The invention relates to a twin-pipe shock absorber, com¬ prising a working cylinder, a piston which is movable in said cylinder and has a passage and upstream valve, an external pipe fitted round the working cylinder, an oil tank between working cylinder and external pipe, a disc shutting off the working cylin- der and having a passage and an upstream valve, and a damper valve.

Such shock absorbers are known. The damper valve in them is located either in the piston rod guide or in a separate bypass.

The oil flow passes through the same damper valve both on the ingoing and on the outgoing stroke of the piston. A disadvantage is that the shock absorption is the same at all frequencies of movement of the unsprung wheel guide part. At high frequencies this is very disturbing. The object of the invention is to eli¬ minate this disadvantage and produce a shock absorber of the type mentioned in the preamble, in which the shock absorption can be adapted very rapidly to the frequency of movement of the unsprung wheel guide part.

According to the invention, the above twin-pipe shock absor¬ ber is characterized in that a central pipe extends through the piston and with its bottom end projects through said disc, said damper valve is disposed at the bottom end of said central pipe, and regulation means are present to regulate the damping exerted by the damper valve depending at least on the frequency at which the working cylinder vibrates.

It is of essential importance that the oil flow on the in- going and outgoing stroke of the piston is conveyed through the central pipe to one single damper valve, and that the damping of this valve can be set rapidly. The piston of the shock absorber and the guide of the piston are simple in design: there is only one upstream valve, which can be placed round the central pipe in such a way that it seals dynamically.

It is pointed out that known shock absorbers which have damping adapted to frequency have two damper valves and two regu-

lation systems. This makes those designs expensive and complex.

GB-A-2, 159,604 discloses a twin-pipe shock absorber having an electromagnetically controlled axially displaceable control tube in the interior of the piston rod. The damper valve at the bottom end of the control tube cannot be regulated depending on the frequency at which the working cylinder vibrates.

Further FR-A-2,560,325 discloses a twin-pipe shock absorber having an electronically controllable damping valve in the connec¬ tion between the oil tank and the space underneath the piston. However, there is no central pipe extending through the piston and with its bottom end projecting through a disc shutting off the working cylinder.

The valve body of the damper valve will be disposed on or form part of an exciting element which moves up and down and has a narrow oil passage, such throttling means being capable of regulat¬ ing the oil pressure under said exciting element.

In order to produce the regulation by mechanical means, provision is made below the exciting element for a mass inertia piston between two springs or spring systems, in which piston there is a passage which is provided with an upstream valve and connects the space above and below the piston, there being above the piston an oil passage with valve which can be opened when the oil pressure is raised above the piston, in order to reduce the pressure below the exciting element. A finer regulation is, however, possible if provision is made below the exciting element for a throttle plate fixed to one or more coils which are placed in the field of at least one perma¬ nent magnet, it being possible to move the throttle plate up and down through more or less exciting of the coils, and provision being made between the throttle plate and the exciting element for an oil chamber having an oil throughflow spout which can be shut off to a greater or lesser extent by the throttle plate.

The exciting element can be a piston which is displaceable in a cylinder. The leakage occurring between cylinder and exciting piston can cause problems in accuracy. It may therefore be prefer¬ able for the exciting element to be suspended in resilient mem¬ branes.

If a spring acts upon the throttle plate, fitting tolerances can be eliminated.

The invention also relates to a vehicle provided with a shock absorber of the above-mentioned type, in which a throttle plate fixed to one or more coils is disposed below the exciting element. According to the invention, the coils have an electrical connection to a regulation system which is in turn connected to sensors for measuring different movements of the vehicle, and which can give signals to the coils depending on the movements measured. The invention will now be explained in greater detail with reference to the figures, in which a number of embodiments are shown in longitudinal section.

Figs. 1, 2 and 3 show respectively three embodiments of shock absorbers according to the invention. Fig. 4 shows the bottom end of the embodiment according to ^' Fig. 3, on a larger scale.

The shock absorber shown in Fig. 1 comprises a working cylin¬ der 1 designed as an internal pipe, an external pipe 2, a tank 3 located between the working cylinder 1 and the external pipe 2, a piston 4 which is displaceable in the working cylinder 1 , a hollow piston rod 5 connected to the sprung part of a wheel suspension part, a cover 6 shutting off the working cylinder and the tank at the top and also forming guidance for the piston rod, a disc 7 shutting off the working cylinder 1 a short distance above its bottom end, and a bottom plate 8 which shuts off the working cylin¬ der 1 and the external pipe 2 and is provided with a fastening eye 9 for fastening the shock absorber to the unsprung part of the wheel guide.

A central pipe 10, projecting with its bottom end through the disc 7, extends through the piston 4.

Just above the piston 4, the piston rod 5 has apertures 11 which connect the space above the piston to the interior of the piston rod. The top end of the central pipe 10 is some distance above the apertures 11. The piston 4 is provided with a cylin- drical valve housing 12, a valve seat 13, and a valve body 14 which is pressed by a spring 15 on the seat 13. The parts 12 to 15 form an upstream valve.

The disc 7 contains passages 16 which are provided with a spring-loaded upstream valve 17 on the top side.

Below the disc 7 is an exciting piston 19 which is displace¬ able up and down in a cylinder 18, and on which the resilient valve body 20 of the damper valve is fitted. This valve body 20 contacts a seat at the bottom side of the disc 7 when the damper valve is in the closed position.

The exciting piston 19 contains a narrow duct 21 (for re¬ striction) which connects the space above and below the piston 19. The pressure above and below the piston 19 can be balanced by means of this duct.

A solid, relatively heavy piston 23 (mass inertia piston) which is displaceable up and down in a cylinder 22 is disposed below the exciting piston 19. This heavy piston is retained between two springs 24, 25. The spring 25 presses a valve body 26 against a seat formed on the cover of the cylinder 22. An aperture 27 is provided in this cylinder cover. The spring 24 rests against the bottom plate 8. The mass inertia piston 23 has a passage 28 with upstream valve 29. The oil tank 3 is connected by means of apertures 30 to the space below the disc 7.

The shock absorber works as follows:

On the outgoing stroke (the piston 4 moves upwards) the oil above the piston 4 is pressed through the apertures 11 into the hollow piston rod 5 and comes to rest in the central pipe 10. Near the foot of this pipe 10 the oil flows through the damper valve to the ducts 16 and via the upstream valve 17 into the space below the piston 4. On the ingoing stroke (piston 4 moves downwards) the oil lying below the piston 4 will flow through the upstream valve 13, 14, 15 to the increasing space above the piston 4. An oil volume corresponding to the volume of the piston rod 5 penetrating the working cylinder 1 also flows through the central pipe 10 to the damper valve 7, 20 and after passing through it goes into the tank

3 through the apertures 30. The upstream valve 13, 14, 15 of the piston 4 causes negligible damping; the damping action is produced by the damper valve 7, 20 on the bottom side of the disc 7. The size of the damping is determined by the system 18, 19, 22 to 29

situated below the valve body 20, in particular by the combination of mass inertia piston 23 and the springs 24, 25, the so-called mass spring system. This system is designed for the actual frequen¬ cy of the non sprung mass (wheel, hub, disc brake, wheel guide part). When the wheel is flung up, the piston 23 remains behind, while oil flows via the ball valve 29 and the passage 28 from the space below the piston 23 to the space above the piston 23. The valve body 26 comes away from its seat, so that the pressure below the exciting piston 19 is reduced or falls off. The valve body 20 of the damper valve is pressed on its seat with little or no pres¬ sure, so that the damping action is very slight. This is restored after the piston 23 is returned to its original place by the springs 24, 25, and the valve body 26 goes onto its seat. The oil flow through the narrow duct 20 can then pressurize the space below the exciting element 19.

The mass inertia piston 23 displaces oil when it moves, so that the recoil movement is damped. With a suitable selection of the characteristics of the springs 24, 25 and the mass of the piston 23, the damping of the shock absorber can decrease to zero at high frequencies of the movement of the unsprung mass. The movements of low frequency and great amplitude can be damped in the optimum manner.

In the embodiment according to Fig. 2, corresponding parts are given the same reference numbers. This embodiment differs from that of Fig. 1 in that the mass spring system is replaced by an electromagnetic system. Moreover, the exciting element 19 is also a damper valve body. The said electromagnetic system comprises a permanent magnet 31, which is fixed to the cylinder 18, and a suspended electrical coil 32 with horizontal choke plate 33. The attracting action of the permanent magnet on the elec¬ trically excited coil causes the choke plate 33 to be pressed against a seat surrounding an outflow port 34 in the bottom of the cylinder 18.

A spring 35 which presses the piston 18 against a seat at the bottom side of the disc 7 is active between the bottom of the cylinder 18 and the exciting piston 19.

The coil 31 is electrically connected to earth at one side,

and at the other is connected by means of a wire 37 to a connecting element 36 which can be connected to an electronic control system. The exciting of the coil 31 will result in a reduction or enlarge¬ ment of the gap between the choke plate 33 and the duct 34. When the gap increases a release in pressure occurs below the exciting piston 19, with the result that the piston is not pressed as hard against the damper valve seat, so that there is less damping. A reduction in the width of the gap results in greater damping.

The said electronic control system is connected to sensors which pass on information to the system on any change in the steer¬ ing angle per second, the speed of travel, the horizontal and vertical, lateral and transversal speed, the height of travel etc. This system excites the coil 34, so that a lower or higher damping level is selected. The switching speed is extremely high. The damper valve reacts to slight movements in the regulating coil. As in Fig. 1 , the damping can be switched off fully or partially if there are disturbing high frequencies in the movement of the un¬ sprung part of the wheel guide.

A disadvantage of the invention according to Fig. 2 can be that leakage between the exciting piston 19 and the cylinder 18 is not constant. Seals cannot be used, due to the additional friction.

The embodiment according to Figs. 3 and 4 does not have these disadvantages. In this embodiment also, corresponding parts are given the same reference numbers. The exciting element 19 is free from the walls of the cylinder 8, 18 and is suspended in resilient plates 49. The valve body 17 of the damper valve also consists of resilient plates. There is no risk of leaking; oil can pass through the exciting element only via duct 21. The regulating process can be accurate; the sensitivity is great. Provision is made below the throttle plate 23 for another leaf spring 41 which holds the throttle plate in the de-energized position.

In the embodiment according to Figs. 2, 3 and 4 the space below the bottom of the cylinder 18 is connected to the tank 3.

It is of essential importance for the invention that the damping takes place on both the outgoing and the ingoing stroke by means of the same damper valve, whose damping action is regulated mechanically or electromagnetically in such a way that at high

frequency of the movement of the unsprung part of the wheel guide it is reduced to a minimum, while at low frequency the normal damping action is achieved. The embodiments described lead to very rapid adaptation of the damping action to the said frequency. The piston 4 in the working cylinder 1 is simple in design and has only one upstream valve which lies round the central pipe in such a way that it gives a dynamic seal. Concentrating the damping in a single regulating element is a great practical advantage. Of course, various additions and modifications are possible within the scope of the invention. For example, the duct 34 can contain a cylin¬ drical shaft with flat side or a tapering shaft which rests on the choke plate 33 and determines the dimensions of the passage accu¬ rately when the choke plate is not resting on its seat.