GB2283799A

GB2283799A - A damper for a vehicle suspension system

Info

Publication number: GB2283799A
Application number: GB9323556A
Authority: GB
Inventors: Richard Hurdwell
Original assignee: Lotus Cars Ltd
Current assignee: Lotus Cars Ltd
Priority date: 1993-11-15
Filing date: 1993-11-15
Publication date: 1995-05-17
Also published as: GB9323556D0

Abstract

The damper (70) comprises a cylinder, a piston moveable in the cylinder and defining with the cylinder first (A) and second (B) variable volume chambers for receiving fluid. Connection means (71, 78) is provided which allows flow of fluid into or out of the first variable volume chamber (A). The cylinder is connectable to one of a vehicle body and a wheel and hub assembly. Compliant means (72, 74) is provided in or connected to both of the first (A) and second (B) chambers, the compliant means (72, 74) deforming an application of force to the damper (70) to allow motion of the piston relative to the cylinder. <IMAGE>

Description

A DAMPER FOR A VEHICLE SUSPENSION SYSTEM The present invention relates to a damper for a vehicle suspension system.

In the past it has been known to provide a passive damper, commonly called a shock absorber, connected between a vehicle body and a wheel and hub assembly. The damper usually acts in parallel with a road spring between the vehicle body and the wheel and hub assembly. The essential components of the damper are a piston which moves through fluid located in the cylinder, with the viscosity of the fluid providing resistance to motion of the piston.

A damper reacts passively to motion inputs, with the resistive force being a function of the velocity of motion of the piston.

There are also adaptive suspension systems which comprise dampers which react passively to input loads but which have variable damping rates. The damping characteristics of the damper are controlled in such systems by a computer which varies the damping rate of the damper.

A damper is generally connected to the vehicle body by means of a compliant rubber isolator. It has been found necessary to include an isolator in the load path between the vehicle body and the vehicle wheel and hub assembly, because a conventional passive damper is not very efficient at attenuating high frequency noise inputs, such as road noise and is also not very efficient at attenuating very sudden shock load inputs. A rubber isolator is commonly provided both to cope with the high frequency noise inputs and the sudden shock load inputs to the vehicle. However, the isolator detracts from the ability of a damper to provide ride and handling control of the vehicle.

The present invention provides a damper for a vehicle suspension system comprising a first cylinder, a piston moveable in the first cylinder which defines with the first cylinder first and second variable volume chambers for receiving fluid, connection means which allows flow of fluid into or out of the first and second variable volume chambers and which has restriction means to restrict the flow of fluid, wherein the first cylinder is connectable to one of a vehicle body and a wheel and hub assembly and the piston is connectable to the other of the vehicle body and the wheel and hub assembly, characterised in that compliant means is provided in or connected to both of the first and second chambers, the compliant means deforming on application of force to the damper to allow motion of the piston relative to the first cylinder.

The compliant means can take any form, for instance; a gas spring, probably requiring separation from the hydraulic fluid in the damper or actuator (by means of a membrane or piston); a compliant oil reservoir or cylinder housing; inclusion of compliant material (e.g. closed cell foam) in a chamber; or the use of a working fluid in the actuator which has a bulk modulus reduced from that of the usual hydraulic fluid used in dampers and actuators.

The conventional rubber isolators present in the damper load path between a wheel and hub assembly and a vehicle body can be replaced by non-compliant connections if the damper or actuator of the present invention is used. Alternatively, very stiff compliant mounts can be used, of a stiffness which would not provide sufficient attenuation in a normal passive or adaptive suspension system.

Preferably the connection means connects the first and second variable volume chambers and the compliant means can allow motion of the piston relative to the cylinder without the need for flow of fluid between the first and second chamber.

Preferably the compliant means comprises first compliant means connected to the first chamber and second separate compliant means connected to the second chamber.

In a first preferred embodiment the compliant means comprises a compliant body located in one of the chambers.

The compliant body could be, for instance, a closed cell foam block. This would have the advantage of cheapness.

In a second preferred embodiment the compliant means comprises a gas spring permanently connected by connection means to one of the chambers.

The use of a gas spring would be more expensive than use of a compliant body, but would provide better compliance characteristics. A gas spring could be provided within a chamber by the use of a floating piston arrangement, or could be connected to a chamber by a suitable conduit.

In a third preferred embodiment the compliant means comprises a compliant fluid container connected by connection means to one of the chambers.

The compliant container could take the form of a compliant (eg. rubber) hose. Such hoses are cheaper and simpler than gas springs. The compliant container could also take the form of a container having a piston movable therein and a spring exerting a biasing force on the piston.

The use of compliant hoses as the connection means would enhance performance gain.

The present invention provides in a second aspect a passive or adaptive suspension system for a vehicle comprising a damper as described above, first connection means for connecting one of the piston and the cylinder of the damper to a vehicle body and second connection means for connecting the other of the piston and the cylinder to a wheel and hub assembly, wherein the first connection means includes no compliant isolator member.

Preferably the second connection means includes no compliant isolator member.

The present invention is advantageous because there is no lost motion in the suspension arrangement caused by flexing of isolators. Therefore more energy is dissipated directly by the damper, the damper cannot vibrate on its mountings and better control of wheel and hub motion is achieved. The invention also allows control of the hysterises in the load/velocity characteristic of the suspension. This provides improved isolation of the vehicle from high frequency loading and shock loading.

The invention also frees the design of suspension systems by removing the need for an isolator.

The damper of the present invention further provides a "hysterisis effect", which will be described later, which has a beneficial effect on vehicle ride.

Preferred embodiments of the present invention will now be described with reference to the accompanying figures in which; Figure 1 shows a vehicle suspension system of the prior art, Figure 2 shows a conventional monotube damper according to the prior art, Figure 3 shows a conventional twin tube damper according to the prior art, Figure 4 shows a monotube damper according to the present invention, Figure 5 shows the monotube damper of figure 4 in use, Figure 6 shows a twin tube damper according to the present invention, Figure 7 shows the twin tube damper of figure 6 in use, Figure 8 shows a second embodiment of monotube damper according to the present invention, Figure 9 shows the monotube damper of figure 8 in use, Figure 10 is a schematic showing a damper according to the invention attached to a vehicle about to pass over a road bump.

Figure 11 is a graph illustrating the hysterisis properties of a damper according to the invention.

Figure 1 shows an example of a conventional passive suspension system for a vehicle. There can be seen in the figure a wheel and hub assembly 10 which is mounted on a suspension system comprising a swing arm 11 which is pivotally connected by pivots 12 to a vehicle body 13. Acting between the swing arm 11 and the vehicle body 13 is a road spring 14. A passive damper 15 is connected between the swing arm 11 and the vehicle body 13 and acts in parallel with the road spring 14. The passive damper 15 comprises a piston 16 moveable in the cylinder 17. The piston 16 is connected by a connecting rod 18 and by a compliant pivot joint 19 to the swing arm 11. The cylinder 17 is connected by a rubber isolator 20 to the vehicle body 13.

An example of a conventional monotube damper is shown in figure 2 and in this damper it can be seen that the piston 16 defines in the cylinder 17 an upper chamber A and a lower chamber B. An orifice 21 is provided in the piston 16 to allow flow of fluid from chamber A to chamber B and vice versa. The orifice 21 is designed to deliberately restrict flow of fluid therethrough to provide damping forces.

A floating piston 22 is provided in the top chamber A and gas is sealed in a chamber C defined between the floating piston 22 and the top of cylinder 17. The floating piston 22 and the chamber C are necessary since the connecting rod 18 reduces the cross-sectional area of the chamber B with respect to the chamber A. Thus, for a given chamber height chamber A can contain more fluid than chamber B and if fluid is to flow effectively from one chamber to the other some degree of compliance must be provided and this is achieved by means of the floating piston 22 in the chamber C.

In the twin tube damper 23 of figure 3 a second cylinder 24 is used instead of a second piston 22.

The first cylinder 17 is located within the outer second cylinder 24 and is fixedly attached thereto.

Gas is provided in the chamber D defined between the outer cylinder 24 and the inner cylinder 17. The gas is located in the top annular portion of the chamber D. The piston 16 moves within cylinder 17 and has two orifices which are shown as 25 and 26. The orifice 26 is controlled by a one way valve 27.

An orifice 28 is also provided between the bottom chamber B and the chamber D. A one way valve 29 is also provided in a second orifice connecting the chamber B and chamber D. In the arrangement it can be seen that the connecting rod 18 will in fact be connected to a vehicle body rather than a swing arm.

The provision of rod 18 in chamber A decreases the cross-sectional area of the piston 16 which acts in chamber A in comparison with the cross-section of the piston 16 acting in chamber B. The gas in chamber D is provided to account for the different cross-sectional areas.

The isolator 20 (shown in figure 1) is necessary in a vehicle suspension system when normal prior art passive dampers are used e.g. the dampers 15 and 23 of figures 2 and 3. The dampers 15 and 23 shown in figures 2 and 3 transmit high frequency noise inputs to the vehicle, such as road noise, and also sharp sudden shock loads to the vehicle. The rubber isolators are needed to provide compliance in the load path between the wheel and hub assembly 10 and the vehicle body 13 to attenuate these inputs. However, the use of the isolators detracts from the handling and some aspects of the ride of the vehicle, as mentioned above.

A monotube damper 30 according to the present invention is shown in figure 4. The monotube damper 30 of the present invention comprises all of the components of the monotube damper 15 of the prior art and identical components have identical reference numerals.

The difference between the monotube 30 of figure 4 and the monotube 15 of the prior art is the inclusion of compliant means in the lower chamber B, the compliant means comprising a closed cell foam block 31. When the monotube damper 30 of figure 4 is used in place of the monotube damper 15 of figure 2, the need for use of a rubber isolator such as isolator 20 is eliminated and the cylinder 17 can be connected directly to a vehicle body via a suitable articulating joint. This is shown in figure 5 where the damper 30 can be seen connected to a vehicle body by an articulating joint 32 which includes no compliant rubber isolator. The damper 30 is also connected to the swing arm by a joint 34 which does not include an isolator.

In figure 6 a twin tube damper 40 according to the present invention is illustrated. In most respects the damper 40 is identical to the damper 23 shown in figure 3 and identical components have been given the same reference numerals.

The difference between the figure 6 twin tube damper 40 and the figure 3 twin tube damper 23 lies in the provision of compliant means connected to chambers A and B. The chamber A is connected to a compliant enclosure 41 via a tube 42. The chamber B is connected to a gas spring 43 via a tube 44. The tube 44 includes a compliant hose portion 44A, which is made of rubber.

If the twin tube damper 23 shown in figure 3 is used in a vehicle suspension system, then the connecting rod 18 will be connected to the vehicle body via a rubber isolator, shown as 20 in figure 1.

If the twin tube damper 23 is replaced by the twin tube damper 40 of the present invention illustrated in figure 6 then the rubber isolator 20 should preferably not be used and the connecting rod 18 can be connected to the vehicle body via a suitable non-compliant joint. This is shown in figure 7, where the damper can be seen connected to a vehicle body by an articulating joint 45 which does not include a compliant rubber isolator and to a wheel and hub assembly 46 by a joint 47 which does not include a compliant isolator member.

A second embodiment of monotube damper according to the invention is shown in figure 8. In figure 8 the monotube damper 70 has a chamber A which is connected by a tube 71 to a gas spring 72. The monotube damper 70 also has a chamber B connected by a tube 73 to a compliant fluid container 74. The compliant fluid container 74 comprises a cylinder 75 in which a piston 76 is movable. The piston 76 is biased by a spring 77 which exerts a biasing force on piston 76 when compressed. The piston 76 and cylinder 75 define together a variable volume chamber E into which and out of which fluid can flow to and from the chamber B of damper 70.

In the tube 71 there is provided a restriction 78 which restricts flow of fluid through the tube 71.

The restriction introduces a controlled element of damping in the fluid flow; this avoids any problems caused by the piston bouncing within the cylinder.

Figure 9 shows the damper 70 in use. The damper 70 is connected to a vehicle body by a joint 79 which does not include a rubber isolator. Also the damper 70 is connected to the swing arm 80 by a joint 81 which does not include a compliant member.

The gas spring 72 could be provided within the cylinder of damper 70, with a dividing wall provided in the chamber A and an orifice in the dividing wall.

The stiffness of the compliant means of a damper of the invention will be chosen to give an effective compliance to the damper arrangement similar to or slightly stiffer than a comparable conventional damper and isolator arrangement. Typically for a light passenger vehicle the stiffness will be in the range 900 N/mm to 2500 N/mm, with a preferred median of around 1600 N/mm, when calculated as an effective stiffness at the wheel.

The selection of a suitable stiffness for compliant means could be made after calculating the natural frequency of the spring/damper arrangement, but more typically the stiffness would be chosen emperically by testing.

The use of compliant means provides a beneficial "hysterisis" effect, as illustrated by figures 10 and 11. Figure 10 shows schematically a wheel 50 connected to a vehicle body 51 by a damper 52, which is a damper according to the invention (eg. damper 15, damper 23 or damper 70). The vehicle body 51 is moving in the direction of the shown arrow and thus the wheel 50 must pass over a bump. There can be seen on the bump various points A to I. Illustrated on the graph of figure 11 is a curve showing the relationship between the vertical component of velocity of the wheel 50 (ie. the velocity towards and away from the vehicle body) and the force transmitted by the damper 52 to the vehicle body 51. A negative velocity indicates that the wheel 50 is moving upwardly, towards the vehicle body 51, and a positive velocity indicates that the wheel 50 is moving downwardly, away from the vehicle body. A negative force indicates a compressive force required to close the damper 51 and a positive force indicates a force required to extend the damper 51. The points A to I are marked on the graph.

As the wheel 50 first engages the bump the wheel 50 is given an upward (negative velocity) which increases in magnitude from point A to point C and decreases in magntitude as the top of the bump is reached from point C to point E. It can be seen from a comparison of the points B and D that the compressive force on the damper 51 at point B is less in magnitude than the compressive force on the damper at point D, even though the damper has the same velocity (ie. is contracting at the same rate) at both points. This is advantageous since it leads to the result that the vehicle body is subjected to a more gradually increasing force than would be provided by a normal damper; the force/velocity characteristics of a known damper are given by the dotted line 60 in figure 7 and it will be seen that the force on a normal damper for a given velocity is the same whether the velocity is increasing or decreasing. With use of a damper according to the invention, the driver experiences less of a jerk on encountering a bump.

After the wheel 50 has reached the top of the bump the damper 51 is extended under the action of a road spring 53 acting in parallel with the damper 51.

This happens from E to I. Again the hysterisis effect results in two different expansive forces for the same rate of extension of the damper, depending on whether the rate of extension is increasing or decreasing in magnitude. This again is advantageous in lowering the downward jerk force on the vehicle body as the wheel passes the top of the bump.

Some prior art dampers do exhibit some degree of hysterisis, but this is not deliberate. The present invention deliberately introduces hysterisis and the hysterises is then tu nable to meet specific vehicle needs.

The stiffnesses of the compliant means of the invention will preferably be adjustable so that the properties of the damper can be adjusted to suit different vehicle needs.

Whilst in the embodiments described fluid flows from one chamber to another via a restriction through the piston of the damper this is not a necessary feature of the invention: instead a restriction could be provided to allow fluid flow out of one chamber to another suitable container.

Whilst in the embodiments described above the compliant means comprises gas springs, compliant enclosures and compliant inserts, it is also envisaged by the applicant that compliant means could comprise compressible hydraulic fluid which is more compressible than the usual hydraulic fluid used for dampers or deliberately compliant sides for the cylinders of the dampers.

The bulk modulus of hydraulic fluid usually used 2 for dampers is 1380 N/mm2, which falls on 2 contamination by air to 780 N/mm2. The present invention would use hydraulic fluid with a bulk modulus of around 30% to 40% of the standard bulk modulus, typically around 500 N/mm2. Alternatively gas springs or compliant containers would be connected to the fluid to lend the fluid an effective stiffness of around 500 N/mm2.

Whilst in the above specification we have only mentioned monotube and twin tube dampers, it should be appreciated that the present invention could be used with any form of damper.

Claims

CLAIM8:

1. A damper for a vehicle suspension system comprising a first cylinder, a piston moveable in the first cylinder and defining with the first cylinder first and second variable volume chambers for receiving fluid, connection means which allows flow of fluid into or out of the first variable volume chamber, wherein the first cylinder is connectable to one of a vehicle body and a wheel and hub assembly and the piston is connectable to the other of the vehicle body and the wheel and hub assembly, characterised in that compliant means is provided in or connected to both of the first and second chambers, the compliant means deforming on application of force to the damper to allow motion of the piston relative to the first cylinder.

2. A damper as claimed in Claim 1 wherein the connection means connects the first and second chambers and the compliant means can allow motion of the piston relative to the first cylinder without the need for flow of fluid between the first and second chambers.

3. A damper as claimed in Claim 1 or Claim 2 wherein the compliant means comprises first compliant means connected to the first chamber and separate compliant means connected to the second chamber.

4. A damper as claimed in any one of the preceding claims wherein the compliant means comprises a compliant body located in one of the chambers.

5. A damper as claimed in any one of the preceding claims wherein the compliant means comprises a gas spring permanently connected by connection means to one of the chambers.

6. A damper as claimed in any one of the preceding claims wherein the compliant means comprises a compliant fluid container connected by connection means to one of the chambers.

7. A damper as claimed in Claim 5 or Claim 6 wherein the connection means comprises a compliant hose.

8. A damper as claimed in any one of the preceding claims comprising additionally: a second cylinder fixed to and at least partially surrounding the first cylinder, a third chamber for receiving fluid which is defined between the exterior of the first cylinder and the interior of the second cylinder, connecting means connecting the first and third chambers, restriction means in the connecting means to restrict flow of fluid between the first and third chambers and mounting means on the exterior of the second cylinder to enable the first cylinder to be connected to one of the vehicle body and the wheel and hub assembly via the second cylinder.

9. A damper as claimed in any one of the preceding claims wherein the compliant means comprises hydraulic fluid of a bulk modulus with a magnitude 30% to 40% of the bulk modulus of uncontaminated hydraulic fluid used in a comparable conventional damper.

10. A damper as claimed in Claim 9 wherein the hydraulic fluid has a bulk modulus in the range of 400 to 600 N/mm.

11. A damper as claimed in any one of the preceding claims wherein the or each compliant means is arranged to provide the damper with a stiffness in the range of 900 N/mm to 2500 N/mm.

12. A damper as claimed in Claim 10 wherein the or each compliant means is arranged to provide the damper with a stiffness of approximately 1600 N/mm.

13. A damper as claimed in any one of the preceding claims wherein the stiffness of the compliant means is adjustable, a user of the damper whereby the user can vary the stiffness of the damper to suit different applications or modes of use.

14. A damper as claimed in any one of the preceding claims wherein there is provided compliant means which can allow the damper to have the same rate of extension for two different extending forces acting on the damper, a first force when the rate of extension is increasing in magnitude and a second high force when the rate of extension is decreasing in magnitude.

15. A damper as claimed in any one of the preceding claims wherein there is provided compliant means which can allow the damper to have the same rate of contraction for two different compressive forces acting on the damper, a first force when the rate of contraction is increasing in magnitude and a second force of greater magnitude when the rate of contraction is decreasing.

16. A damper as claimed in Claim 14 or Claim 15 wherein the second higher force is more than 1.25 times greater than the first lower force.

17. A damper as claimed in Claim 16 wherein the second higher force is more than 1.5 times greater than the first lower force.

18. A damper as claimed in any one of Claims 15, 16 or 17 wherein the compliant means is adjustable by a user of the damper whereby the ratio of the magnitude of the first force to the magnitude of the second force is adjustable by the user.

19. A passive or adaptive suspension system for a vehicle comprising a damper as claimed in any one of the preceding claims, first connection means for connecting one of the piston and the cylinder of the damper to a vehicle body and second connection means for connecting the other of the piston and the cylinder to a wheel and hub assembly, wherein the first connection means includes no compliant isolator member.

20. A passive or adaptive suspension system as claimed in Claim 19 wherein the second connection means includes no compliant isolator member.

21. A damper substantially as hereinbefore described with reference to and as shown in the accompanying figures 2 to 5.