Abstract:
A simple mechanical height adjustment device adjusts the ground clearance of a trailer. The trailer, having the height adjustment device can selectably and reversibly increase its ground clearance. The trailer, having an adjustable ground clearance, can minimize its load height when operating on improved roads in order to meet local and state height regulation, but can increase its ground clearance on uneven roads.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for selectably adjusting the ground clearance of a towed trailer vehicle. 
     2. Description of the Related Art 
     Whenever, a trailer transports a high load it must minimize its load height to avoid the need for special routing or for hiring an escort as may be necessitated by local or state regulations. This problem is especially acute for the long trailers used for hauling oilfield equipment such as coiled tubing rigs used for well drilling and servicing. Coiled tubing rigs normally are rather tall, and the desirability of mounting an entire rig on a single trailer leads to the use of long trailers. 
     Further complicating this problem is the fact that many well locations are only accessible over very rough, uneven roads. Pulling long, low trailers loaded with heavy equipment over rough, uneven roads often leads to problems in expeditiously getting the oilfield equipment to a well site. This is particularly true, if it is desired to deliver the coiled tubing rigs to the well site on a single trailer. 
     A need exists for a simple, inexpensive means of adjusting trailer ground clearance which is easy to use. 
     SUMMARY OF THE INVENTION 
     This invention pertains to a device for adjusting the ground clearance of a trailer. One embodiment of the present invention includes a simple mechanical means for selectably and reversibly increasing the ground clearance of a trailer so that it can operate within a safe overall height limit when operating on improved roads, but can also operate with increased ground clearance on uneven roads. 
     One embodiment of the present invention includes a vehicle trailer comprising: (a) a rear deck; (b) a forward deck having a second end; (c) a central deck having a front end and a rear end, wherein the rear end is attached to the rear deck; and d) a pivotable altitude adjustment mechanism connecting the front end of the central deck to the second end of the forward deck, wherein the altitude adjustment mechanism includes a plurality of selectably engageable pin connections; whereby the forward deck and the central deck have a unique angular alignment with each other for each pin connection. 
     A second embodiment of the present invention includes a vehicle trailer comprising: (a) a rear deck; (b) a forward deck having a first end and a second end; (c) a central deck having a front end and a rear end, wherein the rear end is attached to the rear deck; and (d) a pair of pivotable altitude adjustment mechanisms connecting the front end of the central deck to the second end of the forward deck, wherein each altitude adjustment mechanism includes (i) a pair of rear pivot plates mounted on the front end of the central deck parallel to and offset from a longitudinal midplane of the trailer, wherein each rear pivot plate has a pivot hole and a plurality of position locking pin holes, (ii) a forward pivot plate mounted on the second end of the forward deck parallel to and offset from the longitudinal midplane of the trailer, wherein the forward pivot plate has a pivot hole and a plurality of position locking pin holes, and (iii) a pivot pin connecting the pivot hole of the forward pivot plate to the pivot holes of the pair of rear pivot plates when the pivot holes of the forward pivot plate and the rear pivot plates are coaxially aligned, and (iv) a selectably engageable position locking pin connecting one of a plurality of coaxially aligned position locking pin hole sets, wherein each set of position locking pin holes includes one position locking pin hole of the forward pivot plate and one position locking pin hole in each rear pivot plate, and whereby each set of position locking pin holes is associated with a particular height of ground clearance for the front end of the central deck. 
     Another embodiment of the present invention includes a vehicle trailer comprising: (a) a rear deck; (b) a forward deck having a first end and a second end; (c) a central deck having a front end and a rear end, wherein the rear end is attached to the rear deck; and (d) a pair of pivotable altitude adjustment mechanisms connecting the front end of the central deck to the second end of the forward deck, wherein each altitude adjustment mechanism includes (i) a first rear pivot plate having a first pivot hole and a plurality of position locking pin holes, wherein the first rear pivot plate is mounted on the front end of the central deck parallel to and offset from the longitudinal midplane of the trailer, (ii) a second rear pivot plate having a second pivot hole and a plurality of position locking pin holes, wherein the second rear pivot plate is mounted on the front end of the central deck parallel to the first rear pivot plate and offset from the first rear pivot plate towards the longitudinal midplane of the trailer, (iii) a forward pivot plate mounted on the second end of the forward deck parallel to and offset from the longitudinal midplane of the trailer, wherein the forward pivot plate has a pivot hole and a plurality of position locking pin holes, and (iv) a pivot pin connecting the pivot hole of the forward pivot plate to the pivot holes of the pair of rear pivot plates when the forward pivot plate is positioned between the first and second rear pivot plates and the pivot holes of the rear pivot plates are coaxially aligned with the pivot hole of the forward pivot plate, and (v) a selectably engageable position locking pin connecting one of a plurality of coaxially aligned position locking pin hole sets, wherein each set of position locking pin holes includes one position locking pin hole of the forward pivot plate and one position locking pin hole in each rear pivot plate, and whereby each set of position locking pin holes is associated with a particular ground clearance for the front end of the central deck. 
     The foregoing has outlined rather broadly several aspects of the present invention in order that the detailed description of the invention that follows may be better understood and thus is not intended to narrow or limit in any manner the appended claims which define the invention. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing of the structures for carrying out the same purposes as the invention. It should be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an oblique view of a tractor-trailer combination, wherein the trailer having an altitude adjustment mechanism for adjusting its ground clearance. 
         FIG. 2  is a side profile view of the tractor-trailer combination of  FIG. 1 , showing the trailer in its first configuration for being towed on a highway. 
         FIG. 3  corresponds to  FIGS. 1 and 2 , but shows the jacks located toward the forward deck of the trailer extended to contact the roadway surface. 
         FIG. 4  is an exploded oblique view from the forward side of the underside of the forward deck of the trailer showing the components of the altitude adjustment mechanism. 
         FIG. 5  is an exploded oblique view from above of the rearward side of the forward deck of the trailer showing the same components of the altitude adjustment mechanism as shown in  FIG. 4 . 
         FIG. 6  corresponds to  FIGS. 2 and 3 , but with the position locking pins having been moved to engagement with a second set of locking pin holes after the jacks have elevated the forward deck of the trailer sufficiently in order to align the second set of locking pin holes. 
         FIG. 7  corresponds to  FIG. 6 , but with the jacks retracted so the trailer can be moved without interference with the roadway. 
         FIG. 8  is a detail side view of the outer rear pivot plate, wherein the angular relationships of the position locking pin holes are indicated. 
         FIG. 9  is a detail side view of the forward pivot plate, wherein the angular relationships of the position locking pin holes are indicated. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As a note, the use of the terms “invention”, “present invention” and variations thereof throughout the subject patent application (and headings therein) are intended to refer or relate to one or more embodiments of the present application, not necessarily every embodiment or claim of the application. 
     Referring now to the drawings, it is noted that like reference characters designate like or similar parts throughout the drawings. The figures, or drawings, are not intended to be to scale. For example, purely for the sake of greater clarity in the drawings, wall thicknesses and spacings are not dimensioned as they actually exist in the assembled embodiments. 
     The present invention pertains to a device for adjusting the ground clearance of a trailer. One embodiment of the present invention includes a simple mechanical means for selectably and reversibly increasing the ground clearance of a trailer so that it can operate within a safe overall height limit when operating on improved roads, but can also operate with increased ground clearance on uneven roads. 
     The materials of construction of the structural components of the height adjustment mechanism are typically those employed by other commercial highway vehicles. The tractor is generally a heavy duty commercially available vehicle which has a fifth wheel for towing a trailer and multiple rear axles. The tractor normally will have a steel frame and either a steel or fiberglass cab. 
     For hauling high loads, the trailer will preferably be a “low-boy” trailer with a depressed center section for carrying cargo at a lower height than would a standard trailer with a flat deck. The trailer normally is fabricated from steel because of the high weight and long span between its pivot pin and its multiple rear axles, although aluminum can be used for many parts in order to minimize vehicle weight. The trailer is provided with a pair of either hydraulic or mechanical jacks having foot plates for contacting the ground. The jacks are mounted at or near the forward and back ends of the depressed center section of the trailer. 
     Referring to  FIG. 1 , a tractor/trailer rig  10  utilizing the low-boy trailer  20  is shown in an oblique view hauling a tall rectangular load  70 . The tractor  11  is a heavy duty commercially available vehicle having a fifth wheel  12  for towing the trailer  20  and multiple driven rear axles. The load  70  is shown as a rectangular prism for simplicity, but can have a wide variety of configurations, depending upon the make-up and desired use of the load. 
     For example, when designing a trailer for mounting a coiled tubing rig for use in well drilling or servicing, the rig designer will normally make the reel for the tubing and the other rig components as large as possible to still be compatible with standard unpermitted vehicle height limits. This is done in order to permit using larger diameter tubing or more tubing on the reel or both. 
       FIG. 2  shows a side profile view of the tractor  11  and trailer  20  of the present invention, wherein the trailer is in its lower first transit position, or highway towing position, suitable for unpermitted highway use. The highway ground clearance for the trailer of  FIG. 2  is preselected by the trailer designer to be the minimum compatible with most paved roads, so that the practical load height can be maximized. However, the ground clearance of the trailer  20  as shown in  FIG. 2  is insufficient for rough roads. 
     Referring to both  FIGS. 1 and 2 , the trailer  20  is seen to have a rear deck  51  which is necessarily elevated in order to provide clearance for the rear axle assemblies  52 . The rear deck  51  is flat and typically has a width of approximately 8 feet, while its length is generally somewhat longer in order to accommodate multiple rear axle assemblies. The rear axle assemblies include the axles, brakes, conventional spring supported suspensions, shock absorbers, wheels, and tires common to most conventional heavy duty trailers suitable for both highway and unimproved road use. The flat upper surface of the rear deck  51  is underlain and supported by longitudinal beams with smaller transverse cross beams (not shown). When in the highway towing configuration shown in  FIG. 2 , the rear deck  51  is substantially parallel to the approximately planar roadway  80  supporting the tractor-trailer rig  10 . 
     The rectangular main or central deck  21  of the low-boy trailer  20  has the same width as that of the rear deck  51  and is structurally connected to the rear deck by a short inclined deck segment which slopes downwardly in the forward direction. The central deck  21  is elongated and serves as the main load supporting area for the trailer  20 . When unloaded, the central deck  21  is generally cambered upwardly in its center. However, when the central deck  21  is statically loaded with its design load cargo  70 , the upper surface of the central deck is substantially planar. 
     Both the central deck  21  and the short inclined deck segment are connected to the rear deck  51  by continuous longitudinal beams to support the large bending loads associated with supporting the cargo on the central deck. Transverse cross beams are used to further stiffen and strengthen the deck surface of the central deck  21 , similar to the rear deck  51 . When in the highway towing configuration shown in  FIG. 2 , the main bed  21  of the trailer  20  is substantially parallel to the rear deck  51 . 
     An altitude adjustment mechanism  24  is used to join the central deck  21  to the forward deck  54  of the trailer  20 .  FIGS. 4 and 5  show exploded views of the altitude adjustment mechanism  24 . A pair of altitude adjustment mechanisms  24  are utilized to selectably adjust the ground clearance of the front end of the central deck  21  of the trailer, with one altitude adjustment mechanism  24  mounted on each side of the trailer  20 . The two altitude adjustment mechanisms  24  are symmetrical mounted about the longitudinal midplane of the trailer  20 . 
     Each altitude adjustment mechanism  24  includes three principle components. One pair of elements (the rear pivot plates  25  and  30 ) of the altitude adjustment mechanism  24  is attached to the forward end of the central deck  21 , while the other element (the forward pivot plate) of the mechanism  24  is attached to the rear side of the forward deck  54  of the trailer  20 . 
     The two elements of each altitude adjustment mechanism  24  mounted on the forward end of the central deck  21  of the trailer  20  are a pair of substantially similar rear pivot plates  25  and  30 . The outboard or outer pivot plate  25  is mounted vertically with its outer side flush with the side of the main bed of the central deck  21 . The inboard or inner pivot plate  30  of each rear pivot plate pair is mounted vertically and aligned parallel to the outboard pivot plate  25 . Each inboard pivot plate  30  is spaced apart from its adjacent outboard pivot plate  25  in the direction towards the trailer longitudinal midplane. 
     Referring to  FIG. 8 , it can be seen that each rear outboard pivot plate  25 , and similarly each rear inboard pivot plate  30 , consists of an approximately 90° arcuate sector of a circle with a projecting tab on its lower horizontal straight side. Going counterclockwise from its righthand lower side as seen in  FIGS. 5 and 8 , the perimeter of each pivot plate  25  and  30  has a short first vertical edge  61 , a 90° arcuate edge  62  curving upwardly and to the left, a short first horizontal edge  63 , and a long vertical edge  64 . The short first horizontal edge  63  and the long vertical edge  64  are joined by a small radius arc. 
     Adjoining the long vertical edge  64  on its lower end is a short edge  65  inclined at approximately 45° from the vertical to slope downwardly toward the front portion of the central deck  21  to which the outboard pivot plate  25  is attached. Adjoining the inclined edge  65  is a second vertical edge  66  having a length equal to the vertical thickness of the central deck  21 . A long horizontal edge  67  extends from the second vertical edge  66  to the lower end of the first vertical edge  61 . 
     The center of the 90° arc edge  62  lies in the plane of the second vertical edge  66 . Each of the rear outboard  25  and inner  30  pivot plates is attached by welding to both the forward deck and the upper surface of the central deck  21  of the trailer  20 . The rear pivot plates  25  and  30  are strongly and rigidly connected to the structure underneath the upper load bearing surface of the central deck  21  of the trailer  20 . In particular, loadings are efficiently transferred between the rear pivot plates  25  and  30  and the longitudinal beams supporting the main bed of the central deck  21  of the trailer. 
     A large diameter pivot pin through hole  26  is aligned with the axis of the arcuate edge  62  of the outer pivot plates  25 . A corresponding large diameter pivot pin through hole  31  is aligned with the axis of the arcuate edge  62  of the inner pivot plates  30  and is coaxial with the through hole  26  of the outer pivot plate  25 . 
     For the rear outer pivot plates  25 , a first position locking pin hole  27  is located vertically at radius R above the pivot pin hole  26 . A second position locking pin hole  28  is located, clockwise from hole  27  by angle ΦA, at radius R from the pivot pin hole  26 . The first position locking pin hole  27  and second position locking pin hole  28  have substantially the same diameter, which is smaller than that of the pivot pin hole  26 . The pivot pin hole  26  and the position locking pin holes  27  and  28  are mutually parallel and normal to the longitudinal midplane of the trailer  20 . 
     For the rear inner pivot plates  30 , a first position locking pin hole  32  is located vertically at radius R above the pivot pin hole  31 , and a second position locking pin hole  33  is located at radius R from the pivot pin hole  31  clockwise from hole  32  by angle ΦA. The first  32  and second  33  position locking pin holes have the same diameter which is smaller than that of the pivot pin holes  31 . The pivot pin hole  31  and the position locking pin holes  32  and  33  are mutually parallel and normal to the longitudinal midplane of the trailer  20 . In addition, the first position locking pin hole  27  and the second position locking pin hole  28  of the outboard pivot plate  25  are coaxial with the corresponding first position locking pin hole  32  and the second position locking pin hole  33  of the inner pivot plates  30 . 
     A pair of hydraulic jacks  44  is located at the forward end of the central deck  21  of the trailer  20 . The jacks  44  typically utilize conventional hydraulic double acting single end cylinders with the cylinders oriented perpendicular to the flat deck of the central deck  21  of the trailer  20 . The selectably reciprocable rods of the jacks  44  extend downwardly. The jacks  44  are similar to each other and generally consist of a conventional hollow right circular cylindrical body of revolution  45  having a main bore diameter, a blind end at an upper end, and a reduced bore diameter gland at the lower end. 
     Each jack body  45  has a radially outwardly extending cylinder mounting bracket  46  on one side. A right circular cylindrical jack cylinder rod  47  having an upset transverse cylindrical piston head at its upper end and a male threaded distal portion at its lower end is deployed in the cylinder body  45 . The piston head and the rod shank respectively seal to the main bore and the gland of the cylinder body. A foot plate assembly  48  having from its upper end a female socket threadedly engaged with the male thread of the rod  47 , a pivoting hinge with a foot plate pivot pin  49 , and a rectangular transverse foot plate is located at the bottom of each jack  44 . The axis of the foot plate pivot pin  49  is horizontal and is perpendicular to the longitudinal midplane of the trailer  20 . 
     Each jack  44  is attached to a rear inner pivot plate  30  in its vertical position inboard of and extending forward of the forward vertical edge of the inner pivot plate  30 . The positioning of the jacks  44  is such that their foot plates will not interfere with the forward deck of the central deck  21  of the trailer  20  when the foot plates  48  of the jacks are raised or lowered. 
     The forward deck  54  of the trailer  20  is a short flat section which is somewhat narrower than the rest of the trailer. The forward deck  54  of the trailer has a conventional kingpin extending downwardly on the underside of the forward deck. The kingpin  55  attachment is transverse to the upper surface of the forward deck and is located on the longitudinal midplane of the trailer  20  near the forward deck of the forward deck. The kingpin  55  is configured to be selectably latched and unlatched from the fifth wheel  12  of the tractor  11  so that the trailer  20  can be towed readily. The nature of the kingpin  55  to fifth wheel  12  attachment is such that it can tolerate a reasonably large amount of angular misalignment while still transmitting towing and braking forces between the trailer  20  and the tractor  11 . The kingpin  55  and the upper surface of the forward deck  54  are structurally interconnected by large longitudinal beams and smaller transverse beams. 
     Towards its rear end, the forward deck  54  is connected there to a pair of identical forward pivot plates  56 . One forward pivot plate  56  is attached by lapping welding the pivot plate  56  onto each outboard side of the forward deck  54 . As can be seen in  FIGS. 4 ,  5 , and  9 , the forward pivot plates  56  extend to the rear and below the main portion of the forward deck  54 . 
     The lateral positioning of the pivot plates  25 ,  30 , and  56  relative to the longitudinal centerline of the trailer  20  is such that each forward pivot plate  56  of the forward deck  54  will have a loose slip fit between a rear outboard pivot plate  25  and a rear inboard pivot plate  30  of the central deck  21  when the two portions of the trailer are comated with their pivot holes  26 ,  31 , and  57  placed in a coaxial position and the longitudinal centerlines of the forward deck  54  and the central deck  21  of the trailer aligned. 
     Referring to  FIG. 9 , the configuration of the forward pivot plates  56  is as follows moving counterclockwise from the lower rear corner of the plates. Edge  84  is an arcuate surface extending upwardly and a forward direction and covering an arc length of more than 90°. The upper end of the arcuate edge  84  is located vertically above the axis of the arcuate surface. Horizontal edge  85  is coplanar with the upper deck surface of the forward deck  54  and extends forward past the rear transverse end of the forward deck surface. Vertical edge  86  extends from the upper side to the lower side of the forward deck  54 . First inclined edge  87  slopes downwardly in a rearward direction, where it intersects second inclined edge  88 . 
     The second inclined edge  88  slopes slightly upwardly in the rearward direction until it intersects the arcuate edge  84 . The intersection of the first  87  and second  88  inclined edges is radiused, as is the intersection of the second inclined edge  88  and the arcuate edge  84 . Typically rectangular and triangular plate segments are used to form hollow triangular prismatic transitions between the vertical edges  86  of the two forward pivot plates  56  and the sides of the forward deck  54 . 
     A large diameter pivot pin through hole  57  is aligned with the axis of the arcuate edge  84  of the forward pivot plates  56 . The diameter of the pivot pin holes  57  in the forward pin plates is substantially the same as that of the pivot pin holes  26  and  31  of the rear outer  25  and rear inner  30  pivot plates. 
     Referring to  FIG. 9 , a first position locking pin hole  58  in the forward pivot plate  56  is located vertically at radius R above the pivot pin hole  57 , and a second position locking pin hole  59  is located at radius R from the pivot pin hole  57  clockwise from position locking pin hole  58  by angle ΦB. The first  58  and second  59  position locking pin holes of the forward pivot plate  56  have about the same diameter as that of the position locking pin holes  27 ,  28  of the rear outer pivot plates  25  and the position locking pin holes  32 ,  33  of the rear inner pivot plates  30 . 
     At assembly, the forward pivot plate  56  is positioned between the rear pivot plates  25  and  30  with the pivot pin holes  26 ,  31 , and  57  coaxially aligned and normal to the longitudinal midplane of the trailer  20 . Furthermore, the first position locking pin holes  25  and  32  of the rear pivot plates  25  and  30  are coaxially aligned with the first position locking pin hole  58  of the forward pivot plate  56 , and the second position locking pin holes  28  and  33  of the rear pivot plates  25  and  30  are coaxially aligned with the second position locking pin hole  59  of the forward pivot plate  56 . 
     Two pivot pins  36  and two position locking pins  38  are used to join the elements of the two altitude adjustment mechanisms  24  and thus the forward deck  54  to the central deck  21  of the trailer  20 . The pivot pins  36  are headed on a first end and generally have a slight taper on the second end. The length of the right circular cylindrical central portion of the pivot pins  36  is sufficient to extend through and approximately one inch beyond each set of three interleaved pivot plates  25 ,  30 ,  56 , and the diameter of the pivot pins  36  is such that they are a slip fit to the pivot pin holes  26 ,  31 , and  57 . 
     Typically, transverse diametrical keeper pins holes  40  are offset axially outwardly from the head of the pivot pins  36 . The pivot pins  36  are stabbed through the coaxial pivot pin holes  26 ,  57 , and  31  at assembly of the trailer  20  so that the transverse inner shoulders of the headed pivot pins  36  abut a rear outboard pivot plate surface. Headed elongated cylindrical pivot pin keepers  37  are inserted through the keeper pin holes  40  to retain the pivot pins  36  in place after assembly. Often headed bolts with lock nuts are used for this purpose. Following the insertion of the pivot pins and their keepers  37 , the central deck section  21  and the forward deck section  54  of the trailer  20  are hinged together on a horizontal axis transverse to the longitudinal midplane of the trailer. 
     The two position locking pins  38  are substantially similar to the pivot pins  36 , with the exception that the diameter of the central cylindrical section of the position locking pins  38  is a slip fit both to the first position locking pin holes  27 ,  32 ,  58  and the second position locking pin holes  28 ,  33 ,  59  position locking pin holes. Position locking pin keepers  39 , used for the position locking pins  38 , are similar to the keeper pins  37  used with the pivot pins  36  with the exception that the keeper pins  39  are shorter, since the diameter of the position locking pins  38  is less than that of the pivot pins  36 . The position locking pin keepers  39  are inserted into diametrical cross holes  41  in the exposed nonheaded ends of the position locking pins  38  to provide pin retention. 
     When the trailer has both its pivot pins  36  installed as described above and its position locking pins  38  installed in either the coaxial position locking pin holes  27 ,  32 ,  58  or the coaxial position locking pin holes  28 ,  33 ,  59 , the trailer  20  has its central deck section  21  structurally attached in a rigid and safe load bearing configuration. Either of the two described configurations is structurally competent to transport a heavy trailer cargo  70  on both highways and unimproved roads. 
     OPERATION OF THE INVENTION 
     To complete assembly of the trailer  20 , the position locking pins  38  with their position locking pin keepers  39  are installed in either the coaxial position locking pin holes  27 ,  32 , and  58  or alternatively in the coaxial position locking pin holes  28 ,  33 , and  59 . For the purpose of example, it is assumed herein that the position locking pins are preinstalled in the coaxial first position locking pin holes  27  and  32  of the central deck portion  21  and the mutually coaxial first position locking pin holes  58  of the forward deck  54  portion of the trailer  20 . The result is the first configuration of the trailer  20  shown in both  FIGS. 1 and 2 , with the jacks  44  fully retracted. This first configuration of the trailer  20  provides both an acceptable road clearance and a minimal practical load height for highway use. 
     In order to increase the road clearance for the trailer  20  from that shown in  FIGS. 1 and 2 , the first step is to extend the jacks  44  until they bear fully on the surface of the roadway, as is shown in  FIG. 3 . When the jacks  44  are then extended sufficiently further, the transverse loadings on the position locking pins  38  are reduced sufficiently to permit their extraction after the removal of the position locking pin keepers  39 . Following extraction of the position locking pins  38 , the jacks are extended still further until the second position locking pin holes  28 ,  33 , and  59  are brought into axial alignment, as shown in  FIG. 6 . 
     The change of alignment from the first position shown in  FIGS. 1 and 2  to that of  FIG. 6  is effected by elevating both the forward deck of the central deck  21  and the rear portion of the forward deck  54 , which are still pinned together by the pivot pins  36 . At this point, the position locking pins  38  are then fully inserted into the now aligned and coaxial second position locking pin holes  28 ,  33 ,  59  and individually retained by their respective position locking pin keepers  39  engaged in the holes  41  of the locking pins  38 . The position locking pins  38  are shown in their installed second position in  FIG. 6 . 
     To transition from the arrangement shown in  FIG. 6  to the completed off-highway position shown in  FIG. 7  all that is necessary is to fully retract the jacks  44  so that they do not extend below the bottom of the central deck  21  of the trailer  20 . When the trailer is elevated as is shown in  FIG. 7 , the center portion of the length of the trailer  20  located between the fifth wheel  12  and the rear axles  52  is sufficiently elevated that it will clear high spots on most unimproved roads. The resulting improved road clearance allows the low boy trailer to be brought into and removed from the majority of most remote field locations. 
     Adjusting the trailer from the higher ground clearance configuration to the lower ground clearance configuration, the operator reverses the procedure for transitioning from the low ground clearance configuration of  FIG. 2  to the improved ground clearance configuration of  FIG. 7 . 
     ADVANTAGES OF THE INVENTION 
     The provision of the selectably lockable articulated transverse axis joint intermediate to the length of the low boy trailer of the present invention readily permits adjusting the ground clearance of the trailer to handle different road conditions. The selectable locking of the altitude adjustment mechanism  24  is accomplished easily with the use of the position locking pins  38 . 
     Fabrication costs for the altitude adjustment mechanism are low. The cost of the simple mechanical components is relatively low, and maintenance costs are minimal. The pinned connection is inherently robust while providing sufficient stiffness and strength to ensure long term reliability. Equipment inspection is quite simple. 
     Operator training is relatively simple and equipment inspection is basic. There are numerous advantages of being able to provide a practical maximum load height when operating on highways with vertical height limitations, while still being able to selectably and easily increase the road clearance of the critical central section of the trailer. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. As may be understood readily by those skilled in the art, certain variations in the structure of the present invention may be made without departing from the spirit of the invention. For instance, mechanical rather than hydraulic jacks could be used to raise and lower the trailer. The position locking pins could be made so that they can be withdrawn and engaged using screw threads to reciprocate the pins out of and into full engagement with the position locking pin holes. These and other minor changes can be made without departing from the spirit of the invention.