Abstract:
A linkage assembly of variable length extends between a steerable member and a frame member to define the center position of the steerable member. The linkage assembly includes a resistance assembly for providing a resistance force resisting forces tending to move the steerable member to either side of the center position, and a center holding assembly having a piston that may be locked in position in a cylinder for transmitting steering forces to the resistance assembly. A control valve is operable between a closed position for preventing fluid flow so that the piston is held in its locked position, and an open position for allowing the holding piston to move away from its locked position to permit the length of the linkage to freely change in response to the steering forces. The control valve is operated by an actuator responsive to the amount of the resistance force, such that the piston is free to move in its cylinder when the resistance force exceeds a predetermined amount.

Description:
FIELD OF THE INVENTION 
     This invention relates to vehicle steering systems and more particularly to a device for holding the steerable wheels of a vehicle, such as a motor home, bus, truck, automobile or the like, so that a center steering position is maintained in spite of spurious steering inputs, such as those caused by variable crosswinds, crown curvature or slant of the highway, or other factors tending to adversely affect vehicle steering by the driver. 
     BACKGROUND OF THE INVENTION 
     The steering systems of highway vehicles and the like are designed primarily for driver control. In these systems, the steering force required on the steering wheel and the ratio between steering wheel movement and movement of the steerable ground wheels depend upon the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of extraneous forces can act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise more precise and careful driving control. 
     Vehicles with steering systems having positive caster generally track relatively straight ahead and generally resist normal steering inputs away from center, including those of the driver. Intentional turning maneuvers by the driver therefore require sufficient turning force to overcome this positive resistance to movement away from center. When the driver relaxes the turning force applied to the steering wheel, a positive caster system has a definite tendency to return to its straight ahead position, although it may overshoot the neutral or center position if the steering wheel is entirely released. 
     While positive caster is desirable in some respects, it is not without compromises over the full steering spectrum. For example, the adverse effects of strong gusty cross winds are usually more pronounced with large amounts of positive caster. As its name would imply, the vehicle tends to caster towards the side of the roadway to which it is being pushed by the wind. Similarly, a high crown at the center of the roadway or a slanted roadway can cause vehicles to turn toward the edge of the roadway, that is, in the downhill direction. In addition, generous positive caster provides significant resistance to small radius turns, which can make city driving quite fatiguing. These three adverse effects are some of the negative aspects of achieving steering stability through generous amounts of positive caster. 
     Another drawback of prior art steering systems is that spurious inputs transmitted from the roadway through the steerable wheels affect substantially the entire steering assembly before encountering any stabilizing resistance from the steering wheel. The negative action of the steerable wheels is caused by spurious steering inputs from crosswinds, slanted or crown roads, bad road surfaces, and other adverse dynamic steering forces. Inherent geometric steering characteristics may also be responsible for spurious steering inputs. 
     The transmission of these various inputs between the steerable wheels and the steering wheel causes the interconnecting components of the steering system to repeatedly oscillate between states of tension and compression. Such oscillations cause wear and slack in ball joints and other connections and have long been considered a primary source of stress fatigue which can lead to premature failure of various steering system components. Mechanical slack due to worn parts can also be a cause of steering system oscillations and vehicle wandering that require constant corrections and therefore produce driver fatigue. 
     The ideal driving situation is therefore one where the steering system inherently causes the vehicle to travel in an unswerving straight line unless the driver intentionally turns the vehicle in another direction. Thus, the ideal steering system would require relatively little attention from the driver as the vehicle progresses along a straight line path down the roadway. From a steering standpoint, the vehicle should not respond to anything but the driver&#39;s steering commands and these must be of sufficient magnitude to overcome a significant resistance to turning away from center. In the absence of a steering input by the driver, the vehicle should literally do nothing but progress straight ahead. 
     SUMMARY OF THE INVENTION 
     The invention provides improved on-center holding of the steerable wheels, and significantly reduces driver fatigue because it results in a major reduction in driver steering inputs. The holding assembly is easily activated by the driver while driving the vehicle, and its activation makes driving more pleasurable and less fatiguing. 
     The center holding assembly of the invention comprises linkage means of variable length that extends between the steerable wheels and an axle or frame member such that the length of the linkage means defines the center position of the steering system. The linkage means comprises a resistance assembly that provides a resistance force for resisting steering forces tending to move the steerable wheels to either side of the center position, and a center holding assembly for transmitting the steering forces to the resistance assembly. 
     The holding assembly comprises a holding piston, a holding cylinder providing first and second holding chambers, one on each side of the holding piston, a fluid transfer system for providing a flow of fluid to and from each of the holding chambers, and a solenoid operated valve for controlling the fluid flow. The control valve is operable between a closed position for preventing the fluid flow to hold the piston in a locked center position, and an open position for allowing the piston to move away from its center position in the holding cylinder. Movement of the holding piston causes fluid flow to one of the holding chambers and fluid flow from the other of the holding chambers. This fluid flow permits the length of the linkage assembly to change relatively freely in response to steering forces, which in turn permits the steerable wheels to move freely to either side of their center position in response to applied steering forces for small radius turns as used for going around corners in city driving. 
     The resistance force provided by the resistance assembly is preferably produced either by a dual spring mechanism or by a second fluid system comprising a pair of pistons in a cylinder, and an accumulator. An electrical actuator system is responsive to the resistance force for operating the solenoid control valve so as to prevent holding piston movement away from its locked center position until the resistance force reaches a predetermined level. 
     The actuator system includes a first sensor for providing a first signal in response to the predetermined resistance force, a second sensor for providing a second signal in response to the holding piston being in its locked position, and a control assembly. The control assembly causes the solenoid valve to move to its open position in response to the first signal and thereafter keeps the valve open during holding piston movement away from its locked position. The control assembly actuates the solenoid valve to its closed position only when the holding piston returns to its locked position after the vehicle has completed a small radius turn. 
     The resistance assembly includes a component that moves in response to the applied steering force, and movement of this component resists relatively small and limited changes in the length of the linkage assembly, such that relatively small movements of the steerable wheels to either side of their center position are opposed by the resistance force. These small movements correspond to the very large radius turns that occur when a vehicle is steered through maneuvers at highway speeds (as opposed to the small radius turns that occur when a vehicle turns a corner). Thus, during large radius turns, the resistance assembly provides a resistance force that biases the steerable wheels back toward their center position, and this bias serves as a return force to return the steerable wheels to their center position upon removal of the steering force producing the large radius turn. 
     On the other hand, during small radius turns, the resistance assembly is rendered ineffective (and may be said to be deactivated) by the control assembly causing opening of the solenoid valve, which in turn allows movement of the holding piston away from its locked position in response to the steering force applied to produce the small radius turn. A preferred way of “triggering” the opening of the solenoid valve is for the first sensor to provide its signal in response to a predetermined amount of the aforesaid resistance component movement that is proportional to the resistance force. 
     Once the solenoid valve is opened to allow movement of the holding piston away from its locked center position, this valve remains open until the piston returns to its center position so that no resistance force will be applied to the steering system during small radius turning maneuvers of the steerable wheels. In addition, the actuator system includes a remote switch for turning the actuator system on and off remotely from the driver&#39;s station of a vehicle. When the remote switch is in its off position, the solenoid valve remains open so that no resistance force is applied to the steering system by the linkage assembly because the holding piston is not “locked” and is therefore free to move away from its center position in the holding cylinder. 
     The assembly is preferably connected between the steering system and the front axle or a nearby frame member of the vehicle in a position that allows the steerable member(s) to move through its full range of steering movements while providing sufficient leverage for the apparatus to resist movement of the steerable member away from the center position producing straight ahead travel of the vehicle. The steering system connection may be made to any steering system component providing appropriate range and leverage, such as a tie rod which joins the two front steerable wheels of a highway vehicle, or the pitman arm connected to the reduction gear. The frame connection may be made to any component serving as a fixed mounting relative to the steering system. This fixed component may be a frame member, or an axle or some other part carried by the vehicle frame instead of an actual frame member. 
     The invention may be used with steering systems having a reduction gear between the steering wheel and the steerable wheels. In this application, the assembly is preferably connected to the steering system at a location between the steerable wheels and the reduction gear so as to be unaffected by any slack in the reduction gear or in components and connections between the reduction gear and the steering wheel. It is therefore on the slow side of the reduction gear ratio. The invention thus provides a zero backlash center holding assembly. 
     An adjustment means accessible by a mechanic provides for different levels of steering force to initiate or breakaway into a steering movement away from center, depending on the size, type and steering characteristics of the vehicle. This level of force is sometimes referred to in this specification as the “break away resistance”. Different levels of break away resistance and of resistance force may also be appropriate to compensate for changes in the forces acting upon the vehicle. Thus, the resistance force provided by the invention may be increased or decreased to provide a level of force sufficient to overcome any spurious steering inputs and to suit driver road feel, particularly a feel of the steering wheel that lets the driver know when the steered wheels are beginning to move away from center and are closely approaching return to center. 
     In the absence of the invention, spurious inputs to and/or mechanical slack in the steering assembly require almost constant manipulation of the steering wheel by the driver and make it almost impossible for the driver to hold the vehicle on a true straight ahead course. Use of the invention therefore permits a substantial reduction in the caster angle of vehicles with positive caster, thereby significantly reducing the crosswind effect and provides the driver with a positive touch control not heretofore attainable with positive caster. 
     In other words, the invention provides a distinctive feel when approaching or leaving the center position. Thus, the sense of touch is added to the visual sense to aid control of the vehicle and reduce driver fatigue. The turning resistance selected should satisfy the road feel desired by the driver and be sufficient to overcome anticipated spurious inputs. Positive stability is thereby achieved for previously unstable steering systems. 
     Although the present invention is particularly useful as a center holding assembly for motor vehicles, it can be employed to hold the center position of any steerable member moveable to either side of a preselected position. For example, the assembly can keep an outboard motor centered so that a boat follows a straight course over the water in the presence of spurious steering forces produced by wind and wave action. The assembly can also be used to keep centered such steerable members as the rudders of ships or airplanes and the tongues of tandem trailers or railway cars. The assembly is useable with both power and non-powered steering systems, with the level of holding forces provided usually being less for vehicles without power steering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, both as to its structure and operation, may be further understood by reference to the detailed description below taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a plan view illustrating installation of the invention between the frame and steering system of a motor vehicle; 
     FIG. 2 is a sectional view taken along line  2 — 2  of FIG. 1 showing the structure connecting the turning resistance assembly to the vehicle frame; 
     FIG. 3 is a schematic diagram of the fluid and electrical systems and of the major components of the invention, and includes sectional views showing structural details of the holding cylinder assembly and the turning resistance assembly; 
     FIG. 4 is an enlarged partial sectional view of the locking valve portion of the holding cylinder assembly taken along line  4 — 4  of FIG. 3; 
     FIG. 5 is a schematic diagram of the fluid and electrical systems and of the major components of a modification of the invention, and includes sectional views showing structural details of the holding cylinder assembly and the turning resistance assembly; and 
     FIG. 6 is an enlarged partial sectional view of the locking valve portion of the holding cylinder assembly taken along line  6 — 6  of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The center holding system of the present invention comprises a linkage assembly, generally designated  10 , which may be connected between a front axle or frame member  12  and the tie rod  14  of a conventional motor vehicle as shown in FIG. 1 of the drawings. Steering inputs by the driver are transmitted to the tie rod  14  by the pitman arm (not shown) of the vehicle&#39;s steering gear. The outer end of a holding rod  16  of linkage assembly  10  is connected to the tie rod  14  by means of a ball joint  18  connected between the outer rod end and tie rod  14  by a bracket  20 . 
     The outer end of a resistance rod  22  at the opposite end of linkage assembly  10  is connected to the axle  12  by means of a mounting bracket  24  to which is secured a stub  13  carrying the ball element  15  of a ball joint  26  slidingly carried on a mid-portion of the resistance rod  22  by a sleeve  17  as shown in FIG.  3 . The ball joints  18  and  26  permit pivotal movement in the vertical plane and to a limited extent in the horizontal plane, and are conventional joints wherein an enlarged spherical ball  15  is held for pivotable movement within a surrounding journal structure  19  carried by the stub. 
     The components of the holding system and the way in which they hold center at highway speeds, and return a vehicle steering system to center only from large radius turns at those speeds, will now be described. It is to be understood that the components described are connected together by appropriately sized fluid conduits and electrical wires and that these conduits and wires are represented by the lines interconnecting the components as shown in the drawings. 
     Referring now to FIGS. 3 and 4, the linkage  10  includes a holding assembly  28  and a resistance assembly  30 . Holding assembly  28  comprises a holding cylinder housing  32  enclosing an interior cylinder  33 , which is divided into two chambers  34  and  35  by a holding piston  36  secured to the inner end of holding rod  16 . The inner end of rod  16  also carries a magnetic disk  38 , the purpose of which will be described below. The outer ends of housing  32  and cylinder  33  are closed by a head  40  having a journaled and sealed aperture  41  for sliding passage of holding rod  16 . A set of multiple seals  42  and the head  40  are secured in place by an end cap  44  threaded or crimped into the cylinder housing  32 . The end portion of holding rod  16  projecting beyond cylinder housing  32  is surrounded by a dirt and grease barrier in the form of a flexible boot  48 . 
     The inner end of cylinder  33  is closed by an intermediate head  50  in which is mounted an electrically operated solenoid valve  52 , the purpose of which is also described below. The side of intermediate head  50  opposite to holding cylinder  33  closes a reservoir cylinder  53  that is also enclosed by the housing  32 . The end of cylinder  53  opposite to head  50  is closed by an end head  54  to define a reservoir chamber  55  containing a resilient cylindrical pad  56  made of a foam plastic with closed cells, such that an increase in pressure caused by fluid flowing into chamber  55  will compress and thereby decrease the size of the pad  56 . The closed cell foam material chosen for pad  56  is preferably selected such that the pad will be compressed by about 50% in response to a pressure of about 30 psig. In turn, such a pressure increase will be maintained by the resiliency of the pad  56  to cause an outflow of fluid from chamber  55  when an outflow path is provided as described below. The chamber  55  of cylinder  53  thereby serves as an accumulator for fluid discharged from holding chambers  34  and  35 . 
     As may be seen best in FIG. 4, the solenoid valve  52  is secured to the intermediate head  50  by an adaptor  58  and has a tapered valve stem  59  that fits within a correspondingly tapered valve seat when the valve is actuated to its closed position by an electric current supplied to the solenoid  60  by an electrical switch assembly  62 . In the absence of electrical power, valve stem  59  is pushed to its open position by a compressed return spring  61 . When the valve  52  is in its open position (deactivated), a fluid, preferably a liquid, may be supplied to the holding chambers  34  and  35  and the reservoir chamber  55  via a Schraeder type pressure valve  64  communicating with reservoir chamber  55  via internal passages  65 ,  66 , and  67 ; communicating with holding chamber  34  via internal passages  65  and  68  and an annular passage  69  between the housing  32  and the cylinder  33  leading to chamber port  70 ; and communicating with holding chamber  35  via internal passages  65 ,  71 , and  72  leading to a chamber port  73 . The chambers  34 ,  35 , and  55  and their interconnecting passages are preferably filled with a liquid via valve  64 , and for this liquid fill, a passage  57  in end head  54  connects reservoir chamber  55  to ambient via a bleed valve  63  for bleeding air from these chambers as they are filled with liquid. 
     When valve  52  is open, piston  36  is relatively free to move back and forth in holding chambers  34  and  35  away from a center position C 1 , in which piston  36  is “locked” when valve  52  is closed. The availability of an accumulator reservoir, such as reservoir  55 , is required even though the fluid may flow back and forth between holding chambers  34  and  35  via ports  70  and  73  and interconnecting passages  69 ,  68 ,  71 , and  72 , when valve  52  is open. This is because of the volume of chamber  34  taken up by the presence of holding rod  16 , which makes the volume change in chamber  34  by the movement of piston  36  different from the volume change in chamber  35  by such movement. 
     The reservoir  55  may also receive fluid discharged through internal passages  74  and  67  by a relief valve  75  for preventing an excessive positive pressure in chamber  35 , which could unduly restrict steering system movement. A similar restriction of steering system movement that could be caused by a vacuum in chamber  35  is prevented by an excessive negative pressure relief valve  76  in an integral passage  77 . Similarly, excessive positive pressure relief for holding chamber  34  is provided by a relief valve  78 , and excessive negative pressure relief for chamber  34  is provided by a pressure relief valve  79 , valves  78  and  79  being mounted in corresponding through passages in holding piston  36 . To prevent the possibility of a pressure differential developing across valve stem  59  and interfering with its operation, the valve stem includes a diagonal passage  80  interconnecting the overlying stem retraction chamber  81  with the underlying central passage  66 . 
     The reservoir end head  54  is connected to the frame bracket  24  by the resistance assembly  30 , which includes the ball joint  26  comprising the bracket mounting stub  13 , the ball member  15  and the annular journal member  19  for carrying and permitting articulation of the ball member  15 . Mounted in the ball member  15  is the sleeve  17  for slidingly receiving the resistance rod  22  so that the ball joint may slide back and forth along the rod  22  in its axial directions. 
     An inner compression spring member  86  surrounding rod  22  is mounted between the ball  15  and the outer end face of the reservoir end head  54 , and an outer compression spring member  87  surrounding rod  22  is mounted between ball  15  and a lock nut  89  threaded onto the distal end of rod  22 . The spring members  86  and  87  are surrounded by dirt and grease barriers in the form of flexible boots  90  and  91 , respectively. Although other types of compression spring members may be utilized, the members  86  and  87  are preferably made from a plurality of stacked Belleville spring washers. These are annular washers of spring steel that have a convex shape so as to form an accordion-like spring member when a plurality thereof are placed on the rod  22  with the convex side of adjacent washers facing in opposite directions. 
     When so arranged, the spring force of the spring members  86  and  87  oppose movement of the ball member  15  in either direction away from a center position represented by the centerline C 2 . The number and type of Belleville washers are preferably chosen to provide about ½ inch of movement of the ball joint to either side of the center position, this reciprocal movement being represented by the arrow R and the limits of this movement being represented by the pair of broken lines R 1  and R 2 . Where the resistance rod  22  has a diameter of about ⅝ inches, about 22 Belleville washers may be used for each of the spring members  86  and  87 . 
     Adjustment of the lock nut  89  in either of the axial directions along rod  22  allows adjustment of the initial compression preload applied by the spring members  86  and  87  when the ball joint is in its center position. A preload of about 200 lbs. to about 250 lbs. is preferred, and this amount of preload will compress each spring pack by about ½ of the distance that the spring pack may be compressed before all of the washers are flattened. With this preload, the maximum resistance force opposing turning movement away from center will be about 350 lbs to about 400 lbs., as measured at the tie rod. 
     At the top of the mounting stub  13  of ball joint  26  is a magnet  94  and, mounted in close proximity, is a centering limit switch  95  having a magnetic pickup or sensor  96  centered on the centerline C 2 . Switch  95  is activated by the movement of magnet  94  beyond the range of sensor  96 , this range corresponding to the limit R 1  as rod  22  moves to the right and to limit R 2  as rod  22  moves to the left in FIG.  3 . To ensure that magnet  94  is not moved out of sensor range by rotation of the bearing around the axis of rod  22 , an anti-rotation member  98  is affixed to end head  54  and is in the form of a double-slotted tube in which an upper slot  82  is arranged to engage opposite sides of an upper portion of the bearing body and a lower slot  83  is arranged to engage a lower portion of the bearing body as shown in FIG.  2 . The anti-rotation member  98  also provides a base on which the switch  95  is mounted by a bracket  100 . 
     The operation of the holding assembly  28  and the resistance assembly  30  will now be described in more detail. When the steerable wheel or member of a vehicle is turned to either side of its center position, either by a steering force transmitted from the steering wheel  85  or other steerable member or by spurious steering forces transmitted to the steerable member by a roadway or the like, this steering movement is resisted by the resistance assembly  30  of the linkage assembly  10 . Until the steering force exceeds the compression preload of the spring members  86  and  87 , i.e., 200 lbs., for example, there will be no movement of the steerable member. After the compression preload is exceeded, small turning movements are permitted within the range between R 1  and R 2 , such as for example up to a steering force of about 350 lbs. as applied to the tie rod  14 . This maximum steering force is opposed by an equal amount of resistance force provided by the compression spring force, which biases the ball joint and the entire steering system backs toward their center positions. The small turning movements permitted are sufficient for maneuvering a motor vehicle at highway speeds, and the resistance force bias returns the steering system to center after such maneuvers, which also may be referred to as large radius turns. 
     At the maximum steering force limit, the body of stub  13  is either in the position R 1  or the position R 2  such that the magnet  94  has moved beyond the sensing range of sensor  96 , thereby activating the switch  95 . When activated, switch  95  sends an electrical signal via a wire  102  to a control unit  62 . This signal causes the opening of a switch  103  in a relay  104  which interrupts electrical power provided to the solenoid  60  of valve  52  via electrical lines  105  and  106  from a power supply  107 . 
     The loss of electrical power to solenoid  60  causes retraction of valve stem  59  into chamber  81  in response to return compression spring  61 , thereby placing valve  52  in its open position. When valve  52  is in its open position, fluid is free to flow back and forth in the internal passages in intermediate head  50  as previously described. For example, the piston  36  is free to move toward the right in FIG. 2, thereby causing fluid to flow from holding chamber  35  into holding chamber  34  through passages  72 ,  71 ,  68 , and  69 , and into reservoir chamber  55  through passages  72 ,  71 ,  66 , and  67 . On the other hand, movement of piston  36  to the left in FIG. 2 causes fluid to flow from holding chamber  34  into holding chamber  35  via passages  69 ,  68 ,  71 , and  72 , and into reservoir chamber  55  via passages  69 ,  68 ,  66 , and  67 . However, in this instance, there may be relatively little or no flow into reservoir chamber  55  because the volume of chamber  35  will increase more rapidly than the volume of chamber  34  will decrease, due to the presence of holding rod  16  in chamber  34 . 
     Although the piston  36  is relatively free to move back and forth in the chambers  34  and  35  when solenoid valve  52  is open, it is preferable that the internal passages in intermediate head  50  be sized to provide a dampening action sufficient to prevent overly rapid movements of the steerable member away from its center position, such as may otherwise occur during the blowout of a tire on a steerable wheel. The invention may thus provide a relatively high degree of protection against a loss of vehicle steering control due to tire blowouts or other accidental impacts to a steerable wheel. 
     After the solenoid valve  52  has been opened by a signal from switch  95 , it will remain open until the magnet  38  on the inner end of holding rod  16  returns to its center position C 1  beneath a magnetic pick-up or sensor  109  of a center position switch  110 . When the magnet  38  is centered at the position C 1 , the switch  110  provides a signal to the receiver  112  via electrical line  111 . At this time, the earlier signal provided to the receiver  112  by switch  95  has ceased because the magnet  94  immediately returns to center on the centerline C 2  under the action of spring member  86  or  87  upon the opening of valve  52 . Upon receiving a signal from switch  110  without a signal from switch  95 , the receiver  112  provides a current to coil  114  in relay  104  and thereby closes relay switch  103  of control unit  62 , which in turn closes valve  52  by feeding current via line  108 . 
     The control unit  62  optionally may also include a remote receiver section  116  for receiving a remote signal inputted to an antenna  117  from a remote signal generating unit  118  that may be located on the steering wheel  85  or elsewhere within the driver&#39;s station of a motor vehicle using the invention for centering the vehicle&#39;s front steerable wheels. The signal generating unit  118  and the signal receiving section  116  are of conventional design and allow the driver of the vehicle to turn the system on and off as desired. With this option, the switch  103  of relay  104  cannot be closed until the receiver section  116  receives an ON signal from the remote unit  118 . Similarly, an OFF signal from unit  118  opens switch  103 . 
     Referring now to FIGS. 5 and 6, there is shown a modification  30 ′ of the resistance assembly wherein a cylinder and dual piston arrangement is substituted for the spring arrangement of FIG. 3. A resistance cylinder housing  149  encloses two separate interior cylinders  150  and  151 , the adjacent ends of which are connected together by a collar  152 . The cylinders  150  and  151  contain resistance pistons  142  and  144 , respectively, and these components together define respective fluid chambers  146  and  148 . An enlarged rod head  140  is keyed and fastened to the inner end of a resistance rod  22 ′. Head  140  is positioned between opposing faces of pistons  142  and  144  and serves as the actuator for these pistons. 
     A breather port  147  is provided in collar  152  to allow air to enter and leave the space between pistons  142  and  144  as they reciprocate in cylinders  150  and  151 . Port  147  vents to an annular space S between the exterior of cylinders  150  and  151  and the interior of housing  149 . In turn, the annular space S may vent to ambient via a breather line or to a vent reservoir (not shown). The end portion of resistance rod  22 ′ projecting beyond cylinder housing  149  is connected to the mounting bracket  24  by a conventional ball joint (not shown), and also may be surrounded by a dirt and grease barrier in the form of a flexible bellows (not shown) similar to that shown for holding rod  16  in FIG.  1 . 
     The end of cylinder  150  opposite to collar  152  is closed by a head  158  having a journaled and sealed aperture  160  for sliding passage of resistance rod  22 ′. A set of multiple seals  162  and the head  158  are secured in place by an end cap  164  threaded or crimped into the cylinder housing  149 . Piston  142  has a sealed aperture  166  for sliding passage of rod  22 ′ during its movement of piston  144 . A piston rod seal  168  and a journal  170  are secured in piston  142  by a snap ring  172 . The end of cylinder  151  opposite to collar  152  is closed by one end of the valve body  50 ′ and the two cylinders  150  and  151  are secured to the collar  152  and the opposite respective closures by crimping at positions beyond seal rings  174 ,  175 ,  176  and  177 . Pistons  142  and  144  are shown in their rest positions corresponding to a centered steering system. These positions are offset toward the valve body  50 ′ (to the left in FIG. 5) in order to equalize the volumetric capacity of chambers  146  and  148  since chamber  146  also contains a portion of rod  22 ′. Both pistons are arranged for compressive movement toward the opposite ends of their respective chambers, piston  142  traveling in chamber  146  and piston  144  traveling in chamber  148  within cylinders  150  and  151 , respectively. 
     The rest or retracted position of each piston is defined by the internal annular collar  152  which serves as a piston stop and preferably has an axial width substantially (preferably within one-ten thousandth of an inch) equal to the axial thickness of rod head  140 . A stop width greater than the head thickness is undesirable because gaps between opposing surfaces would allow unbiased movement (slack) between rod  22 ′ and cylinders  150  and  151 . A stop width less than the head thickness is also undesirable because this would let fluid flow back and forth between chambers  146  and  148  through a connecting conduit  153  so that the pistons  142  and  144  would move (drift) together until one of them bottoms out against the collar  152 , there being no pressure differential applied to the rod head during such joint piston movement. 
     A fluid pressurization system  84  constantly biases the resistance pistons  142  and  144  into substantially simultaneous engagement with both the stop collar  152  and the piston rod head  140  at all times when the assembly is pressurized and in its center position, such that there is no significant slack or drift at any time during its operation. Near the end of each centering chamber opposite to the retracted piston position is a port for communicating fluid pressure to the chamber, port  154  serving chamber  146  and port  156  serving chamber  148 . Ports  154  and  156  are connected together by the annular conduit  153  so that both ports communicate with chamber  148 , which in turn is connected via a conduit  157  to an accumulator  162 , which preferably provides hydraulic fluid under pressure to the centering chambers  146  and  148 . 
     The vehicle steering system is properly centered when pistons  142  and  144  abut collar  152 . In order to move or break away from collar  152 , these pistons must overcome the resistance provided by accumulator pressure acting through the conduit  157 , which may contain an in-line filter (not shown). An important feature of the fluid resistance unit  30 ′ is the difference in diameters between cylinders  150  and  151 , the diameter of cylinder  150  being larger by an amount sufficient to produce equal resistance forces on pistons  142  and  144  in spite of the area of piston  142  lost because the resistance rod  22 ′ passes therethrough. Thus, to provide equal working areas, the cross-sectional area of cylinder  150  is greater than the cross-sectional area of cylinder  151  by an amount sufficient for the annular surface area of piston  142  to be equal to the disk surface area of piston  144 , the difference in the cylinder cross-sectional areas being equal to the amount of piston area lost by reason of rod aperture  166  in piston  142 . 
     The accumulator  162  has a gas chamber  163  over a liquid chamber  164 , both chambers being pressurizable to a selected pressure by a one-way valve gas  159 , such as a Schraeder valve, in a cap  161  closing a liquid fill tube  186 . Although a gas such as air could also be used for the resistance fluid in chambers  146  and  148 , a liquid resistance fluid is preferred because it is substantially incompressible as compared to a gaseous fluid, and therefore provides the capability of a viscous dampening action as the liquid flows through a restricted passage, such as that provided by an orifice  165 . 
     The liquid chamber  164  of accumulator  162  should be large enough to receive the entire volume of fluid from either chamber  146  or  148  without unduly compressing the gas in chamber  163 . The fill tube  186 , which is normally closed by cap  161  carrying valve  159 , allows liquid chamber  164  to be filled with hydraulic fluid up to the desired level as determined by a sightglass  165 . 
     Gas pressure in chamber  163  acts against the liquid surface to store fluid energy received from the hydraulic side of the system, and to maintain fluid pressure on the liquid side of the respective pistons  142  and  144  until such time as the gas pressure is released to ambient atmosphere by actuation of Schraeder valve  159 , which may be actuated to release air in the same manner as a vehicle tire valve. A pressure relief valve (not shown) may communicate with the gas chamber side of the accumulator to provide an upper limit to the resistance and return forces that may be generated by compressive contact of the piston rod head with one or the other of the resistance pistons. 
     Accumulator  162  also allows hydraulic pressure in the centering chambers  146  and  148  to be precisely set at a selected value within a relatively wide range corresponding to the gas pressures available. The gas trapped in gas chamber  163  provides a spring-like resistance force and this chamber may be sized such that the gas pressure and corresponding resistance force do not vary significantly with compressive piston movement. The gas pressure in chamber  163  is preferably in the range of about 200 to about 400 psig, and more preferably about 250 to about 300 psig. The gas pressure control may comprise the Schraeder valve  159  in combination with pressure gauge  176  to indicate accumulator pressure. 
     Alternately, a pressure regulator (not shown) may be used for maintaining a manually selected gas pressure in accumulator  162  where the vehicle has an air system, such as for air brakes. The regulator may be provided with a control knob to permit varying the pressure settings of the regulator by hand. By varying the gas pressure in gas chamber  163  through adjusting the pressure, the break away resistance and the centering return force produced by the pistons can be increased or decreased as desired. The range of pressures available should be selected so that break away resistance remains relatively high since the system is preferably used for centering at highway speeds. 
     For lighter vehicles, such as automobiles and pickup trucks, the accumulator pressure and other compensator parameters may be chosen so that a linear break away steering force of at least about 30 pounds, preferably at least about 50 pounds, more preferably at least about 100 pounds, and most preferably at least about 150 pounds, must be applied to the tie rod by the pitman arm in order to initiate a break away turning movement of the steerable wheels. For heavier vehicles, such as eighteen wheel trucks and motor homes, these parameters may be chosen to require a linear break away steering force of at least about 200 pounds, preferably at least about 225 pounds, and more preferably at least about 250 pounds. These turning forces are opposed by equal turning resistances which should be maintained for at least small turning angles away from center, preferably 0°-5°, more preferably 0°-3°, and most preferably within one degree on either side of center. Small turning angles correspond to large radius turning maneuvers. 
     When piston  144  reaches the end of its travel in chamber  148 , this position is detected by a sensor  193  that sends a signal to control unit  62 ′ via a wire  194 . When piston  142  reaches the end of its travel in chamber  146 , this position is detected by a sensor  195  that sends a signal via a wire  196  to control unit  62 ′, which operates in the same way as control unit  62  of FIG. 2, except that two signal wires are used instead of one signal wire to signal the maximum permitted degree of turn to either side of center before deactivation and opening of valve  52  to permit free movement of piston  36  in cylinder  33 . 
     In the embodiment of FIG. 5, the reservoir cylinder and its internal components have been replaced by a second pressure system  85  having a separate accumulator  120  that is essentially the same as accumulator  162  previously described. Pressure system  85  also includes a sight glass  121 , a pressure gauge  122 , and a liquid fill tube  123  closed by a fill cap  124  carrying a Schraeder type air valve  125 . Removal of the cap  124  allows filling of the lower section  126  of the accumulator with a liquid, and Schraeder valve  125  allows filling the upper section  127  with a gas, such as air, preferably at a pressure in the range of about 20 to about 40 psig, more preferably about 30 psig. 
     The liquid under pressure in accumulator  120  is provided to the holding chambers  34  and  35  via a line  129  connected to a passageway  67 ′ in the intermediate head  50 ′ by a fitting  130 . The remaining passageways in the head  50 ′ are essentially the same as in the intermediate head  50  of FIG. 3, except the relief valve passages  74 ′ and  76 ′ are side by side instead of one over the other, and passage  65  is closed by plug  195  instead of valve  64  (FIG.  4 ), as seen best in FIG.  6 . The remaining passages and elements are the same and have the same numerical designations as in FIG.  3 . As the solenoid valve  52 ′, the control unit  62 ′, and the holding assembly  28 ′ also are essentially the same as in FIG. 3, the same numerical designations are used for the parts of these components in FIGS. 5 and 6. 
     Persons skilled in the art, upon learning of the present disclosure, will recognize that various modifications to the units, and to the components and the elements of the units, of the invention are possible without significantly affecting their functions. For example, a gas such as air may be used as the fluid in both the holding assembly and the resistance assembly of FIG.  5 . Also, other steering resistance units may be substituted for the resistance units  30  and  30 ′ of the present invention. In this context, the term “resistance unit” means any unit capable of applying a resistance force to the steering system that tends to return it to a centered position for straight-ahead travel of a steerable member. 
     Similarly, other pressurizing units utilizing the same or other fluids may be substituted for the hydraulic accumulator  120  and/or  162 . One such alternative is to replace these accumulators with pressure regulators  190  and  192  in gas supply lines  191  and  193  that are respectively connected to resistance unit supply lines  129  and  157 , as shown by broken lines in FIG.  5 . In this alternative, a pressurized gas instead of a pressurized liquid is used as the working fluid in both the holding assembly  28 ′ and the resistance assembly  30 ′. Accordingly, while the preferred embodiments have been shown and described in detail by way of example, further modifications and embodiments are possible without departing from the scope of the invention as defined by the claims set forth below.