Abstract:
A wheel clutch includes a clutch pack mounted in a housing wherein either the friction plates or clutch plates of the clutch pack are fixed for rotation with a corresponding drive wheel, and the other of the friction plates or clutch plates is fixed for rotation with a drive shaft of the vehicle, a resilient member within the clutch housing normally biasing the clutch pack into a fully locked mode, a selectively actuable actuator within the clutch housing engaging the resilient member to selectively and progressively reduce a resilient force or to selectively and progressively allow the increase of the resilient force applied by the resilient member to the clutch pack, whereby the clutch pack is completely or progressively unlocked or locked respectively to provide an optimized amount of rotational traction versus rotational slippage.

Description:
TECHNICAL FIELD 
     The invention relates to methods and apparatus for driving motor vehicle wheels, and more particularly to a method and apparatus for independently and variably reducing or optimizing torque distribution to motor vehicle drive wheels. 
     BACKGROUND 
     Motor vehicles typically use a mechanical differential to distribute engine torque between paired left and right drive wheels. A differential allows paired left and right drive wheels driven by the same input to rotate at different speeds. As compared with a solid axle, this provides superior handling during cornering by allowing the inside drive wheel to turn more slowly than the outside drive wheel. A conventional rear differential is located underneath a car in the middle between the back tires. A differential is a group of gears that revolve around each other and let the two tires, the laterally paired left and right tires, travel at different speeds. A differential is really important for vehicle handling when going around corners, because the tire on the inside of the turn is going in a smaller circle than the outside and therefore is going slower. Without a differential one tire would be dragging, and the car would be difficult to turn and would want to keep going straight. The drawback of a differential is that it causes the vehicle to have less traction because it lets one tire spin without the other and allows the drive force to automatically go to the drive wheel with the least traction. 
     The reason we need differentials has not changed, but modern higher performing vehicles sometimes need more traction, so in the prior art traction adding devices have been added to try to compensate for the differential. 
     Some vehicles use limited slip differentials that limit the amount of torque supplied to an idly rotating drive wheel, such as may occur when wheel traction is lost. In vehicles with limited slip differentials, the drive torque applied to the inner drive wheel produces a moment counteracting the moment which tends to turn the motor vehicle. As a result, turning performance with a limited slip differential is lowered as compared with an open differential. 
     In some circumstances, such as may occur in off-road driving, it is desirable that left and right drive wheels turn in unison regardless of the traction (or lack thereof) available to either wheel individually. Some vehicles provide locked or selectively lockable differentials, which fix, permanently or selectively, the relative rotational orientations of paired left and right drive wheels. 
     Since open, limited-slip and locked differentials are more or less advantageous depending on circumstances, there is a desire for methods and apparatus that distribute engine torque to paired left and right drive wheels according to selectable and/or automatically adjustable modes. There is further a desire for methods and apparatus that permit the torque at which left and right drive wheels slip relative to one another to be selectively progressively or infinitely varied. 
     In the prior art applicants are aware of U.S. Pat. No. 7,111,702 which issued Sep. 26, 2006, to Perlick et al. Perlick describes a system for steering angle control of independent rear clutches. Perlick teaches that the vehicle must have four wheel drive and have a conventional differential as the primary drive, which is taught to be in the front end and to receive two-thirds of the power, with the remaining one third of the power going to the secondary drive, ie, to his system. Perlick also calls for a center differential and brake manipulation. In contra-distinction the apparatus described herein works in front wheel drive, rear wheel drive, four or more wheel drive, and equally well with live or independent axles. The present apparatus, that is, as described herein, does not need a primary drive, or as described by Perlick, a conventional front differential, and in fact does not need a conventional differential at all, but will work equally well in conjunction with one. The apparatus described herein can be a stand alone two wheel drive, primary drive, secondary drive, or four or more wheel drive. Perlick requires a micro-processor, multiple sensors and brake manipulation, wherein the present apparatus may advantageously be controlled by the steering or by a separate actuating circuit such as a separate hydraulic circuit, or may for example, like Perlick, utilize processors and sensors. 
     Perlick&#39;s system is either engaged or disengaged, wherein the present apparatus is progressive from completely engaged to completely disengaged as required. According to Perlick, the primary purpose is to improve cornering in primarily front wheel drive applications and is thus not a traction device per se, whereas the present apparatus works in all applications and provides improved steering and handling performance with the least compromise in traction. Perlick teaches only regulating the back wheels and leaving the front wheels with traction and steering compromises, whereas the present application may regulate all driven wheels. 
     In the prior art applicants are also aware of U.S. Pat. No. 6,817,434 to Sweet which issued Nov. 16, 2004, for an Active Hydraulically Actuated On-demand Wheel End Assembly. Sweet describes a system which is normally 100% unlocked, there thus being zero traction during the normal 100% unlocked mode of each wheel end assembly. 
     The present apparatus is the opposite, that is, each clutch is normally 100% locked and thus starts with 100% traction. Sweet requires pressure to increase traction and drive the vehicle, whereas the present apparatus is again opposite. In an embodiment of the present apparatus which employs a hydraulic actuator, as the hydraulic system is pressurized traction is decreased only in order to steer the vehicle. Sweet teaches the reverse; viz, manipulating pressure to increase traction. The presently described system manipulates pressure to increase steering performance, whereas the Sweet system takes a wheel spinning under power and attempts to stop or slow it by applying pressure and increasing clutch friction. Typically this condition would be caused while increased power is called for during driving. The present device is the opposite as it keeps the axles locked while the largest amounts of power and traction are required, and then only releases holding force in the clutch when the vehicle is negotiating a turn. Typically this would take place only during reduced throttle applications, and so the present apparatus works in synergy with the reality of the driving dynamics: traction when power is applied, reduced or zero hold in any one independently controlled wheel clutch when handling is required and power requirements typically are reduced. 
     In the present apparatus a hydraulic pressure failure results in the full traction, i.e., each affected wheel clutch remains fully locked, thereby providing power and 100% traction to the wheels. In the Sweet design, if there is a pressure failure, transmitted power and traction goes to zero and the vehicle cannot move. 
     The present apparatus acts like a conventional vehicle when parked, i.e. the vehicle is locked in position even without use of a hand-brake. The Sweet design requires a hand-brake or the like or pressure as the wheels are unlocked when parked in the event that pressure bleeds down. Further, the Sweet design does not work on live axles, whereas the present apparatus does. In the Sweet design the axles are short, limiting wheel travel and ground clearance. In the present design longer axles may be used than currently exist, increasing wheel travel and ground clearance. The Sweet design may not easily be retro fitted to existing cars whereas in the present apparatus the wheel clutches may replace existing hubs, differentials or axles in whole or in part, and may be added to the outside of the wheel brakes as a form of locking/unlocking hub. 
     As with the Perlick design, the Sweet design requires complex sensors, computers or other processors (computers, processors, programmable logic controllers, etc collectively referred to as processors herein) and requires brake control first, then rear traction control followed by front traction control. The present apparatus may in one embodiment be simply controlled by the steering, advantageously with a manual over-ride to help the wheel clutches in the locked position. Unlike in the present apparatus, Sweet makes no mention of unlocking inside tires in a corner, whereas, as described herein, the present apparatus automatically unlocks the inside tires, dramatically improving performance in all conditions. 
     The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will be apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
     SUMMARY OF ASPECTS OF THE INVENTION 
     The design described herein is simple, lighter, and may be less costly and easier to produce than conventional systems, has low maintenance requirements and provides an optimized performance system for vehicles (ie., cars, trucks, all terrain vehicles, etc and other wheeled devices having at least one driven wheel, collectively referred to herein as a vehicle) when cornering while also providing the most traction the vehicle tires can use. 
     This may thus result in a vehicle which will be cheaper to buy, easier to drive and park, get better gas mileage, have cheaper maintenance costs and have better traction when required, making it potentially safer. The present system eliminates the need for a conventional differential and all conventional traction adding devices, including limited slip differentials, locking differentials and locking/unlocking hubs. The present system consists of clutches on the drive axles, also referred to herein as wheel clutches, for use on two wheel drive, front or rear, four wheel drive, or all wheel drive. The wheel clutches are normally in a locked state or mode, by default. They provide 100% torque/drive force to all driven wheels all the time, until the vehicle needs to turn. Upon turning, in one embodiment which is not intended to be limiting, the vehicle&#39;s steering system releases pressure to the wheel clutches as required to improve handling or cornering ability of the vehicle. The present system thus only reduces the torque/traction required by the minimum amount required by any given wheel to attain the required handling performance for that particular driving application, always automatically maintaining the optimized or highest combined level of handling and torque/traction. 
     In one embodiment, when the vehicle is turning, the steering system variably or progressively reduces torque on the inside driven tires, thereby reducing steering effort, increasing performance, handling and fuel economy, and reducing body roll, tire and steering wear. The present system automatically and variably or progressively returns to a mechanically locked state when the corner is complete, automatically returning to maximum torque/traction when cornering performance is not required. The present system can be installed on existing wheel hubs, inside existing differential housings, or, in place of conventional wheel hubs, axles and or conventional differentials. It may also replace locking hubs on four wheel drives. 
     Thus as will be apparent, the present invention works opposite to most traction devices. It starts locked and only unlocks to steer. It is a unique combination of existing components that excels in its simplicity. 
     To summarize, aspects of the present invention may be characterized as follows: it is a drive system that will work on all vehicles and does not need a conventional differential; it is 100% engaged and only disengaged to steer; unlike all other traction devices for primary differentials, it unlocks the inside tire; it is infinitely adjustable from 0 to 100% engaged; it works on 2, 4, all wheel etc systems; it works on live and independent axles; it can be retro fitted into existing vehicles or built new; it can be used in place of a variety of existing drive line components; it does not require but can use micro processors, wheel speed sensors etc.; it does not require brake manipulation; one configuration allows increased ground clearance and wheel travel over all existing technology when used in independent axles; it also allows for more useable floor space in passenger compartments; it is simple, automatic, inexpensive, lighter; it automatically provides more traction and better steering than all existing conventional systems and can be enhanced with sensors etc.; it would allow the rest of the drive-train to be downsized for mass production because of reduced shock loading on the drive-train; it may save weight and cost and improve vehicle gas mileage. 
     The system includes a wheel clutch for a drive wheel, and advantageously wheel clutches for two or more drive wheels, or one or more laterally opposite pairs of drive wheels or for example all drive wheels. Each wheel clutch includes a clutch pack mounted in a housing wherein either the friction plates/washers or clutch plates of the clutch pack are fixed for rotation with a corresponding drive wheel, for example for rotation with part or all of the housing, and the other of the friction plates/washers or clutch plates is fixed for rotation with a drive shaft of the vehicle whether or not with an intervening differential or axles or hubs etc. A resilient member is mounted within the clutch housing so as to normally bias the clutch pack into a fully locked mode, that is, so that the clutch pack is compressed to frictionally lock the clutch plates against the friction plates/washers. A selectively actuable actuator operates within the clutch housing to engage the resilient member and to thereby selectively and progressively reduce a resilient force of the resilient member or to selectively and progressively allow the increase of the resilient force applied by the resilient member to the clutch pack, whereby the clutch pack is completely or progressively unlocked or locked respectively to provide an optimized amount of rotational traction versus rotational slippage. 
     In embodiments described herein and illustrated, the actuator is a hydraulically actuated piston and the resilient member is a spring washer, although these are meant to be examples and not limiting, as one skilled in the art would know that other actuation means and other resilient members would also work, especially at the surprisingly low actuation pressures that have been found to work, as better described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings show non-limiting example embodiments wherein like reference numerals denote corresponding parts in each view. 
         FIG. 1  is a schematic plan view of a part of a vehicle powertrain comprising an axle assembly according to an example embodiment. 
         FIG. 2  is a side elevation view of an axle assembly according to an example embodiment. 
         FIG. 3  is an exploded perspective view of the axle assembly shown in  FIG. 2   
         FIG. 4  is a partially-exploded perspective view of the axle assembly shown in  FIG. 2 . 
         FIG. 4 a    is a diagrammatic sectional view of an alternative embodiment of a wheel clutch. 
         FIG. 5  is a schematic drawing of a hydraulic control system for controlling a vehicle&#39;s wheel clutch assemblies according to a first example embodiment employing the vehicle&#39;s power steering pump and steering box. 
         FIG. 6  is a schematic drawing of a hydraulic control system for controlling a vehicle&#39;s wheel clutch assemblies according to a second example embodiment employing a secondary hydraulic system and optionally a processor and sensors. 
         FIG. 7  is a diagrammatic partially cut-away view of an alternative embodiment of a wheel clutch. 
     
    
    
     DESCRIPTION 
     Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail in order to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
       FIG. 1  is a schematic plan view of part of a motor vehicle powertrain  10 . As shown in  FIG. 1 , power from an engine  14  is transmitted via a transmission  16  to a driveshaft  18 . Torque from driveshaft  18  is distributed by a torque transmitting device  20  to shafts  22 . In some embodiments, torque transmitting device  20  rotates to drive shafts  22  at the same rotational speed. For example, torque transmitting device  20  may comprise a spool, a locking differential, or the like. Shafts  22  are each coupled to a wheel hub  24  by an axle assembly  30 . Importantly, a differential in the conventional sense described above in the background section of this specification, that is, which allows different rotational speeds of corresponding left and right wheels during a turn by conventional means is not required in embodiments employing the present invention. 
       FIGS. 2, 3 and 4  show, respectively, an exploded side elevation view, an exploded perspective view, and a partially-exploded perspective view of an axle assembly  30  according to an example embodiment. Axle assembly  30  transmits torque from shaft  22  to wheel hub  24 . Assembly  30  comprises a spindle  32  having an integrally formed mounting portion, namely flange  32 A, that is removably mounted to a flange  22 A formed at the outward end of shaft  22 . A rotating seal  36  is mounted on spindle  32  adjacent flange  32 A. Rotating seal  36  provides for the transmission of hydraulic fluid from an input port  38 A to an output port  38 B that may rotate relative to one another about spindle  32 . Wheel hub  24  is mounted outwardly adjacent of rotating seal  36  for rotation about spindle  32 . 
     A clutch drum  40  is mounted laterally outward of wheel hub  24  for rotation about spindle  32 . Clutch drum  40  comprises a hollow cylinder  42  closed at one end by an integrally formed base  44  and at its other end by a removable plate  46 . A plurality of bolts  41  extend through base  44  and into wheel hub  24  to fixedly connect clutch drum  40  to wheel hub  24 , such that they rotate in unison about spindle  32 . A spool  50  is mounted for rotation with spindle  32  inside clutch drum  40 . More particularly, grooves or internal splines formed on a central aperture  50 A formed in spool  50  mate with external splines formed at end  32 B of spindle  32 . 
     A plurality of friction discs, washers or plates  52  interleaved with clutch plates  54  provide for variable transmission of rotation between spool  50  and clutch drum  40 . For convenience, the interleaved stack of friction discs  52  and clutch plates  54  may be referred to herein as a clutch pack  56 . Clutch pack  56  is housed within clutch drum  40 . Friction discs  52  are mounted to rotate in unison with spool  50  around spindle  32 . More particularly, teeth  52 A formed along central apertures defined through friction discs  52  mate with external splines  50 B formed on the outer sidewall of spool  50 . Clutch plates  54  are mounted to rotate in unison with clutch drum  40  about spindle  32 . More particularly, teeth  54 A formed on the outside edges of clutch plates  54  mate with a plurality of radially-arrayed posts  48  that project inwardly into clutch drum  40  from plate  46 . The free ends of posts  48  are received in a corresponding radial array of recesses  44 A formed in base  44  of clutch drum  40 . In some embodiments, plate  46  is sealed against clutch drum  40 , and clutch drum  40  may be filled with fluid. 
     A spring washer  58  compresses friction discs  52  and clutch plates  54  in direction A towards base  44  of drum  40 . Spring washer  58  may have a waffle or corrugated structure, or a dished structure, or a slightly conical structure, or other means for providing resiliency for the washer so as to resiliently urge the compression of clutch pack  56  into drum  40  in direction A. Thus, clutch pack  56  is normally biased into a fully engaged mode, locking rotation of spindle  32  to rotation of plate  46 . That is, compression of clutch pack  56  by the resilient urging of spring washer  58  increases frictional engagement between adjacent friction discs  52  and clutch plates  54 , which provides a connection between clutch drum  40  and spool  50 . 
     Bolts  49  are mounted to the laterally outer surface  46   a  of cap plate  46 . Bolts  49  allow mechanically pre-loading the spring washer  58 . 
     In the illustrated embodiment, a hydraulic actuator  60  is provided on base  44 . Actuator  60  comprises an annular barrel  62  defined in base  44  and an annular piston  64  sealingly disposed in and movable along barrel  62 . The axis of barrel  62  is parallel to the longitudinal axis of spindle  32 , and in the illustrated embodiment is co-axial with the longitudinal axis of spindle  32 . Piston  64  is controllably movable outward along barrel  62  by increasing the supply of hydraulic fluid to a chamber  66  defined between barrel  62  and piston  64 . Hydraulic fluid is supplied to chamber  66  by way of a fluid line (fluid line  67  in  FIG. 7 , but not shown in the other drawings) connected between output port  38 A of rotating seal  36  and a port located on clutch drum  40 . 
     In a preferred embodiment the hydraulic fluid is hydraulic steering fluid from the hydraulic steering pump and hydraulic steering circuit of the vehicle. Thus as the vehicle&#39;s steering wheel is turned, the gradually increasing pressure of the hydraulic steering fluid variably or progressively actuates actuator  60 , thereby correspondingly allowing limited slippage in the clutch pack  56 . This allows for increasingly differential rates of rotation of the inner versus the outer wheels of the pair of left and right driven wheels. 
     The actuation of the actuator  60  in direction B compresses spring washer  58  in a direction opposite to direction A. This unlocks clutch pack  56 , thereby allowing at least limited rotary slippage between friction discs  52  and clutch plates  54 . 
     The force from actuation of actuator  60  is imparted to spring washer  58  by the use of a pair of book-end clutch plates or caging washers  54 B and  54 C which in conjunction with pins  55  form an open, cylindrical frame acting between piston  64  and spring washer  58  when actuator  60  is urging spring washer  58  so as to de-compress clutch pack  56 . 
     Teeth  54 A on clutch plates  54  form an array of radially spaced apart gaps  54 D and  54 E therebetween, radially equally spaced apart around longitudinal axis of rotation C. Gaps  54 D and  54 E alternate around the perimeter of each clutch plate  54 . Gaps  54 D and  54 E may be identical to one another. Clutch plates  54  are aligned relative to one another so that all of the gaps  54 D line up and all of the gaps  54 E line up, so that gaps  54 D and  54 E each form corresponding longitudinal channels running longitudinally the length of clutch pack  56  in an array of radially spaced apart longitudinal channels. Posts  48  extend through and along some or all of the channels formed by gaps  54 E, and pins  55  extend through and along some or all of the channels formed by gaps  54 D. Book-end clutch plates or caging washers  54 B and  54 C do not have gaps  54 D. Instead, they have rigid flanges (or widened teeth)  54 F where gaps  54 D would be. Consequently, pins  55  bear against flanges  54 F. That is, each pin  55  is sandwiched, length-wise, between corresponding pairs of flanges  54 F on each plate or washer  54 B. Posts  48  are longer than pins  55 . Posts  48  extend all of the way from plate  46  to base  44  in drum  40 . Pins  55  only extend between book-end clutch plates or caging washers  54 B and  54 C. 
     The open, cylindrical frame formed by pins  55  sandwiched on-end between the pair of book-end clutch plates or caging washers  54 B and  54 C transmits a load, in compression, against spring washer  58  as piston  64  is extended away from base  44 , thereby relieving and reducing the compressive load on clutch pack  56 . This allows for de-compression of clutch pack  56 . As piston  64  retracts the loading is gradually removed from the open frame of pins  55  and the book-end clutch plates or caging washers  54 B and  54 C allowing spring washer  58  to re-compress clutch pack  56  against thrust plate or shoulder washer  57 . Compression of clutch pack  56  is enabled because ends  55   a  of pins pass through corresponding holes  57   a  in thrust plate or shoulder washer  57  as piston  64  retracts towards base  44 . Decompression of the clutch pack  56 , which is otherwise normally clamped between spring washer  58  and shoulder washer  57 , is sufficient to allow selectively limited or progressive slippage or unlimited slippage between clutch plates  54  and friction washers  52 . As clutch pack  56  is decompressed, the individual clutch plates  54  are free to translate slightly along posts  48  and pins  55 , and friction washers  52  are free to translate slightly along the external splines of spool  50  so as to slightly expand clutch pack  56 . They translate so as to re-compress within clutch pack  56 , and lock the axle assembly  30 , as the spring washer  58  is allowed to re-engage against the clutch pack  56 , pressing clutch pack  56  against shoulder washer  57  as the actuator piston  60  is retracted and pins  55  retract through holes  57   a . A diagrammatic partially cutaway view of such an arrangement is shown in the sectional view of  FIG. 4   a.    
     The torque at which wheel hub  24  will slip relative to the shaft  22  is thus controllable by varying amount of compression exerted by spring washer  58  on clutch pack  56  as regulated by actuation of hydraulic actuator  60 . Thus by retracting piston  64  to allow the full compression of clutch pack  56  by spring washer  58 , axle assembly  30  may transfer torque from shaft  22  to wheel hub  24  in the mode of a locked differential. Limited extension of piston  64 , resulting in limited reduction in the compression of clutch pack  56  by spring washer  58  provides the functionality of a limited slip differential. The functionality of an open differential is achieved by full extension of piston  64  to relieve the compression of clutch pack  56  by spring washer  58 . Those skilled in the art will appreciate that in one embodiment, which is not intended to be limiting, clutch drum  40 , clutch pack  56 , spool  50  and hydraulic actuator  60  comprise the components of a clutch in which spool  50  and friction plates  52  comprise driving members and clutch drum  40  and clutch plates  54  comprise driven members. 
     In an example prototype embodiment, which again is not intended to be limiting, clutch pack  56  comprises eight ⅛″ thick friction discs  52  and nine ⅛″ thick clutch plates  54  (including book-end clutch plates or caging washers  54 B and  54 C), and movement of piston  64  by less than 5 thousandths of an inch is sufficient to change the torque transfer mode of axle assembly  30  from the mode of an open differential (for example when the hydraulic steering circuit is pressurized during turning) to the mode of a locked differential (the normal mode of each clutch). Experiments with this prototype embodiment have shown that hydraulic fluid at a surprisingly low pressure of 200 PSI is sufficient to cause axle assembly  30  to behave in the mode of an un-locked limited slip differential during normal turning conditions. In other embodiments, more or fewer friction discs and clutch plates may be used, friction discs and clutch plates may be of large or smaller diameter, and limited or completely un-locked differential behavior may be obtained driving hydraulic actuator  60  at a correspondingly lower or greater pressure respectively. 
     Example advantages provided by axle assembly  30  include the following: 
     The force applied to clutch packs  56  may be varied continuously. 
     The force applied to one clutch pack  56  may be varied independently of the force applied to the other clutch pack  56 . 
     The force applied to clutch packs  56  can be varied quickly (e.g., on the order of milliseconds in an example prototype embodiment) and remotely (e.g., by steering in embodiments where actuator  60  is actuated by the hydraulic steering circuit, or otherwise from a control mounted in the vehicle passenger compartment). 
     Axle assembly  30  may provide the functionality of a wheel locking and unlocking device. In particular, assembly  30  has been shown in an example prototype embodiment to reliably maintain wheel hub  24  in its normally locked mode relative to shaft  22 . In the context of, for example, off road vehicle driving, the capability provided by assembly  30  to quickly and remotely lock and unlock wheel hub  24  and shaft  22  advantageously permits a vehicle to be normally operated in locked mode and easily fully or variably unlocked as circumstances require. 
     Axle assembly  30  provides performance characteristics similar to a locked differential while providing reduced risk of breaking power-train components when a sudden traction event occurs. In particular, the force exerted by spring washer  58  on clutch packs  56  may be configured such that the torque required to displace friction discs  52  relative to clutch plates  54  (e.g., the torque at which the static friction limit of clutch pack  56  is exceeded) is marginally less than the torque that would break power-train components. Where axle assembly  30  is so configured, a potentially drive-train-breaking torque will cause clutch plates  54  to slip relative to friction discs  52 , which may prevent transmission of the potentially damaging torque to drive-train components. 
     Axle assembly  30  may be controlled to provide desired steering dynamics. For example, the force applied to the clutch pack  56  coupling the driveshaft to the inside wheel of a turn may be made less than the force applied to the clutch pack  56  coupling the driveshaft to the outside wheel. For another example, the force applied to the clutch pack  56  coupling the driveshaft to the inside wheel of a turn may be made greater than the force applied to the clutch pack  56  coupling the drive shaft to the outside wheel. 
     Wheel hub  24  is mounted relatively close to flange  22 A at the end of shaft  22 . As a result, axle assembly  30  increases vehicle track by only a small amount as compared with mounting a wheel on flange  22 A. 
     In embodiments where clutch drum  40 , spool  50  and clutch pack  56  are located outward of wheel hub  24 , such that a wheel mounted on wheel hub  24  may protect clutch drum  40 , spool  50  and clutch pack  56  from radially-inwardly directed forces. 
     A wheel may be mounted on wheel hub  24  and clutch drum  40 , spool  50  and clutch pack  56  installed outward of the interface between the wheel and wheel hub  24 , so that maintenance and the like may be performed on clutch drum  40 , spool  50 , clutch pack  56 , actuator  60  and the like without removing the wheel. 
     The continuously variable and independent control of torque transmission through axle assemblies  30  provides flexibility in control of torque transfer between driveshaft  18  and wheel hubs  24 . While the steering angle is within a range about center the force regained to lock clutch packs  56  on both left and right drive wheels is applied by the spring washers  58  in each clutch  30 . When the steering angle is moved outside of the center range the actuator&#39;s force from actuators  60  on spring washers  58  relieves the locking compression of the clutch pack  56  on the inside (or outside) of the turn indicated by the steering angle to permit the inside wheel to slip relative to its shaft  22  (or to allow the outside shaft to slip relative to the outside wheel). 
     A sensor may detect the force acting on the suspension, and when the suspension force indicates the vehicle is airborne, actuators  60  may be actuated to unlock clutch packs  56 , and the actuator force removed to re-lock the clutch packs  56  after the suspension force indicates that the vehicles is no longer airborne. Advantageously, this manner of control may reduce the risk of breaking powertrain components when the wheels suddenly gain traction upon landing. 
     Where a component is referred to above, unless otherwise indicated, reference to that component (including a reference to “means”) should be interpreted as including as equivalents of that components any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example: Though in the described example embodiments a normally locked spring-biased hydraulically unlocked multiple-plate clutch is used, other types of clutches may be used, including electromagnetically actuated multiple-plate clutches, electromagnetic powder clutches, and the like. In embodiments where an electrically operated clutch is used to, electrical power may be provided to the actuating element of the clutch (functionally to unlock the clutch pack) via a slip ring or the like. 
     The components of axle assembly  30  may be simplified, re-arranged, integrated and/or connected differently. For example, clutch drum  40  may be adapted to be the drive element, and spool  50  may be the driven element (i.e., clutch drum  40  may comprise the driving member and spool  50  may comprise the driven member of a clutch). For another example, clutch drum  40  and spool  50  may be located inward of wheel hub  24 , for example mounted where a conventional differential would be located, even possibly as a retro-fit within an existing differential housing. Clutches  30  may replace the axle shafts in whole or in part. For a further example, clutch drum  40  and wheel hub  24  may comprise a single component (e.g., an integrally formed component). In certain embodiments it may not be required to use the actual hydraulic steering system to actuate actuator  60 , as it may be a completely separate system that is mechanically or electrically or hydraulically or otherwise activated by the steering system. Thus it could be done with no sensors, wires or valves etc. For yet another example, clutch drum  40  may be mounted on flange  22 A of shaft  22 , and rotating seal  36  mounted outward of clutch drum  40 . In the embodiment illustrated diagrammatically in  FIG. 7 , piston  64 ′ in actuator  60 ′ acts directly against spring washer  58  so as to remove the need to use the cylindrical open frame of pins  55  between caging washers  54 B and  54 C. Instead, piston  64 ′, illustrated to be hydraulically actuated, although this is not intended to be limiting, is mounted between clutch pack  56  and spring washer  58 , where spring washer  58  is mounted within the actuator chamber, for example interleaved between the piston and an expansion chamber  65 ′, where hydraulic fluid acts on the opposite side of piston  64 ′, entering through port  60   a′.    
     Components of axle assembly  30  may be integrated with shaft  22 . For example, shaft  22  may comprise spindle  32 . 
     The components of axle assembly may have relative dimensions that are different from the hydraulic actuator  60 . As seen in the diagrammatic views of  FIGS. 5 and 6 , hydraulic circuit  100  may be connected to the steering box  102 , itself connected to power steering pump  104 . Hydaulic circuit  100  provides hydraulic pressure to actuator  60  within each assembly  30 . In the embodiment of  FIG. 6 , steering system  110  may contain a hydraulic pump, and may include a secondary static hydraulic system that can be for example mechanically or electrically or hydraulically actuated (or any combination thereof), and may include a processor cooperating with sensors such as described above. Steering system  110  is connected to assemblies  30  by circuits  100 . 
     While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.