Abstract:
Improvements for a vehicle stabilizer bar assembly having a pair of stabilizer bar members that are selectively uncoupled via a clutch assembly to provide a vehicle with improved traction. A first improvement concerns a reserve powering apparatus for powering the clutch assembly in the event of an electrical failure that prevents the vehicle power source from transmitting electrical power to the clutch assembly. A second improvement concerns a speed sensing apparatus for inhibiting the operation of the stabilizer bar assembly in a disengaged condition if the speed of the vehicle is greater than or equal to a predetermined speed threshold. A third improvement concerns a logic for controlling the engagement and disengagement of a pair of stabilizer bar assemblies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/273,779 filed Mar. 3, 2001 and of commonly assigned co-pending U.S. patent application Ser. No. 09/663,613 filed Sep. 18, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention generally relates to a semi-active anti-roll suspension system having a pair of independently mounted stabilizer bar halves, which may be coupled to one another to control vehicle roll and more particularly to an apparatus and method for controlling the coupling of the stabilizer bar halves.  
         BACKGROUND OF THE INVENTION  
         [0003]    Traditional vehicle suspension systems include resilient devices, such as coil springs and leaf springs, to flexibly support a portion of a vehicle and enable all of the wheels to maintain contract with the ground when traversing uneven terrain. Segregating the vehicle into unsprung and sprung portions in this manner is also useful for preventing severe impulsive forces from being transmitted to the vehicle occupants.  
           [0004]    It is known that when vehicle travels around a corner, centrifugal forces acting on the vehicle tend to cause the sprung portion of the vehicle to roll. In severe instances, the effects of roll can cause instability and impede the ability of the driver to control the vehicle. Although the effects of roll are more pronounced with vehicles having a comparatively high center of gravity, such as vans or trucks, every vehicle is affected by roll.  
           [0005]    In tuning the ride and handling of a vehicle, it is often desirable to soften or lower the spring rate of the suspension&#39;s springs to provide a softer, less harsh ride. One of the main drawbacks associated with this approach is that a suspension system having springs with a relatively low spring rate permits the vehicle body to roll at a relatively higher rate. Accordingly, it would seem that the combination of springs with a very low spring rate and a relatively stiff stabilizer bar would optimize both the ride and handling of the vehicle.  
           [0006]    The relatively stiff stabilizer bar, however, tends to directly connect the vehicle wheels such that the motion of one wheel is copied to another wheel. If a vehicle so equipped was to strike a bump with one wheel, for example, the upward force (i.e., jounce) imparted to that wheel would be transmitted through the stabilizer bar to the opposite wheel, causing the opposite wheel to move in an upward direction. This “cross-talk” between the vehicle wheels is highly undesirable as it adversely affects vehicle ride.  
           [0007]    Another drawback of stabilizer bars is that their torsional stiffness inhibits the free travel of the vehicle wheels. Modern materials and design techniques have substantially reduced the weight of the vehicle wheels and mounting structures to such an extent that the weight of a wheel and its mounting structure is typically insufficient to cause the stabilizer bar to rotate. While this problem is rarely, if ever, noticed on the relatively flat surfaces of modern roads, it can become apparent when the vehicle is operated over un-even terrain. In some situations, it is possible for one of the vehicle wheels to remain in an elevated position over a dip in the terrain due to the torsional resistance of the stabilizer bar. While situations of this severity are not routinely encountered, the fact remains that the stabilizer bar reduces vehicle traction in some situations.  
           [0008]    One solution to the above-mentioned drawbacks is disclosed in copending and commonly assigned U.S. patent application Ser. No. 09/663,613 entitled “Semi-Active Anti-Roll System”, the disclosure of which is hereby incorporated by reference as if fully set forth herein. This system employs a pair of stabilizer bar members, which are selectively uncoupled via a clutch assembly to improve vehicle traction in some situations. Despite the numerous advantages of this system, it is susceptible to improvement.  
           [0009]    For example, it is highly desirable to provide a clutch controller for causing the stabilizer bar members to couple in response to a fault in the transmission of electrical power to the clutch assembly. Configuration in this manner may compromise vehicle traction in some situations, but effectively ensures that the vehicle will not be operated at high rates of speed with the stabilizer bar members disconnected.  
           [0010]    In another example wherein a pair of these stabilizer bar systems are incorporated into a vehicle for the front and rear axles of the vehicle, it is highly desirable to control the coupling and disconnecting of the stabilizer bar members so as to improve vehicle stability and eliminate any roll-over steering effect that is generated as a result of the method by which the stabilizer members are connected and disconnected.  
         SUMMARY OF THE INVENTION  
         [0011]    In one preferred form, the present invention provides a reserve powering apparatus for powering a clutch assembly in the event of an electrical failure, which prevents the vehicle power source from transmitting electrical power to the clutch assembly.  
           [0012]    In another preferred form, the present invention provides a speed sensing apparatus for inhibiting the operation of the stabilizer bar assembly in a disengaged condition if the speed of the vehicle is greater than or equal to a predetermined speed threshold.  
           [0013]    In yet another preferred form, the present invention provides a methodology for controlling the engagement and disengagement of a pair of stabilizer bar assemblies.  
           [0014]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein:  
         [0016]    [0016]FIG. 1 is a perspective view of an exemplary independent front wheel suspension into which the stabilizer bar assembly of the present invention is incorporated;  
         [0017]    [0017]FIG. 2 is an exploded perspective view of the stabilizer bar assembly of FIG. 1;  
         [0018]    [0018]FIG. 3 is a longitudinal cross-section of a portion of the stabilizer bar assembly taken along the line  3 - 3 ;  
         [0019]    [0019]FIG. 4 is a front view of a portion of the clutch assembly illustrating the coupling member and the second gear member in greater detail;  
         [0020]    [0020]FIG. 5 is a longitudinal cross-section similar to that of FIG. 3 but illustrating an alternately constructed clutch assembly;  
         [0021]    [0021]FIG. 6 is a schematic illustration of the clutch controller of the present invention;  
         [0022]    [0022]FIG. 7 is a schematic illustration of an exemplary vehicle constructed in accordance with another embodiment of the present invention; and  
         [0023]    [0023]FIG. 8 is a schematic illustration in flowchart form of the methodology of the present invention for controlling the operation of the front and rear stabilizer bar assemblies. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    With reference to FIG. 1 of the drawings, an exemplary independent front wheel suspension, generally indicated by reference numeral  10 , of the type having suspension components at each wheel, which are suspended from the vehicle frame. Reference will be made to a vehicle frame in the present disclosure, yet those skilled in the art will recognize that many current vehicles do not have a frame as such, but instead have regions of the body, which act as an integrated frame structure. With this in mind, frame  12  is shown to partially include a pair of longitudinal side rails  14  and a crossbeam  16 .  
         [0025]    At each wheel, suspension  10  includes a lower control arm  18  and an upper control arm  20  which are both pivotally attached to frame  12 . A strut assembly having a helical coil spring  22  and a strut damper  24  is retained between an intermediate portion of lower control arm  18  and frame  12  to support the weight of the vehicle body and any loads which are transmitted through lower control arm  18 . Upper control arm  20  is connected to lower control arm  18  by a steering knuckle  26 . A hub and rotor assembly  28  is rotatably attached to a spindle portion (not shown) of steering knuckle  26  such that a wheel and tire (also not shown) may be mounted thereon. Suspension system  10  further includes an anti-roll system  29  comprised of a stabilizer bar assembly  30  and a pair of end links  32  which connect the ends of stabilizer bar assembly  30  to lower control arms  18 . Stabilizer bar assembly  30  is shown to include a pair of stabilizer bar members  40   a ,  40   b  and a clutch assembly  42 . Each of the stabilizer bar members  40   a ,  40   b  is shown to be generally L-shaped having a central segment  44 , which extends laterally across the vehicle and an arm segment  46 , which extends longitudinally along the vehicle at the distal end of central segment  44 . Each of the central segments  44  is rotatably attached to frame rails  14  by a mounting bracket  48 . The opposite end of each arm segment  46  is connected to a corresponding one of the lower control arms  18  by one of the end links  32 .  
         [0026]    In FIGS. 2 and 3, the exemplary clutch assembly  42  provided is illustrated to include a housing assembly  60 , first and second gear members  62  and  64 , respectively, a coupling member  66  and an actuator assembly  68 . Housing assembly  60  includes a pair of housing members  70  which cooperate to define a central cavity  72  into which the central segments  44  of the stabilizer bar members  40   a ,  40   b  are received.  
         [0027]    First and second gear members  62  and  64  are illustrated to include a journal portion  76 , an external gear portion  78  and an internal spline portion  80 . Journal portion  76  is supported for rotation by an associated bearing  82  in central cavity  72 . The outer face  84  of each external gear portion  78  sized to engage an associated thrust washer  86  disposed between the housing member  70  and the external gear portion  78  to prevent contact therebetween. Internal spline portion  80  is sized to receive an external spline portion  90  formed into the proximal end of a corresponding one of the stabilizer bar members  40   a ,  40   b . Meshing engagement of the external spline portion  90  of stabilizer bar member  40   a  and the internal spline portion  80  of first gear member  62  couples stabilizer bar member  40   a  and first gear member  62  for rotation about the of stabilizer bar members  40   a ,  40   b . Likewise, meshing engagement of the external spline portion  90  of stabilizer bar member  40   b  and the internal spline portion  80  of second gear member  64  couples stabilizer bar member  40   b  and second gear member  64  for rotation about rotational axis  94 .  
         [0028]    Coupling member  66  is shown to include an internal gear portion  78  and a pair of annular flanges  100 , which cooperate to define a clutch fork groove  102 . Internal spline portion  80  includes an aperture  104  that defines a plurality of gear teeth  106 . Aperture  104  is configured in a manner, which is complementary to first gear member  62  such that the gear teeth  106  of coupling member  66  meshingly engage the gear teeth  110  of first gear member  62 . Configuration in this manner permits coupling member  66  to slide along rotational axis  94  while remaining in meshing engagement with first gear member  62 . Coupling member  66  may be slid along first gear member  62  and into meshing engagement with second gear member  64 . The teeth  106  and  112  of coupling member  66  and second gear member  64 , respectively, are chamfered to improve the ability with which they meshingly engage.  
         [0029]    The position of coupling member  66  along rotational axis  94  is controlled by actuator assembly  68 . In the embodiment illustrated, actuator assembly  68  is shown to include a clutch fork  120 , an actuator device  122 , a controller  124  and a spring  126 . Clutch fork  120  includes a generally U-shaped body portion  130  having a pair of arms  132 , which are disposed within clutch fork groove  102 . A pin member  134  intersects the body portion  130 . A first portion  136  of the pin member  134  is disposed in cavity  72 . A second portion  138  of pine member  134  includes a bore  140  that is sized to receive spring  126 .  
         [0030]    Actuator device  122  includes a positionable actuator member  142 , which contacts the first portion  136  of pin member  134  and may be selectively positioned between a first actuator position and a second actuator position. Controller  124  is operable for generating an actuator signal, which is received by actuator device  122 . In its most basic form, actuator signal is a discreet signal that toggles between two predetermined values (e.g., 0 and 1) which are indicative of the desired actuator position. Upon receipt of an actuator signal, actuator device  122  repositions actuator member  142  as necessary to affect the position of clutch fork  120 . Placement of actuator member  142  in the first actuator position causes clutch fork  120  to slide coupling member  66  away from and out of meshing engagement with second gear member  64 . Operation of stabilizer bar assembly  29  in this manner effectively de-couples stabilizer bar members  40   a  and  40   b  such that they are permitted to rotate independently of one another about rotational axis  94 . Placement of actuator member  142  in the second actuator position causes clutch fork  120  to slide coupling member  66  toward and into meshing engagement with second gear member  64 . Operation of stabilizer bar assembly  29  in this manner effectively couples stabilizer bar members  40   a  and  40   b  such that they are coupled for common rotation about rotational axis  94 . Those skilled in the art will understand that coupling of stabilizer bar members  40   a  and  40   b  in this manner will permit the transmission of torque therebetween, which resists the roll of a vehicle body (not shown).  
         [0031]    Preferably, controller  124  is coupled to a vehicle controller  144  via a data bus  146 , thereby providing controller  124  with various vehicle data, such as the vehicle speed and an actively engaged transmission ratio. Configuration in this manner is advantageous in that controller  124  may then be employed to automatically position coupling member  66  in a desired position in response to a predetermined set of vehicle data. For example, upon the detection of a vehicle speed in excess of a predetermined amount, controller  124  may be programmed to generate the actuator signal to cause clutch fork  120  to slide coupling member  66  into meshing engagement with second gear member  64 . Similarly, controller  124  may be programmed to cause clutch fork  120  so slide coupling member  66  out of engagement with second gear member  64  if a predetermined gear ratio has been engaged (e.g., four-wheel drive, low gear) and the vehicle speed is less than a predetermined amount.  
         [0032]    With additional reference to FIG. 6, controller  124  is shown to include a reserve powering apparatus  160 , a speed sensing apparatus  162  and a power circuit  164  for providing power to the actuator device  122 . The power circuit  182  is shown to include a positive terminal  166 , a ground terminal  167  that is coupled to an electrical ground, and a control signal terminal  168  that receives a control signal from a remotely mounted 2-position switch  169  that is employed by the vehicle operator to select the mode in which the anti-roll system  29  is operated.  
         [0033]    In the particular embodiment illustrated, the reserve powering apparatus  160  includes a relay  170  with a normally open contact  174  and a normally closed contact  176 , an electrical power storage device  178  and preferably, means  180  for preventing the electrical power storage device  178  from back-feeding to the contact  174 . The contact  174  is coupled to a vehicle power supply  182  that provides electrical power only when the vehicle is operating and the contact  176  is coupled to the control signal terminal  168 . When electrical power having a voltage in excess of a predetermined voltage is present at the relay  170 , the contact  174  is closed so as to provide power to the positive terminal  166  and the contact  176  is opened so as to maintain the integrity of the electrical signal from the switch  169 . When electrical power having a voltage in excess of a predetermined voltage is not present at the relay  170 , the contact  174  is opened so as to interrupt the circuit between the vehicle poser source  182  and the positive terminal  166  and the contact  176  is closed so as to couple the control signal terminal  168  to the ground terminal  167 .  
         [0034]    In general, when the vehicle is operating and the switch  169  is placed in the first switch position, the switch  169  provides a first signal of a first predetermined voltage to the control signal terminal  168 , causing the power control circuit  164  to apply power to the actuator device  122  to drive the actuator member  142  into the first actuator position to thereby decouple the stabilizer bar members  40   a  and  40   b . Similarly, when the vehicle is operating and the switch  169  is placed in the second switch position, the switch  169  provides a second signal of a second predetermined voltage that is less than the first predetermined voltage to the control signal terminal  168  (e.g., couples control signal terminal  168  to the earth ground), causing the power control circuit  164  to apply power to the actuator device  122  to drive the actuator member  142  into the second actuator position to thereby couple the stabilizer bar members  40   a  and  40   b . Regardless of the position of the switch  169 , some of the electrical power that is fed toward the positive terminal  166  is employed to continuously charge the electrical power storage device  178 .  
         [0035]    In the event of an electrical fault wherein electrical power is no longer provided to the relay  170 , contact  174  is switched to its open position and contact  176  is switched to its closed position as discussed above. Accordingly, the contact  174  is opened so as to interrupt the circuit between the vehicle power source  182  and the positive terminal  166  and the contact  176  is closed so as to couple the control signal terminal  168  to the ground terminal  167 . In response to the coupling of the control signal terminal  168  to the ground terminal  167 , the voltage at the control signal terminal  168  is less than the first predetermined voltage to the control signal terminal  168 , causing the power control circuit  164  to apply power to the actuator device  122  to drive the actuator member  142  into the second actuator position to thereby couple the stabilizer bar members  40   a  and  40   b . With the positive terminal  166  no longer receiving power from the vehicle power source  182 , the electrical power storage device  178  is employed to provide power to the power circuit  164  to ensure that the actuator member  142  of the actuator device  122  is positioned in the second actuator position. In the particular example provided, the electrical power storage device  178  is illustrated to be a capacitor  178   a , but may be any electric power storage device including a battery  178   b.    
         [0036]    The back-feed preventing means  180  is shown to be a diode  180   a  in the particular embodiment illustrated and is interposed between the contact  174  and the electrical power storage device  178  to ensure that power from the electrical power storage device  178  does not back-feed to the contact  174  rather than feed into the power circuit  164 . Accordingly, those skilled in the art will understand that the back-feed preventing means  180  may also or alternatively include a switch, relay or other device that would open or otherwise prevent electrical power from back-feeding from the electrical power storage device  178  to the contact  174 .  
         [0037]    The speed sensor apparatus  162  includes a comparison circuit  300  and a relay  302  having a normally open contact  304  that is coupled to the control signal terminal  168 . The comparison circuit  300  is shown to be coupled to a speed sensor  306  that monitors the speed of the vehicle and generates a speed signal in response thereto. In the example provided, the speed sensor  306  actually senses the rotational speed of the transmission output shaft, but as those skilled in the art will understand, the sensor may sense the velocity of any component whose speed is proportional to that of the vehicle. If while monitoring the speed signal the comparison circuit  300  determines that the speed of the vehicle is greater than or equal to a predetermined vehicle speed (e.g., 15 miles per hour), the relay  302  is actuated to cause the contact  304  to close, thereby coupling the control signal terminal  168  to the ground terminal  167 . When this occurs, the voltage at the control signal terminal that is less than the first predetermined voltage and as such, this will cause the power control circuit  164  to apply power to the actuator device  122  to drive the actuator member  142  into the second actuator position to thereby couple the stabilizer bar members  40   a  and  40   b . Configuration of the controller  124  in this manner is advantageous in that it prevents operation of the vehicle at high speed when the stabilizer bar members  40   a  and  40   b  are disconnected.  
         [0038]    When the speed signal the comparison circuit  300  determines that the speed of the vehicle is less than the predetermined vehicle speed, the relay  302  is actuated to cause the contact  304  to open, thereby uncoupling the control signal terminal  168  from the ground terminal  167 . Assuming that the vehicle power source  182  is providing the relay  170  with power, the voltage at the control signal terminal  168  is then responsive to the position in which the switch  169  has been placed (i.e., if the switch  169  has been positioned in the first switch position, the voltage at the control signal terminal  168  is greater than the first predetermined voltage and as such, this will cause the power control circuit  164  to apply power to the actuator device  122  to drive the actuator member  142  into the first actuator position to thereby uncouple the stabilizer bar members  40   a  and  40   b ).  
         [0039]    Although various signals, such as the speed signal and the switch signal, have been illustrated and described herein as being discrete signals, those skilled in the art will understand that some or all of this data may also be transmitted over a conventional in-vehicle network. For example, commercially available vehicle and/or transmission controllers typically have the capability of determining the vehicle speed and providing this information over an in-vehicle network to other modules. Accordingly, the vehicle controller  144  may, in the alternative, calculate the speed of the vehicle, transmit this information over the vehicle network  146  where it is received by the controller  124 . If the vehicle speed is greater than or equal to a predetermined speed, the controller  124  will output a digital signal to the relay  302  causing the contact  304  to open.  
         [0040]    Spring  126  provides a degree of compliance in the positioning of clutch fork  120  to prevent damage to clutch assembly  42  in situations where coupling member  66  cannot be immediately engaged to or disengaged from second gear member  64 . If, for example, coupling member  66  cannot engage second gear member  64  simultaneously with the movement of actuator member  142  into the second actuator position, spring  126  is compressed between housing  70  and body portion  130 . The force generated by the compression of spring  126  is exerted onto clutch fork  120  and tends to push coupling member  66  into meshing engagement with second gear member  64  at an appropriate time (e.g., when the rotational speed and/or alignment of coupling member  66  and second gear member  64  are consistent).  
         [0041]    Preferably, coupling member  66  and second gear member  64  are configured such that they can only be meshingly engaged when stabilizer bar members  40   a  and  40   b  are in a predetermined orientation. Methods for keying the orientation of one shaft to another are commonly known in the art and need not be discussed in detail herein. In the example provided, stabilizer bar members  40   a  and  40   b  are keyed to one another via a blocking tooth  150  and a blocking slot  152  which are shown in greater detail in FIG. 4. Blocking tooth  150  is formed, for example, in second gear member  64  and has a shape which is relatively wider than the other teeth  112  of second gear member  64 . Blocking slot  152  is formed in coupling member  66  and configured to receive blocking tooth  150 . Blocking tooth  150  and blocking slot  152  cooperate to prevent the engagement of coupling member  66  to second gear member  64  unless blocking tooth  150  and blocking slot  152  are aligned to one another. Those skilled in the art will understand that in the alternative, blocking tooth  150  may be formed into coupling member  66  and blocking tooth  150  may be formed into second gear member  64 .  
         [0042]    Also preferably, clutch assembly  42  also includes a de-lashing mechanism  180  (FIG. 4) for removing the gear lash between the first and second gear members  62  and  64  and the coupling member  66 . Such de-lashing mechanisms are well known in the art and need not be discussed in detail. In the particular embodiment illustrated, a portion of the teeth  112  of the second gear member  64  are formed on a helix so that the second gear member  64  will rotate about the rotational axis  94  in response to meshing engagement with coupling member  66  to cause the teeth  106  of the coupling member  66  to contact the teeth  112  of the second gear member  64 . Those skilled in the art will understand that the rotational motion of the second gear member  64  will also cause coupling member  66  to rotate relative to the first gear member  62  to cause their teeth  106  and  110 , respectively, to contact one another.  
         [0043]    In FIG. 5, an alternately constructed stabilizer bar assembly  29 ′ is illustrated. A pair of seals  200  sealing engage the central segments  44 ′ of stabilizer bar members  40   a ′ and  40   b ′. First gear member  62 ′ is illustrated to be fixedly coupled to the proximal end of stabilizer bar member  40   a ′ and second gear member  64 ′ is illustrated to be fixedly coupled to the proximal end of stabilizer bar member  40   b ′. Stabilizer bar member  40   b ′ is supported by bearing  204  for rotation within housing assembly  60 ′. Stabilizer bar member  40   b ′ is illustrated to include a pilot aperture  208  which is configured to receive a pilot portion  210  that is coupled to stabilizer bar member  40   a ′. Pilot aperture  208  and pilot portion  210  cooperate to align stabilizer bar members  40   a ′ and  40   b ′ about rotational axis  94 ′.  
         [0044]    Clutch fork  120 ′ is illustrated to include a unitarily formed body portion  130 ′ and a unitarily formed pin member  134 ′. Pin member  134 ′ extends through an aperture (not specifically shown) formed into body portion  130 ′ and is slidable along an axis that is generally parallel rotational axis  94 ′. First and second spring members  212  and  216 , respectively, are disposed about pin member  134 ′ and spaced axially apart by body portion  130 ′. Actuator device  122 ′ is constructed such that actuator member  142 ′ is normally maintained in the second actuator position. As first spring  212  has a spring rate that is relatively larger than that of second spring  216 , the force generated by first spring  212  will be applied to body portion  130 ′ such that coupling member  66  is normally maintained in a condition wherein it is engaged with second gear member  64 ′ (i.e., clutch assembly  42 ′ is normally maintained in the second condition which is indicated in phantom).  
         [0045]    Upon the receipt of an actuator signal indicative of the desire to shift actuator member  142 ′ to the second actuator position, actuator device  122 ′ will extend actuator member  142 ′ to contact pin member  134 ′, causing pin member  134 ′ to translate relative to body portion  130 ′ and compress second spring  216 . In this condition, the force generated by second spring  216  exceeds the biasing force that is exerted by the first spring  212 , causing body portion  130 ′ to slide axially and disengage coupling member  66  from second gear member  64 ′. In this arrangement, if the actuator signal is lost (e.g., in the event of a power failure), actuator device  122 ′ will not maintain actuator member  142 ′ in the first actuator position, thereby permitting the biasing force of first spring  212  to shift body portion  130 ′ so that coupling member  66  engages second gear member  64 ′.  
         [0046]    Stabilizer bar assembly  29 ′ is also shown to include a stop device  220  which is coupled to stabilizer bar member  40   a ′. Stop device  220  includes a stop member  222  that is operable for restraining stabilizer bar member  40   a ′ from moving axially along rotational axis  94 ′ by an amount that exceeds a predetermined amount. In the particular embodiment illustrated, stop member  222  is a retaining ring  224  which is coupled to stabilizer bar member  40   a ′, the retaining ring  224  being disposed in a pair of retaining ring grooves  226  and  228  formed in the housing assembly  60 ′ and the stabilizer bar member  40   a ′, respectively, in a manner that is well known in the art.  
         [0047]    In FIG. 7, a vehicle constructed in accordance with the teachings of another alternate embodiment of the present invention is generally indicated by reference numeral  400 . Vehicle  400  is shown to be a four-wheel drive vehicle having a conventional powertrain  402  with an engine  404 , a transmission  406 , a transfer case  408 , front and rear propshafts,  410  and  412 , respectively, and front and rear axle assemblies  414  and  416 , respectively, for driving a set of front wheels  417  and a set of rear wheels  418 . The transfer case  408  conventionally permits the front axle assembly  414  to be disengaged from the engine  404  so that engine power is transmitted only to the rear wheels  418  to permit the vehicle  400  to be operated in a 2-wheel drive mode. The vehicle  400  also includes front and rear stabilizer bar assemblies  420  and  422 , both of which are identical in construction to that of stabilizer bar assembly  29  except that a common controller  124 ″ is employed to control the operation of both stabilizer bar assemblies  420  and  422 .  
         [0048]    With additional reference to FIG. 8, the methodology employed by the controller  124 ″ to control the operation of both stabilizer bar assemblies  420  and  422  is illustrated schematically in flowchart form. Advantageously, the methodology of the present invention permits the stabilizer bar assemblies  420  and  422  to be coupled and uncoupled in a manner that prevents the vehicle  400  from being subjected to roll steer. The methodology begins at bubble  500  wherein the stabilizer bar member  40   a  of each of the stabilizer bar assemblies  420  and  422  are engaged to their associated stabilizer bar member  40   b . The methodology progresses to decision block  504  where the methodology determines whether the vehicle operator has selected a predetermined operational mode (e.g., offroad). Those skilled in the art will understand that the manner in which the predetermined operational mode is selected my comprise a switch input from a toggle or pushbutton switch that is mounted, for example, on the vehicle instrument panel, or via the selection of a predetermined gear ratio, as when, for example, the transfer case  408  is placed in a 4-wheel drive mode. If the vehicle operator has not selected the predetermined operational mode, the methodology loops back to bubble  500 . If the vehicle operator has selected the predetermined operational mode in decision block  504 , the methodology proceeds to decision block  508 .  
         [0049]    In decision block  508 , the methodology determines whether the powertrain  402  is operating in the predetermined operational mode. If, for example, the predetermined operation mode is the operation of the vehicle  400  in a 4-wheel drive mode, the methodology determines in decision block  508  whether power from the engine  404  is being transmitted to the front axle assembly  414 . If the power train  402  is not operating in the predetermined operational mode, the methodology loops back to bubble  500 . If the power train  402  is operating in the predetermined operational mode, the methodology proceeds to decision block  512 .  
         [0050]    In decision block  512 , the methodology determines whether the speed of the vehicle  400  is less than a predetermined threshold speed. If the speed of the vehicle  400  is not less than the predetermined threshold speed, the methodology loops back to bubble  500 . If the speed of the vehicle  400  is less than the predetermined threshold speed in decision block  512 , the methodology proceeds to block  516  wherein the stabilizer bar members  40   a  and  40   b  of the stabilizer bar assembly  422  associated with the rear axle assembly  416  are disconnected from one another. The methodology then proceeds to block  520  to permit a predetermined amount of time, such as 2 seconds, to elapse so as to ensure that the stabilizer bar members  40   a  and  40   b  of the stabilizer bar assembly  422  are completely disconnected before proceeding to the next step of the methodology.  
         [0051]    After the predetermined amount of time has elapsed in block  520 , the methodology proceeds to block  524  wherein the stabilizer bar members  40   a  and  40   b  of the stabilizer bar assembly  420  associated with the front axle assembly  414  are disconnected from one another. The methodology then proceeds to decision block  528 .  
         [0052]    In decision block  528 , the methodology determines if the speed of the vehicle  400  is greater than the predetermined speed threshold. If the speed of the vehicle  400  is not less than the predetermined speed threshold, the methodology proceeds to block  536 . If the speed of the vehicle  400  is less than the predetermined speed threshold, the methodology proceeds to decision block  532 .  
         [0053]    In decision block  532 , the methodology determines whether the vehicle operator has de-selected the predetermined operational mode (e.g., canceled the offroad mode to revert to an on-road mode). If the vehicle operator has not de-selected the predetermined operational mode, the methodology loops back to decision block  528 . If the vehicle operator has de-selected the predetermined operational mode, the methodology proceeds to block  536 .  
         [0054]    In block  536 , the methodology causes the stabilizer bar members  40   a  and  40   b  of the stabilizer bar assembly  420  associated with the front axle assembly  414  to be connected to one another. The methodology then proceeds to block  540  to permit a predetermined amount of time, such as 2 seconds, to elapse so as to ensure that the stabilizer bar members  40   a  and  40   b  of the stabilizer bar assembly  420  are completely disconnected before proceeding to the next step of the methodology.  
         [0055]    After the predetermined amount of time has elapsed in block  540 , the methodology proceeds to block  544  wherein the stabilizer bar members  40   a  and  40   b  of the stabilizer bar assembly  422  associated with the rear axle assembly  416  are connected to one another. The methodology then loops back to bubble  500 .  
         [0056]    Control of the stabilizer bar assemblies  420  and  422  in this manner is highly advantageous in that because the front stabilizer bar  420  is never disengaged while the rear stabilizer bar assembly  422  is engaged, vehicle stability is maintained and the vehicle is not subjected to a roll-over steering effect that would render it difficult to maneuver.  
         [0057]    While the methodology has been described thus far as being at least partially dependent upon the speed of the vehicle  400  relative to a predetermined threshold speed to attach or detach the stabilizer bar assemblies  420  and  422 , those skilled in the art will appreciate that the invention, in its broader aspects, may be performed somewhat differently. In this regard, the determination to attach or detach the stabilizer bar assemblies  420  and  422  may be based upon a statistical moving average of the speed of the vehicle  400 . Operation of the stabilizer bar assemblies  420  and  422  in this manner would permit operation of the vehicle  400  at a speed that shifts from a point somewhat above the speed threshold to another point somewhat below the speed threshold without requiring the stabilizer bar assemblies  420  and  422  engage and disengage each time the speed of the vehicle  400  exceeds or falls below the predetermined speed threshold. In this regard, the speed of the vehicle  400  may be analyzed as a function of time and the threshold signal for engaging the stabilizer bar assemblies  420  and  422  being generated in response to determining that an area under a curve defined by the vehicle speed as a function of time exceeds a predetermined threshold.  
         [0058]    As another alternative, two discrete speed thresholds may be employed to trigger the engagement and disengagement of the stabilizer bar assemblies  420  and  422 . In this regard, it is presently preferred that the speed threshold for triggering the engagement of the stabilizer bar assemblies  420  and  422  be greater than the speed threshold for triggering the disengagement of the stabilizer bar assemblies.  
         [0059]    While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.