The present invention relates to a piston for a damping-adjustable shock-absorber, intended to be used in particular in a vehicle suspension. More specifically, the present invention relates to a piston for a damping-adjustable shock-absorber, arranged to be slidably mounted inside a cylinder of the shock-absorber so as to split this latter into a lower chamber and an upper chamber, the piston comprising

a first pair of passive flow-control valves for controlling the flow of a damping fluid between the lower chamber and the upper chamber via a first flow path, said first pair of passive flow-control valves comprising a compensation valve and a rebound valve which are made as check valves arranged to control the flow of the damping fluid from the lower chamber to the upper chamber (compression phase) and from the upper chamber to the lower chamber (rebound phase), respectively,

a second pair of passive flow-control valves for controlling the flow of the damping fluid between the lower chamber and the upper chamber via a second flow path separate from the first one, said second pair of passive flow-control valves comprising a compensation valve and a rebound valve which are made as check valves arranged to control the flow of the damping fluid from the lower chamber to the upper chamber (compression phase) and from the upper chamber to the lower chamber (rebound phase), respectively, and

a flow-dividing solenoid valve shiftable between a first operating position, in which it allows the flow of the damping fluid between the upper chamber and the lower chamber both via the first pair of flow-control valves and via the second pair of flow-control valves, i.e. both via the first flow path and via the second one, and a second operating position, in which is allows the flow of the damping fluid between the upper chamber and the lower chamber only via the second pair of flow-control valves, i.e. only via the second flow path, wherein each of the passive flow-control valves is made as a passive valve comprising resilient means adapted to keep the valve normally closed, and

wherein the resilient means associated to the compensation and rebound valves forming said first pair of flow-control valves have a stiffness lower than that of the resilient means associated to the compensation and rebound valves, respectively, forming said second pair of passive flow-control valves, in such a manner that when the flow-dividing solenoid valve is in the first operating position the damping fluid can flow between the upper chamber and the lower chamber via the first pair of passive flow-control valves to which the less stiff resilient means are associated (hereinafter referred to as minimum-curve passive flow- control valves) and therefore the shock-absorber generates a lower damping force, whereas when the flow-dividing solenoid valve is in the second operating position the damping fluid can flow between the upper chamber and the lower chamber via the second pair of passive flow-control valves to which the stiffer resilient means are associated (hereinafter referred to as maximum-curve passive flow-control valves) and therefore the shock- absorber generates a higher damping force.

It is an object of the present invention to provide a piston for a damping-adjustable shock- absorber of the above- specified type, which allows to reduce the energy consumption to the minimum.

A further object of the present invention is to provide a piston for a damping-adjustable shock-absorber of the above- specified type, which allows to adjust independently of each other the operating characteristics of the shock-absorber (compression with open solenoid valve, rebound with open solenoid valve, compression with closed solenoid valve and rebound with closed solenoid valve) and which allows to obtain pressure-flow rate operating characteristics (or, equivalently, force-speed operating characteristics) of so-called "degressive" type, i.e. comprising, immediately after a first ascending section (low flow rate or low speed), a second constant or at least not ascending section (high flow rate or high speed).

A still further object of the present invention is to provide a piston for a damping- adjustable shock-absorber of the above- specified type, which has a simple structure and a reliable operation .

These and other objects are fully achieved according to the invention by virtue of a piston for a damping-adjustable shock-absorber having the features set forth in the enclosed independent claim 1. Advantageous embodiments of the invention are specified in the dependent claims, the content of which is to be regarded as an integral and integrating part of the following description.

In short, the invention is based on the idea of providing a piston for a damping-adjustable shock-absorber of the above- specified type, in which the flow-dividing solenoid valve is made as a normally-open solenoid valve, whereby in the non-energized condition of the solenoid the damping fluid can flow between the upper chamber and the lower chamber of the shock-absorber via the pair of minimum-curve passive flow-control valves and hence the shock-absorber generates a lower damping force. Therefore, in normal driving conditions, in which a soft response of the suspensions is required to ensure driving comfort, the solenoids of the flow-dividing solenoid valves of the damping-adjustable shock-absorbers can be kept in the non-energized condition and therefore do not contribute to increase the energy consumption of the vehicle.

The passive flow-control valves are advantageously made as valves of the same type as that forming the subject-matter of Italian Patent application No. TO2009A000681 (or of the corresponding International Patent application No. WO2011/027314) in the Applicant's name. Preferably, at least the two maximum-curve passive flow-control valves (compensation valve and rebound valve) and the minimum-curve rebound valve are made as valves of the above-mentioned type. Such a construction of the passive flow-control valves allows to obtain pressure-flow rate (or force-speed) operating characteristics of the shock-absorber of "degressive" type. The pressure-flow rate (or force-speed) operating characteristics of the shock-absorber can also be adjusted independently of each other by acting both on the resilient elements and on the number and geometry of the adjustment discs of the passive flow-control valves.

Further features and advantages of the present invention will appear more clearly from the following detailed description, given purely by way of non-limiting example with reference to the appended drawings, in which:

Figure 1 is an axial section view of a piston for a damping-adjustable shock- absorber according to a preferred embodiment of the present invention; Figure 2 is an axial section view showing on an enlarged scale the bottom end portion of the piston of Figure 1 with the two pairs of passive flow-control valves, that is to say, the pair of minimum-curve passive flow-control valves and the pair of maximum- curve passive flow-control valves, respectively;

Figure 3 is an axial section view showing on an enlarged scale the flow-dividing solenoid valve of the piston of Figure 1 ; and

Figure 4 is a pressure-flow rate diagram illustrating examples of operating characteristics which can be obtained with a damping-adjustable shock-absorber comprising a piston according to the present invention.

In the following description and claims, terms such as "upper" and "lower" are to be intended as referring to the normal mounting condition of the shock-absorber on the vehicle, in which the valve assembly (passive flow-control valves) of the piston is located in the bottom end portion of the piston.

With reference first to Figure 1, a piston for a damping-adjustable shock-absorber, particularly for use in a vehicle suspension, is generally indicated 10 and basically comprises a rod 12 inside which a flow-dividing solenoid valve 14 (hereinafter simply referred to as solenoid valve) is mounted, a valve assembly 16 which is mounted at the bottom end of the piston and comprises a pair of minimum-curve passive flow-control valves 18 and 20, namely a minimum-curve compensation valve 18 and a minimum-curve rebound valve 20, and a pair of maximum-curve passive flow-control valves 22 and 24, namely a maximum- curve compensation valve 22 and a maximum-curve rebound valve 24, and a coupling body 26 axially interposed between the rod 12 and the valve assembly 16. The expression "minimum-curve" associated to the compensation valve 18 and to the rebound valve 20 means that these valves determine the minimum pressure-flow rate (or force-speed) characteristic curve of the shock-absorber, i.e. the one in which the minimum value of the pressure (or of the force), and hence the minimum value of the braking force generated by the shock-absorber, is associated to the same value of the flow rate (or of the speed). On the other hand, the expression "maximum-curve" associated to the compensation valve 22 and to the rebound valve 24 means that these valves determine the maximum pressure-flow rate (or force-speed) characteristic curve of the shock-absorber, i.e. the one in which the maximum value of the pressure (or of the force), and hence the maximum value of the braking force generated by the shock-absorber, is associated to the same value of the flow rate (or of the speed).

With reference also to Figure 3, the coupling body 26 integrally forms an upper cylindrical portion 28 and a lower cylindrical portion 30 and is fixed to the rod 12, for instance by means of a threaded connection. The upper cylindrical portion 28 extends inside the rod 12 coaxially thereto. The lower cylindrical portion 30 integrally forms in turn an upper plate 34, an intermediate portion 36 and a lower plate 38. The upper plate 34 extends inside the rod 12 and has on its outer cylindrical surface an external threading 32 cooperating with a corresponding internal threading of the rod 12 to provide the aforesaid threaded connection, whereas the intermediate portion 36 and the lower plate 38 project downwards from the rod 12. A central axial bore 40 passes both through the upper plate 34 and through the intermediate portion 36. The intermediate portion 36 also has a plurality of radial bores 42 which put the central axial bore 40 in fluid communication with the outside of the piston. The lower plate 38 also has a plurality of oblique bores 44 which extend downwards and outwards and put the central axial bore 40 in fluid communication with the valve assembly 16.

The solenoid valve 14 basically comprises a poppet 46, a spring 48 and a solenoid 50. The poppet 46 is mounted so as to be axially slidable in the central axial bore 40 of the coupling body 26 between an upper end-of-travel position or open position (shown in Figures 1 and 3), in which it leaves the radial bores 42 open, and a lower end-of-travel position or closed position (not shown in the drawings), in which it closes the radial bores 42. The spring 48 applies on the poppet 46 a resilient force tending to keep it in the open position. In the illustrated embodiment, the spring 48 is made as a cylindrical helical spring, axially interposed between the upper plate 34 of the coupling body 26 and a flange 52 of the poppet 46, but it might also be a spring of different type. The solenoid 50 is received inside the rod 12 and is operable by an electronic control unit (not shown) to apply on the poppet 46 an electromagnetic repulsion force tending to move the poppet 46 into the closed position by overcoming the biasing resilient force applied by the spring 48. The solenoid valve 14 is therefore of the normally-open type, i.e. in the condition in which the solenoid 50 is not energized the poppet 46 is in the open position in which it allows the damping fluid to flow between the lower chamber and the upper chamber of the shock-absorber via the minimum-curve flow-control valves (compensation valve 18 and rebound valve 20). In normal driving conditions, in which a soft response of the shock-absorber is generally required, the solenoid 50 of the solenoid valve 14 is thus kept in the non-energized state and the energy consumption of the vehicle is thus not increased. The solenoid 50 is energized only in case of need, when an increase in the braking force generated by the shock-absorber is required. The solenoid valve 14 may have a discrete way of operation, i.e. may be moved only into the two open and closed positions, or may have a continuous way of operation, i.e. may be moved continuously between the two open and closed positions. In the first case, the shock-absorber will only have the following four operating characteristics: rebound with open solenoid valve (minimum curve), rebound with closed solenoid valve (maximum curve), compression with open solenoid valve (minimum curve) and compression with closed solenoid valve (maximum curve). An example of operating characteristics which can be obtained with a shock-absorber provided with a piston according to the invention is shown in the pressure-flow rate diagram of Figure 4. In the second case, the operating characteristic curve of the shock-absorber can be adjusted each time continuously, both in the rebound phase and in the compression phase, between a minimum curve (open solenoid valve) and a maximum curve (closed solenoid valve).

With reference now in particular to Figure 2, the valve assembly 16 comprises, in addition to the above-mentioned valves 18, 20, 22 and 24, an inner body 54, an outer body 56 and a cover 58.

The inner body 54 integrally forms a hollow cylindrical upper portion 60, a hollow cylindrical lower portion 62 and a hollow frusto-conical intermediate portion 64 connecting the upper portion 60 and the lower portion 62. The upper portion 60 is fixed to the coupling body 26 and encloses a chamber 66 in which the minimum-curve compensation valve 18 and the minimum-curve rebound valve 20 are received and into which the oblique bores 44 of the coupling body 26 debouch. The lower portion 62 has a central axial bore 68 debouching at its bottom into the lower chamber of the shock-absorber. Therefore, with the solenoid valve 14 in the open position, the damping fluid can flow from the lower chamber to the upper chamber of the shock-absorber in order through the central axial bore 68 of the lower portion 62 of the inner body 54 of the valve assembly 16, through the minimum- curve compensation valve 18 in the chamber 66 of the inner body 54 of the valve assembly 16 and through the oblique bores 44, the central axial bore 40 and the radial bores 42 of the coupling body 26, and, in the opposite direction, that is to say, from the upper chamber to the lower chamber of the shock-absorber, in order through the radial bores 44, the central axial bore 40 and the oblique bores 44 of the coupling body 26, through the minimum- curve rebound valve 20 in the chamber 66 of the inner body 54 of the valve assembly 16 and through the central axial bore 68 of the lower portion 62 of the inner body 54 of the valve assembly 16.

A partition plate 70 is received in the chamber 66 of the inner body 54 and has a series of first axial through bores 72 (only one of which can be seen in the section view of Figure 2) located along a first circumference of larger diameter and a series of second axial through bores 74 (only one of which can be seen in the section view of Figure 2) located along a second circumference of smaller diameter, said first and second axial through bores 72 and 74 putting the upper portion of the chamber 66, which is at the same pressure as that of the upper chamber of the shock-absorber, in fluid communication with the lower portion of the chamber 66, which is at the same pressure as that of the lower chamber of the shock- absorber. The minimum-curve compensation valve 18 is associated to the first axial through bores 72, whereas the minimum-curve rebound valve 20 is associated to the second axial through bores 74.

The minimum-curve compensation valve 18 is a unidirectional valve comprising a closing element 76 and a spring 78. The closing element 76 is a thin disc of annular shape mounted so as to be axially movable between a closed position (shown in Figure 2), in which it is in contact with the top face of the partition plate 70 and closes the axial through bores 72, thereby preventing the damping fluid from flowing from the upper chamber to the lower chamber of the shock-absorber, and an open position (not shown in the drawings), in which it is raised with respect to the top face of the partition plate 70 and opens the axial through bores 72, thereby allowing the damping fluid to flow from the lower chamber to the upper chamber of the shock-absorber (compression phase). The spring 78 acts on the closing element 76 so as to apply on this latter a resilient force tending to keep it in the closed position. In the illustrated example, the spring 78 is a conical helical spring which abuts at its top against the lower plate 38 of the lower cylindrical portion 30 of the coupling body 26 and at its bottom against the closing element 76.

The minimum-curve rebound valve 20 is a unidirectional valve of the same type as the one forming the subject-matter of the above-mentioned Italian Patent application No. TO2009A000681, and basically comprises a movable element 80, a spring 82 and a plurality of adjustment discs 84. The movable element 80 is received in the chamber 66 of the inner body 54 so as to slide in the axial direction of the piston and is made as a cup-shaped element integrally forming a bottom wall 86 facing towards the partition plate 70 and a cylindrical lateral wall 88 guided along the cylindrical lateral surface of a guide element 90 which is received in the chamber 66 and extends in the axial direction of the piston. The spring 82, which in the illustrated example is a cylindrical helical spring, but which might also be a spring (or a plurality of springs) of different type, is axially interposed between an abutment surface 92 formed by the inner body 54 and the movable element 80 so as to apply on this latter a resilient force which is directed upwards and tends to urge it towards the partition plate 70. The partition plate 70 forms, on its bottom face, a radially inner annular projection 94 and a radially outer annular projection 96, which radially delimit the second axial through bores 74. The partition plate 70 also has, on its bottom face, an annular cavity 98 which is open downwards and is radially delimited between the cylindrical lateral surface of the guide element 90 and the radially inner annular projection 94. The assembly formed by the adjustment discs 84 stacked on one another is axially interposed between the partition plate 70 and the movable element 80 and is capable of sliding axially along the cylindrical lateral surface of the guide element 90. In the closed condition of the valve 20 (condition illustrated in Figure 2), the assembly formed by the adjustment discs 84 rests on the annular projections 94 and 96 of the bottom face of the partition plate 70. The spring 82, acting on the adjustment discs 84 via the movable element 80, tends to keep the valve 20 in this condition. At least one radial opening 100 is provided in the adjustment discs 84 (or better, at least in the top adjustment disc, i.e. in the adjustment disc which is directly in contact, in the closed condition of the valve 20, with the annular projections 94 and 96) and extends astride the radially outer annular projection 96 to allow, even in the closed condition of the valve 20, the damping fluid coming from the upper chamber of the shock-absorber through the second axial through bores 74 to flow towards the lower chamber of the shock-absorber passing over the radially outer annular projection 96. Moreover, the adjustment discs 84 advantageously have an internal diameter slightly larger than that of the cylindrical lateral surface of the guide element 90 so as to define with this latter a restrictor 102 (that is to say, a passage having a reduced cross-section) for the damping fluid flowing from the annular cavity 98 to the lower chamber of the shock- absorber. Moreover, in the area of connection between the bottom wall 86 and the cylindrical lateral wall 88 of the movable element 80 one or more openings 104 are provided, which are made in such a manner as to put the annular cavity 98 in fluid communication with the lower chamber of the shock-absorber through the restrictor 102. The openings 104 have therefore the function of allowing the damping fluid collected in the annular cavity 98 as a result of the movable element 80 moving away from the partition plate 70 to flow towards the lower chamber of the shock-absorber.

In general terms, it can be said that the minimum-curve rebound valve 20 has:

a first variable restrictor (formed in the present case by the passage between the radially outer annular projection 96 and the set of adjustment discs 84), the restriction amount of which depends on the position of the movable element 80 and through which the upper chamber of the shock-absorber is in fluid communication with the lower chamber of the shock-absorber,

a second variable restrictor (formed in the present case by the passage between the radially inner annular projection 94 and the set of adjustment discs 84), the restriction amount of which depends on the position of the movable element 80 and through which the upper chamber of the shock-absorber is in fluid communication with an auxiliary chamber of the valve (formed in the present case by the annular cavity 98),

a first fixed restrictor (formed in the present case by the passage 102 between the adjustment discs 84 and the cylindrical lateral surface of the guide element 90) through which the auxiliary chamber of the valve is in fluid communication with the lower chamber of the shock-absorber, in such a manner that the value of the pressure in the auxiliary chamber 98 of the valve, along with the opening force of the valve which opposes the resilient force produced by the spring 82, increases as result of an increase in the opening amount of the valve; and

a second fixed restrictor (formed in the present case by the radial opening 100 provided at least in the top adjustment disc 84) through which the upper chamber of the shock- absorber is in fluid communication with the lower chamber of the shock-absorber in parallel to the first variable restrictor.

The aforesaid second fixed restrictor might also be omitted.

The outer body 56 of the valve assembly 16 comprises a partition plate 106 and a cylindrical sleeve 108, preferably made as a single piece. The partition plate 106 is fixed to the inner body 54, namely to the hollow cylindrical lower portion 62 of the inner body 54, and has a series of first axial through bores 110 (only one of which can be seen in the section view of Figure 2) located along a first circumference of larger diameter and a series of second axial through bores 112 (only one of which can be seen in the section view of Figure 2) located along a second circumference of smaller diameter, said first and second axial through bores 110 and 112 putting the upper chamber in fluid communication with the lower chamber of the shock-absorber. The maximum-curve compensation valve 22 is associated to the first axial through bores 110, whereas the maximum-curve rebound valve 24 is associated to the second axial through bores 112. The cylindrical sleeve 108 is arranged with its outer lateral surface tightly slidable along the inner cylindrical surface of the shock-absorber and comprises an upper sleeve portion 114 extending upwards from the partition plate 106 and a lower sleeve portion 116 extending downwards from the partition plate 106. Between the upper sleeve portion 114 of the outer body 56 and the hollow cylindrical upper portion 60 of the inner body 54 a passage 118 is defined through which the damping fluid flowing from the lower chamber of the shock-absorber through the maximum-curve compensation valve 22 can reach the upper chamber of the shock-absorber and vice versa the damping fluid flowing from the upper chamber of the shock-absorber can reach the second axial through bores 112 and from here enter the lower chamber of the shock-absorber under control of the maximum-curve rebound valve 24. Likewise, between the lower sleeve portion 116 of the outer body 56 and the cover 58 a passage 120 is defined through which the damping fluid flowing from the lower chamber of the shock-absorber can reach the first axial through bores 110 and from here enter the upper chamber of the shock-absorber under control of the maximum-curve compensation valve 22 and vice versa the damping fluid flowing from the upper chamber of the shock-absorber through the maximum-curve rebound valve 24 can enter the lower chamber of the shock-absorber. The partition plate 106 forms, on its top face, a radially inner annular projection 122 and a radially outer annular projection 124, which radially delimit the first axial through bores 110. The partition plate 106 also has, on its top face, an annular cavity 126 which is open upwards and is radially delimited between the radially outer annular projection 124 and the inner cylindrical lateral surface of the upper sleeve portion 114. The partition plate 106 forms, on its bottom face, a radially inner annular projection 128 and a radially outer annular projection 130, which delimit radially the second axial through bores 112. The partition plate 106 also has, on its bottom face, an annular cavity 132 which is open downwards and is radially delimited between the radially inner annular projection 128 and the outer cylindrical lateral surface of the hollow cylindrical lower portion 62 of the inner body 54.

The maximum-curve compensation valve 22 is also, like the minimum-curve rebound valve 20, a unidirectional valve of the same type as the one forming the subject-matter of the above-mentioned Italian Patent application No. TO2009A000681 and basically comprises a movable element 134, a spring 136 and a plurality of adjustment discs 138. The movable element 134 is slidably mounted in the axial direction of the piston and is made as a cup- shaped element integrally forming a bottom wall 140 facing towards the partition plate 106 and a cylindrical lateral wall 142 guided along the inner cylindrical lateral surface of the upper sleeve portion 114. The spring 136, which in the illustrated example is a cylindrical helical spring, but which might also be a spring (or a plurality of springs) of different type, is axially interposed between an abutment surface 144 formed by the inner body 54 and the movable element 134 so as to apply on this latter a resilient force which is directed downwards and tends to urge it towards the partition plate 106. The assembly formed by the adjustment discs 138 stacked on one another is axially interposed between the partition plate 106 and the movable element 134 and is capable of sliding axially along the inner cylindrical lateral surface of the upper sleeve portion 114. In the closed condition of the valve 22 (condition illustrated in Figure 2), the set of adjustment discs 138 rests on the annular projections 122 and 124 on the top face of the partition plate 106. The spring 136, acting on the adjustment discs 138 through the movable element 134, tends to keep the valve 22 in this condition. At least one radial opening 146 is provided in the adjustment discs 138 (or better, at least in the bottom adjustment disc, i.e. in the adjustment disc directly in contact, in the closed condition of the valve 22, with the annular projections 122 and 124) and extends astride the radially outer annular projection 124 to allow, even in the closed condition of the valve 22, the damping fluid coming from the lower chamber of the shock-absorber via the first axial through bores 110 to flow towards the upper chamber of the shock-absorber passing over the radially outer annular projection 124. Moreover, the adjustment discs 138 advantageously have an external diameter slightly smaller than that of the inner cylindrical lateral surface of the upper sleeve portion 114 so as to define with this latter a restrictor 148 (that is to say, a passage with reduced cross-section) for the damping fluid flowing from the annular cavity 126 to the upper chamber of the shock- absorber. Moreover, in the area of connection between the bottom wall 140 and the cylindrical lateral wall 142 of the movable element 134 one or more openings 150 are provided, which are made in such a manner as to put the annular cavity 126 in fluid communication with the upper chamber of the shock-absorber through the restrictor 148. The openings 150 have therefore the function of allowing the damping fluid collected in the annular cavity 126 as a result of the movable element 134 moving away from the partition plate 106 to flow towards the upper chamber of the shock-absorber.

In general terms, it can be said that the maximum-curve compensation valve 22 has:

a first variable restrictor (formed in the present case by the passage between the radially inner annular projection 146 and the set of adjustment discs 138), the restriction amount of which depends on the position of the movable element 134 and through which the lower chamber of the shock-absorber is in fluid communication with the upper chamber of the shock-absorber,

a second variable restrictor (formed in the present case by the passage between the radially outer annular projection 124 and the set of adjustment discs 138), the restriction amount of which depends on the position of the movable element 134 and through which the lower chamber of the shock-absorber is in fluid communication with an auxiliary chamber of the valve (formed in the present case by the annular cavity 126),

a first fixed restrictor (formed in the present case by the passage 148 between the adjustment discs 138 and the inner cylindrical lateral surface of the upper sleeve portion 114 of the outer body 56) through which the auxiliary chamber of the valve is in fluid communication with the upper chamber of the shock-absorber, in such a manner that the value of the pressure in the auxiliary chamber 126 of the valve, along with the opening force of the valve which opposes the resilient force produced by the spring 136, increases as a result of the increase in the opening amount of the valve, and

a second fixed restrictor (formed in the present case by the radial opening 146 provided at least in the bottom adjustment disc 138) through which the lower chamber of the shock-absorber is in fluid communication with the upper chamber of the shock-absorber in parallel to the first variable restrictor.

The aforesaid second fixed restrictor might also be omitted.

The maximum-curve rebound valve 24 is also, like the minimum-curve rebound valve 20 and the maximum-curve compensation valve 22, a unidirectional valve of the same type as the one forming the subject-matter of the above-mentioned Italian Patent application No. TO2009A000681, and basically comprises a movable element 152, a spring 154 and a plurality of adjustment discs 156. The movable element 152 is slidably mounted in the axial direction of the piston and is made as a cup- shaped element integrally forming a bottom wall 158 facing towards the partition plate 106 and a cylindrical lateral wall 160 guided along the outer cylindrical lateral surface of the hollow cylindrical lower portion 62 of the inner body 54. The spring 154, which in the illustrated example is a cylindrical helical spring, but which might also be a spring (or a plurality of springs) of different type, is axi- ally interposed between an abutment surface 162 formed by the cover 58 and the movable element 152 so as to apply on this latter a resilient force which is directed upwards and tends to urge it towards the partition plate 106. The set of adjustment discs 156 stacked on one another is axially interposed between the partition plate 106 and the movable element 152 and is capable of sliding axially along the outer cylindrical lateral surface of the hollow cylindrical lower portion 62 of the inner body 54. In the closed condition of the valve 24 (condition illustrated in Figure 2), the set of adjustment discs 156 rests against the annular projections 128 and 130 on the bottom face of the partition plate 106. The spring 154, acting on the adjustment discs 156 through the movable element 152, tends to keep the valve 24 in this condition. At least one radial opening 164 is provided in the adjustment discs 156 (or better, at least in the top adjustment disc, i.e. in the adjustment disc directly in contact, in the closed condition of the valve 24, with the annular projections 128 and 130) and extends astride the radially outer annular projection 130 to allow, even in the closed condition of the valve 24, the damping fluid coming from the upper chamber of the shock- absorber via the second axial through bores 112 to flow towards the lower chamber of the shock-absorber passing over the radially outer annular projection 130. Moreover, the adjustment discs 156 advantageously have an internal diameter slightly larger than that of the outer cylindrical lateral surface of the hollow cylindrical lower portion 62 of the inner body 54 so as to define with this latter a restrictor 166 (that is to say, a passage of reduced cross- section) for the damping fluid flowing from the annular cavity 132 to the lower chamber of the shock-absorber. Moreover, in the area of connection between the bottom wall 158 and the cylindrical lateral wall 160 of the movable element 152 one or more openings 168 are provided, which are made in such a manner as to put the annular cavity 132 in fluid communication with the lower chamber of the shock-absorber through the restrictor 166. The openings 168 have therefore the function of allowing the damping fluid collected in the annular cavity 132 as a result of the movable element 152 moving away from the partition plate 106 to flow towards the lower chamber of the shock-absorber.

In general terms, it can be said that the maximum-curve rebound valve 24 has:

a first variable restrictor (formed in the present case by the passage between the radially outer annular projection 130 and the set of adjustment discs 156), the restriction amount of which depends on the position of the movable element 152 and through which the upper chamber of the shock-absorber is in fluid communication with the lower chamber of the shock-absorber,

a second variable restrictor (formed in the present case by the passage between the radially inner annular projection 128 and the set of adjustment discs 156), the restriction amount of which depends on the position of the movable element 152 and through which the upper chamber of the shock-absorber is in fluid communication with an auxiliary chamber of the valve (formed in the present case by the annular cavity 132),

a first fixed restrictor (formed in the present case by the passage 166 between the adjustment discs 156 and the outer cylindrical lateral surface of the hollow cylindrical lower portion 62 of the inner body 54) through which the auxiliary chamber of the valve is in fluid communication with the lower chamber of the shock-absorber, in such a manner that the value of the pressure in the auxiliary chamber 132 of the valve, along with the opening force of the valve opposing the resilient force produced by the spring 154, increases as a result of the increase in the opening amount of the valve, and

a second fixed restrictor (formed in the present case by the radial opening 164 provided at least in the top adjustment disc 156) through which the upper chamber of the shock-absorber is in fluid communication with the lower chamber of the shock-absorber, in parallel to the first variable restrictor.

The aforesaid second fixed restrictor might also be omitted.

The spring (or the assembly of springs) of each valve of minimum-curve has a lower stiffness than that of the spring (or of the set of springs) of the corresponding maximum-curve valve. The spring 78 of the minimum-curve compensation valve 18 has therefore a lower stiffness than that of the spring 136 of the maximum-curve compensation valve 22, and likewise the spring 82 of the minimum-curve rebound valve 20 has a lower stiffness than that of the spring 154 of the maximum-curve rebound valve 24. Accordingly, with the solenoid valve 14 in the normal open condition (solenoid 50 in the non-energized state), the flow of the damping fluid between the upper and lower chambers of the shock-absorber occurs through the axial bores 72 and 74 in the inner body 54 of the valve assembly 16 under control of the minimum-curve compensation valve 18 and of the minimum-curve rebound valve 20, whereas with the solenoid valve 14 in the closed condition (solenoid 50 in the energized state), the flow of the damping fluid between the upper and lower chambers of the shock-absorber occurs through the axial bores 110 and 112 in the outer body 56 of the valve assembly 16 under control of the maximum-curve compensation valve 22 and of the maximum-curve rebound valve 24.

The operation of the minimum-curve rebound valve 20 will be described now by way of example, being it clear that what will be said in connection with this valve is equally well applicable to the maximum-curve compensation valve 22 and to the maximum-curve rebound valve 24. In the closed condition of the valve 20, the movable element 80 is subject to the resilient force of the spring 82, which tends to urge this element, along with the adjustment discs 84, against the annular projections 94 and 96, i.e. to keep the valve closed, and to the force applied by the pressure of the damping fluid contained in the upper chamber of the shock- absorber (pressure which is higher than that in the lower chamber of the shock-absorber). In this condition, the radial opening (or the radial openings) 100 provided at least in the top adjustment disc 84 allows the passage of fluid (although in a very small amount) from the upper chamber to the lower chamber of the shock-absorber. The annular cavity 98 is in fluid communication, through the restrictor 102, with the lower chamber of the shock- absorber, whereby the value of the pressure in this cavity is close to the value of the pressure in the lower chamber of the shock-absorber. When the pressure of the fluid in the upper chamber of the shock-absorber is such as to overcome the resilient force of the spring 82, the movable element 80 moves away from the projections 94 and 96 of the partition plate 70, thus allowing also the adjustment discs 84 to move away from these projections. The damping fluid contained in the upper chamber of the shock-absorber can now flow towards the lower chamber of the shock-absorber not only directly through the passage defined between the top adjustment disc 84 and the radially outer annular projection 96, but also indirectly through the annular cavity 98, through the restrictor 102 and through the openings 104 provided in the movable element 80. The pressure loss due to the restrictor defined between the top adjustment disc 84 and the radially inner annular projection 94 causes the pressure in the annular cavity 98 to be lower than the pressure in the upper chamber of the shock-absorber, wherein the difference between these two pressures decreases progressively with the increase in the opening amount of the valve until it becomes close to zero. Accordingly, the effective value of the area on which the pressure of the upper chamber of the shock-absorber acts varies from a minimum value equal to the area of the second axial through bores 74 to a maximum value tending to be equal to the sum of the areas of the second axial through bores 74 and of the annular cavity 98.

This effect of amplification of the force applied by the pressure of the damping fluid against the elastic reaction of the spring allows to obtain the second "degressive" section of the pressure-flow rate (or force-speed) characteristic curve of the shock-absorber in the rebound phase with open solenoid valve. Similar pressure-flow rate (or force-speed) charac- teristics of degressive type can be obtained with the solenoid valve in the closed condition, both in the compression phase and in the rebound phase, tinder control of the maximum- curve compensation valve and of the maximum-curve rebound valve, respectively, as shown in the diagram of Figure 4.

The minimum-curve compensation valve 18, which in the proposed example is of conventional type, might also be made as a unidirectional valve of the same type as the one forming the subject-matter of the above-mentioned Italian Patent application No. TO2009A000681. In this connection, the partition plate 70 of the inner body 54 of the valve assembly 16 forms, on its top face, a radially inner annular projection 170 and a radially outer annular projection 172, which delimit radially the first axial through bores 72. The partition plate 70 also has, on its top face, an annular cavity 174 which is open upwards and is radially defined between the radially outer annular projection 172 and the inner cylindrical lateral surface of the upper hollow cylindrical portion 60 of the inner body 54. It is therefore possible to replace the closing element 76 of the valve 18 illustrated in Figure 2 with a movable element and a set of adjustment discs such as those described above with reference to the other three passive flow-control valves 20, 22 and 24.

In view of the above description, the advantages obtained with a piston for a damping- adjustable shock-absorber according to the present invention are evident.

First of all, the use of a flow-dividing solenoid valve in combination with four passive flow-control valves allows to obtain, both in the compression phase and in the rebound phase, a pair of limit characteristic curves corresponding to the open condition and to the closed condition of the solenoid valve, respectively.

Secondly, the use of a normally-open solenoid valve allows to obtain a soft response of the shock-absorber, both in the compression phase and in the rebound phase, with no need to energize the solenoid of the solenoid valve and without increasing therefore the energy consumption of the vehicle on which the shock-absorber is installed.

Moreover, the four passive flow-control valves can be adjusted independently of each other, by changing the stiffness characteristics of the springs and, in case of use of valves of the same type as the one forming the subject-mater of Italian Patent application No. TO2009A000681, by changing the number and the geometry of the adjustment discs, which allows to adjust the four limit characteristic curves of the shock-absorber independently of each other.

Finally, the use of valves of the same type as the one forming the subject-mater of Italian Patent application No. TO2009A000681 as passive flow-control valves allows to obtain pressure-flow rate (or force-speed) characteristics of the shock-absorber having a "degressive" profile.

Naturally, the principle of the invention remaining unchanged, the embodiments and the constructional details may vary widely from those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of the invention as defined in the appended claims.