WO2007098559A1

WO2007098559A1 - Hydraulic system for a vehicle suspension

Info

Publication number: WO2007098559A1
Application number: PCT/AU2007/000257
Authority: WO
Inventors: Raymond Andrew Munday
Original assignee: Kinetic Pty Ltd
Priority date: 2006-03-02
Filing date: 2007-03-02
Publication date: 2007-09-07
Also published as: EP1989072A1

Abstract

A hydraulic system for a vehicle suspension for a vehicle has at least one front pitch control ram (11) activated by change in ride height of the front of the vehicle body with respect a forward wheel assembly, and at least one rear pitch control ram (13) activated by a change in ride height of the rear of the vehicle body with respect to a rearward wheel assembly. The front pitch control ram(s) and the rear pitch control ram(s) including at least a compression chamber (45, 46, 47) and a rebound chamber (49, 50, 51). The compression chamber of the front pitch control ram(s) is in fluid communication (53, 54, 59) with the rebound chamber of said at least one rear pitch control ram to provide a front compression volume, and the . compression chamber of the rear pitch control ram(s) is in fluid communication (55, 57, 58) with the rebound chamber of the front pitch control ram(s) to provide a rear compression volume. The hydraulic system provides increased pitch stiffness decoupled from heave stiffness of the suspension system.

Description

HYDRAULIC SYSTEM FOR A VEHICLE SUSPENSION TECHNICAL FIELD

The present invention is generally directed to suspension systems for vehicles, and in particular to a hydraulic system providing control of one or more suspension parameters.

BACKGROUND OF THE INVENTION

There are known many alternative interconnected suspension systems which have the ability to passively differentiate between different modes of wheel motion with respect to the vehicle body and therefore provide a variety of alternatives in functionality. For example, the applicant's international application PCT/AUOO/00312 published as WO 00/61394 discloses a passive system providing high roll stiffness with low warp stiffness and negligible heave stiffness and providing high roll damping with lower, more comfortable and isolating heave damping. As the system does not provide significant heave stiffness, separate support springs are required.

An example of a system having just roll and/or pitch damping can be found in Yamaha's US patent number 5,486,018 and Kayaba's US6,024,366. The system in these documents uses a device between a pair of wheel damping rams, each wheel damping ram having a damper valve in its piston to provide double-acting damping but make the ram single-acting (i.e. there is only fluid port). The device provides for independent levels of damping for in-phase (i.e. heave) and out of phase (i.e. roll and/or pitch) motions. However this system does not provide significant stiffness in any mode, so in addition to the need for support springs, generally anti-roll bars will be required for a good balance between bounce and roll stiffness. Additionally, as the wheel rams are effectively single acting (having only one fluid port) the amount of damping that the device can provide is limited. There are improvements made to the system to combat this problem, which can be found in Japanese patent office publication number 11291737, but these add to the complexity of the system by providing more plumbing and spool valves.

Yamaha also discloses a pitch stiffness and damping system for a motorcycle in Japanese Patent Application Number 59-009418 to reduce squat and dive using a single conduit connection between the front and rear wheel shock absorbers.

An example of systems having roll and/or pitch stiffness is shown in French Patent Number 1.535.641 in which double acting wheel rams are interconnected in pairs, the compression chamber of one ram being connected to the rebound chamber of the other ram of the pair. If the wheel rams are interconnected between laterally spaced wheels, then primarily roll stiffness (and a related warp stiffness) is provided, with a lower heave and related pitch stiffness component. If the wheel rams are interconnected between longitudinally spaced wheels, then primarily pitch stiffness (and a related warp stiffness) is provided, with a lower heave and related roll stiffness component. If the wheel rams are interconnected between diagonally spaced pairs of wheels, then the high stiffness differential motion modes of the rams provide a high roll and pitch stiffness with similar motion modes of the rams (warp and heave) having a lower stiffness. In vehicles with aerodynamic aids, particularly motor racing vehicles, the height of the front and rear of the vehicle and the pitch angle of the vehicle body have a significant effect on the efficiency and stability of the aerodynamic aids. It is therefore important to maintain the front and rear ride heights and particularly the pitch attitude of the vehicle body within a tight operating range. Commonly the front and rear suspension systems of race cars include a "third spring" or "compensating spring" arranged to provide additional two-wheel stiffness for a laterally spaced pair of wheels. This arrangement provides increased heave and pitch stiffness with no additional roll or warp stiffness.

However, decoupling the additional pitch stiffness from any other modal stiffness (heave, roll or warp) can provide the benefit of more stable aerodynamic loads with less detrimental effect on the variation of wheel loads over uneven surfaces.

It is therefore an object of the present invention to provide a hydraulic system for a vehicle suspension that provides higher pitch stiffness than heave stiffness and provides substantially zero roll or warp stiffness.

It is a preferred object of the invention to provide (at least in part) damping of pitch motions of the vehicle decoupled from damping of other vehicle wheel modes to again reduce the detrimental effect on the variation of wheel loads over uneven surfaces. STATEMENT OF INVENTION

With this in mind, according to one aspect of the present invention there is provided a hydraulic system for a vehicle suspension for a vehicle, the vehicle including a vehicle body, at least one forward wheel assembly and at least one rearward wheel assembly, the hydraulic system including: at least one front pitch control ram activated by change in ride height of the front of the vehicle body with respect to the at least one forward wheel assembly, and at least one rear pitch control ram activated by a change in ride height of the rear of the vehicle body with respect to the at least one rearward wheel assembly, the at least one front pitch control ram and the at least one rear pitch control ram each including at least a compression chamber and a rebound chamber; wherein the compression chamber of said at least one front pitch control ram is in fluid communication with the rebound chamber of said at least one rear pitch control ram to provide a front compression volume, and the compression chamber of said at least one rear pitch control ram is in fluid communication with the rebound chamber of said at least one front pitch control ram to provide a rear compression volume, the hydraulic system thereby providing increased pitch stiffness decoupled from heave stiffness of the suspension system.

Thus, advantageously, the hydraulic system is arranged to provide higher pitch stiffness than heave stiffness and provides substantially zero roll or warp stiffness.

It will be appreciated that the front and rear compression volumes may be termed pitch compression volumes since these act as to transfer pressures during pitch motions of the front and rear pitch control rams to the respective rear and front pitch control rams.

In addition, in the hydraulic system, it will also be appreciated that the front and rear (pitch) compression volumes may each be referred to as fluid volumes in acting to transfer fluid pressure between respective compression and rebound chambers. In such an arrangement, the front pitch compression fluid volume is the fluid volume which increases in pressure during braking motions when the front of the vehicle experiences compression forces and the rear of the vehicle experiences rebound forces. Similarly the rear pitch compression fluid volume is the volume which increases in pressure during acceleration motions when the rear of the vehicle experiences compression forces and the front of the vehicle experiences rebound forces. It should be understood that front compression relates to motion of the vehicle where the front ram(s) contract and the rear ram(s) extend. Likewise, rear compression relates to motion of the vehicle where the rear ram(s) contract and the front ram(s) extend.

Resilience may be provided in each (the front and rear compression) fluid volume of the hydraulic system. The resilience may be provided by compliance of the fluid (such as a silicon based fluid) compliance of components of the hydraulic system (such as flexible hoses) and/or by resilient means (such as fluid pressure accumulators). If fluid pressure accumulators are provided, their resilience contribution can be damped by the provision of one or more fluid restrictions or damper valves in the connection between the accumulator and the associated fluid volume.

The vehicle suspension may include front and rear resilient vehicle support means between the vehicle body and the wheel assemblies for resiliently supporting at least a portion of the load of the vehicle above the wheel assemblies. The front and rear resilient vehicle support means may be independent of the hydraulic system, i.e. they may be coil springs which operate in parallel with the hydraulic system.

If the vehicle support means are the primary means of vehicle support, all of the volumes in the hydraulic system can be run at the same pressure. The vehicle support means may support a substantial proportion of the load of the vehicle, and all of the volumes in the hydraulic system may be charged at substantially the same pressure.

Also, as the system contains hydraulic fluid and possibly gas, both of which expand with increasing temperature, a pressure compensation arrangement is required in order to maintain the system static pressure and roll stiffness within a design range over the design temperature. This pressure compensation arrangement can also be used to compensate for any fluid loss over time. Therefore, there is provided a pressure maintenance device connected to each of the system volumes through respective restrictions or valves. For example, the pressure maintenance device may be connected to the front and rear pitch compression fluid volumes. So, additionally or alternatively, at least one fluid pressure accumulator may be connected to at least one fluid volume through a restriction or valve device. The accumulator(s) provides a degree of compensation for any changes in the volume and pressure of fluid in an associated fluid volume due, for example, to leakages and temperature changes. The restriction is determined by compromising between the attitude change after prolonged acceleration for example versus the time taken for pressure equalisation between the fluid volume and the fluid pressure accumulator. If a valve device is used, its operation may be controlled electronically or by any other known means. The fluid pressure accumulator may be replaced with a hydraulic pressure supply system including a pump and reservoir.

The hydraulic system may be applied to a motorcycle to add pitch stiffness to a vehicle having only one front wheel and one rear wheel.

Alternatively, the hydraulic system may be provided on a vehicle having three wheels. In this case there are two main equivalent configurations which suit different wheel geometry styles. Firstly, if the pair of wheels at a first end of the vehicle include a common support means for providing support of at least a portion of the load on said pair of wheels, a single first pitch control ram can be used in place of or in parallel with the support means. For example, on many cars with race-car type push rod suspension support arrangements, a "third spring" is commonly used (to provide bounce support for a laterally spaced pair of wheels whilst providing zero additional roll stiffness) which can be replaced by a pitch control ram.

Secondly, particularly where dictated by packaging, a single pitch control ram can be replaced by a pair of pitch control rams for two wheels at one end of the vehicle. So, for example, at a first end of the vehicle, there may be a laterally spaced pair of wheels, and at a second end of the vehicle there is a single wheel, there may be provided two first pitch control rams and a second pitch control ram. The compression chamber of each first pitch control ram being connected to the compression chamber of the other first pitch control ram and the rebound chamber of the second pitch control ram forming a first pitch compression fluid volume, the rebound chamber of each first pitch control ram being connected to the rebound chamber of the other first pitch control ram and the compression chamber of the second pitch control ram forming a second pitch compression fluid volume. Interconnecting like chambers of rams at the same end of the vehicle in this manner provides pitch control without providing any roll stiffness.

If the second end of the vehicle also has a pair of laterally spaced wheels, then the single pitch control ram can be used, the hydraulic configuration of the system being as above. Alternatively, a pair of second pitch control rams can be used, in which case: the first pitch compression volume includes the compression chambers of each first pitch control ram and the rebound chambers of each second pitch control ram; and the second pitch compression volume includes the compression chambers of each second pitch control ram and the rebound chambers of each first pitch control ram

In any of the above configurations of the hydraulic system, the compression chamber and rebound chamber of the pitch control rams may have the same effective piston area, such as having a dummy rod extending through one of the chambers to match the piston rod extending through the other chamber.

The vehicle may be primarily supported by the resilient vehicle support means.

The vehicle support means may be any known support means such as coil springs, air springs, torsion bars, leaf springs and rubber cones. The vehicle support means can, in the case of coil springs and air springs, be mounted around the pitch control rams or mounted separately. Additional valves may be provided to block the two fluid circuits for failsafe and/or wheel lift lockouts.

The accompanying drawings illustrate preferred embodiments of the present invention. Other arrangements are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an isometric view of a first preferred embodiment of a hydraulic system having pitch stiffness and pitch damping without providing roll or warp stiffness (or damping), according to the present invention; Figure 2 is a schematic view of the first preferred embodiment of the hydraulic system;

Figure 3 is a schematic view of a second preferred embodiment of a hydraulic system according to the present invention, providing the same basic functionality as the first embodiment, but adapted to an alternative packing envelope at one end of the vehicle. A pressure maintenance device is also shown;

Figure 4 is a schematic view of a third preferred embodiment of a hydraulic system according to the present invention, being similar in basic functionality to the first and second embodiments, the connection sequence having been extended to incorporate a fourth pitch control ram, providing further packaging and damping options. An alternative pressure maintenance device is also shown; and

Figure 5 is a schematic view showing variations to the third preferred embodiment of the hydraulic system. DESCRIPTION OF PREFERRED EMBODIMENT

Referring initially to figures 1 and 2, there is shown a suspension system for a vehicle. Two pitch control rams (11 , 13) are located between the vehicle body (1) and the front and rear axles (6, 7) of the vehicle. Although the front and rear axles are shown in Figure 1 , locating two longitudinally spaced sets of wheels (2, 3 and 4, 5) of the vehicle, the front and rear pitch control rams could act on pairs of laterally spaced, independently located wheels, or on a single wheel at one or both ends of the vehicle (such as on a motorcycle). Each pitch control ram includes a cylinder (15, 17) connected to the vehicle body (either directly or through a bushing or any other known means), a piston (19, 21 ) slidably housed within the cylinder, and a rod (23, 25) fixed between the piston and the axle (6, 7) or a wheel hub or other suspension geometry to move with the axle, wheel(s) or suspension geometry respectively. The pitch control rams may substantially support the vehicle, the supporting force applied to the vehicle being a function of the system pressure and the front and rear rod sizes. The relationship between the pitch stiffness and the heave stiffness provided by the hydraulic system is a function of the front and rear rod and bore diameters. The pitch control rams may be any form of fluid displacing device, such as air springs or hydro-pneumatic rams.

Alternatively the system may be used in conjunction with separate resilient support means such as coil springs, etc, it should be understood that the resilient support means may be of any alternative known type, such as for example, air springs and may be located either around the ram, or separate to each pitch control ram, which broadens the alternatives. For example, the separate resilient support means may be two torsion bars connected to the geometry providing wheel location. In some installations, especially when the support means are located around the ram or there are other sources of side-load present on the pitch control ram, the cylinder of the ram may be located inside an outer tube, the rod being fixed to the outer tube. The advantage of using separate resilient support means, other than packaging flexibility, is that they can be chosen in combination with the hydraulic pitch control system and any other vibration control system on the vehicle to give the desired balance between the four modal stiffness rates of heave, pitch, roll and warp, as independent support springs on a four wheeled vehicle provide a stiffness in each of these four modes.

The pitch control rams may be inverted with the cylinder connected to the wheel hub or other suspension geometry and the piston rod connected to the vehicle body. The pitch control system may be used in combination with another vibration control system (in addition to the aforementioned separate resilient vehicle support means), such as separate wheel dampers, roll control systems, etc. If another system is provided it can be advantageous to remove any pitch stiffness that the other system may provide; for example a series of supporting air bags interlinked front to rear of the vehicle in order to remove pitch stiffness from the system. Care should also be taken in the amount of warp stiffness present in any other systems. The pitch control rams shown in the figures are basically conventional double-acting rams for simplicity. Taking the front pitch control ram 11 as an example, the piston 19 (which may be formed as an integral part of the rod 23) has two grooves containing a bearing 39 and a seal 40. In some cases, the individual bearing and seal parts can be replaced by a single item (not shown) which may be bonded to or formed around the piston for ease of assembly and low cost. The cylinder end (41) has three grooves containing a rod seal 42, a bearing 43 and a rod wiper 44 or other form of secondary seal such as an excluder. Each ram therefore has a compression chamber (45, 47) and a rebound chamber (49, 51) formed by the piston (19, 21 ) within each cylinder (15, 17).

The two double-acting rams are connected by one circuit to provide passive de-coupling of pitch stiffness from heave stiffness (and thus do not contribute any roll or warp stiffness to the suspension system). The said circuit comprises two fluid volumes, a front compression volume and a rear compression volume. The front compression volume includes the front compression chamber 45, a front compression conduit 53, a rear rebound conduit 59 and a rear rebound chamber 51. The rear compression volume similarly includes a rear compression chamber 47, a rear compression conduit 55, a front rebound conduit 57 and a front rebound chamber 49.

This simple connection arrangement would displace fluid as follows: in pitch, fluid volumes corresponding to a compression chamber/s and the annular rebound chamber of the longitudinally spaced pitch control ram are displaced (into or out of any resilient device such as an accumulator); whereas in heave, only a fluid volume corresponding to a rod volume (compression chamber minus annular rebound chamber) would be displaced (into or out of the resilient device). The high relative volume displaced in the pitch mode relative to heave gives a higher pitch stiffness than heave stiffness.

It should be noted that the simple arrangement of a single front pitch control ram and a single rear pitch control ram cannot provide a roll or warp stiffness, thus the hydraulic system does not provide roll or warp stiffness to a suspension system. One arrangement for providing resilience in the two compression volumes is to provide a single accumulator (73, 74) for each volume. A higher amount of pitch damping than heave damping can be achieved (since flow into the accumulator in pitch motions is significantly larger) by including at least one pitch damper (75, 76) between each accumulator and the associated compression volume.

To be able to tune the front and rear pitch stiffness, the front and rear pitch control ram sizes and the pitch accumulators can be altered. This can be done both to change the ratio of front to rear pitch stiffness and to provide different pitch stiffness in the acceleration and braking directions. To be able to tune the front and rear pitch damping forces, the front and rear pitch control ram sizes and the pitch damper valves can be altered to affect fore and aft pitch motions at alternative rates. Similarly different damping can be provided in the acceleration and braking directions. The pitch damper valves may also be passive or controlled, the control being of any known form, from a simple switched damper valve to a completely continuously variable damper valve. The simple switched damper valve may be of any known type such as a switchable bypass around each pitch damper valve or a simple controlled bleed orifice. Preferably each damper valve 75 and 76 is a bidirectional damper valve providing damping of fluid both into and out of each accumulator. Ideally, the damping can be tuned independently in each direction of each valve to enable control of the damping of the hydraulic system in the braking and accelerating pitch directions while producing an effect on or minimising the extremes of low and high pressure in each fluid volume. Damper valves may also be provided for each pitch control ram to damp the flow into and/or out of at least one of the compression or rebound chambers. In this case, preferably a uni-directional damper valve is provided for each chamber of each ram to damp fluid as it exits the cylinder, with a non-return or intake valve being provided in parallel with each damper valve to permit free flow of fluid into each chamber of each ram. This permits an alternative front to rear balance between pitch stiffness forces and pitch damping forces.

When the hydraulic system of Figure 2 is applied to a vehicle having (at least at one end of the vehicle) two laterally spaced wheels with independent suspension geometry, the pitch control ram can act on both laterally spaced wheels. For example, vehicle suspension geometry is known wherein left and right wishbones actuate a first end of left and right torsion bar springs, the second end of these torsion bar springs being connected to left and right lever arms, the left and right lever arms being interconnected by a lateral beam or a ram. In this case, the ram may be a pitch control ram and the left and right lever arms may be actuated directly by the wishbones, excluding the torsion bars. Similarly, particularly on race cars using rod and crank mechanism support linkages which actuate the suspension springs, the use of a common central or "third" spring is well know. The pitch control ram can be substituted for the third springs, or used in parallel with the third springs. In this way, the hydraulic system shown in Figure 2 can be used to control the pitch motions of a race car having rod and crank mechanism support linkages and thereby enable more consistent performance of the aerodynamic aids on the race car. To clarify, the operation of the hydraulic system in Figure 2 is basically as follows:

In heave, fluid volumes corresponding to the front and rear rod volumes are displaced into or out of the accumulators. The rod volumes are generally small in relation to the accumulator gas volumes, so the pressure change in heave is also small, which acting over the rod areas, provides little change in wheel force. Therefore the hydraulic system has a very low heave stiffness.

In pure roll and warp motions, there should be no displacement of either the front pitch control ram or the rear pitch control ram. Therefore, due to the mechanical arrangement of the two pitch control ram hydraulic system into a suspension system for a vehicle with 3 or more wheels, the hydraulic system has negligible roll and warp stiffness.

In pitch, for example when braking, the front compression volume increases in pressure (due to fluid from the front compression chamber 45 and the rear rebound chamber 51 entering the accumulator 73) and the rear compression volume reduces in pressure (due to fluid entering the rear compression chambers 47 and the front rebound chamber 49 from the accumulator 74) thereby providing the necessary pitch couple. During pitch motions of the opposite sense, for example when accelerating, fluid flows in the opposite direction to the accumulators 73 and 74. Therefore it is preferable to use different damper characteristics for flow in the two different pitch directions, to thereby give independent control of the braking direction pitch damping and the acceleration direction pitch damping. This is desirable as the pitch velocities, accelerations, peak forces and general requirements (such as trade off between forces for control and comfort) can be very different in the two directions.

Depending on the pitch (stiffness and damping) force distribution of the hydraulic system, the front and rear chamber volume changes may or may not be matched. If the pitch force distribution is close to 50% then the pitch damping provided can be of similar magnitude at both ends of the vehicle. If the pitch force distribution of the hydraulic system is not 50%, or due to geometry or other effects, the pitch dampers may produce a larger magnitude of damping force on one end of the vehicle than the other. Conventional wheel damper valves or pitch control ram damper valves can be used to tune the pitch damping of the hydraulic system to the desired front to rear balance when the pitch force distribution of the hydraulic system does not provide the required balance.

So in pitch, the fluid transferred from front or rear compression volumes to respective accumulators (73 or 74) passes through the associated pitch damper valves (75 or 76) thereby providing pitch damping. The overall pitch damping provided for a given input is therefore dependent on the flow restriction and pressure balance between the pitch damper valves and the accumulators.

As the pitch stiffness distribution of the hydraulic system may not match the pitch stiffness distribution of the suspension system as a whole (due to the coil spring rates, front to rear bushing stiffness differences, etc) it may be necessary to provide different levels of pitch damping on the front wheels compared to the rear wheels. This can be done as shown in figure 2 by using alternative front and rear pitch damping valves 75 and 76.

As discussed, this two pitch control ram system may be applied to a vehicle with two wheels (such as a motorcycle), 3 wheels or more (ATVs₁ Quads, Cars, Trucks, etc.) For a vehicle with 3 or more, wheels, the incorporation of resilient support means (such as a coil spring, air spring or hydro-pneumatic strut between each wheel assembly and the vehicle body) offers the advantage of pitch stiffness completely independent of roll stiffness. The support means can provide the sole source of roll stiffness and/or anti-roll bars or any other known means of providing roll stiffness can be used. Alternatively, the support means can provide substantially no roll stiffness. For vehicles with 4 or more sets of wheels, the use of separate resilient support means offer the further advantage of providing a known warp stiffness independent of the heave and pitch stiffness of the suspension. It is also possible to make the heave stiffness of the hydraulic pitch system zero by having zero displacement of fluid in the accumulators in heave, which can for example be accomplished by using pitch control rams with the same effective piston areas in compression and rebound directions. Referring now to figure 3, there is shown a suspension system for a vehicle similar to that shown in Figures 1 and 2, however two pitch control rams are provided at one end of the vehicle to overcome potential packaging issues associated with the application of the single ram arrangement to some vehicle and geometry types. Although twin pitch control ram arrangements can be applied to either end of the vehicle, in this example, the two pitch control rams (11 , 12) are located towards the front of the vehicle between the vehicle body (not shown) and the suspension geometry of at least one (preferably two transversely opposed) front wheel(s) of the vehicle (not shown). As in the previous figures, a single rear pitch control ram (13) is located between the vehicle body and the suspension geometry of at least one (preferably another pair of transversely opposed) rear wheel(s) of the vehicle longitudinally spaced from the front wheel(s). Each pitch control ram includes a cylinder (15, 16, 17) connected to a wheel hub or other suspension geometry to move with the wheel or suspension geometry respectively, a piston (19, 20, 21 ) slidably housed within the cylinder, and a rod (23, 24, 25) fixed between the piston and the body of the vehicle. The connection of the rod to the vehicle body may be by any known means, usually through a rubber bushing which may include a bearing. As in Figures 1 and 2, the pitch control rams may be inverted.

The pitch control ram (13) at the rear is shown with a dummy rod extending through the compression chamber (47). Using this type of arrangement, the effective piston areas in compression and rebound can be sized as required. For example, the effective piston areas in compression and rebound can be equal. The system is connected and behaves similarly to that of Figures 1 and 2, however, the front rams have like chambers laterally interconnected: the front left and front right compression chambers (45, 46) of the respective pitch control rams (11 , 12) are interconnected to each other through front left and front right compression conduits (53, 54) and via the rear rebound conduit (59) to the rear rebound chamber (51) of the rear pitch control ram (13) forming the front compression fluid volume; and the front left and front right rebound chambers (49, 50) of the respective pitch control rams (11 , 12) are interconnected to each other through front left and front right rebound conduits (57, 58) and via the rear compression conduit (55) to the rear compression chamber (47) of the rear pitch control ram (13) forming the rear compression fluid volume. The connection of the three double-acting rams into one fluid circuit comprising the front compression fluid volume and the rear compression fluid volume provides passive decoupling of pitch stiffness from heave stiffness and does not provide a roll or warp stiffness.

Also shown in Figure 3 is a pressure maintenance system. If the hydraulic system is not the primary means of support (ie the coil springs - or air, torsion, etc. springs provide a large proportion of the vehicle support e.g. 70% or more) both of the volumes in the system (the previously defined front and rear compression volumes) can be operated at a common static pre-charge pressure. The advantage of operating both systems at the same static pre-charge pressure is that pressure differentials across piston seals throughout the system are eliminated and so therefore are pitch attitude changes caused by fluid leakage between the systems, and the need for a powered control system which can pump fluid between the volumes.

The front and rear compression volumes are connected via restrictions (81 , 82) to a common conduit or passage 83 which is in turn connected to a pressure maintenance device 84. Each restriction is typically a micro orifice with filters either side to prevent blockage, although any known restrictive means may be used. The orifice is sized to provide the characteristics required to maintain the pressures in the front and rear compression volumes within an acceptable range whilst preventing significant fluid flow during major pitch motions to maintain the static pitch attitude within an acceptable range when returning to normal minimum pitch motions.

Although the pressure maintenance device 84 may be omitted, changes in the volumes of fluid and gas in the hydraulic system and its accumulators through the operating temperature range of the vehicle are usually large enough to require some form of compensation device. The complexity of this device can vary significantly, depending on the design parameters and the functionality required.

In its simplest form, the pressure maintenance device (84) can be a simple accumulator with any known construction (for example bladder-type with gas spring, piston-type with gas spring or with mechanical spring).

Alternatively the pressure maintenance device (84) can use a fluid pressure source (such as a tank with a pump, or another vehicle system such as the power steering) to maintain the pressure in the hydraulic suspension volumes to either a fixed or a variable pressure. If a fixed pressure is chosen, the components required can be simple, cheap, passive, mechanical parts, however as the system temperature changes, the system stiffness will change slightly. To maintain the system stiffness characteristics constant with varying temperature, the pressure in the systems must be adjusted in dependence on their temperature, which generally requires one or more temperature sensors, at least one variable pressure switch or pressure transducer and an electronic control unit.

Also, the pitch stiffness of the hydraulic suspension system can be adjusted by changing the pressure in the systems, so if a pressure maintenance device (84) with variable pressure set-points is used, the pressure can be varied in dependence on the load in the vehicle and/or by a driver operated mode selector or a variable selector.

If the pressure maintenance device is designed to provide two regulated pressures and is connected to the front and rear compression volumes by independent conduits (with or even without restrictions) then the pressures in the two volumes can be controlled to be different to each other to offset a static pitch load on the vehicle for example due to an offset payload. Alternatively, or additionally, active pitch control components can be added on to the system to provide an offset in the passive pitch angle of the vehicle during pitch motions, or completely compensate for all resilience in the hydraulic system and maintain a zero pitch angle. The components (pump, tank, valves, sensors and controllers for many different algorithms) are all well known. It should be noted that this three pitch control ram system may be applied to a vehicle with two wheels (such as a motorcycle), 3 wheels or more (ATVs, Quads, Cars, Trucks, etc.). For a vehicle with 3 or more wheels, the use of separate resilient support means offers the advantage of pitch stiffness independent of roll stiffness. The support means can provide the sole source of roll stiffness and/or anti-roll bars or any other known means of providing roll stiffness can be used. Alternatively, the support means can provide substantially no roll stiffness. As the pitch control hydraulic system can support the vehicle, only an anti-roll system may be required. For vehicles with 4 or more sets of wheels, separate resilient support means offer the further advantage of providing a known warp stiffness independent of the heave and pitch stiffness of the suspension. It is also possible to make the heave stiffness independent of the pitch stiffness by having zero displacement of fluid in the accumulators in heave. This may be partially achieved by using a ram design at one end of the vehicle (or completely achieved by using a ram design at both ends of the vehicle) which has similar effective piston areas for the compression and rebound chambers. An example of a two wheeled, three pitch control ram system is that of a (pedal or motor powered) cycle with front forks (two rams on opposite sides of the front wheel) that are interconnected and furthermore interconnected to a rear pitch control ram. The suspension system can have two pitch control rams at each end of the vehicle as shown in Figure 4. The operation of this four pitch control ram system is similar to that of the systems in Figures 1 to 3. As in Figure 3, the two front pitch control rams (11 , 12) are located between the vehicle body (not shown) and the suspension geometry of at least one (preferably two transversely opposed) front wheel(s) of the vehicle (not shown). Similarly the two rear pitch control rams (13, 14) are located between the vehicle body and the suspension geometry of at least one (preferably two transversely opposed) rear wheel(s) of the vehicle. The additional rear pitch control ram is a rear left pitch control ram 14 having similar construction to the other pitch control rams, a cylinder 18, a rod 26 connected to a piston 22 slidably located inside the cylinder forming a rear left pitch control compression chamber 48 and a rear left pitch control rebound chamber 52.

The system is connected and behaves similarly to that of the previous figures, the compression chambers of the pitch control rams at one end of the vehicle are connected to each other and to the rebound chambers of the pitch control rams at the opposite end of the vehicle. More specifically, the front left and front right compression chambers (45, 46) of the respective pitch control rams (11 , 12) are interconnected to each other through front left and front right compression conduits (53, 54) and to the rear right and rear left rebound chambers (51 , 52) via the rear right and rear left rebound conduits (59, 60) and the front compression longitudinal conduit (69) forming a front compression fluid volume. Similarly, the rear right and rear left compression chambers (47, 48) of the respective pitch control rams (13, 14) are interconnected to each other through rear right and rear left compression conduits (55, 56) and to the front left and front right rebound chambers (49, 50) via the front left and front right rebound conduits (57, 58) and the rear compression longitudinal conduit (70) forming a rear compression fluid volume. The connection of the four double-acting rams into one fluid circuit comprising the front compression fluid volume and the rear compression fluid volume provides passive decoupling of pitch stiffness from heave stiffness and does not provide a roll or warp stiffness.

The direct damping of each pitch control ram can be accomplished by the provision of compression (61-64) and rebound (65-68) damper valves mounted on conduits (53-60) close to the compression and rebound chambers of each pitch control ram. Indeed damper valves can be provided to directly damp the fluid flow into and/or out of any of the pitch control rams in any of the two three of four ram embodiments described herein. These damper valves can be single- acting, working on restricting fluid flow out of either a compression or a rebound chamber, or they may be double acting, in which case only one valve (on either the compression or preferably on the rebound chamber) may be utilised. These damper valves can be located in the pitch control ram body where there is package space, or attached to the pitch control ram body or in the conduits as shown. Damping of the hydraulic system can alternatively be provided by damper valves in any of the conduits of the hydraulic system to enable any inter- ram mount, tyre or other compliance induced resonances to be reduced or controlled.

A basic version of the pressure maintenance device is shown in Figure 4 utilising a simple fluid pressure accumulator. As in any other forms of the pressure maintenance system, the restrictions (81 , 82) to the front and rear compression volumes can be replaced by valves, the valves typically being low flow rate electronically control solenoid valves. The valves can also be mechanically controlled (such as spool valves operated by a pendulum reactive to longitudinal acceleration). The valves can be high flow and operated to selectively remove any pitch stiffness effect from the suspension system.

It should be noted that this four pitch control ram system may be applied to a vehicle with two wheels (such as a motorcycle), 3 wheels or more (ATVs, Quads, Cars, Trucks, etc.) For a vehicle with 3 or more wheels, the use of separate resilient support means offers the advantage of pitch stiffness independent of roll stiffness, although it can still be desirable to provide separate support means in any 2, 3 or 4 wheeled and 2, 3 or 4 pitch control ram versions of the system.

Figure 5 shows independent support means in the form of coil springs (87, 88, 89, 90) mounted between the body and wheels (not shown) in parallel with the pitch control rams. The support means can provide the sole source of roll stiffness and/or anti-roll bars or any other known method of providing roll stiffness can be used. Alternatively, the support means can provide substantially no roll stiffness. As the pitch control hydraulic system can support the vehicle, only an anti-roll system may be required. For vehicles with 4 or more sets of wheels, separate resilient support means offer the further advantage of providing a known warp stiffness independent of the heave and pitch stiffness of the suspension. Alternatively or additionally roll stiffness can be provided by separate roll control means such as anti-roll bars and/or by a front to rear interconnected anti-roll system having zero warp stiffness or any other known form of roll control. It is also possible to make the heave stiffness independent of the pitch stiffness by having zero displacement of fluid in the accumulators in heave. This may be partially achieved by using a ram design at one end of the vehicle (or completely achieved by using a ram design at both ends of the vehicle) which has similar effective piston areas for the compression and rebound chambers.

Figure 5 also shows further modifications to figure 4 including an optional front and rear volume connecting lockout valve (or "bridge valve" 93) which can also be operated to selectively remove any pitch stiffness from the hydraulic system. This valve and function can be combined with the pressure maintenance system valves or applied in parallel with the pressure maintenance restrictions or valves. An optional bridge damper (94) is shown in series with the bridge valve to permit some pitch damping to remain even when the pitch stiffness is removed through the opening of the bridge valve.

Front to rear lockout valves (91 , 92) are also provided and can be used for safety (for example to break the system into four fluid volumes to reduce the effect of fluid loss from one point due to a failure) and/or deliberate changes in stiffness rates and/or in stiffness balance. If these front to rear lockout valves are provided, preferably at least two additional accumulators (not shown) are provided, one in fluid communication with the front rebound chambers and conduits and one in fluid communication with the rear rebound chambers and conduits. These additional accumulators can optionally include damper valves to damp the flow of fluid between the system volumes and the accumulator. The additional accumulators can be provided for any embodiment of the system (including those with only two or three pitch control rams).

Throughout all of the preceding drawings, each damper valve indicated can be either: a single damper valve having the same characteristics in both directions; a single valve having different characteristics from one direction of fluid flow to the other; a single valve having flow restriction characteristics in one direction and being relatively free-flowing in the opposite direction; two single- acting valves, one damper valve to control the restriction to flow in one direction and a second damper valve to control the restriction to flow in the opposite direction, the two valves being used in parallel, or in series with a non-return valve in parallel with each valve as is known in conventional damper valve technology.

Also as is well known, any of the damper valves can be of any known construction. For example some damper or all damper valves can be passive devices such as simple orifice restrictions, multi-stage dampers using stacks of shims part of which can be preloaded, blow-off damper valves using preloaded springs such as coil springs or Belleville washers or any other known construction of passive damper valve. Additionally or alternatively, some or all of the damper valves can be controlled to vary or switch the damping characteristics between different valves, different low flow restrictions, varying blow-off preloads or any other known adjustment of the damping characteristics.

The drawings should not be taken as limiting the possible location of the damper valves. For example, the front left compression damper valve 61 does not have to be located in the front left compression conduit, it can be formed into the end of the compression chamber as is known in existing hydro-pneumatic wheel ram designs.

In each of the figures the basic arrangement may be altered in that the accumulators can be positioned anywhere along the associated volumes and similarly the pressure maintenance may be connected to any point along each volume.

The lockout valves can be provided in alternative positions to those shown in Figure 5 and can be applied to the two or three pitch control ram embodiments of the invention. As mentioned previously, the valves can be used in the event of a detected loss of fluid pressure in one of the fluid volumes, or the fluid pressure not matching a mapped fluid pressure for the dynamic conditions on a vehicle, ie in the event of a system failure or malfunction.

An alternative to the bridge valve method of changing the pitch stiffness of the hydraulic system is to use additional accumulators which can be connected to the hydraulic system through switchable lockout valves. This allows for the pitch stiffness to be switched between a 'high and a low setting. These settings are generally either side of what would be chosen as a multi-purpose single setting, so that the system can provide improved control with high stiffness and improved comfort with low stiffness, as selected or controlled automatically by any known means (acceleration sensors, throttle and brake sensors, position sensors, etc). These additional accumulators can be provided with damper valves to damp the fluid flowing into or out of the hydraulic system. An alternative to hydraulically switching a whole accumulator in and out of the systems, is to use an accumulator design with two gas volumes, then simpler, cheaper gas switching valves can be used to vary the gas volumes available to the systems by switching the lock-out valve to between the two gas volumes to isolate one of the volumes.

To minimise unnecessary actions of the pressure maintenance device, the hydraulic system components can be sized to ensure that the pitch force distribution is matched to the typical distribution of loads experienced by the vehicle in normal operation. For example if a vehicle spends significant time pitching due to squat caused by acceleration or due to differing front to rear aerodynamic loads, if the pitch force distribution is matched to this load change ratio front to rear, then the pressure maintenance device will not be making unnecessary changes to the pressure in the two system volumes.

Many other alterations to the basic arrangement of the components whilst maintaining the connection sequence essential for the functionality of the hydraulic system are considered to fall within the scope of this application. For example, in a four pitch control ram system the conduit packaging may permit shorter conduit lengths and lower fluid volumes if longitudinally spaced rams are cross connected and then lateral conduits can be added to connect the two pairs of fluid volumes formed to achieve the same connection sequence between pitch ram chambers as in Figures 4 and 5. Similarly, it will be appreciated that, in a production design of the system, it is possible to incorporate not only the wheel damper valves (53-60) into the main body of the wheel ram, but also the accumulators, roll and pitch valves. For example, the front left wheel ram may include the wheel damper valves 53 and 57, the accumulator 73, the roll damping valves 75 and 78 and the pitch damping valve 71. It will be appreciated that, while the present invention provides modal pitch stiffness and damping, there is little or no roll stiffness from the hydraulic system. Although roll stiffness can be added by conventional means, such as anti-roll bars, for vehicles with at least four wheels it can be advantageous to provide roll stiffness using a modal roll system that provides roll stiffness and little or no pitch stiffness. Roll stiffness can be provided by separate roll control means such as anti-roll bars and/or by a front to rear interconnected anti-roll system having zero warp stiffness or any other known form of roll control. Although the dummy rod has only been illustrated for the single rear pitch control ram in figure 3, dummy rods can be used in the ram or rams at either or both ends of the vehicle.

It should be understood that although the term wheels has been used through the specification, they may be any form of surface engaging means such as skis, tracks, floats for engaging any commonly traversed surface such as tarmac or other road or pavement, mud, sand, water, snow or ice.

Claims

CLAIMS:

1. A hydraulic system for a vehicle suspension for a vehicle, the vehicle including a vehicle body, at least one forward wheel assembly and at least one rearward wheel assembly, the hydraulic system including: at least one front pitch control ram activated by change in ride height of the front of the vehicle body with respect to the at least one forward wheel assembly, and at least one rear pitch control ram activated by a change in ride height of the rear of the vehicle body with respect to the at least one rearward wheel assembly, the at least one front pitch control ram and the at least one rear pitch control ram each including at least a compression chamber and a rebound chamber; wherein the compression chamber of said at least one front pitch control ram is in fluid communication with the rebound chamber of said at least one rear pitch control ram to provide a front pitch compression volume, and the compression chamber of said at least one rear pitch control ram is in fluid communication with the rebound chamber of said at least one front pitch control ram to provide a rear pitch compression volume, the hydraulic system thereby providing increased pitch stiffness decoupled from heave stiffness of the suspension system.

2. A hydraulic system according to claim 1 , wherein the front pitch compression volume is a fluid volume which increases in pressure during braking motions when the front of the vehicle experiences compression forces and the rear of the vehicle experiences rebound forces and the rear pitch compression volume is a fluid volume which increases in pressure during acceleration motions when the rear of the vehicle experiences compression forces and the front of the vehicle experiences rebound forces.

3. A hydraulic system according to any one of the preceding claims, further including resilience in each of the pitch compression volumes of the hydraulic system.

4. A hydraulic system according to claim 3, wherein the resilience is provided by compliance of system fluid, compliance of components of the hydraulic system, and/or by resilient means such as one or more fluid pressure accumulators, or combinations thereof.

5. A hydraulic system according to claim 4, wherein, when one or more fluid pressure accumulators are provided, a resilience contribution therefrom is damped by provision of one or more fluid restrictions or damper valves in a connection between the accumulator and an associated compression or fluid volume.

6. A hydraulic system according to any one of the preceding claims, wherein the vehicle suspension includes front and rear resilient vehicle support means between the vehicle body and the wheel assemblies for resiliently supporting at least a portion of the load of the vehicle above the wheel assemblies.

7. A hydraulic system according to claim 6, wherein at least the front or rear resilient support means includes first support means providing roll stiffness.

8. A hydraulic system according to claim 6, wherein at least the front or rear resilient support means includes second support means providing substantially zero roll stiffness.

9. A hydraulic system according to claim 6, 7 or 8, wherein the vehicle support means supports a substantial proportion of the load of the vehicle and all of the volumes in the hydraulic system are charged at substantially the same pressure.

10. A hydraulic system according to any one of the preceding claims, further including a pressure compensation arrangement.

11. A hydraulic system according to claim 10, wherein the pressure compensation arrangement maintains system static pressure and roll stiffness within a design range over a design temperature.

12. A hydraulic system according to claim 10 or 11 , wherein the pressure compensation arrangement compensates for system fluid loss over time.

13. A hydraulic system according to any one of claims 10 to 12, wherein the pressure compensation arrangement includes a pressure maintenance device connected to each of the system volumes through respective restrictions or valves.

14. A hydraulic system according to claim 13, wherein the pressure maintenance device is connected to the front and rear pitch compression fluid volumes.

15. A hydraulic system according to claim 13 or 14, wherein the pressure maintenance device includes at least one fluid pressure accumulator.

16. A hydraulic system according to claim 15, wherein the at least one fluid pressure accumulator is connected to at least one respective fluid volume through at least one restriction or valve device.

17. A hydraulic system according to claim 13 or 16, wherein the pressure maintenance device includes a hydraulic pressure supply system including a pump and reservoir.

18. A hydraulic system according to any one of the preceding claims, the system configured for a two wheeled vehicle, wherein the system includes pitch stiffness to a single front wheel and single rear wheel of vehicle.

19. A hydraulic system according to any one of the preceding claims, the system configured for a three or more wheeled vehicle having at least a pair of wheels at a first end of the vehicle and at least one wheel at a second end of the vehicle.

20. A hydraulic system according to any one of the preceding claims, the system configured for a four wheeled vehicle having at least a pair of transversely spaced wheels at a first end of the vehicle and at least a pair of transversely spaced wheels at a second end of the vehicle.

21. A hydraulic system according to any one of the preceding claims, wherein the compression chamber and rebound chamber of the pitch control rams have substantially the same effective piston areas.

22. A hydraulic system according to claim 21 , wherein at least one of the pitch control rams includes a dummy rod extending through one of the chambers to match the piston rod extending through another of the chambers.

23. A hydraulic system according to any one of the preceding claims, wherein the vehicle is primarily supported by resilient support means, such as coil springs, air springs, torsion bars, leaf springs, rubber cones or combinations of one or more thereof.

24. A hydraulic system according to claim 23, wherein the resilient support means includes one or more members mounted around the pitch control rams or mounted separately with respect thereto.

25. A hydraulic system according to any one of the preceding claims, including one or more valves provided to block the two fluid circuits for failsafe and/or wheel lift lockout.

26. A hydraulic system according to any one of the preceding claims, wherein the front and rear pitch compression volumes are interconnected by a bridge or lockout valve to control fluid flow between the front and rear pitch compression volumes.

27. A hydraulic system according to claim 26, including a damper valve in series with the bridge or lockout valve to damp flow between the front and rear pitch compression volumes.

28. A multi wheeled vehicle including a hydraulic system according to any one of the preceding claims.