WO1999062740A1

WO1999062740A1 - Suspension system for a vehicle seat having improved vibration reduction

Info

Publication number: WO1999062740A1
Application number: PCT/US1999/012188
Authority: WO
Inventors: Thomas A. Mckenzie
Original assignee: Holland Neway International, Inc.
Priority date: 1998-06-02
Filing date: 1999-06-01
Publication date: 1999-12-09
Also published as: AU4952799A; WO1999062740B1

Abstract

A seat suspension system (10) for a vehicle seat includes a lower support (14) securable in a stationary relationship with a vehicle body, an upper support (12) arranged generally above the lower support for supporting a vehicle seat, and a cooperating pair of elongated arm assemblies (16, 18) which each have opposite ends pivotally connected at opposite ends thereof to a corresponding one of the upper and lower supports so that the upper support is vertically movable with respect to the lower support. Various solutions are provided for reduction of vibration transmitted from the lower support to the upper support so that the vibration is not noticeably encountered by an occupant of a seat mounted to the seat suspension. In one embodiment, the upper and lower arm assemblies (16, 18) are braced by diagonal bracing (28) or with overlying plates (64) or with tubular side arms (70). In another embodiment, low friction bearings (32) are provided in the pivotal joints between the elongated arm assemblies and the upper and lower supports (14). In another embodiment, a shock absorber (44) is mounted at one end to the lower support transversely with respect to the movement between the upper and lower supports and is connected at another end to one of the upper support and one of the arm assemblies through a link (46) which translates vertical motion o the upper support or rotational movement of the arm assembly into transverse movement of the other end of the shock absorber. In yet another embodiment, an air spring (20) is mounted between the upper support (12) and one of the arm assemblies (16) to cushion the movement of the upper support (12) with respect to the lower support (14) and has a body of liquid which is connected to a pump for selectively changing the volume of liquid in the air spring to selectively adjust the height of the upper support with respect to the lower support, and control the natural frequency of vertical vibration of the assembly.

Description

SUSPENSION SYSTEM FOR A VEHICLE SEAT HAVING IMPROVED VIBRATION REDUCTION

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application

No. 60/087,668, filed on June 2, 1998 and of U.S. Provisional Patent Application No. 60/119,052, filed on February 8, 1999.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a pneumatic suspension system for a vehicle seat. In one of its aspects, the invention relates to a pneumatic suspension incorporating a base assembly. In another of its aspects, the invention relates to a pneumatic suspension system for a vehicle seat wherein road vibrations are minimized.

State of the Prior Art

Air or pneumatically suspended seat assemblies have been in use for many years in the trucking and construction equipment industry. The primary function of the seat is to provide the occupant with some level of physiological protection from the adverse effects of prolonged vehicle vibration.

The vast majority of all air seats utilize either a parallelogram or scissors mechanism configuration which constrains the seat cushion vertical movement to a linear direction without rotation (seat cushion translates). Pneumatic sleeve-type springs are utilized in the mechanism as the spring or suspension element to provide the required seat elevation or height and a low natural frequency of vibration to isolate the occupant from the harmful effects of vehicle vibration.

Examples of prior art pneumatic seat suspension systems are shown in the following U.S. Patents: 5,234,203 to Smith issued August 10, 1993 titled "Pneumatic Spring for a Vehicle Seat", 4,809,944 to Smith issued March 7, 1989 titled "Air

Spring Seat and Air Pump", 5,388,801 to Edrich et al. issued February 14, 1995 titled "Vehicle Seat, Especially for Commercial Vehicles", 5,651,585 to Van Duser titled "Knee Action Suspension Seat", 4,954,051 to Smith issued September 4, 1990 titled "Air Spring Seat and Air Pump" and 5,542,638 to Smith issued August 6, 1996 titled "Base Assembly for Use With a Suspension System of a Vehicle Seat."

Although pneumatic seats are a rather mature industry as represented by the above prior art patents, a number of complaints continue to exist relative to their performance, durability and longevity. First, the seat occupant is typically exposed to a side-to-side roll motion due to the lack of adequate stability in the basic parallelogram or scissors mechanism. Second, the pivot connections in the basic mechanism exhibit poor durability or develop unacceptable looseness due to wear. Third, when the seat experiences resonant input, the lack of adequate hydraulic damping control results in unacceptable vertical vibratory motion to the operator. These problems in prior art seat suspensions have given rise to additional problems. It becomes difficult to properly control the vehicle operation when the driver is utilizing significant effort to stop or attenuate the vibratory motion of the seat. Seat belts/restraints are not used or left extremely loose to prevent excessive rub/chaffing of straps on shoulders. Consequently, the safety provided by a loose or disconnected seat belt in the event of an accident is seriously degraded.

The lack of adequate damping in prior art seat suspensions is primarily due to a lack of adequate mechanical multiplication of the available damping forces. The dampers are developing the maximum amount of resistance possible and typically operate at design limits of mechanical hardware.

Prior art seat suspensions have been unsuccessful in overcoming these problems. Further, there are no available means of altering the undamped natural frequency of the suspension system in the seat, to provide adequate separation between the cab suspension resonance and the seat suspension resonant behavior.

SUMMARY OF THE INVENTION

The invention relates to a suspension system for a vehicle seat and includes a lower support securable in a stationary relationship with a vehicle body; an upper support arranged generally above the lower support for supporting a vehicle seat; and a cooperating pair of elongated arm assemblies which each have opposite ends pivotally connected at opposite ends thereof to a corresponding one of the upper and lower supports so that the upper support is vertically movable with respect to the lower support.

In one embodiment, at least one of the elongated arm assemblies comprises a pair of spaced side rails and a bracing configuration for the side rails, wherein the bracing configuration is constructed to achieve torsional stiffness of the at least one arm assembly sufficient to minimize roll deflection due to lateral acceleration or offset vertical loading of the suspension system.

The bracing configuration can comprise at least one diagonal brace rigidly mounted to each side rail which can intersect the side rails at a generally 45-degree angle. The at least one diagonal brace can comprise a plurality of braces forming an x-shaped configuration between the side rails.

Alternatively, the bracing configuration can comprise at least one cover plate mounted to one of upper and lower edges of the spaced side rails. Preferably, first and second cover plates are mounted to respective upper and lower edges of the spaced side rails.

In another embodiment, the bracing configuration comprises elongated tubular members forming the spaced side rails. At least one of the elongated tubular members can have a rectangular or circular cross section. In another embodiment, the pivotal connections between each of the arm assemblies and the respective upper and lower supports comprising a shaft mounted to one of the corresponding upper and lower supports and one of the arm assemblies, and a tube mounted to the other of the corresponding upper and lower supports and one of the arm assemblies, and a bearing located between the shaft and the tube. The bearings thereby minimize the effects of frictional resistance between the arm assemblies and the corresponding upper and lower supports.

The bearing can any type of suitable bearing such as a lubricated rolling element bearing or a cup-type needle bearing. In a preferred embodiment, the tube is rigidly mounted to the corresponding arm assembly. In a further embodiment, a shock absorber is pivotally mounted between the lower support and the upper support or one of the arm elongated arm assemblies to dampen movement of the upper support relative to the lower support, wherein the shock absorber is pivotally mounted at one end to the lower support and generally transverse to the movement of the upper support relative to the lower support. A linkage is connected at one end to one of the upper support and one of the elongated arm assemblies and is pivotally connected at another end to another end of the shock absorber. The linkage and the shock absorber thereby dampen vertical oscillations of the upper support relative to the lower support. The one end of the linkage can be mounted to the upper support.

Alternatively, the one end of the linkage can be non-rotatably attached to the upper elongated arm assembly at the pivotal connection between the upper elongated arm assembly and the upper and/or lower support. The linkage can be an arcuate member whereby pivotal movement of the one end of the linkage is translated to linear movement at the other end thereof.

In another embodiment, a foot rest system can be mounted to the upper support and can have a foot rest adapted to support a portion of an occupant seated on the upper support whereby the occupant is isolated from vibrations encountered by the lower support. In yet another embodiment, an air spring is fluidly interconnected with a source of pressurized air and mounted between the upper support and one of the arm assemblies to cushion the movement of the upper support with respect to the lower support. A source of liquid can be in fluid communication with a first portion of the air spring for providing a volume of liquid into the first portion of the air spring. The pressurized air can be in fluid communication with a second portion of the air spring and a pump can be provided for selectively changing the volume of liquid in the first portion of the air spring. The height of the upper support with respect to the lower support can thereby be changed by changing the volume of liquid in the first portion of the air spring. A first valve can be provided for selectively changing the volume of the pressurized air in the second portion of the air spring to selectively change the vibration frequency of the upper support with respect to the lower support. The pump can comprise an accumulator having a first portion in fluid communication with the first portion of the air spring, and a second portion in fluid communication with the pressurized air source. A second valve can be provided in fluid communication between the accumulator second portion and the pressurized air source for controlling the pressure of the air in the accumulator and thereby controlling the amount of liquid pumped into the first portion of the air spring. The first valve can be a three-way valve selectively moveable between a first position wherein the second portion of the air spring is opened to the pressurized air source, a second position wherein the second portion of the air spring is closed to the pressurized air source and a third position wherein the second portion of the air spring is in communication with the atmosphere. The second valve can be a three-way valve selectively moveable between a first position wherein the second portion of the accumulator is opened to the pressurized air source, a second position wherein the second portion of the accumulator is closed to the pressurized air source and a third position wherein the second portion of the accumulator is in communication with the atmosphere. The liquid can be any suitable liquid, preferably one that freezes below 0°

Fahrenheit, for example, ethylene glycol. Alternatively, the liquid can be water or a mixture of water and ethylene glycol. The liquid can further have anti-corrosive properties with respect to the air spring and accumulator.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a side elevational view of a suspension system according to the invention;

FIG. 2 is a perspective view of a lower arm assembly used in the suspension system illustrated in FIG. 1 ; FIG. 3 is plan view of a modified lower arm assembly for use in the suspension system illustrated in FIG. 1 ;

FIG. 4 is a partial end elevational view, in section, of the suspension system illustrated in FIG. 1 taken along lines 4-4 of FIG. 1 ; FIG. 5 is a schematic view of an air control system used in the suspension system illustrated in FIG. 1 ;

FIG. 6 is side elevational view, like FIG. 1, of a modified suspension system according to the invention;

FIG. 7 is a side elevational view, like FIG. 1, of a modified suspension system according to the invention;

FIG. 8 is a perspective view, like FIG. 2, of a modified lower arm assembly used in the suspension system illustrated in FIG. 1 ;

FIG. 9 is an end view of the lower arm assemble illustrated in FIG. 8;

FIG. 10 is a perspective view, like FIG. 2, of another modified lower arm assembly used in the suspension system illustrated in FIG. 1 ;

FIG. 11 is an end view of the lower arm assemble illustrated in FIG. 10; and

FIG. 12 is a view like FIG. 6 of another modified form of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and to FIG. 1 in particular, a suspension system 10 which is useful for a vehicle seat, for example comprises an upper support frame 12, including an upper platform, a lower support frame 14, a lower control arm assembly 16, an upper control arm assembly 18 and a sleeve type air spring 20. The lower and upper control arm assemblies 16 and 18, respectively, are pivotally mounted to the lower support frame 14 at one end and to the upper support frame 12 at the other ends thereof. The sleeve type air spring 20 with a pedestal 30 is mounted between the upper portion of the lower control arm assembly 16 and the lower side of the upper surface of the upper support frame 12. The upper support frame 12 can be attached to a vehicle seat, for example, and the lower support frame 14 can be attached to a floor 22 of a vehicle, for example. The roll stability of the parallelogram suspension system 10 in the pneumatic seat base is dependent directly on the torsional stiffness of the upper and lower arm assemblies 18 and 16, respectively, to resist roll moments. The existing upper and lower arm assemblies 18 and 16, respectively, are rectangular frames, utilizing circular cross-section tubes 24 as cross members, and rectangular flat stock for side rails 26. The tubes 24 are welded to the side rails 26 where the components abut each other as illustrated in FIG. 2. Since the side rails 26 are flat plate members, the arm assemblies may lack adequate roll stiffness due to poor torsional stiffness of the flat plates. By introducing X-frame or diagonal bracing 28 interior to the lower arm assembly (FIG. 3), an extremely torsionally stiff assembly is achieved. The diagonal or X-bracing preferably intersects the side rails at 45° inclinations to achieve the maximum torsional stiffness for the assembly. With the X-frame bracing installed in the lower arm assembly 16, the seat base can be operated over the entire height variation range and the seat assembly 10 will experience negligible roll deflection due to lateral acceleration experienced by the operator, or offset (non-centered) vertical loading.

Reference is now made to FIGS. 8-11 which show modified forms of one of the upper and lower arm assemblies 18 and 16 where like numbers have been used to designate like parts. The torsional stiffness of the upper and lower arm assemblies 18, 16 can be increased significantly by introducing rectangular top and bottom cover plates 64, 66, as shown in FIGS. 8 and 9. These relatively thin gage plates 64,65 are welded to the upper and lower edges of the rectangular side rails 26, resulting in a "closed" section design between the tubular cross members 24. As shown in FIGS. 10 and 11, by replacing the control arm assembly side rails

26 with tubular members 70 (either circular or rectangular cross section tubes) to form a rectangular frame composed entirely of closed section (tubular) members 24 and 70. The tubes 24 are welded to the rectangular tubular members 70 through welds 72. Typically only one arm assembly 16 or 18 must be torsionally stiff to achieve the desired roll stiffness in the suspension system. Large levels of rotational compliance are required between the support frames 12 and 14 and the upper and lower control arm assemblies 18 and 16, respectively, to accommodate the full range of height variation in the seat base. To minimize the effects of frictional resistance on the overall system performance, low-friction lubricated rolling element bearings 32 are incorporated at all pivot connections between frames 12 and 14 and control arms 16 and 18 (eight locations) as illustrated in FIG. 4. As shown by example in FIG. 4, bearings such as cup-type needle roller bearing assemblies 32 are installed in the outboard ends of each of the control arm cross tubes 24. The bearing assemblies 32 are supported by a journal/shaft assembly 34 which is housed in aligned bores 36 in each of the support frames 12 and 14. The bearing design is a modification of a standard cup-type needle bearing used with conventional Cardan or universal-type joints, providing a cost-effective and a long life (no wear) assembly.

Referring now to FIG. 5, the seat suspension/height positioning system comprises a conventional small sleeve type air spring assembly 20 pneumatically interconnected with an accumulator/storage chamber 38 through conduit 48.

Broadly speaking, it is an important inventive feature of this invention for an occupant of the seat supported by the seat suspension described herein to adjust the height of the upper support 12 relative to the lower support 14 while maintaining or independently adjusting the natural frequency of the suspension. Thus, as the occupant changes the height of the upper support 12, the natural frequency of the suspension system is maintained. Alternatively, the the natural frequency of the suspension system can be adjusted independently of the height of the seat, as described below. This isolation is provided by an inventive liquid/gas mixture present in the air spring assembly and the accumulator 38. The accumulator 38, when interconnected to a source of pressurized air 42 (typically already provided on trucking vehicles for braking brake actuator systems), acts as a pump to move liquid between the air spring 20 and the accumulator 38. The liquid can comprise a predetermined volume of ethylene glycol, such as automotive antifreeze, and water mixture which is contained within the spring/accumulator 38 to establish the total gas volume in the air spring assembly 20 (ultimately, the vertical natural frequency of vibration for the seat/person combination) through line 48.

A conduit 74 having a three way manually operated slide first valve 76 is connected to the upper portion of the air spring assembly 20 and the conduit 50 between the three way valve 42 and the air supply 40. The first valve 76 is adapted to open to connect the source of pressurized air 40 to the air spring assembly 20 in one position, close in a second position and open to exhaust gas from the air spring assembly 20 in a third position.

The accumulator/storage chamber 38 is connected to a source of pressurized air 40 through a conduit 50 which has a manually operated three way second valve 42. The second valve 42 is adapted to open to connect the source of pressurized air 40 to the accumulator/storage chamber 38 in one position, close in a second position and open to exhaust gas from the accumulator/storage chamber 38 in a third position.

Liquid thereby resides in a lower first portion of the accumulator 38 and the air spring 20 and within the interconnecting conduit 48. Thus, the first valve 76 controls the introduction of pressurized air into a second or upper portion of the air spring 20 and the second valve 42 controls the introduction of pressurized air into a second or upper portion of the accumulator 38.

The accumulator 38 separates the air/fluid medium in the air spring 20 from the source of pneumatic pressure 40 used to raise and lower the seat cushion. The accumulator/storage chamber 38 can be any appropriate air/fluid divided chamber such as (1) a linear cylinder employing a free piston with adequate seals to separate the fluid medium from the working pneumatic medium, (2) a closed vessel or chamber employing a diaphragm to separate the fluid medium from the working pneumatic medium, or (3) a sufficiently large closed vessel or chamber, in which the free surface of the fluid serves as the barrier between the fluid medium and the working pneumatic medium. The chamber must be of sufficient size that the free surface remains essentially intact for all heights or elevations of the seat.

With the first and second alternatives of the accumulator/storage chamber 38, a measured volume of fluid can be introduced into the system during assembly without leaks and are preferred embodiments. The use of the third alternative presents a possible leak problem during assembly or field disassembly and must be carefully sealed and monitored but will function identical to the first and second options discussed above.

By utilizing a sleeve type air spring 20 with pneumatic connections on each end of the assembly, the overall ride frequency of the system can be adjusted by the user during operation.

To increase the ride frequency of the seat assembly (increase stiffness) the first valve 76 is opened and air is exhausted from the gas volume above the fluid in the spring 20. As gas is exhausted from the spring 20, the associated stiffness is increased.

To decrease the ride frequency of the seat assembly (decrease stiffness), the manually operated slide valve 76 is opened to the air supply and air enters the gas volume above the fluid in the spring. This action will force some of the fluid out of the spring 20 into the reservoir, resulting in an increase in gas volume. Increased gas volume in the pneumatic spring results in an associated reduction in stiffness of the spring.

To increase the height of the seat, the manually operated slide valve 42 in an air line 50 is "opened," allowing air supply 40 to force fluid from the accumulator chamber 38 (movement of free piston or diaphragm) into the air spring 20, resulting in a sleeve of increased height.

Conversely, to decrease the elevation of the seat, the manually operated slide valve 42 is operated to vent air from the pneumatic side of the accumulator chamber 38. This reduction in accumulator supply pressure allows fluid to enter the accumulator 38 (due to motion of the piston or diaphragm). In either case, when the desired seat height or elevation is achieved, the manually operated slide valve 42 is returned to the center biased "off position, separating the air supply 40 from further interaction with the spring/accumulator chamber system 38.

Since the gas volume in the air spring assembly 20 remains constant (the liquid is added or removed from the spring 20 to alter seat height), regardless of design height, the vibratory natural frequency of the seat/spring system remains constant, independent of seat elevation.

To control the amplitudes of resonant vibration in the seat suspension system 10, a double-acting hydraulic damper 44 is installed. Referring now to FIG. 6, the damper 44 is positioned horizontally, pivotally mounted at one end to the bottom of the lower support frame 14 through pin 52 generally transverse to the movement of the upper support 12 relative to the lower support 14. The damper 44 is pivotably attached at the other end through pin 54 to a lever arm 46 which is rigidly attached to the cross tube 24 of the upper control arm 18. This physical arrangement provides the maximum amount of hydraulic damping to control vertical oscillations of the seat base.

The length of the arm assembly 46 from its attachment on the cross tube 24 of the upper control arm 18 provides the maximum possible velocities across the damper 44 (within the allowable space envelope provided by the seat base). Further, the length of the arm assembly 46 provides the maximum amount of multiplication of damper output force acting to control rotation of the upper control arm assembly 18 relative to the fixed base.

The damper assembly 44 can contain an internal gas bag or cell (not shown) to insure the air in the reservoir section cannot enter the valve orifices which generate the hydraulic resistance. To utilize the isolation benefits of the seat assembly to maximum potential, a footrest system 56 is incorporated the upper support frame as shown in FIG. 7, where like numerals have been used to indicate like parts. A flat side bar is pivotally mounted at an upper end thereof to the upper support frame 12 through pin 66 and is retained in a given position by stop plate 64. A footrest bar 60 is pivotably mounted to the lower ends of the side bars 58 through pin 62. This footrest system supports the legs and feet of the seat occupant at any desired elevation (within travel limits of seat base), and isolates vibration for the entire body of the occupant since the feet and lower legs are also supported by the upper support frame 12. Referring now to FIG. 12, there is show a further modification of the invention in which like numerals have been used to identify like parts. The embodiment shown in FIG. 12 is similar to the embodiment shown in FIG. 6 except the double acting cylinder 44 is connected to the pin 24 of the upper control arm 18 through a lever arm 78. The lever arm 78 is non-rotatably attached to the cross tube 24 at the upper end of the upper control arm assembly 18 and rotatably mounted to the extendible rod of the damping cylinder 44 through pin 54. With this configuration, the axis of the damper remains essentially parallel with the plane of the upper arm 18, to which it is attached. This construction provides more consistent or uniform level of damping performance over the complete range of seat elevations as compared to the arrangement shown in FIG. 6.

While particular embodiments of the invention have been shown, it will be understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. Reasonable variation and modification are possible within the scope of the foregoing disclosure of the invention without departing from the spirit of the invention which is defined is the accompanying claims.

Claims

CLAIMSWhat is claimed is:

1. In a suspension system for a vehicle seat including: a lower support securable in a stationary relationship with a vehicle body; an upper support arranged generally above the lower support for supporting a vehicle seat; a cooperating pair of elongated arm assemblies which each have opposite ends pivotally connected at opposite ends thereof to a corresponding one of the upper and lower supports so that the upper support is vertically movable with respect to the lower support; the improvement comprising: at least one of the elongated arm assemblies comprises a pair of spaced side rails and a bracing configuration for the side rails, wherein the bracing configuration is constructed to achieve torsional stiffness of the at least one arm assembly sufficient to minimize roll deflection due to lateral acceleration or offset vertical loading of the suspension system.

2. The suspension system of claim 1 wherein the bracing configuration comprises at least one diagonal brace rigidly mounted to each side rail which intersects the side rails at a generally 45-degree angle.

3. The suspension system of claim 2 wherein the at least one diagonal brace comprises a plurality of braces forming an x-shaped configuration between the side rails.

4. The suspension system of claim 1 wherein the bracing configuration comprises at least one cover plate mounted to one of upper and lower edges of the spaced side rails.

5. The suspension system of claim 4 wherein the bracing configuration comprises first and second cover plates mounted to respective upper and lower edges of the spaced side rails.

6. The suspension system of claim 1 wherein the bracing configuration comprises elongated tubular members forming the spaced side rails.

7. The suspension system of claim 6 wherein at least one of the elongated tubular members has one of a rectangular and circular cross section.

8. In a suspension system for a vehicle seat including: a lower support securable in a stationary relationship with a vehicle body; an upper support arranged generally above the lower support for supporting a vehicle seat; a cooperating pair of elongated arm assemblies which each have opposite ends pivotally connected at opposite ends thereof to a corresponding one of the upper and lower supports so that the upper support is vertically movable with respect to the lower support; the improvement comprising: each of the pivotal connections between each of the arm assemblies and the respective upper and lower supports comprises a shaft mounted to one of the corresponding upper and lower supports and one of the arm assemblies and a tube mounted to the other of the corresponding upper and lower supports and one of the arm assemblies; and a bearing located between the shaft and the tube; whereby the bearing minimizes the effects of frictional resistance between the arm assemblies and the corresponding upper and lower support.

9. The suspension system of claim 8 wherein the bearing comprises a lubricated rolling element bearing.

10. The suspension system of claim 8 wherein the bearing comprises a lubricated cup-type needle bearing.

11. The suspension system of any of claims 8-10 wherein the tube is rigidly mounted to the corresponding arm assembly.

12. In a suspension system for a vehicle seat including: a lower support securable in a stationary relationship with a vehicle body; an upper support arranged generally above the lower support for supporting a vehicle seat; a cooperating pair of elongated arm assemblies which each have opposite ends pivotally connected at opposite ends thereof to a corresponding one of the upper and lower supports so that the upper support is vertically movable with respect to the lower support; a shock absorber pivotally mounted to the lower support to dampen movement of the upper support relative to the lower support; the improvement comprising: the shock absorber is pivotally mounted at one end to the lower support and generally transverse to the movement of the upper support relative to the lower support; and a linkage connected at one end to one of:

(a) the upper support; and

(b) one of the elongated arm assemblies; and pivotally connected at another end to another end of the shock absorber; whereby the linkage and the shock absorber control vertical oscillations of the upper support relative to the lower support.

13. The suspension system of claim 12 wherein the one end of the linkage is mounted to the upper support.

14. The suspension system of claim 12 wherein the one end of the linkage is non-rotatably attached at the pivotal connection between the upper elongated arm assembly and the upper support.

15. The suspension system of claim 12 wherein the one end of the linkage is non-rotatably attached at the pivotal connection between the upper elongated arm assembly and the lower support.

16. The suspension system of any of claims 12-15 wherein the linkage is an arcuate member whereby pivotal movement of the one end of the linkage is translated to linear movement at the other end thereof.

17. The suspension system of any of claims 1-16 and further comprising a foot rest system mounted to the upper support and having a foot rest adapted to support a portion of an occupant seated on the upper support whereby the occupant is isolated from vibrations encountered by the lower support.

18. In a suspension system for a vehicle seat including: a lower support securable in a stationary relationship with a vehicle body; an upper support arranged generally above the lower support for supporting a vehicle seat; a cooperating pair of elongated arm assemblies which each have opposite ends pivotally connected at opposite ends thereof to a corresponding one of the upper and lower supports so that the upper support is vertically movable with respect to the lower support; an air spring fluidly interconnected with a source of pressurized air and mounted between the upper support and one of the arm assemblies to cushion the movement of the upper support with respect to the lower support; the improvement comprising: a source of liquid in fluid communication with a first portion of the air spring for providing a volume of liquid into the first portion of the air spring; the pressurized air is in fluid communication with a second portion of the air spring; a pump for selectively changing the volume of liquid in the first portion of the air spring; whereby the height of the upper support with respect to the lower support can be changed by changing the volume of liquid in the first portion of the air spring.

19. The seat suspension of claim 18 and further comprising a first valve for selectively changing the volume of the pressurized air in the second portion of the air spring to selectively change the vibration frequency of the upper support with respect to the lower support.

20. The seat suspension of claim 18 or 19 wherein the pump comprises an accumulator having a first portion in fluid communication with the first portion of the air spring, and having a second portion in fluid communication with the pressurized air source.

21. The seat suspension of any of claims 18-20 and further comprising a second valve in fluid communication between the accumulator second portion and the pressurized air source for controlling the volume of the air in the accumulator and thereby controlling the amount of liquid pumped into the first portion of the air spring.

22. The seat suspension of any of claims 19-21 wherein the first valve is a three-way valve selectively moveable between a first position wherein the second portion of the air spring is opened to the pressurized air source, a second position wherein the second portion of the air spring is closed to the pressurized air source and a third position wherein the second portion of the air spring is in communication with the atmosphere.

23. The seat suspension of claim 21 or 22 wherein the second valve is a three-way valve selectively moveable between a first position wherein the second portion of the accumulator is opened to the pressurized air source, a second position wherein the second portion of the accumulator is closed to the pressurized air source and a third position wherein the second portion of the accumulator is in communication with the atmosphere.

24. The seat suspension of any of claims 18-23 wherein the liquid is ethylene glycol.

25. The seat suspension of any of claims 18-23 wherein the liquid is water.

26. The seat suspension of any of claims 18-23 wherein the liquid is an antifreeze composition.

27. The seat suspension of any of claims 18-23 wherein the liquid has a freezing point below 0 degrees Fahrenheit.

28. The seat suspension of any of claims 18-27 wherein the liquid has anti- corrosive properties with respect to the air spring and accumulator.