Abstract:
The trailer steering mechanism automatically steers the wheels of a trailer in accordance with the tow vehicle steering and articulation between the tow vehicle and trailer during backing maneuvers in order to facilitate backing maneuvers and to prevent or greatly reduce the possibility of jackknifing between tow vehicle and trailer. The mechanism includes a pivotally attached steering frame connected between the steerable wheels of the trailer and one or more hydraulic cylinders that, when locked, create a very strong link between the tow vehicle and the trailer and its steering system. The hydraulic cylinders are able to withstand great loads, and more importantly, are able to disengage while under great load without jamming. The hydraulic cylinders provide links that exert lateral force to the trailer to push or pull the trailer into a turn while backing, in addition to controlling the steering mechanism of the trailer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to vehicle steering systems, and particularly to a trailer steering mechanism for towing a trailer behind a motor vehicle. 
   2. Description of the Related Art 
   The towing vehicle and trailer combination is an inherently unstable one during backing maneuvers. Even a simple trailer having a single axle with non-steerable wheels requires some skill by the driver of the towing vehicle during backing maneuvers. This is because of the articulation between the towing vehicle and trailer, which allows the trailer to turn relative to the towing vehicle. Once the trailer begins to turn, even continued straight backing by the towing vehicle will cause the trailer to turn ever more sharply, resulting in the jackknifing of the trailer and towing vehicle in short order. 
   While the above problem can be overcome by a skilled driver operating a single axle trailer, other trailer configurations can be impossible to back. For example, truck tractors, or other towing vehicles towing multiple articulated trailers in tandem, can only successfully back the single trailer connected directly to the towing vehicle. There is no way for the driver of the towing vehicle to steer both trailers of such a configuration while backing. Another example is the farm wagon-type trailer, having a front axle with steerable wheels and a rear axle with directionally fixed wheels. This combination cannot be backed due to the instability of the steerable front wheels of the trailer, which causes a castering action when backing. 
   A number of different trailer steering mechanisms have been developed over the years in attempts to solve at least some of the above-described problems. 
   German Patent No. 3,538,338, published on Apr. 30, 1987, describes (according to the drawings and English abstract) an electrical system incorporating servomotors to drive the trailer steering. The drawings are primarily directed to the electronic circuitry used in the system. 
   German Patent No. 4,216,543, published on Dec. 3, 1992, describes (according to the drawings and English abstract) a rigid linkage of two or more struts or tow bars between the towing vehicle and trailer, which angularly lock the trailer relative to the towing vehicle and prevent articulation therebetween. The trailer wheels are not actively steered, but, rather, caster to follow the turns of the towing vehicle during both forward and reverse travel. The lateral hydraulic cylinder disclosed in the drawings is not a steering actuation link or element, but is a shimmy damper, as conventionally applied to castering wheel systems. 
   Finally, German Patent No. 10,236,335, published on Feb. 27, 2003, indicates (according to the English title and abstract) that the subject trailer has a steerable front axle. However, no such steering mechanism is apparent in the drawings of the two-axle trailer. It appears that the trailer actually has castering front wheels, with no positive steering system being apparent. 
   None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, a trailer steering mechanism solving the aforementioned problems is desired. 
   SUMMARY OF THE INVENTION 
   The trailer steering mechanism facilitates the backing of a trailer by steering the trailer wheels in the same direction as the towing vehicle during backing maneuvers. The system automatically locks the trailer wheels straight for forward travel. A pivotally mounted steering frame is provided beneath the trailer frame, with the steering frame acting upon the tie rod(s) of the steerable wheels of the trailer. The steering frame receives its input from one or more hydraulic cylinders extending between the steering frame and a hitch bar, with the hitch bar being rigidly and immovably secured to the towing vehicle. The hydraulic cylinder(s) is/are hydrostatically locked during backing maneuvers, thus causing the steering frame of the trailer to pivot according to articulation between the trailer and towing vehicle when backing. The trailer follows (or more accurately leads, during backing maneuvers) the path of the towing vehicle during such backing maneuvers. 
   The locking of the hydraulic cylinder(s) or strut(s) is controlled by one or more valves, which control hydraulic flow between the two ends of the double acting cylinder(s). The valves are, in turn, controlled by an electric motor or other electric drive, which is, in turn, controlled by the conventional electrical back-up signal (backing lights, alarm bell or signal, etc.) provided for most motor vehicles. When no signal or electrical power is provided for backing, the valve(s) is/are open to allow relatively free hydraulic flow through the cylinder(s) or strut(s). This allows the cylinder(s) to extend and retract with articulation between the trailer and towing vehicle, while the steering frame of the trailer remains locked straight ahead to lock the wheels of the trailer. When the tow vehicle is placed in reverse, electrical power is received from the backing circuit of the tow vehicle to close the valve(s), thereby locking the hydraulic cylinder(s) to cause the trailer steering frame to move with the tow vehicle, thereby steering the trailer wheels. 
   The trailer steering mechanism may make use of various systems for actuating the hydraulic valve mechanism of the trailer, including multiple valves actuated by a single motor and chain or cable drive, or even a rigid rod linkage between the motor and valves, or a single valve having multiple ports, electrically operated solenoid valves, etc. Alternatively, the valve mechanism may be actuated by a self-contained electrical system on the trailer, or by manual operation if so desired. The hydraulic cylinder control valve(s) or hydraulic circuit may include one or more relief valves to relieve extreme pressures and forces in the system in the event of extreme steering angles by the tow vehicle during backing maneuvers. One or more steering frame and axle centering springs and adjustable steering stops may be provided to assist in centering the trailer steering and limit the steering angle of the trailer wheels. A mechanical latch may be provided to lock the trailer steering frame, and thus the trailer wheels, straight ahead for forward travel, with the latch automatically releasing for backing maneuvers and automatically engaging for forward travel. The mechanism may be applied to trailers having a forward and a rearward axle, i.e., farm wagons, with a link provided to a modified rear axle assembly permitting steering of the rear wheels in concert with the freely steering front wheels of the trailer. 
   These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an environmental, perspective view of a towing vehicle with a trailer incorporating a trailer steering mechanism according to the present invention, showing its operation during a backing maneuver. 
       FIG. 2  is a bottom plan view of a trailer incorporating the trailer steering mechanism of the present invention, showing the general configuration thereof. 
       FIG. 3  is a schematic top plan view showing a towing vehicle and trailer steering mechanism according to the present invention during forward travel, with the trailer wheels being locked straight for forward travel. 
       FIG. 4  is a schematic top plan view showing a towing vehicle and trailer steering mechanism according to the present invention during a backing maneuver, with the trailer wheels automatically turning to align the trailer with the path of the towing vehicle. 
       FIG. 5  is a schematic top plan view of an alternative embodiment of trailer and trailer steering mechanism according to the present invention, wherein the trailer has forward free-steering wheels and a rearward axle having steerable wheels thereon controlled by a steering mechanism. 
       FIG. 6  is a detailed top plan view showing a first embodiment of an actuating mechanism for the hydraulic valves of a trailer steering mechanism according to the present invention. 
       FIG. 7  is a detailed top plan view showing another embodiment of an actuating mechanism for the hydraulic valves of a trailer steering mechanism according to the present invention, wherein the system incorporates electrically actuated solenoid valves to control hydraulic flow. 
       FIG. 8  is a plan view in section of an exemplary two-way or double-acting pressure relief valve for use in the trailer steering mechanism of the present invention. 
       FIG. 9  is a detailed top plan view of an alternative embodiment of a valve mechanism for a trailer steering mechanism according to the present invention, incorporating the entire valve mechanism in a single housing. 
       FIG. 10  is a detailed side elevation view in partial section of one of the steering centering and limit devices of a trailer steering mechanism according to the present invention. 
       FIG. 11  is a left side elevation view of the locking mechanism for securing the trailer wheels in a straight ahead position for forward travel in a trailer steering mechanism according to the present invention. 
   

   Similar reference characters denote corresponding features consistently throughout the attached drawings. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention comprises various embodiments of a steering mechanism for trailers having steerable wheels, and various trailer configurations incorporating such a steering mechanism. The steering mechanism allows the steerable wheels of the trailer to be steered positively according to articulation between the towing vehicle and the trailer during backing maneuvers, while locking the steerable wheels straight ahead for forward travel of the trailer. 
     FIG. 1  of the drawings provides an environmental perspective view of a trailer  10  with its steering mechanism, shown hitched behind a towing vehicle V, with  FIG. 2  providing a bottom plan view of the trailer  10  and its steering mechanism. The trailer  10  includes a frame  12  having a forwardly extending rigid tongue  14 , with the tongue  14  being removably and pivotally hitched to a lateral three-ball hitch bar  16  attached to the tow vehicle V. The hitch bar  16  is shown most clearly in the top plan view illustrations of  FIGS. 3 ,  4 , and  5 , and is rigidly and immovably secured to the tow vehicle V by means of a conventional receiver hitch or the like. The hitch bar  16  comprises a three-ball hitch bar, having a central ball for the removable connection of the trailer tongue  14  coupler thereto and opposite left and right balls for the removable connection of the couplers of the left and right hydraulic cylinders (described in detail further below) thereto. 
   The steerable trailer  10  includes a steering frame  18  attached beneath the trailer frame  12  at a pivot  20 . The location of the pivot point  20  for the steering frame  18  may vary from that shown, so long as the geometry of the steering linkage (i.e., various arm lengths, arcuate and linear movements, etc.) is adjusted accordingly. In the example of  FIG. 2 , the steering frame  18  includes a rearwardly extending steering arm  22 , which connects to an intermediate link  24  between the distal end of the arm  22  and the tie rod(s)  26  of the steerable trailer wheels  28   a ,  28   b , which extend from each end of the axle  30  of the trailer  10 . The wheels  28   a ,  28   b  are attached to the trailer axle  30  by conventional articulated joints, e.g., king pins, etc., to allow the trailer wheels  28   a ,  28   b  to be steered when the steering frame  18  pivots to move the tie rod(s)  26  by means of the intermediate link  24 . 
   The steering frame  18  includes a crossmember  32  which provides for attachment of at least one hydraulic cylinder to the steering frame, with there preferably being one hydraulic strut or cylinder  34   a  installed to the left side of the trailer tongue  14  and a second strut or cylinder  34   b  installed to the right side of the tongue  14 . Alternatively, plural struts or cylinders may be installed to each side of the central tongue  14 , if so desired, depending upon the rating of each hydraulic cylinder or strut and the size of the trailer and maneuvering forces required. Each of the cylinders  34   a ,  34   b  has a hitch coupling end  36   a  and  36   b  and an opposite steering frame attachment end  38   a  and  38   b , with the forward coupling ends  40   a ,  40   b  of the cylinders being pivotally coupled to, and extending from, the corresponding ends of the hitch bar  16 , and the steering-frame attachment ends  38   a ,  38   b  attaching to the ends of the steering frame crossmember  32 . The struts or cylinders  34   a ,  34   b  are parallel to the trailer tongue  14  and to one another, with the tongue  14  centered between the two struts or cylinders. 
   The trailer steering mechanism is selectively actuated according to forward or reverse travel of the tow vehicle V and trailer  10 , with the steering mechanism being locked in a neutral position with the trailer wheels  28   a ,  28   b  straight ahead regardless of articulation between the towing vehicle V and trailer  10  during forward motion, and with the hydraulic cylinders  34   a,    34   b  being hydraulically locked to transmit articulation between the hitch bar  16  of the tow vehicle V and the steering frame  18  of the trailer  10  to steer the trailer wheels  28   a ,  28   b  during backing maneuvers. This is controlled by a hydraulic strut or cylinder control mechanism  42 , shown in detail in  FIG. 6  of the drawings. 
   In the mechanism  42  embodiment of  FIGS. 1 through 4  and  FIG. 6 , a series of four hydraulic control valves  44  through  50  are controlled by a single valve actuator  52 . The first control valve  44  is positioned between a source of hydraulic fluid or reservoir  54  and the hitch bar attachment end  36   a  of the first or left side hydraulic cylinder or strut  34   a . The second control valve  46  is positioned between the hydraulic reservoir  54  and the steering frame attachment end  38   a  of the first or left side cylinder or strut  34   a . The third control valve  48  is positioned between the hydraulic reservoir  54  and the hitch bar attachment end  36   b  of the second or right side hydraulic cylinder or strut  34   b , with the fourth control valve  50  being positioned between the hydraulic reservoir  54  and the steering frame attachment end  38   b  of the second or right side cylinder or strut  34   b.    
   The four valves  44  through  50  are linked to the actuator  52  by a roller chain  56 , which is driven by a drive sprocket  58  extending from the actuator, with the drive sprocket  58 , in turn, driving a valve sprocket  60  extending from each of the valves. When the actuator  52  is rotated, its sprocket  58  drives the chain  56  to rotate each of the driven sprockets  60  of the four valves, thereby turning the valves on or off according to the operation of the actuator. It will be seen that other mechanisms may be used in lieu of the above-described mechanism, e.g., a cable drive, a series of rigid pushrods between the valves, etc., or even manual actuation, as desired. Alternatively, electric solenoid valves may be used in lieu of mechanically or manually actuated valves, if so desired. 
   The actuator  52  is driven by an electric motor  62 , which is controlled by a solenoid switch  64 , with the solenoid  64  receiving power from the conventional backup signal  66  (backup lights, bells, etc.) provided on tow vehicles. A separate manually-controllable cutout switch  68  may be included in the circuit, if so desired, in order that the trailer wheels may be locked straight during backing maneuvers, e.g., when maneuvering in tight quarters. Assuming the cutout switch  68  is closed, shifting the tow vehicle into reverse provides electrical power to the solenoid switch  64  to actuate the valve actuator control motor  62 . This rotates the actuator  52 , thereby driving the chain  56  to rotate each of the valve sprockets  60  to close their corresponding valves  44  through  50  to hydraulically lock the lengths of the two cylinders  34   a  and  34   b . This results in articulation between the tow vehicle V and trailer  10  turning the steering frame  18  of the trailer, thereby turning the trailer wheels  28   a  and  28   b , as described further above. This is the preferred means of carrying out the control of the system, i.e. providing electrical power from the tow vehicle. However, electrical valve actuation may be accomplished by electrical power supplied by a source on the trailer, if so desired. 
   Shifting of the tow vehicle to a forward gear (or opening the cutout switch  68 ) shuts off the electrical signal to the solenoid  64 , thereby shutting off power to the actuator motor  62 . This allows the actuator  52  to return to its original position, driving the chain  56  and valve sprockets  60  to open the various hydraulic cylinder or strut control valves  44  through  50 . Alternatively, a separate circuit may be provided to drive the motor  62  in the opposite direction to open the valves for forward travel. This allows hydraulic fluid to flow relatively freely through the system, with the two hydraulic cylinders  34   a  and  34   b  extending and retracting with articulation between the tow vehicle V and trailer  10  as the assembly turns during forward motion. The viscosity of the hydraulic fluid flowing through the system during turning maneuvers does provide some beneficial damping of any unwanted sway between the trailer and tow vehicle. Since the hydraulic cylinders are no longer hydraulically locked, they do not impart any significant thrust or force upon the steering frame  18  to steer the wheels of the trailer. However, a positive mechanical lock may be provided for the steering frame  18  to assure that the trailer wheels are locked straight ahead during forward travel for stability of the trailer and tow vehicle assembly. This locking mechanism is illustrated in  FIG. 10 , and discussed further below. 
   It will be seen that there is no hydraulic pump installed in this hydraulic system, as none is required. The hydraulic system of the present mechanism merely serves to selectively lock the lengths of the hydraulic cylinders or struts  34   a  and  34   b  to lock the relative orientation of the steering frame  18  relative to the hitch bar  16  of the tow vehicle V when backing, thus causing the locked hydraulic cylinders to steer the steering frame to turn the trailer wheels. Conversely, free flow of hydraulic fluid is permitted back and forth between the cylinders  34   a  and  36   a  when in forward travel, thus allowing articulation between the trailer  10  and tow vehicle V while the steering frame  18  is locked to lock the trailer wheels straight ahead. While no hydraulic pump is required, the closed and sealed system may be pneumatically pressurized, if so desired, by adding some predetermined air (or other gas) pressure to the reservoir  54  to serve as a preload for the system. The provision of a closed, sealed system provides additional benefits as well in preventing contaminants (e.g., road dirt and debris, moisture, etc.) from entering the system, thus adding to the longevity of the hydraulic fluid in the system as well as to the components of the system. 
     FIG. 7  provides a top plan view in which the above-described hydraulic system (illustrated in  FIG. 6 ) is controlled by electrohydraulic solenoid valves  44   a  through  50   a . The electrohydraulic solenoid valves  44   a,    46   a ,  48   a , and  50   a  each include a hydraulic valve selectively permitting or stopping the flow of hydraulic fluid therethrough, with the hydraulic valve being controlled by the mechanical action of an electrically actuated solenoid. Accordingly, the mechanical system shown in  FIG. 6  is not needed for control of the electrically actuated valves  44   a - 50   a  of the embodiment of  FIG. 7 . Electrical power to the electrohydraulic solenoid valves  44   a  through  50   a  may be controlled through an electrical circuit identical to that used for the control and operation of the electric motor  62  driving the valve actuator  52  for the mechanical system of  FIG. 6 , i.e., a solenoid  64  receiving power from the back-up system  66  of the tow vehicle. A separate conventional electrical power source may be provided from the tow vehicle or an electrical source on the trailer (e.g., battery, etc.) if more electrical power is required than can be provided through the back-up system  66 , with the actuation of the separate power source being controlled by another solenoid similar to the solenoid  64  shown in  FIGS. 6 and 7 , or a double pole solenoid or the like. 
     FIG. 8  of the drawings provides a side elevation view in section of an exemplary double-acting or two-way pressure relief valve  100 , which may be incorporated in the trailer steering mechanism. This two-way valve  100  serves to relieve excessive hydraulic pressure in the system in either direction of flow through the valve. The valve  100  includes a stationary frame  101 , which is secured within a passage through the valve housing or body, e.g., the passage  92  of the rotary valve member  80  of  FIG. 9 , by a retaining ring  103  or the like. A first valve spring  105  extends between the stationary frame  101  and a first or outer poppet valve  107 , with the first valve  107  opening toward the first spring  105  when sufficient pressure is developed to overcome spring pressure. 
   The first or outer valve  107  includes a central opening or passage  109  therethrough, which is sealed by a second poppet valve  111 . The second valve  111  is held in position over the first valve opening  109  by a second frame  113 , which extends from the first valve  107  opposite the first or stationary frame  101 , with a second valve spring  115  being captured between the second frame  113  and second valve  111  to urge the second valve  111  against the inner face of the first valve  107 . Thus, a first pressure P 1  of sufficient force to compress the first valve spring  105  will force the second valve  111  against its seat on the first valve  107 , but will push the first valve  107  open from its seat on its stationary frame  101  to relieve the excessive pressure P 1 . However, an opposite second pressure P 2  acts in the same direction as the force developed by the first valve spring  105 , with the first valve  107  accordingly remaining closed. But such a pressure P 2  acts against the face of the second valve  111  and against its spring  115 , pushing the second valve  111  open to relieve excessive pressure in the direction indicated by P 2  in  FIG. 8 . 
   It will thus be seen that the pressure relief valve  100  can relieve excessive hydraulic pressure in either direction using only a single valve. Moreover, the two springs  105 ,  115  may be provided with adjustable compressive force by means of threaded retainers between the respective valve frames and valves in order to allow the opening force for each valve  107 ,  111  to be adjusted as desired. It will be seen that such a two-way or double-acting pressure relief valve(s)  100  may be incorporated in the hydraulic circuits of  FIGS. 6  and/or  7 , wherein four separate control valves are used, with each incorporating a single two-way pressure relief valve  100 . Alternatively, single directional relief valves may be incorporated by means of two separate parallel hydraulic paths between each control valve or port and corresponding hydraulic cylinder or strut end. 
     FIG. 9  of the drawings illustrates an alternative valve assembly  70 , which may be incorporated in the trailer steering mechanism in lieu of the four separate valves and actuator illustrated in detail in  FIGS. 6 and 7  and described above. The valve assembly  70  of  FIG. 9  incorporates four separate valve ports or outlets, respectively  72  through  78 , which communicate hydraulically with the respective hitch coupling end  36   a  and steering frame arm attachment end  38   a  of the first or left hydraulic cylinder or strut  34   a  and the hitch coupling end  36   b  and steering frame arm attachment end  38   b  of the second or right hydraulic cylinder or strut  34   b . A single rotary member  80  rotates within the valve body  82  to selectively align the four hydraulic passages  84  through  90  of the rotary member  80  with the four outlet ports  72  through  78  of the valve assembly  70  to allow flow between the ends of the hydraulic cylinders  34   a  and  34   b  so that the trailer wheels  28   a ,  28   b  may be locked straight ahead for forward travel of the trailer  10 . The valve assembly  70  is illustrated in this open flow configuration for forward travel in  FIG. 9 . 
   When the trailer  10  is to be backed, the rotary center member  80  of the valve assembly  70  is rotated (manually, or by means of an actuator motor and electrical circuit, such as the motor  62  and circuit shown in  FIG. 6  and described further above) through forty-five degrees to align the passages  84  through  90  of the rotary member  80  with the closed interior sidewalls of the valve body  82 , thereby shutting off flow between those passages  84  through  90  and their respective hydraulic cylinders  34   a  and  34   b  to lock the cylinders hydraulically. 
   However, it will be noted that the valve assembly  70  includes additional restricted flow passages  92  through  98 , each of which includes a two-way pressure relief valve  100  therein; such a two-way pressure relief valve  100  is shown in detail in  FIG. 8  of the drawings, and has been described in detail above. When the valve  70  has been repositioned to close the primary passages  84  through  90  for backing operations, the pressure relief or restricted flow passages  92  through  98  are aligned with the four ports  72  through  78 . So long as hydraulic pressure remains below a certain predetermined level(s) within the cylinders  34   a ,  34   b  and their hydraulic lines, the relief valves  100  remain closed. 
   However, in the event that, e.g., an excessive turning angle is reached that might cause damage to the mechanical steering mechanism, the relief valve(s)  100  will open briefly to relieve hydraulic pressure in the system, and then reset automatically once the pressure has been relieved. This limits the extension and/or retraction of the hydraulic cylinders or struts  34   a ,  34   b , rather than having the locked cylinders exert excessive force upon the mechanical structure of the trailer  10  and its steering mechanism and possibly overstressing and/or damaging the structure. 
     FIG. 10  of the drawings provides a detailed side elevation view in partial section of an exemplary steering centering spring and stop limit assembly, which may be incorporated into the trailer steering mechanism. The assembly of  FIG. 10  installs between the trailer frame  12  and the pivotal steering frame  18 , or more precisely one (or both) of the lateral arms or crossmembers  32  of the steering frame. A steering angle limit stop  102  is secured (e.g., welded, etc.) to a bracket  104 , which extends from the trailer frame  12 . A steering stop bumper  108  is adjustably secured to the steering frame arm  32  by a threaded adjuster  110 . The span between the end of the limit stop  102  and the end of the bumper  108  may be adjusted by means of the adjuster  110  to adjust the maximum angle or limit of the steering geometry of the trailer  10 . A steering centering coil spring  112  is installed concentrically over and between the steering limit stop  102  and bumper  108  to urge the wheels to a straight position when the hydraulics are released during forward movement, and to limit the impact forces between the steering limit stop  102  and bumper  108  during extreme turning maneuvers when backing. 
   The above-described trailer steering centering mechanism serves to resist angular steering input at and near the predetermined geometric steering limits for the trailer, but the spring force does relatively little to resist steering input from the neutral or straight ahead position of the trailer wheels, particularly when identical centering mechanisms are installed upon each crossmember  32  of the steering frame  18 . The lack of any means for holding the trailer wheels precisely aligned would result in instability and trailer wandering during forward travel. Accordingly, a steering lock mechanism is provided for the trailer steering, as shown in the left side elevation view of  FIG. 11 . 
     FIG. 11  illustrates the distal forward end  114  of the forwardly extending arm  116  of the trailer steering frame  18 , disposed generally beneath the hydraulic valve actuator  52  and its motor  62  and other componentry. The valve actuator  52  includes an actuator shaft and arm assembly  118  extending therefrom, with the actuator shaft serving to rotate the hydraulic valve drive sprocket  58  (shown in  FIG. 6 ). A steering frame lock cable  120  (or chain or other flexible member, etc.) extends in tension from the arm of the actuator shaft and arm assembly  118 , ultimately connecting to a steering frame arm lock  122 , which is pivotally attached laterally to the trailer frame  12 , or more precisely to a crossmember  124  extending between the two lateral members of the frame  12 . The cable  120  may pass over or through one or more pulleys  126  or guides, fairleads, etc. in order to achieve the proper routing and direction of operation. A down spring  128  urges the steering frame arm lock  122  down to its engaged position to capture the forward end portion  114  of the forwardly extending steering frame arm  116 . The steering frame arm lock  122  may comprise a laterally disposed length of material having a generally L-shaped cross section, e.g., “angle iron” or the like. The side or face opposite its attachment hinge  130  includes a steering arm engagement slot  132  therein (shown in broken lines in  FIG. 11 ), which fits fairly precisely over the forward arm portion  116  of the steering frame. 
   When the actuator  52  is operated to close the hydraulic control valves  44  through  50  (shown in  FIG. 6 ; a conventional solenoid may be used to reposition a lever or arm, for the solenoid valve system of  FIG. 7 ), or alternatively, the single valve  70  (shown in  FIG. 9 ) to allow the trailer wheels to be steered for backing maneuvers, the actuator shaft and arm  118  rotate to the right, as shown in the side view of  FIG. 11 , pulling the upper portion of the cable  120  to the right and the lower end of the cable upwardly, thereby drawing the steering arm lock  122  upwardly generally to its position as shown in broken lines in  FIG. 11 . This allows the forward end portion  114  of the steering frame arm  116  to swing laterally in accordance with steering input due to the articulation between the towing vehicle V and trailer  10 , to the limits defined by the steering centering and stop mechanism of  FIG. 10  and/or any other mechanical limits. 
   However, when the towing vehicle is placed in a neutral or forward gear, i.e., the backup signal is no longer activated, the valve actuator  52  rotates the actuator shaft and arm assembly  118  to the position shown in  FIG. 11 , thereby allowing the down spring  128  to draw the cable  120  and steering arm lock  122  downwardly against the top of the steering frame arm  116 . The lower edge of the steering arm lock  122  will ride against the upper surface of the steering frame arm  116  if the arm is not centered. However, when the trailer  10  is pulled forward slightly, the wheel centering spring(s)  112  cause the steerable wheels  28   a ,  28   b  of the trailer  10  to move to a straight ahead position, thus forcing the arm  116  to center. This allows the engagement slot  132  of the steering arm lock  122  to drop downwardly over the steering frame arm  116 , thus capturing the arm  116  within the slot  132  of the steering arm lock  122  to lock the arm  116  and its steering frame  18  and trailer wheels  28   a ,  28   b  straight for stable towing of the trailer  10  during forward travel. The engagement of the steering arm lock  122  with the steering frame arm  116  is easily detected by the driver of the tow vehicle, as the lock  122  is pulled sharply down over the steering arm  116  when the arm centers. Alternatively, a solenoid actuator may be used to control the steering arm lock  122 . The solenoid actuator would be controlled by the same electrical circuit as used to control a solenoid valve system, e.g., the system of  FIG. 7  of the drawings. 
   The above-described trailer towing mechanism and trailer with its steerable wheels is directed to a trailer having a single axle with two steerable wheels, one on each end of the single axle. However, the trailer steering mechanism may also be applied to trailers having multiple axles, such as the farm wagon type trailer  10   a  of  FIG. 5 . The trailer  10   a  of  FIG. 5  has a frame  12   a  (shown as a solid peripheral edge for the trailer  10   a  in  FIG. 5 ) carrying a forward axle  30   a  and a rearward axle  30   b , with the forward axle  30   a  having free steering left and right wheels  28   c ,  28   d  at the opposite ends thereof and the rearward axle  30   b  having directionally steerable wheels  28   e,    28   f  at the opposite ends thereof. The front wheels  28   c ,  28   d  are free steering in the sense that they are directionally articulating, but do not include a steering control mechanism. A tie rod  26   a  extends between the steering mechanism of each steerable rear wheel  28   e ,  28   f , with the tie rod  26   a  being connected to the steering frame  18   a  by an intermediate steering link  24   a.    
   The steering mechanism for the trailer  10   a  with its rearward axle  30   b  and its directionally steerable rear wheels  28   e ,  28   f  is generally the same steerable wheel mechanism as described further above for the trailer  10  with its single axle  30  and steerable wheels  28   a ,  28   b , shown in  FIGS. 2 through 4  of the drawings. However, the pivotally mounted steering frame  18   a  includes a rearward steering arm extension  22   a  to which the intermediate steering link  24   a  is attached, with the distal end of the link  24   a  being attached to the rear axle tie rod  26   a . Thus, only the rear wheels  28   e ,  28   f  extending from the rear axle  30   b  of the trailer  10   a  may be steered, with the front wheels  28   c,    28   d  being free steering, i.e. not connected to or directionally controlled by a steering mechanism. 
   The rear axle steering mechanism of the rear wheel steering trailer  10  is controlled by means of the same apparatus as provided with the trailer  10 . The frame  12   a  of the trailer  10   a  includes a forwardly extending rigid tongue  14 , with the tongue  14  being removably and pivotally hitched to a lateral hitch bar  16  rigidly attached to the tow vehicle V. The steering frame  18   a  includes a crossmember  32   a  which provides for attachment of at least one hydraulic cylinder to the steering frame, with there preferably being one hydraulic strut or cylinder  34   a  installed to the left side of the trailer tongue  14  and a second strut or cylinder  34   b  installed to the right side of the tongue  14 . Alternatively, plural struts or cylinders may be installed to each side of the central tongue  14 , if so desired, particularly in the case of a relatively large and heavy multiple axle farm wagon-type trailer  10   a . Each of the cylinders  34   a ,  34   b  has a hitch coupling end  36   a  and  36   b  and an opposite steering frame attachment end  38   a  and  38   b , with the forward ends  40   a ,  40   b  of the cylinder struts being pivotally coupled to, and extending from, the corresponding ends of the hitch bar  16 , and the steering frame attachment ends  38   a ,  38   b  attaching to the ends of the steering frame crossmember  32   a.  The struts or cylinders  34   a ,  34   b  are parallel to the trailer tongue  14  and to one another, with the tongue  14  being centered between the two struts or cylinders, just as in the case of the single axle trailer  10  embodiment. 
   The above-described steering mechanism is controlled by a hydraulic cylinder or strut control system  42 , as illustrated in  FIGS. 6 and 7  of the drawings and described in detail further above. Alternatively, the multiple valve mechanism of the system  42  may be replaced by the single valve mechanism  70  of  FIG. 9 , if so desired. The system operates in essentially the same manner as described above for the single-axle trailer  10 , but steers only the wheels and tires  28   e  and  28   f  of the rear axle  30   b,  with the forward wheels and tires  28   c ,  28   d  of the front axle  30   a  being able to free steer at all times during forward or rearward travel. 
   When the tow vehicle V towing the multiple axle trailer  10   a  of  FIG. 5  is towing the trailer forward, the hydraulic system is open to allow hydraulic fluid to flow freely between the two cylinders  36   a  and  36   b , thereby allowing the two cylinders to extend and retract in accordance with articulation between the tow vehicle V and trailer  10   a  without actuating the steering frame  18   a  and associated steering linkage to the rear axle  30   b.  Thus, the rear wheels and tires  28   a ,  28   f  track in a straight line relative to the centerline of the trailer  10   a , during forward travel. The two front wheels  28   c,    28   d  steer freely to allow the trailer  10   a  to negotiate turns while traveling forward. 
   When the twin-axle trailer  10   a  is to be backed, the two hydraulic cylinders or struts  36   a ,  36   b  are hydraulically locked, as in the case of the single-axle trailer  10 . This results in any articulation between the tow vehicle V and trailer  10   a  rotating the steering frame  18   a , thus steering the rear wheels  28   e ,  28   f . The front wheels  28   c ,  28   d  are free to steer or articulate directionally as required to enable the tow vehicle V and two axle trailer  10   a  combination to negotiate turns as required. 
   In conclusion, the trailer steering mechanism in its various embodiments provides a means for greatly simplifying the steering or guidance of a trailer during backing maneuvers. The system may automatically switch from a directionally locked mode for forward travel to steering mode for backing maneuvers, or may, alternatively, be manually switched, if so desired. While the trailer steering mechanism is particularly well suited to trailers having a single axle and two wheels, it may be adapted to other types of trailers having multiple axles as well, with little modification. Accordingly, the trailer steering mechanism will prove to be extremely popular with vehicle operators who have need to tow and back a trailer from time to time. 
   It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.