JP2025527810A - Hydraulic shock absorber for vehicle suspension with hydraulic end stop members operating during the compression stroke - Google Patents

Abstract

The hydraulic shock absorber 10 includes a first cylindrical tube 14, a piston rod 18 disposed coaxially relative to the first cylindrical tube 14 and projecting from an upper side of the first cylindrical tube 14, a main piston 20 attached to the piston rod 18 and slidably mounted within the first cylindrical tube 14 to divide the interior volume of the first cylindrical tube 14 into a extension chamber 22 and a compression chamber 24 containing a damping fluid, and hydraulic end stop members 36, 38, 58 disposed within the compression chamber 24 and operable to hydraulically dissipate the kinetic energy of the piston rod 18 during the final section of the compression stroke of the shock absorber 10. The hydraulic end stop members 36, 38, 58 include a cup-shaped body 36 attached to the piston rod 18 below the main piston 20 and arranged to be open at its bottom, a secondary piston 38 arranged to slide within the cup-shaped body 36 during the final section of the compression stroke of the shock absorber 10 to compress damping fluid contained in a working chamber 52 defined between the cup-shaped body 36 and the secondary piston 38, and a pin 58 attached to the bottom region of the first cylindrical tube 14 and extending coaxially relative to the cup-shaped body 36, the pin 58 associated with the secondary piston 38 to enable the secondary piston 38 to slide within the cup-shaped body 36 during the final section of the compression stroke of the shock absorber 10.

Description

The present invention relates generally to hydraulic shock absorbers for vehicle suspensions. More specifically, the present invention relates to hydraulic shock absorbers with compression-activated hydraulic end stop members, i.e., hydraulic end stop members positioned to damp relative motion between the piston rod and the shock absorber body during the final portion of the compression stroke.

The present invention is described herein with particular reference to hydraulic shock absorbers for vehicle suspensions of the so-called twin-tube type, but is intended to be applicable to any other type of hydraulic shock absorber for vehicle suspensions.

A twin-tube hydraulic shock absorber for a vehicle suspension typically includes an outer cylindrical tube, an inner cylindrical tube coaxial with the outer cylindrical tube and defining an annular chamber with the outer cylindrical tube, a piston rod coaxially disposed relative to the two cylindrical tubes and partially projecting therefrom, and a main piston slidably mounted within the inner cylindrical tube and fixed to the lower end of the piston rod. The main piston separates the inner cylindrical tube's internal volume into a rebound chamber and a compression chamber, which contain a damping fluid, typically oil. The main piston includes a first pair of one-way valves: a compensation valve that controls the flow of damping fluid from the compression chamber to the rebound chamber during the shock absorber's compression phase, and a rebound valve that controls the flow of damping fluid from the rebound chamber to the compression chamber during the shock absorber's extension phase. The valve assembly is located at the bottom of the shock absorber and includes a second pair of one-way valves: a compression valve that controls the flow of damping fluid from the compression chamber to the annular chamber during the compression phase, and an intake valve that controls the flow of damping fluid from the annular chamber to the compression chamber during the extension phase.

It is known to provide hydraulic shock absorbers of this type with hydraulic end stop members that are activated upon compression.

A hydraulic shock absorber for a vehicle suspension having a hydraulic end stop member that operates during compression is described, for example, in WO 2016/146660. This document, in particular, discloses a hydraulic shock absorber for a vehicle suspension in which the hydraulic end stop member has a cup-shaped body coaxially mounted within the shock absorber's compression chamber and a secondary piston coaxially mounted on the shock absorber's piston rod and below the main piston, so that the secondary piston slides within the cup-shaped body during the compression stroke as the shock absorber approaches its end-stroke position. The cup-shaped body has side and bottom walls that, together with the secondary piston, define an operating chamber in which damping fluid is compressed by the secondary piston as it slides within the operating chamber toward the bottom wall of the cup-shaped body. Additionally, an axial groove or channel is provided on the inner surface of the side wall of the cup-shaped body, allowing damping fluid to flow axially out of the working chamber as the secondary piston slides within the working chamber toward the bottom wall of the cup-shaped body. In such a hydraulic shock absorber, damping of the shock absorber's piston rod movement during the final section of the compression stroke is therefore achieved by damping fluid flowing out of the cup-shaped body of the hydraulic end stop member through the axial channel provided in the cylindrical side wall of the body.

This known solution has several drawbacks.

First, the cup-shaped body must be manufactured with high precision to ensure a perfect fit with the inner surface of the inner cylindrical tube. This, along with the need to provide multiple axial channels on the inner surface of the side wall of the cup-shaped body, which preferably have a cross-section whose area continuously decreases axially toward the bottom wall of the cup-shaped body, makes manufacturing the cup-shaped body rather complex and expensive. Furthermore, calibration of the hydraulic end stop member, i.e., adjusting the damping force acting on the shock absorber's piston rod, requires changing the cross-section of the axial channels in the cup-shaped body and therefore removing and replacing the axial channels, operations that are by no means quick or easy to perform.

WO 2016/146660

The object of the present invention is to provide a hydraulic shock absorber for a vehicle suspension having a hydraulic end stop member that operates upon compression, thereby overcoming the above-mentioned drawbacks of the prior art.

This and other objects are fully achieved according to the present invention by a hydraulic shock absorber for a vehicle suspension having the features defined in independent claim 1.

Advantageous embodiments of the present invention are defined in the dependent claims, the subject matter of which is intended to form an integral part of the following description. In summary, the present invention is based on the idea of providing a hydraulic end stop member, which, like the prior art, also has a cup-shaped body and a secondary piston, but unlike the prior art, the cup-shaped body is fixed to the piston rod of the shock absorber, in particular below the main piston, and the secondary piston is configured to slide within the cup-shaped body during the final section of the shock absorber's compression stroke due to relative movement between the cup-shaped body, which is drivingly connected to the shock absorber's piston rod, and a valve assembly at the bottom of the shock absorber. According to the present invention, the hydraulic end stop member further has a pin attached to the valve assembly at the bottom of the shock absorber, extending coaxially relative to the cup-shaped body, and associated with the secondary piston so that the pin can slide within the cup-shaped body during the final section of the shock absorber's compression stroke. This configuration of the hydraulic end stop member offers the advantage of being easily adaptable to existing hydraulic shock absorbers, as it does not require any special modifications to the existing shock absorber. Indeed, the cup-shaped body can be easily attached to the lower end of the shock absorber's piston rod using the threads typically provided on its end for attaching a nut, with the nut used to attach the shock absorber's main piston to the piston rod. Similarly, the pin can be easily attached to the valve assembly at the bottom of the shock absorber using the threaded pin typically provided for attaching a nut, with the nut used to attach the valve assembly to the bottom of the shock absorber.

Calibrating the hydraulic end stop member is also very quick and easy, as it is sufficient to remove the shock absorber's piston rod, disassemble the cup-shaped body attached to it, and replace it with a new cup-shaped body.

According to one embodiment, the secondary piston is mounted within the cup-shaped body so as to be biased by the elastic means towards a rest position located at a maximum distance from the bottom wall of the cup-shaped body, and a pin mounted in a valve assembly at the bottom of the shock absorber is configured to penetrate into the cup-shaped body during the final section of the shock absorber's compression stroke, thus biasing the secondary piston along the cup-shaped body towards the bottom wall of the cup-shaped body.

According to another embodiment, the secondary piston is mounted on the outside of the cup-shaped body, specifically on the end of a pin attached to the valve assembly at the bottom of the shock absorber, so that it penetrates into the cup-shaped body during the final section of the shock absorber's compression stroke.

Further features and advantages of the present invention will become apparent from the following detailed description, given purely by way of non-limiting example.

The following detailed description of the invention refers to the accompanying drawings.

1 is a schematic diagram of a hydraulic shock absorber for a vehicle suspension, in particular a hydraulic shock absorber having a twin tube architecture with hydraulic end stop members that operate during compression, according to one embodiment of the present invention; 2A to 2C are axial cross-sectional views of a hydraulic shock absorber for a vehicle suspension according to the embodiment of FIG. 1 in different operating positions; 2A to 2C are axial cross-sectional views of a hydraulic shock absorber for a vehicle suspension according to the embodiment of FIG. 1 in different operating positions; 2A to 2C are axial cross-sectional views of a hydraulic shock absorber for a vehicle suspension according to the embodiment of FIG. 1 in different operating positions; 2 is an axial cross-sectional view, on an enlarged scale, of an assembly formed by the cup-shaped body of the shock absorber of FIG. 1 and the secondary piston of the hydraulic end stop member; FIG. 2 is an axial cross-sectional view, on an enlarged scale, of the assembly formed by the pin of the hydraulic end stop member of the shock absorber of FIG. 1 and the bottom valve assembly. 1 is a schematic diagram of a hydraulic shock absorber for a vehicle suspension according to a further embodiment of the present invention, in particular a hydraulic shock absorber having a twin tube architecture with hydraulic end stop members that operate during compression; 8A to 8C are axial cross-sectional views of a shock absorber for a vehicle according to the embodiment of FIG. 7 in different operating positions; 8A to 8C are axial cross-sectional views of a shock absorber for a vehicle according to the embodiment of FIG. 7 in different operating positions; 8A to 8C are axial cross-sectional views of a shock absorber for a vehicle according to the embodiment of FIG. 7 in different operating positions; 8 is an axial cross-sectional view, on an enlarged scale, of the cup-shaped body of the hydraulic end stop member of the shock absorber of FIG. 7. 8 is an axial cross-sectional view on an enlarged scale of the assembly formed by the pin and secondary piston of the hydraulic end stop member and the valve assembly at the bottom of the shock absorber of FIG. 7; FIG.

In the following description and claims, the terms "axially" and "axially" identify the direction of the longitudinal axis of the shock absorber and the longitudinal axis of the hydraulic end stop member. Furthermore, the terms "upper" and "lower" are intended to refer to the shock absorber arrangements shown in Figures 1 and 7, respectively, with reference to the first and second embodiments of the present invention proposed herein, in which the main piston of the shock absorber is attached to the lower end of the piston rod, and therefore the piston rod and main piston move downward during the shock absorber's compression stroke and move upward during the shock absorber's extension stroke.

As noted above, this description of the present invention relates to hydraulic shock absorbers having a twin-tube architecture. However, the present invention is not intended to be limited to such shock absorber architectures, as it is equally applicable to single-tube hydraulic shock absorbers.

Referring first to FIG. 1, a twin-tube hydraulic shock absorber for a vehicle suspension is generally designated 10 and comprises, in a manner known per se, an outer cylindrical tube 12, an inner cylindrical tube 14 coaxial with the outer cylindrical tube 12 and defining, together with the outer cylindrical tube 12, an annular chamber 16 at its upper portion which is filled with gas, a piston rod 18 arranged coaxially with the cylindrical tubes 12 and 14 and projecting partially from the upper side of the shock absorber, and a piston 20 (hereinafter referred to as the main piston) slidably mounted within the inner cylindrical tube 14 and fixed to the lower end of the piston rod 18. The longitudinal axis of the shock absorber 10 is designated by z.

The main piston 20 separates the interior volume of the inner cylindrical tube 14 into an upper chamber 22 or extension chamber and a lower chamber 24 or compression chamber, which contains a damping fluid, typically oil.

The main piston 20 is provided, in a manner known per se, with a valve assembly having a pair of one-way valves, i.e., compensation valves 26, which control the flow of damping fluid from the compression chamber 24 to the rebound chamber 22 during the compression phase of the shock absorber, and a rebound valve 28, which controls the flow of damping fluid from the rebound chamber 22 to the compression chamber 24 during the rebound phase of the shock absorber.

At the bottom of the shock absorber 10, i.e., at the bottom of the inner cylindrical tube 14, there is provided, in a manner known per se, a valve assembly 30 (hereinafter referred to as the bottom valve assembly) having a pair of one-way valves, namely a rebound valve 32 which controls the flow of damping fluid from the annular chamber 16 to the compression chamber 24 during the extension phase, and a rebound valve 34 which controls the flow of damping fluid from the compression chamber 24 to the annular chamber 16 during the compression phase.

The shock absorber 10 also includes a hydraulic end stop member disposed within the compression chamber 24 that operates to hydraulically dissipate the kinetic energy of the suspension during the shock absorber's compression stroke, i.e., as the shock absorber approaches its end-of-compression position.

The hydraulic end stop member first has a cup-shaped body 36 attached to the lower end of the piston rod 18 below the main piston 20 and extending coaxially with the main piston, and a piston 38 (hereinafter referred to as the secondary piston to distinguish it from the main piston 20) mounted within the cup-shaped body 36 so as to slide along the longitudinal axis z.

The cup-shaped body 36 has a bottom wall 40 and a cylindrical side wall 42 and is positioned upside down, i.e., with the bottom wall 40 facing upward, i.e., open at the bottom toward the main piston 20. The cylindrical side wall 42 of the cup-shaped body 36 has a plurality of holes 44 through which the damping fluid contained within the cup-shaped body 36 flows outward, i.e., toward the compression chamber 24, during the shock absorber's compression stroke, while the damping fluid flows from the compression chamber 24 into the cup-shaped body 36 during the shock absorber's extension stroke.

The secondary piston 38 also has a cup-shaped configuration with a bottom wall 46 and a cylindrical side wall 48 in the illustrated example. In this case, the bottom wall 46 faces the open end, i.e., the lower end, of the cup-shaped body 36. Advantageously, the secondary piston 38 is provided with sealing means (not shown), for example formed by a sealing ring, to ensure a tight seal between the cylindrical side wall 48 of the secondary piston 38 and the cylindrical side wall 42 of the cup-shaped body 36. In the illustrated example, the secondary piston 38 has a through hole 50 provided in the bottom wall 46 to allow the flow of damping fluid in the direction from the compression chamber 24 to the interior of the cup-shaped body 36.

A working chamber 52 is therefore defined within the cup-shaped body 36, the working chamber 52 being bounded at its top by the bottom wall 40 of the cup-shaped body 36, laterally by the cylindrical side wall 42 of the cup-shaped body 36, and at its bottom by the bottom wall 46 of the secondary piston 38.

Resilient means are provided within the cup-shaped body 36, and the resilient means are formed, for example, by a conical spring 54, which acts on the secondary piston 38 to exert a downward resilient force thereon, thereby tending to hold the secondary piston 38 in a downward, end-of-stroke position in which the secondary piston 38 abuts a stop ring 56 attached to the lower end of the cup-shaped body 36.

The hydraulic end stop member further includes a pin 58 that is attached to the bottom valve assembly 30 of the shock absorber and extends upward, i.e., toward the assembly formed by the cup-shaped body 36 and the secondary piston 38. The pin 58 is coaxially disposed with respect to the shock absorber, i.e., its axis is substantially coincident with the longitudinal axis z.

As shown in FIG. 1, the pin 58 is normally positioned some distance from the secondary piston 38. In this state, the hydraulic end stop member is not activated and therefore the damping force generated thereby is zero, or substantially zero.

Now, referring to Figures 2 to 4, the operation of the hydraulic end stop member is described during the final section of the shock absorber's compression stroke (Figures 2 and 3), and in the next stage when the operation reverses, and thus during the first section of the shock absorber's extension stroke (Figure 4).

As shown in FIG. 2, at a certain point during the compression stroke of the shock absorber, the secondary piston 38, which is housed within the cup-shaped body 36 and, as described above, is held by the conical spring 24 at the lower end of the cup-shaped body 36, comes into contact with a pin 58 attached to the bottom of the shock absorber. As a result, the through hole 50 provided in the secondary piston 38 is closed by the pin 58.

As the shock absorber continues its compression stroke (FIG. 3), the secondary piston 38 penetrates further into the cup-shaped body 36, thereby increasingly urging the secondary piston 38 toward the bottom wall 40 of the cup-shaped body 36, i.e., upward against the action of the conical spring 24. As a result, the volume of the working chamber 52 of the cup-shaped body 36 gradually decreases, thereby forcing the damping fluid contained therein to flow out through the holes 44 in the cylindrical side wall 42 of the cup-shaped body 36. However, as the secondary piston 38 moves toward the bottom wall 40 of the cup-shaped body 36, the number of holes 44 in the cylindrical side wall 42 of the cup-shaped body 36 through which the damping fluid can flow out of the working chamber 52 of the cup-shaped body 36 decreases, thus reducing the overall cross-sectional flow area for the outflow of damping fluid from the working chamber 52 of the cup-shaped body 36. This results in greater resistance to the downward movement of the shock absorber piston rod 18, and therefore, an increase in damping force. The relationship between the increase in damping force and the displacement of the secondary piston 38 within the cup-shaped body 36 toward the bottom wall 40 (a displacement that coincides with the displacement of the piston rod 18 toward the bottom of the shock absorber) can be set by appropriately selecting the number, location, and/or shape of the holes 44 in the cylindrical side wall 42 of the cup-shaped body 36.

To limit the maximum pressure in the working chamber 52 of the cup-shaped body 36, and therefore the maximum force transmitted to the shock absorber piston rod 18 and thus to the vehicle, the cup-shaped body 36 is provided with a pressure limiting valve, generally designated 60. The pressure limiting valve 60 essentially comprises a set of discs 62 (or, more generally, at least one disc 62) arranged against the upper surface of the bottom wall 40 of the cup-shaped body 36, i.e., the surface facing the main piston 20, thereby preventing damping fluid from leaving the working chamber 52 through a through-hole 64 provided in the bottom wall 40 of the cup-shaped body 36 up to a predetermined pressure limit in the working chamber 52. If the pressure in the working chamber 52 exceeds this limit, the discs 62 undergo elastic deformation, thereby leaving a free passage between them and the upper surface of the bottom wall 40 of the cup-shaped body 36. Thus, damping fluid can flow from the working chamber 52 of the cup-shaped body 36 through the through-hole 64 and the free passage between the bottom wall 40 and the disc 62, thereby avoiding a further increase in pressure within the working chamber 52 and, consequently, a further increase in the damping force on the piston rod 18.

Starting from the state shown in FIG. 3 (or a similar state in which the secondary piston 38 is displaced toward the bottom wall 40 of the cup-shaped body 36 relative to its initial position abutting the stop ring 56), as the piston rod 18 changes direction of movement, i.e., moves upward (extension), the pin 58 gradually moves away from the working chamber 52 of the cup-shaped body 36, and the secondary piston 38 gradually moves away from the bottom wall 40 of the cup-shaped body 36, thereby increasing the volume of the working chamber 52. Therefore, damping fluid can enter the working chamber 52 from the shock absorber's compression chamber 24 through the through-hole 50 in the secondary piston 38 and through the hole 44 in the cylindrical side wall 42 of the cup-shaped body 36, which gradually re-connects with the working chamber 52 as a result of the secondary piston 38's downward movement. This operating state is shown in FIG. 4.

As shown in the enlarged scale view of FIG. 5, the cup-shaped body 36 is preferably fixed to the lower end of the shock absorber piston rod 18 using a threaded portion, generally designated 66, provided at its end for attachment of a nut, which secures the shock absorber's main piston 20 to the piston rod 18. Accordingly, in this case, the bottom wall 40 of the cup-shaped body 36 has an axially threaded hole 68 into which the aforementioned threaded portion 66 of the piston rod 18 can be screwed. Of course, if necessary, one or more spacers 70 may also be mounted on the piston rod 18 between the main piston 20 and the cup-shaped body 36 to accurately position the cup-shaped body 36 relative to the main piston 20, so as to avoid interference between the aforementioned components, particularly between the main piston 20 and the disk 62 of the pressure limiting valve mounted on the bottom wall 40 of the cup-shaped body 36. In this way, the cup-shaped body 36 can be easily mounted onto the piston rod 18 of a conventional shock absorber without requiring any special modification of the same piston rod.

As shown in the enlarged scale view of FIG. 6, the pin 58 is preferably secured to the shock absorber bottom valve assembly 30 by a nut using the threads of a bolt 72 normally provided for securing the bottom valve assembly 30 to the bottom of the shock absorber. In this case, the pin 58 therefore has a blind threaded hole 74 at its lower end to allow it to be screwed onto the bolt 72. In this way, the pin 58 can be easily installed on a conventional shock absorber bottom valve assembly 30 without requiring any special modifications to the same valve assembly. To ensure good stability of the pin 58, it may be necessary to replace the bolt provided for mounting the conventional shock absorber bottom valve assembly 30 with a new, longer bolt 72; however, this replacement can obviously be carried out in this case as a very simple and inexpensive operation.

Further embodiments of hydraulic shock absorbers according to the present invention are shown in Figures 7 to 12, in which parts or elements that are identical to or correspond to those of the shock absorber embodiments of Figures 1 to 6 are designated by the same reference numerals.

This further embodiment differs substantially from the previous embodiment in that the secondary piston 38 is not slidably mounted within the cup-shaped body 36 but is fixed to the free (upper) end of the pin 58, and in that the cup-shaped body 36 does not have a stop ring 56 such that, during the compression stroke of the shock absorber, starting at a certain point until the end of the compression stroke, the secondary piston 38 penetrates the cup-shaped body 36 and begins to slide within it, thereby reducing the volume of the working chamber 52 of the cup-shaped body 36. In this regard, FIG. 8 illustrates the secondary piston 38 beginning to enter the cup-shaped body 36 during the compression stroke of the shock absorber, thereby closing the working chamber 52; FIG. 9 illustrates the secondary piston 38 still completely inside the cup-shaped body 36 during the compression stroke of the shock absorber; while FIG. 10 illustrates the secondary piston 38 still inside the cup-shaped body 36 but moving downward relative to the cup-shaped body 36 during the extension stroke of the shock absorber.

Further, according to this embodiment, the secondary piston 38 is provided with a check valve 76 that functions to allow damping fluid to enter the working chamber 52 of the cup-shaped body 36 during the shock absorber's extension stroke. The check valve 76 essentially includes one or more disks 78 disposed on the upper surface of the secondary piston 38 and adapted to close a plurality of axial through-holes 80 extending through the secondary piston 38 parallel to its axis (i.e., parallel to the longitudinal axis z). The disks 78 keep the axial through-holes 80 closed during the compression stroke, while during the extension stroke, the axial through-holes 80 deflect or move relative to the upper surface of the secondary piston 38 (e.g., tend to hold the disks 78 against the action of resilient means (not shown)) to allow fluid to flow through the axial through-holes 80. Thus, during the shock absorber's extension stroke, when the secondary piston 38 is still inside the cup-shaped body 35 (as shown in FIG. 10), the check valve 76 allows damping fluid to flow from the shock absorber's compression chamber 24, through the axial through-hole 80 in the secondary piston 38, and through the hole 44 in the cylindrical side wall 42 of the cup-shaped body 36, to the working chamber 52 of the cup-shaped body 36.

1-6 also apply in all other respects. In particular, in this case too, the cup-shaped body 36 is preferably fixed to the lower end of the shock absorber piston rod 18 by a threaded portion 66 provided at its end, typically with one or more spacers 70 possibly interposed between the main piston 20 and the cup-shaped body 36, as shown in the enlarged scale view of FIG. 11. Furthermore, as shown in the enlarged scale view of FIG. 12, it is preferably fixed to the shock absorber's bottom valve assembly 30 by a bolt 72 provided to lock the bottom valve assembly 30 to the bottom of the shock absorber by a nut.

As is clear from the above description, by using the hydraulic end stop member according to the present invention, it is possible to achieve an increased damping force on the shock absorber piston rod in the final section of the shock absorber's compression stroke, without changing the behavior of the shock absorber under other operating conditions. The hydraulic end stop member can be easily attached to existing shock absorbers without requiring any modifications to the shock absorber's cylinder or piston rod. Furthermore, with the hydraulic end stop member according to the present invention, calibration of the end stop member can be performed very simply and quickly, as the cup-shaped body and secondary piston (if not located within the cup-shaped body) can be easily removed from the shock absorber.

Furthermore, experiments conducted by the applicant have shown that the hydraulic end stop member according to the present invention is capable of increasing the damping force acting on the piston rod in the final section of the shock absorber's compression stroke, even at low speeds, ensuring good progress in the force increase, avoiding discontinuities at the start of the final section of the shock absorber's compression stroke and also avoiding delays in the increase of the damping force.

The present invention has been described herein with reference to preferred embodiments thereof. It is understood that other embodiments may be envisioned that share the same inventive core as described herein, as defined by the following claims.

Claims (11)

A hydraulic shock absorber (10), in particular for a vehicle suspension, comprising:
a first cylindrical tube (14),
a piston rod (18) arranged coaxially with respect to said first cylindrical tube (14) and projecting from the upper side of said first cylindrical tube (14);
a main piston (20) attached to said piston rod (18), said main piston (20) slidably mounted within said first cylindrical tube (14) so as to divide the internal volume of said first cylindrical tube (14) into a tension chamber (22) and a compression chamber (24) containing a damping fluid;
a hydraulic end stop member (36, 38, 58) disposed within the compression chamber (24), the hydraulic end stop member (36, 38, 58) configured to operate during a final section of the compression stroke of the shock absorber (10) to hydraulically dissipate kinetic energy of the piston rod (18);
a hydraulic shock absorber (10) wherein the hydraulic end stop member (36, 38, 58) has a cup-shaped body (36) and a secondary piston (38), the secondary piston (38) being arranged to slide within the cup-shaped body (36) during the final section of the compression stroke of the shock absorber (10) to compress damping fluid contained in a working chamber (52) defined between the cup-shaped body (36) and the secondary piston (38);
the cup-shaped body (36) is attached to the piston rod (18) below the main piston (20) and is positioned so as to be open at its bottom; and the hydraulic end stop member (36, 38, 58) further comprises a pin (58) attached to a bottom region of the first cylindrical tube (14), the pin (58) extending coaxially with the cup-shaped body (36), the pin (58) being associated with the secondary piston (38) to allow the secondary piston (38) to slide within the cup-shaped body (36) during the final section of the compression stroke of the shock absorber (10). 2. The shock absorber of claim 1, wherein the cup-shaped body (36) has a bottom wall (40) facing the main piston (20) and a cylindrical side wall (42), the cylindrical side wall (42) having a plurality of first holes (44) for fluidly connecting the working chamber (52) of the cup-shaped body (36) with the compression chamber (24) of the shock absorber (10), the first holes (44) being configured such that as the secondary piston (38) slides within the cup-shaped body (36) toward the bottom wall (40), the overall flow cross-sectional area for the outflow of the damping fluid from the working chamber (52) through the first holes (44) decreases. 3. The shock absorber of claim 2, wherein the bottom wall (40) of the cup-shaped body (36) has a plurality of second holes (64) arranged to fluidly connect the working chamber (52) of the cup-shaped body (36) with the compression chamber (24) of the shock absorber (10), and the cup-shaped body (36) is provided with a pressure limiting valve (60) arranged to limit the maximum pressure in the working chamber (52) of the cup-shaped body (36) during the final section of the compression stroke of the shock absorber (10), thereby allowing the damping fluid to flow out of the working chamber through the second holes (64) when the pressure in the working chamber (52) exceeds a predetermined limit value. A shock absorber as claimed in any one of claims 1 to 3, wherein the cup-shaped body (36) is twisted onto the threaded portion (66) of the piston rod (18) at the lower end of the piston rod (18). The shock absorber of any one of claims 1 to 4, further comprising: a second cylindrical tube (12) disposed outside the first cylindrical tube (14) and coaxially relative to the first cylindrical tube (14), the second cylindrical tube (12) defining an annular chamber (16) with the first cylindrical tube (14) whose upper portion is filled with gas; and a valve assembly (30) disposed at the bottom of the first cylindrical tube (14), the valve assembly (30) having a compression valve (34) that controls the flow of the damping fluid from the compression chamber (24) to the annular chamber (16) during the compression phase, and an intake valve (32) that controls the flow of the damping fluid from the annular chamber (16) to the compression chamber (24) during the extension phase. The shock absorber of claim 5, wherein the valve assembly (30) is attached to the bottom of the first cylindrical tube (14) by a bolt (72), and the pin (58) of the hydraulic end stop member (36, 38, 58) is torsionally attached onto the bolt (72). A shock absorber according to any one of claims 2 to 6, wherein the secondary piston (38) is mounted within the cup-shaped body (36), the cup-shaped body (36) comprising elastic means (54) configured to exert an elastic force on the secondary piston serving to urge the secondary piston towards a rest position located at a predetermined distance from the bottom wall (40) of the cup-shaped body (36), and the pin (58) is configured to penetrate into the cup-shaped body (36) in the final section of the compression stroke of the shock absorber (10), thereby urging the secondary piston (38) along the cup-shaped body (36) towards the bottom wall (40) against the action of the elastic means (54). The shock absorber of claim 7, wherein the cup-shaped body (36) is provided with a stop means (56) configured to prevent the secondary piston (38) from being biased by the elastic means (54) beyond the rest position. The shock absorber of claim 7 or 8, wherein the secondary piston (38) has at least one axial through-hole (50) for fluidly connecting the working chamber (52) of the cup-shaped body (36) with the compression chamber (24) of the shock absorber (10), and the at least one axial through-hole (50) is closed by the pin (58) when the pin (58) abuts against the secondary piston (38) in the final section of the compression stroke of the shock absorber (10). A shock absorber as claimed in any one of claims 2 to 6, wherein the secondary piston (38) is mounted on the pin (58), in particular at the end of the pin (58) facing the main piston (20), thereby penetrating into the cup-shaped body (36) in the final section of the compression stroke of the shock absorber (10). 11. The shock absorber of claim 10, wherein the secondary piston (38) has at least one axial through-hole (80) and is provided with a check valve (76), the check valve (76) being arranged to prevent the damping fluid from exiting the working chamber (52) of the cup-shaped body (36) through the at least one axial through-hole (80) during the final section of the compression stroke of the shock absorber (10), but to allow the damping fluid to enter the working chamber (52) of the cup-shaped body (36) through the at least one axial through-hole (80) during the extension stroke of the shock absorber (10) as long as the secondary piston (38) is inside the cup-shaped body (36).