Vehicle roll control system

A vehicle roll control system, comprising a front torsion bar(22), attached to the front hydraulic actuator (34) and a rear torsion bar (24) attached to the rear hydraulic actuator (34′). A pressure control valve (99) is fluidly connected between a pressure source (80) and a reservoir (81). A directional valve (82) is fluidly connected between the pressure control valve (99) and the hydraulic actuators (34,34′). A pressure relief valve (83,84) fluidly connects the directional valve to the pressure source (80) or the reservoir (81) and is actuated to create pressure differential between first fluid chambers (58,58′) of the hydraulic actuators (34,34′)and/or to create pressure differential between second fluid chambers (60,60′) of the hydraulic actuators (34,34′).

TECHNICAL FIELD

The present invention relates to a roll control system for a motor vehicle.

BACKGROUND OF THE INVENTION

EP-A-1103395 discloses a vehicle roll control system in which a pair of directional valves and a pressure control valve are used to control the movement of the piston of hydraulic actuators associated with the front and rear axles of a motor vehicle. WO-A-03/093041 discloses a vehicle roll control system in which a pair of pressure control valves and a directional valve are used to control the movement of the piston of hydraulic actuators associated with the front and rear axles of a motor vehicle. In both cases, each hydraulic actuator has a first fluid chamber positioned on one side of the piston, and a second fluid chamber positioned on the other side of the piston. The first fluid chambers of the front and rear hydraulic actuators receive hydraulic fluid at substantially the same pressure; and the second fluid chambers of the front and rear hydraulic actuators receive hydraulic fluid at substantially the same pressure.

SUMMARY OF THE INVENTION

The aim of the present invention is to provide a roll control system which is an improvement to the above mentioned arrangements.

A vehicle roll control system in accordance with the present invention is characterised by the features specified in claim1.

In the present invention, the control means for the hydraulic circuit is capable of providing fluid pressure to the first fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to the first fluid chamber of the rear hydraulic actuator; and/or is capable of providing fluid pressure to the second fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to second fluid chamber of the rear hydraulic actuator.

The present invention provides a system which allows an aggressive roll control strategy and balance strategy which leads to improvements in motion, turning, and stability (braking in turn at high speed). The present invention also provides continuous control between right turn and left turn.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 1, a vehicle10is shown schematically and comprises a pair of front wheels12each rotatably mounted on an axle14, a pair of rear wheels16each rotatably mounted on an axle18, and a shock absorbing system20associated with each wheel. A portion22of a vehicle roll control system in accordance with the present invention is associated with the front wheels12, and a portion24of the vehicle roll control system in accordance with the present invention is associated with the rear wheels16. The portions22,24are substantially the same but with modifications made solely to allow fitting to the vehicle10.

Referring in more detail toFIGS. 2 to 4, the portion22of the vehicle roll control system for the front of the vehicle comprises a torsion bar26, a first arm28, a second arm30, a lever arm32, and a hydraulic actuator34. The torsion bar26is mounted on the vehicle by a pair of resilient mounts36in conventional manner to extend longitudinally between the wheels12. The first arm28(FIG. 3) is fixed at one end38by a splined connection40to the torsion bar26. The other end42of the first arm28is connected to the axle14of one of the front wheels12by a tie rod43. The second arm30(FIG. 4) is rotatably mounted at one end44on the torsion bar26by way of a bearing46. The other end48of the second arm30is connected to the axle14of the other front wheel12by a tie rod49. The first and second arms28,30extend substantially parallel to one another when the vehicle is stationary, and substantially perpendicular to the torsion bar26.

The lever arm32(FIG. 4) is fixed at one end50to the torsion bar26by a splined connection52substantially adjacent the one end44of the second arm30and the bearing46. The lever arm32extends substantially perpendicular to the torsion bar26to a free end54. The front hydraulic actuator34(FIG. 4) extends between, and is connected to, the free end54of the lever arm32and the other end48of the second arm30. The front hydraulic actuator34comprises a housing56which defines first and second fluid chambers58,60separated by a piston62which makes a sealing sliding fit with the housing. As shown inFIG. 4, the housing56is connected to the other end48of the second arm30, and the piston62is connected to the free end54of the lever arm32by a piston rod64which extends through the second fluid chamber60. It will be appreciated that these connections may be reversed. The fluid chambers58,60contain hydraulic fluid and are fluidly connected to fluid lines66,68respectively. The portion24of the vehicle roll control for the rear of the vehicle is substantially the same, but with the components (which are primed) having a different layout. The rear hydraulic actuator34′ is substantially the same as the front hydraulic actuator34.

The hydraulic and electrical control circuit of the vehicle roll control system ofFIGS. 1 to 4is shown inFIG. 5. The hydraulic circuit includes a fluid pump80, a fluid reservoir81, a directional valve82, a first pressure relief valve83, a second pressure relief valve84, and a pressure control valve99. The directional valve82has eight ports85-92. The first pressure relief valve83has three ports93-95. The second pressure relief valve84has three ports96-98. The pressure control valve99is fluidly connected between the pump80and the reservoir81. Fluid filters may be positioned after the pump80and/or before the reservoir81.

The directional valve82has a first port85fluidly connected to the fluid pump80; a second port86fluidly connected to the first port93of the first pressure relief valve83; a third port87fluidly connected to the fluid pump80; a fourth port88fluidly connected to the first port96of the second pressure relief valve84; a fifth port89fluidly connected to the first chamber58′ of the rear actuator34′ by way of fluid line66′; a sixth port90fluidly connected to the second chamber60′ of the rear actuator34′ by way of fluid line68′; a seventh port91fluidly connected to the first chamber58of the front actuator34by way of fluid line66; and an eighth port92fluidly connected to the second chamber60of the front actuator34by way of fluid line68. The directional valve82is solenoid actuated, and has a de-actuated state (shown inFIG. 5) in which the first and second ports85,86are fluidly connected; the third and fourth ports87,88are fluidly connected; the fifth and seventh ports89,91are fluidly connected; and the sixth and eighth ports90,92are fluidly connected. In the actuated state of the directional valve82, the first and eighth ports85,92are fluidly connected; the second and seventh ports86,91are fluidly connected; the third and sixth ports87,90are fluidly connected; and the fourth and fifth ports88,89are fluid connected. In an alternative arrangement, the directional valve82may be hydraulically actuated by first and second pilot (on/off) valves (not shown).

The second port94of the first pressure relief valve83is fluidly connected to the pump80. The third port95of the first pressure relief valve83is fluidly connected to the reservoir81. In the de-actuated state of the first pressure relief valve83(shown inFIG. 5), the first port93is fluidly connected to the third port95, and the second port94is fluidly isolated. In the actuated state of the first pressure relief valve83, the first port93is fluidly connected to the second port94, and the third port95is fluidly isolated.

The second port97of the second pressure relief valve84is fluidly connected to the pump80. The third port98of the second pressure relief valve84is fluidly connected to the reservoir81. In the de-actuated state of the second pressure relief valve84(shown inFIG. 5), the first port96is fluidly connected to the third port98, and the second port97is fluidly isolated. In the actuated state of the second pressure relief valve84, the first port96is fluidly connected to the second port97, and the third port98is fluidly isolated.

The first and second pressure relief valves83,84are preferably solenoid actuated as shown inFIG. 5. Alternatively, the pressure relief valves83,84may be hydraulically actuated by first and second pilot (on/off) valves (not shown).

The pump80may be driven by the vehicle engine and hence continuously actuated. Alternatively, the pump80may be driven by an electric motor or any other suitable means, either continuously, or variably. The pressure control valve99is actuated to adjust the fluid pressure in the hydraulic system between a predetermined minimum pressure and a predetermined maximum pressure. The pressure control valve99is also actuated to adjust the pressure differential between the first and second chambers58,58′,60,60′ of the hydraulic actuators34,34′ respectively (when the directional valve82and pressure relief valves83,84are also actuated as required).

The electrical control circuit includes an electronic and/or computerised control module70. The control module70operates the fluid pump80, the directional valve82, the pressure control valve99, and the pressure relief valves83,84, when required. The control module70actuates the valves82-84,99dependent on predetermined vehicle conditions which are determined by signals from one or more sensors, such as a first pressure sensor76(which detects the fluid pressure associated with the first chamber58of the front hydraulic actuator34), a second pressure sensor77(which detects the fluid pressure associated with the first chamber58′ of the rear hydraulic actuator34′), a third pressure sensor75(which detects the fluid pressure associated with the second chambers60,60′ of the actuators34,34′), a lateral g sensor74(which monitors the sideways acceleration of the vehicle), a steering sensor72(which monitors the steering angle of the front wheels12), a vehicle speed sensor78, and/or any other relevant parameter.

If the control module70detects that roll control is required (due, for example, to cornering of the motor vehicle10), the control module determines if the module has to generate a force F, F′ which acts on the piston rods64,64′ respectively to extend the front and/or rear actuators34,34′, or to compress the front and/or rear actuators, in an axial direction. In the present invention, the force F on the front actuator34may be different from the force F′ on the rear actuator34′ dependent on the actuation of the pressure relief valves83,84; and the value of the pressure differential is set by the pressure control valve99.

In this embodiment, the roll control system can be operated in four different modes when the directional valve82is actuated and the pressure control valve99is actuated. In a first mode, when the first pressure relief valve83is actuated and the second pressure relief valve84is de-actuated, the second fluid chambers60,60′ of the front and rear hydraulic actuators34,34′ are at substantially the same pressure, the first fluid chamber58of the front hydraulic actuator is at a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve83, and the first fluid chamber58′ of the rear hydraulic actuator is at a different pressure. In a second mode, when the first pressure relief valve83is de-actuated and the second pressure relief valve84is actuated, the second fluid chambers60,60′ of the front and rear hydraulic actuators34,34′ are at substantially the same pressure, the first fluid chamber58′ of the rear hydraulic actuator is a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve84, and the first fluid chamber58of the front hydraulic actuator is at a different pressure. In a third mode, when the pressure relief valves83,84are de-actuated, the first fluid chambers58,58′ of the hydraulic actuators34,34′ are at substantially the same pressure, and the second fluid chambers60,60′ of the hydraulic actuators are at substantially the same pressure but at a different pressure to the first fluid chambers. In a fourth mode, when the pressure relief valves83,84are actuated, the second fluid chambers60,60′ of the front and rear hydraulic actuators34,34′ are at substantially the same pressure, the first fluid chamber58of the front hydraulic actuator is at a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve83, and the first fluid chamber58′ of the rear hydraulic actuator is at a pressure which is substantially equal to or less than the pressure in the second chambers dependent on the pressure relief valve84. Also, in this fourth mode, the pressures in the first chambers58,58′ may be different from one another dependent on the pressure relief valves83,84. In all of the above modes, the value of any pressure differential is control by the pressure control valve99and the pressure relief valves83,84. This arrangement provides improvement management of the compression or expansion of the hydraulic actuators, and hence provides improved roll control of the vehicle.

FIG. 6illustrates a first alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 6is a modification of the hydraulic circuit shown inFIG. 5, in which changes have been made to the directional valve82. In this alternative, in the de-actuated state of the directional valve82, the fifth, sixth, seventh and eighth ports89-92are fluidly isolated from one another and from the other ports85-88. The operation of this first alternative is substantially the same as the operation of the arrangement shown inFIG. 5.

FIG. 7illustrates a second alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 7is a modification of the hydraulic circuit shown inFIG. 5, in which changes have been made to the directional valve482. In this alternative, the directional valve482comprises six ports, a first port485, a second port486, a third port487, a fourth port488, a fifth port489and a sixth port490. The first port485is fluidly connected to the pump80. The second port486is fluidly connected to the first port93of the first pressure relief valve83. The third port487is fluidly connected to the first port96of the second pressure relief valve84. The fourth port488is fluidly connected to the first fluid chamber58′ of the rear actuator34′. The fifth port489is fluidly connected to the first fluid chamber58of the front actuator34. The sixth port490is fluidly connected to the second fluid chambers60,60′ of the front and rear actuators34,34′. In the de-actuated state of the directional valve482, the first, second, third and sixth ports485-487,490are fluidly isolated from one another and from the other ports488,489(which are fluidly connected). The operation of this second alternative is substantially the same as the operation of the arrangement shown inFIG. 5.

FIG. 7Aillustrates a third alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 7Ais a modification of the hydraulic and electrical circuit shown inFIG. 7, in which changes have been made to the operation of the directional valve482′. In this alternative, the directional valve482′ is hydraulically actuated (rather than solenoid actuated) by first and second pilot (on/off) valves491,492. The pilot valve491,492are fluidly connected in series between the pump80and the reservoir81. The actuator493for the directional valve482′ is fluidly connected to the flow path between the pilot valves491,492. The operation of the pilot valves491,492is controlled by the control module70. Other than the actuation of the directional valve482′ by the pilot valves491,492, the operation of this third alternative is substantially the same as the operation of the arrangement shown inFIG. 5.

FIG. 8illustrates a fourth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 8is a modification of the hydraulic circuit shown inFIG. 7, in which changes have been made to the directional valve482. In this alternative, in the de-actuated state of the directional valve482, the first, second and third ports485-487are fluidly connected to one another. The operation of this fourth alternative is substantially the same as the operation of the arrangement shown inFIG. 5.

FIG. 8Aillustrates a fifth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 8Ais a modification of the hydraulic and electrical circuit shown inFIG. 8, in which changes have been made to the operation of the directional valve482′. In this alternative, the directional valve482′ is hydraulically actuated (rather than solenoid actuated) by first and second pilot (on/off) valves491,492. The pilot valve491,492are fluidly connected in series between the pump80and the reservoir81. The actuator493for the directional valve482′ is fluidly connected to the flow path between the pilot valves491,492. The operation of the pilot valves491,492is controlled by the control module70. Other than the actuation of the directional valve482′ by the pilot valves491,492, the operation of this fifth alternative is substantially the same as the operation of the arrangement shown inFIG. 5.

FIG. 9illustrates a sixth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 9is a modification of the hydraulic circuit shown inFIG. 7, in which changes have been made to the directional valve482. In this alternative, in the de-actuated state of the directional valve482, the second and third ports485-487are fluidly connected to one another, whilst the first and sixth ports485,490are fluidly isolated. The operation of this sixth alternative is substantially the same as the operation of the arrangement shown inFIG. 5.

FIG. 9Aillustrates a seventh alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 9Ais a modification of the hydraulic and electrical circuit shown inFIG. 9, in which changes have been made to the operation of the directional valve482′. In this alternative, the directional valve482′ is hydraulically actuated (rather than solenoid actuated) by first and second pilot (on/off) valves491,492. The pilot valve491,492are fluidly connected in series between the pump80and the reservoir81. The actuator493for the directional valve482′ is fluidly connected to the flow path between the pilot valves491,492. The operation of the pilot valves491,492is controlled by the control module70. Other than the actuation of the directional valve482′ by the pilot valves491,492, the operation of this seventh alternative is substantially the same as the operation of the arrangement shown inFIG. 5.

The above-described embodiments all operate in substantially the same way, but provide different hydraulic circuit arrangements for their respective fail-safe modes, as illustrated in the drawings. Also, the selection is dependent on the type of hydraulic actuator that is used.

FIG. 10illustrates an eighth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 10is a modification of the hydraulic circuit shown inFIG. 5, in which the second pressure relief valve has been removed, and a second directional valve184is positioned between the first directional valve82and the pump80and reservoir81.

The second directional valve184has a first port185fluidly connected to the fluid pump80; a second port186fluidly connected to the reservoir81; a third port187fluidly connected to the first port93of the pressure relief valve83; a fourth port188fluidly connected to the reservoir81; a fifth port189fluidly connected to the fourth port88of the first directional valve82; a sixth port190fluidly connected to the third port87of the first directional valve82; a seventh port191fluidly connected to the second port86of the first directional valve82; and an eighth port192fluidly connected to the first port85of the first directional valve82. The second directional valve184is solenoid actuated, and has a de-actuated state (shown inFIG. 10) in which the first and eighth ports185,192are fluidly connected; the second and seventh ports186,191are fluidly connected; the third and sixth ports187,190are fluidly connected; and the fourth and fifth ports188,189are fluidly connected. In the actuated state of the second directional valve184, the first, seventh and eighth ports185,191,192are fluidly connected; the third, fifth and sixth ports187,189,190are fluidly connected; and the second and fourth ports186,188are fluidly isolated.

Also, in this eighth alternative, there are only two pressure sensors76′,77′. The first pressure sensor76′ detects the fluid pressure. associated with the front hydraulic actuator34, and a second pressure sensor77′ detects the fluid pressure associated with the rear hydraulic actuator34′.

In this embodiment, the roll control system can be operated in four different modes when the directional valve82is actuated. In a first mode, when the first pressure relief valve83is de-actuated and the second directional valve184is de-actuated, the first and second fluid chambers58′,60′ of the rear hydraulic actuator34′ and the first fluid chamber58of the front hydraulic actuator34are at substantially the same pressure, and the second fluid chamber60of the front hydraulic actuator is at a different pressure. In a second mode, when the first pressure relief valve83is actuated and the second differential valve184is de-actuated, the first fluid chambers58,58′ of the front and rear hydraulic actuators34,34′ are at substantially the same pressure, the second fluid chamber60of the front hydraulic actuator is at a different pressure to the first fluid chambers, and the second fluid chamber60′ of the rear hydraulic actuator is a pressure which is substantially equal to or less than the pressure in the second chamber of the front hydraulic actuator dependent on the pressure relief valve83. In a third mode, when the pressure relief valve83is de-actuated and the second direction valve184is actuated, the first and second fluid chambers58,60of the front hydraulic actuator34are at substantially the same pressure, and the first and second fluid chambers58′,60′ of the rear hydraulic actuator34′ are at substantially the same pressure but at a different pressure to the fluid chambers of the front actuator. In a fourth mode, when the pressure relief valve83and the second directional valve184are actuated, the first and second fluid chambers58,60of the front hydraulic actuator34are at substantially the same pressure, and the first and second fluid chambers58′,60′ of the rear hydraulic actuator34′ are at substantially the same pressure which is substantially the same as or less than the pressure in the fluid chambers of the front actuator dependent on the pressure relief valve83. In the above modes, the value of any pressure differential is control by the pressure control valve99and the pressure relief valve83. This arrangement provides improvement management of the compression or expansion of the hydraulic actuators, and hence provides improved roll control of the vehicle.

Although the directional valves82,184are shown as solenoid actuated, these valves may, as an alternative, be hydraulically actuated by pilot (on/off) valves.

FIG. 11illustrates a ninth alternative of the hydraulic and electrical control circuit of the vehicle roll control system shown inFIGS. 1 to 4.FIG. 11is a modification of the hydraulic circuit shown inFIG. 11, in which changes have been made to the first directional valve82. In this alternative, in the de-actuated state of the first directional valve82, the fifth, sixth, seventh and eighth ports89-92are fluidly isolated from one another and from the other ports85-88. The operation of this ninth alternative is substantially the same as the operation of the arrangement shown inFIG. 10.

The above-described embodiments ofFIGS. 10 and 11both operate in substantially the same way, but provide different hydraulic circuit arrangements for their respective fail-safe modes, as illustrated in the drawings. Also, the selection is dependent on the type of hydraulic actuator that is used.

In the present invention, in all of the above embodiments, the valves of the hydraulic circuit are actuable to provide fluid pressure to the first fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to the first fluid chamber of the rear hydraulic actuator; and/or actuable to provide fluid pressure to the second fluid chamber of the front hydraulic actuator which is different from the fluid pressure provided to second fluid chamber of the rear hydraulic actuator.

The present invention is also applicable for use with a vehicle roll control system, the front portion122of which is as shown inFIG. 12and the rear portion of which is substantially identical to the front portion. In this embodiment in accordance with the present invention, the front portion122comprises a torsion bar126, a first arm128, and a hydraulic actuator134. The first arm128is fixed at one end138to one end140of the torsion bar126. The other end142of the first arm128is connected to one of the shock absorbers120. The hydraulic actuator134has a piston rod164which is fixed to the other end187of the torsion bar126. The housing156of the actuator134is connected to the other shock absorber120. The hydraulic actuator134is substantially the same as the actuator34described above with reference toFIGS. 1 to 5, and has a fluid line166connected to a first fluid chamber inside the housing, and another fluid line168connected to a second fluid chamber inside the housing. The first and second fluid chambers inside the housing156are separated by a piston secured to the piston rod164. The fluid lines166,168for each hydraulic actuator are connected to a hydraulic circuit as shown inFIG. 5, which is controlled by a control circuit as shown inFIG. 5, or any one of the arrangements shown inFIGS. 6 to 11. The roll control system is operated in substantially the same manner as that described above with reference toFIGS. 1 to 5, or any one ofFIGS. 6 to 11.

The present invention is also applicable for use with a vehicle roll control system as shown inFIG. 13. In this third embodiment in accordance with the present invention, the front portion222of the system comprises a torsion bar226, a first arm228, a second arm228′, and a hydraulic actuator234. The rear portion of the system is substantially identical. The first arm228is fixed at one end238to one end240of the torsion bar226. The other end242of the first arm228is connected to one of the shock absorbers220. The second arm228′ is fixed at one end238′ to the other end287of the torsion bar226. The other end242′ of the second arm228′ is connected to the other shock absorber220′. The torsion bar226is split into first and second parts290,292, respectively. The first and second parts290,292of the torsion bar226have portions294,296, respectively, which are axially aligned. The axially aligned portions294,296are connected by a hydraulic actuator234.

The hydraulic actuator234, as shown inFIG. 14, comprises a cylindrical housing256which is connected at one end239to the portion294of the first part290of the torsion bar226. The actuator234further comprises a rod241positioned inside the housing256, extending out of the other end243of the housing, and connectable to the portion296of the second part292of the torsion bar226. The rod241has an external screw thread249adjacent the housing256. Balls251are rotatably positioned in hemispherical indentations253in the inner surface255of the housing256adjacent the screw thread249. The balls251extend into the screw thread249. The rod241is slidably and rotatably mounted in the housing256at the other end243by way of a bearing259positioned in the other end243. This arrangement allows the rod241to rotate about its longitudinal axis relative to the housing256, and to slide in an axial direction A relative to the housing. A piston chamber261is defined inside the housing256. The rod241sealing extends into the piston chamber261to define a piston rod264, and a piston262is secured to the end of the piston rod inside the piston chamber. The piston262makes a sealing sliding fit with the housing256and divides the chamber261into a first fluid chamber258and a second fluid chamber260. The first fluid chamber258is fluidly connected to fluid line266, and the second fluid chamber260is fluidly connected to fluid line268.

The fluid lines266,268are connected to a hydraulic circuit as shown inFIG. 5, which is controlled by a control circuit as shown inFIG. 5, or any one of the arrangements shown inFIGS. 6 to 11. The roll control system222is operated in substantially the same manner as that described above with reference toFIGS. 1 to 5, or any one ofFIGS. 6 to 11

An alternative arrangement for the hydraulic actuator ofFIG. 14is shown inFIG. 15. In this alternative embodiment, the actuator334comprises a cylindrical housing356which is connected at one end339to the portion294of the first part290of the torsion bar226. The actuator334further comprises a rod341positioned inside the housing356, extending out of the other end343of the housing, and connectable to the portion296of the second part292of the torsion bar226. The rod341has an external screw thread349adjacent the housing356. Balls351are rotatably positioned in hemispherical indentations353in the inner surface355of the housing356adjacent the screw thread349. The balls351extend into the screw thread349. The rod341is sidably and rotatably mounted in the housing356at the other end343of the housing by way of a bearing359positioned in the other end. The rod341makes a sliding guiding fit with the inner surface355of the housing356at its end341′ remote from the second part292of the torsion bar226. This arrangement allows the rod341to rotate about its longitudinal axis relative to the housing356, and to slide in an axial direction A relative to the housing. First and second fluid chambers358,360are defined inside the housing356. The rod341makes a sealing fit with the inner surface355of the housing356by way of seal371to define a piston362. The first fluid chamber358is positioned on one side of the piston362, and the second fluid chamber360is positioned on the other side of the piston. A seal369is positioned adjacent the bearing359. A portion364of the rod341defines a piston rod which extends through the second fluid chamber360. The first fluid chamber358is fluidly connected to fluid line366, and the second fluid chamber360is fluidly connected to fluid line368. The fluid lines366,368are fluidly connected with one of the hydraulic circuits shown inFIGS. 5 to 11to actuate the actuator334.

A further alternative arrangement of hydraulic actuator334′ is shown inFIG. 16. In this further alternative embodiment, the actuator334′ is substantially the same as the actuator334shown inFIG. 15, but without the sliding guiding fit of the free end341′ of the rod341with the housing356.

In a preferred arrangement, the cross-sectional area of the first fluid chamber of each hydraulic actuator described above is substantially double the cross-sectional area of the piston rod of the hydraulic actuator, when considered on a radial basis. Such an arrangement provides the same output force from the hydraulic actuator in either direction, using the same fluid pressure.

In the preferred arrangement described above, a hydraulic actuator is provided for both the front of the vehicle and the rear of the vehicle, and these hydraulic actuators are substantially the same. In an alternative arrangement, the hydraulic actuator for the front of the vehicle may be a different type to the hydraulic actuator for the rear of the vehicle.

In any of the roll control systems described above, the hydraulic actuator may include a check valve (not shown, but preferably mounted in the piston) which allows flow of hydraulic fluid from the first fluid chamber to the second fluid chamber only when the fluid pressure in the first fluid chamber is greater than the fluid pressure in the second fluid chamber. With such an arrangement, the second fluid chamber can be connected to a reservoir during servicing of the actuator to bleed air from the hydraulic fluid. Also, the presence of the check valve reduces the risk of air being sucked into the second fluid chamber should the fluid pressure in the second fluid chamber fall below the fluid pressure in the first fluid chamber, and provides further improvements in ride comfort.