SUSPENSION AND SPRING ELEMENT THEREFOR

Abstract: A spring element (23) is of lune shape with arcuate sides so that when it is bent into a bowed configuration it provides a substantially constant force to oppose movement of its ends towards one another. The element (23) is usable in a vehicle suspension with a device (22) to vary the geometry of the suspension to correct for changing vehicle loading and dynamic conditions.

Description: i ' itle. SUSPENSION AND SPRING ELEMENT 1ΗEREFOR Description of Invention This invention relates in its broadest aspect to a spring element which is able to exert a springing force which remains substantially constant as the element is deformed in use. It also relates to a suspension device including such a spring element, the suspension device being able to provide a constant or substantially constant force to oppose displacement of one part relative to another, which force is able to be adjusted. The invention has been developed in relation to a suspension for a vehicle. When we use the term vehicle, we mean to include, but not be limited to passenger cars and commercial vehicles of all types including trailers and semi¬ trailers. The invention is also applicable more generally, for example to plant or equipment wherein a suspension system is required to support a part or assembly in such as way as to isolate it from movement or vibration affecting another part of the equipment. Conventional springs as used in vehicle suspensions, e.g. coil or leaf springs, resist spring deflection caused by movement of the vehicle's wheels relative to the sprung structure of the vehicle with a force that increases with such deflection. The force may increase linearly with spring deflection (this condition being referred to as constant rate springing) or may increase more rapidly with spring deflection (rising rate springing). In a vehicle suspension incorporating conventional springing means, selection of the spring characteristics is always a compromise between the requirements of providing a comfortable ride for the occupants of a vehicle, providing the vehicle with satisfactory stability and handling characteristics, and being able to accommodate a wide range of vehicle loading conditions. When one considers a vehicle travelling over an irregular surface, assuming the surface does not have any large scale gradients, it is theoretically desirable if the wheels of the vehicle can follow the irregular surface while the sprung structure of the vehicle follows a level path, assuming the size of the surface irregularities does not exceed the range of wheel movement provided by the vehicle suspension. Ideally, this requires that the springing means employed in the suspension does not increase or decrease the support force applied to the vehicle, so that the vehicle is not deflected from its level path. This would be a constant force (zero rate) suspension system. However, such a suspension system in itself would not establish a vehicle ride height and maintain vehicle attitude and stability under different loading and dynamic conditions. It is desirable therefore to provide a vehicle suspension with springing means which provides a constant or substantially constant springing force (zero rate or substantially zero rate), but which force is adjustable enabling it to be controlled to stabilise the vehicle under varying operating conditions. Such desiderata are also applicable to the other examples of applications of the invention. It is the object of the present invention to provide such a suspension, and a spring element therefor- According to one aspect of the present invention, we provide a spring element operable between two parts movable relative to one another to change the distance between them, the element being of elongate configuration and having end portions adapted to be connected to the two parts whereby the element is deformed into an increasingly buckled configuration as the parts move towards one another, the shape of the element being such as to provide substantially uniform stress in its surface when it is so deformed and a substantially constant force exerted by the element on said parts during said relative movement therebetween. A spring element according to the invention is preferably in the form of an elongate strip of material having a centre line which when the spring element is buckled, lies in a single plane. To achieve the required uniform stress in the surface of the element when it is deformed, if the element is of constant thickness, the amount of material of the element in its width dimension (by which we mean its dimension in the direction peφendicular to the plane occupied by the centre line of the strip) has to vary, from a minimum at or adjacent its ends to a maximum midway between its ends. The thickness of the element is its dimension in the direction parallel to the plane in which the centre line of the buckled element lies. The distribution of material required in an element of constant thickness, to achieve the condition of uniform stress in its surface, is the same as the distribution of material in an element of lune shape, its lune shape being defined by the expression: X L cos - - COS - R 2R W = Wo L 1 - cos - 2R Where L is the overall length of the undeformed element. Wo is the maximum width of the element, at a point midway between its ends. W is the width of the element at a distance X from its mid-way point. R is the radius of the arc into which the element is bent at its condition of maximum deformation. Preferably the spring element is of such a lune shape, but a distribution of the material of the spring element along its length the same as the distribution of material in such a lune element could be achieved, for example, by an element with parallel outer sides and an appropriately shaped cut-out portion between its sides. The spring element may be of metal or composite (fibre-reinforced plastics) material. The ends of the spring element may be adapted for pivotal connection to the two relatively movable parts. For example, the ends of the spring element may be provided with bearing surfaces for the element to abut the movable parts. According to another aspect of the invention, we provide a suspension for suspending a first member relative to a second member, comprising means constraining the first member to a required path of movement relative to the second member; springing means disposed between two parts associated with said members the distance between said two parts changing as the one member moves relative to the other, said springing means comprising at least one spring element connected to said parts so as to exert a substantially constant force therebetween as said parts move relative to one another; and means for changing the geometrical relationship between the at least one spring element and at least one of said members to vary the force which is exerted between the members by the spring means. Preferably the or each spring element is one according to the first aspect of the invention as above set forth, and is connected to said parts so as to be deformed into said buckled configuration and to exert a force urging said parts away from one another and to be increasingly buckled as said parts move towards one another. The change in geometrical relationship preferably comprises a change in the position of the connection of the at least one spring element to one of said parts. As above referred to, the invention has been devised in relation to a vehicle suspension. Thus, according to another aspect of the invention, we provide a vehicle suspension comprising means constraining a wheel carrier member to a required generally vertical path of movement relative to the vehicle structure, and springing means disposed between two parts whose distance from one another decreases as the wheel carrier moves upwardly relative to the vehicle structure, said springing means comprising at least one element according to the first aspect of the invention as above set forth, with its ends pivotally connected to said parts so as to be deformed into said buckled configuration and to exert a force urging said parts away from one another, and to be increasingly buckled as said parts move towards one another, the position of connection of the elongate element to at least one of said parts being adjustable to vary the force which is exerted on the wheel carrier member by the springing means. In a vehicle suspension according to the invention, such a configuration of at least one elongate element constituting the springing means provides a substantially constant springing force (zero rate). As the force created on the wheel carrier member is adjustable, by adjustment of the position of connection of the springing means (in effect, by adjusting the leverage between the wheel carrier member and the springing means) the suspension can be controlled to stabilise the vehicle under different loading and dynamic conditions. Particularly in a vehicle suspension according to the invention, there must be provided a control system which operates on a suitable means for adjusting the position of connection of an end of the elongate element or elements, in order to vary the force that is exerted on the wheel carrier member. This will ensure that the vehicle suspension operates, in response to surface irregularities as above described, about an appropriate mean position. By way of example, the operation of such a control system will be considered in relation to the vehicle at rest whose load is increased. The increased load on the vehicle's wheels would exceed the force exerted by the springing means of the suspension on the wheels, which would cause the vehicle to move downwardly until the limit of suspension travel is reached. Suitable means for detecting commencement of such movement of the vehicle would be provided, and the control system would operate to cause an increase in the force exerted by the springing means of the suspension until the vehicle rises to resume its former position. When such former position is reached, the control system adjusts the force exerted by the springing means until it matches the new vehicle weight, so that the vehicle remains steady at the desired suspension height. When the vehicle is driven, the suspension moves about the mean position as above determined. When the vehicle corners, which would cause roll of the vehicle, the control system would respond to detection of incipient roll to compensate therefor by adjustment of the springing means. Similarly pitch of the vehicle, i.e. changes in attitude thereof due to fore and aft accelerations, would be compensated for. Means could be provided to detect the profile of the road some distance ahead of the vehicle, to add another parameter to those to which the control system is responsive. The control system would have a response time such that no alteration of spring force results from transient high frequency wheel movements as from an uneven road surface when the vehicle is travelling at speed. The control system would operate with a longer time constant, so as to respond only to load changes, gradients, cornering, and fore and aft acceleration. A spring element according to the first aspect of the invention as above set forth does not necessarily provide an absolutely constant force opposing its deformation to an increasingly buckled condition. In practice the force provided by the spring element may vary by up to about 10% from its nominal value, and when we refer to a substantially constant force we include such variations. When one or more such spring elements is or are used in a vehicle suspension, the geometry of the suspension may be arranged so that a more nearly constant force or zero rate is provided at the wheel carrier member. The path of movement of the position of the connection of the end of the or each spring element to (preferably) the vehicle structure may be selected to give either the required spring rate at the wheel or, alternatively, minimum force required to be exerted to change the position of connection of the or each spring element. Examples of such paths of movement are described hereafter. The invention will now be described by way of example with reference to the accompanying drawings, of which:- Figure 1 is a diagrammatic illustration of a vehicle suspension according to the invention; Figure 2 is a diagrammatic illustration of how spring elements of the suspension may be connected to parts of the suspension; Figure 3 is a diagrammatic representation of the shape of a spring element of the suspension; Figure 4 is a diagram showing the geometry of the suspension. The vehicle suspension illustrated in Figure 1 comprises a wheel carrier member 10 which supports a stub axle with a flange 11 for wheel mounting. An arm 12 extending from the wheel carrier member provides for connection to a steering link. Also shown is part of a drive shaft 13 and a boot 14 covering a constant velocity ratio universal joint providing a torque transmitting connection to the stub axle. The wheel carrier member is connected, by swivelling joints 15, 16, to an upper wishbone and lower wishbone 17, 18 respectively. The wishbones provide for pivotal connection, at bushes 19, 20, to the chassis or integral body-chassis structure of a vehicle, not shown. The wishbones provide for vertical or substantially vertical movement of the wheel carrier member 10 relative to the vehicle structure in a predetermined path. Springing is provided by a number, in this case four, of elongate elements whose one ends engage a part of the wheel carrier member, not visible in the drawing, and whose other ends engage an abutment member 21. The abutment member 21, which is supported by the vehicle structure, has its position adjustable relative to the vehicle structure by a fluid pressure operated piston and cylinder device 22. The abutment member 21 is positioned so that when the wheel carrier member moves upwardly relative to the vehicle structure, the distance between the abutment member and the part of the wheel carrier member engaged the other ends of the spring elements decreases. When the position of the abutment member is adjusted by the device 22, the leverage between the wheel carrier member and the spring elements is varied. The spring elements, the uppermost one of which is visible at 23, may be of spring steel sheet or other suitable resilient material, and are buckled into a bowed configuration so that they exert a force between the wheel carrier member and abutment member 21 to resist upward movement of the former relative to the vehicle structure. The shape of the spring elements 23 is such that they act as substantially constant force or zero rate springs, i.e. as they are increasingly bowed with decrease in the linear distance between their ends, the force does not increase substantially. As illustrated (and described hereafter in greater detail), the spring elements are of a lune shape with sides which are generally arcuate (actually, part of a sine curve). Each end of each spring element, as indicated at 24 for the uppermost element 23 where it engages the abutment member 21, is widened to provide an elongate knife edge pivot. Referring now to Figure 2 of the drawings, this illustrates how ends of spring elements may engage parts such as the abutment member 21 and the wheel carrier member 10. A saddle member 30 comprises two spaced generally V- shaped parts 31, 32, between which there extends a portion 33 provided with a number of spaced V-shaped recesses 34 for engagement by the ends of spring elements two of which are shown at 35, 36. The ends of the spring elements are provided with knife edged pivot formations. The number of such V-shaped recesses 34 is, of course, dependent on the number of spring elements required to be used. The saddle member 30 fits on a support member 37 which has two spaced upstanding parts 38, 39 with aligned knife-edged support surfaces 40, 41. These engage in the internal V-shaped recesses of the parts 31, 32 of the saddle member, so that the latter is able to pivot to a small extent about an axis indicated at 42. Because the portion 33 with which the ends of the spring elements engage is, in the orientation of the drawing, beneath the axis 42, the saddle member 30 will align itself in accordance with the direction of the force exerted by the spring elements. As hereinbefore referred to, a vehicle suspension as above described requires a control system which causes the means such as the fluid pressure operated piston and cylinder device 22 to adjust the position of the abutment member 21 to ensure that the suspension operates about the required mean position. As depicted diagrammatically in Figure 1, a control system as represented at 45 includes a height or distance measuring device to provide a signal representative of the position of the member of the suspension, e.g. of the upper wishbone 17. Suitably processed, this signal controls a valve which determines the flow of fluid to or from the device 22 to establish the required position of the abutment member 21. Other vehicle operational parameters to which the control system may respond include, for example, lateral, vertical or angular acceleration; steering angle; or detected features or characteristics of the road surface which the vehicle is approaching. Figure 3 shows diagrammatically the lune shape of one of the spring elements 23, from which the widened knife-edged end portions have been omitted. The shape of the element is defined by the expression: X cos cos R 2R W = Wo L cos 2R Where L is the overall length of the undeformed element. Wo is the maximum width of the element, at a point midway between its ends. W is the width of the element at a distance X from its mid-way point. R is the radius of the arc into which the element is bent at its condition of maximum deformation. 10 Such a shape gives, for an element of constant thickness, substantially uniform stress in the stressed surface of the element when it is buckled. Springing elements of the above configuration may be made of composite (fibre-reinforced plastics) material, or any other suitable resilient material. Referring now to Figure 4 of the drawings, this shows diagrammatically the geometry of a suspension as shown in Figure 1. A pivoted suspension member such as a wishbone is indicated by the line A-B, pivoted at B about an axis peφendicular to the plane of the drawing. At A, a wheel carrier member or the like can be secured. A spring element according to the invention is pivotally connected to the suspension member at point C, and a suitable position for the abutment of other end of the spring element is determined by constructing a semi-circle on the line C-B (centre of curvature O) when the suspension member A-B is horizontal. The other end of the spring element is pivotally connected to a point on this semi-circle, e.g. as indicated at D. Then the force at A will bear a substantially constant relationship to the force exerted by the spring element itself, and will thus itself be substantially constant. If the position of the point D is moved, for example to D 1 , the force at A will increase in the ratio D α B:DB, but again will remain substantially independent of wheel position. In this way, the force at the wheel can be adjusted to an appropriate value, by a control system as above referred to. The range of adjustment of the point D may be provided, e.g. as indicated from E to E 1 . The point may actually be movable on a straight line as indicated; this approximates sufficiently closely to the theoretical path of movement to operate in the required manner. Linear movement is readily provided, e.g. by a fluid pressure piston and cylinder device as illustrated at 22 in Figure 1, or by an arrangement of a screw and nut driven by an electric motor. As an alternative to the movement of the abutment of the spring element on the semi-circle with centre of curvature O (or on the chord of such semi-circle, the abutment point may move on an arc whose centre of curvature is at the point of connection C of the spring element to the suspension member, when the suspension member is horizontal (or on a chord of such an arc). Such a range of adjustment of the spring element abutment requires a minimum force to be exerted by the actuator in changing the position of abutment, but there is a greater variation in the force exerted at the wheel carrier member as the suspension member pivots. Although described above in relation to a vehicle suspension, it will be appreciated that the suspension, and spring element therefor according to the invention, are applicable more generally where similar or analogous requirements arise. For example, the invention is applicable to plant or equipment wherein a suspension is required to support a part or assembly in such a way as to isolate it from movement or vibration affecting another part of the plant or equipment. In the case where dynamic changes in the operational parameters of the equipment do not occur in the same manner as for a vehicle, it would be possible to provide for manual adjustment of the position of connection of the spring element or elements to one or more of the suspended parts. Yet further examples of applicability of the invention are in rail vehicles or water craft where some form of suspension is required. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, 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: 12 CLAIMS 1. A spring element operable between two parts movable relative to one another to change the distance between them, the element being of elongate configuration and having end portions adapted to be connected to the two parts whereby the element is deformed into an increasingly buckled configuration as the parts move towards one another, the shape of the element being such as to provide substantially uniform stress in its surface when it is so deformed and a substantially constant force exerted by the element on said parts during said relative movement therebetween. 2. A spring element according to Claim 1 comprising an elongate strip of material having a centre line which when the spring element is buckled lies in a single plane, and wherein the amount of material in the width dimension of the strip varies from a minimum at or adjacent its ends to a maximum midway between its ends. 3. A spring element according to Claim 2 wherein the variation of the amount of material in the width dimension of the strip is the same as the variation of width of an element of lune shape. 4. A spring element according to Claim 3 which is of a lune shape. 5. A spring element according to Claim 3 or Claim 4 wherein said lune shape is defined by the expression: X L W = Wo Where L is the overall length of the undeformed element. Wo is the maximum width of the element, at a point midway between its ends. W is the width of the element at a distance X from its mid-way point. R is the radius of the arc into which the element is bent at its condition of maximum deformation. 6. A spring element according to any one of the preceding claims which is of metal. 7. A spring element according to any one of Claims 1 to 5 which is of composite material. 8. A spring element according to any one of the preceding claims wherein the ends thereof are adapted for pivotal connection to the two relatively movable parts. 9. A spring element according to Claim 8 wherein the ends thereof have bearing surfaces for abutting the movable parts. 10. A suspension for suspending a first member relative to a second member, comprising means constraining the first member to a required path of movement relative to the second member; springing means disposed between two parts associated with said members, the distance between said two parts changing as one member moves relative to the other, said springing means comprising at least one spring element connected to said parts so as to exert a substantially constant force therebetween as said parts move relative to one another; and 14 means for changing the geometrical relationship between the at least one spring element and at least one of said members, to vary the force exerted between the members by the springing means. 11. A suspension according to Claim 10 wherein the or each said spring element is an element according to any one of Claims 1 to 9, and is connected to said part so as to be deformed into said buckled configuration and to exert a force urging said parts away from one another and to be increasingly buckled as said parts move towards one another. 12. A suspension according to Claim 10 or Claim 11 wherein said change in geometrical relationship comprises a change in the position of the connection of the at least one spring element to one of said parts. 13. A suspension according to any of Claims 10 to 12 which is a vehicle suspension, and said members comprise a wheel carrier member and a part of the sprung vehicle structure. 14. A suspension according to any one of Claims 10 to 13 further comprising control means operable to cause a change in said geometrical relationship in response to at least a measurement of the position of the two members relative to one another. 15. A vehicle suspension comprising means constraining a wheel carrier member to a required generally vertical path of movement relative to the vehicle structure, and springing means disposed between two parts whose distance from one another decreases as the wheel carrier moves upwardly relative to the vehicle structure, said springing means comprising at least one spring element according to any one of Claims 1 to 9 with its ends pivotally connected to said parts so as to be deformed into said buckled configuration and to exert a force urging said parts away from one another, and to be increasingly buckled as said parts move towards one another, the position of connection of said at least one elongate element to at least one of said parts being adjustable to vary the force which is exerted on the wheel carrier member by the springing means. 16. A vehicle suspension according to Claim 15 further comprising control means operative in response to at least one vehicle operational parameter to cause adjustment of said position of connection of said at least one elongate element to said at least one part. 17. A vehicle suspension according to Claim 16 wherein said control means is operative to cause the suspension to operate about a desired mean position. 18. A vehicle suspension according to any one of Claims 15 to 17 wherein said position of connection of said at least one elongate element to said at least one part is movable along a path such that the force exerted at the wheel carrier member as it moves relative to the vehicle structure is substantially constant at each position of connection. 19. A vehicle suspension according to any one of Claims 15 to 17 wherein said position of connection of said at least one elongate element to said at least one part is movable along a path such that minimum force is required to be exerted to change said position. 20. A suspension according to Claim 11, or a vehicle suspension according to any one of Claims 15 to 19, wherein the ends of said at least one spring element engage members pivotable relative to the parts to which they are connected, to self-align in accordance with the direction of the force exerted by the at least one spring element. 21. A spring element substantially as hereinbefore described with reference to the accompanying drawings. 22. A suspension substantially as hereinbefore described with reference to the accompanying drawings. 23. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.

Inventors: PULLEN JOHN, PULLEN, JOHN
Applicants: GKN TECHNOLOGY LTD, GKN TECHNOLOGY LIMITED
Application year: 1991
Main classification: B60G17
IPC codes: B60G11, B60G17, F16F1