WO1993022581A1

WO1993022581A1 - Flow sensitive, acceleration sensitive shock absorber

Info

Publication number: WO1993022581A1
Application number: PCT/US1993/004257
Authority: WO
Inventors: Donald G. Richardson; David A. Shirley
Original assignee: Richardson Technologies Ltd., D.B.A. Ricor
Priority date: 1992-05-05
Filing date: 1993-05-05
Publication date: 1993-11-11
Also published as: AU4235193A

Abstract

An acceleration sensitive shock absorber has a tubular housing (11) and a piston assembly dividing the housing into chambers. The piston (14) is connected to the wheel of a vehicle and the housing is connected to the chassis of the vehicle. Fluid can pass between the chambers during either extension or compression of the shock absorber. A movable inertial mass (46) in the piston assembly opens a port (48) in the piston when acceleration of the wheel of the vehicle is greater than predetermined magnitude for increasing flow between the chambers. There is an orifice downstream from the port for creating hydraulic pressure which biases the inertial mass (46) towards a port-open position in response to fluid flow between the chambers. A similar result is provided in a twin tube shock absorber which has concentric inner and outer tubes connected to the wheel of a vehicle.

Description

!0 FLOW SENSmVE, ACCELERATION SENSITIVE SHOCK ABSORBER

Background

This invention relates to vehicle shock absorbers which are typically mounted between the wheels and chassis of an automobile, truck, motorcycle, etc. The invention relates to a

15 shock absorber with damping characteristics that change depending upon the acceleration of parts of the shock absorber, most importantly, upon downward acceleration of the vehicle wheel. More specifically, it relates to addition of fluid flow or pressure sensitivity to acceleration sensitivity in the shock absorber. 20 Hydraulic shock absorbers are universally employed in automotive vehicles. Each wheel of the vehicle is coupled to the vehicle chassis or frame by a spring so that bumps or dips in the road are not transmitted directly to the passengers or vehicle load. A spring alone, however, would still give a rough ride. Shock absorbers are therefore mounted in parallel with the springs to damp the accelerations applied to the chassis from the wheel.

25 There is a long history of shock absorber development to obtain desired characteristics of passenger ride, comfort, handling for steering, road traction and the like.

Most shock absorbers are designed to have a certain operating characteristic or load- velocity curve which is a compromise of the characteristics desired for a variety of road

-_ conditions. The characteristics suitable for driving on relatively smooth road may, however, be inappropriate where the vehicle wheels may encounter short range bumps or dips. Such conditions are not limited to vehicles like those used on off-road terrain, but also include ordinary passenger and freight vehicles which may unexpectedly encounter chuck holes, speed bumps or foreign objects on the roadway.

35 There are two principal types of shock absorbers employed on automotive vehicles.

One of them comprises a single tube or cylinder with a piston in the cylinder. A variety of 1 valves and orifices are used for controlling flow of hydraulic shock absorber fluid from one end of the cylinder to the other end through the piston.

The other principal type of shock absorber has twin tubes. In this type of shock absorber there are concentric tubes with a piston in the inner tube. The annulus between the

5 inner and outer tubes serves as a reservoir for hydraulic fluid and may serve as a pressure accumulator.

Applicant's acceleration sensitive shock absorbers have demonstrated a remarkable ability to improve the performance of vehicles equipped with such shock absorbers. For _jø example, race cars equipped with acceleration sensitive shock absorbers regularly show increased lap speeds of one or two seconds as compared with identical cars equipped with shock absorbers without acceleration sensitivity.

Even though a couple seconds reduction in lap time is very important in a race car, it is still desirable to provide additional improvement in an acceleration sensitive shock 15 absorber. It is desirable to provide the enhanced performance with no additional parts in the shock absorber.

It is also desirable in some cases to provide for external adjustability of the stiffness of a shock absorber. Since volume is limited it is also desirable to "package" all of the internal parts of the shock absorber as compactly as possible.

Brief Summary of the Invention

There is, therefore, provided in practice of this invention according to a presently preferred embodiment an acceleration sensitive shock absorber having a tubular housing and 25 a piston assembly in the housing, dividing the housing into an upper chamber and a lower chamber. The shock absorber is connected at one end to the chassis of a vehicle and the other end to a wheel of the vehicle. Fluid can pass between the upper and lower chambers with a restricted flow rate during either extension or compression of the shock absorber.

There is a port for providing fluid communication between the upper and lower chambers and

30 a movable inertial mass for opening the port when acceleration of the wheel of the vehicle is greater than a predetermined magnitude for increasing flow between the chambers. In addition, the acceleration sensitive shock absorber has means for biasing the inertial mass towards a port-open position in response to fluid flow between the chambers.

- _ In an embodiment for a single tube shock absorber, the inertial mass is mounted in the piston assembly for normally keeping the port closed and opening the port upon extension of the shock absorber, i.e., acceleration of the vehicle wheel downwardly. The port is kept 1 open by having an orifice downstream from the port which has a slower rate of opening than the port when the port is partly open.

In an embodiment for a twin tube shock absorber, a sleeve-like inertial mass is mounted in a fluid reservoir surrounding the chambers for normally keeping a port between one of the chambers and the reservoir closed and opening the port upon acceleration of the vehicle wheel downwardly. The port is kept open by having an orifice downstream from the port which has a flow area smaller than the flow area of the port during at least a portion of the travel of the inertial mass between a port-open position and a port-closed position.

10 In another embodiment, the means for biasing an inertial valve open comprises a diagonally extending fluid flow passage through an inertial mass. In a twin tube shock absorber, means are provided for stiffening the shock absorber shortly before the end of its stroke.

1 Brief Description of the Drawings

These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates in longitudinal cross-section an acceleration sensitive shock absorber 20 constructed according to principles of this invention when there is no acceleration of the vehicle wheel;

FIG. 2 is a fragmentary longitudinal cross-section of the piston assembly when the vehicle wheel is accelerating downwardly and an acceleration sensitive valve has opened; 25 FIG. 3 is a semi-schematic longitudinal cross-section of another embodiment of flow sensitive shock absorber of the twin tube variety when there is no acceleration of the shock absorber;

FIG. 4 illustrates the twin tube shock absorber in longitudinal cross section as the tubes accelerate longitudinally toward the rod shaft end or away from the ground;

30 FIG. 5 illustrates the shock absorber when the tubes are accelerating away from the shaft end or toward the ground, and

FIG. 6 is a fragmentary semi-schematic longitudinal cross-section of another embodiment of twin tube shock absorber with a flow sensitive inertial mass.

^■

35

Detailed Description 1 The first two drawings illustrate a piston assembly of a single tube shock absorber which is acceleration sensitive and flow sensitive. The piston assembly is on a piston rod 10 connected to the wheel (not shown) of a vehicle. The piston assembly is mounted in the hollow cylindrical body 11 of the shock absorber, which is connected to the frame or chassis

5

(not shown) of the vehicle. The piston assembly divides the interior of the cylinder 11 into a lower chamber 12 below the piston and an upper chamber 13 above the piston. The rest of the shock absorber, including means for connecting to the vehicle is conventional and need be illustrated for an understanding of this invention.

10 It will be understood that references are made to an upper chamber and a lower chamber, since this is the way the shock absorber is normally mounted in a vehicle. It may alternatively be inverted in some arrangements. When mounted, as illustrated, movement of the piston assembly downwardly occurs upon extension of the shock absorber such as, for example, when the wheel moves away from the vehicle as the terrain drops away beneath the

-^** vehicle or the wheel rebounds from compression. Alternatively, upon compression of the shock absorber, the wheel and piston assembly move upwardly within the cylinder.

The piston assembly has a hollow piston 14 threaded onto the upper end of the hollow piston rod 10. A set screw 15 prevents the piston from unscrewing from the piston rod. A hollow inertia valve retainer 16 is threaded into a smaller diameter end of the piston. A set 20 screw (not shown) in a diagonal hole 20 in the inertia valve retainer bites into the end of the piston to prevent the retainer from unscrewing from the piston. The perimeter of the piston is sealed to the inside of the cylinder by a circumferentially extending scarf-cut wear band 17 made of polytetrafluoroethylene or the like. The wear band is backed up an O-ring 18 which 25 acts as a "spring" for biasing the wear band against the inside of the cylinder.

An adjustment rod 19 extends through the hollow piston rod and piston. The upper end of the adjustment rod is hollow and is closed by a threaded plug 21. The exterior of the upper end of the adjustment rod is hexagonal and fits in a hexagonal hole of a rebound adjuster 22 which is held in the inertia valve retainer by a snap ring 23. An annular rebound

30 valve 24 has a larger diameter portion that seats against a shoulder inside the inertia valve retainer and is biased against the shoulder by a rebound spring 26. There are four diagonally extending slots 27 in the outside of a reduced diameter portion of the rebound valve.

During the extension or rebound of the shock absorber the piston moves downwardly

- _ in the cylinder, raising the pressure in the lower chamber and decreasing pressure in the upper chamber. This causes fluid to flow through radial openings 28 in the piston rod and additional radial openings 29 communicating with the hollow interior of the adjustment rod. 1 The increased fluid pressure against the rebound valve 24 moves the valve upwardly against the rebound spring, moving the diagonal slots 27 past the shoulder in the retainer so that fluid can flow past the valve and through holes 31 through the rebound adjuster at the upper end of the piston assembly. It will also be noted that the changing position of the threaded

5 rebound adjuster also changes the total travel of the rebound valve. This affects the maximum opening of the slots adjacent the shoulder and hence the flow rate of fluid through the valve.

As mentioned above, the end of the adjustment rod 19 is hexagonal and fits in a

JO hexagonal hole in the rebound adjuster. The rebound adjuster is threaded into the inertial valve retainer. Thus, rotation of the adjustment rod can move the rebound adjuster longitudinally in the threads. This changes the force on the rebound spring and hence the opening force of the rebound valve. The adjustment rod extends through the lower end of the shock absorber for adjustment of the rebound characteristics.

*^■•- An annular compression valve 32 fits around the adjustment rod and has a shoulder which seats against the end of a smaller diameter portion of the rebound valve 24. A smaller diameter portion of the compression valve fits within a portion of the rebound valve. The smaller diameter portion of the compression valve has diagonally extending slots 33 on the outside surface facing toward the inside of the rebound valve. The compression valve is biased toward the closed position against the rebound valve by a compression spring 34. The other end of the compression spring bears against a compression adjuster 36 which fits onto the adjustment rod and seats against a shoulder 37. The compression adjuster is captive between that shoulder and a snap ring 40.

25 To adjust the opening force for the compression valve one moves the adjustment rod longitudinally. As the compression adjuster moves away from the compression valve, the force on the compression spring 34 is relaxed, reducing the opening force of the valve. Conversely, as the adjustment rod is moved upwardly toward the valve the opening force is increased.

30

Four longitudinal extensions 39 on the compression adjuster are positioned for engagement with the bottom of the compression valve. When the adjustment rod is in its fully up position the extensions actually bear against the end of the compression valve and prevent it from opening. This provides the maximum stiffness of the shock absorber in

,. _ compression. This adjustment also changes the travel of the compression valve. When the adjustment rod is moved downwardly, the extensions are spaced away from the end of the compression valve 32 so that the valve can open. Typically, a longitudinal travel of 2.5 1 millimeters is appropriate for adjustment to the softest desired compression resistance. Thus, the compression adjuster sets both the compression opening force for the compression valve and the travel of the valve. The limitation on travel of the valve regulates the amount of opening of the slots 33 and meters the quantity of fluid that can flow through the compression valve.

In the event of the vehicle hitting a bump, for example, so that the shock absorber is compressed, the fluid pressure in the upper chamber 13 becomes greater than the pressure in the lower chamber. Shock absorber fluid flows through the holes 31 in the rebound

10 adjuster, through the center hole of the rebound valve 24, through the slots 33 in the compression valve, through openings (not shown) between the extensions 39 on the compression adjuster, through the radial holes 29 in the hollow end of the adjustment rod, and through the openings 28 through the piston rod into the lower chamber.

The compression spring 34 biasing the compression valve 32 against the rebound valve

" 24 has a sufficient travel that it keeps the compression valve closed even when the rebound valve moves toward its open position. The pressure from the lower chamber during rebound also helps keep the compression valve closed.

The arrangement of an annular rebound valve with a compression valve coaxial and partly nested within the rebound valve provides a very compact valving arrangement for the tightly confined space in a shock absorber. In this arrangement the compression valve and rebound valve are dueling, with the compression valve biased towards opening the rebound valve. The rebound spring 26 has a higher spring constant than the compression spring 38 so that when there is no pressure differential across the piston assembly the rebound valve

25 remains closed against the shoulder in the inertia valve retainer and the compression valve remains closed against the end of the rebound valve.

The edge of the shoulder in the inertia valve retainer cooperates with the diagonal slots 27 in the rebound valve to meter the flow through the valve during the rebound or extension stroke of the shock absorber. As the rebound valve moves away from the shoulder as

30 pressure in the lower chamber increases, the slots progressively open and more fluid can flow through the valve. It will be apparent that the same function can be achieved with diagonal slots in the inertia valve retainer and a cooperating cylindrical surface on the outside of the rebound valve.

„ _ Similarly, the smaller diameter end of the rebound valve cooperates with the diagonal slots 33 in the compression valve to meter flow during the compression stroke of the shock absorber. When the compression valve is in its maximum nested position inside the rebound 1 valve, the slots 33 are completely closed and no fluid flows through the valve. As the compression valve moves out of its nested position, the area of the slots through which fluid can flow progressively increases. The limitation on travel of the compression valve before encountering the extensions 39 on the compression adjuster determines the maximum flow

5 rate of fluid and the compression stiffness of the shock absorber. If desired the slots can be provided inside the rebound valve.

The force of the compression valve on the rebound valve tending to open the rebound valve varies depending on the opening force adjustment of the compression valve. Thus, 10 when it is desired to adjust the stiffness of the shock absorber, it is best to adjust the compression before the rebound.

It is also desirable to have a "blow off" of pressure in the event of rapid compression of the shock absorber. For this purpose there is a conventional flapper valve 41 held in place against the bottom of the piston by a flapper valve retainer 42. In the event of substantially -^*5 increased pressure in the upper chamber, fluid flows through diagonal passages 43 through the piston and pops the flapper valve open to permit direct fluid flow from the upper chamber to the lower chamber.

An important feature of the shock absorber is acceleration sensitivity. This is ' ; ovided by a somewhat massive inertia valve 46 which fits closely around a smaller diameter 20 longitudinal extension 47 of the piston. There is a close fit between the outside diameter of the piston extension and the inside diameter of the inertia valve member for minimizing fluid leakage when the valve is closed. For example, the diametral clearance is about 60 to 65 micrometers.

25 There are generally radially extending ports 48 through the piston extension adjacent to the inside surface of the inertia valve member 46 when it is closed as illustrated in FIG. 1.

In the illustrated embodiment the ports are machined diagonally through the wall of the extension 47 to avoid interference during machining with a circumferentially extending rib

49 on the upper end of the piston. In an exemplary embodiment there are four such ports,

30 each with an area of 20 mm², or a total flow area through the ports of 80 mm².

In the event of downward acceleration of the wheel to which the piston rod is attached, the piston accelerates downwardly. Because of the inertia of the inertia valve member, it tends to remain at a fixed location in space and the piston moves away from it. Upon

, _ sufficient acceleration the inertia valve member can move upwardly (relative to the piston) until it engages the inertia valve retainer 14. When it moves to this upward or open position as illustrated in FIG. 2, the lower portion of the inertia valve member no longer obstructs the 1 ports through the piston. Fluid from the lower chamber can therefore flow through the radial ports 28 in the hollow piston rod, through a check valve 51, and through the ports 48 into the upper chamber.

Thus, when the downward acceleration of the wheel exceeds some selected magnitude, the inertia valve is completely opened to permit relatively rapid fluid flow from the lower chamber to the upper chamber. This, of course, reduces the resistance to extension of the vehicle spring and wheel, permitting the wheel to travel downwardly rapidly and maintain engagement with the road surface.

1 An optional feature is a light weight spring 52 between the upper face of the piston and the inertia valve member. The spring is selected so that when the inertia valve is completely closed as illustrated in FIG. 1, the spring supports only 80 to 90% of the weight of the inertia valve member. This means that gravity closes the inertia valve against the spring force, bringing the lower end of the inertia valve member against the upper face of the piston as

15 illustrated in FIG. 1. Also, when the spring is fully extended as illustrated in FIG. 2, with the inertia valve member against the inertia valve retainer, the spring supports from 10 to

20% of the weight of the inertia valve member. The addition of such a spring assists in promoting lift-off of the inertia valve member and promotes rapid opening of the inertia valve. 0

The check valve 51 is biased closed by a relatively light spring 53. The check valve permits flow from the lower chamber to the upper chamber when the ports 48 through the piston are opened by upward displacement of the inertia valve member. The check valve, however, closes quickly and prevents reverse flow in the event of compression before the 5 inertia valve member is completely closed. This check valve is optional, and there are embodiments where it appears better not to have a check valve in this flow stream.

It has been found desirable to maintain the inertia valve in an open position (as illustrated in FIG. 2) even after acceleration has diminished. An orifice is therefore provided downstream from the ports 48 controlled by the inertia valve member for hydraulically

30 biasing the inertia valve member toward its open position as fluid flows from the lower chamber to the upper chamber.

This orifice is provided by a small annular clearance between the inside diameter of the rib 49 and the circumferential outside surface 54 on the inertia valve member. When the

_ _ inertia valve is completely closed as illustrated in FIG. 1, an exemplary clearance between the inside of the rib and the outside of the inertia valve member is as low as 0.6 millimeter.

The relative areas and spacings of the ports 48 and the orifice between the rib and inertia 1 valve member are such that the orifice has a smaller area than the ports when the ports are open, except for a short distance when the ports are almost closed.

Thus, when the inertia valve is partly or fully open, the cross-sectional area for fluid flow through the orifice is less than the cross-sectional area for fluid flow through the ports.

5

Because of the restricted flow path downstream from the ports there is a higher pressure in the space between the piston and the inertia valve member than there is in the upper chamber

13. This hydraulic pressure differential between the lower end of the inertia valve member and its upper end biases the acceleration sensitive valve toward its open position.

10 The outside edge of the lower end of the inertia valve member has a radius 56, and there is a radius 57 on the inside of the top of the rib on the piston. The orifice for flow control downstream from the ports has an area controlled by the clearance between the rib and inertia valve member until near the upper end of the travel of the inertia valve member when the two radii begin to enlarge the distance between these members, and the flow area

15 increases. Even when fully open as illustrated in FIG. 2, the flow area through the orifice between the radii 56 and 57 is less than the flow area through the ports. Conversely, when the inertia valve starts to close, the area of the orifice decreases for part of the stroke and then remains essentially constant for the rest of the stroke.

As the inertia valve member moves from its open position toward its closed position die pressure in the space between the end of the inertia valve member and the piston face increases while fluid is flowing through the ports and orifice from the lower chamber to the upper chamber. The increased pressure retards closing of the valve, thereby permitting rapid flow of fluid for a longer period.

25 As suggested above, the check valve 51 inhibits reverse flow in the event of compression before the inertia valve closes.

The radial clearance and the radii help determine the pressure in the space under the inertia valve member, and hence, the tendency of the valve to remain open. It has been found that making the radial clearance rather tight can make the inertia valve stay open too

30 long. Increasing the clearance makes the inertia valve close sooner. The exemplary clearance mentioned above is suitable for an off-road race car which encounters rough terrain at high speed where rapid shock absorber performance is required. For an automobile for more customary street usage where bumps and dips are encountered at a slower pace, a

„ _ smaller clearance is preferable for a slower closing inertia valve.

Adjustment features as described above are suitable for costly race cars, for example, but are probably too expensive for most production line cars. The adjustment features can 1 be used in development work, however, to determine the appropriate parameters best suited to a selected vehicle. Those parameters can then be duplicated in fixed parameter shock absorbers for production vehicles.

A representative flow sensitive, acceleration sensitive twin tube shock absorber is

5 illustrated semi-schematically in FIG. 3. The shock absorber has an outer tube 110 sealed at its lower end by a lower end cap 111 having a conventional fitting 112 for bolting the end cap to the mounts for a vehicle wheel 115. The upper end of the outer tube is sealed by an upper end cap 113. An inner tube 114 is also sealed to both the upper and lower end caps.

10 This defines an annular fluid reservoir 116 between the inner and outer tubes.

A movable piston 117 is sealed in the inner tube, dividing its interior into an upper chamber 118 and lower chamber 119. The piston is connected to a shaft 121 which extends through the upper end cap and terminates in a fitting 122 which is used for bolting the shaft to a vehicle chassis 125, for example. In the drawings, various seals (such as a seal around

15 the shaft), threaded connections and the like are not illustrated since they are not required for an understanding of this invention and are conventional features well known to those skilled in the art.

The piston has an orifice 123 for metered fluid flow between the upper and lower chambers. The piston also includes a disk-type check valve 124 for permitting fluid to flow from the lower chamber to the upper chamber, and preventing fluid flow from the upper chamber to the lower chamber. The piston and check valve are conventional in twin tube shock absorbers, although not all shock absorbers have such a check valve or they may use other types of check valves.

25 The shock absorber includes two additional check valves to provide fluid flow between the annular reservoir between the tubes and the chambers in the inner tube. What is commonly known as a foot valve 127 is provided in the lower end cap. The foot valve is a large-area check valve which permits fluid to flow from the reservoir into the lower chamber, and prevents flow from the lower chamber into the reservoir. In some embodiments of shock

30 absorber, the foot valve is replaced by a fluid passage which meters flow in either direction between the reservoir and lower chamber.

An adjustable pressure relief valve 128 is arranged to permit fluid flow from the lower chamber into the annular reservoir and prevent reverse flow. The pressure relief valve has

- _ a rather high opening force, and is provided to release fluid displaced as the piston shaft moves downwardly. In principle, the shock absorber described to this point is conventional. It is merely exemplary of a twin tube shock absorber. Some of these features may be deleted in specific embodiments, and other features may be present which are commonly found in shock absorbers. For example, an external pressure accumulator may be connected to the annular reservoir for maintaining pressure in the shock absorber and accommodating fluid displaced by the piston. Likewise, a closed-cell foam (not Ulustrated) may be included in the reservoir to act as an internal pressure accumulator to accommodate pressure changes as the piston shaft displaces fluid. The shock absorber responds at zero or low acceleration as a conventional shock absorber. Thus, if the vehicle is forced downwardly by cornering or braking, for example, causing the piston to move downwardly relative to the tubes, fluid flows through the orifice 123 and check valve 124 in the piston from the lower chamber to the upper chamber. The pressure relief valve 128 opens to release fluid displaced by the shaft 121 entering the upper chamber. Restricted flow through the orifice and valves limits the rate of compression of the shock absorber. During compression fluid flows rather readily through the piston to avoid cavitation in the upper chamber. Metering of fluid flow through the pressure relief valve to limit movement of fluid displaced by the piston shaft provides the principal resistance to compression.

When the compression is relieved and the vehicle rebounds, fluid flows from the reservoir through the foot valve into the lower chamber, and from the upper chamber into the lower chamber through the piston orifice. The rebound force on the piston comes from the vehicle spring (not shown), and in some cases from gas pressure, in a conventional manner. In the event the wheel extends from a mid-range position at a very low acceleration, such as when the road surface gradually drops away, the shock absorber expands in essentially the same way as when the vehicle is rebounding.

Assume, however, that the wheel hits a bump or foreign object in the road. This drives the tubes upwardly with rapid acceleration. It is desirable under those circumstances to have the shock absorber suddenly become "softer" so that the impact of the bump is alleviated and less shock is transmitted to the body of the vehicle. For this purpose, a normally closed valve 131 opens for increasing the flow of fluid from the lower chamber 119 into the annular reservoir.

A lower movable sleeve 131 in the reservoir surrounds the inner tube. The lower sleeve is supported by a spring 132 which at least supports the weight of the sleeve and holds it against a stop ring 133 at the top of the sleeve. When the tubes accelerate upwardly, the inertia of the lower sleeve leaves it essentially immovable in space, and the tubes move upwardly relative to the sleeve until the sleeve reaches the lower end cap.

When the sleeve reaches this position, a row of passages 134 through the sleeve align with a row of lower ports 136 through the side wall of the inner tube. This alignment permits fluid to flow from the lower chamber into the annular reservoir, thereby permitting the tubes to travel faster relative to the piston. This permits the wheel to move quickly relative to the vehicle for alleviating the shock transmitted to the vehicle.

When the wheel acceleration diminishes below a predetermined value, the spring causes the lower sleeve to return to its normally closed position against the stop, thereby closing the port and returning the shock absorber to its normally stiffer characteristic. Thus, in the event the wheel hits a raised bump or object in the roadway, the rapid acceleration causes the shock absorber to temporarily become soft for minimizing the shock transmitted to the vehicle body, while under normal road conditions the shock is relatively stiff for good handling. It may also be noted that the lower orifices 136 through the side wall of the inner tube are spaced above the bottom of the tube. This assures that in the event of an extreme compression of the shock absorber, its characteristic of being soft does not continue for the full stroke; but instead, when the piston passes the orifices, the shock absorber again becomes stiff before the end of the stroke is reached. In this embodiment, three orifices at different distances from the end of the stroke are used (with suitable passages through the lower sleeve) so that the shock absorber becomes progressively stiffer near the end of the stroke as less and less fluid can flow through the orifices.

It is also desirable that the shock absorber respond with different characteristics if a wheel drops away suddenly from the vehicle body. This may occur for example, if the wheel encounters an unexpected hole, or it may occur immediately following shock absorber compression as the wheel passes over a bump or object in the road. It can be quite desirable under those conditions for the wheel to rapidly move toward the road service with minimum inhibition by a stiff shock: absorber since this enhances road traction. It is undesirable that the shock of such rapid wheel movement be transmitted to the vehicle, passengers or load.

Normally closed valve means are also provided for changing the stiffness of the shock absorber upon rapid downward acceleration of the tubes. For this purpose, a movable upper sleeve 141 surrounds the inner tube near its upper end. A significant portion of the weight of the upper sleeve is supported by a low spring rate coil spring 142. During normal use of the shock absorber on a reasonably smooth road, the upper sleeve rests on a stop sleeve 143 inside the coil spring. The upper sleeve includes a generally radial fluid passage 144. 1 In the event of rapid downward acceleration of the shock absorber tubes at more than a predetermined magnitude, the upper sleeve essentially remains fixed in space due to its inertia, and the tubes move away from the sleeve until the sleeve reaches the upper end cap. In this position, the passage through the upper sleeve aligns with an upper port 146 through the side wall of the inner tube near its upper end. This permits fluid to flow rapidly from the upper chamber 118 into the annular reservoir between the tubes. This relatively large port permits rapid extension of the shock absorber for outstanding rebound characteristics and accommodation of chuckholes, or the like, without transmitting significant shock loading to

10 the vehicle.

The spring 142 for the upper sleeve is optional. By supporting a significant portion of the weight of the sleeve, the sensitivity of the shock absorber to low magnitudes of acceleration is improved. It is found, however, that the upper sleeve valving for the extension stroke of the shock absorber works without a spring. The spring 132 supporting 5 the lower sleeve for compression valving is important for maintaining the sleeve against the stop.

The upper orifice is also spaced apart from the upper end cap so that the piston passes the upper orifice before the piston reaches the end of its stroke, thereby, increasing the stiffness near the end of the stroke. The distance of the orifice from the end of the stroke for 20 extension can be appreciably less than for compression, since the violence of extension tends to be less than that of compression.

It is often desirable during rebound or extension for the shock absorber to remain soft after acceleration has decreased below the magnitude that caused the inertia valve to open.

25 The passages 144 through the upper sleeve are therefore angled downwardly and outwardly so that fluid flowing from the upper chamber into the reservoir is deflected downwardly and the sleeve is, therefore, biased upwardly. These diagonal passages tend to hold the inertia valve open after the acceleration has decreased beyond the magnitude that originally opens the valve and the valve remains open while fluid flow continues during shock absorber extension.

Providing orifices through the side wall of the inner tube in positions where they may be closed by passage of the piston provides the shock absorber with the capability of being soft (under appropriate accelerations), in a mid-portion of the stroke and stiff near each end .m of the stroke, this capability minimizes the shock that may be transmitted to the vehicle upon

"bottoming" of a shock absorber. This capability is not readily available in shock absorbers which have variable characteristics due to valving mounted in the piston. 1 It might be noted that the mass of the acceleration sensitive valve in a shock absorber as described herein can be appreciably greater than where the acceleration sensitive valve is located in the piston. In this arrangement, the sleeve can be relatively thick and long to obtain a desired mass, since there is ample space in the annular reservoir for accommodating

5 the sleeve. Space is much more limited in the piston. Thus, any desired sensitivity may be obtained.

The sensitivity may be affected by fluid drag on the movable sleeves, thus streamlining in the form of rounded ends 147 may be provided on one or more of the sleeves for 0 minimizing this effect.

An exemplary automotive shock absorber has a tube length in the order of 25 cm and a diameter of about 5 cm. The diameter of the inner tube is about 3 cm. In such an embodiment, a typical lower orifice comprises two rows of three drilled holes about 1.5 millimeters in diameter. The upper orifice is appreciably larger, and the collective area of

15 several holes drilled through the side wall of the inner tube is equivalent to the cross section of a round hole about 12 millimeters in diameter. Several holes about 2.5 millimeter diameter are typically used for virtually unrestricted fluid flow upon rapid acceleration.

Larger orifices give a softer shock absorber and smaller holes give a stiffer shock absorber.

The clearance between the sleeves and the inner tube should be sufficient that fluid 20 shear in the clearance does not substantially retard the relative displacement between the tube and sleeve, yet the clearance must be small enough that the orifices are closed when the sleeve is over them. A clearance of about 130 microns appears to be sufficient. The sleeve should also be sufficiently long relative to the diameter of the tube that it will not cock and

25 can move freely.

It will be recognized that the shock absorber, illustrated herein, is semi-schematic and that some features are omitted for clarity. For example, the piston seal is omitted. A circumferential recess in the sleeve adjacent to the passages is also not shown. This is used as a fluid passage so that it is not necessary to keep the sleeve passages aligned with the

30 orifices through the tube wall when the respective valves are open.

It will also be apparent that the actual construction may differ from the illustration.

For example, stop rings for the sleeves may not be practical in small size shock absorbers, since the wall thickness of the inner tube may be too thin for a groove to receive the stop

,, _ ring. Thus, the stop ring 133 and sleeve 143 may be in the form of shoulders on the outside of a machined tube. The interior of each sleeve may be provided with a circumferential groove so that the passages need not be aligned with the orifices through the wall of the tube. Instead of passages through the sleeves, the inside of the sleeve may be enlarged or counterbored nearer one end so that fluid can flow longitudinally between the sleeve and inner tube wall when the orifices are not covered by the closer fitting portion of the sleeve. In effect, this is a passage through the sleeve, but discharging at an end of the sleeve. Such a counterbore can also serve to direct fluid flow in a direction that keeps the normally closed valve in its open position. If desired, different degrees of stiffness may be provided for different acceleration rates. It may be desirable to have a somewhat soft shock absorber for mild accelerations and a considerably softer shock absorber for severe accelerations. Such performance may be achieved by employing a pair of concentric sleeves around the inner tube. One of the sleeves, has a suitable mass and spring so that it displaces a sufficient distance to open one set of orifices at a relatively lower magnitude of acceleration. The other sleeve has a sufficient mass and spring constant for opening another orifice at a higher acceleration. Thus, at low acceleration, fluid may flow through one orifice and at higher accelerations, may flow through both orifices for a softer effect.

It is considered that the pressure differential technique for biasing an inertia valve toward its open position is more readily controlled than a diagonal passage. Such an arrangement is illustrated in FIG. 6.

FIG. 6 illustrates the upper end of a twin tube shock absorber of the same general type as illustrated in FIGs. 3 to 5. This embodiment illustrates fluid flow sensitivity employing the principle of a downstream orifice smaller than a flow port for keeping an inertia valve open for a longer period. The shock absorber has an outer tube 210 sealed at its upper end by an upper end cap 213. An inner tube 214 is also sealed to the upper end cap. This defines an annular fluid reservoir 216 between the inner and outer tubes. A movable piston

217 is sealed in the inner tube, dividing its interior into an upper chamber 218 and a lower chamber 219. The piston is connected to a shaft 221 which extends through the upper end cap and terminates in a fitting 222 which is used for bolting the shaft to a vehicle chassis 225.

The piston assembly is the same as illustrated in FIG. 3.

A rebound or extension acceleration sensitive valve is provided at the upper end of the inner tube for permitting fluid flow from the upper chamber 218 into the annular reservoir

216 in the event of rapid acceleration of the wheel downwardly. An axially movable upper sleeve 241 surrounds the inner tube near its upper end. A significant portion of the weight of the upper sleeve is supported by a low spring rate coil spring 242. The sleeve serves as an inertia mass for controlling the rebound valve. The spring is sufficiently light that it will not support the entire weight of the inertia mass, but simply offsets a portion of that weight so that the inertia mass can displace more quickly.

When the sleeve is in its lower position, i.e., when there is no downward acceleration of the wheel, the bottom of the sleeve rests on a stop shoulder 243 on an inner sleeve 244. As described above, when the wheel of the vehicle encounters a dip in the terrain or passes over the top of a bump, the wheel rebounds or accelerates downwardly. Sufficiently rapid acceleration leaves the inertial mass 241 in place as the inner tube of the shock absorber accelerates downwardly. This opens the acceleration sensitive valve. When the outer sleeve 241 moves toward its upper or open position as illustrated in FIG. 6, the upper end of the sleeve clears radial ports 246 through the wall of the inner tube. When the inertial mass is in its lower position against the stop 243, the end of the sleeve covers the ports and prevents fluid flow from the upper chamber into the annular reservoir.

The inner sleeve 244 has a conical external surface which tapers from the relatively smaller diameter at the upper end toward a relatively larger diameter near the lower stop shoulder 243. The inside surface of the outer inertia mass 241 is essentially cylindrical. The relative dimensions of these parts and the angle of the taper provide an annular orifice 247 between the inner sleeve and the outer sleeve so that throughout most of the travel of the outer inertial mass the flow area through the orifice is less than the flow area through the radial ports in the inner tube.

Thus, when the valve is substantially completely open with the outer inertial mass in its uppermost position as illustrated in FIG. 6, there is a maximum flow area through both the ports and downstream orifice. Since the flow area through the orifice is smaller than the ports, the pressure in the space between the movable outer sleeve and the fixed inner sleeve is greater than the pressure in the annular reservoir. This tends to bias the inertial mass toward the open position. Furthermore, in this arrangement when the valve is, for example, one-third open and two-thirds closed, the remaining flow area through the radial ports is still larger than the annular orifice between the sleeves because of the taper.

In the illustrated embodiment the taper extends the full length of the inner sleeve so that at substantially all positions of the outer sleeve the downstream orifice has a smaller flow area than the ports. If desired, the taper may extend only part of the way along the inner sleeve and nearer the larger diameter lower end, the sleeve may become cylindrical. In such an embodiment, as valve approaches its closed position, the orifice area stops getting smaller, thereby minimizing or eliminating the pressure differential between space between the sleeves and the surrounding annular reservoir. In such an embodiment the increased pressure tends to hold the valve open when it is most of the way open and permits it to close more readily when the outer sleeve has moved most of the way towards the closed position. Small radial slots (not shown) may be provided at the stop 243 so that there is a small opening adjacent to the orifice when the valve is completely closed and the inertial mass is against the stop.

Although the inner taper is illustrated in this embodiment on a separate sleeve it will be apparent that part of the structure providing the annular orifice may be integral with the inner tube. It will also be apparent that the variable area orifice may be provided by an internal taper inside the inertial mass which moves adjacent to a shoulder on the outside of the inner tube. Other arrangements for providing a downstream orifice having an area smaller than the upstream ports will be apparent to those skilled in the art.

A flow sensitive arrangement for biasing the inertia valve open also assists in preventing "chatter" when the valve is only partly open.

Although just three embodiments of flow sensitive, inertia sensitive shock absorber have been described and illustrated herein, it will be apparent that there may be many modifications, variations and embellishments of flow sensitive, acceleration sensitive shock absorbers. Some of the check valves may be omitted or replaced by flow restricting passages in specific embodiments. The shape of the orifices and passages may be varied or chamfers provided so that the change between stiff and soft characteristics of the shock absorber change at a controlled rate. Since there are many such modifications and variations, which will be apparent to those skilled in the art, it is to be understood that the invention may be practiced, otherwise than as specifically described.

Claims

What Is Claimed Is;

1. An acceleration sensitive shock absorber comprising: a tube containing shock absorber fluid for connection to one portion of a vehicle; a piston in the tube dividing the interior of the tube into an upper chamber and a lower chamber, for connection to another portion of a vehicle; inertia valve means for changing the stiffness of the shock absorber when the shock absorber is subjected to acceleration, the stiffness being greater upon lower acceleration and smaller upon higher acceleration; and means for maintaining the inertia valve means open after acceleration has decreased in response to fluid flow through the valve means.

2. An acceleration sensitive shock absorber as recited in claim 1 wherein the means for maintaining the inertia valve means open comprises a fluid flow passage extending at least partly axially for deflecting fluid flow through the valve means at least partly axially at least partly in an axial direction opposite to the direction of opening of the valve means.

3. An acceleration sensitive shock absorber as recited in claim 1 wherein the means for maintaining the inertia valve means open comprises a fluid flow port adjacent one chamber and a fluid flow orifice downstream from the port, the orifice having a smaller flow area than the port when the port is open.

4. An acceleration sensitive shock absorber as recited in claim 3 wherein the port is through the piston assembly and the inertial valve means comprises an inertial mass mounted in the piston assembly for opening or closing the port.

5. An acceleration sensitive shock absorber as recited in claim 4 wherein the orifice 0 comprises an annular clearance between the inertial mass and a portion of the piston assembly.

6. An acceleration sensitive shock absorber as recited in claim 3 wherein the shock - absorber comprises an inner tube and an outer tube with an annular fluid reservoir therebetween, the port is through the wall of the inner tube and the inertial valve means comprises an inertial mass mounted in the annular reservoir for opening or closing the port.

1 7. An acceleration sensitive shock absorber as recited in claim 6 wherein the orifice comprises an annular clearance between the inertial mass and a portion of the inner tube.

8. An acceleration sensitive shock absorber as recited in claim 7 wherein the annular

5 clearance comprises a tapered surface for changing the orifice area as a function of inertia mass position.

9. An acceleration sensitive shock absorber connected at one end to the chassis of a 0 vehicle and at the other end to a wheel of the vehicle comprising: a tubular housing for connection to one portion of a vehicle; a piston assembly in the housing comprising a piston dividing the housing into an upper chamber and a lower chamber, and a piston rod for connection to another portion of the vehicle, one of said portions being the chassis of the vehicle and the other portion being a 15 wheel of the vehicle; means for passing shock absorber fluid between the upper chamber and the lower chamber with a restricted flow rate during compression of the shock absorber; means for passing shock absorber fluid between the upper chamber and the lower chamber with a restricted flow rate during extension of the shock absorber; 20 a port for providing fluid communication between the upper and lower chambers; a movable inertial mass in the shock absorber for opening the port when acceleration of the wheel of the vehicle is greater than a predetermined magnitude for increasing flow of fluid between the upper chamber and the lower chamber; and 25 means for biasing the inertial mass toward a port-open position in response to fluid flow between the chambers.

10. An acceleration sensitive shock absorber as recited in claim 9 wherein the port is through the piston assembly and the inertial mass is mounted in the piston assembly. 30

11. An acceleration sensitive shock absorber as recited in claim 9 wherein the means for biasing comprises an orifice downstream from the port, the orifice having a smaller area for fluid flow than the port during at least a portion of the stroke of the inertial mass from

, _ a port-closed position to a port-open position.

12. An acceleration sensitive shock absorber as recited in claim 11 wherein the orifice comprises an annular clearance between the inertial mass and a portion of the piston assembly.

13. An acceleration sensitive shock absorber as recited in claim 9 wherein the inertial mass is responsive to downward acceleration of the wheel of the vehicle.

14. An acceleration sensitive shock absorber as recited in claim 9 wherein the tubular housing comprises an outer tube and an inner tube fixed within the outer tube defining an annular reservoir between the inner and outer tubes for shock absorber fluid; the piston is sealed within the inner tube; and the inertia valve means comprises a port through the sidewall of the inner tube adjacent one of the chambers, and a movable sleeve in the reservoir surrounding the portion of the inner tube containing the port for opening the port upon acceleration of the tubes in one longitudinal direction and increasing flow of fluid from one of the chambers into the reservoir in the event of longitudinal acceleration of the shock absorber tubes at more than a preselected magnitude.

15. An acceleration sensitive shock absorber as recited in claim 9 wherein the orifice comprises an annular clearance between the sleeve and a portion of the inner tube.

16. An acceleration sensitive shock absorber as recited in claim 15 wherein the annular clearance comprises a tapered surface for changing the orifice area as a function of sleeve position.

17. An acceleration sensitive shock absorber as recited in claim 9 wherein the port comprises: an upper port through the sidewall of the inner tube near the top of the inner tube; and wherein the movable sleeve surrounds an upper portion of the inner tube for opening the upper port through the sidewall of the inner tube upon downward acceleration of the shock absorber at more than a preselected magnitude.

18. An acceleration sensitive shock absorber as recited in claim 9 further comprising: means for passing shock absorber fluid between the upper chamber and the lower chamber with a restricted flow rate during compression of the shock absorber; means for passing shock absorber fluid between the upper chamber and the lower chamber with a restricted flow rate during extension of the shock absorber; and wherein 5 the fluid flow port is in the piston for flow between the upper and lower chambers; the fluid flow orifice is for flow between the upper and lower chambers; and the inertial mass being arranged so that the orifice has a smaller fluid flow area than the port during most of the travel of the inertial mass from the port-closed position toward

10 the port-open position.

19. An acceleration sensitive shock absorber as recited in claim 9 wherein the port comprises a generally radially extending passage, the inertial mass moves between a position obstructing the passage in the port-closed position and a position exposing the passage in the port-open position, the orifice comprising an annular clearance around the inertial mass through which fluid from the ports must flow.

20. An acceleration sensitive shock absorber comprising: an outer tube;

20 means for connecting the outer tube to a portion of a vehicle; an inner tube fixed within the outer tube defining an annular reservoir between the inner and outer tubes for shock absorber fluid; a piston sealed within the inner tube and connected to a shaft extending out of the 25 shock absorber, the shaft including means for connecting the shaft to another portion of the vehicle, the piston dividing the inside of the inner tube into an upper chamber and a lower chamber; means for passing fluid between the upper and lower chambers and the reservoir upon compression or extension of the shock absorber;

30 an orifice through the sidewall of the inner tube adjacent one of the chambers; and a movable sleeve in the reservoir surrounding the portion of the inner tube containing the orifice for opening the orifice upon acceleration of the tubes in one longitudinal direction and increasing flow of fluid from one of the chambers into the reservoir in the event of

_ _ longitudinal acceleration of the shock absorber tubes at more than a preselected magnitude.

1 21. An acceleration sensitive shock absorber as recited in claim 20 wherein the orifice is spaced apart from the end of the stroke of the piston for closing the orifice before the end of the stroke.

22. An acceleration sensitive shock absorber as recited in claim 20 comprising: a plurality of orifices through the sidewall of the inner tube at differing distances from the end of the tube.

10 23. An acceleration sensitive shock absorber as recited in claim 20 wherein the orifice comprises: a lower orifice through the sidewall of the inner tube near the bottom of the inner tube; and the movable sleeve surrounds a lower portion of the inner tube for opening the lower 5 orifice through the sidewall of the inner tube upon upward acceleration of the shock absorber at more than a preselected magnitude.

24. An acceleration sensitive shock absorber as recited in claim 20 wherein orifice comprises:

20 an upper orifice through the sidewall of the inner tube near the top of the inner tube; and the movable sleeve surrounds an upper portion of the inner tube for opening the upper orifice through the sidewall of the inner tube upon downward acceleration of the shock 25 absorber at more than a preselected magnitude.

25. An acceleration sensitive shock absorber as recited in claim 24 an end of the upper sleeve is streamlined for minimizing fluid drag on the sleeve.

30 26. An acceleration sensitive shock absorber comprising: a tube containing shock absorber fluid for connection to one portion of a vehicle; a piston in the tube dividing the interior of the tube into an upper chamber and a lower chamber, for connection to another portion of a vehicle; „ valve means for changing the stiffness of the shock absorber when the shock absorber is subjected to acceleration, the stiffness being greater upon lower acceleration and smaller upon higher acceleration; and means for closing the valve means before an end of the stroke of the piston in the tube for permitting smaller stiffness in at least a mid-portion of the stroke and providing greater stiffness in an end portion of the stroke.

27. An acceleration sensitive shock absorber as recited in claim 26 wherein the valve means comprises an orifice through a sidewall of the tube spaced apart from an end of the tube a sufficient distance that the piston can pass the orifice before reaching the end of the stroke of the piston.

28. An acceleration sensitive shock absorber as recited in claim 27 wherein the orifice comprises a plurality of orifices through a sidewall of the inner tube at differing distances from the end of the tube.