in the fluid conduits have effectively been bypassed. The valves  81  could be digital (on or off only), multi-position or proportional depending on the level of damping control required in comfort mode. The valve  81  must seal when fully closed. The construction of the valve may be different to that illustrate d, such as a disc with holes arranged in it and attached to the shaft  83 . The piston may have spring steel shims on the holes on either side of the piston to provide damping control. The holes in the disc may be in the form of tapered slots to provide variable flow areas and therefore a degree of variable damping control. 
     FIG. 11  shows a preferred embodiment of the roll control system which utilises the fluid conduit layout of the system shown in  FIG. 7 , but further includes a respective flow control valve  26  for each wheel cylinder. 
   This layout minimises fluid inertia effects even when the flow control valves  26  are closed and provides a further reduction of fluid inertia effects with the flow control valves  26  open. It should however be appreciated that the flow control valves  26  could alternatively be inside the pistons  80  as shown in  FIGS. 10   b  and  10   c.    
   The preferred embodiment shown in  FIG. 12  is the same as the arrangement shown in  FIG. 11  but with additional rear accumulators  16  for the same reasons as in the arrangement shown in  FIG. 8 .