GB2372792A

GB2372792A - A magnetorheological fluid damper

Info

Publication number: GB2372792A
Application number: GB0102676A
Authority: GB
Inventors: Patrick Lowe
Original assignee: Trysome Ltd
Current assignee: Trysome Ltd
Priority date: 2001-02-02
Filing date: 2001-02-02
Publication date: 2002-09-04
Anticipated expiration: 2021-02-02
Also published as: GB2372792B; GB0102676D0

Abstract

A fluid damper of the piston-and-cylinder type in which the damping fluid is a magneto-rheological fluid and the piston having a piston head which includes a fluid pathway 218 through which fluid may flow from one side of the piston head to the other. The piston head comprises an electromagnet 219 which is capable of applying a magnetic field (220 in Fig. 17) across magneto-rheological fluid in the pathway 218 to alter its viscosity an thereby alter the response of the damper; and at least two distinct portions 200, 201 each formed of a material having a relative magnetic permeability greater than 3000 and forming a path for magnetic flux from the electromagnet 219, the portions being weldlessly fixed together to define the fluid pathway.

Description

DAMPERS This invention relates to dampers, for example for vehicle suspension arrangements.

Figure 1 shows a typical damper usable in a vehicle suspension. The damper comprises a cylinder 1 enclosing a chamber 2 of fluid. A piston having a head 3 and a piston rod 4 can slide longitudinally in the chamber 2. Passages 5 through the piston head allow fluid to pass against resistance from one side of the piston head to the other. Fixing points 6 and 7 for the vehicle's body and the vehicle's wheel structure are provided on the cylinder and the piston rod. A chamber 7 in the cylinder contains a gas and is separated from the chamber 2 by a diaphragm 9.

The force required to move the piston in the cylinder is dependant on the relative velocity of the piston and the cylinder. Figure 2 illustrates a typical force/velocity response curve for a vehicle damper. In figure 2 the curve 10 shows the force required to move the piston in the bump (compression) direction. Curve 11 shows the force required to move the piston in the rebound (extension) direction.

In designing vehicle suspension the response of the damper is engineered to approach a desired forcelvelocity response. In a damper of the type shown in figure 1 this is generally done by means of mechanical adjustments to valves 8 set across the orifices 5. However, this arrangement has a number of disadvantages: 1. Since the valves are located inside the sealed chamber 2 it is difficult to arrange for mechanical adjustment of the valves.

2. Access to the damper is often hindered by other elements of the vehicle suspension, so it is difficult for an engineer to gain access to the damper to adjust it. For this reason, the damper cannot in normal circumstances be adjusted whilst the vehicle is running.

3. The valves only allow a finite number of adjustments to be made to the damper, so only an approximation to the desired response can normally be achieved; and it is difficult to determine the correct set of valve adjustments to be made.

4. It is normally desired to have different forms of response in bump and rebound and one-way valving therefore has to be provided and adjusted separately for each direction of response.

5. The response of the damper is additionally dependant on factors such as temperature, which alters the viscosity of the fluid in the chamber 2. Since it is difficult to adjust the damper, viscosity changes cannot easily be compensated for, and the characteristics of the damper are therefore likely to vary during use.

These problems are especially acute in suspension dampers for motor racing, where it is often necessary to make rapid adjustments to dampers in an attempt to achieve an ideal car set-up.

Figure 3 shows another form of damper that has been proposed. The damper of figure 3 is a magnetorheological damper. The fluid in chamber 20 contains particles of a magnetically susceptible material such as iron. The piston head 21 contains a coil 22 which can be caused to induce a magnetic field by application of an alternating current electrical signal from drive unit 23 through wires 24,25 that lead through the piston rod 26. The field from coil 22 extends across orifices 27 through the piston head 21. When a field is applied by the coil the arrangement of the particles in the fluid changes and the viscosity of the fluid in the region of the field (including in the orifices 27) increases.

Figure 4 shows typical force/velocity response curves for the bump direction for a damper of the type shown in figure 3 at a range of applied voltages across the coil 22. The applied voltage can be selected by selector switch 28 in figure 3. By selecting one of the applied voltages a desired one of the response curves can be chosen. This provides a convenient way of selecting the response of the damper from one of the available curves. However, other response curves are not available unless mechanical adjustments of the orifice dimensions are also made, for example by means of additional mechanically adjustable valves set across the orifices 27, this arrangement does not allow for precise variation of the form of the response curve in order to achieve a desired set-up.

One difficulty in the design of magnetorheological dampers is that in order to improve the performance of a magnetorheological damper, especially at high frequencies of operation, it is desirable to use a high permeability material for the piston head. However, the shape of the piston head is generally complex and formed of a number of parts, and high permeability materials such as electrical steels are very difficult to weld. Therefore, easily weldable but low permeability materials such as mild steel have generally been used.

According to the present invention there is provided a damper comprising a cylinder defining a chamber for containing magnetorheological fluid; and a piston having a piston head movable in the chamber and including a fluid pathway through which fluid may flow from one side of the piston head to the other, the piston head comprising: an electromagnet capable of applying a magnetic field across magnetorheological fluid in the passage to alter its viscosity and thereby alter the response of the damper; and at least two distinct pieces each formed of a material having a relative magnetic permeability greater than 3000 and forming a path for magnetic flux from the electromagnet, the parts being weldlessly fixed together to cooperatively define the fluid pathway.

I Preferably the relative magnetic permeability is greater than 50000 or more preferably greater than 100000. Preferably the said two pieces are formed of the same material. The said pieces may be formed of an iron-silicon alloy, for example an alloy comprising from 40 to 60% iron with the balance nickel, or of an iron nickel alloy.

The electromagnet may be actuable to define two magnetic poles with a magnetic flux therebetween extending across the pathway. The piston head may be arranged such that the flux at a only single pole of the electromagnet is capable of substantially influencing the viscosity of magnetorheological fluid in the pathway.

The fluid pathway may include a first portion at the said single one of the poles and a second portion at the other of the poles. The piston head may define at least one bridge for magnetic flux across the second portion of the fluid pathway.

The first portion may be a constricted portion of the pathway.

The piston head may comprise a first region of magnetically susceptible material adjacent to the electromagnet, a second region of magnetically susceptible material spaced from the first region by the fluid pathway and a third region of magnetically susceptible material bridging the fluid pathway between the first region and the second region to provide the said bridge for magnetic flux. The second region may surround the first region. The piston head may comprise magnetically susceptible material defining a longitudinally extending central portion, a first end piece extending radially outward from a first end of the central portion, a longitudinally extending side wall surrounding the central portion and a second end piece joining the side wall to the second end of the central portion, the first portion of the fluid pathway being defined between the outer rim of the first end piece and the side wall, and the second end piece defining the bridge and including a plurality of fluid passages therethrough. The central portion and the first end piece may be integral with each other and the side wall and the second end piece may be integral with each other.

The said pieces may be joined by screwing. The piston may include a piston rod having a threaded end. The piston rod may pass through a central hole in the second end piece with its threaded end received in a corresponding threaded hole in the central portion.

The piston rod may include a shoulder for bearing against the second end piece to draw the second end piece against the central portion as the end of the piston rod is threaded into threaded hole.

The invention may also provide a vehicle comprising a body and a ground engaging wheel and a damper as claimed in any preceding claim mounted with its cylinder coupled to one of the body and the wheel and its piston coupled to the other of the body and the wheel.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: figure 1 shows a conventional fluid damper; figure 2 shows an example of a response curve for a damper of the type shown in figure 1; figure 3 shows a magnetorheological damper arrangement; figure 4 shows examples of response curves for a damper of the type shown in figure 3; figure 5 shows a magnetorheological damper; figure 6 shows response curves of the damper of figure 5; figure 7 illustrates a control process for the damper of figure 5; figure 8 illustrates a damper of the type shown in figure 5 installed in a vehicle ; figure 9 illustrates apparatus for implementing the control process of figure 7 for a pair of vehicle dampers ; figure 10 is a cross sectional view of a piston head in a cylinder showing the inner and outer assembled ; figure 11 is a view of an outer from one end; figure 12 is a side view of an outer; figure 13 is a view of an outer from the other end; figure 14 is a view of an inner from one end; figure 15 is a side view of an inner; figure 16 is a view of an inner from the other end; and figure 17 illustrates magnetic flux paths.

Figure 5 shows generally a damper suitable for use in accordance with the present invention. The damper of figure 5 is a magnetorheological damper and comprises a cylinder 50 enclosing a space which is divided into two chambers 51, 52 by a separating piston 53. Chamber 51 is filled with a gas, which can be pressurised by means of valve 54. Chamber 52 is filled with a magnetorheological fluid, whose viscosity is dependant on the magnetic field across it. The fluid may be a suspension of particles of a magnetically susceptible material in oil.

A piston is slidably moveable relative to the cylinder. The piston comprises a piston head 56 located in chamber 52 and a piston rod 59 which is fixed to the piston head and passes through a hole in an end cover 60 of the cylinder. The end cover has seals 61 which bear against the piston rod 59 to prevent the magnetorheological fluid from escaping from the chamber 52. The piston head has seals 62 which bear against the inner surface of the cylinder to prevent the flow of magnetorheological fluid around the outside of the piston head from one side of the piston head to the other. The piston head has fluid passages 63 passing through it from one side of the piston head to the other by means of which the magnetorheological fluid can communicate between the regions of the chamber 52 on either side of the piston head. Sliding movement of the piston in the cylinder is thus resisted by the viscosity of the fluid in the chamber 52 as it is forced to flow through the restrictive passages 63 from one side of the piston to the other.

The piston head also comprises a coil 64 wound around the interior of the piston head, which constitutes an electromagnet. Wires 65 connected to either end of the coil pass to the coil through the piston rod to allow current to be applied to the coil from outside the cylinder. The piston head 56 and the piston 50 are formed of steel or another magnetically susceptible material, and the coil 64 is arranged in the piston head so that when activated it can apply a magnetic flux across the fluid passages 63 to alter the viscosity of the magnetorheological fluid in the passages. The viscosity of the fluid, and therefore its resistance to movement of the piston relative to the cylinder, is dependant on the intensity of the magnetic field applied. Figures 10 to 16 show the structure of a piston head 56 in more detail. The piston head is formed of an inner 200 and an outer 201 into which the inner is mounted. The piston rod 59 is attached to the inner through a hole in the outer and draws the inner rigidly against the outer (see figure 10).

Referring to figures 11 to 13, the outer 201 is a one-piece component generally of the form of a perforate cap. It comprises an annular side wall 202 and a circular end wall 203 together defining a cylindrical interior recess 204. A groove 205 for receiving an o-ring 206 for sealing against the cylinder 50 is formed around the outer surface of the side wall. Six bore formations 207 are equally spaced around the outer 201. Each bore formation passes through the end wall 203 and partially in to the side wall 202, providing a means for fluid communication through the outer 201, from the outer side of the end wall 203 to interior recess 204. The bore formations are formed by drilling through the end wall 203 and include orifices of circular cross section (e. g. at 206) through the end wall. The bore formations are formed at radial locations such that the projection of each circular orifice in the end wall 203 intersects the inner surface of the side wall 202. As a result, the bore formations continue partially into the side wall 202 as semicylindrical recesses (e. g. at 207) in the inner surface of the side wall. The semicylindrical recesses run for approximately half the length of the side wall.

Peripheral orifices (e. g. at 208) pass from the bore formations through the side wall to provide for further fluid communication from the exterior of the outer to interior recess 204.

Referring to figures 14 to 16, the inner 200 is also a one-piece component and comprises a body 209 and an end piece 210. The body 209 is in the form of a flat-sided cylinder having a threaded blind hole 217 extending into it from one end for receiving the end of the piston rod 59. The flat-sided cylinder is sized and shaped to fit snugly into a corresponding seat 211 in the inner surface of the end wall 203 of the outer. The end piece 210 is extends radially outwards from the other end of the body 209. The end piece has an annular outer surface which is sized to be slightly radially smaller than the inner surface of the side wall 202 of the outer. Two holes 212 pass through the end piece 210 and are co-ordinated with the flats on the body 209 so that they are clear of the body. A channel 213 runs across the top surface of the end piece, connecting the orifices 212, and intersecting with a central hole 214.

To assemble the piston and piston head, a coil 219 is wound around the body 209 of the inner, and the end wires of the coil passed through the holes 212, along the channel 213 and through the central hole 214. A cap 215 is then fitted over then top surface of the end piece to protect the wiring. The wires are threaded through central hole 216 of the outer and the central bore of the piston rod. Then the inner is mounted in the outer with the body 209 of the inner snugly in the seat 211 of the outer and the end of the piston rod is passed through the central hole of the outer and into the blind hole 217 of the inner. The piston rod is screwed tightly into the blind hole 217, holding the inner tightly against the outer.

When the piston head is located in the cylinder 50, a path for fluid is provided from one side of the piston head to the other through the bore formations 207 and then through the annular gap 218 that exists between the outer surface of the end piece 210 of the inner and the inner surface of the side wall 202 of the outer. To pass through the bore formations 207 fluid may travel through the circular orifices (e. g. 206) into the interior 204 of the outer or from the outside of the side wall 202 through the peripheral orifices (e. g. 208).

The inner 200 and the outer 201 are formed of a soft magnetic material such as mild steel or Permenorm 5000H2 steel. When the coil 219 is activated a magnetic field is developed as illustrated generally at 220 in figure 17. The field flows through the body 209 of the inner, outwards along the end piece 210 of the inner, across the gap 218 and back through the side wall 202 and the end wall 203 of the outer. The material of the inner and the outer is greatly more magnetically susceptible than the magnetorheological fluid that is used in the cylinder, so the fluid is substantially unaffected by the magnetic circuit as it passes through the bore formations 207 of the outer. In contrast, as the fluid passes through the gap 218 the full magnetic field is directed across the fluid. Thus although the field crosses and returns across the fluid path, the field at only pole of the coil has an influence on the fluid flow : the end wall of the outer 201 forms a bridge for magnetic flux that is fully hydraulically permeable, carrying the full magnetic flux around the bore formations.

The general performance of the damper may be set by the width of the gap 218.

Preferred values depend on factors such as the viscosity of the magnetorheological fluid. In a typical embodiment the width of the gap may be in the range from 0.3mm to 1. 5mm.

This design provides a number of significant advantages over prior designs such as those of WO 94/00704. First, in this design only one pair of magnetic poles has an influence on the fluid flow. As a result the present design allows for easier tuning of the performance of the damper by means of the width of the fluid gap across which the flux has an influence. Second, the present piston head can be securely assembled without welding. This allows high performance soft magnetic materials, which are typically very difficult to weld, to be used for the piston head.

As a result, the magnetic performance of the piston head can be greatly enhanced over prior designs using piston heads of mild steel.

The following table shows the properties of examples of materials of which the

inner and outer of the piston head may be formed :

Material Description Resistivity Coercivity Saturation Permeability (Ohm. cm x O') (A/m) (T) (reL., max) 220M07 (EN1A) Mild steel 12 240-400 1. 95 1200 SGM SPM-SP4/4 Sintered Fe-63 34 2. 00 7000 Si (4%) VACPermenorm5000H2 Fe-Ni (50%) 45 5 1. 55 120000 VAC Vacoflux 50 Fe-Co (50%) 35 140 2. 35 9000

VAC Vacoflux 17 Fe-Co (17%) 39 150 2. 22 4000 Carpenter B-FM Fe-Si (2. 5%) 40 56 2. 06 5000 VAC Trafoperm N3 Fe-Si (3%) 40 20 203 30000 VAG Vacofer S1 Pure Fe 10 6 2. 15 40000 VAC Vacoperm 100 Fe-Ni (-80%) 601 . 74 250000 VAC Mumetal Fe-Ni (-80%) 55 1. 5 0. 80 350000

Of these materials, mild steel is easily welded but offers the worst magnetic performance in this application. Since there is no need for the present piston head to be welded the difficulties of welding, for instance, Fe-Si alloys are not important. In general, softer magnetic materials (those having a greater permeability) are preferred in the present application for good magnetic performance-for example good response to rapid changes in applied voltage.

Figure 6 shows a set of response curves 70-75 of the damper for a range of voltages applied to the coil. Each curve shows the response of the damper as a damping force versus velocity curve for a single applied voltage. The greater the voltage of the signal applied to the coil, the greater the damping force. Therefore, the damper can be controlled to have the response indicated by one of the curves 70-75 by applying the appropriate voltage to the coil. In practice, the response desired from the damper may not be that of any of the curves 70-75. For example, the desired response may be as indicated by curve 76. To allow the damper to be controlled to have a desired response of that form the closed-loop control process illustrated by the block diagram in figure 7 is employed.

Referring to figure 7, sensors are provided for sensing the position of the piston relative to the cylinder (as illustrated at 80 in figure 7) and the load on the piston rod in the displacement direction (as illustrated at 81 in figure 7). These sensors produce analogue output signals illustrated at 82 and 83 indicating the measured relative position and measured load respectively. The analogue load signal at 83 is converted to a corresponding digital signal by A-D converter 108. The analogue position signal is differentiated by analogue differentiation unit 109, and the resulting analogue output, which is representative of the relative velocity of the piston and the cylinder, is converted to a corresponding digital signal by A-D converter 110.

A response processing unit as illustrated at 84 is pre-programmed with the desired force/velocity response. The response processing unit receives the measured velocity, and from the pre-programmed desired response determines the currently required force which is output as a signal illustrated at 85. A modulus unit as illustrated at 86 determines the absolute value of the currently required force as an absolute load signal illustrated at 87.

A preload sampling unit as illustrated at 88 receives the measured load and velocity and samples the measured load to generate a signal illustrated at 89 representing the measured static load. This signal is received by a subtraction unit as illustrated at 90 which subtracts from the measured static load a signal illustrated at 91 representing the load due to the steady state force on the damper when it is at rest (e. g. due to the mass of an object supported by the damper and the force due to gas precharge acting on the piston rod), to give a signal at 92 representing the offset of the measured load from the steady state. A modulus unit as illustrated at 93 forms a signal at 94 representing the absolute value of the signal at 92.

A comparator as illustrated at 95 compares the measured absolute load at 92 with the required absolute load at 87 to generate an error signal which is split to give proportional (P) and integral (I) signals at 96 and 97 respectively. These signals are amplified by respective amplifiers as illustrated at 98 and 99 respectively to generate scaled P and I signals 100 and 101 respectively. In this example the P signal is scaled by a factor of 0.003 and the I signal is scaled by a factor of 0.2, but other suitable values could be used. The scaled I signal is integrated in a discrete time integration step as illustrated at 102 to generate an integrated I signal at 103. The scaled P signal and the integrated I signal are summed as illustrated at 104 to generate a current demand signal at 105 from which a drive signal 106 to the damper is generated by drive unit 107.

Most of the components used to implement the control system of figure 7 may be implemented on a single digital microcontroller, as illustrated at 111.

The closed-loop control signal shown in figure 7 allows for accurate real-time control of the damper response to match a desired force/velocity curve. The use of inputs of both damper load (force) and velocity (derived in this case from differentiation of a position measurement) is especially preferred in order to allow the desired curve to be matched precisely. The relative velocity of the damper's cylinder and piston could be sensed directly by means of a velocity sensor.

However, it has been found that the use of an analogue position sensor, whose output is then differentiated (preferably but not necessarily by means of an analogue circuit) is highly preferable because it is less expensive and significantly easier to implement. The differentiation of the position signal may be performed in digital or analogue electronics, but analogue differentiation is preferred since it gives superior high frequency performance.

Figure 8 illustrates apparatus installed in a vehicle to implement the process of figure 7 in the damping of the vehicle's suspension. A wheel 120 of the vehicle is movable relative to the vehicle's body 121. The wheel is coupled to the body by a spring 122 and a magnetorheological damper 123 as described above. The damper is fitted with a load cell 124 in its piston rod 125 and a position sensor 126 implemented as a potentiometer connected between the piston rod and the damper's cylinder 127. The potentiometer outputs a voltage which varies with the relative position of the piston and the cylinder. The output 128 from the position sensor is differentiated by analogue differentiation unit 129 to give an analogue signal at 130 representative of the relative velocity of the piston and the cylinder.

The analogue outputs of the differentiation unit 129 and the load cell 124 are converted to digital signals by respective A-D converters 131, whose outputs are fed to a digital control unit 132. The digital control unit implements the remaining steps illustrated in figure 7 and generates a digital output at 133 indicating the current to be applied to the coil 134 of the damper. The signal at 133 is converted to analogue by a D-A converter 135 whose output passes to a drive unit 136 which generates a corresponding drive signal for the coil 134.

The digital control unit receives an input at 136 (see also figure 7) which is used to indicate the desired response curve. The control unit could be pre-programmed with a number of response curves, one of which may be selected by means of the input 136, and/or the input 136 could be used to program the control unit with a response curve.

Each wheel of the vehicle may have a similar arrangement to that shown in figure 8. One control unit could be used for each wheel or the control unit could be common to all wheels, receiving velocity and force measurements from each wheel and generating current signals for the coil of the damper of each wheel.

The desired response curves for each wheel could be the same or different. The dampers of the front wheels of the vehicle could be controlled to have the same response. The dampers of the front and/or rear and/or left and/or right wheels could be controlled to have the same response as each other.

Figure 9 illustrates the apparatus for installation to one pair of left and right dampers 150,151 respectively, for the front or rear wheels of a vehicle. The potentiometers for sensing the relative location of each cylinder 152,153 and piston 154,155 of the dampers are fitted in sensor cylinders 156,157 mounted on the side of the damper cylinders 152,153 and having sliding pistons 158,159 attached to the damper pistons 154,155. The damper cylinders and the damper pistons have mounting bushes 162-165 at their distal ends to allow them to be mounted to the vehicle. The load cells 166,167 are located in the piston rods between their mounting bushes and the cylinder. Flexible cables 168-171 lead from the dampers to a sealed housing 172 which contains the circuitry of the control system. The housing is connected by a cable 173 to the vehicle's battery and to means such as an on-board controller for controlling and/or monitoring its operation. The apparatus of figure 9 could be duplicated for the other set of the wheels.

In the application illustrated in figure 9 the control sample rate of the control system may suitably be 2ms (500Hz) or thereabouts.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS 1. A damper comprising: a cylinder defining a chamber for containing magnetorheological fluid ; and a piston having a piston head movable in the chamber and including a fluid pathway through which fluid may flow from one side of the piston head to the other, the piston head comprising: an electromagnet capable of applying a magnetic field across magnetorheological fluid in the passage to alter its viscosity and thereby alter the response of the damper; and at least two distinct pieces each formed of a material having a relative magnetic permeability greater than 3000 and forming a path for magnetic flux from the electromagnet, the parts being weldlessly fixed together to cooperatively define the fluid pathway.
2. A damper as claimed in claim 1, wherein the said pieces are each formed of a material having a relative magnetic permeability greater than 50000.
3. A damper as claimed in claim 1, wherein the said pieces are each formed of a material having a relative magnetic permeability greater than 100000.
4. A damper as claimed in any preceding claim, wherein the said pieces are formed of an iron-silicon alloy.
5. A damper as claimed in any preceding claim, wherein the said pieces are formed of an iron nickel alloy.
6. A damper as claimed in any preceding claim, wherein the electromagnet is actuable to define two magnetic poles with a magnetic flux therebetween

extending across the pathway. zu
7. A damper as claimed in claim 6, wherein the piston head is arranged such that the flux at a only single pole of the electromagnet is capable of substantially influencing the viscosity of magnetorheological fluid in the pathway.
8. A damper as claimed in claim 7, wherein the fluid pathway includes a first portion at the said single one of the poles and a second portion at the other of the poles, and the piston head defines at least one bridge for magnetic flux across the second portion of the fluid pathway.
9. A damper as claimed in claim 9, wherein the first portion is a constricted portion of the pathway.
10. A damper as claimed in any of claims 7 to 9, wherein the piston head comprises a first region of magnetically susceptible material adjacent to the electromagnet, a second region of magnetically susceptible material spaced from the first region by the fluid pathway and a third region of magnetically susceptible material bridging the fluid pathway between the first region and the second region to provide the said bridge for magnetic flux.
11. A damper as claimed in claim 10, wherein the second region surrounds the first region.
12. A damper as claimed in claim 11, wherein the piston head comprises magnetically susceptible material defining a longitudinally extending central portion, a first end piece extending radially outward from a first end of the central portion, a longitudinally extending side wall surrounding the central portion and a second end piece joining the side wall to the second end of the central portion, the first portion of the fluid pathway being defined between the outer rim of the first end piece and the side wall, and the second end piece defining the bridge and including a plurality of fluid passages therethrough.
13. A damper as claimed in claim 12, wherein the central portion and the first end piece are integral with each other and the side wall and the second end piece are integral with each other.
14. A damper as claimed in any preceding claim, wherein the said pieces are joined by screwing.
15. A damper as claimed in any of claims 12 to 14, wherein the piston includes a piston rod having a threaded end and the piston rod passes through a central hole in the second end piece with its threaded end received in a corresponding threaded hole in the central portion.
16. A damper as claimed in claim 15, wherein the piston rod includes a shoulder for bearing against the second end piece to draw the second end piece against the central portion as the end of the piston rod is threaded into threaded hole.
17. A damper substantially as herein described with reference to figures 5 to 17 of the accompanying drawings.
18. A vehicle comprising a body and a ground engaging wheel and a damper as claimed in any preceding claim mounted with its cylinder coupled to one of the body and the wheel and its piston coupled to the other of the body and the wheel.
19. A vehicle substantially as herein described with reference to figures 5 to 17 of the accompanying drawings.