TECHNICAL FIELD

The present invention relates to a shock absorber. An aspect of the invention relates to a vehicle incorporating a shock absorber. In particular, aspects of the invention relate to a shock absorber incorporating a rebound damping system.

BACKGROUND

Among other things, shock absorbers damp the rate of compression of a spring supporting the mass of a vehicle. They can be regarded as providing a force that is proportional to speed, although other variables are possible. A vehicle has wheels suspended from the vehicle on suspension arms attached to a body of the vehicle. The wheels are therefore unsuspended with respect to the ground and, as the vehicle moves over the ground, shocks caused by uneven ground are absorbed, in the wheels, by the tyres of the vehicle. However, a spring is generally disposed between the suspension arms and the body which is compressed as the wheel moves over uneven ground, absorbing the shock to the vehicle body that would otherwise be caused by the uneven ground. To avoid the vehicle wallowing due to unfettered resilience of the spring, a damper or shock absorber is disposed, often within the coils of the spring, where that is a coil spring, between the suspension arm and body. Air springs are not excluded, of course. It is to be understood that suspension arm means any system of levers or members connecting the wheels of the vehicle to its body. The nature of the suspension arm and spring forms no part of the present invention.

A shock absorber to which the present invention relates comprises a cylinder tube having a bottom end and an open end; a piston rod having an attachment end and a piston end; an endcap closing the open end of the cylinder tube and having an aperture to receive the piston rod therethrough; a main piston mounted on the piston end of the piston rod and being slidable in the cylinder tube and dividing the cylinder tube into an extension chamber between the main piston and the endcap and a compression chamber on the other side of the main piston; damper passages between the compression chamber and extension chamber; and, hydraulic fluid in the compression chamber and extension chamber, whereby movement of the main piston in the cylinder tube is controlled by flow of hydraulic fluid in the damper passages between the chambers, wherein a rebound piston is slidably arranged around the piston rod in said extension chamber to divide said extension chamber into an endcap rebound chamber and a main piston rebound chamber; a rebound spring is disposed in said endcap rebound chamber between the endcap and said rebound piston, whereby rebound of the main piston towards said endcap is retarded over at least part of its travel at least partially by compression of said spring and at higher rates of rebound by an increase in hydraulic pressure in said endcap rebound chamber. Leak formations are provided in the wall of the endcap rebound chamber to allow hydraulic fluid in the endcap rebound chamber to leak past the rebound piston into the main piston rebound chamber as the piston rebounds. The formations are shaped so that the leak passage they form reduces in section as the rebound piston moves towards the endcap. This progressively increases the retardation of the rebounding piston, possibly sufficiently, at high speeds of rebound, to prevent blocking of the coils of the rebound spring. The rebound piston moves over the entire length of compression of the rebound spring, requiring the leak formations to also extend over much of such length. The leak formations may comprise swaging of the cylinder tube and it would be desirable to reduce the extent of such swaging without substantially altering the rebound response of the shock absorber.

It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide a shock absorber comprising: a cylinder tube having a bottom end and an open end;

a piston rod having an attachment end and a piston end;

an endcap closing the open end of the cylinder tube and having an aperture to receive the piston rod therethrough;

a main piston mounted on the piston end of the piston rod and being slidable in the cylinder tube and dividing the cylinder tube into an extension chamber between one side of the main piston and the endcap and a compression chamber on the other side of the main piston; damper passages between the compression chamber and extension chamber; and,

hydraulic fluid in the compression chamber and extension chamber, whereby movement of the main piston in the cylinder tube is controlled by flow of hydraulic fluid in the damper passages between the chambers, wherein

a rebound piston is slidably arranged around the piston rod in said extension chamber to divide said extension chamber into an endcap rebound chamber and a main piston rebound chamber;

a first rebound spring is disposed in said endcap rebound chamber between the endcap and said rebound piston; and

a second rebound spring is disposed in said main piston rebound chamber between said rebound piston and said main piston, whereby

rebound of the main piston towards said endcap is retarded over at least part of its travel by compression of said first and second springs and at higher rates of rebound by an increase in hydraulic pressure in said endcap rebound chamber retarding movement of the rebound piston.

Said first rebound spring may be fixed with respect to the endcap. Said damper passages may be arranged in the main piston.

The first and second springs may be coil springs. Other springs are not excluded, for example Belleville washer springs, or rubber springs, are feasible. By disposing a second spring between the rebound piston and the main piston, the first spring can be reduced in size, whereby the rebound piston can be arranged closer to the endcap and its travel shortened, without significantly altering the response profile of the shock absorber. The spring rate of the second spring may be less than the spring rate of the first spring such that the second spring is coil bound before the first spring is coil bound. "Coil bound" should be understood throughout this specification to include any form of blocking of the spring (be it a coil spring or otherwise) that has the effect of increasing its apparent spring rate significantly.

Thus, rebound of the main piston is retarded over at least part of its rebound travel at least partially by compression of said first and second springs, as well as by an increase in hydraulic pressure, at higher rebound rates, in the endcap rebound chamber retarding movement of the rebound piston, which in turn retards movement of the main piston via the second spring. The second spring may be coil bound during such retardation.

The spring rate of the second spring may be at least 50% of the spring rate of the first spring. The spring rate of the second spring may be between 75% and 95% of the spring rate of the first spring. The stroke length before blocking of the second spring may be at least 25% of the stroke length before blocking of the first spring. The stroke length before blocking of the second spring may be between 50 and 75% of the stroke length before blocking of the first spring. The first and second springs may have substantially the same spring rates and stroke lengths before blocking, or at least within 10% of each other in each respect.

Hydraulic retardation force is generally arranged to be of a much higher magnitude than spring retardation force, whereby high accelerations of the main piston can be safely damped. As a consequence, it is essential in this scenario for the second spring to block before the first spring blocks, so that there remains some capacity for travel of the rebound piston while it is directly driven by the main piston when the second spring blocks, whereby the greater hydraulic retardation force can be applied to, and, optimally, slow the travel of the main piston to zero, before the first spring is also blocked and the damper becomes solid. Of course, if hydraulic retardation begins to have an effect before the springs become coil bound, then the second spring will become coil bound almost immediately because its compression is not prevented by retardation of the rebound piston, whereas the first spring is supported/assisted by the hydraulic retardation of the rebound piston and continues to assist retardation of the rebound piston, even though its contribution as such will be relatively minor.

In an embodiment, said piston rod has a rebound collar fixed on the piston rod between said main piston and rebound piston. In this event, the rebound collar engages said second spring on rebound of the main piston.

In an embodiment, said damper passages in the main piston are controlled by pressure control valves or check valves mounted on the piston. In an embodiment, volume relief passages are formed in a reserve chamber defined by a reserve tube surrounding said cylinder tube and communicating with said compression chamber, whereby unequal change in the volumes of the extension and compression chambers as the main piston moves, caused by introduction of the piston rod to and from the extension chamber, is accommodated. Pressure control valves or check valves may be disposed in a base of the cylinder tube to control flow of hydraulic fluid into and from the reserve tube.

In one embodiment, the reserve tube comprises a drawn tube having an open top end and a blind end integrally formed with the remainder of the reserve tube, and wherein the bottom end of the cylinder tube comprises a body fitted to the end of the cylinder tube.

In another embodiment, the shock absorber has a monotube construction with a displaceable partition in the compression chamber defining a compressible volume, whereby the variation in overall volume of the compression and extension chambers as the piston rod moves in and out of the extension chamber is accommodated through compression and expansion of the compressible volume.

In this case, the cylinder tube may comprises a drawn tube and wherein the bottom end is integrally formed with the remainder of the cylinder tube. A leak formation may be provided in the cylinder tube whereby hydraulic fluid can transfer between the endcap chamber and main piston chamber around the rebound piston over its range of movement. A purpose of said leak formation may be to alter the response of the rebound piston in dependence upon the position of the rebound piston in the extension chamber. That is, the cylinder tube may have said leak formation comprising a channel in the side of the cylinder tube defining a channel passage adapted to co-operate with the rebound piston to define the response of the rebound piston to movement forces on it in dependence on its velocity and position.

The channel passage may have a section which changes along its length from a start position to a finish position, wherein said start position is nearer said main piston and said finish position is nearer said endcap.

The rebound piston may have a limit of movement away from said endcap. Said limit may be provided by the first spring having a first end fixed with respect to the cylinder tube near or at the endcap and a second end fixed with respect to the rebound piston. At its limit of movement, the rebound piston may be disposed at a channel passage position where the channel passage has a maximum dimension, whereby movement of the rebound piston is restricted to a minimal extent by hydraulic fluid moving through the channel passage between the endcap rebound chamber and the main piston rebound chamber.

The section of the channel passage may diminish to a minimum dimension from said channel passage position towards said endcap, whereby the rate of transfer of hydraulic fluid along said channel between said endcap rebound chamber and the main piston rebound chamber affects the rate of movement of the rebound piston towards said endcap to an increasing extent as the rebound piston moves towards said endcap.

Said channel may be formed by a swaging of the cylinder tube.

In one embodiment, the channel has a constant section along its length, terminating at the open end of the cylinder tube, the channel passage being modified by an insert received in said channel. The swaging may be a general increase in the diameter of the tube at its open end, the channel thereby entirely surrounding the extension chamber, a sleeve insert being received in said channel to define said channel passages. The insert may be a plastics material. The second spring may be free between the rebound and main pistons. Alternatively, one end of the second spring may be fixed with respect to one of the rebound piston and main piston.

The main piston may be free to move a free distance in the cylinder tube from a compressed condition of the shock absorber, in which the compression chamber has a minimum volume, towards an extended condition of the shock absorber, in which the compression chamber has a greater volume, without compressing the second spring. Thus, while moving the free distance from the compressed condition of the shock absorber, the second spring does not affect extension of the shock absorber. When the main piston moves the free distance from the compressed condition of the shock absorber at above a threshold speed, such movement may increase the hydraulic pressure in the main piston rebound chamber so as to: move the rebound piston, compress the first spring, and hydraulically retard the movement of the main piston. After movement of the main piston over the free distance in the extension direction from the compressed condition of the shock absorber, the piston, or said rebound collar when present, may contact the second spring and commence to compress it. As the second spring is compressed, the rebound piston is itself urged by the second spring in the extension direction, whereby the first spring is also compressed. Compression of the first and second springs applies a retarding force against further movement of the main piston.

The contribution made by the first and second springs to the retardation of the main piston, compared with the retardation of the main piston movement through increase in hydraulic pressure in the endcap rebound chamber and/or the main piston rebound chamber, depends on the rate of movement of the main piston, and may be insignificant with higher rates of movement, or a significant majority of the retardation with lower rates of movement. The spring force applied by the first and second springs in series is a function of their respective spring rates. The spring rates of the first and second springs may be adjusted to tune the rebound response profile of the shock absorber. Likewise the channel passages may be adjusted to tune the hydraulic rebound response profile of the shock absorber, which is also dependent on other parameters including the viscosity of the hydraulic fluid and the resistance to flow of the damper passages. Conveniently, adjustment of the channel passages can be achieved by employing different inserts in the same cylinder tube.

As mentioned above, the main piston may be protected from contact with the second spring and rebound piston by a rebound collar fixed on the piston rod adjacent the main piston. This may be desirable to protect components of the main piston that may include valving arrangements for the damper passages interconnecting the extension and compression chambers. In one embodiment, the damper passages are in the main piston and do not restrict flow of fluid on rebound of the main piston between the main piston rebound chamber and the compression chamber, such that hydraulic retardation of rebound of the main piston is caused only by increased hydraulic pressure in the endcap rebound chamber on connection between the main piston or rebound collar and the rebound piston through the second spring. In another aspect of the invention there is provided a shock absorber comprising a) a main piston mounted on a piston rod;

b) a rebound piston slidably disposed on the piston rod defining a main piston rebound chamber between the main piston and one side of the rebound piston and an endcap rebound chamber on the other side of the rebound piston, and

c) first and second rebound springs disposed one on either side of the rebound piston, whereby

rebound of the main piston is retarded over at least part of its rebound travel at least partially by compression of said first and second springs and by an increase in hydraulic pressure in said endcap rebound chamber.

Another aspect of the present invention provides a vehicle comprising a shock absorber as described above. In particular, the invention also provides a vehicle comprising a body, a suspension arm mounted on the body and a wheel mounted on the suspension arm, wherein a shock absorber as defined above is disposed between the wheel and body or between the suspension arm and body so as to dampen movements of the suspension arm and wheel with respect to the body.

The shock absorber, and its arrangement in a vehicle, may be such that the main piston is free to move a free distance in the cylinder tube without compressing the second spring, from a neutral position of the main piston towards said extended condition of the shock absorber, said neutral position being adopted when the vehicle is unloaded and standing on flat, horizontal ground. Here, unloaded means without additional loading of passengers or other payload as may be carried by the vehicle.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are now described by way of example only, with reference to the accompanying figures, in which: Fig. 1 shows part of a prior art arrangement of a shock absorber;

Fig. 2 shows in side section a schematic illustration of part of a shock absorber in accordance with the present invention; Fig. 3 shows a variation of the shock absorber shown in Fig. 2;

Fig. 4 shows a further variation of the shock absorbers shown in Figs. 2 and 3;

Fig. 5 shows in side section a schematic illustration of part of a further embodiment of a shock absorber in accordance with the present invention;

Fig. 6 shows a simulation of low speed response profile of shock absorbers, including shock absorbers in accordance with the present invention; Fig. 7 shows a simulation of high speed response profile of shock absorbers, including shock absorbers in accordance with the present invention; and

Fig. 8 is an illustration of a vehicle in which the shock absorbers shown in Figs. 1 to 6 may be fitted.

DETAILED DESCRIPTION

Referring now to the drawings in which like reference numerals designate like or corresponding parts throughout the several views, there is shown in Fig. 8 a vehicle 1 10 which includes a suspension system incorporating shock absorbers in accordance with the present disclosure. Vehicle 1 10 includes a rear suspension 1 12, a front suspension 1 14 and a body 1 16. Rear suspension 1 12 has a transversely extending rear axle assembly (not shown) adapted to operatively support a pair of rear wheels 1 18 of vehicle 1 10. The rear axle assembly is operatively connected to body 1 16 by means of a pair of shock absorbers 120 and a pair of helical coil springs 122. Similarly, front suspension 1 14 includes a transversely extending front axle assembly (not shown) to operatively support a pair of front wheels 124 of vehicle 1 10. The front axle assembly is operatively connected to body 1 16 by means of a second pair of shock absorbers 126 and by a pair of helical coil springs 128. Shock absorbers 120 and 126 serve to dampen the relative motion of the unsprung mass (i.e., front and rear suspensions 1 12 and 1 14, respectively) and the sprung mass (i.e., body 1 16) of vehicle 1 10. While vehicle 1 10 has been depicted as a passenger car having front and rear axle assemblies, shock absorbers 120 and 126 may be used with other types of vehicles or in other types of applications such as vehicles incorporating independent front and/or independent rear suspension systems. Further, the term "shock absorber" as used herein is meant to refer to dampers in general and thus will include MacPherson struts. The foregoing paragraph is commonly known (US2009/0057079 is acknowledged). The invention applies to any vehicle that suffers linear shocks, and to any shock absorber within the scope of the claims. In general terms, a vehicle comprises a body, a movable suspension arm mounted on the body, and a wheel mounted on the suspension arm, wherein a shock absorber is disposed between the wheel and the body, or between the suspension arm and the body, so as to dampen movements of the suspension arm and wheel with respect to the body. This arrangement thus includes the possibility of knuckles and uprights, as may be present in a MacPherson strut suspension, for example. In Fig. 1 , a prior art shock absorber, or damper, A, is shown, comprising a cylinder tube 12 having an open end 14 and a bottom end 16, the open end 14 being closed by an endcap 18. The endcap 18 has an aperture 20 through which a piston rod 22 extends, being sealed with respect thereto by a ring seal 24. An attachment end 26 of the piston rod 22 is for connection to a vehicle body by means well known and not forming part of the present invention and therefore not illustrated or discussed further herein.

A piston end 28 of the piston rod 22 carries a main piston 30 that divides the cylinder tube 12 into a compression chamber 32 and an extension chamber 34. The main piston 30 is slidable in the cylinder tube 12 between a compressed condition of the shock absorber, where the piston is near the bottom end 16 of the cylinder tube, and an extended condition in which the piston is nearer the endcap 18. In all the Figures, the main piston 30 is shown at an intermediate or neutral position between these two extreme conditions.

Chambers 32 and 34 are filled with hydraulic fluid, which may be liquid, such as oil. As the main piston 30 slides in the cylinder tube 12, hydraulic fluid must move between the extension and compression chambers 32,34. In Fig. 1 , this is provided by damper passages 39 in the main piston 30. The damper passages may comprise pressure control valves (not shown) controlling the rate of transfer whereby hydraulic pressure in the compression and extension chambers dampen, that is retard, movement of the piston. As the piston 30 moves towards the bottom end 16 of the cylinder tube, the volume of the compression chamber reduces. This is partially compensated by an increase in volume of the extension chamber 34, but because the piston rod 22 has a volume and is introduced into the extension chamber 34 as the main piston moves, the overall volume of the extension and compression chambers reduces. The hydraulic fluid is in most cases incompressible, therefore a compensation volume of air or other compressible material is provided to permit introduction of the piston rod and merely pressurise the compensation volume.

In Fig. 1 , bottom end 16 of the cylinder tube 12 comprises a valve body 36 including damper passages 37 by which hydraulic fluid may access a reserve chamber 38 defined between a reserve tube 40 and the cylinder tube 12. The reserve tube 40 may be fixed on the endcap 18 and the reserve chamber 38 may include an air or other gas volume at its top end near the endcap 18 (which is normally uppermost, in use). The rate of flow of hydraulic fluid from the compression chamber to the reserve chamber may be controlled by restrictions in the damper passages 37 in the valve body 36. However, such restrictions may be in the reserve chamber 38 itself.

The hydraulic fluid entering the reserve chamber 38 compresses and pressurises the gas therein. The reverse applies when the piston 30 moves in the extension direction and the gas expands as more fluid is required to fill the compression chamber than is delivered by the extension chamber.

However, one benefit of the reserve tube 40 is that it facilitates the provision of a means of dynamic control of the damping response of the shock absorber. That is to say, flow of hydraulic fluid through the reserve chamber 38 may be diverted through a unit that has a variable restriction. Such an arrangement is disclosed in WO-A- 2009/017608, although many alternative arrangements are feasible and the present invention is not restricted to any such arrangement being present. In that event, pressure control valves may be absent in one or both the damper passages 37 and 39. As will be well appreciated, in a vehicle application, compression of the shock absorber A is generally accompanied, and hence modulated, by compression of springs 122,128 that support the weight of the vehicle. However, after compression of the shock absorber, the springs tend to accelerate rebound of the main piston 30. The present invention is concerned with a shock absorber that includes a rebound piston 50 and a rebound spring 60.

In Fig. 1 , rebound piston 50 is a close sliding fit, both on the piston rod 22 and in the extension chamber 34. It divides the extension chamber into an endcap rebound chamber 34a and a main piston rebound chamber 34b. The spring 60 is disposed in the endcap rebound chamber 34a between the endcap 18 and rebound piston 50. A rebound collar 70 is fixed on the piston rod 22 between the pistons 30,50. When rebound commences, the main piston 30 moves (leftwardly in the drawings) a free distance F before rebound collar 70 contacts rebound piston 50. When the rebound collar contacts the rebound piston 50, the piston moves with the piston rod 22 and begins to compress the spring 60. The channel passages 80 are shaped along the length of travel of the rebound piston (between positions of full extension of the spring 60 and its fully blocked position) so that the leak passage they represent diminishes with travel, whereby hydraulic resistance to further travel of the rebound piston increasingly plays a part in retarding rebound movement of the main piston 30.

Thus, there is a combined rebound retardation effect: primarily by the spring 60, at low rebound speeds; primarily by hydraulic action at high speeds; and by a combination of the two effects at intermediate speeds. The desired performance of the shock absorber with regard to rebound can be adjusted by suitable choice of spring 60 and channel passage 80.

However, a potential issue with this design of rebound mechanism is the length of the shock absorber that must be employed purely for the purpose of rebound control. That length is determined to a significant extent by the free length of the spring 60 that must be employed to achieve desired response for low speed rebounds. The present disclosure, as illustrated in an embodiment shown in Fig. 2, proposes a shock absorber B in which the spring is split into first and second springs 60a, 60b, disposed respectively in the endcap rebound chamber 34a and the main piston rebound chamber 34b. The first spring 60a performs exactly the same function as the spring 60 described above, but is here shorter than the spring 60, whereby the rebound piston 50 is positioned nearer to the endcap 18. Furthermore, its travel is less, so that the channel passage 80' can not only be shorter in length but also positioned nearer to the endcap 18.

Thus, at low rebound speeds, the rebound piston contributes little to retardation of rebound movement of the main piston 30, just as in the arrangement of Fig. 1 . Furthermore, when collar 70 strikes second spring 60b after the same length F of free travel, the combined springs 60a,b acting in series can behave exactly as the spring 60.

Likewise, at high speeds, the rebound piston 50 can, as before, control hydraulic pressure in the main piston rebound chamber 34b and likewise hydraulically retard rebound movement of the main piston 30. However, there can be a difference here, since only the first spring 60a resists movement of the rebound piston 50, but, since it is hydraulic forces that are primary at high speeds, this difference can be insignificant. Consequently, a shock absorber B in accordance with the present invention can have substantially similar operating characteristics as the arrangement described above with reference to Fig. 1 .

The first spring 60a may be fixed, both with respect to the endcap 18 and with respect to the rebound piston 50. The former may be achieved by the final coil of the first spring 60a being a press-fit in the cylinder tube 12. The latter may be achieved by a clip (not shown) on the rebound piston engaging the last coil at the other end of the first spring 60a. This serves to position and retain the rebound piston, defining a limit of movement L away from the endcap 18, when the main piston 30 is in a neutral position (as shown in the drawings). In this positon, the rebound piston is within the confines of the channel 80', whose passage may have a maximum cross- sectional area at this position, whereby resistance to transfer of hydraulic fluid between the endcap rebound chamber 34a and the main piston rebound chamber 34b is a minimum. The second spring 60b may likewise be clipped to the other side of the rebound piston 50, with its other end being free. Alternatively, as shown in Fig. 3, the second spring 60b can instead be fixed to the rebound collar 70. In either event, a free distance F exists between the collar 70 and spring 60b (or between the spring 60b and rebound piston 50 in the case of the embodiment of shock absorber C shown in Fig. 3) over which the main piston travels from its neutral position before compression of the springs 60a,b begins. The distance F is with respect to the neutral position of the main piston 30 shown in the drawings. The distance is F+ with respect to a fully compressed condition of the shock absorber, which distance and condition is not shown in the drawings (except see description of Figs. 6 and 7 below).

In Fig. 4, as in all the drawings, the cylinder tube 12 of shock absorber D is shown as a drawn steel tube, in which the channel passages 80" are formed by a swaging 82" of the tube, whereby the external profile of the cylinder tube reflects such swaging as external bulges. In the Fig. 4 arrangement, the swaging 82" is taken to the open end 14 of the cylinder tube, which is an easier manufacturing step than arranging undercut swaging 82,82' as in the arrangements described above. In Fig. 4, however, the channel passage 80" is formed in part by an insert 84 within the swaging 82". In one embodiment, the swaging 82" is around the entire circumference of the open end 14, and the insert 84 is a cylindrical sleeve that has a length equal to the depth of the swaging 82", but it has cut-outs around the circumference to form the channel passages 80". An issue with the external bulges 82 of the arrangement described with reference to Fig. 1 is that they interfere with the internal space of the reserve chamber 38 and alter the external profile of the cylinder tube 12 over a considerable length. It is therefore impossible in this region to apply seals, for example, or openings (neither shown), that may be required where the shock absorber is to be rendered dynamically controllable in respect of its damping characteristics. (See, for example, the arrangements disclosed in WO-A-2009/017608, although such features form no part of the present invention, as such.) The arrangement proposed by the present invention enables the swaging 82', 82" to be restricted to nearer the endcap 18 whereby there is less interference with other features of the shock absorber that may be provided (none shown, however). In Fig. 5, a further embodiment of the present invention is shown, wherein the shock absorber E is a mono-tube arrangement. Here, the cylinder tube 12' is shown as a drawn tube with a blind-end 16'. As above, damper passages 39 are disposed on the main piston 30' to permit transfer of hydraulic fluid between the compression chamber 32 and extension chamber 34. In order to compensate for the difference in volume between the two chambers depending on the position of the main piston 30', a compressible volume 90 is defined by a displaceable partition such as a floating piston (or possibly a diaphragm) 92. The volume 90 may be filled with gas. Otherwise, the operation of the rebound arrangements are as described above. In this case, the benefit of being able to relocate and/or reduce the size of bulges 82"' enables to cylindrical form of the cylinder tube to be retained over more of its length than would be the case if a single spring was employed.

In all the embodiments of shock absorbers B to E described above, the first and second springs 60a,b are shown to have substantially equal spring rates and spring strokes (from neutral to spring block positions, not shown). However, the present invention enables different springs 60a,b to be employed having different spring rates and strokes, whereby the performance of the shock absorber may be tuned to meet particular requirements. For example, it may be preferred to restrict hydraulic reaction to rebound movements of the main piston to only greater deflections of that piston. In this case, second spring 60b may have a longer stroke than first spring 60a, which itself may have a stronger spring rate than the second spring 60b. Alternatively, the reverse situation may be employed. Finally, with reference to Figs. 6 and 7, the rebound response of a shock absorber in accordance with the present invention is compared with the response of an arrangement according to Fig. 1 . In this simulation, the first and second springs 60a,b are merely the spring 60 cut into two pieces. That is, each spring 60a,b has the same characteristics as the spring 60, but each is shorter so that their combined length is the same as the spring 60. However, as just mentioned, it should be understood that this is not essential and both spring rate and length of travel before becoming coilbound (or otherwise being blocked from further spring travel) can be different in the springs 60a,b, depending on the desired operating characteristics of the shock absorber.

Fig. 6 represents the response of shock absorbers at a low rebound rate of 0.1 Hz, at which the piston may be moving at a speed of 0.03 m/s. The graph shows on the x- axis displacement in the extension direction of the main piston 30 in metres from the neutral position shown in Figs. 1 to 5. The y-axis displays the force required to achieve the displacement at the speed mentioned. Thus, over the initial displacement to 0.01 m (distance F in Figs. 1 to 5), no force is required as the collar 70 advances towards the rebound piston 50 (in the case of the arrangement of Fig. 1 ) and the second spring 60b in the case of an arrangement in accordance with the present invention.

Between 0.01 m and 0.03 m displacement, the response is the same for all three situations. In the case of a rebound stop that is a spring only, then the force required to displace the spring increases constantly, along line 138/140 in Fig. 6, with respect to displacement. Even in the known arrangement having an hydraulic stop, because the speed is low, the hydraulic effect is minimal and the rebound follows the same course along line 138 as with a spring only until the spring 60 is coil bound at a deflection of about 0.05 m at point 160. However, with the present invention, there is a potential change in resistance at a point where the second spring 60b becomes coil bound (or otherwise blocked) after 0.03 m of displacement (at 142 in Fig. 6), whereupon resistance to further movement is provided only by first spring 60a, which is necessarily greater than the combined effect of both springs in series (bearing in mind the inverse relationship of springs in series) and proceeds along line 144 until it also is coil bound and further travel is impossible at point 170.

In Fig. 7, the fuller compression stroke F+ is shown, but this has no relevance to the present invention. Again, over the free distance F from the neutral position 0.00, there is no resistance to rebound movement. Initially, the same response applies in all three scenarios and again, along line 148. Where there is spring resistance only, the rebound force remains constant over the full deflection of the spring along line 150 up to a displacement of 0.05 m, point 160. However, with the known arrangement shown in Fig. 1 , as the rebound piston moves into the region where the by-pass channel passages 80 reduce significantly at about 0.018 m displacement (point 152 on line 148), the retardation force increases rapidly. The simulation shown in Fig. 7 results from a pulsed, substantially sinusoidal, acceleration whereby the retardation force increases along line 154. But, as the speed of rebound begins to reduce at point 156, the retardation force falls away along line 158 until the speed of movement of the main piston slows so that the retardation force again becomes largely only spring derived and line 158 returns to the same point 160, just before 0.05 m deflection, as the Fig. 6.

With the present invention, substantially the same performance can be arranged. With the same rebound speed profile, spring retardation continues along line 148 until a later point 162, before second spring 60b would otherwise become coil bound. However, it is only at that point that the by-pass channel passages 80 reduce significantly and the rebound piston 50 slows its own movement through hydraulic retardation, whereby the second spring 60b becomes coil bound sooner than it does in Fig. 6 with slow movement. Here, it goes coil bound at a deflection of about 0.024 m. Now the main piston is directly retarded by hydraulic action and the force of retardation increases rapidly along line 164 until point 166 whereupon speed of movement of the main piston has slowed and the force of retardation decays along line 168 until it reaches the same point 170 as in Fig. 6 above.

The point 142 in the graph in Fig.6 is where the second spring becomes coil bound when the main piston rebounds slowly. To the extent that the line 138/140 is an ideal response of the shock absorber, the deviation of that response to line 144 at point 142 represents a compromise. If both springs 60a,b were identical in all respects then the line 144 would approach the line 140, with each spring blocking at the same time. Moreover, the position of point 142 could be moved either leftwardly or rightwardly (in the drawing) in dependence on the characteristics of the first and second springs 60a,b. The further leftwardly the point is moved the arrangement becomes more akin to the prior art shock absorber A, shown in Fig. 1 (that is, the effect of second spring 60b is reduced to the extent that it is effectively non-existent). The further rightwardly the point is moved, the less scope there is for hydraulic retardation over a long distance, because the hydraulic retardation only occurs once the second spring 60b is coil bound or otherwise blocked. Whilst reducing the length of the channel formations 80 (of the shock absorber A) to 80', 80" or 80"' (of shock absorbers B to E in accordance with the invention and as shown in Figs 2 to 5 above) provides a packaging advantage, it can (potentially) reduce the length of main piston travel over which hydraulic rebound damping occurs, and this is a further optional compromise. Thus, the arrangement of present disclosure enables an improved packaging of the shock absorber through relocation of the channel passages 80 without significant alteration of the performance of the shock absorber. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.