Abstract:
A vehicle roll control system comprises a torsion bar and a first arm extending-substantially perpendicular to the torsion bar. The first arm is fixed to the torsion bar at one end and connectable to one of the axles at the other end. A hydraulic actuator is attached to the torsion bar; and a control connected to the hydraulic actuator controls the operation thereof on detection of a predetermined vehicle condition. The hydraulic actuator comprises a housing, a piston making a sealing sliding fit inside the housing to define a first fluid chamber and a second fluid chamber, and a piston rod connected to the piston and extending through the second fluid chamber and out of the housing. The control acts on detection of the predetermined vehicle condition alternatively (1) to apply substantially the same fluid pressure to the first and second fluid chambers when the piston tends to move in a first direction to extend the hydraulic actuator, or (2) to apply a fluid pressure to the second fluid chamber above the fluid pressure in the first fluid chamber when the piston tends to move in a second direction to compress the hydraulic actuator. The control comprises a first directional valve and a second directional valve, each directional valve being capable of moving between first and second positions either to tend to move the piston in the first direction or to tend to move the piston in the second direction.

Description:
RELATED APPLICATIONS 
     This application is related to applications U.S. Ser. No. 09/721,459, Vehicle Roll Control System, and U.S. Ser. No. 09/721,443, Hydraulic Actuator for a Vehicle Roll Control System, both filed on the same day as this application and assigned to the same assignee. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a roll control system for a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     GB-A-2230237 discloses a roll control system comprising a torsion bar, a first arm fixedly connected to one end of the torsion bar, and a second arm rotatably connected to the other end of the torsion bar by way of a rotary actuator. The rotary actuator is operable to effect relative angular movement between the second arm and the said other end of the torsion bar. This arrangement is such that the actuator has to generate a large amount of force in order to provide the required roll control. 
     GB-A-2284184 describes a roll control system in which a hydraulic cylinder is used to prevent or allow rotation of an arm attached to a torsion bar between the torsion bar and an axle of a wheel. This arrangement provides a limited amount of roll control. 
     EP-A-0783986 describes an arrangement which is similar in layout to GB-A-2284184 but in which the hydraulic actuators are powered to provided active roll control for the vehicle. EP-A-0512358 describes a twin-axle roll control system which makes use of an attitude sensor for controlling roll. 
     SUMMARY OF THE INVENTION 
     The vehicle roll control system of this invention comprises a torsion bar, a first arm attached to the torsion bar at one end of the first arm and connectable to one of the axles at the other end of the first arm, a hydraulic actuator attached to the torsion bar, and control means connected to the hydraulic actuator and controlling the operation thereof on detection of a predetermined vehicle condition. The hydraulic actuator comprises a housing, a piston making a sealing sliding fit inside the housing to define a first fluid chamber and a second fluid chamber, and a piston rod connected to the piston and extending through the second fluid chamber and out of the housing. The control means acts on detection of the predetermined vehicle condition alternatively (1) to apply substantially the same fluid pressure to the first and second fluid chambers when the piston tends to move in a first direction to extend the hydraulic actuator, or (2) to apply a fluid pressure to the second fluid chamber above the fluid pressure in the first fluid chamber when the piston tends to move in a second direction to compress the hydraulic actuator. The control means comprises a first directional valve and a second directional valve, with each directional valve being capable of moving between first and second positions so as to tend to move the piston in the first direction or to tend to move the piston in the second direction. The system provides active roll control with reduced hydraulic fluid flow which reduces the risk of cavitation in the fluid, and/or reduces the difference between response times during extension and compression of the hydraulic actuators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic presentation of a vehicle incorporating a vehicle roll control system in accordance with the present invention; 
     FIG. 2 is an enlarged view of the front and rear portions of the vehicle roll control system shown in FIG. 1; 
     FIG. 3 is a side view of the first arm of the vehicle roll control system shown in FIG. 2; 
     FIG. 4 is a side view of the second arm, hydraulic actuator (shown in cross-section) and lever arm of the vehicle roll control system shown in FIG. 2; 
     FIG. 5 is a schematic diagram of the hydraulic and electrical control circuit of the vehicle roll control system shown in FIG. 1 when the control valves are de-energised; 
     FIG. 6 is a schematic diagram of the hydraulic and electrical control circuit of FIG. 5 when one of the control valves is actuated such that the pistons of the hydraulic actuators are moving in one direction; 
     FIG. 7 is a schematic diagram of the hydraulic and electrical control circuit of FIG. 5 when the other of the control valves is actuated such that the pistons of the hydraulic actuators are moving in the other direction; 
     FIG. 8 is a schematic diagram of a first alternative arrangement for the hydraulic circuit of a vehicle roll control system in accordance with the present invention when the control valves are de-energised; 
     FIG. 9 is a schematic diagram of a second alternative arrangement for the hydraulic circuit of a vehicle roll control system in accordance with the present invention when the control valves are de-energised; 
     FIG. 10 is a schematic diagram of a third alternative arrangement for the hydraulic circuit of a vehicle roll control system in accordance with the present invention when the control valves are de-energised; 
     FIG. 11 is a schematic diagram of a fourth alternative arrangement for the hydraulic circuit of a vehicle roll control system in accordance with the present invention when the control valves are de-energised; 
     FIG. 12 is a view of a portion of a vehicle roll control system in accordance with a second embodiment of the present invention; 
     FIG. 13 is a view of a vehicle roll control system in accordance with a third embodiment of the present invention; 
     FIG. 14 is a cross-section view of the hydraulic actuator of the vehicle roll control system of FIG. 13; 
     FIG. 15 is a cross-sectional view of an alternative embodiment of hydraulic actuator for the vehicle roll control system of FIG. 13; and 
     FIG. 16 is a cross-sectional view of a further alternative embodiment of hydraulic actuator for the vehicle roll control system of FIG.  13 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a vehicle  10  is shown schematically and comprises a pair of front wheels  12  each rotatably mounted on an axle  14 , a pair of rear wheels  16  each rotatably mounted on an axle  18 , and a shock absorbing system  20  associated with each wheel. A portion  22  of a vehicle roll control system in accordance with the present invention is associated with the front wheels  12 , and a portion  24  of the vehicle roll control system in accordance with the present invention is associated with the rear wheels  16 . The portions  22 ,  24  are substantially the same but with modifications made solely to allow fitting to the vehicle  10 . 
     Referring in more detail to FIGS. 2 to  6 , the portion  22  of the vehicle roll control system for the front of the vehicle comprises a torsion bar  26 , a first arm  28 , a second arm  30 , a lever arm  32 , and a hydraulic actuator  34 . The torsion bar  26  is mounted on the vehicle by a pair of resilient mounts  36  in conventional manner to extend longitudinally between the wheels  12 . The first arm  28  (FIG. 3) is fixed at one end  38  by a splined connection  40  to the torsion bar  26 . The other end  42  of the first arm  28  is connected to the axle  14  of one of the front wheels  12  by a tie rod  43 . The second arm  30  (FIG. 4) is rotatably mounted at one end  44  on the torsion bar  26  by way of a bearing  46 . The other end  48  of the second arm  30  is connected to the axle  14  of the other front wheel  12  by a tie rod  49 . The first and second arms  28 , 30  extend substantially parallel to one another when the vehicle is stationary, and substantially perpendicular to the torsion bar  26 . 
     The lever arm  32  (FIG. 4) is fixed at one end  50  to the torsion bar  26  by a splined connection  52  substantially adjacent the one end  44  of the second arm  30  and the bearing  46 . The lever arm  32  extends substantially perpendicular to the torsion bar  26  to a free end  54 . The hydraulic actuator  34  (FIG. 4) extends between, and is connected to, the free end  54  of the lever arm  32  and the other end  48  of the second arm  30 . The hydraulic actuator  34  comprises a housing  56  which defines first and second fluid chambers  58 , 60  separated by a piston  62  which makes a sealing sliding fit with the housing. As shown in FIG. 4, the housing  56  is connected to the other end  48  of the second arm  30 , and the piston  62  is connected to the free end  54  of the lever arm  34  by a piston rod  64  which extends through the second fluid chamber  60 . It will be appreciated that these connections may be reversed. The fluid chambers  58 , 60  contain hydraulic fluid and are fluidly connected to fluid lines  66 ,  68  respectively. The portion  24  of 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 hydraulic and electrical control circuit of the vehicle roll control system is shown in FIGS. 5 to  7 . The hydraulic circuit includes a fluid pump  80 , a fluid reservoir  82 , a first directional valve  84 , a second directional valve  86 , and a pressure control valve  88 . Each directional valve  84 , 86  has a first port  90  fluidly connected to the output of pump  80 , a second port  92  fluidly connected to input to the reservoir  82 , a third port  94  fluidly connected to the fluid line  66  and the first fluid chamber  58  of each hydraulic actuator  34 , 34 ′, and a fourth port  96  fluidly connected to the fluid line  68  and the second fluid chamber  60  of each hydraulic actuator. The first directional valve  84  is solenoid actuated and has a de-energised state (FIGS. 5 and 7) in which the ports  90 - 96  are isolated from one another, and an energised or actuated state (FIG. 6) in which the first port  90  is fluidly connected with the third and fourth ports  94 , 96 , and in which the second port  92  is closed or isolated. The second directional valve  86  is solenoid actuated and has a de-energised state (FIGS. 5 and 6) in which the ports  90 - 96  are isolated from one another, and an energised or actuated state (FIG. 7) in which the first port  90  is fluidly connected with the fourth port  96  and the second port  92  is fluidly connected with the third port  94 . The pressure control valve  88  is fluidly connected between the output of the pump  80  and the input of the reservoir  82 . In a preferred arrangement, the pump  80  is driven by the vehicle engine and hence continuously actuated, and the pressure control valve  88  is actuated to adjust the fluid pressure in the hydraulic system between a predetermined minimum pressure and a predetermined maximum pressure. Alternatively, the pump  80  may be driven by an electric motor or any other suitable means, either continuously, or variably (in which case the pressure control valve may be omitted). 
     The electrical control circuit includes an electronic and/or computerised control module  70 . The control module  70  operates the fluid pump  80 , the directional valves  84 , 86 , and the pressure control valve  88 , when required. The control module  70  actuates the valves  84 , 86 , 88  dependent on predetermined vehicle conditions which are determined by signals from one or more sensors, such as a pressure sensor  76  (which detects the presence of fluid pressure in the hydraulic circuit), a lateral g sensor  74  (which monitors the sideways acceleration of the vehicle), a steering sensor  72  (which monitors the steering angle of the front wheels  12 ), a vehicle speed sensor  78 , and/or any other relevant parameter. 
     If the control module  70  detects that roll control is not required (that is, the vehicle is travelling in a straight line), the control module actuates the pressure control valve  88  to provide the predetermined minimum pressure, and de-energises the directional valves  84 , 86 . Fluid can freely flow within the hydraulic system between the pump  80  and the reservoir  82 . As the directional valves  84 , 86  are closed, the actuators  34 , 34 ′ are effectively locked. 
     If the control module  70  detects that roll control is required (due, for example, to cornering of the motor vehicle  10 ), the control module determines if the motion will generate a force F which acts on the piston rod  64  to extend the actuators  34 , 34 ′, or to compress the actuators, in an axial direction. If the former case, the control module  70  actuates the pressure control valve  88  to provide a fluid pressure in the hydraulic system which correlates with the force F, and sets the first directional valve  84  in the actuated position as shown in FIG. 6, but does not actuate the second directional valve  86 , so that the same fluid pressure is generated in each of the fluid chambers  58 , 60  of each actuator  34 , 34 ′. If the latter case, the control module  70  actuates the pressure control valve  88  to provide a fluid pressure in the hydraulic system which correlates with the force F, and sets the second directional valve  86  in the actuated position as shown in FIG. 7, but does not actuate the first directional valve  84 , so that the fluid in the second fluid chamber  60  of each actuator  34 , 34 ′ is pressurised, but the first fluid chamber  58  of each actuator is connected to the reservoir  82 . By restricting connection of the first fluid chamber  58  of each actuator  34 , 34 ′ to the reservoir  82  only during compression of the actuators, the amount of fluid flow within the hydraulic circuit is reduced during roll control. By suitable dimensions for the actuators  34 , 34 ′, the output force from the actuators can be made substantially the same irrespective of the direction of motion of the piston  62 . 
     FIG. 8 shows a first alternative arrangement for the hydraulic circuit in which (in comparison to FIGS. 5 to  7 ) like parts have been given the same reference numeral. In this first alternative, the first directional valve  184  has the first port  90  fluidly connected to the output of pump  80 , the second port  92  fluidly connected to input to the reservoir  82 , the third port  94  fluidly connected to the second port  92  of the second directional valve  186 , and the fourth port  96  fluidly connected to the first port  90  of the second directional valve. The third port  94  of the second directional valve  186  is fluidly connected to the fluid line  66  and the first fluid chamber  58  of each hydraulic actuator  34 , 34 ′, and the fourth port  96  of the second directional valve is fluidly connected to the fluid line  68  and the second fluid chamber  60  of each hydraulic actuator. The first directional valve  184  is solenoid actuated and has a de-energised state in which the first port  90  is fluidly connected with the fourth port  96  and the second port  92  is fluidly connected with the third port  94 , and an energised or actuated state in which the first port  90  is fluidly connected with the third and fourth ports  94 , 96 , and in which the second port  92  is closed or isolated. The second directional valve  186  is solenoid actuated and has a de-energised state in which the ports  90 - 96  are isolated from one another, and an energised or actuated state in which the first port  90  is fluidly connected with the fourth port  96  and the second port  92  is fluidly connected with the third port  94 . When the need for roll control is determined which requires that the actuators  34 , 34 ′ extend in an axial direction, both the first and second directional valves  184 , 186  are energised or actuated. When the need for roll control is determined which requires that the actuators  34 , 34 ′ compress in an axial direction, the first directional valve  184  is de-energised and only the second directional valve  186  is energised or actuated. Other features and operation of this first alternative hydraulic circuit in a vehicle roll control system in accordance with the present invention are as described above in respect of FIGS. 1 to  7 . The first alternative hydraulic circuit of FIG. 8 may also include a relief pressure device  198  fluidly in parallel with the pressure control valve  88  to substantially prevent excessive fluid pressure being generated in the hydraulic system. 
     FIG. 9 shows a second alternative arrangement for the hydraulic circuit in which (in comparison to FIGS. 5 to  7 ) like parts have been given the same reference numeral. In this second alternative, the first directional valve  284  has the first port  90  fluidly connected to the output of pump  80 , the second port  92  fluidly connected to input to the reservoir  82 , the third port  94  fluidly connected to the second port  92  of the second directional valve  286 , and the fourth port  96  fluidly connected to the first port  90  of the second directional valve. The third port  94  of the second directional valve  286  is fluidly connected to the fluid line  66  and the first fluid chamber  58  of each hydraulic actuator  34 , 34 ′, and the fourth port  96  of the second directional valve is fluidly connected to the fluid line  68  and the second fluid chamber  60  of each hydraulic actuator. The first directional valve  284  is solenoid actuated and has a de-energised state in which the first port  90  is fluidly connected with the fourth port  96  and the second port  92  is fluidly connected with the third port  94 , and an energised or actuated state in which the first port  90  is fluidly connected with the third and fourth ports  94 , 96 , and in which the second port  92  is closed or isolated. The second directional valve  286  is solenoid actuated and has a de-energised state in which the first and second ports  90 , 92  are fluidly connected by a flow restriction, and the third and fourth ports  94 , 96  are isolated from one another, and an energised or actuated state in which the first port  90  is fluidly connected with the fourth port  96  and the second port  92  is fluidly connected with the third port  94 . When the need for roll control is determined which requires that the actuators  34 , 34 ′ extend in an axial direction, both the first and second directional valves  284 , 286  are energised or actuated. When the need for roll control is determined which requires that the actuators  34 , 34 ′ compress in an axial direction, the first directional valve  284  is de-energised and only the second directional valve  286  is energised or actuated. Other features and operation of this second alternative hydraulic circuit in a vehicle roll control system in accordance with the present invention are as described above in respect of FIGS. 1 to  7 . 
     FIG. 10 shows a third alternative arrangement for the hydraulic circuit in which (in comparison to FIGS. 5 to  7 ) like parts have been given the same reference numeral. In this third alternative, the first directional valve  384  has the first port  90  fluidly connected to the output of pump  80 , the second port  92  fluidly connected to input to the reservoir  82 , the third port  94  fluidly connected to the second port  92  of the second directional valve  386  and to the fluid line  66  and the first fluid chamber  58  of the rear hydraulic actuator  34 ′, and the fourth port  96  fluidly connected to the first port  90  of the second directional valve and to the fluid line  68  and the second fluid chamber  60  of the rear hydraulic actuator  34 ′. The third port  94  of the second directional valve  386  is fluidly connected to the fluid line  66  and the first fluid chamber  58  of the front hydraulic actuator  34 , and the fourth port  96  of the second directional valve is fluidly connected to the fluid line  68  and the second fluid chamber  60  of the front hydraulic actuator. The first directional valve  384  is solenoid actuated and has a de-energised state in which the first port  90  is fluidly connected with the fourth port  96  and the second port  92  is fluidly connected with the third port  94 , and an energised or actuated state in which the first port  90  is fluidly connected with the third and fourth ports  94 , 96 , and in which the second port  92  is closed or isolated. The second directional valve  386  is solenoid actuated and has a de-energised state in which the first and second ports  90 , 92  are fluidly connected by a flow restriction, and the third and fourth ports  94 , 96  are isolated from one another, and an energised or actuated state in which the first port  90  is fluidly connected with the fourth port  96  and the second port  92  is fluidly connected with the third port  94 . When the need for roll control is determined which requires that the actuators  34 , 34 ′ extend in an axial direction, both the first and second directional valves  384 , 386  are energised or actuated. When the need for roll control is determined which requires that the actuators  34 , 34 ′ compress in an axial direction, the first directional valve  384  is de-energised and only the second directional valve  386  is energised or actuated. When no roll control is required, the front hydraulic actuator  34  is fluidly isolated from the rear hydraulic actuator  34 ′ to effectively lock the front actuator  34 , but allow the rear actuator  34 ′ to float. Such an arrangement provides, where required, additional understeer on the vehicle. Other features and operation of this third alternative hydraulic circuit in a vehicle roll control system in accordance with the present invention are as described above in respect of FIGS. 1 to  7 . In an alternative to the above described third alternative, the flow restriction in the second displacement valve  386  may be omitted and replaced by the relief pressure device  198  of FIG.  8 . 
     FIG. 11 shows a fourth alternative arrangement for the hydraulic circuit in which (in comparison to FIGS. 5 to  7 ) like parts have been given the same reference numeral. In this fourth alternative, the first directional valve  484  has the first port  90  fluidly connected to the output of pump  80 , the second port  92  fluidly connected to input to the reservoir  82 , and the third port  94  fluidly connected to the first port  90  of the second directional valve  486  by way of a pressure actuated one-way valve  400  (which substantially prevents fluid flow to the first directional valve) and to the fluid line  66  and the first fluid chamber  58  of each hydraulic actuator  34 , 34 ′. The second and third ports  92 , 94  of the second directional valve  486  are fluidly connected to the input to the reservoir  82 . The fluid line  68  and the second fluid chamber  60  of each hydraulic actuator  34 , 34 ′, and the first port  90  of the first directional valve  484 , are fluidly connected to the output of the pump  80  by way of a bypass valve  402 . The first directional valve  484  is solenoid actuated and has a de-energised state in which the first port  90  is fluidly isolated and the second port  92  is fluidly connected with the third port  94 , and an energised or actuated state in which the first port  90  is fluidly connected with the third port  94  and the second port  92  is fluidly isolated. The second directional valve  486  is substantially identical to the first directional valve  484  and is also solenoid actuated. The bypass valve  402  is solenoid actuated (and controlled by the control module  70 ) and has a first port  404  fluidly connected to the output of the pump  80 , and a second port  406  fluidly connected to the first port  90  of the first directional valve  484  and the lines  68 . In the de-energised state, the first port  404  and the second port  406  of the bypass valve  402  are fluidly connected by a pressure actuated one-way valve  408  which substantially prevents fluid flow towards the pump  80 . In the energised or actuated state, the first and second ports  404 , 406  of the bypass valve  402  are directly fluidly connected. When the need for roll control is determined which requires that the actuators  34 , 34 ′ extend in an axial direction, the bypass valve  402  and the first directional valve  484  are energised or actuated, and the second directional valve  486  is de-energised. When the need for roll control is determined which requires that the actuators  34 , 34 ′ compress in an axial direction, the first directional valve  484  is de-energised and the second directional valve  486  and the bypass valve  402  are energised or actuated. Other features and operation of this fourth alternative hydraulic circuit in a vehicle roll control system in accordance with the present invention are as described above in respect of FIGS. 1 to  7 . In an alternative arrangement, the one-way valve  408  may be omitted from the bypass valve  402  to leave the first and second ports  404 , 406  fluidly isolated from one another in the de-energised state of the bypass valve. 
     The present invention is also applicable for use with a vehicle roll control system, the front portion  122  of which is as shown in FIG.  12  and the rear portion of which is substantially identical to the front portion. In this embodiment in accordance with the present invention, the front portion  122  comprises a torsion bar  126 , a first arm  128 , and a hydraulic actuator  134 . The first arm  128  is fixed at one end  138  to one end  140  of the torsion bar  126 . The other end  142  of the first arm  128  is connected to one of the shock absorbers  120 . The hydraulic actuator  134  has a piston rod  164  which is fixed to the other end  187  of the torsion bar  126 . The housing  156  of the actuator  134  is connected to the other shock absorber  120 . The hydraulic actuator  134  is substantially the same as the actuator  34  described above with reference to FIGS. 1 to  7 , and has a fluid line  166  connected to a first fluid chamber inside the housing, and another fluid line  168  connected to a second fluid chamber inside the housing. The first and second fluid chambers inside the housing  156  are separated by a piston secured to the piston rod  164 . The fluid lines  166 , 168  for each hydraulic actuator are connected to a hydraulic circuit as shown in FIGS. 5 to  7 , which is controlled by a control circuit as shown in FIGS. 5 to  7 , or any one of the arrangements shown in FIGS. 8 to  11 . The roll control system is operated in substantially the same manner as that described above with reference to FIGS. 1 to  7 , or any one of FIGS. 8 to  11 . 
     The present invention is also applicable for use with a vehicle roll control system as shown in FIG.  13 . In this third embodiment in accordance with the present invention, the system  222  comprises a torsion bar  226 , a first arm  228 , a second arm  228 ′, and a hydraulic actuator  234 . The first arm  228  is fixed at one end  238  to one end  240  of the torsion bar  226 . The other end  242  of the first arm  228  is connected to one of the shock absorbers  220 . The second arm  228 ′ is fixed at one end  238 ′ to the other end  287  of the torsion bar  226 . The other end  242 ′ of the second arm  228 ′ is connected to the other shock absorber  220 ′. The torsion bar  226  is split into first and second parts  290 , 292 , respectively. The first and second parts  290 , 292  of the torsion bar  226  have portions  294 , 296 , respectively, which are axially aligned. The axially aligned portions  294 , 296  are connected by a hydraulic actuator  234 . 
     The hydraulic actuator  234 , as shown in FIG. 14, comprises a cylindrical housing  256  which is connected at one end  239  to the portion  294  of the first part  290  of the torsion bar  226 . The actuator  234  further comprises a rod  241  positioned inside the housing  256 , extending out of the other end  243  of the housing, and connectable to the portion  296  of the second part  292  of the torsion bar  226 . The rod  241  has an external screw thread  249  adjacent the housing  256 . Balls  251  are rotatably positioned in hemispherical indentations  253  in the inner surface  255  of the housing  256  adjacent the screw thread  249 . The balls  251  extend into the screw thread  249 . The rod  241  is slidably and rotatably mounted in the housing  256  at the other end  243  by way of a bearing  259  positioned in the other end  243 . This arrangement allows the rod  241  to rotate about its longitudinal axis relative to the housing  256 , and to slide in an axial direction A relative to the housing. A piston chamber  261  is defined inside the housing  256 . The rod  241  sealing extends into the piston chamber  261  to define a piston rod  264 , and a piston  262  is secured to the end of the piston rod inside the piston chamber. The piston  262  makes a sealing sliding fit with the housing  256  and divides the chamber  261  into a first fluid chamber  258  and a second fluid chamber  260 . The first fluid chamber  258  is fluidly connected to fluid line  266 , and the second fluid chamber  260  is fluidly connected to fluid line  268 . 
     The fluid lines  266 , 268  are connected to a hydraulic circuit as shown in FIGS. 5 to  7 , which is controlled by a control circuit as shown in FIGS. 5 to  7 , or any one of the arrangements shown in FIGS. 8 to  11 . The roll control system  222  is operated in substantially the same manner as that described above with reference to FIGS. 1 to  7 , or any one of FIGS. 8 to  11 . 
     An alternative arrangement for the hydraulic actuator of FIG. 14 is shown in FIG.  15 . In this alternative embodiment, the actuator  334  comprises a cylindrical housing  356  which is connected at one end  339  to the portion  294  of the first part  290  of the torsion bar  226 . The actuator  334  further comprises a rod  341  positioned inside the housing  356 , extending out of the other end  343  of the housing, and connectable to the portion  296  of the second part  292  of the torsion bar  226 . The rod  341  has an external screw thread  349  adjacent the housing  356 . Balls  351  are rotatably positioned in hemispherical indentations  353  in the inner surface  355  of the housing  356  adjacent the screw thread  349 . The balls  351  extend into the screw thread  349 . The rod  341  is slidably and rotatably mounted in the housing  356  at the other end  343  of the housing by way of a bearing  359  positioned in the other end. The rod  341  makes a sliding guiding fit with the inner surface  355  of the housing  356  at its end  341 ′ remote from the second part  292  of the torsion bar  226 . This arrangement allows the rod  341  to rotate about its longitudinal axis relative to the housing  356 , and to slide in an axial direction A relative to the housing. First and second fluid chambers  358 , 360  are defined inside the housing  356 . The rod  341  makes a sealing fit with the inner surface  355  of the housing  356  by way of seal  371  to define a piston  362 . The first fluid chamber  358  is positioned on one side of the piston  362 , and the second fluid chamber  360  is positioned on the other side of the piston. A seal  369  is positioned adjacent the bearing  359 . A portion  364  of the rod  341  defines a piston rod which extends through the second fluid chamber  360 . The first fluid chamber  358  is fluidly connected to fluid line  366 , and the second fluid chamber  360  is fluidly connected to fluid line  368 . The fluid lines  366 , 368  are fluidly connected with one of the hydraulic circuits shown in FIGS. 5 to  11  to actuate the actuator  334 . 
     A further alternative arrangement of hydraulic actuator  334 ′ is shown in FIG.  16 . In this further alternative embodiment, the actuator  334 ′ is substantially the same as the actuator  334  shown in FIG. 15, but without the sliding guiding fit of the free end  341 ′ of the rod  341  with the housing  356 . 
     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 and equal amounts of fluid. 
     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 controlled in unison. It will be appreciated that the hydraulic actuators may be controlled individually, and in certain cases the portion of the roll control system at the rear of the vehicle may be omitted. Also, 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.