VEHICLE SUSPENSION SYSTEM.

Abstract

A fluid suspension system for a land vehicle consists of an actuator connected to a chassis and a separate axle with a pivoting connection. A fluid pressure circuit works in conjunction with the actuator(s). A controller is in operational communication with the fluid pressure circuit and is programmed to receive input indicative of the land vehicle's travel speed. The fluid pressure circuit is adjusted to limit fluid flow or reduce fluid pressure within a low travel speed range, allowing the axle to pivot in response to variations in the underlying support surface.The fluid pressure circuit is adjusted to a higher displacement speed range for selective actuation of the actuator(s) or for higher fluid pressure actuation of the actuator(s), in response to variations in the underlying surface.

Description

VEHICLE SUSPENSION SYSTEM CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority over U.S. application serial number 17 / 477.026 filed on September 16, 2021, which is a continuation in part of U.S. application serial number 17 / 089.016 filed on November 4, 2020, the disclosures of which are incorporated herein by reference in their entirety. TECHNICAL FIELD Several implementations refer to suspension systems for land vehicles. BACKGROUND U.S. Patent No. 5,447,331 granted to Genie Industries, Inc., on September 5, 1995 for a Vehicle Axle Oscillation System with Positive Ground Contact, disclosing a fluid suspension system for a land vehicle. COMPENDIUM According to at least one embodiment, a fluid suspension system for a land vehicle is provided with at least one actuator adapted to be connected to a chassis and axle of a land vehicle, separate from a pivoting axle connection. A fluid pressure circuit cooperates with the actuator(s). A controller is in operational communication with the fluid pressure circuit and is programmed to receive an input indicative of the land vehicle's travel speed. The fluid pressure circuit closes at a low travel speed to limit fluid flow and allow the axle to pivot in response to variations in an underlying support surface. The fluid pressure circuit opens, and the actuator(s) are selectively actuated at a higher travel speed in response to variations in the underlying support surface. According to another embodiment, the axle is further defined as a first axle. The fluid suspension system is also provided with a flow control valve in fluid cooperation with the fluid pressure circuit and a second axle that is connected to the chassis via permissible pivoting. According to another further embodiment, a solenoid valve is in fluid cooperation with a pressurized fluid source and the flow control valve. According to another additional embodiment, the solenoid valve closes in response to the low displacement speed range. According to another additional embodiment, the solenoid valve opens in response to the higher displacement speed range. According to another further embodiment, a flow limiter is in fluid cooperation with the pressurized fluid source and the flow control valve. According to another embodiment, the flow limiter is in fluid communication in parallel with the solenoid valve. According to another additional embodiment, the solenoid valve is a normally open valve. According to another additional embodiment, the solenoid valve is a normally closed valve. According to another embodiment, a land vehicle is provided with a chassis. An axle is connected to the chassis with permissible pivoting about a horizontal geometric axis perpendicular to the axle. A pair of wheels is mounted on the axle and separated by a pivoting connection between them to support the axle and chassis as they travel on an underlying support surface. At least one actuator is connected to the chassis and the axle, separated by the pivoting connection. A fluid pressure circuit cooperates with the actuator(s). A controller is in operational communication with the fluid pressure circuit and is programmed to receive an input indicative of the land vehicle's travel speed. The fluid pressure circuit closes at a low travel speed to limit fluid flow and allow the axle to pivot in response to variations in the underlying support surface.The fluid pressure circuit opens and the actuator(s) are selectively actuated at a higher travel speed range in response to variations in the underlying support surface. According to another embodiment, a speed sensor cooperates with the ground vehicle to determine the ground vehicle's travel speed and is in communication with the controller to provide indicative input of the travel speed. According to another further embodiment, the axle is further defined as a first axle. A second axle is pivotally connected to the chassis about the horizontal geometric axis perpendicular to the second axle and separate from the first axle. A second pair of wheels is mounted on the second axle and separated by the pivoting connection between the second axle and the chassis in order to support the second axle and the chassis as they move on the underlying support surface. A flow control valve is in fluid cooperation with the fluid pressure circuit and the second axle. According to another embodiment, a solenoid valve is in fluid cooperation with a pressurized fluid source and the flow control valve. According to another embodiment, a flow limiter is in fluid cooperation with the pressurized fluid source and the flow control valve. According to another embodiment, the flow limiter is in fluid communication in parallel with the solenoid valve. According to one embodiment, a land vehicle is provided with a chassis. A first axle is pivotally connected to the chassis about a horizontal geometric axis perpendicular to the first axle. A second axle is pivotally connected to the chassis about a horizontal geometric axis perpendicular to the second axle and separate from the first axle. A first pair of wheels is mounted on the first axle and separated by a pivoting connection between the first axle and the chassis to support the first axle and chassis as they move on an underlying support surface. A second pair of wheels is mounted on the second axle and separated by a pivoting connection between the second axle and the chassis to support the second axle and chassis as they move on the underlying support surface. A speed sensor determines the travel speed of the land vehicle.A pressurized fluid source is provided. A first actuator is connected to the chassis, and the first shaft is detached from the pivot point. A second actuator is also connected to the chassis, and its first shaft is detached from both the pivot point and the first actuator. A flow control valve is in fluid operation with the second shaft. A solenoid valve is in fluid operation with the pressurized fluid source and the flow control valve. A flow restrictor is in fluid operation with the pressurized fluid source and the flow control valve. The flow restrictor's operation is parallel to that of the solenoid valve. A controller is in operational communication with the solenoid valve and is programmed to receive an input indicative of the ground vehicle's travel speed from the speed sensor.The solenoid valve closes at a low travel speed to limit fluid flow and allow the first shaft to pivot in response to variations in the underlying bearing surface. The solenoid valve opens and selectively actuates the first and second actuators at a higher travel speed in response to variations in the underlying bearing surface. According to another embodiment, a suspension system for a land vehicle is provided with at least one actuator adapted to be connected to a chassis and axle of a land vehicle, separate from a pivoting axle connection. A suspension circuit cooperates with the actuator(s). A controller is in operational communication with the suspension circuit and is programmed to receive an input indicative of the land vehicle's travel speed. The actuator(s) are selectively actuated proportionally to the land vehicle's travel speed, at a higher travel speed range, in response to variations in the underlying support surface. According to another embodiment, the actuator(s) also include an electromechanical actuator. According to another embodiment, a fluid suspension system for a land vehicle is provided with at least one actuator adapted to be connected to a chassis and axle of a land vehicle, separate from a pivoting axle connection. A fluid pressure circuit cooperates with the actuator(s). A controller is in operational communication with the fluid pressure circuit and is programmed to receive an input indicative of the land vehicle's travel speed. The fluid pressure circuit is adjusted to limit fluid flow or reduce fluid pressure in a low travel speed range, in order to allow the axle to pivot in response to variations in an underlying support surface.The fluid pressure circuit is adjusted to a higher displacement speed range for selective actuation of the actuator(s) or for higher fluid pressure actuation of the actuator(s), in response to variations in the underlying support surface. According to another embodiment, the axle is further defined as a first axle. The fluid suspension system is provided with a flow control valve in fluid cooperation with the fluid pressure circuit and a second axle that is connected with the chassis via permissible pivoting. According to another further embodiment, a solenoid valve is in fluid cooperation with a pressurized fluid source and the flow control valve. According to another further embodiment, a pressure reducing valve is in fluid cooperation with the solenoid valve and is in communication with the controller. According to another further embodiment, the pressure reducing valve is in fluid communication between a pressurized fluid source and the solenoid valve. According to another additional embodiment, the controller is programmed to actuate the pressure reducing valve in order to reduce the fluid pressure in a displacement speed range of 0.4 meters per second or less. According to another additional embodiment, the controller is programmed to actuate the pressure-reducing valve in order to increase the fluid pressure in a displacement speed range of 0.13 meters per second or higher. According to another further embodiment, a sequential valve is in fluid communication with the solenoid valve to block fluid flow at a predetermined pressure. According to another embodiment, a land vehicle is provided with a chassis. An axle is connected to the chassis with permissible pivoting about a horizontal geometric axis perpendicular to the axle. A pair of wheels is mounted on the axle and separated by a pivoting connection between them to support the axle and chassis for movement on an underlying support surface. At least one actuator is connected to the chassis and the axle by the pivoting connection. A fluid pressure circuit cooperates with the actuator(s). A controller is in operational communication with the fluid pressure circuit and is programmed to receive an input indicative of the land vehicle's travel speed.The fluid pressure circuit is adjusted to limit fluid flow or reduce fluid pressure at low travel speeds to allow the shaft to pivot in response to variations in the underlying bearing surface. At higher travel speeds, the fluid pressure circuit is adjusted for selective actuation of the actuator(s) or for increased fluid pressure actuation of the actuator(s), in response to variations in the underlying bearing surface. According to another embodiment, a speed sensor cooperates with the ground vehicle to determine the ground vehicle's travel speed and is in communication with the controller to provide indicative input of the travel speed. According to another further embodiment, the axle is further defined as a first axle. The land vehicle is also provided with a second axle, pivoting with the chassis about the horizontal geometric axis perpendicular to the second axle and separate from the first axle. A second pair of wheels is mounted on the second axle and separated by a pivoting connection between the second axle and the chassis, in order to support the second axle and the chassis as they move on the underlying support surface. A pressure-reducing valve is in fluid cooperation with the fluid pressure circuit and the second axle. According to another further embodiment, a solenoid valve is in fluid cooperation with a pressurized fluid source and the pressure reducing valve. According to another further embodiment, a sequential valve is in fluid communication with the solenoid valve to block fluid flow at a predetermined pressure. According to another further embodiment, the shaft is further defined as a first shaft. The fluid pressure system is further provided with a flow control valve in fluid cooperation with the fluid pressure circuit and a second shaft that is connected to the chassis via permissible pivoting. According to another further embodiment, a solenoid valve is in fluid cooperation with a pressurized fluid source and the flow control valve. According to another further embodiment, a pressure reducing valve is in fluid cooperation with the solenoid valve and is in communication with the controller. According to another further embodiment, the pressure reducing valve is in fluid communication between a pressurized fluid source and the solenoid valve. According to another further embodiment, the controller is programmed to actuate the pressure reducing valve in order to reduce the fluid pressure in a displacement speed range of 0.4 meters per second or less. According to another further embodiment, the controller is programmed to actuate the pressure-reducing valve in order to increase the fluid pressure in a displacement speed range of 0.13 meters per second or higher. According to another embodiment, a land vehicle is provided with a chassis. A first axle is pivotally connected to the chassis about a horizontal geometric axis perpendicular to the first axle. A second axle is pivotally connected to the chassis about a horizontal geometric axis perpendicular to the second axle and separate from the first axle. A first pair of wheels is mounted on the first axle and separated by a pivoting connection between the first axle and the chassis to support the first axle and chassis as they move on an underlying support surface. A second pair of wheels is mounted on the second axle and separated by a pivoting connection between the second axle and the chassis to support the second axle and chassis as they move on the underlying support surface. A speed sensor determines the travel speed of the land vehicle.A pressurized fluid source is provided. A first actuator is connected to the chassis, and the first axle is detached from the pivot point. A second actuator is also connected to the chassis, and its first axle is detached from both the pivot point and the first actuator. A pressure-reducing valve is in fluid communication with the pressurized fluid source. A solenoid valve is in fluid cooperation with the pressure-reducing valve. A controller is in operational communication with the pressure-reducing valve and is programmed to receive an input indicative of the ground vehicle's travel speed from the speed sensor. The pressure-reducing valve is actuated to reduce the fluid pressure at low travel speeds, allowing the axle to pivot in response to variations in the underlying bearing surface.The pressure reducing valve operates at a higher displacement speed range for higher fluid pressure actuation of the first and second actuators, in response to variations in the underlying bearing surface. According to another embodiment, the controller is programmed to actuate the pressure-reducing valve in order to reduce the fluid pressure in a displacement speed range of 0.4 meters per second or less. According to another additional embodiment, the controller is programmed to actuate the pressure-reducing valve in order to increase the fluid pressure in a displacement speed range of 0.13 meters per second or higher. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of an aerial lifting vehicle according to one embodiment, illustrated in a partially extended position; FIGURE 2 is a perspective view of an aerial lifting vehicle according to another embodiment, which is illustrated partially extended; FIGURE 3 is a schematic end view of an axle assembly of a land vehicle according to another embodiment; FIGURE 4 is a schematic end view of another axle assembly of the land vehicle of Figure 3; FIGURE 5 is a diagram of a fluid circuit of the land vehicle of Figure 3 according to one embodiment; FIGURE 6 is an enlarged part of the fluid circuit diagram in Figure 5; FIGURE 7 is a diagram of the fluid circuit of the land vehicle of Figure 3 according to another embodiment; and FIGURE 8 is a diagram of a fluid circuit of the land vehicle of Figure 3 according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION As necessary, detailed embodiments of the present invention are disclosed herein; however, it should be understood that the disclosed embodiments are merely examples of the invention that may be incorporated in various and alternative ways. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, the specific structural and functional details described herein should not be construed as limiting, but merely as a representative basis for teaching a person skilled in the art how to employ the present invention in various ways. Aerial lift assemblies provide an operator platform on an articulated assembly that pivots and / or translates to raise the operator platform to an elevated working position. Conventional aerial lift assemblies include several adjustable structures to raise an operator platform to a height required to perform a work operation. Aerial lift assemblies frequently include a stackable articulated assembly. Aerial lift assemblies frequently include an articulated arm assembly, which can be provided by a four-bar linkage or an extendable lifting joint. Six aerial lifts are frequently arranged on ground vehicles for transporting the operator's platform to the work site. Figure 1 illustrates an aerial lifting assembly 20 according to one embodiment. The aerial lifting assembly 20 is a mobile aerial lifting assembly 20, such as a ground vehicle, that can be folded for transport over an underlying support surface 22, such as the ground or a floor (Figure 1). The aerial lifting assembly 20 can also be transported as a trailer and carried on a trailer behind a truck. The aerial lifting assembly 20 can be expanded by operator control to raise an operator to an elevated working position. The aerial lifting assembly 20 is analyzed relative to the ground 22. Therefore, terms such as upper, lower, and other height-related terms are relative to the height from the ground 22 and do not limit the aerial lifting assembly 20 to specific applications on the ground 22. The aerial lifting assembly 20 includes a lifting structure that provides significant stability and performance characteristics when raising a worker to a position advantageous for reaching while providing stability. The aerial lifting assembly 20 includes a chassis 24 to support the aerial lifting assembly 20 above the ground 22 or any supporting surface. The chassis 24 is supported on a plurality of wheels 26 that are in contact with the ground 22. An articulated assembly 28 is connected to the chassis 24 to extend from and retract into the chassis 24. A platform 30 is provided on the articulated assembly 28 to extend from and retract into the chassis 24. The platform 30 includes a perimeter guardrail 32 that extends upward from the platform 30 to enclose an operator workspace above the platform 30. The overhead hoist assembly 20 is used to lift the platform 30 and workers to elevated work locations for performing work operations. The articulated assembly 28 is a stackable articulated assembly 28, with a series of stackable links 34 connected with the permitted pivoting, which retract to fold and stack on the chassis 24 to increase storage and transport compactness. The overhead hoist assembly 20 also includes an actuator assembly 36 for extending and retracting the articulated assembly 28 and, consequently, extending and retracting the platform 30. Figure 2 illustrates an aerial lifting assembly 38 according to another embodiment. The aerial lifting assembly 38 includes a chassis 40 for supporting the aerial lifting assembly 38 above the ground 22. The chassis 40 is supported on a plurality of wheels 42 that are in contact with the ground 22 for the support and mobility of the aerial lifting assembly 38. An articulated assembly 44 is connected to the chassis 40 to extend from and retract into the chassis 40. A platform 46 with a perimeter railing 48 is provided on the articulated assembly 44. The articulated assembly 44 includes a plurality of four-bar linkages 50 with an extendable arm 52. Actuator assemblies 54 are provided for pivoting the four-bar linkages 50 and the extendable arm 52. An actuator assembly 56 is provided for extending the arm 52. Figure 3 illustrates an end view of an axle assembly 58 of the land vehicle 20, which may be a rear axle assembly 58 according to one embodiment. Figure 4 illustrates an end view of an axle assembly 60 of the land vehicle 20, which may be a front axle assembly 60. The chassis 24 is supported on the pair of axle assemblies 58 and 60. The chassis 24 is connected with permissible pivoting to a rear axle 62 of the rear axle assembly 58 at a rear pivot pin 64. The rear pivot pin 64 is located at the center of the rear axle 62 and is oriented horizontally in a front-to-rear direction of the land vehicle 20, perpendicular to the rear axle 62. The chassis 24 is connected with permissible pivoting to a front axle 66 of the front axle assembly 60 at a front pivot pin 68.The front pivot pin 68 is located at the center of the front axle 66 and is also oriented horizontally in the front-to-rear direction of the land vehicle 20, which is perpendicular to the front axle 66. The front axle assembly 60 can be oriented, as is known in the art. The wheels 26 are mounted so that they rotate on the axle assemblies 58, 60 to support the axle assemblies 58, 60 on the ground 22. Some or all of the wheels 26 can be driven by a motor or a plurality of motors, as is known in the art. The pivoting of axle assemblies 58, 60 allows a suspension of the land vehicle 20 to maintain contact of the wheels 26 with the ground 22 when the wheels 26 encounter irregularities in the ground 22. Pivoting axle assemblies 58, 60 are frequently referred to as a swing suspension system. One function of the swing suspension system is to maintain normal ground contact force to prevent loss of traction. Another function of the swing suspension system is to lock the axle in place when movement reduces the vehicle's stability. A further function of the swing suspension system is to prevent any of the wheels 26 from lifting significantly off the ground 22, which is undesirable since changes in terrain can cause a rollover onto the lifted wheel 26 when the projected center of gravity crosses a diagonal line between the wheels 26 at opposite corners of the land vehicle 20. The pivoting of the rear axle assembly 58 is limited in its angular pivot range. For example, the rear axle assembly 58 may be allowed to pivot approximately one degree in any angular direction. Rigid stops 70, 72 are provided on the chassis 24 and extend toward the rear axle 62. As the rear axle assembly 58 approaches irregularities in the ground 22, the rear axle 62 may pivot in any direction until it contacts one of the rigid stops 70, 72, thereby limiting the pivot range. The suspension system cooperates with the front and rear axle assemblies 58, 60 to adjust the front axle assembly 60 in response to the pivoting of the rear axle assembly 58.The adjustment can cooperate so that if one of the stops 70, 72 comes into contact with the rear axle 62, the front axle 66 pivots in a reverse angular direction to keep the tires 26 in contact with the ground. The suspension system detects the pivoting of the right or left pivoting steering of the rear axle 62. An input element 74 is connected to the pivoting allowed with the rear axle 62 to translate as the rear axle 62 pivots. The input element 74 cooperates with the suspension system so that the suspension system detects the translation of the input element 74. Figure 5 illustrates a portion of the suspension system 76 as a circuit diagram. According to one embodiment, the suspension system 76 is a fluid suspension system 76. According to another embodiment, the suspension system 76 is a hydraulic suspension system 76. The hydraulic suspension system 76 receives pressurized hydraulic fluid from a functional manifold 78. A supply line 80 provides fluid communication of the hydraulic fluid from the functional manifold 78 to the hydraulic suspension system 76. Referring to Figure 5, the hydraulic suspension system 76 includes a flow control valve, called the swing valve 82. The inlet element 74 of the rear axle assembly 58 in Figure 3 is connected to the swing valve 82. The swing valve 82 detects the pivot direction of the rear axle 62 based on the translation of the inlet element 74. The swing valve 82 is a directional control valve unit with a partially extended neutral position when the rear axle 62 is in a balanced position. In the neutral position, the swing valve 82 does not direct the flow. Further extension of the inlet element 74 indicates a pivot direction, allowing flow in one direction. Retraction of the inlet element 74 from the neutral position indicates pivoting in the opposite direction, thus allowing fluid to flow in another direction.Although one input element 74 is illustrated and described, any number of input elements 74 can be used. Referring again to Figure 4, a pair of actuators 84, 86 is provided on the front axle 66. According to one embodiment, the actuators 84, 86 are hydraulic cylinder actuators 84, 86. Each of the hydraulic cylinder actuators 84, 86 is pivotally connected to the front axle 66 and the chassis 24, and the actuators 84, 86 are separated by the chassis 24. According to another embodiment, each of the actuators 84, 86 is an electromechanical actuator, such as a ball screw assembly or the like. According to yet another embodiment, any number of actuators 84, 86 may be employed as a single actuator. Referring again to Figure 5, the swing valve 82 controls the flow between the functional manifold circuit 78 and the swing cylinder circuits 88, 90, each of which cooperates with one of the hydraulic cylinder actuators 84, 86. The swing cylinder circuits 88, 90 are illustrated in greater detail in Figure 6. Each swing cylinder circuit 88, 90 includes a pair of blocking valves. 92, 94, 96, 98. Each block valve 92, 94, 96, 98 is in parallel with a respective check valve 100, 102, 104, 106 located in a branch line 108, 110, 112, 114. The block valves 92, 94, 96, 98 are pilot-operated and are moved to a continuous flow position against the included return springs by pressurizing the respective cross-pilot lines 116, 118, 120, 122. Purge lines 124, 126, 128, 130 are provided at the spring end of the lock valves 92, 94, 96, 98 to bleed any oil leakage at the lock valves 92, 94, 96, 98. As illustrated in Figures 5 and 6, flow lines 132 and 134 connect the shut-off valves 92, 94, 96, and 98 to the swing valve 82. Referring to Figure 6, the inlet flow line 132 is connected to the shut-off valves 94 and 98, and the outlet flow line 134 is connected to the shut-off valves 92 and 96. Flow lines 136 and 138 connect the shut-off valves 92 and 98 to the cylinder ends of the hydraulic cylinder actuators 84 and 86. Flow lines 140 and 142 connect the rod ends of the hydraulic cylinder actuators 84 and 86 to shut off the valves 94 and 96. In the event of excessive pressure buildup in the system due to unusual thermal conditions, the thermal relief lines 144, 146, 148, 150 connect lines 136, 140, 142, 4 138 respectively with the locking valves 92, 94, 96, 98 to open the locking valves 92, 94, 96, 98 sufficiently to relieve excess pressure in the hydraulic cylinder actuators 84, 86. Returning to Figure 5, the functional manifold 78 is connected to a pump 166 and a reservoir 168. The functional manifold 78 includes a pressure relief valve 152 in parallel with the supply line 80. In the embodiment shown, the pressure relief valve 152 relieves hydraulic pressure exceeding 900 pounds per square inch (psi) back to the reservoir 168 within the functional manifold 78. The swing valve 82, when not in its centered locking position, functions to connect the pressurized supply line 80 of the functional manifold 78 to line 132 or 134 in the suspension system 76, and simultaneously connects an outlet line 154 to whichever of lines 132 or 134 is not connected to the supply line 80. Referring again to Figure 6, when line 132 is pressurized, the check valves 102 and 106 are not seated, and pressurized fluid flows through the bypass lines 110 and 114 and lines 140 and 138 to retract the hydraulic cylinder actuator 84 and extend the hydraulic cylinder actuator 86. Simultaneously, flow from line 132 moves through the cross-pilot lines 116 and 120 to open the valves. 92, 96 locking.When this occurs, the fluid at the cylinder end of hydraulic cylinder actuator 84 and the fluid at the rod end of hydraulic cylinder actuator 86 return along lines 136, 142, and 134 to the swing valve 82 (Figure 5) and the outlet line 154 (also Figure 5). At the same time, and referring again to Figure 6, the rod end of hydraulic cylinder actuator 84 is charged from line 140, thereby retracting hydraulic cylinder actuator 84 as hydraulic cylinder actuator 86 extends. When the swing valve 82 is moved in the opposite direction, so that line 134 is connected via swing valve 82 to supply line 80 instead of outlet line 154, and so that line 134 is connected via swing valve 82 to outlet line 154 instead of supply line 80, the result is that check valves 100, 104 are not seated and pressurized fluid flows through bypass lines 108, 112 and lines 136, 142 to extend hydraulic cylinder actuator 84 and retract hydraulic cylinder actuator 86. Simultaneously, flow from line 134 moves through cross-pilot lines 118, 122 to open block valves 94, 98.When this occurs, the fluid at the rod end of hydraulic cylinder actuator 84 and the fluid at the cylinder end of hydraulic cylinder actuator 86 return along lines 140, 138, and 132 to the swing valve 82 (Figure 5) and the outlet line 154 (also Figure 5). At the same time, and referring again to Figure 6, the cylinder end of hydraulic cylinder actuator 84 is charged from line 136, thereby extending hydraulic cylinder actuator 84 as hydraulic cylinder actuator 86 retracts. The hydraulic suspension system 76 is suitable for controlling axle actuators 84, 86 at certain travel speeds. The oscillation function allows all four wheels 26 to maintain contact with the ground 22 to maintain stability. At low speeds, the movement of the axles 62, 66 may be faster than required by the terrain, which can cause the axles 62, 66 to move with a gradual motion, potentially causing discomfort to the operator. This gradual motion can initiate and amplify vehicle dynamics that are fed back through the oscillation valve 82, resulting in a dynamic motion that is uncomfortable for the operator. Low speeds can be 1.0 miles per hour (mph) and lower, or even slower, such as 0.5 mph and lower, 0.4 mph and lower, or 0.3 mph and lower. Conversely, top speeds are usually 4-5 mph, depending on the particular suspension system.Low-speed control dynamics occur when the retractable hydraulic cylinder actuator 84 or 86 is subjected to a high load while the ground vehicle 20 is moving very slowly (0.3 mph and below, or 0.5 mph and below) over an obstacle. When the swing valve 82 opens, the hydraulic cylinder actuators 84 and 86 extend or retract rapidly, although there is a hysteresis delay before the swing valve 82 closes again. This results in a jump in position and inertia imparted to the lifting structure of the ground vehicle 20. When the ground vehicle 20 is moving at higher speeds (greater than 0.3 mph or greater than 0.5 mph), the swing motion is not significantly faster than required by the terrain, so the swing system cannot over-rotate and cause dynamic effects while the swing valve 82 remains open.At low forward speeds, the oscillation exceeds the speeds required to follow the irregularities of the terrain, and it over-rotates and closes the oscillation valve 82, which subsequently opens again with the forward movement, resulting in an uncomfortable cyclical motion. Referring again to Figure 5, a suspension controller 156 is provided. The suspension controller 156 is in electrical communication with a vehicle controller 158 of the ground vehicle 20. The suspension controller 156 can be incorporated within the vehicle controller 158 according to an alternative embodiment. The vehicle controller 158 provides a vehicle travel speed to the suspension controller 156. The vehicle controller 158 can provide the vehicle travel speed from a speed sensor in the ground vehicle 20. The ground vehicle 20 can employ an open-loop control system with hydrostatic drive to monitor the vehicle travel speed. The ground vehicle 20 can employ a closed-loop control system with an engine speed sensor, such as an electric motor drive.The suspension controller 156 is also in electrical communication with the functional collector 78. Functional manifold 78 includes a fixed control valve 160 between the pressure source and the supply line 80. According to one embodiment, the flow control valve 160 limits the flow, for example, to a limit of 0.5 gallons per minute (GPM). Functional manifold 78 also includes a normally open solenoid valve 162 in parallel with the flow control valve 160 and in fluid communication with the pressurized fluid source and the supply line 80. Valve 162 is in electrical communication with the controller 156. A pressure control relief valve 164 may also be provided in the supply line 80, between the flow control valve 160, the switching valve 162, and the swing valve 82 to limit the pressure to 750 psi according to one embodiment. When the ground vehicle 20 is traveling in the higher travel speed range, the switching valve 162 is left in the open position, bypassing the flow control valve 160 and allowing the swing valve 82 to balance the axle assemblies 58 and 60 as previously described. The hydraulic fluid pressure source is approximately eight to nine GPM, enabling a rapid swing response. However, when the ground vehicle 20 is traveling in the lower travel speed range, the suspension controller 156 closes the switching valve 162. When the switching valve 162 is closed, the fluid to the supply line 80 is directed through the flow control valve 160, thus limiting the hydraulic fluid flow rate to 0.5 GPM.Under flow-restricted conditions, the balancing of the swing valve 82 slows to a comfortable rate that matches the travel speed. When the vehicle 20 returns to a higher travel speed, the suspension controller 156 stops closing the switching valve 162, thereby allowing the valve 162 to reopen for uncontrolled swing balancing. According to another embodiment, the switching valve 162 can be a normally closed valve 162 that opens the controller 156 in the higher displacement speed range. According to another embodiment, the flow circuit can be proportional to the vehicle's travel speed, instead of switching between two flow rates. Figure 7 illustrates a portion of a suspension system 170 as a circuit diagram according to another embodiment. According to one embodiment, the suspension system 170 is a fluid suspension system 170. According to another embodiment, the suspension system 170 is a hydraulic suspension system 170. The hydraulic suspension system 170 receives pressurized hydraulic fluid from a functional manifold 78, as illustrated in the previous embodiment. A supply line 172 provides fluid communication of the hydraulic fluid from the functional manifold 78 to the hydraulic suspension system 170. The hydraulic suspension system 170 includes a flow control valve, called the swing valve 174. The inlet element 74 of the rear axle assembly 58 in Figure 3 is connected to the swing valve 174. The swing valve 174 detects the pivot direction of the rear axle 62 based on the translation of the input element 74. The swing valve 174 is a directional control valve unit with a partially extended neutral position when the rear axle 62 is in a balanced position. In the neutral position, the swing valve 174 does not direct the flow. Further extension of the input element 74 indicates a pivot direction, allowing flow in one direction. Retraction of the input element 74 from the neutral position indicates pivoting in the opposite direction, thus allowing fluid to flow in another direction. Although one input element 74 is illustrated and described, any number of input elements 74 may be used. The swing valve 174 controls the flow between the functional manifold circuit 78 and the swing cylinder circuits 88, 90, each of which is in cooperation with one of the hydraulic cylinder actuators 84, 86. The swing cylinder circuits 88, 90 are illustrated in more detail in Figure 6. Flow lines 132, 134 connect the shut-off valves 92, 94, 96, 98 of the swing cylinders 88, 90 and the swing valve 174. The hydraulic suspension system 170 includes a pressure relief valve 176 in parallel with the supply line 172. In the embodiment shown, the pressure relief valve 176 relieves hydraulic pressure exceeding 670 psi back to the reservoir within the functional manifold 78. The swing valve 174, when not in its centered locking position, functions to connect the pressurized supply line 172 of the functional manifold 78 to line 132 or 134 in the suspension system 170, and simultaneously connects an outlet line 178 to whichever line 132 or 134 is not connected to the supply line 172. Referring again to Figure 6, when line 132 is pressurized, the check valves 102 and 106 are not seated, and pressurized fluid flows through the bypass lines 110 and 114 and lines 140 and 138 to retract the hydraulic cylinder actuator 84 and extend the hydraulic cylinder actuator 86. Simultaneously, flow from line 132 moves through the cross-pilot lines 116 and 120 to cause the Open the blocking valves 92, 96.When this occurs, the fluid at the cylinder end of hydraulic cylinder actuator 84 and the fluid at the rod end of hydraulic cylinder actuator 86 return along lines 136, 142, and 134 to the swing valve 174 (Figure 7) and the outlet line 178 (also Figure 7). At the same time, and referring again to Figure 6, the rod end of hydraulic cylinder actuator 84 is charged from line 140, thereby retracting hydraulic cylinder actuator 84 as hydraulic cylinder actuator 86 extends. When the swing valve 174 is moved in the opposite direction, so that line 134 is connected via swing valve 174 to supply line 172 instead of outlet line 178, and so that line 134 is connected via swing valve 174 to outlet line 178 instead of supply line 172, the result is that check valves 100, 104 are not seated and pressurized fluid flows through bypass lines 108, 112 and lines 136, 142 to extend hydraulic cylinder actuator 84 and retract hydraulic cylinder actuator 86. Simultaneously, flow from line 134 moves through cross-pilot lines 118, 122 to open block valves 94, 98.When this occurs, the fluid at the rod end of hydraulic cylinder actuator 84 and the fluid at the cylinder end of hydraulic cylinder actuator 86 return along lines 140, 138, and 132 to the swing valve 174 (Figure 7) and the outlet line 178 (also Figure 7). At the same time, and referring again to Figure 6, the cylinder end of hydraulic cylinder actuator 84 is charged from line 136, thereby extending hydraulic cylinder actuator 84 as hydraulic cylinder actuator 86 retracts. The hydraulic suspension system 170 is suitable for balancing the land vehicle 20 at certain speeds. The oscillation function allows all four wheels 26 to maintain contact with the ground 22 to maintain stability. At low speeds, the settling of the suspension system 170 may not follow the variations in vehicle dynamics, which can cause discomfort to an operator. Low speeds can be 0.4 meters per second (m / s) and lower or even slower. This low-speed control dynamics occur when the retractable hydraulic cylinder actuator 84 or 86 is subjected to a high load while the land vehicle 20 is moving very slowly (at 0.4 m / s and less) over an obstacle.When the swing valve 174 opens, the hydraulic cylinder actuators 84 and 86 extend or retract rapidly. However, there is a hysteresis delay before the swing valve 174 closes again, resulting in a jump in position and imparting inertia to the lifting structure of the ground vehicle 20. When the ground vehicle 20 travels at higher speeds (greater than 0.4 m / s), the swing motion is not significantly faster than required by the terrain, so the swing system cannot over-rotate and cause dynamic effects while the swing valve 174 remains open. At low forward speeds, the swing exceeds the speeds required to follow the terrain irregularities, and it over-rotates and closes the swing valve 174, which subsequently opens again with the forward movement, resulting in an uncomfortable cyclic motion. Referring again to Figure 7, the hydraulic suspension system 170 is provided with a suspension controller 180. The suspension controller 180 is in electrical communication with a vehicle controller 158 of the ground vehicle 20. The suspension controller 180 can be incorporated within the vehicle controller 158 according to an alternative embodiment. The vehicle controller 158 provides a vehicle travel speed to the suspension controller 180. The vehicle controller 158 can provide the vehicle travel speed from a speed sensor on the ground vehicle 20. The hydraulic suspension system 170 includes a pressure reducing valve 182 between the supply line 172 and the swing valve 174. The pressure reducing valve 182 provides a high pressure, such as 600 psi, to the swing valve 174 when the drive speed is high enough to avoid dynamic effects, such as greater than 0.15 m / s. The pressure reducing valve 182 provides a reduced pressure, such as 520 psi, to the swing valve 174 at lower travel speeds, such as 0.4 m / s or less, to limit dynamic effects. Figure 8 illustrates a portion of a suspension system 190 as a circuit diagram according to another embodiment. According to one embodiment, the suspension system 190 is a fluid suspension system 190. According to another embodiment, the suspension system 190 is a hydraulic suspension system 190. The hydraulic suspension system 190 receives pressurized hydraulic fluid from a functional manifold 78, as illustrated in the above embodiments. A supply line 192 provides fluid communication of the hydraulic fluid from the functional manifold 78 to the hydraulic suspension system 190. The hydraulic suspension system 190 includes a flow control valve, called the swing valve 194. The inlet element 74 of the rear axle assembly 58 in Figure 3 is connected to the swing valve 194. The swing valve 194 detects the pivot direction of the rear axle 62 based on the translation of the inlet element 74. The swing valve 194 is a directional control valve unit with a partially extended neutral position when the rear axle 62 is in a balanced position. In the neutral position, the swing valve 194 does not direct the flow. Further extension of the inlet element 74 indicates a pivot direction, allowing flow in one direction. Retraction of the inlet element 74 from the neutral position indicates pivoting in the opposite direction, thus allowing fluid to flow in another direction.Although one input element 74 is illustrated and described, any number of input elements 74 can be used. Swing valve 194 controls the flow between the functional manifold circuit 78 and the swing cylinder circuits 88 and 90, each of which is in cooperation with one of the hydraulic cylinder actuators 84 and 86. The swing cylinder circuits 88 and 90 are illustrated in more detail in Figure 6. Flow lines 132 and 134 connect the shut-off valves 92, 94, 96, and 98 of the swing cylinders 88 and 90 to swing valve 194. The hydraulic suspension system includes a sequential valve 196 in series with the supply line 192. Flow is permitted from port 198 to port 200 until the downstream pressure reaches a setpoint of 650 psi. Once the downstream pressure reaches 650 psi, the sequential valve 196 blocks the downstream flow and redirects the flow to an outlet line 202 and back to the reservoir inside the functional manifold 78. The swing valve 194, when not in its centered locking position, functions to connect the pressurized supply line 192 of the functional manifold 78 to line 132 or 134 in the suspension system 190, and simultaneously connects the outlet line 202 to whichever of lines 132 or 134 is not connected to supply line 192. Referring again to Figure 6, when line 132 is pressurized, check valves 102 and 106 are not seated, and pressurized fluid flows through bypass lines 110 and 114 and lines 140 and 138 to retract hydraulic cylinder actuator 84 and extend hydraulic cylinder actuator 86. Simultaneously, flow from line 132 moves through cross-pilot lines 116, 120 to open the locking valves 92, 96.When this occurs, the fluid at the cylinder end of hydraulic cylinder actuator 84 and the fluid at the rod end of hydraulic cylinder actuator 86 return along lines 136, 142, and 134 to the swing valve 194 (Figure 8) and the outlet line 202 (also Figure 8). At the same time, and referring again to Figure 6, the rod end of hydraulic cylinder actuator 84 is charged from line 140, thereby retracting the hydraulic cylinder actuator as hydraulic cylinder actuator 86 extends. When swing valve 194 is moved in the opposite direction, so that line 134 is connected via swing valve 194 to supply line 192 instead of outlet line 202, and so that line 134 is connected via swing valve 194 to outlet line 202 instead of supply line 192, the result is that check valves 100, 104 are not seated and pressurized fluid flows through bypass lines 108, 112 and lines 136, 142 to extend hydraulic cylinder actuator 84 and retract hydraulic cylinder actuator 86. Simultaneously, flow from line 134 moves through cross pilot lines 118, 122 to open block valves 94, 98.When this occurs, the fluid at the rod end of hydraulic cylinder actuator 84 and the fluid at the cylinder end of hydraulic cylinder actuator 86 return along lines 140, 138, and 132 to the swing valve 194 (Figure 81) and outlet line 202 (also Figure 8). At the same time, and referring again to Figure 6, the cylinder end of hydraulic cylinder actuator 84 is charged from line 136, thereby extending hydraulic cylinder actuator 84 as hydraulic cylinder actuator 86 retracts. The hydraulic suspension system 190 is suitable for balancing the land vehicle 20 at certain speeds. The oscillation function allows all four wheels 26 to maintain contact with the ground 22 to maintain stability. At low speeds, the settling of the suspension system 190 may not follow the variations in vehicle dynamics, which can cause discomfort to an operator. Low speeds can be 0.4 meters per second (m / s) and lower or even slower. This low-speed control dynamics occur when the retractable hydraulic cylinder actuator 84 or 86 is subjected to a high load while the land vehicle 20 is moving very slowly (at 0.4 m / s and less) over an obstacle.When the swing valve 194 opens, the hydraulic cylinder actuators 84 and 86 extend or retract rapidly. However, there is a hysteresis delay before the swing valve 194 closes again, resulting in a jump in position and imparting inertia to the lifting structure of the ground vehicle 20. When the ground vehicle 20 travels at higher speeds (greater than 0.4 m / s), the swing motion is not significantly faster than required by the terrain, so the swing system cannot over-rotate and cause dynamic effects while the swing valve 194 remains open. At low forward speeds, the swing exceeds the speeds required to follow the terrain irregularities, and it over-rotates and closes the swing valve 194, which subsequently reopens with the forward movement, resulting in an uncomfortable cyclic motion. Referring again to Figure 8, the hydraulic suspension system 190 is provided with a suspension controller 204. The suspension controller 204 is in electrical communication with a vehicle controller 158 of the ground vehicle 20. The suspension controller 204 can be incorporated within the vehicle controller 158 according to an alternative embodiment. The vehicle controller 158 provides a vehicle travel speed to the suspension controller 204. The vehicle controller 158 can provide the vehicle travel speed from a speed sensor on the ground vehicle 20. The hydraulic suspension system 190 includes a pressure reducing valve 206 between the supply line 192 and the swing valve 194. The pressure reducing valve 206 provides a high pressure, such as 600 psi, to the swing valve 194 when the drive speed is high enough to prevent dynamic effects, such as greater than 0.15 m / s. The pressure reducing valve 206 provides a reduced pressure, such as 520 psi, to the swing valve 194 at lower travel speeds, such as 0.4 m / s or less, to limit dynamic effects. Although several embodiments are described above, these embodiments are not intended to describe all possible forms of the invention. Rather, the words used in the specification are descriptive rather than limiting, and it is understood that various changes can be made without departing from the nature and scope of the invention. Furthermore, features of various implementation embodiments can be combined to form additional embodiments of the invention. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. A fluid suspension system for a land vehicle, comprising: at least one actuator adapted to be connected to a chassis and axle of a land vehicle separate from a pivoting axle connection; a fluid pressure circuit cooperating with the actuator(s);and a controller in operational communication with the fluid pressure circuit and programmed to: receive indicative input of a ground vehicle travel speed, adjust the fluid pressure circuit to limit fluid flow or reduce fluid pressure in a low travel speed range, in order to allow the axle to pivot in response to variations in an underlying bearing surface, and adjust the fluid pressure circuit in a higher travel speed range for selective actuation of the actuator(s) or higher fluid pressure actuation of the actuator(s), in response to variations in the underlying bearing surface.

2. The fluid suspension system of claim 1, wherein the axle is further defined as a first axle; and wherein the fluid suspension system further comprises a flow control valve in fluid cooperation with the fluid pressure circuit and a second axle that is connected with the chassis via permissible pivoting.

3. The fluid suspension system of claim 2, further comprising a solenoid valve in fluid cooperation with a pressurized fluid source and the flow control valve.

4. The fluid suspension system of claim 3, further comprising a pressure reducing valve in fluid cooperation with the solenoid valve and in communication with the controller.

5. The fluid suspension system of claim 4, wherein the pressure reducing valve is in fluid communication between a pressurized fluid source and the solenoid valve.

6. The fluid suspension system of claim 4, wherein the controller is programmed to actuate the pressure reducing valve in order to reduce the fluid pressure in a displacement speed range of 0.4 meters per second or less.

7. The fluid suspension system of claim 4, wherein the controller is programmed to actuate the pressure reducing valve in order to increase the fluid pressure in a displacement speed range of 0.13 meters per second or higher.

8. The fluid suspension system of claim 3, further comprising a sequential valve in fluid cooperation with the solenoid valve to block fluid flow at a predetermined pressure.

9. A land vehicle, comprising: a chassis; an axle connected with the chassis, pivoting as permitted, about a horizontal geometric axis perpendicular to the axle; a pair of wheels mounted on the axle and separated by the pivoting connection between them in order to support the axle and the chassis for movement on an underlying support surface; and at least one actuator connected to the chassis and the axle, separated by the pivoting connection; a fluid pressure circuit cooperating with the actuator(s);and a controller in operational communication with the fluid pressure circuit and programmed to: receive indicative input of a ground vehicle travel speed, adjust the fluid pressure circuit to limit fluid flow or reduce fluid pressure in a low travel speed range, in order to allow the axle to pivot in response to variations in the underlying bearing surface, and adjust the fluid pressure circuit in a higher travel speed range for selective actuation of the actuator(s) or higher fluid pressure actuation of the actuator(s), in response to variations in the underlying bearing surface.

10. The land vehicle of claim 9, further comprising a speed sensor cooperating with the land vehicle to determine the travel speed of the land vehicle and communicating with the controller to provide the indicative travel speed input.

11. The land vehicle of claim 9, wherein the axle is further defined as a first axle; wherein the land vehicle further comprises: a second axle connected with the permitted pivoting to the chassis about the horizontal geometric axis perpendicular to the second axle and separated from the first axle, a second pair of wheels mounted on the second axle and separated by the pivoting connection of the second axle and the chassis between them in order to support the second axle and the chassis to move on the underlying support surface, and a pressure reducing valve in fluid cooperation with the fluid pressure circuit and the second axle.

12. The land vehicle of claim 11, further comprising a solenoid valve in fluid cooperation with a pressurized fluid source and the pressure reducing valve.

13. The land vehicle of claim 12, further comprising a sequential valve in fluid cooperation with the solenoid valve to block fluid flow at a predetermined pressure.

14. The land vehicle of claim 9, wherein the axle is further defined as a first axle; and wherein the fluid pressure circuit further comprises a flow control valve in fluid cooperation with the fluid pressure circuit and a second axle that is connected with the permitted pivoting to the chassis.

15. The land vehicle of claim 14, further comprising a solenoid valve in fluid cooperation with a pressurized fluid source and the flow control valve.

16. The land vehicle of claim 15, further comprising a pressure reducing valve in fluid cooperation with the solenoid valve and in communication with the controller.

17. The land vehicle of claim 16, wherein the pressure reducing valve is in fluid communication between a pressurized fluid source and the solenoid valve.

18. The land vehicle of claim 16, wherein the controller is programmed to actuate the pressure reducing valve in order to reduce the fluid pressure in a travel speed range of 0.4 meters per second or less.

19. The land vehicle of claim 16, wherein the controller is programmed to actuate the pressure reducing valve in order to increase the fluid pressure in a travel speed range of 0.13 meters per second or higher.

20. A land vehicle, comprising: a chassis; a first axle connected with the chassis by permissible pivoting about a horizontal geometric axis perpendicular to the first axle; a second axle connected with the chassis by permissible pivoting about the horizontal geometric axis perpendicular to the second axle and separate from the first axle; a first pair of wheels mounted on the first axle and separated by the pivoting connection between the first axle and the chassis in order to support the first axle and the chassis for movement on an underlying support surface; a second pair of wheels mounted on the second axle and separated by the pivoting connection between the second axle and the chassis in order to support the second axle and the chassis for movement on the underlying support surface; a speed sensor for determining a travel speed of the land vehicle; a pressurized fluid source;a first actuator connected to the chassis and the first shaft separate from the pivot connection; a second actuator connected to the chassis and the first shaft separate from the pivot connection and the first actuator; a pressure reducing valve in fluid communication with the pressurized fluid source; a solenoid valve in fluid cooperation with the pressure reducing valve;and a controller in operational communication with the pressure reducing valve and programmed to: receive an input indicative of the ground vehicle's travel speed from the speed sensor, actuate the pressure reducing valve to reduce fluid pressure in a low travel speed range, in order to allow the axle to pivot in response to variations in the underlying bearing surface, and actuate the pressure reducing valve in a higher travel speed range for higher fluid pressure actuation of the first and second actuators, in response to variations in the underlying bearing surface.

21. The land vehicle of claim 20, wherein the controller is programmed to actuate the pressure reducing valve in order to reduce the fluid pressure in a travel speed range of 0.4 meters per second or less.

22. The land vehicle of claim 20, wherein the controller is programmed to actuate the pressure reducing valve in order to increase the fluid pressure in a travel speed range of 0.13 meters per second or higher.

23. A fluid suspension system for a land vehicle, comprising: at least one actuator adapted to be connected to a chassis and axle of a land vehicle separate from a pivoting axle connection; a fluid pressure circuit cooperating with the actuator(s); and a controller in operational communication with the fluid pressure circuit and programmed to: receive input indicative of a travel speed of the land vehicle, close the fluid pressure circuit to limit fluid flow in a low travel speed range, in order to allow the axle to pivot in response to variations in an underlying support surface, and open the fluid pressure circuit in a higher travel speed range for selective actuation of the actuator(s), in response to variations in the underlying support surface.

24. The fluid suspension system of claim 23, wherein the axle is further defined as a first axle; and wherein the fluid suspension system further comprises a flow control valve in fluid cooperation with the fluid pressure circuit and a second axle that is connected with the permitted pivoting to the chassis.

25. The fluid suspension system of claim 24, further comprising a solenoid valve in fluid cooperation with a pressurized fluid source and the flow control valve.

26. The fluid suspension system of claim 25, wherein the solenoid valve closes in response to the low travel speed range.

27. The fluid suspension system of claim 26, wherein the solenoid valve opens in response to the higher displacement speed range.

28. The fluid suspension system of claim 25, further comprising a flow limiter in fluid cooperation with the pressurized fluid source and the flow control valve.

29. The fluid suspension system of claim 28, wherein the flow limiter is in fluid communication in parallel with the solenoid valve.

30. The fluid suspension system of claim 25, wherein the solenoid valve is a normally open valve.

31. The fluid suspension system of claim 25, wherein the solenoid valve is a normally closed valve.

32. A land vehicle, comprising: a chassis; an axle connected with the chassis, pivoting freely, about a horizontal geometric axis perpendicular to the axle; a pair of wheels mounted on the axle and separated by a pivoting connection between them in order to support the axle and the chassis for movement on an underlying support surface; and at least one actuator connected to the chassis and the axle, separated by a pivoting connection; a fluid pressure circuit cooperating with the actuator(s);and a controller in operational communication with the fluid pressure circuit and programmed to: receive indicative input of a ground vehicle travel speed, close the fluid pressure circuit to limit fluid flow in a low travel speed range, in order to allow the first axle to pivot in response to variations in the underlying support surface, and open the fluid pressure circuit in a higher travel speed range for selective actuation of the actuator(s), in response to variations in the underlying support surface.

33. The land vehicle of claim 32, further comprising a speed sensor cooperating with the land vehicle to determine the travel speed of the land vehicle and communicating with the controller to provide the indicative travel speed input.

34. The land vehicle of claim 33, wherein the axle is further defined as a first axle; wherein the land vehicle further comprises: a second axle connected with the permitted pivoting to the chassis about the horizontal geometric axis perpendicular to the second axle and separated from the first axle, a second pair of wheels mounted on the second axle and separated by the pivoting connection of the second axle and the chassis between them in order to support the second axle and the chassis to move on the underlying support surface, and a flow control valve in fluid cooperation with the fluid pressure circuit and the second axle.

35. The land vehicle of claim 34, further comprising a solenoid valve in fluid cooperation with a pressurized fluid source and a flow control valve. 5 5 36. The land vehicle of claim 35, further comprising a flow limiter in fluid cooperation with the pressurized fluid source and the flow control valve.

37. The land vehicle of claim 36, wherein the flow limiter is in fluid communication in parallel with the solenoid valve.

38. A land vehicle, comprising: a chassis; a first axle connected with the chassis by permissible pivoting about a horizontal geometric axis perpendicular to the first axle; a second axle connected with the chassis by permissible pivoting about the horizontal geometric axis perpendicular to the second axle and separated from the first axle; a first pair of wheels mounted on the first axle and separated by the pivoting connection between the first axle and the chassis in order to support the first axle and the chassis for movement on an underlying support surface; a second pair of wheels mounted on the second axle and separated by the pivoting connection between the second axle and the chassis in order to support the second axle and the chassis for movement on the underlying support surface; a speed sensor for determining a travel speed of the land vehicle; a pressurized fluid source;a first actuator connected to the chassis and the first shaft separate from the pivot connection; a second actuator connected to the chassis and the first shaft separate from the pivot connection and the first actuator; a flow control valve in fluid cooperation with the second shaft; a solenoid valve in fluid cooperation with the pressurized fluid source and the flow control valve; a flow limiter in fluid cooperation with the pressurized fluid source and the flow control valve, the fluid cooperation of the flow limiter being in parallel with the solenoid valve;and a controller in operational communication with the solenoid valve and programmed to: receive an input indicative of the ground vehicle's travel speed from the speed sensor, close the solenoid valve to limit fluid flow in a low travel speed range, in order to allow the first shaft to pivot in response to variations in the underlying bearing surface, and open the solenoid valve and selectively actuate the first actuator and the second actuator in a higher travel speed range, in response to variations in the underlying bearing surface.

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