Abstract:
This invention relates to personal transportation devices and more specifically to folding bicycles. The invention consists of a folding frame comprised of a main tube with tuned cross section, a folding pivot axis perpendicular to the vehicle axis, and a locking mechanism further comprising a seat post rigidly retained by two clamping sections of a seat tube. The invention also consists of a rear wheel retention element—a “dropout”—with horizontal axle slots and a replaceable derailleur hanger.

Description:
BACKGROUND 
   This invention relates to personal transportation devices and more specifically to folding bicycles. 
   Many attempts have been made to create folding bicycles and many products are available commercially on the market. A key problem with existing devices is that frame structures which accommodate folding typically are less rigid in the deployed configuration than those of a conventional bicycle. As a result, folding bicycles typically feel more flexible and less stable than conventional bicycles when ridden vigorously. 
   A second problem with folding bicycles is that they are often equipped with idiosyncratic transmission components, such as unusually small rear sprockets or special-purpose hubs. These components present barriers to servicing the bicycle and to adaptation of a folding bicycle frame to different applications. 
   It is the object of this invention to provide a bicycle whose frame delivers the essential performance characteristics of a full-sized bicycle frame, and yet is foldable. A further object of the invention is to provide a single frame that can be configured with a wide variety of different standard transmission elements. 
   SUMMARY 
   The invention consists of a folding frame comprised of a main tube with tuned cross section, a folding pivot axis perpendicular to the vehicle axis, and a locking mechanism further comprising a seat post rigidly retained by two clamping sections of a seat tube. The invention also consists of a rear wheel retention element—a “dropout”—with horizontal axle slots and a replaceable derailleur hanger. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings, where: 
       FIG. 1  is an overall perspective view of an embodiment of the invention in the deployed configuration. 
       FIG. 2  is a perspective view of an embodiment of the invention shown in the folded configuration. 
       FIG. 3  is a cross sectional view of the main tube of an embodiment of the invention. 
       FIG. 4  is a cross sectional view of the pivot region of the main tube of an embodiment of the invention. 
       FIG. 5  is perspective view of the rear dropouts of an embodiment of the invention shown with a derailleur hanger. 
       FIG. 6  is a perspective view, from the inside of the frame, of the rear dropouts of an embodiment of the invention shown with a derailleur hanger. 
       FIG. 7  is a perspective view of the rear dropouts of an embodiment of the invention shown without a derailleur hanger. 
   

   DETAILED DESCRIPTION 
   A preferred embodiment of the invention is shown in  FIG. 1 . This embodiment is shown in the deployed position. A frame  10  connects a front wheel  11  and a rear wheel  12 . For clarity, the bicycle is shown without seat, handlebars, pedals, and several other components not unique to the invention. For the purposes of this description and associated claims, we adopt the following definitions as indicated in  FIG. 1 . The road plane is a horizontal plane on which the bicycle is ridden. The vehicle plane is a vertical plane, perpendicular to the road plane and perpendicular to the axes of rotation of the wheels of the bicycle. The fore direction is defined as the direction towards the front wheel  11 . The rear direction is defined as the direction towards the rear wheel  12 . The starboard side of the vehicle is to the right when facing in the fore direction. The port side of the vehicle is to the left when facing in the fore direction. Up and the upward direction are normal to the road plane as shown in  FIG. 1 . Down and the downward direction are defined as the direction normal to the road plane as shown in  FIG. 1 . A pivot axis is perpendicular to the vehicle plane and intersects the frame as shown. 
     FIG. 2  shows a preferred embodiment of the invention in the folded position. A main tube  21  connects a steering pivot  29  and an upper seat tube  24 . An upper seat tube clamp  26  acts on the upper seat tube  24  to releasably retain seat post  23 . A rear frame structure  22  pivots about the pivot axis intersecting the main tube  21  at pivot  28 . The rear frame structure comprises a lower seat tube  25 , which further comprises a lower seat tube clamp  27 . Seat post  23  mates axially with the upper seat tube  24 , and with both the upper seat tube  24  and lower seat tube  25  when the bicycle is in the deployed position. When in the deployed position, and when both upper seat tube clamp  26  and lower seat tube clamp  27  are in the clamped position, seat post  23  is rigidly retained relative to upper seat tube  24  and lower seat tube  25 . The stiffness of the seat post  23  and the clamping forces retain the deployed frame in a highly rigid configuration, forming a truss-like structure comprising the main tube  21 , seat post  23 , and rear frame structure  22 . 
     FIG. 3  is a cross sectional view of main tube  21 . The tube is hollow with a wall thickness that varies around the perimeter of the section. The tube is axially symmetric and can be manufactured by extrusion. In a preferred embodiment of the invention the tube is extruded from 6000-series aluminum alloy, which is readily extruded and provides for reasonably high strength at modest cost. In order to provide the main tube with high stiffness, relative to bending in the vehicle plane, without excessive weight, the wall thickness of the main tube is substantially thicker in the upward and downward regions  30  than in the port and starboard regions  31 . In a preferred embodiment, the tube is 6061 T6 aluminum and the moment of inertia for bending in the vehicle plane is at least 200,000 mm 4 , resulting from an overall section depth of approximately 70 mm, a section width of approximately 30 mm, a wall thickness in regions  31  of approximately 2.0 mm and a wall thickness in regions  30  of approximately 6.5 mm. 
     FIG. 4  shows the pivot  28  in cross section. Main tube  21  includes sleeve  44 . In a preferred embodiment, sleeve  44  is welded to main tube  21 . Seat stay  40  and seat stay  41  are attached to pivot bosses  42  and  43 . A shoulder bolt  45  and nut  46  connect pivot bosses  42  and  43  to sleeve  44 , and shoulder bolt  45  acts as the pivot pin for pivot  28 . In a preferred embodiment pivot bosses  42  and  43  are welded to seat stays  40  and  41  respectively. The use of shoulder bolt  45  and nut  46  allows the pivot  28  to be disassembled and thus allows the separation of main tube  21  from rear frame structure  22 . Such disassembly facilitates the transport of the frame in a suitcase or other small package. 
     FIG. 5  shows the starboard rear dropout  50  of the invention. The dropout includes a slot  53  for retaining a rear-wheel axle. The horizontal orientation of slot  53  allows a rear wheel to be adjusted in the forward or rearward position to properly tension the drive chain in a single-gear configuration or in an internal hub configuration. The use of a horizontal dropout thus makes the invention adaptable to a wide variety of transmission options. 
   Hole  57 , which in a preferred embodiment is threaded for an M5 fastener, allows for a fastener to be attached to dropout  50 . The head of a fastener, so positioned, prevents a rear wheel from sliding out the rear of the dropout by interfering with the axle nut, quick release lever, or axle skewer nut of the rear wheel, thus preventing rearward disengagement of the rear axle. 
   Derailleur hanger  54  is attached to dropout  50  by means of a fastener inserted through hole  55 . The mating surfaces  56  of the derailleur hanger  54  and the dropout  50  in combination with a fastener through hole  55  prevent the relative motion of derailleur hanger  54  and dropout  50 . This element of the invention allows the derailleur hanger  54  to be replaced if damaged, or to be removed altogether in the event that a transmission without derailleur is used on the bicycle. The position of hole  55  and mating surfaces  56  allow the derailleur hanger to be positioned correctly for proper gear shifting, while not interfering with the horizontal motion of a rear wheel axle in the slot  53 . 
     FIG. 6  shows dropout  50  from the perspective of the port side of the bicycle. A second pair of mating surfaces  67  contribute to rigidly retaining the dropout  50  and the derailleur hanger  54 . Port dropout  60  is shown with port axle slot  63 . 
     FIG. 7  shows dropout  50 , with no derailleur hanger attached, from the perspective of the port side of the bicycle. Pocket  70  is positioned on the inside of dropout  50  and is largely invisible from the outside of the bicycle when no derailleur hanger is present. 
   The combination of horizontal slot  53  and derailleur hanger  54  allow the invention to accommodate essentially any common bicycle transmission, whether derailleur, internal hub, fixed gear, or single-speed. 
   The foregoing discussion should be understood as illustrative and should not be considered to be limiting in any sense. While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the claims.