SUSPENSION SYSTEM

Abstract: A suspension system suitable for use in a road vehicle comprises a piston and cylinder unit, respective elements (12) and (14) of which are connected with a chassis (10) and a bearing of a wheel (11) of the vehicle. An outlet (16) of the cylinder is connected in series with series connected constriction (17), accumulator (19), constriction (22) and accumulator (18).

Description: Title: Suspension system Description of invention The present invention relates to a suspension system, by which we mean a system which is used to control relative movement of two members in a manner such that the relative movement is yieldingly opposed and is damped. The majority of vehicles intended for use on land have suspension systems interposed between a body of the vehicle and wheels or other ground-engaging elements to protect the vehicle body and passengers or a load carried thereby from mechanical shocks. The importance of suspension systems in freight-carrying road vehicles for minimising the damage caused to road surfaces by such vehicles has been recognised. Although the effect of traffic on road surfaces is not well understood, it is generally believed that it is desirable to minimise the forces exerted by vehicle tyres on a road surface and that low-frequency oscillation of a vehicle wheel is more damaging to a road surface than is a higher frequency oscillation which imposes the same maximum force on the road surface. In many suspension systems, fluids are used to transmit forces. In recent years, there have been developed active suspension systems. An active suspension system comprises a motor which is energised from a source of power extraneous to the suspension system and which applies energy to the suspension system, typically by driving a pump which transfers fluid under pressure from a reservoir into a pressurised part of the suspension system. Active suspension systems are expensive and have therefore been developed only for use in relatively expensive passenger cars and more specialised vehicles. The present invention is primarily concerned with a suspension system which can be operated as a passive system, that is to say without the transfer of energy from an extraneous source to the suspension system, except for the incidental transfer of energy required to operate valves for controlling the flow of fluid and/or the minor transfer of energy arising upon the gradual addition of fluid under pressure under the control of a self-levelling system which compensates for uneven static loading of a vehicle. A theoretical analysis of a class of hypothetical suspension systems described as basically passive suspensions has been published in 1984 in a paper entitled "Adaptive Suspension Concepts for Road Vehicles", by D. Karnopp and D. Margolϊs (Vehicle System Dynamics, 13 (1984) pp 145-160, published by Swets & Zeitlinger). This paper mentions the requirement for reducing the stiffness of a suspension system, when the vehicle concerned is operating on a smooth road, and for increasing the stiffness of the suspension system when the vehicle is on a rough road. Reference is made to the possibility of varying the damping of the suspension system with forward speed and/or with retardation of the vehicle. Reference is also made to the desirability of varying the characteristics of a spring incorporated in a suspension system. Karnopp et al state that changing the damping alone of a suspension system is not a good way to stiffen or soften a suspension and that if the spring constant alone is changed from an optimum value for a given road input, then a softened system will have too much damping and a stiffened system will have too little damping. They put forward theoretical analyses to support these statements. Karnopp et al propose that the damping coefficient and the spring stiffness of a suspension system should be varied concurrently and suggest an arrangement of variable dampers and compliant elements which might be used to achieve this. A schematic representation of such an arrangement is shown in Figure 6a of the Karnopp et al paper. The suspension arrangement represented schematically in Figure 6a of the Karnopp et al paper comprises a pair of variable dampers and a pair of air springs. There is presented a theoretical analysis of the behaviour of the system represented and Figure 10 comprises a graphical representation of the expected behaviour of the hypothetical system, when both of the dampers are varied simultaneously. The suspension system contemplated by Karnopp et al and represented schematically in Figure 6a of their paper would require an elaborate system for controlling both of the dampers in accordance with various parameters. Reference is made in the Karnopp et al paper to varying the suspension system in a manner which is dependent upon vehicle speed, mean square acceleration of the vehicle, diving due to braking, squatting due to acceleration, manoeuvre induced roll, brake pressure, steering wheel angular velocity and steering wheel angular acceleration. A control system which uses only a selection of these inputs would be expensive to develop and provide and would be a potential source of unreliability. So far as the present applicants have been able to ascertain, no-one has constructed and tried a suspension system as represented schematically in Figure 6 of the Karnopp et al paper. Publication of the paper in 1984 has not lead to any advance in the art of suspension systems for vehicles. Subsequent to publication of the Karnopp et al paper, resources have been expended on a large scale in attempts to achieve improved suspension systems for vehicles, primarily passenger cars. These attempts have not been based on the Karnopp et al disclosure and that disclosure has evidently been discarded by those skilled in the art as leading to no useful result. The present applicants have devised an alternative suspension arrangement which, like the Karnopp et al arrangement, is essentially a passive system, but which differs from the Karnopp et al arrangement, in that the present invention does not require the simultaneous variation of a pair of dampers and an elaborate control system to change the dampers in accordance with various inputs from the vehicle. According to a first aspect of the present invention, there is provided a suspension system comprising a chamber containing a fluid, the chamber having an outlet through which the fluid can be expelled from the chamber, a displacer which displaces fluid in the chamber when movement which is to be controlled by the suspension system occurs, a first impedance which impedes movement of the displacer to displace fluid in the chamber, a first fluid capacitor in communication with the chamber for accornmodating displaced fluid when the pressure in the fluid rises and a second impedance for impeding flow of fluid between the chamber and the first capacitor, wherein one or both of the first and second impedances has a value which is unchanged during at least a substantial period of use of the system. The chamber in which fluid is displaced by the displacer may contain a compressible fluid, in which case some of the displaced fluid will be accommodated in the chamber and the chamber will act as a second capacitor. In this case, the first impedance may be connected mechanically in parallel with the fluid in the chamber so that both the pressure of that fluid and the first impedance oppose movement of the displacer in a direction to compress fluid in the chamber. Alternatively, the chamber may contain an incompressible fluid. In this case, the first impedance is preferably connected in series with the outlet from the chamber to impede flow of fluid through the outlet and there is preferably interposed between the first and second impedances a second fluid capacitor for accommodating displaced fluid when the pressure in the fluid rises. In use of this preferred system, some of the fluid displaced by the displacer can flow from the chamber through the first impedance only to the second capacitance and a further part of the displaced fluid can flow through both the first and second impedances to the first capacitance. According to a second aspect of the invention, there is provided a method of controlling up and down movement of a vehicle body relative to a ground-engaging wheel of the vehicle, wherein there is provided means for defining a chamber containing a fluid and a displacer, the chamber having an outlet through which fluid can flow into and from the chamber, interposing said means defining the chamber and the displacer between the wheel and the body of the vehicle, associating a first impedance with the means defining the chamber to impede movement of the displacer in a direction to expel fluid from the chamber through the outlet and connecting a first capacitance via a second impedance with the chamber to receive fluid expelled therefrom, wherein, during operation of the vehicle, fluid flows between the chamber and the first capacitor via the second impedance and wherein, during a substantial period of operation of the vehicle, the second impedance is maintained substantially constant. It will be understood that the value of one or both of the impedances may be adjusted during manufacture of a vehicle or preparation of the vehicle for use. Furthermore, an adjustment of one or each of the impedances may be made to change one or more characteristics of the vehicle for a substantial period. For example, a characteristic may be changed when the vehicle is to be used for carrying a relatively heavy load or for carrying a relatively light load or is to be used in a substantially unladen condition. A characteristic may be changed, in preparation for use of the vehicle on a relatively smooth road or changed in preparation for use of the vehicle off-road or on a relatively rough road. Such adjustments of the impedances may be effected manually and do not require the provision of an elaborate control system which responds to various inputs from the vehicle, as would be required to operate the suspension system represented in the Karnopp et al paper. The vehicle body may be a cab which is distinct from a load-platform of the vehicle and the means defining the chamber and the displacer may be interposed between the cab and the load-carrying platform, that platform being supported on ground-engaging wheels of the vehicle. Examples of suspension systems embodying the present invention will now be described, with reference to the accompanying drawings, wherein: FIGURE 1 is a diagrammatic representation of the suspension system and associated parts of a vehicle, FIGURES 2 AND 3 are diagrams representing the behaviour of the system of Figure 1, of a known, passive suspension system and of a known hydroelastic suspension system and FIGURE 4 is a diagram representing a modified suspension system. The suspension system represented in Figure 1 of the accompanying drawing is incorporated in a road vehicle having a chassis 10 and running wheels, one of which is represented at 11. The wheel 11 may incorporate a pneumatic tyre and be intended for running on the ground. Alternatively, the wheel may be intended for running on a rail or other member of a track, for example a track which is a part of a vehicle. The chassis 10 may support a cabin for a driver of the vehicle and for one or more passengers and may also support a platform for carrying a load. As in known vehicles, the chassis also supports an engine for driving at least some of the mnning wheels. The vehicle incorporates a suspension system for transmitting a downwardly directed load from the chassis 10 to the running wheel 11. The suspension system may be interposed mechanically between the chassis and an axle housing or a bearing which carries the wheel 11. There may be a respective suspension system for each of the running wheels of the vehicle. Alternatively, a suspension system may be common to a plurality of running wheels. The suspension system illustrated in the drawing comprises a cylinder 12 defining a chamber 13 and a displacer in the form of a piston 14 which operates in the cylinder in a known manner to displace fluid in the chamber 13. The cylinder and a piston rod 15 of the piston 14 are attached to respective ones of the members, relative movement of which is to be controlled by the suspension system so that the volume of the space available in the chamber 13 to accommodate fluid changes whenever there is relative movement of these members. In the arrangement illustrated, the cylinder 12 is attached to the chassis 10 and the piston rod 15 is attached to the axle housing or bearing of the wheel 11. It will be understood that the reverse of this arrangement may be used. The chamber 13 and associated parts of the suspension system are occupied by a substantially incompressible fluid, for example oil. The oil in the chamber 13 is maintained under pressure to oppose downward movement of the chassis 10 relative to the wheel 11. The pressure in the chamber 13 is dependent upon the static load borne by the chassis and upon the dynamic conditions when the vehicle is in use. The particular example of suspension system illustrated in the drawing is a passive system and the volume of oil contained in the system does not change. It will be appreciated that provision may be made for replacement of any oil which escapes from the system by leakage or which is drained from the system, in order that the oil can be replaced with fresh oil. The chamber 13 has an outlet 16 and whenever the piston 14 moves relative to the cylinder 12, oil flows through the outlet, leaving the chamber when the piston moves in a direction to reduce the volume of the chamber and entering the chamber when the piston moves in the opposite direction. A first impedance 17 is provided for impeding flow of oil through the outlet 16 in either direction. The suspension system further comprises first and second fluid capacitors 18 and 19 respectively for accommodating oil which is expelled from the chamber 13 through the outlet 16 by movement of the piston 14 relative to the cylinder 12. Oil accommodated by the capacitors can subsequently be released by the capacitors when the piston 14 moves relative to the cylinder in a direction to increase the volume of the chamber 13. The capacitors 18 and 19 may be known devices, commonly called accumulators, and each comprising a canister containing a volume of gas under pressure and a volume of oil, the oil being adjacent to respective ducts 20 and 21, via which the respective interiors of the canisters are connected with the first impedance 17 and the outlet 16. Each canister may contain a diaphragm separating the gas from the oil. It will be understood that the gas in the canister acts as a spring and that other forms of spring may be provided in the capacitors to enable the capacitors to accept additional volumes of oil when the pressure in the oil is increased and to expel oil from the canisters when the pressure in the oil falls. The first capacitor 18 differs from the second capacitor 19 in that the first capacitor is adapted to act as a softer spring than is the second capacitor 19. For example, the capacitor 18 may contain a substantially larger volume of gas than does the capacitor 19. This means that the volume of additional oil which the capacitor 18 will accept when there is a given increase in the pressure in the oil is substantially greater than the volume of additional oil which the capacitor 19 will accept, when subjected to the same increase in the pressure of the oil. The suspension system further comprises a second impedance 22 which is interposed between the first capacitor 18, on the one hand, and the second capacitor 19 and the first impedance 17, on the other hand. The duct 20 leads from the first capacitor 18 to the second impedance 22. Each of the impedances 17 and 22 may comprise a known orifice plate mounted in a known housing connected in a known manner by suitable pipes with other components of the suspension system. Thus, the first and second impedances may have fixed values which may differ from each other. It would also be within the scope of the invention to provide adjustable impedances in place of the fixed impedances 17 and 22 or in place of one of them. Furthermore, there may be connected in series with the second impedance 22 a shut-off valve which can be operated to isolate the first capacitance 18 from the remainder of the suspension system. When the vehicle is in use, the first impedance 17 damps upward and downward movement of the chassis 10 relative to the wheel 11. That is to say, the first impedance dissipates energy when oil is caused to flow in either direction through the outlet 16 by movement of the piston 14 relative to the cylinder 12. This energy is dissipated as heat. Oil flowing from the chamber 13 through the outlet 16 flows partly to the first capacitance 18 and partly to the second capacitance 19. The respective proportions of the flow which are accommodated by these capacitances will vary in a manner dependent upon the rate of flow through the outlet 16. When the flow rate is high, flow to the first capacitance 18 is impeded significantly by the second impedance 22 and a relatively smaller proportion of the flow through the outlet 16 is accommodated by the first capacitance 18. When the flow rate through the outlet 16 is relatively low, the effect of the second impedance 22 is smaller and a relatively larger portion of the flow is accommodated by the first capacitance 18. The "stiffness" of each of the capacitances 18 and 19 and the relative values of the impedances 17 and 22 are selected to provide the required operating characteristics of the suspension system. In operation of the suspension system illustrated in the accompanying drawing, the entire volume of fluid expelled from the chamber 13 through the outlet 16 is accommodated in the capacitors 18 and 19 collectively. These capacitors also release the entire volume of fluid which enters the chamber 13 through the outlet 16, when the piston 14 moves relative to the cylinder 12 in a direction to increase the volume of the chamber 13. The energy required to drive fluid into the chamber 13 and to drive the piston 14 relative to the cylinder 12 in a direction to increase the volume of the chamber is stored in the capacitors 18 and 19 when the piston moves in a direction to reduce the volume of the chamber 13. There is no supply of energy or of fluid under pressure to the suspension system from an extraneous source, during normal operation. It would be within the scope of the invention to supplement the suspension system illustrated in the accompanying drawing by means of one or more additional capacitors connected in series with the capacitors 18 and 19 and for each additional capacitor to have its own series-connected impedance. It will be noted that, in the arrangement illustrated in the drawing, the impedances 17 and 22 and the capacitors 18 and 19 are connected in series with one another and with the outlet 16. In the case of a vehicle having more than one suspension system as represented in the accompanying drawing, these systems may be modified by interconnecting the systems, either directly or via one or more valves. The suspension system illustrated in the accompanying drawing is considered to be sufficiently simple and inexpensive to be used in freight - carrying road vehicles. Whilst it is preferred that the impedances 17 and 22 shall be non- adjustable, in order to rninimise cost, it would be within the scope of the invention for either one or both of these impedances to be adjustable. This would facilitate adjustment during construction of a vehicle to achieve required characteristics of the suspension system. It would also facilitate adjustment of the suspension system, after construction of the vehicle, in order to prepare the vehicle for use in different conditions. For example, the impedances which are preferred when the vehicle is used in a fully laden condition may differ from the impedances which are preferred when the vehicle is used in an unladen condition. Adjustment of the impedances may be by means of manually operated, remote controls. Alternatively, adjustment may be achieved by means of power-operated valves or actuators, either under manual control or under the control of a control device which is responsive to the load carried by the vehicle. In none of these cases, would adjustment be effected continuously during operation of the vehicle. Comparative tests have been carried out on a suspension system embodying the invention and arranged as shown in Figure 1, a known passive system and a representative active suspension system. For this purpose, there was used a test rig comprising one half of a commercially available suspension system supplied by Rubery Owen - Rockwell, designated "Indair" system and which is intended for use in freight-carrying vehicles. Thus, the rig incorporated a single wheel station. A stack of "Belville" washers was used to simulate the action of a tyre and displacements of the tyre representative of the displacement caused by an uneven road surface were imparted to the stack of springs by an hydraulic actuator which was controlled in a manner to represent a road surface having characteristics corresponding to a selected one of three ISO standard classifications identified as very good, good and average. The rig included instrumentation to facilitate the recording of sprung an unsprung mass accelerations, tyre force (force exerted on the road surface by the tyre), suspension deflection and velocity and tyre deflection. The test rig was used to obtain measurements with the springing being provided by air spring supplied as a part of the "Indair" suspension. The air spring and damper supplied in the Indair system were then removed and replaced by suitable components to provide the suspension arrangement represented in Figure 1. The measurements were then repeated. Finally, the rig was again modified to achieve an hydro-pneumatic suspension system, substantially as hydro- pneumatic systems which have been proposed prior to the invention. The results obtained are represented graphically in Figure 2 and Figure 3. In Figure 2, there is represented the force which would be exerted by the tyre on the road surface in consequence of oscillation of the wheel at frequencies within the range 0 to 20Hz for the test rig with the known damper and air spring arrangement, for the test rig with the hydro-pneumatic suspension arrangement and for the test rig with the suspension arrangement represented in Figure 1. The input to the test rig was controlled to be representative of an ISO "very good" profile. Figure 2 shows that the forces exerted on the road surface by a suspension system as represented in Figure 1 are, over a frequency range from approximately 1 to approximately 4Hz, much lower than the forces which resulted from use of the known air spring suspension and also much lower than the forces resulting from use of the known hydro-pneumatic suspension system. Figure 3 shows that a suspension system as represented in Figure 1 achieves a similar improvement in a case where the input to the rig is representative of a road surface classified as ISO "medium". Measurements obtained from the rig when the input was representative of a road surface classified as ISO "good" also show a similar relationship between the force exerted by the tyre on the road surface when a suspension system as represented in Figure 1 is used, and the force exerted on the road when the air spring and hydro- pneumatic suspension systems are used. It is believed that forces exerted on the road surface in the frequency range 1 to 4Hz are more harmful to the structure of the road than are forces exerted at higher frequencies. It is evident from Figures 2 and 3 that the suspension system represented in Figure 1 subjects the road surface to forces within this frequency range which are much smaller than the forces to which road surfaces are subjected by known suspension systems incorporating air springs and by hydro-pneumatic suspensions. It is believed that the performance of the suspension system represented in Figure 1 is not significantly inferior to that of active suspension systems which have been proposed. It is further believed that the cost of a suspension system as represented in Figure 1 is sufficiently low for such systems to be fitted as normal practice to freight-carrying vehicles; whereas active suspension systems are prohibitively expensive for application to freight carrying vehicles and have not been developed for freight-carrying vehicles. These beliefs are supported by a theoretical analysis using parameters which are typical of trucks manufactured for use on roads at the present time. This analysis has been carried out for each of an active suspension system, a suspension system as represented in Figure 1 and a passive suspension system. In a case where the analysis is based on a road surface categorised as "very good" under the ISO classification and assuming a vehicle speed of 20 metre per second, the root mean square values for the forces exerted by a vehicle wheel on the road surface has been computed to be 1.14kN for an active suspension, 0.93kN for an equivalent suspension as represented in Figure 1 and 1.36kN for an equivalent passive suspension. In an analysis based on the assumption of a road surface categorised as "good" under the ISO classification and also assuming a vehicle speed of 20 metre per second, the root mean square values of the forces exerted by a vehicle wheel on the road surface were computed to be 2.99kN for an active suspension, 2.44kN for a suspension as represented in Figure 1 and 3.59kN for a passive suspension. Figure 4 shows a diagrammatic representation of an alternative suspension system which embodies the present invention. In Figure 4, parts corresponding to those herein before described with reference to Figure 1 are identified by like reference numerals with the prefix 1. The preceding description is deemed to apply, except for differences hereinafter mentioned. The interior of the cylinder 112 is divided by a displacer which is represented as a double-acting piston 114. The cylinder contains an incompressible fluid on which each of the opposite faces of the piston can act directly. The volume of incompressible fluid in the system does not change. Provision may be made, but is not represented in the drawing, for replacing fluid which leaks from the system or which is drained from the system, in order to be replaced. The suspension system of Figure 4 defines a first chamber 113 which is immediately adjacent to one face of the piston 114. A boundary of the chamber 113 opposite to the piston is defined by an internal fixed partition 124 of the cylinder. In the partition there is an orifice which provides communication between the chamber 113 and a further chamber 125 inside the cylinder. The structure defining the orifice in the partition 124 constitutes an impedance and is typically a separately-formed component mounted in the partition. The chamber 125 communicates with a capacitor 119 which, in the arrangement represented in Figure 4, is disposed within the cylinder 112. The capacitor 119 may comprise a quantity of a compressible fluid contained in a flexible envelope. This capacitor may, for example, lie at an end of the chamber 125 remote from the piston 114 and partition 124. A third chamber 126 is defined in the cylinder 112 between the piston 114 and a further partition 127, also having an orifice which provides communication between the third chamber and a fourth chamber 128 inside the cylinder. The structure defining the orifice in the partition 127 constitutes a further impedance for impeding flow of fluid between the chambers 126 and 128. The chamber 128 communicates directly with a respective capacitor 129 which also lies inside the cylinder 112, for example at an end of the chamber 128 which is remote from the piston 114 and the partition 127. The capacitor 129 also may comprise a body of gas separated from the incompressible fluid in the chamber 128 by a flexible envelope, a diaphragm or a piston. The chamber 125 has a port 116 leading to a duct 130 outside the cylinder 112 which provides communication between the second chamber 125 and the fourth chamber 128. The duct 130 communicates with the fourth chamber at a port 131. A further impedance 132 is provided in the duct 130 for impeding flow of fluid between the chambers 125 and 128. The suspension systems represented in Figure 4 further comprises a capacitor 133 which lies outside the cylinder 112 and which is not in communication with the chambers inside the cylinder. The capacitor 133 may be in the form of a known air-spring. When a vehicle having the suspension system illustrated in Figure 4 is in use and the wheel 111 travels over a hump in a road surface, so that an upwardly directed force is transmitted from the wheel 111 to the chassis 110 via the suspension system, the piston 114 moves relative to the cylinder 112 in a direction to reduce the volume of the chamber 113 and increase the volume of the chamber 126. Accordingly, incompressible fluid is caused to flow via the impedance 132 to the chamber 128. A first proportion of the flow is accommodated by the capacitor 119 and a further proportion of that flow is lead along the duct 130 to the chamber 128. Since only a proportion of the flow from the chamber 113 is lead to the chamber 128, the capacitor 129 will expand to expel incompressible fluid from the chamber 128 through the impedance incorporated in the partition 127 so that the flow through that impedance will correspond to the flow through the impedance incorporated in the partition 124. Upward movement of the wheel 111 relative to the chassis 110 is also opposed by the capacitor 133, which stores energy during mutual approach of the chassis 110 and the axis of rotation of the wheel 11, that energy being expended subsequently to contribute to driving of incompressible fluid along the duct 130 in a direction from the chamber 128 to the chamber 125. The features disclosed in the foregoing description, or the following claims, or the accompanying drawing, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims: CLAIMS 1. A suspension system comprising a chamber containing a fluid, the chamber having an outlet through which the fluid can be expelled from the chamber, a displacer which displaces fluid in the chamber when movement to be controlled by the system occurs, a first impedance which impedes movement of the displacer to displace fluid in the chamber, a first capacitor in communication with the chamber for accommodating displaced fluid when the pressure in the fluid rises and a second impedance for impeding flow of fluid between the chamber and the first capacitor. 2. A system according to Claim 1 wherein the chamber contains a substantially incompressible fluid. 3. A system according to Claim 1 wherein the chamber contains a substantially incompressible fluid, the first impedance is connected in series with the outlet from the chamber to impede flow of fluid through the outlet and the system further comprises a second capacitor connected between the first and second impedances. 4. A system according to Claim 3 wherein the volume of additional incompressible fluid accepted by the first capacitor upon a given increase in the pressure in said fluid is substantially greater than the volume of additional incompressible fluid accepted by the second capacitor upon the same increase in the pressure in the fluid. 5. A system according to any preceding claim wherein the chamber is defined by a cylinder and the displacer is a piston operating in the cylinder. 6. A suspension system according to Claim 1 or Claim 2 comprising a cylinder, within which there are defined said chamber and a further chamber, wherein the displacer is a piston operating in the cylinder and dividing said chambers from each other and wherein the system comprises a duct providing for flow of fluid between said chambers. 7. A system according to Claim 6 wherein said first impedance is in said duct. 8. A system according to Claim 7 wherein said second impedance is interposed between said chamber and said duct. 9. A system according to Claim 1 or Claim 2 defining a further chamber, wherein the chambers are divided from each other by the displacer and wherein means is provided for directing to one of the chambers fluid expelled from the other of the chambers by movement of the displacer. 10. A method of controlling up and down movement of a vehicle body relative to a ground-engaging wheel of the vehicle, wherein there is provided means for defining a chamber containing a fluid and a displacer, the chamber having an outlet through which fluid can flow into and out of the chamber, interposing said means defining the chamber and the displacer between the wheel and the body of the vehicle, associating a first impedance with the means defining the chamber to impede movement of the displacer in a direction to expel fluid from the chamber through the outlet and connecting a first capacitance via a second impedance with the chamber to receive fluid expelled therefrom, wherein, during operation of the vehicle, fluid flows between the chamber and the first capacitor via the second impedance and, wherein, during a substantial period of operation of the vehicle, the second impedance is maintained substantially constant. 11. A method according to Claim 10 wherein said means defining a chamber and a displacer defines a pair of chambers separated from each other by the displacer, wherein a proportion of the fluid expelled from a first of the chambers during operation of the vehicle flows to the first capacitor and wherein a further proportion of the fluid expelled from the first chamber flows to the second chamber. 12. A suspension system substantially as herein described with reference to Figure 1 of the accompanying drawings. 13. A suspension system substantially as herein described with reference to Figure 4 of the accompanying drawings. 14. Any novel feature or novel combination of features disclosed herein or in the accompanying drawings.

Inventors: WILKINSON PAUL ADRIAN, WILKINSON, PAUL, ADRIAN
Applicants: MOTOR IND RES ASS, THE MOTOR INDUSTRY RESEARCH ASSOCIATION
Application year: 1992
Main classification: F16F9
IPC codes: B60G17, F16F9