Abstract:
A steering stabilizer apparatus for the steering system of a vehicle having a steerable member movable away from a selected center position in response to a steering movement. The apparatus includes a rotary member having a corresponding face with centering detents, a piston member having a corresponding face arranged opposite to the rotary member face with a centering detent aligned with each rotary member detent when the rotary member is in a centered position corresponding to the selected position of the steerable member. Bearing members are arranged to be pressed between the rotary and piston members and to be in contact with a seat of each of a pair of aligned detents when the steerable member is in the preselected position. The rotary member is rotated relative to the piston member in response to movement of the steerable member. A fluid system causes the piston and rotary members to be pressed together so that contact pressure between the bearing members and the seats of the aligned centering detents resists relative movement between the rotary member and the piston member and prevents movement of the steerable member away from the selected position until the steering force exceeds a predetermined value. Each of the detents include a ramp sloped outwardly from the seat to a track in the corresponding face, and the ramp is formed by a groove having substantially the same radius as the bearing member.

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 a 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. Thus, the adverse steering inputs caused by crosswinds are directly related to the amount of positive caster offset, which is a classic example of having to balance a benefit with a detriment. The small amount of stability gained from castering the steerable wheels on a non-windy day may be paid for many times over when driving in a crosswind because of the destabilizing effect of the crosswind caused by positive caster offset. Positive caster offset also allows steering inputs from rutted and other imperfect roadway surfaces to steer back against the driver and thereby cause road wander, which is a universal driving complaint, particularly by driver&#39;s of heavy vehicles such as trucks and motor homes. 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 adverse effects are some of the negative aspects of achieving steering stability through generous amounts of positive caster. 
     For the lack of a more advanced method, steerable wheel castering has been accepted by the industry as a low-cost method of achieving steerable wheel returnability. Accordingly, many over-the-road vehicles are provided with generous amounts of positive caster. Not much thought has been given to the self-defeating side effects of steerable wheel castering. The failure of the industry to recognize the critical need to provide directional stability by replacing steerable wheel castering with another method of achieving steerable wheel returnability may go down in history as one of the longest enduring vehicle design oversights. 
     Thus, a highly important consideration that has long been overlooked by the industry is that steerable wheel castering is directly responsible for road wander, crowned road steering wheel pull and cross wind steering problems. Keeping a vehicle tracking straight and under control currently requires an inordinate amount of driver steering corrections to counteract the adverse side effects of castered wheels. The repetitive task of making thousands of precise steering corrections mile after mile weighs heavily on a driver&#39;s physical and mental well-being, and may result in extreme driving fatigue. Thus, vehicle directional stability can best be achieved by stabilizing the on-center behavior of the steerable wheels with a more suitable method than the traditional steerable wheel castering used on many current production vehicles. 
     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 transmission of these 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 a center stabilizer assembly for 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 stabilizer assembly is easily activated by the driver while driving the vehicle, and its activation makes driving more pleasurable and less fatiguing. The stabilizer assembly 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 unit that provides a resistance force for resisting steering forces tending to move the steerable wheels to either side of the center position, and a trim unit for transmitting the steering forces to the resistance unit. 
     The trim unit comprises a trim piston, a trim cylinder providing first and second trim chambers, one on each side of the trim piston, a fluid transfer system for providing a flow of fluid to and from each of the trim 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 centering position, and an open position for allowing the piston to move to a new centering position in the trim cylinder. Movement of the trim piston causes fluid flow to one of the trim chambers and fluid flow from the other of the trim 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 a new center position in response to an applied steering force. 
     The resistance unit includes a component that moves with the steering system in response to steering wheel movement, and resistance to movement of this component provides a resistance force opposing relatively small movements of the steerable wheels to either side of their center position. 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 unit 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 unit may be rendered ineffective to permit easy, away from center movements during such turns. 
     More specifically, the stabilizer has a pair of detent members with opposing faces, each with at least one centering detent. At least one bearing member is arranged to simultaneously contact rim bands around undercut portions of two opposing centering detents when a steerable member is in its preselected position. One of the detent members is connected to the steerable member and the other of these members is connected to the frame of the vehicle so that the bearing member and the detent members move relative to each other in response to movement of the steerable member away from its preselected position. A compressed spring is arranged to press the bearing member between the two opposing detent members so that sufficient contact pressure is maintained at all times to keep the bearing member firmly within the centering detent or in a groove defining a corresponding track in the face of each detent member for guiding the bearing member when it is moved outside of the detent. There are two tracks associated with each detent, one extending away in a direction opposite from the other. Each detent includes two sloped ramp segments each with one end adjacent to the centering detent and the other end fared into a corresponding one of the tracks, which may be flat (no slope) for providing substantially zero resistance or may have a gradually sloped (constant or changing) portion for providing a relatively small amount of resistance as described below. The ramp is also formed by a groove and the track and ramp grooves both have substantially the same radius of curvature as the bearing member so as to snugly fit and frictionally engage the bearing member to cause it to travel out of the detent, up the ramp, and along the track when the steering force exceeds a break away level of resistance. 
     There are preferably a plurality of bearings and a plurality of opposing of detent pairs, one pair being associated with each bearing. The pressing force between the bearing members and their centering detents resists relative movement between the bearing members and the detent members, and the pressing force provided by the compression spring is preferable supplemented by air pressure in a piston chamber adjacent one of the detent members that is in the form of a detent piston. Because the bearing and detent members are arranged between the steerable member and the frame of the vehicle, resistance to relative movement between these members prevents substantial movement of the steerable member away from its preselected center position until the steering force applied to the steering system exceeds a predetermined value corresponding to the level of break away resistance provided by the contact pressure between the bearing members and the centering detents. 
     The detent members are preferably opposing plates, namely a rotary plate arranged for rotary movement relative to a piston plate restrained from rotation. The bearing members are preferable a plurality of spherical ball bearings arranged in spaced relation to each other with a disc-like separator retaining this spaced relation. The bearings may have other shapes with curved outer surfaces, such as an oval shape. Each detent plate has a plurality of centering detents arranged relative to the bearing members and bearing retainer so that one of the bearings is received in each opposing pair of centering detents when the steering system is centered. The detents or depressions in each detent plate have a spaced relation corresponding to the spaced relation of the bearings. The bearings are pressed into the centering detents of the detent plates by a retaining spring to keep the plates and bearings in position and by means of air pressure in a cylindrical resistance chamber adjacent the piston plate, which serves as a reciprocating piston. The contact pressure between the bearing members and the detent plates, and thereby the resistance force, may be varied by varying the air pressure in the resistance chamber. 
     The invention also includes a feature for eliminating mechanical slack in the interaction between each bearing member and its corresponding centering detent. This slack-removal feature comprises providing each detent with an undercut bottom portion having a radius of curvature that is smaller than the radius of curvature of the bearing member. A narrow contact band may also be provided around the rim of the undercut portion. Although it may be slightly rounded by a convex shape, the width of this band extends generally along a line tangent to the curved surface of the bearing member, such that contact between the bearing member and the centering detent will occur substantially only along a line of contact. Where the transition between the rim of the undercut portion and the adjacent surface of the detent ramp would otherwise be relatively sharp, the contact band along which contact pressure occurs may be slightly convex (rounded) so as to minimize wear at the rim of the undercut portion. If this transition is relatively sharp, repeated travel of the bearing member over the rim may wear off the sharp edge in an uneven manner, resulting in intermittent bands of contact separated by areas of no contact. In each of these alternatives, contact between each bearing member and the rim of the undercut portion of its corresponding detent occurs along substantially a continuous line of contact. 
     The stabilizer includes means for remotely and selectively varying both the amount of resistance to movement away from center and the preselected position of the steerable member relative to the vehicle frame. Both of these remote adjustments can be made by the driver while the vehicle is in operation. A control system is employed for operating a solenoid and a pressure regulator and the switch and dial for actuating these devices are preferably located at the driver&#39;s station of the vehicle. The switch preferably has a toggle that is biased by a spring to the circuit opening position. These types of switches are closed only momentarily when the toggle is held in a depressed position against the spring bias. Thus, the solenoid is actuated only while the toggle is actually depressed. Release of the toggle opens the circuit and stops the adjustment at the point selected. 
     The level of resistance to movement away from center may be remotely adjusted either by such a manual control system operable by the driver or by a microprocessor control system responsive to the speed of the vehicle. Thus, the turning resistance of the present invention is readily adjustable to provide a low level at low speeds and a high level at speeds of about 35 mph or greater. In this regard, the centering stabilizer of the present invention is much less complex than prior art arrangements, such as those which combine high positive caster near the center position and complex power steering systems for varying the level of power assist from a low assist level for large radius turns to a high assist level for small radius turns. 
     The centering return force provided by positive wheel caster follows a force curve that may provide relatively little, if any, turning resistance in the straight ahead position or for large radius turns immediately adjacent to the straight ahead position. The bearings and detent plates are sized and the centering detents are sized and shaped or “cut” so as to provide a resistance force which blends with any return force provided by the normal geometry of the front end of a motor vehicle. The invention can increase substantially the turning resistance available at and immediately adjacent to either side of the straight ahead position of the steerable wheels. At greater turning angles (small radius turns), the resistance force provided by the invention preferably tapers off as positive caster return force increases. The turning resistance provided by the invention at or near the centered wheel position should be sufficiently large to resist spurious steering inputs generated either by the driver or by an overactive power steering system. 
     In a preferred embodiment, the shape of the centering detent and other stabilizer parameters are chosen so that a total break away steering force of at least 100 pounds, preferably at least 200 pounds, and more preferably at least 300 pounds must be applied to the tie rod in order to initiate break away turning movement of the steerable wheels at vehicle speeds above about 35 miles per hour. For city driving at vehicle speeds of about 35 miles per hour or less, the break away force required is preferably lowered to about 100 pounds, more preferably below about 50 pounds, at the tie rod. Where steerable wheels are provided with positive caster, which is usually the case with highway motor vehicles, the grooves in which the bearing members ride adjacent to the upper end of the ramp of the centering detents are shaped to form a neutral (no slope) cam surface which provides no further turning resistance. In other words, interaction between a bearing member and its corresponding detent in the detent member provides a decreasing level of resistance force as the steerable wheels move away from center, until the caster return force, which increases in proportion to turning angle with positive caster, is of sufficient magnitude to alone provide stabilizing resistance. However, the resistance force need not go to zero, but instead the slope of the track groove surfaces beyond the outer ends of the ramps may provide a resistance force effective over the entire range of turning angles, which for highway vehicles is usually limited to 45 degrees on either side of the straight ahead position (the “0” position). Preferably, there should be sufficient positive caster for the resistance force to be effective over the range of 0-10 degrees, more preferably 0-5 degrees and most preferably 0-3 degrees on either side of center. 
     The stabilizer 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. 
     The invention may be used with steering systems having a reduction gear between the steering wheel and the steerable wheels. In this application, the stabilizer 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 stabilizer assembly. 
     The level of steering force required to initiate or breakaway into a steering movement away from center is sometimes referred to in this specification as the “break away resistance”. Different levels of break away resistance and of resistance force may 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 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. 
     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 or elimination of the caster angle of vehicles with positive caster, thereby significantly reducing the crosswind effect and providing the driver with a positive touch control not heretofore attainable with positive caster. Positive stability is thereby achieved for previously unstable steering systems. 
     Although the present invention is particularly useful as a center stabilizer 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 stabilizer 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 stabilizer 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 stabilizer is useable with both power and non-powered steering systems, with the level of resistance 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 centering unit and the trim unit of the invention between the axle and the tie rod of a motor vehicle; 
     FIG. 2 is a diagrammatic illustration of the fluid and control systems connected to the centering and trim units of FIG. 1; 
     FIG. 3 is a horizontal cross section of the trim unit of FIG. 2; 
     FIG. 4 is an enlarged fragmentary view of the solenoid valve seat within the area identified by the broken line circle  4  in FIG. 3; 
     FIG. 5 is a diagrammatic illustration of the fluid passages and valves within the trim unit  7  of FIG. 3; 
     FIG. 6 is an exterior plan view of the centering unit shown as rotated counterclockwise by 90 degrees relative to its orientation in FIG. 2; 
     FIG. 7 is an elevational cross-sectional view of the centering unit as taken along line  7 — 7  of FIG. 6; 
     FIG. 8 is an elevational cross-sectional view of the centering unit as taken along line  8 — 8  of FIG.  6  and shows the unit in its centered or rest position; 
     FIG. 9 is a plan and partially fragmentary cross-sectional view of the centering unit as taken along line  9 — 9  of FIG. 8; 
     FIG. 10 is a plan and partially fragmentary view showing the centering unit in a moved position relative to the rest position of FIG. 8; 
     FIG. 11 is an elevational cross-sectional view of the centering unit as taken along line  11 — 11  of FIG. 10; 
     FIG. 12 is an enlarged fragmentary view showing details of a piston guiding feature when the centering unit is in the rest position of FIG. 8; 
     FIG. 13 is an enlarged fragmentary view showing details of the piston guiding feature when the centering unit is in the moved position of FIG. 10; 
     FIG. 14 is a plan cross-sectional view of the centering unit similar to FIG. 9, but with the balls and ball retainer removed to show the face of the piston plate; 
     FIG. 15 is an enlarged fragmentary view showing details of the piston plate face within the area identified by the broken line circle  15  in FIG. 14; 
     FIG. 16 is an elevational cross-sectional view of the piston plate as taken along line  16 — 16  of FIG. 15; 
     FIG. 17 is an enlarged fragmentary view showing details of one of the detents in the piston plate face within the area identified by the broken line circle  17  in FIG. 16; 
     FIG. 18 is an enlarged cross-sectional view of the piston plate face taken along line  18 — 18  in FIG.  15  and also shows a ball bearing member fully seated within an undercut seat segment of its detent; 
     FIG. 19 is an enlarged cross-sectional view of the piston plate face taken along line  19 — 19  in FIG.  15  and also shows a ball bearing member at an intermediate position along a ramp segment of its detent; 
     FIG. 20 is an enlarged cross-sectional view of the piston plate face taken along line  20 — 20  in FIG.  15  and also shows a ball bearing member entering its track at the upper end of its detent ramp; and, 
     FIG. 21 is an elevational cross-sectional view of the piston plate similar to FIG. 16 showing dimensional details of a detent relative to its ball member. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The steering stabilizer system of the present invention comprises a stabilizer assembly, generally designated  20 , which may be connected between a front axle or other frame member  21  and the tie rod  22  of a conventional motor vehicle as shown in FIG. 1 of the drawings. The steering system components shown are conventional and include bell cranks  23 ,  23  carried by knuckles  24 ,  24 , which support steerable wheels  26 ,  26  for pivotal turning movement about kingpins P,P mounted on the vehicle frame. Steering inputs by the driver are transmitted to the tie rod  22  by a Pittman arm (not shown) of the steering gear. The outer end of a centering rod  28  of stabilizer  20  is connected to the tie rod  22  by means of a ball joint  27  carried by a mounting bracket  29 . As may be seen best in FIG. 2, the stabilizer includes a resistance unit  33 , having a centering lever  35  with an inner end connected to a rotary shaft  36 . The outer end of lever  35  is connected by a ball joint  38  to a trim unit  31  having a trim cylinder housing  37  housing a trim piston  48  carried by the inner end of a trim rod  40  as described further below. The trim unit  31  provides a remotely adjustable connecting linkage between the vehicle tie rod  22  and the centering lever  35  of the rotary centering unit  33 . In other words, the centering arm  35  is held aligned with a centerline C by resistance means that opposes movement of this arm away from the centerline C. The resistance means referred to here is described in detail below. The trim rod  40  is extensible and contractible relative to trim housing  37  to provide remote adjustment of the center position of the steering system to be maintained by the stabilizer  20 . This adjustment is accomplished by changing the length of the linkage between the ball joints  27  and  38 . 
     The components of the centering system and the way in which they center and stabilize a vehicle steering system 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. The resistance unit  33  through the lever  35  and its connecting linkages provides a resistance force resisting movement of the steerable wheels  26  away from a selectable center position, the level of this force being adjustable and dependent upon the amount of fluid pressure supplied to a resistance chamber in the centering unit  33  via a fluid conduit  41  as shown in FIG.  2 . The fluid is preferably a gas and the gas pressure control may comprise a manual throttle valve (not shown) upstream of conduit  41 , in combination with a pressure gauge  43  to indicate the gas pressure. Alternately, a pressure regulator  45  may be used for maintaining a manually selected system pressure. A selector knob  49  is provided to permit varying the pressure settings of the regulator by hand. By varying the gas pressure in a gas chamber  109  (FIG. 7) by adjustments to pressure regulator  45 , the break away resistance and the centering return force produced by the stabilizer of the invention can be increased or decreased as desired. The pressure gauge and the regular may be mounted on a control panel  89 , preferably located at or near the driver&#39;s station of the vehicle. The range of pressures available should be selected so that break away resistance can be varied from relatively low at low speeds to relatively high at high speeds. 
     Pressure regulator  45  is connected to a compressed gas source  101  via a conduit  103  containing a solenoid operated cut-off valve  105 . The gas pressure in chamber  109  is indicated by the pressure gauge  43 , which is connected to pressure regulator  45  by a conduit  107 . The gas is preferably air. The electrical components of the control system are activated by an on-off switch  113 , which is connected to an electrical bus  115  by a line  117  containing a circuit breaker  119 . As it is best to deactivate stabilizer  20  in the event of a failure of the power steering system, a switch  121  for interrupting electrical power to the solenoid valve  105  may be provided for vehicles with power steering systems. Switch  121  is mounted on a pressure sensor  123  located in a hydraulic line  125  in fluid communication with the outlet of the power steering pump (not shown). A loss of pressure at the pump outlet causes switch  121  to open, thereby causing gas supply valve  105  to close in the absence of electrical power to its solenoid. 
     As an alternative to manual adjustment, the output pressure of regulator  45  may be adjusted by a reversible electric motor (not shown) controlled by an on-board computer  51 , which comprises a microprocessor  53 , an encoder  55  and a decoder  63 . Encoder  55  converts to digital signals an analog signal  65  input from a pressure sensor  67  in the gas supply conduit  41 , an analog signal  69  input from a vehicle speed sensor  75 , and an analog signal  81  input from a position sensor (not shown) within regulator  45 . Decoder  63  converts digital control signals generated by microprocessor  53  to an analog signal  83  for controlling the reversible electric motor which adjusts the output pressure provided by regulator  45 . The gas pressure in gas chamber  109  and the resulting resistance and centering forces are thereby made automatically responsive to the speed of the vehicle to provide “speed sensitive stabilizing” of the vehicle&#39;s steering system. It may be desirable in some applications that the resistance to turning movements away from the center position be increased automatically as the speed of the vehicle increases because the effects of small off-center movements in response to spurious steering inputs increase dramatically with vehicle speed. The trim unit  31  is also operated remotely by a solenoid  56  in response to a driver of the vehicle pushing a trim button  85  on the control panel  89 . Actuation of the solenoid  56  by pushing the button allows the trim rod  40  to move freely relative to the trim cylinder housing  37 , thereby permitting a change in the center position of the steerable wheels relative to the centered position of the center lever  35  maintained by the centering unit  33  as explained further below. 
     Operation of the components of the trim unit  31  will now be described on more detail. As may be seen best in FIG. 3, the trim cylinder housing  37  has an end cap  39  that is integrally formed with the centering rod  28 , the outer end of rod  28  being threaded for engagement with corresponding threads of the ball joint  27 . At the opposite end of trim cylinder housing  37  is a second end cap  42  for holding in place a trim cylinder head  44 , through which passes the trim rod  40 . Cap  42  and head  44  carry appropriate seals  76  and  77  to confine hydraulic fluid in the chamber  46  on one side of a trim piston  48 . The trim unit  31  includes an intermediate head  50  that incorporates a trim valve assembly  52 . Trim valve assembly  52  comprises a valve member  54  on a plunger  96  actuated by the solenoid  56  in response to a driver of the vehicle pushing the trim button  85  on the control panel  89 , which is preferably located at or near the driver&#39;s station of the vehicle. 
     A trim cylinder housing  37  encloses two separate interior cylinders  60  and  61 , the inner ends of which are connected together by the intermediate head  50 , which includes circumferential head seals  78 ,  78  and  79 ,  79 . The cylinder  60  contains the trim piston  48  and cooperates with this piston to define the two hydraulic chambers  46  and  47 . The piston  48  is keyed and fastened to the inner end of the trim rod  40 , such that the pressure differences between the fluid in chambers  46  and  47 , respectively, cause piston and trim rod movement. An annular conduit  62  is provided between the exterior of cylinder  60  and the interior of housing  37 , a port  64  is provided in end head  44 , and passages  66  and  68  are provided in intermediate head  50 , so that fluid flow paths are available between chambers  46  and  47  when solenoid valve  52  is in its open position. Check valves  71 ,  72 ,  73  and  74  are provided in the flow of passages of intermediate head  50  so that fluid can flow only one way from chamber  46  to chamber  47  when trim rod  40  is being extended, and can flow only one way from chamber  47  to chamber  46  when trim rod  40  is being contracted. 
     The passages  66 ,  68 ,  92 ,  90 ,  93 ,  94  and  95  are formed within the intermediate head  50  and are shown diagrammatically in FIG. 5 since they cannot all be readily shown in a cross-sectional view such as FIG.  3 . Passage  93  is connected to trim chamber  46  through an annular conduit and a head port (not shown) similar to the annular conduit  62  and the head port  64  that connect chamber  46  to passage  66 . As illustrated in FIGS. 4 and 5, the valve element  54  is mounted on the inner end of the reciprocating plunger  96  of valve assembly  52  and is pulled away from a valve seat member  97  in head  50  against the tension of a spring  99  upon actuation of the solenoid  56  by the pushing of trim button  85 . Positioned between valve seat member  97  and head  50  is an annular filter member  98  for filtering fluid passing through the valve from passage  95  to passage  90 . 
     To ensure that the hydraulic chambers  46  and  47  are kept completely filled with hydraulic fluid in the event of leakage past the rod and head seals  76 ,  77 ,  78  and  79 , the inner cylinder  61  defines a hydraulic reservoir  82  containing a pressure piston  87  that is pressed against hydraulic fluid in reservoir chamber  82  by a compression spring  84 , which preferably has sufficient compression to provide at least about 40 psig pressure in chamber  82 . Chamber  82  is filled with hydraulic fluid through a servicing fixture  86  containing a check valve  88 . Servicing fluid introduced through fixture  86  passes through check valve  88  and a passage  94  into chamber  82 , reverse flow out of chamber  82  through fixture  86  being prevented by the check valve. Passages  92  and  93  allow any makeup of fluid needed to pass from chamber  82  into either of trim chambers  46  and  47  independent of solenoid valve  52 . The solenoid  56  of valve  52  is energized to its open position by the electrical line  57  from the driver&#39;s control station. 
     For sealingly engaging trim cylinder  60 , trim piston  48  carries two sets  80 , 80  of dual circumferential seals. Although not specially shown, each set may comprise an outer seal of square cross section concentrically stacked on a more resilient seal of oval cross section to provide a close tolerance seal arrangement for substantially preventing any leakage past the trim piston. These multiple seals preclude any significant drift of trim piston  48  away from its locked position for setting the on-center position of the centering detents  112  and  124 . Also provided is a wear band  81 . A similar close tolerance circumferential seal  80 ′ and a similar wear band  81 ′ are also preferably provided on the reservoir piston  87 . 
     With reference to FIG. 5, the remotely controlled trim unit  31  operates as follows. If there is a roadway pull to the right, straight ahead travel will require a compensating steering force to the left from the steering wheel to move the bearing members slightly away from their seated positions in the detents. Such movement of the bearing members produce a differential pressure across trim piston  48  in trim cylinder  60 . While holding the steering wheel in the position giving straight ahead travel, the trim button  85  is pushed momentarily to briefly open solenoid valve  52  as shown in FIG. 4, which allows fluid to be discharged from trim chamber  46  and supplied to trim chamber  47  such that cylinder housing  37  moves to the right and the differential pressure across trim piston  48  is removed by equalizing the pressures in trim chambers  46  and  47 . The force moving housing  37  is provided by the return forces generated by the bias tending to return each of the bearing members to their seats in the detents. The movement of trim piston  48  in trim cylinder  60  causes each of the bearing members to be reseated in their rest or seated positions in the detents. After its momentary actuation, the trim button  85  is then released to close solenoid valve  52  and thereby lock trim piston  48  in its changed position corresponding to a new on-center position in which the detent seats are realigned with their corresponding bearing members. This new on-center position will then maintain the vehicle steering system in a newly centered condition, which provides straight ahead travel of the vehicle that is free from the previously experienced roadway pull to the right and will be maintained even when the steering wheel is released. 
     Fluid flowing out of chamber  46  follows a return flow path to reservoir  82  through passages  66 ,  95  and  90  and valves  71  and  52 . Fluid supplied to trim chamber  47  follows a supply flow path from reservoir  82  through passage  92  and valve  74 . Similarly, a compensating steering force to the right with the trim button  85  pushed causes fluid to be discharged from trim chamber  47  and supplied to trim chamber  46  such that cylinder housing  37  moves to the left. Fluid flowing out of chamber  47  follows a return flow path to reservoir  82  through passages  68 ,  95  and  90  and valves  73  and  52 . Fluid supplied to trim chamber  46  follows a supply flow path from reservoir  82  through passage  93  and valve  72 . 
     Referring now to FIGS. 6,  7 ,  8  and  9 , there is shown the resistance unit  33  with its components in their rest or centered positions. The resistance unit  33  has a housing  100  comprising a resistance cylinder  102  held between a base  104  and a cover  106  by a plurality of bolts  108  (FIG.  6 ). Arranged for reciprocal movement within the resistance cylinder  102  is a piston detent plate  110  having a detent generally designated  112 , and a lower ball track  114 . The detent  112  comprises a ramp  116 , a seat band  118 , and undercut bottom  120 , the details of which are described further below. Integrally formed with shaft  36  at its base is a rotary detent plate  122  having an upper ball track  126  and a detent  124  with a ramp  131 . The detent  124  and track  126  are substantially identical to the lower detent  112  and the lower track  114  so that only the details of the lower detents and the lower tracks will be described hereinafter. 
     Passing through a central portion of detent plate  122  and into the lower end of shaft  36  is a guide bore  130  containing a sleeve bushing  132  for receiving a guide pin  134  formed integrally with and upstanding from a central portion of piston detent plate  110 . Guide pin  134  engages bushing  132  during reciprocal movement of detent plate  110  in response to movement of a plurality of ball bearing members  136  out of their corresponding detents in piston plate  110  and rotary plate  122 . The piston detent plate  110  is held against rotary movement by means of a pair of depending lugs  138  and  140 , which respectively slide between a pair of ears  142  and a pair of ears  144  upstanding from the housing base  104 . Centering chamber  109  also contains a compression spring  146  positioned by a recess  148  on the underside of piston plate  110  and a recess  150  in the housing base  104 . The compression spring  146  provides enough upward pressing force against piston plate  110  to hold both it and the bearing members  136  in their proper positions at all times. The spring force provided by spring  146  in many cases is not sufficient to provide the desired turning resistance represented by the resistance of the ball bearings to movement out of their corresponding detents, so that the force of spring  146  is preferably supplemented by providing a pressurized fluid in fluid chamber  109  through a fluid port  152 . Also provided is a bearing spacer  154  for maintaining the spacing between the ball bearings  136  the same as the spacing between the detents when the ball bearings move away from their seated positions in the detents during rotation of the detent plate  122  in response to turning movements of the vehicle steering system as transmitted through the lever  35  and the shaft  36 . The rotary plate  122  rotates about the rotational axis of shaft  36  and its movement along this axis is prevented by a thrust bearing  128  as shown in FIGS. 8 and 11. 
     FIGS. 10 and 11 illustrate a moved position of the resistance unit  33  in response to a turning movement of the vehicle. In these views, the ball bearings  136  have moved to the end of their respective ramps  116  and  131  away from the seat bands  118 , such that the balls will thereafter move along the lower ball track  114  and the upper ball track  126  upon further turning movement of the vehicle. As the ball bearings  136  progress along the ramps  116  and  131  away from their respective seat bands  118 , the piston plate  110  and its depending lugs  138  and  140  move from the positions shown in FIG. 8 to the depressed positions shown in FIG.  11 . The movement of the ball bearings  136  out of the detents  112  and  124  is driven by rotation of the rotary plate  122  with its ramp  131  in firm frictional engagement with the ball bearings. This firm engagement is provided by forming the ramps  116  and  131  and the tracks  114  and  126  as a groove having substantially the same radius as the ball bearings, which will be explained further below. The breakaway turning force required to initiate ball movement away from seat bands  118  and to maintain ball movement along ramps  116  and  131  are functions of both the slope of the ramps  116  and  131  and the compression force applied to the balls by the fluid pressure in chamber  109 . Although ball tracks  114  and  126  may also have some amount of slope to provide continuing resistance through all turning angles, it is preferred in many applications that the tracks  114  and  126  be substantially without slope (flat), such that piston plate  110  does not move further away from rotary plate  122  as the balls travel along these tracks. 
     As shown in FIGS. 12 and 13, it is preferable that the depending lugs  138  and  140  have an exterior shim  156  secured to their opposite exterior faces by pairs of recessed screws  158 ,  158 . The shims  156 ,  156  are preferably made of a low friction material such as brass or a hard plastic, and are machined or otherwise formed to provide a close tolerance sliding fit relative to the retaining ears  142 ,  142  and  144 ,  144  carried by the housing base  104 . 
     In FIGS. 14-21, there are shown structural details of the detent  112  and the ball track  114  in piston plate  110 , which are substantially identical to the details of the detent  124  and the ball track  126  of the rotary plate  122 . FIG. 15 is a blow-up of the segment of piston plate  110  identified by the broken line circle  15  in FIG.  14 . As shown by the transverse cross-sections illustrated in FIGS. 19 and 20, both the ramp  116  and the track  114  are formed by a groove cut on substantially the same radius as the radius of the ball member  136  to provide a snug frictional fit between the ball member and the ramp and track. This snug frictional fit ensures that rotation of the rotary plate  122  relative to the piston plate  110  will cause the ball member to smoothly and consistently ride up the ramp  116  and out of the detent onto the ball track  114 , even under relatively high compressive loads between plates  110  and  122  with fluid pressures in chamber  109  as high as 100 psig. 
     As shown in FIGS. 15-18 and  21 , a seat band  118  is provided around the rim of an undercut bottom portion  120  of the detent  112 . The width of band  118  is preferably machined to be a flat or slightly convex surface tangent to the curvature of the ball member so that when the ball member is fully seated in the detent, it&#39;s outer curved surface rests against the seat band  118  substantially along a line of contact represented by the broken line  160  in FIG.  17 . To ensure such a line of contact and to minimize wear at the upper edge of band  118 , the surface of this band may have a slight convex curvature across its width instead of a straight line width. 
     Referring now to FIG. 18, the opposite sidewalls  123 ,  123  adjacent to the seat band  118  are cut on a radius R 1  that is slightly greater than the radius R of the ball  136  to provide a gap G between the surface of the ball and the surface of the adjacent sidewalls. This gap G insures that the line of contact  160  between the ball and the seat band  118  extends completely around the seat band when the ball is fully seated in the detent  112 . By reason of the cut of sidewalls  123 ,  123 , the bottom of ramp  116  intersects these sidewalls along an imaginary line  125  as seen best in FIG.  15 . The top of ramp  116  intersects the adjacent track  114  along an imaginary line  127  as also shown in FIG.  15 . 
     An illustrative example of one way in which the band  118  and undercut  120  may be formed is illustrated in FIG.  21 . In this figure, R 2  represents the radius of the undercut, such as {fraction (5/16)} inch, and R 3  and R 4  each represent the radius of the ball, such as ½ inch. Also shown is a vertical imaginary axis A that passes through the center C 1  of the undercut and the center C of the ball. To establish the upper and lower limits of the band  118 , R 3  is drawn at an angle of 19 degrees from the vertical axis A and R 4  is drawn at an angle of 27 degrees from the vertical axis A, so that the band width W covers an arc of 8 degrees. These illustrative dimensions yield an undercut  120  with its bottom at a distance U of about 0.045 inch below the bottom of the seated ball, and a band width at W of about {fraction (1/16)} inch. Where E represents the edge of the groove at the late surface  129 , the bottom of the seated ball may be {fraction (5/16)} inch below the edge E, and the bottom or depth D of the groove of track  114  may be ⅛ inch below the edge E to give a vertical rise V of {fraction (3/16)} inch as the ball moves from its seated position on the band  118  to its position in the track  114  at the top of the ramp  116 . In this case, the horizontal distance L over which the ball travels while on ramp  116  may be about 1.25 inches, and the sloped ramp surface may have a convex shape defined by a radius R 5  of about 4 inches. 
     If the diameter of piston member  110  is about 7.0 inches, the air chamber  109  of centering unit  33  may be pressurized by air to a pressure of, for example, about 40 psig to provide a linear resistance force of about 320 pounds as measured at the tie rod  22  for opposing off-center movement of the steerable wheels. An air pressure of about 65 psig will provide about the same resistance force with about a 6 inch diameter piston. Since many conventional steering system geometries provide a linear resistance force of about 15 to 20 pounds as measured at the tie rod, the present invention may be used to increase the resistance and re-centering forces of these steering systems by a multiple of about 5 to about 30 or more, preferably about 10 to about 25. A resistance force of 300 pounds or more is particularly effective in eliminating the adverse effects of crosswinds on large vehicles. 
     The particularly important trimming feature of the invention may be achieved through drive means other than the hydraulic trimming unit  31 . For example, movement of trim rod  40  may be accomplished by controllably varying its position with a reversible electric motor pivotally mounted on the vehicle axle  21  in place of trimming unit  31 . Such trimming arrangements are described in my prior U.S. Pat. No. 4,418,931 and U.S. Pat. No. 4,534,577 which are incorporated herein by reference. However, the piston and cylinder trimming arrangement shown in the drawings is preferably for its simplicity and its precision and ability to provide remote trimming responsive to steering wheel movement. 
     It is also important to recognize that the centering stabilizers of the present invention engage the vehicle steering system at a location between the steerable wheels and the steering gear assembly from which extends the pitman arm. As a result, spurious inputs from the steering column and/or from a power steering unit are absorbed by the stabilizer before these inputs can reach the steerable wheels. Likewise, spurious forces transmitted from the roadway are immediately absorbed in the stabilizer, rather than being transmitted through the entire steering assembly before encountering any stabilizing resistance from the steering wheel. As a result, the centering stabilizer protects the interior components of the steering assembly from the wear caused by repeated oscillations between states of tension and compression. 
     In the operation of roadway motor vehicles, spurious inputs may be caused by road forces acting on the vehicle wheels, environmental forces acting on the vehicle body, driver forces acting on the steering wheel, off-center bias inherent in the steering system itself, or any combination of one or more of these forces. Without the power centering stabilizer of the present invention, such spurious steering forces of relatively small magnitude can cause vehicle steering systems to move to one side or the other of center or to oscillate back and forth from one side to the other, thereby producing corresponding movements of the vehicle away from the desired direction of vehicular travel. The invention also may be used with other tie rod arrangements and with steering systems that do not require tie rod arrangements, such as those with only one steerable member, such as the rudder of a ship or an airplane. 
     The variable resistance and return force components of the invention can be used alone as a centering unit without the remote trimming features. On the other hand, the remote trimming features of the invention are useable not only with the centering unit disclosed herein, but also in combination with centering mechanisms of the prior art. Thus, the remotely operable trimming unit of the present invention can be combined with centering devices of known types to provide adjustment of the center position during vehicle operation. 
     The invention can be used on vehicles with or without power steering systems. The invention can provide centering compensation for the steering system of a wide variety of vehicles, including automobiles, trucks, motorcycles and other on the highway and off the highway motor vehicles, and also small boats, large ships, and aircraft. The invention also has a wide range of other industrial applications and can be utilized to automatically center any device having a steering member interconnected by suitable linkages to a steerable member. 
     The resistance components or remote trimming components of the present invention may be combined with one or more such components of the prior art, such as those disclosed in my prior U.S. Pat. Nos. 4,410,193; 4,418,931; 4,534,577; 5,527,053; 5,536,028; 6,003,887; and 6,267,395, the entire contents of each of these patents being expressly incorporated herein by reference. These components also may be combined with one or more features of U.S. patent application Ser. No. 09/699,520 on a Center Holding Assembly For Vehicle Steering Systems, the entire contents of which is incorporated herein by reference. As a further example, the remotely operable resistance unit of the invention can be combined with remote trimming devices of known types to provide adjustment of the center position during vehicle operation. In addition, a number of other modifications to both the variable resistance components and to the trimming components specifically disclosed are possible without departing from the scope of the invention as defined by the claims set forth below.