Abstract:
A junction for the head tube of a bicycle frame with top tube and down tube members is fabricated in such a way that the top tube and down tube members are jointed. The method of mitering the top tube and down tube members to form the requested joint is cutting off a portion of walls of both the top tube and down tube members at the head ends to form a miter joint. The method of mitering provides for optimum contact area and ease of manufacturing, especially for tubes with non-traditional cross-sectional shapes.

Description:
This application claims benefit of provisional No. 60/133,796 filed May 11, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a bicycle frame, particularly to a method of constructing a head tube junction of a bicycle frame which has a top tube and down tube jointed together at the head tube junction end. 
     BACKGROUND OF THE INVENTION 
     Bicycles with frames fabricated from aluminium tubing have become increasingly popular. Unlike steel, aluminium cannot be brazed, so that joints between the tubes of most aluminium bicycle frames have to be welded. The joints between the tubes of the frame usually involve welding of each wall of one tube, ordinarily an edge at the end of the tube, to the surface of the wall of another tube. A critical joint in the manufacture of modern bicycle frames is the joint between the head tube, the top tube, and the down tube. The fork acts as a long lever arm and can exert significant amounts of stress on the head tube junction. The arrival of suspension bikes in the market place with stiff long-travel suspension forks have made the design of this junction even more critical. 
     Top tubes and down tubes have been getting bigger to achieve greater strength. At the same time, head tubes have been getting shorter to allow for the increased length of front suspension forks. The result is that many frames have top tubes and down tubes that overlap. In the prior art, the top or down tube is mitered to the head tube using hole saws and special fixtures to set up a drill press to position the tube for forming the edge cut. For example, the down tube has a certain diameter and forms a certain angle with the head tube. To make the end cut in the down tube for the welded joint with the head tube, the hole saw having a diameter equal to the diameter of the head tube is installed in a drill press, and the down tube is held in a fixture so that the axis of the hole saw is at the same angle to the down tube as the head tube is in the completed frame. The top tube is mitered to the head tube in the same manner. 
     The bicycle frames with overlapping down tubes and top tubes are traditionally constructed such that one of the top tube and down tube is mitered to the head tube only, and the other of the top tube and down tube is mitered to both the head tube and the one tube. This method of manufacture is limited to top tubes and down tubes with simple cross-sectional shapes. For example, when both the top tubes and down tubes are cylindrical, a portion of the wall of the top tube can be cut by a hole saw having a diameter equal to the diameter of the down tube at the same angle to the top tube as the down tube is in the completed frame. 
     However, the traditional method of mitering one of the top and down tubes to form the requested overlapping is impractical for tubes with non-round cross-sections. The welder is either left with a large gap to fill or else the miter itself is extremely complicated. As an example, U.S. Pat. No. 5,249,818, issued to Patterson on Oct. 5, 1993, describes a method of making a bicycle frame having the top and the down tubes overlapping at the head tube junction ends. A notch in the wall of the down tube is shaped to conform to and abut a portion of the surface of the wall of the top tube, and in the finished joint the weld is formed around the juncture between the notch and the wall portion of the tube that the notch abuts. The notch and other cut edges of the tubes are made by a cutting beam such as a laser beam. Relative movement is provided between the beam and the tube being cut such that the beam transverses a predetermined path over the wall of the tube and makes the desired cut. The relative movement includes a rotation of the tube about its axis and a simultaneous movement of the beam axially relative to the tube. This type of cutting process needs either expensive equipment if the cutting process is automatically completed or skillful operators if the cutting process is done manually. 
     Therefore, there is a need for a method of constructing a head tube conjunction of a bicycle frame which provides for optimum contact area and ease of manufacturing, especially for tubes with non-traditional cross-sectional shapes. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of constructing a head tube junction of a bicycle frame which provides optimum contact area and ease of manufacturing, especially with tubes of non-traditional cross-sectional shapes. 
     Another object of the present invention is to provide a bicycle frame having a head tube junction with increased strength. 
     In accordance with one aspect of the present invention, there is a bicycle frame provided, comprising at least a head tube, a top tube, a down tube and a head tube junction formed therebetween. The top tube has one end cut at a first predetermined angle with respect to a longitudinal axis of the top tube. The end cut has an edge conforming to and abutting a portion of an external periphery of a wall of the head tube, whereby the top tube is mitered to the head tube at the first predetermined angle. Similarly, the down tube has one end cut at a second predetermined angle with respect to a longitudinal axis of the down tube. The cut end has an edge conforming to and abutting a portion of the external periphery of the wall of the head tube; thereby the down tube is mitered to the head tube at the second predetermined angle. An abutting surface is formed by cutting a portion of the wall of the top tube at the cut end thereof, and another abutting surface is formed by cutting a portion of the wall of the down tube at the cutting end thereof so that the abutting surfaces of the top and down tubes mate to form the head tube junction. 
     The head tube is preferably cylindrical, and each of the top and down tubes preferably tapers from a round cross-sectional shape to a rectangular cross-sectional shape at the end which forms the junction. 
     In accordance with another aspect of the present invention, a method is provided for constructing a head tube junction of a bicycle frame including at least one head tube, a top tube, and a down tube. The method comprises steps of cutting one end of the top and down tubes. The one end of the top tube is cut at a first predetermined angle with respect to a longitudinal axis of the top tube so that the top tube is enabled to be mitered to the head tube at the first predetermined angle. The one end of the down tube is cut at a second predetermined angle with respect to the longitudinal axis of the down tube so that the down tube is enabled to be mitered to the head tube at the second predetermined angle. The method comprises further steps of cutting a portion of the wall of the top tube at the cut end thereof to form an abutting surface, and cutting a portion of the wall of the down tube at the cut end thereof to form a mating abutting surface. Finally, the head tube junction is completed by welding at interfaces between the wall of the head tube and the respective cut ends of the top and down tubes, and between the abutting surfaces of the top and down tubes. 
     The portion of the wall of the top tube and down tube is preferably cut along a straight line at an angle with respect to the longitudinal axis of the respective top and down tubes. 
     By mitering the top tube and the down tube to each other, a number of benefits can be realized. The bicycle frame constructed in this manner has an increased strength because the jointed top tube and down tube sections act as a built-in gusset to greatly stiffen the head tube junction. The increased strength also results from the perfect contact between the top tube and the down tube through the entire length of the weld. The miter is greatly simplified and can in one form be accomplished with a straight cut and, therefore, the fit up for manufacturing is simpler than those used in the prior art. The weld between the top tube and down tube is at a preferred orientation, and the resulting joint is more aesthetically pleasing. Other advantages of the invention will be better understood with the description of a preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: 
     FIG. 1 is a side elevational view of a bicycle frame incorporating the present invention; 
     FIG. 2 is a perspective exploded view of the head tube junction showing the miters between the head tube, the top tube, and the down tube; and 
     FIG. 3 is a side cross-sectional view of the head tube junction, taken through the centre plane of the bicycle frame in FIG. 1, showing the manner of assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The frame shown in FIG. 1, generally indicated by numeral  10 , is a typical bicycle frame which is one embodiment incorporating the present invention. The joint of interest is the head tube junction  12  formed between a top tube  14 , a down tube  16 , and a head tube  18 . In this particular embodiment of the invention, the top tube  14  and the down tube  16  include sections  15 ,  17  of a round cross-sectional shape connected at the end  20 ,  22  to a seat tube  24  respectively. The top tube  14  and the down tube  16  further include sections  19 ,  21  of a rectangular cross-sectional shape which taper from the round cross-sectional shape of the sections  15 ,  17  to the ends  26 ,  28  connected to the head tube  18  respectively. In addition, the edges  30  of the rectangular sections  19 ,  21  of the top tube  14  and the down tube  16  are rounded, and the surfaces  32  of the sections  19 ,  21  of the tubes are not perfectly flat, as shown in FIG.  2 . Although the invention is not limited to such tubes, the method described is particularly applicable to these types of non-standard tapering tubes because of their varying cross-sectional shape. 
     As more clearly shown in FIG. 2, the end  26  of the top tube  14  has an edge  34  formed at a first predetermined angle with respect to a centre longitudinal axis of the top tube  14 . The edge  34  conforms to and abuts a portion of the external periphery of the head tube  18  when the top tube  14  is mitered to the head tube  18  at the first predetermined angle. Similarly, the end  28  of the down tube  16  has an edge  36  which is formed at a second predetermined angle with respect to the central longitudinal axis of the down tube  16 . The edge  36  is provided to conform to and abut a portion of the external periphery of the head tube  18  when the down tube is mitered to the head tube  18  at the second predetermined angle. Even though the top tube  14  and down tube  16  have a non-standard rectangular shape, the edges  34  and  36  are easily accomplished through the use of a hole saw or a similar cutting procedure because the head tube  18  has a standard cylindrical shape. 
     The top tube  14  and the down tube  16  are mitered to the head tube  18  at an axial position on the head tube  18  so that the top tube  14  and the down tube  16  are also mitered to one another at the respective ends  26 ,  28 . The traditional method of accomplishing the required connection is by overlapping of the top tube  14  and down tube  16 . The overlap requires that a cut edge of one of the top and down tubes  14 ,  16  matches the surface of the other of the top and down tubes  14 ,  16 . However, because of the rounded edges  30 , non-flat surfaces  32 , and tapered nature of the rectangular sections  19 ,  21  of the top and down tubes in this embodiment, such miter would be too complicated and cannot be achieved with traditional methods. 
     Therefore, the preferred method of mitering for situations similar to the one demonstrated by this particular embodiment is described below. Through this method, the mitering of the top and down tubes  14 ,  16  through the overlap is otherwise accomplished with a simple miter that does not require any special equipment, and the resulting joint provides advantages in terms of a better fit up, strength, and aesthetic appearance. 
     A portion of the wall of the top tube  14  at the end  26  is cut in a plane along straight line  38  intersecting the top tube  14  to form flat abutting surfaces  40  at both intersected wall segments. Similarly, a portion of the wall of the down tube  16  at the end  28  is cut in a plane along a straight line  42  to form flat abutting surfaces  44  which mate the abutting surfaces  40  when the top tube  14  and the down tube  16  are mitered to one another. This cutting can be very simply accomplished through the use of a cut-off saw or the like. The angle and location of the planes along lines  38 ,  42  can be chosen in a variety of ways. In the preferred embodiment, for best results to be accomplished, the height of the plane relative to the head tube junction  12  and the angle of the lines  38 ,  42  with respect to the respective central longitudinal axis of the top and down tubes  14 ,  16 , are carefully chosen so that roughly equal amounts of the wall of the top tube  14  and the down tube  16  are cut off and a maximum contact area is achieved between the abutting surfaces  40 ,  44  of the top and down tubes  14 ,  16 . In this embodiment, the angle and location were determined with the aid of a Computer Aided Design package to model the joint. Choosing the location and angle of the plane results in an abutting surface  40  roughly identical to the abutting surface  44 . In this embodiment, for best results to be accomplished, the angle of the lines  38 ,  42  with respect to the respective central longitudinal axis of the top and down tubes  14 ,  16  is chosen so that roughly equal amounts of the wall of the top tube  14  and the down tube  16  are cut off. As a result, a maximum contact area is achieved between the abutting surfaces  40 ,  44  of the top and down tubes  14 ,  16  because the abutting surface  40  is roughly identical to the abutting surface  44 . 
     It is noted that the cut edges  34 ,  36  at the respective ends  26 ,  28  are symmetric about the centre plane of the frame  10 , and the abutting surface  40  and the abutting surface  44  are respectively formed in a plane orthogonal to the central plane of the frame  10  to ensure that the frame  10  is accurately aligned. 
     FIG. 3 shows a cross-section of the head tube junction  12  assembled together. When the top tube  14 , down tube  16 , and head tube  18  are prepared ready as shown in FIG. 2, the top and down tubes  14 ,  16  and the head tube  18  are placed together in such a manner that the respective top and down tubes  14 ,  16  are mitered to the head tube  18  at the first and second predetermined angles at an axial position on the head tube  18 , in which position the abutting surfaces  40 ,  44  of the top and down tubes  14 ,  16  mate with each other. Then the head tube junction  12  is welded at interfaces between the wall of the head tubing  18  and the respective cut edges  34 ,  36  of the top and down tubes  14 ,  16 . The mating abutting surfaces  40  and  44  are also welded together. 
     The rectangular sections  19 ,  21  of the top and down tubes  14 ,  16  at the head tube  18  result in significant strength advantages. Welds between two round tubes are problematic because cracks can initiate at stress points located at the crown and saddle points of the weld. The present embodiment of the invention using the rectangular sections  19 ,  21  of the top tube  14  and down tube  16  eliminates the stress risers in addition to allowing for significantly higher weld length between the top tube  14 , the down tube  16 , and the head tube  18 . The stress is then distributed more evenly over this greater length of a weld, resulting in a stronger structure. The frame described is significantly stronger than a frame with round non-overlapping tubes. 
     In addition to a greater weld length between the top tube  14  and the head tube  18 , the down tube  16  and the head tube  18 , the mating abutting surfaces  40 ,  44  between the top tube  14  and down tube  16  also provides additional support for the head tube  18 . In fact, the structure formed by the top and down tubes mitering together acts as a built-in gusset for the head tube  18 . Because of the greater strength of the joint, the top tube  14  and the down tube  16  do not have to be as thick as they would for round tubes, which results in weight saving. In this embodiment, the wall thickness of the down tube  16  at the end  28  is 2.4 mm. The wall thickness of the top tube  14  at the end  26  is 1.9 mm. The head tube  18  is 4.0 mm thick. A similar embodiment with non-overlapping circular tubes having the same wall thickness would be significantly weaker. 
     For this particular embodiment, the top tube and down tube have a rectangular cross-sectional shape at the head tube. However, the invention is equally applicable to tubes of almost any constant or varying cross-sectional shape, such as tubes with aerodynamic shapes, oval tubes, and traditional round tubing, as long as the abutting surface of the top tube can mate up with the abutting surface of the down tube. The method applies equally well to aluminium tubing, steel tubing, or tubing made from any other material. 
     For this particular embodiment, the abutting surfaces  40 ,  44  of the top and down tubes  14 ,  16  are made from a simple straight cut. A straight cut is the easiest for manufacturing. However, the abutting surfaces of the top and down tubes are not necessarily restricted to being a straight cut. The abutting surfaces could also be a round cut, or almost any other shape, as long as abutting surface  40  of the top tube  14  matches the abutting surface  44  of the down tube  16 . 
     The design can equally be applied to all types of bicycles, including, but not limited to, hardtail mountain bikes, full-suspension bikes, BMX bikes, and road bikes.