Abstract:
A tow bar-activated brake system capable of engaging two tires or tire sets of a dolly or trailer where the tires or tire sets are of substantially different diameter. The system includes a first brake assembly and second brake assembly commonly actuated yet capable of rotating independently of each other to provide engagement with both tires/tire sets, even when the tires/tire sets are of substantially different diameter, for example due to uneven wear.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/105,049, filed Jan. 19, 2015, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure is directed generally to brake systems for trailers and towed vehicles, and more specifically to tow bar actuated brake systems. 
     BACKGROUND 
     Towed vehicles are often used as ground support equipment for aircraft. Examples include baggage carts, mail carts, dollies, and supply trailers. The towed vehicles often include brake systems that are designed to engage the tread of the vehicle tires to provide braking when the towed vehicle is in a parked configuration. Many such brake systems are designed to engage two of the vehicle tires to provide stability by maintaining the orientation of the vehicle in the parked orientation. 
     It has been reported that some vehicles utilizing such brake systems become unstable in the parked configuration. This has resulted in the vehicle spuriously rotating about one of the braked tires and colliding with adjacent objects. The collisions can lead to expensive repairs, particularly when involving an aircraft. 
     An improved brake system that reduces or eliminates instabilities of parked vehicles would be welcomed. 
     SUMMARY 
     Various embodiments of the disclosure include a brake system that assures stabilization of a parked trailer or vehicle by compensating for different tire diameters that arise, for example, due to uneven tire wear. The disclosed embodiments accomplish the braking of the tires passively or automatically upon raising of the tow bar of the vehicle from a towing configuration to a parking configuration. Some embodiments also include a latching system that secures the tow bar in the parking configuration. Also, various embodiments include an actuation mechanism that is centrally located beneath the trailer bed, thereby reducing susceptibility to becoming accidentally ensnared during vehicle operation. 
     Many conventional braking systems operate by engaging the tread of two tires or two tire sets of a towed vehicle (e.g., the rear tires of a 4-wheel trailer). For various reasons, a condition can develop where the tires of towed vehicle experience differential tread wear, wherein a first of the braked tires or tire sets has worn to an effective diameter that is substantially different from a second of the braked tires or tire sets. For example, one of the braked tires or tire sets may become irreparably damaged and require replacement, while the remaining of the braked tires or tire sets is left on the vehicle to continue service. Often, the replaced tire or tire set has a different degree of tread wear than the remaining tire or tire set, thereby causing differing effective diameters between the braked tires. 
     Many conventional towed vehicles include brake systems that do not accommodate differing effective diameters of the braked tires. In instances where the difference between the diameters becomes large enough, certain conventional brake systems adequately engage one tire or tire set, but not the other, resulting in instability of the braked vehicle. That is, when in the parking configuration, the inadequately braked tire or tire set can pivot about the adequately engaged tire or tire set, for example due to gravity forces, wind, or the thrust of air commonplace on airport tarmacs due to the thrusting of prop or jet engines. The pivoting vehicle may collide with nearby objects, such as other ground support equipment or event aircraft. Various embodiments of the disclosure alleviate this problem by uniformly engaging the braked tires or tire sets, regardless of any diameter differences between the tire or tire sets that may exist. 
     Structurally, in various embodiments of the disclosure, a tow bar-activated brake system for a towed vehicle is disclosed, including a tow bar adapted to rotate from a towing configuration to a parking configuration. A first brake assembly includes a first axle defining a first rotation axis and a first brake bar offset from the first rotation axis for contacting a tread of a first tire of the towed vehicle, the first brake assembly being operatively coupled with the tow bar for rotation of the first axle about the first rotation axis. A second brake assembly includes a second axle defining a second rotation axis and a second brake bar offset from the second rotation axis for contacting a tread of a second tire of the towed vehicle, the second brake assembly being operatively coupled with the tow bar for rotation of the second axle about the second rotation axis. In some embodiments, the second rotation axis is concentric with the first rotation axis. In some embodiments, the second axle is configured for rotation about the second rotation axis independent of the first axle. 
     In various embodiments of the disclosure, a method for braking a towed vehicle is disclosed, including: rotating a first axle of a first brake assembly through a first rotational displacement and into contact with a tread of a first tire of a towed vehicle; and rotating a second axle of a second brake assembly through a second rotational displacement and into contact with a tread of a second tire of a towed vehicle, wherein the second rotational displacement is greater than the first rotational displacement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective, partial cutaway view of a tow bar-activated brake system in an embodiment of the disclosure; 
         FIG. 2  is an enlarged, partial perspective view of the tow bar-activated brake system of  FIG. 1  depicting a linkage assembly in an embodiment of the disclosure; 
         FIG. 2A  is an enlarged, partial perspective view of an alternative linkage assembly in an embodiment of the disclosure; 
         FIG. 3  is an exploded, partial view of the tow bar-activated brake system of including a differential rotation joint in an embodiment of the disclosure; 
         FIG. 4  is an elevational view of the differential rotation joint of  FIG. 3  in assembly with the linkage assembly of  FIG. 2A ; 
         FIG. 5  is a schematic representation of the tow bar-activated brake system of  FIG. 1  in an embodiment of the disclosure; 
         FIG. 6  is a schematic representation of the tow bar-activated brake system of  FIG. 1  in initial engagement with a first tire in an embodiment of the disclosure; 
         FIG. 7  is a schematic representation of the tow bar-activated brake system of  FIG. 1  in engagement with both tires in an embodiment of the disclosure; 
         FIG. 8A  is a partial perspective view of the tow bar-activated brake system of  FIG. 1  depicting operation of a linkage assembly during differential rotation corresponding to  FIG. 7  in an embodiment of the disclosure; 
         FIG. 8B  is an enlarged perspective view of  FIG. 8A ; 
         FIG. 9A  is a partial perspective view of the tow bar-activated brake system of  FIG. 1  in a compensating orientation in an embodiment of the disclosure; 
         FIG. 9B  is an enlarged perspective view of  FIG. 9A ; and 
         FIG. 10  is a schematic representation of the tow bar-activated brake system of  FIG. 1  fully engaged with the tow bar in a locked configuration in an embodiment of the disclosure; 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 through 5 , a tow bar-activated brake system  30  is depicted in an embodiment of the disclosure. The tow bar-activated brake system  30  includes a tow bar assembly  32  adapted to pivot from a towing configuration to a parking configuration. The depiction of  FIG. 1  presents brake system in the towing configuration, wherein the brake system is in a non-braking configuration. The parking configuration is depicted schematically at  FIG. 10 , described in detail below. 
     A tie rod  34  includes a first end  36  coupled to the tow bar assembly  32 , the tie rod  34  defining an actuation axis  35  of the tow bar-activated brake system  30 . A linkage assembly  40  is pivotally connected to a second end  42  of the tie rod  34 , the linkage assembly  40  including a cross member  44 , a first linkage  46 , and a second linkage  48 . The forward ends of the linkages  46  and  48  are mounted to the cross member  44  about vertical pivots  52 . The cross member  44  may be rotatable about a cross member pivot  56  that also extends vertically. In some embodiments, the rearward ends of the linkages  46  and  48  include horizontal pivots  54 . In various embodiments, the pivots  52 ,  54 ,  56  include pins or rods that span end slots formed at the ends of the linkages  46  and  48 . The pins or rods may be coupled to bushings (not depicted) disposed in the linkages  46 ,  48  and tie rod  34 . 
     The depicted embodiment includes a first brake assembly  60  including a first axle  62  defining a first rotation axis  64  and a first brake bar  66  offset from the first rotation axis  64 . The first linkage  46  of the linkage assembly  40  is operatively coupled with the first brake assembly  60 , for example by a first standoff  68 , for rotation of the first axle  62  about the first rotation axis  64 . The depicted embodiment further includes a second brake assembly  70  including a second axle  72  defining a second rotation axis  74  and a second brake bar  76  offset from the second rotation axis  74 , the second linkage  48  of the linkage assembly  40  being operatively coupled with the second brake assembly  70  for rotation of the second axle  72  about the second rotation axis  74 , for example by a second standoff  78 . In various embodiments, the first and second brake bars  66  and  76  are capable of being aligned along an offset axis  77 . The first and second axles  62  and  72  can be of a hollow cylindrical structure, for example a schedule  80  pipe. In various embodiments, the first rotation axis  64  and the second rotation axis  74  are concentric. 
     The rotation of the second axle  72  is independent of the rotation of the first axle  62 , so that the first and second brake assemblies  60  and  70  can undergo what is referred to herein as a “differential rotation.” Differential rotation is established when one of the brake assemblies  60  or  70  ceases rotation due to set engagement with the respective tire/tire set  204  or  206 , while the other of the brake assemblies  70  or  60  continues to rotate for lack of engagement with the respective tire/tire set  206  or  204 . During actuation of the tow bar-activated brake system  30  differential rotation ceases when the other of the brake assemblies  70  and  60  is in set engagement with the other of the tire/tire sets  206  or  204 . 
     In reference to  FIG. 2A , an alternative linkage assembly  40   a  is depicted in an embodiment of the disclosure. The linkage assembly  40   a  includes many of the same components and attributes as the linkage assembly  40 , which are identified with same-numbered numerical references. Instead of the slotted bar linkages  46 ,  48 , the linkages  46   a  and  48   a  include eyed ball joints  52   a  and  54   a  (known by the trade name HEIM joints) formed on opposing ends, the eyed ball joints  52   a  and  54   a  including a ball joint that defines an eye therethrough. Standard fasteners pass through the eyes of the eyed ball joints  52   a  and  54   a  to join the linkages  46   a  and  48   a  to the cross member  44  and the standoffs  68  and  78 . 
     Functionally, the eyed ball joints  52   a  and  54   a  accommodate rotation of the forward ends of the linkages  46   a  and  48   a  toward the actuation axis of the tie rod  34 . During a differential rotation between brake assemblies  60  and  70 , the cross member  44  rotates about the pivot  56 , such that both ends of the cross member  44  rotate towards the actuation axis  35 . This cause the forward ends of both cross members  46   a  and  48   a  to also rotate toward the actuation axis  35 . The differential rotation thereby reduces lateral distance (dimension in the y-direction of the x-y-z coordinate of  FIG. 1 ) between the eyed ball joints  52   a  at the forward ends of the linkages  46   a  and  48   a , while the lateral distance between the rearward ends of the linkages  46   a  and  48   a  remains fixed. The eyed ball joints  52   a  and  54   a  enable the linkages  46   a  and  48   a  to rotate without introducing slop or play in the assembly to accommodate the rotation. 
     In various embodiments, the first axle  62  and the second axle  72  are joined at a differential rotation joint  80 . In one embodiment, the differential rotation joint  80  includes a centering shaft  81  disposed between the first axle  62  and the second axle  72 , the centering shaft  81  being coupled to one of the first and the second axles  62  or  72 , for example by a fastener  79  (depicted as coupled to the first axle  62  herein). In one embodiment, the fastener  79  is disposed through a lateral through-passage  83  that is defined as passing through both the first axle  62  and the centering shaft  81  ( FIG. 4 ), thereby fixing the centering shaft  81  with respect to the first axle  62 . The other of the first and second axles  62  or  72  rotates freely about the centering shaft  81 . 
     In some embodiments, the first brake assembly  60  is connected to and in direct contact with the second brake assembly  70 . In one embodiment, the differential rotation joint  80  includes a first flange  82  coupled to the first axle  62  and concentric about the first rotation axis  64  of the first axle  62 . The first flange  82  includes an inner face  84  and an outer face  86  that defines a thickness  88  therebetween. The first flange  82  also defines circular through holes  90  that pass therethrough. The circular through-holes  90  may be defined along axes  89  that are substantially parallel to the first rotation axis  64 . 
     In various embodiments, the differential rotation joint  80  includes a second flange  92  coupled to the second axle  72  and concentric about the second rotation axis  74  of the second axle  72 . The second flange  92  includes an inner face  94  and an outer face  96  that defines thickness  98  therebetween. The second flange  92  is coupled to the first flange  82  and adapted to rotate about the second rotation axis  74  relative to the first flange  82 . In the depicted embodiment, the second flange  92  defines arcuate slots  102 , each arcuate slot  102  having a width  104  and an arcuate length  106 . The arcuate slots  102  and the circular through-holes  90  are centered at a common radius  108  from the rotation axes  64 ,  74 . Also, the arcuate slots  102  and the circular through-holes  90  are configured to be centered at common angular intervals  109  when the first and second brake bars  66  and  76  are in alignment along the offset axis  77 . 
     In various embodiments, the first and second flanges  82  and  92  are coupled by fasteners  110 , each fastener  110  including a bolt  112 , a washer  114 , and a nut  116 . The bolts  112  pass through the circular through-holes  90  of the first flange  82  in a close tolerance or a press fit, and thus are in a substantially fixed rotational relationship with the first flange  82  of the first brake assembly  60 . Alternatively, studs (not depicted) may be implemented that are welded to the inner face  84  of the first flange  82  and centered at the common angular intervals  109 . The threaded ends of the bolts  112  also pass through the arcuate slots  102 . In some embodiments, the bolts  112  are shoulder bolts, with the shoulder extending through the first flange  82 , a shim  120  (when implemented), and at least a large fraction of the thickness  98  of the second flange  92 . In some embodiments, the washers  114  are oversized in both thickness  117  and an outer diameter  118 , the outer diameter  118  being substantially oversized with respect to the width  106  of the arcuate slots  102  ( FIG. 4 ). In some embodiments, the washers  114  include a low friction and/or self-lubricating material, such as polytetrafluoroethylene (PTFE), NYLON®, or DELRIN®. In some embodiments, the self-lubricating material is a coating over a metallic washer substrate. Optionally, the washers  114  are formed of the self-lubricating material. 
     Optionally, the differential rotation joint  80  includes a spacer or shim  120  disposed interstitially between the first flange  82  and the second flange  92 . In one embodiment, the shim  120  includes a low friction and/or self-lubricating material, such as PTFE, NYLON®, or DELRIN®. The shim  120  defines fastener apertures  122  for passage of the bolts  112  distributed about a center aperture  124  for passage of the centering shaft  81 . Optionally, instead of a separate shim, one or both of the inner faces  84 ,  94  of the flanges  82 ,  92  may be provided with a lubricant. In other embodiments, instead of a shim, a thrust bearing (not depicted) is disposed interstitially between the first and second flanges  82  and  92 . 
     In some embodiments, the differential rotation joint  80  includes bushings  130 , each bushing  130  defining a length  132  and being concentric about a respective one of the bolts  112 . Each bushing  130  rides within a respective one of the arcuate slots  102 . In some embodiments, the bushings  130  include a low friction and/or self-lubricating material, such as polytetrafluoroethylene (PTFE), NYLON®, or DELRIN®. For example, the self-lubricating material may be a coating over a metallic bushing substrate. In various embodiments, each bushing  130  includes an inner metallic sleeve inserted within an outer self-lubricating sleeve, for example in a press fit arrangement. In some embodiments, the bushings  130  may be dimensioned to rotate about the bolts  112 , with the inner diameter of the bushings  130  being lubricated, for example by a dry lubricant, to facilitate the rotation. In the latter embodiments, shoulder bolts used for the bolts  112  as described above provides a smooth surface to facilitate rotation of the bushings  130  about the bolts  112 . 
     In assembly, the centering shaft  81  is inserted into the first axle  62  of the first brake assembly  60  and aligned so as to define the lateral through-passage  83 , and the fastener  79  inserted therethrough and secured such that a protruding portion  142  of the centering shaft  81  protrudes from the first flange  82  of the first brake assembly  60 . The center aperture  124  of the shim  120  is slid over the protruding portion  142  of the centering shaft  81  and the shim  120  rotationally oriented to that the fastener apertures  122  are substantially aligned with the circular through-holes  90  of the first flange  82 . The second brake assembly  70  is slid over the protruding portion  142  of the centering shaft  81  so that the shim  120  is sandwiched between the first and second flanges  82  and  92 . A clearance fit between the brake assembly  70  and the centering shaft  81  enables the brake assembly  70  to be rotated about the center shaft  81 . 
     For the initial assembly of the differential rotation joint  80 , the second brake assembly  70  may be rotated about the second rotation axis  74  so that the first and second brake bars  66  and  76  of the first and second brake assemblies  60  and  70  are aligned along the offset axis  77  ( FIG. 1 ). With the brake bars so aligned, the circular through-holes  90  of the first flange  82  and the fastener apertures  122  of the shim  120  are substantially aligned with the center of the arcuate slots  102  of the second flange  92 . The bolts  112  are fed through the aligned through-holes  90 , fastener apertures  122 , and arcuate slots  102 . The bushings  130  are slid over the bolts  112 , into the arcuate slots  102 , and, if the fastener apertures  122  are sized to accommodate, into the fastener apertures  122  and registered against the inner face  84  of the first flange  82 . The washers  114  are slid over the threaded ends of the bolts  112  (now protruding through the arcuate slots  102 ) and the nuts  116  threaded onto the bolts  112  to register the washers  114  against the outer face  96  of the second flange  92  and to couple the first brake assembly  60  to the second brake assembly  70 . 
     The coupling of the first brake assembly  60  to the second brake assembly  70  must be such that the first brake assembly  60  can rotate independently of the second brake assembly  70 . In some embodiments, to facilitate the independent rotation while still enabling lock tightening of the nuts  116  onto the bolts  112 , the lengths  132  of the bushings  130  are dimensioned so that, in assembly, the bushings  130  protrude slightly proud with respect to the outer face  96  of the second flange  92 . That is, the lengths  132  of the bushings  130  may be slightly greater than the combined thickness of the second flange  92  and shim  120 . In such an embodiment, the nuts  116  are tightened onto the bolts  112  so that the washers  114  register against the bushings  130 . 
     Functionally, the arcuate slots  102  enable the first flange  82  of the differential rotation joint  80  to rotate with respect the second flange  92 . The fasteners  110  that are in substantially fixed relation with the first flange  82  can rotate within the arcuate slots  102 , thus enabling rotation of the first flange  82  relative to the second flange  92 . The rotational displacement of fasteners  110  within the arcuate slots  102  at respective extremes of rotational displacement is illustrated in  FIGS. 8B and 9B . In this way, the first brake assembly  60  is able to rotate relative to the second brake assembly  70 . 
     The centering shaft  81  provides stability and maintains alignment of the rotation axes  64 ,  74  at the joint between the first brake assembly  60  and the second brake assembly  70 . The bushings  130  enable for sliding engagement with the arcuate slots  102  and, as discussed above, may provide the desired spacing between the washers  114  and the inner face  84  of the first flange  82 . In this way, the nuts  116  and washers  114  can be drawn tight against the bushings  130  to securely register the bushings  130  against the inner face  84  of the first flange and to rotationally secure the nuts  116  to the threads of the bolts  112  while enabling the first brake assembly  60  to rotate independently of the second brake assembly  70 . By having the bushing provide the desired clearance, assembly as well as field servicing of the tow bar-activated brake system  30  is simplified. 
     Returning to  FIGS. 1 and 5 , the tow bar assembly  32  includes a tongue  152  and a pivot member  154 , the tow bar assembly  32  configured to pivot about a pivot axis  156  that passes through the pivot member  154 . Gussets  158  may extend between the tongue  152  and the pivot member  154 . In some embodiments, a yoke assembly  160  depends from the pivot member  154 , the yoke assembly  160  including a yoke pin  162  supported by a pair of standoffs  164 . The tow bar assembly  32  is pivotally mounted to a frame or carriage  166  of the towed vehicle. 
     In various embodiments, a spring module  170  is coupled between the tow bar assembly  32  and the first end  36  of the tie rod  34 , best seen in  FIG. 5 . In the depicted embodiment, the spring module  170  includes a housing  172  having a proximal end  174  and a distal end  176 . A lateral passage  178  is defined near the distal end  176 , through which the yoke pin  162  is disposed. The proximal end  174  of the housing  172  may include a reducing flange  182  defining an aperture  184  through which the tie rod  34  passes. In some embodiments, a head member  186  is coupled to the first end  36  of the tie rod  34 , and a biasing element  188  disposed within the housing  172 , the biasing element captured between the reducing flange  182  and the head member  186 . The biasing element  188  may be, for example, a coiled spring (depicted) or an elastic bushing, such as a rubber or polymer material that is compressible yet resilient. In some embodiments, a stop  189  is affixed to the tie rod  34  rearward of the proximal end  174  of the housing  172 . 
     In various embodiments, the tow bar assembly  32  includes a latch member  190  pivotally coupled to the yoke pin  162 . The latch member  190  defines a notch or hook  192  configured to engage the frame  166  when the tow bar assembly  32  is in the parking configuration. In some embodiments, the latch member  190  is loosely coupled to the yoke pin  162 , so as to pivot freely about the yoke pin  162  and rotate downward under force of gravity. In some embodiments, the yoke pin is biased to rotate into a latching position by a spring (not depicted) coiled about the yoke pin  162 . 
     Referring to  FIGS. 6 and 7 , operation of the tow bar-activated brake system  30  is depicted in an embodiment of the disclosure. The  FIGS. 6 and 7 , as well as  FIGS. 5 and 8 , are a schematic representation of the tow bar-activated brake system  30 , and as such depict certain components and attributes in simplified or exaggerated form to illustrate the operational concept. To set the brakes of the tow bar-activated brake system  30 , the tow bar  32  is rotated from the towing configuration or a downward position ( FIG. 6 ) into the parking configuration ( FIG. 10 ), the tongue  52  of the tow bar  32  being pointed substantially upward in the parking configuration. Upon rotation of the tow bar  32  from the towing configuration toward the parking configuration, the off center rotation of the yoke assembly  160  about the pivot axis  156  translates the tie rod  34  along the actuation axis  35  in a forward or distal direction  202 , translating the linkage assembly  40  therewith. The translation of the linkage assembly  40  exerts a forward force on the standoffs  68 ,  78 , causing rotational displacements  214  and  216  of the first and second brake assemblies  60  and  70 , respectively, about their respective rotation axes  64 ,  74 . The rotational displacements  214 ,  216  cause the first and second brake bars  66  and  76  to rotate into contact with the treads of the respective first and second tires or tandem tire sets  204  and  206 . 
     If one of the tires/tire sets  204  or  206  is of substantially larger effective diameter than the other of the tires/tire sets  206  or  204  (tire  204  in  FIGS. 6 and 7 ), the respective one of the brake bars  66  or  76  (brake bar  66  in  FIGS. 6 and 7 ) engages the corresponding tire/tire set  204  or  206  before the other of the brake bars  76  or  66  (brake bar  76  in  FIGS. 6 and 7 ) engages the other tire/tire set  206  or  204 . The rotational displacement  214  or  216  of the contacting brake bar  66  or  76  (rotational displacement  214  in  FIG. 6 ) ceases as the brake bar  66  or  76  becomes set against the larger diameter tire/tire set  204  or  206 . 
     As the tow bar  32  continues to rotate upwards, the stroke of the tie rod  34  in the forward direction  102  also continues, so that the other of the brake bars  76  or  66  (brake bar  76  in  FIGS. 6 and 7 ) continues the rotational displacement  216  or  214  towards the respective tire/tire set  206  or  204 , thus establishing a differential rotation. The rotational displacement  216  or  214  (rotational displacement  216  in  FIG. 7 ) ceases as the other of the brake bars  76  or  66  becomes set against the respective tire/tire set  206  or  204  ( FIG. 7 ). At this point, both tire/tire sets  204  and  206  are braked. 
     Referring to  FIGS. 8A and 8B , the effect of the differential rotation on the linkage assembly  40  is depicted in an embodiment of the disclosure. The independent rotations of the first and second brake assemblies  60  and  70  are driven by the linkage assembly  40 , which is pivotable about the cross member pivot  56 . In the event of a differential rotation, the linkage assembly  40  pivots about the cross member pivot  56 , thereby enabling rotation of one of the brake assemblies  60 ,  70  while the other of the brake assemblies  70 ,  60  remains rotationally stationary. The  FIGS. 8A and 8B  depictions present operation of the linkage assembly  40  as depicted in  FIGS. 6 and 7 , when the first tire/tire set  204  is of a larger diameter than the second tire/tire set  206 . In this scenario, the first brake assembly  60  contacts with the first tire/tire assembly  204  before the second brake assembly  70  contacts the second tire/tire assembly  206 , such that the second brake assembly  70  undergoes a larger rotational displacement to become seated with the treads of the second tire/tire assembly  206 . The linkage assembly  40  pivots about the cross member pivot  56  in a first direction (clockwise in  FIGS. 8A and 8B ) to accommodate the differential rotation between the rotational displacements  214  and  216  of the first and second brake assemblies  60  and  70 . 
     Referring to  FIGS. 9A and 9B , operation of the linkage assembly  40  in an alternative to the scenario of  FIGS. 6 and 7  is depicted in an embodiment of the disclosure. Here, the first brake assembly  60  undergoes a larger rotational displacement than does the second brake assembly  70 , for example because the second tire/tire set  206  is of a larger diameter than the first tire/tire set  204 . In this scenario, the second brake assembly  70  contacts with the second tire/tire assembly  206  before the first brake assembly  60  contacts the first tire/tire assembly  204 , such that the first brake assembly  60  undergoes a larger rotational displacement to become seated with the tread(s) of the first tire/tire assembly  204 . The linkage assembly  40  pivots about the cross member pivot  56  in a second direction (counterclockwise in  FIGS. 9A and 9B ) to accommodate the differential rotation between the first and second brake assemblies  60  and  70 . 
     Referring to  FIG. 10  and again to  FIGS. 5 through 7 , the operation of the spring module  170  and the latch member  190  are described. When tow bar  32  is down ( FIG. 5 ), the yoke assembly  160  pushes the housing  172  of the spring module  170  against the stop  189  to rotate and/or maintain the brake bar  66  and  76  away from the treads of the tires/tire sets  204  and  206  in a disengaged position. In the  FIG. 5  configuration, the biasing element  188  resides passively in the housing  172 , without compression. 
     As the tow bar  32  is further rotated upward, the housing  172  of the spring module  170  is further translated in the forward direction  102  by the yoke assembly  160 . As depicted in  FIG. 6 , the biasing element  188  in the housing  172 , being pushed along by the reducing flange  182  of the spring module  170 , is thereby pushed against the head member  186  affixed to the distal end  36  of the tie rod  34  to further translate the tie rod  34  in the forward direction  102 . The translation of the tie rod  34  causes the further rotational displacements  214  and  216  of the brake assemblies  60  and  70  to pull the brake bars  66  and  76  toward the tire/tire sets  204  and  206 . The rotational displacements  214  and  216  continue until at least one of the brake bars  66  and/or  76  register against the tire/tire sets  204  and/or  206  ( FIG. 6 ). During the rotational displacements  214  and  216 , the force exerted on the bias element  188  is only enough force to overcome the friction and other imperfections of the tow bar-activated brake system  30 . 
     After engagement of a single brake bar  66  or  76  (brake bar  66  in  FIG. 6 ) the other of the brake assemblies  70  or  60  (second brake assembly  70  in  FIG. 7 ) continues to rotate until the other of the brake bars  76  or  66  engages the other of the tire/tire sets  206  or  204  ( FIG. 7 ). During the rotational displacement  216  of  FIG. 7 , the force exerted on the bias element  188  is only enough force to overcome the friction and other imperfections of the tow bar-activated brake system  30 , including the friction associated with differential rotation of the differential rotation joint  80 . 
     When both brake bars  66  and  76  are set against the treads of the tire/tire sets  204  and  206 , the tow bar  32  may not yet be in the parking configuration and the latch member  190  may still be disengaged from the frame  166  ( FIG. 7 ). In this configuration, the tie rod  34  is essentially immobile. Continued raising of the tow bar causes yoke assembly  160  to continue translating the housing  172  of the spring module  170  in the forward direction  102 , so that the aperture  184  of the housing  172  slides over the now immobile tie rod  34 . As the housing  172  slides forward, the reducing flange  182  of the spring module  170  compresses the biasing element  188  against the head member  186  of the now immobile tie rod  34 . The compression of the bias element  188  continues as the tow bar  32  is further rotated into the parking configuration. 
     The parking configuration is fully achieved when the hook  192  of the latch member  190  engages the frame  166  ( FIG. 10 ). When in the parking configuration, the compressed biasing element  188  exerts a rearward force on the housing  172 , which acts to bias the tow bar  32  toward rotation in the downward position, but is prevented from going into the downward position by the engaged latch member  190 . 
     The latch  190  can be released by forcing the tow bar  32  slightly past the parking configuration, which disengages the hook  192  from the frame  166 . The latch  190  can then be rotate upward so that the hook  192  slides over the frame  166 . The tow bar  132  is then free to rotate downward into the towing configuration. When the tow bar  32  is lowered, the off center rotation of the yoke assembly  160  causes the housing  172  of the spring module  170  to exert a rearward force against the stop  189 , thereby causing the brake assemblies  60  and  70  to disengage from the tires/tire sets  204  and  206  and to maintain the disengagement. 
     Functionally, spring module  170  serves three purposes. First, the spring module  170  enables the length of the stroke of the tie rod  34  in the forward direction  102  to vary, which may happen over time as the tires/tire sets  204 ,  206  wear, or as different tires/tire sets  204 ,  206  are substituted into the system. Second, the biasing force exerted by the compression of the biasing element  188  positively sets the latch member  190  against the frame  166  for a more secure engagement. Third, when the tow bar  32  is in the towing or downward configuration, the spring module  170  cooperates with the stop  189  to maintain the brake assemblies  60  and  70  in disengagement from the tires/tire sets  204  and  206 . 
     Each of the additional figures and methods disclosed herein can be used separately, or in conjunction with other features and methods, to provide improved devices and methods for making and using the same. Therefore, combinations of features and methods disclosed herein may not be necessary to practice the disclosure in its broadest sense and are instead disclosed merely to particularly describe representative and preferred embodiments. 
     Various modifications to the embodiments may be apparent to one of skill in the art upon reading this disclosure. For example, persons of ordinary skill in the relevant art will recognize that the various features described for the different embodiments can be suitably combined, un-combined, and re-combined with other features, alone, or in different combinations. Likewise, the various features described above should all be regarded as example embodiments, rather than limitations to the scope or spirit of the disclosure. 
     Persons of ordinary skill in the relevant arts will recognize that various embodiments can comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the claims can comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     References to “embodiment(s)”, “disclosure”, “present disclosure”, “embodiment(s) of the disclosure”, “disclosed embodiment(s)”, and the like contained herein refer to the specification (text, including the claims, and figures) of this patent application that are not admitted prior art. 
     For purposes of interpreting the claims, it is expressly intended that the provisions of 35 U.S.C. 112(f) are not to be invoked unless the specific terms “means for” or “step for” are recited in the respective claim.