The invention relates to a shock absorber according to the preamble of claim 1.

A shock absorber of this kind is by way of example disclosed in NL-C. 1016147. In this a shock absorber for a motorcycle is disclosed, which is . composed, in a known way, of a damper piston-cylinder system and a coil spring extending around it. Securing eyelets are provided at the topside and underside of the system. The prestress of the coil spring can be adjusted using what is referred to as a preload adjuster. For this purpose, the topside of the coil spring is supported against a spring cup which can be displaced up and down in the axial direction of the shock absorber by means of a hydraulically acting system. The hydraulically acting system comprises an annular displacement piston-cylinder system, which acts on the .spring cup and the pressure of which can be varied by means of an actuating member. The actuating member comprises a control piston-cylinder system which is mounted to the side of the damper piston-cylinder system and in which the position of the control piston can .be adjusted by hand by means of a. rotary knob, with a threaded spindle. It is in this way possible to vary the pressure in the control piston-cylinder system and to alter the position of the spring cup in the axial direction accordingly.

One drawback of this arrangement is that the adjustment quality leaves something to be desired. The adjustment is carried out by ■ hand; in practice, it has been found that it is relatively difficult' to turn the rotary knob. Furthermore, the adjustment has to be carried out when the motorcycle is stationary. Adjustment when the motorcycle is moving is irresponsible. If more than one shock absorber of this type is used on the same vehicle, it is not easy to set a desired adjustment to be exactly equal for each of them. This can lead to different deviations, with adverse consequences for the driving performance and ride comfort.

It is an object of the present invention to at least partially overcome the above drawbacks and/or to provide a usable alternative. In particular, it is an object of the invention to provide a user- friendly shock absorber whereof the damping properties and characteristics can be adapted quickly and easily.

This object is achieved by a shock absorber according to claim 1. In this case, the shock absorber comprises a hydraulic damper cylinder with a damper piston which can be displaced therein. A piston rod is connected to the damper piston. Assembly parts are provided at the free ends of the damper cylinder and the piston rod. First and second spring bases are provided in the vicinity of the assembly , parts, with a compression spring enclosed between them. Further there is a ■ hydraulically acting system for adjusting the axial position of at least one of the spring bases with respect to the associated assembly part. The hydraulically acting system comprises a control cylinder with a control chamber and a control piston displaceable therein, as well as a setting chamber, in which a setting piston which interacts with the spring base located in its vicinity is axially displaceable. At one end, the control chamber is in hydraulic ■communication with the setting chamber. There are pressure means for exerting a pressure greater than ambient .pressure on the opposite side of ' the control piston from the hydraulic connection to the setting chamber. As a result, the force required for the adjustment can advantageously be reduced, and/or the adjustment time shortened.

In particular,, the pressure originating from the pressure means is related to the- pressure which is present" in the damping medium as a result of the minimal preloading in the compression spring. More particularly-, the pressure means are designed to deliver a pressure greater than 5 bar.

In one advantageous embodiment, there is a separate gas chamber which is in communication with the control chamber on the opposite side from -.the hydraulic connection to the setting chamber. A pressurized gas volume is present in the gas chamber. The level of the gas pressure is in this case related to the pressure which occurs in the working medium of the hydraulic system as a result of the minimum spring preloading.

The pressure means can be used in conjunction with a manual operating feature, for example by means of rotary knob adjustment as used in the prior art. In a preferred embodiment, the pressure means are used in a shock absorber in which the hydraulically acting system also comprises an actuable electric motor for adjusting the axial position of the control piston. It is advantageously now possible to ..electronically adjust the preloading of .the coil spring and thereby the height of the shock absorber in the compressed position by suitable supply of current to the electric motor. This can be done both when the motorcycle is stationary and when it is being driven, and thus makes it possible to adjust the damping properties and characteristics of the shock absorber at any desired moment, for example by means of a switch on the handlebars of a motorcycle on which the shock absorber is used. The pressure means in this case advantageously ensure that the power required of the electric motor is reduced and/or the adjustment time is shortened. The reduction in the power required is important in particular for use on motorcycles, since motorcycles have a relatively weak battery as power source.

Further preferred embodiments are defined in the subclaims.

The invention also relates to a motorcycle according to claim 17.

The invention will be explained in more detail with reference to the appended drawings, in which: Fig. 1 shows a perspective view, partially in section, of a preferred' embodiment of the shock absorber . for a motorcycle according to the invention; Fig. 2 shows a bottom view of Fig. 1; Fig. 3 shows a cross-sectional view on line III-III in Fig. 1; Fig. 4 shows a cross-sectional view on line IV-IV in Fig. 2; Fig. 5 shows a view of part of Fig. 4 on an enlarged scale; and Fig. 6 shows a view of part of Fig. 5 on an enlarged scale.

The shock absorber is denoted overall by reference numeral 1 in Fig. 1-6 and comprises an oil-filled damper cylinder 2 and a damper piston 3 which is displaceable therein. The damper piston 3 is fitted to the end of a piston rod 4. At both ends of the damper cylinder 2 and the piston rod 4 there are assembly parts 5, which are in this case formed by assembly eyelets in which there is a bearing arrangement. Between the assembly parts 5 there are a lower spring base 6 and an upper spring base 7, against which a coil spring 8 is supported. The lower spring base 6 is delimited at the bottom with respect to the bottom assembly part 5.

The upper- spring base 7 is connected to the lower end of an annular setting piston 10 which is accommodated in a setting ■.chamber 11 in such a manner that it can be moved up and down. The setting chamber 11 is delimited by a cylindrical housing part 12 which is connected to the outer circumferential wall of the damper cylinder 2. The setting piston 10 and the outer circumferential wall of the damper cylinder 2 are provided with projecting edge parts 15 which together prevent the setting piston 10 from being able to shoot too far out of the setting chamber 11.

The setting piston 10 and the setting chamber 11 form part of a hydraulically acting system, within which the pressure can be varied in order to displace the setting piston 10 in the setting chamber 11 and to adjust it upwards or downwards according to the position of the spring base 7. For this purpose, the hydraulically acting system also comprises a control cylinder 16 which delimits a " control chamber 16a, a control piston 17 which can be displaced therein and an electric motor 18 for displacing the control piston 17 in the axial direction. These components are accommodated in a housing part 20 whrch adjoins the housing part 12 and is illustrated in partially transparent form in Fig. 1. The action of the control cylinder 16, the control piston 17 and the electric motor 18 will now be explained in more detail with reference to Fig. 3.

The control piston 17 can be moved to and fro in the axial direction within the control chamber 16a and is limited with respect to rotation by wall parts of the control cylinder 16. The control piston 17 is provided with an internally threaded bore 23 into which a threaded spindle 24 is screwed. The threaded spindle 24 can be driven by the electric motor 18 and then ensures that the control piston 17 moves in the axial direction. The control chamber 16a, on the left-hand side of the control piston 17, is in hydraulic communication with the setting chamber 11 via a through-flow passage, which is not shown in more detail.

On the right-hand side of the control piston .17, there, is gas pressure in the control chamber 16a which is related to the pressure which occurs as a result of the minimum preloading of the coil spring .8 in the hydraulic system. This pressure is at least greater than ambient pressure. In the further load-free state of the shock absorber 1, the gas pressure is set to a value greater than 5 bar, more particularly greater than 8 bar. This pressure contributes to moving the control piston 17 to the left, and thereby reduces the' power required, of the electric motor 18 if the spring base 7 needs to be pushed downwards and/or shortens the time . required for ■ adjusting the position of the spring base 7 downwards.

Beneath the control chamber 16a there is a gas chamber 27 which, via a groove 28, is in communication with that part of the control chamber 16a which is on the right-hand side of the control piston 17. The gas. chamber has a volume which is greater .than half the volume of the control chamber 16a. The gas chamber 27 is provided with an inlet valve 29 for externally pressurizing the gas chamber 27 with a quantity of gas. The pressurized gas cannot mix with the hydraulic medium, in this case formed by oil, which is on the other side of the control piston 17 in the hydraulic system. For this purpose, the control piston 17 is provided with a gasket ring 30. Furthermore, the control chamber 16a and the gas chamber 27 are sealed on the side of the • electric motor 18. For this purpose', the threaded spindle 24 is guided rotatably and in sealed fashion through a closure plate with gasket rings .33 accommodated therein.

The electric motor 18 is formed by a DC motor, in particular a stepper motor, which if desired can be actuated remotely, for example from the handlebars of the motorcycle, using an operating switch.

At the topside, the damper cylinder 2 is in communication, via a connecting passage 40, with a pressure pot 41. On the left-hand side, the pressure pot 41 is closed off by a cover 42, and on the right-hand side it is in communication to a greater or lesser extent with the damper cylinder 2 via an adjustable valve body 43. A displaceable separating element 44, which is in this case formed by a piston body, is accommodated in the pressure pot 41. On the left- hand side, the piston body delimits a gas reservoir 45 which is used to collect the oil displaced by the volume of the piston rod 4 during a compressing movement of the shock absorber 1. The compressing movement is in this case partly damped to a greater or lesser extent by inhibiting the flow of oil from the damper cylinder 2 to the pressure pot 41 by suitable adjustment of the valve body 43 in the connecting passage 40. Furthermore, a through-flow passage 46, in which a resistance is incorporated by means of spring plates, is provided as a pressure relief feature. For return flow of the oil from the pressure pot 41 to the damper cylinder 2, there is a one¬ way valve which does not exert a particularly high resistance to the flow of oil in this direction.

The valve body 43 is in this case formed by a needle shut-off member which extends partly within the connecting passage 40. The axial position of the valve body 43 in the connecting passage 40 can be adjusted by means of a threaded spindle which can be rotated by an electric motor 47. The electric motor 47 is formed by a stepper motor which if desired can be actuated remotely, for example from the handlebars of the motorcycle, using an operating switch. In this way, the inward damping of the shock absorber 1 can be adjusted quickly and efficiently independently of the outward damping.

In the piston rod 4 there is a through-flow passage 55 which extends between chambers above and below the damper piston 3. In the through-flow passage 55 there is an adjustable valve body 56, via which the through-flow passage 55 can be opened to a greater or lesser extent. The valve body 56 comprises a needle shut-off member which extends within the through-flow passage 55. The axial position of the needle shut-off member can be adjusted by means of a threaded spindle which can be rotated by an electric motor 59. The electric motor 59 is formed by a stepper motor which if desired can be actuated remotely, for example from the handlebars of the motorcycle, using an operating switch.

The damper piston 3 is provided with through-flow passages, in which a resistance can be installed by means of spring plates in a known way. Therefore, together with the through-flow passage 55 in the piston rod 4, this allows oil to move from one side of the damper piston 3 to the other if the shock absorber 1 compresses or rebounds . The outward damping of the shock absorber 1 together with a fraction of the inward damping can now be adjusted quickly and efficiently by adapting the axial position of the valve body 56 in the through-flow passage 55 by suitable actuation of the electric motor 59.

At the underside of the valve body 56 there is a pin 60 which has a smaller diameter than the largest diameter of the valve body 56. The pressurized oil in the damper cylinder 2 also passes via the through-flow passage 55 on the underside of the valve body 56. On account of the presence of the pin 60, the effective surface area on the underside of the valve body 56 on which the oil can exert an upward pressure is smaller than the effective surface area on the topside of the valve body 56 on which the oil can exert a downward pressure. This produces a resultant force on the valve body 56 which is directed downwards and therefore advantageously prevents the needle shut-off member from shooting out of the through-flow passage 55 at the top. The upwards pressure on the underside of the valve body 56 advantageously contributes to facilitating displacement of the valve body 56, and therefore reduces the power required of the electric motor 59 if the valve body 56 needs to be adjusted and/or shortens the time required for adjusting the position of the valve body 56.

In addition to the embodiment shown, numerous variants are possible. For example, it is possible to use different types of valve bodies, the position of which can be adjusted with the aid of an electric motor. As an alternative to gas pressure, the pressure means may also comprise a spring element which bears against the control piston. As an alternative to threaded spindles, it is also possible to provide other types of transmissions between the electric motors and the control piston or the valve bodies. The threaded spindle can be connected to the electric motor, but may also directly form the output shaft of the electric motor. The electric motors can optionally be provided with current from the vehicle battery directly via switches which can be operated by the rider. However, it is also possible to provide an automated control system with ■ sensors for actuating the electric motors. The pressure means, the control of the position of the control piston by the electric motor for adjusting the spring preloading, the control of the position of • 5 the valve body in the connecting passage to the pressure pot by the . electric motor for adjusting the inward damping, and the control of the position of the valve body in the through-flow passage through the piston ■ rod by the electric motor for adjusting the outward damping, can also be used independently of one another for a shock 10 ' absorber, i.e. the others may then not be adjustable or may be adjustable in a different way. It is preferable for the three . adjustment options to be used in combination with one another, resulting in a multiply adjustable shock absorber which is reliable ■ in use and the properties of which can be adapted automatically or 15 manually by a rider as desired, as a function of ■ the particular conditions of use.