Title of Invention

A CAST BOTTOM BRIDGE FOR USE IN A STEERING ASSEMBLY

Abstract

A cast bottom bridge for use in a steering assem¬bly has a central region having a central hole defined therein for receiving a stem pipe therein and a pair of arms extending away from each other from the central region, the arms having respective holes defined therein for receiving front fork members, respectively. Each of the arms has a cavity defined in a lower surface thereof between the cen¬tral hole and one of the holes by front, rear, and upper walls of the arm, the upper wall being thinner than the front and rear walls. Specifically, the upper wall has a thickness in a range of about 40 to 80 % of the thickness of the front and rear walls. PRICE: THIRTY RUPEES

Full Text

9 Field of the Invention: The present invention relates to a bottom bridge of cast iron for use in a steering assembly for two- or three-wheeled vehicles such as motorcycles, motor scooters, or the like. Description of the Related Art: Steering assemblies for two-wheeled vehicles such as motorcycles, motor scooters, or the like have, as one component, a bottom bridge connected centrally to the lower end of a stem pipe and having a pair of spaced holes defined one on each side of the center thereof and receiving respec¬tive front fork members therein. The bottom bridge is re¬quired to be sufficiently strong aqainnt torsional loads or stresses which are transmitted through the front fork mem¬bers while the two-wheeled vehicle is cornering. For this reason, it has been customary practice to manufacture bottom bridges according to the forging process* which is advanta¬geous for providing desired mechanical strength. The bottom bridge also has a pair of arms extend¬ing away from each other from the central region thereof which is connected to the stem pipe. The arms have respec¬tive cavities of a predetermined width which are defined in their lower surfaces by front, rear, and upper walls and having a downwardly open channel-shaped cross section* While the forged bottom bridge has a sufficient mechanical strength, the forging process is relatively com¬plex and time consuming, and it is difficult to machine the forged bottom bridge. Another disadvantage is that the forged bottom bridge is of an increased overall weight. One solution is to employ a simpler casting proc¬ess to manufacture bottom bridges. Although cast bottom bridges are lighter in weight, they have smaller fracture angle of torsion and torsional rigidity, and fail to give the rider of the two-wheeled vehicle a feel which is equiva¬lent to that which is provided by the forged bottom bridges while the rider is driving and steering the two-wheeled ve¬hicle. FIG. 1 of the accompanying drawings shows the relationship between fracture angles of torsion and tor¬sional rigidities of a forged bottom bridge made of steel equivalent to steel S35C and cast bottom bridges made of cast iron equivalent to iron FCD450, which were measured when the bottom bridges were subjected to a torsional torque applied about one of the arms thereof while the central re¬gion was being fixed. The characteristic curve FO repre¬sents a conventional forged bottom bridge having cavities defined in their respective arms by front, rear, and upper walls each having a thickness of about 8 mm. The character¬istic curve Dl represents a cast bottom bridge having cavi- ties defined in their respective arms by front, rear, and upper walls each having a thickness of about 8 mm. The characteristic curve D2 represents a cast bottom bridqe hav-ing cavities defined in their respective arms by front, rear, and upper walls each having a thickness of about 10 mm. The characteristic curve D3 represents a cast bottom bridge according to the present invention, which will be described later on. While motorcycles are running in normal situa¬tions, the bottom bridges (one of the arms) are subject to a torsional torque in the range of up to 50 kg«m. The meas¬ured data shown in FIG. 1 indicate that in such a torsional torque range, the cast bottom bridge represented by the characteristic curve Dl has a greater fracture angle of tor¬sion, and hence gives the rider a much softer feel, than the forged bottom bridge represented by the characteristic curve F0. The cast bottom bridge represented by the characteris¬tic curve D2 has a smaller fracture angle of torsion, and hence gives the rider a much harder feel, than the forged bottom bridge represented by the characteristic curve F0. The fracture angle of torsion is 5b derjreoB and the tor¬sional strength was 300 kg-m with respoct to the forged bot¬tom bridge represented by the characteristic curve F0. SUMMARY OF THE IHVEN'J'tON It is therefore an object ol the present inven¬tion to provide a bottom bridge for use in a steering assem¬bly for two-wheeled vehicles such as motorcycles, motor ! scooters, or the like, which can easily be manufactured, has a reduced weight and a required level of mechanical strength, and can give a feel in steering operation equivalent to that which is provided by forged bottom bridges. Accordingly, the present invention provides a cast bottom bridge for use in a steering assembly having a stem pipe and a pair of front fork members, comprising a central region having a central hole defined therein for receiving the stem pipe therein; a pair of arms extending away from each other from said central region, said arms having respective holes defined therein for receiving the front fork members, respectively; each of said arms having a cavity defined in a lower surface thereof between the central hole and one of said holes by front, rear and upper walls of the arm, the upper wall being thinner than the front and rear walls, wherein the upper wall has a thickness in a range of about 40 to 80% of the thickness of the front and rear walls. The central region and said arms may be made of ductile cast iron equivalent to FCD380, FCD450, or FCD550. The cast bottom bridge may further comprise a stem pipe of soft steel having an end inserted through said central hole and having a corner projecting from the bottom bridge and joined thereto by a ductile weld bead formed by arc welding. The corner may be joined to said bottom bridge by the ductile weld bead which is formed by arc brazing using a copper-based welding wire having a lower welding point than the bottom bridge. Alternatively, the corner may be joined to said bottom bridge by the ductile weld bead which is ( formed by arc welding using a nickel-based welding wire. The above and further objects, details and ad¬vantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship be¬tween fracture angles of torsion and torsional rigidities of conventional bottom bridges and a bottom bridge according to the present invention; FIG. 2 is a fragmentary perspective view of a steering assembly for a motorcycle, which incorporates a cast bottom bridge according to an embodiment of the present 1 invention; FIG. 3 is a plan view of the cast bottom bridge shown in FIG. 2; FIG. 4 is a side elevational view, partly in cross section, of the cast bottom bridge shown in FIG. 2; FIG. 5 is a cross-sectional view taken along line V - V of FIG. 3; FIG. 6 is a cross-sectional view similar to FIG. 5, showing a conventional bottom bridge; FIG. 7 is a graph showing how the torsional ri¬gidity of a bottom bridge of ductile cast iron corresponding to FCD450 varies as the wall thickness of the bottom bridge varies; FIG. 8 is a graph showing how the fracture angle of torsion of the bottom bridge of ductile cast iron corre¬sponding to FCD450 varies as the wall thickness of the bot¬tom bridge varies; and FIG. 9 is a cross-sectional view of a cast bottom bridge according to another embodiment of the present inven¬tion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 2, a two-wheeled vehicle such as a motorcycle, a motor scooter, or the like has a stem pipe 3 rotatably inserted in a head pipe 2 on the front end of a vehicle frame 1 and having a lower end which is fitted in a central hole 5 defined in a bottom bridge 4 and firmly joined to the bottom bridge 4. The bottom bridge 4 has a pair of arms 6 (see also FIG. 3) extending As shown in FIGS. 4 and 5, the arms 6 have re¬spective cavities 8 of a predetermined Width which.are de¬fined in their lower surfaces by front, rear, and upper walls 8a, 8b, 8c and having a downwardly open channel-shaped cross section. The cavities 8 are positioned between the central hole 5 and the holes 7. FIG. 6 shows a conventional forged bottom bridge made of steel such as steel S35C. As shown in FIG. 6, each of the arms of the conventional forged bottom bridge has a cavity 51 of a predetermined width which are defined in the lower surface of the arm by front, rear, and upper walls 51a, 51b, 51c and having a downwardly open channel-shaped cross section. The front, rear, and upper walls 51a, 51b, 51c have an equal thickness of about 8 mm, for example. The conventional forged bottom bridge has a weight of about 1602 g. According to the present invention, the bottom bridge 4 is made of cast iron and each of the cavities 8 has a cross-sectional shape different, from thai; of the conven-r tional forged bottom bridge in order to reduce the weight of the bottom bridge, increase the fracture angle of torsion (the angle of torsion at which the arm 6 is cracked) and the torsional rigidity (the torsional load at which the arm 6 is cracked) when a torsional torque is applied about the axis P (see FIG. 3) of the arm 6, and give tho rider of the two-wheeled vehicle a feel which is equivalent to that which is provided by the forged bottom bridges while the rider is driving and steering the two-wheeled vehicle. The bottom bridge 4 is made of cast iron is nilatlvely light in weight and can subsequently be machined with ease. FIG. 7 shows the torsional i/Ujidities of bottom bridge test pieces made of ductile east iron equivalent to cast iron FCD450 at different ratios of the upper wall 8c to the front and rear walls 8a, 8b when a torsional load was applied around the axis p of the arm at a torsional rate of 30 degrees/min. In FIG. 7, the vertical axis represents the torsional rigidity, and the horizontal axis represent^ the ratio (B/A) of the thickness of the upper wall 8c (B) to the thickness of the front and rear walls 8a, 8b (A). FIG. 8 shows the fracture angles of torsion of the same bottom bridge test pieces at different ratios of the upper wall 8c to the front and rear walls 8a, 8b when a torsional load was applied around the axis P of the arm at a torsional rate of 30 degrees/min. In FIG. 8, the vertical axis represents the fracture angle of torsion, and the horizontal axis repre¬sents the ratio (B/A) of the thickness of the upper wall 8c (B) to the thickness of the front and rear walls 8a, 8b (A). The cavities 8 of the bottom bridge test pieces had substan¬tially the same cross-sectional are*a. The torsional load was applied around the axis P of the arm because the same type of torsional load is ap¬plied to bottom bridge 4 through the front fork members when the rider steers the two-wheeled vehicle with the handlebar. It can be seen from the Measured data shown in FIGS. 7 and 8 that the torsional rigidity is of about 190 kg«m or greater and the fracture angle of torsion is of about 170 degrees or greater when the tat to (B/A) of the thickness of the upper wall 8c (B) to the thickness of the front and rear walls 8a, 8b (A) ia In the range of from 40 * to 80 %. Therefore, the mechanical strength of the cast bottom bridge is satisfactory if the ratio (B/A) is in the range of from 40 to 80 %. It was also confirmed that the rider of the two-wheeled vehicle was given a feel at a sat¬isfactory level during vehicle steering operation if the ratio (B/A) is in the range of from 40 to 00 %. 1 It was found out that if the ratio (B/A) was lower than 40 %, then it was difficult, to make bottom bridges by casting, and the torsional strength and the frac¬ture angle of torsion were so low that cracking would start developing in the upper wall 8c, resulting in a failure to achieve the desired level of mechanical strength. It was also found out that if the ratio (B/A) was higher than 80 %, then the torsional strength and the frac¬ture angle of torsion were too low to achieve the desired level of mechanical strength. In this case, cracking starts developing in a lower end of the front wall 8a or a lower end of the rear wall 8b which was expanded when twisted un¬der the torsional torque because maximum tensile stresses act on such a lower end. In view of the above findings, according to the present invention, the thicknesses A of the front and rear walls 8a, 8b are selected to be substantially equal to each other and the thickness B of the Upper wall 8c is selected to be 40 - 80 % of the thicknesses A of the front and rear walls 8a, 8b. The bottom bridge 4 thus constructed has in¬creased fracture angle of torsion and torsional strength at the time torsional torques are applied about the axis P of the arms, and gives the rider of the two-wheeled vehicle an improved feel during vehicle steering operation. The rea¬sons why the fracture angle of torsion and torsional strength are increased by making the thickness B of the up¬per wall 8c smaller than the thicknesses A of the front and rear walls 8a, 8b appear to be that when torsional forces are applied to the bottom bridge 4, the bottom bridge 4 has an increased ability to be elastically deformed under the applied torsional forces and hence can be elastically de¬formed to a greater extent, thus delaying its fracture under the torsional forces. The bottom bridge test pteceo made of ductile cast iron equivalent to cast iron FCP450 wore tested to measure the data shown in FIGS. 7 and 8 It was confirmed that bottom bridge test pieces made of duct.Lie cast iron equivalent to cast iron FCD380 and FCD550 also produced similar test data. f Specific examples of the bottom bridge according to the present invention will be described below. A primary bottom bridge prototype made of ductile cast iron equivalent to cast iron FCD450 and having a weight of 1476 g was produced. The primary bottom bridge prototype ? had front and rear walls of a thickness of 10 mm and an up¬per wall of a thickness of 6.5 mm intermediate between the central hole 5 and the holes 7. A secondary bottom bridge prototype made of duc¬tile cast iron equivalent to cast iron FCD450 and having a weight of 1468 g was produced. The secondary bottom bridge prototype had a front wall of a thickness of 10 mm, a rear walls of a thickness of 9 mm, and an upper wall of a thick¬ness of 5.5 mm intermediate between tho central hole 5 and the holes 7. The secondary bottom bridge prototype is rep¬resented by the characteristic curve D'i In FIG. 1. Each of the primary and secondary bottom bridge prototypes was tested for its fracture angle of torsion, its torsional rigidity, and a feel which it can give during steering operation, by applying a torsional load ground the axis P of the arm at a torsional rate of 30 degrees/min. while the central region of the bottom bridge prototype was being fixed in position. With respect to the primary bottom bridge prototype, the fracture angle of torsion was 18 de¬grees and the torsional strength was 203 kg>m, and wi+h re-spect to the secondary bottom bridge prototype, the fracture angle of torsion was 19 degrees and the torsional strength was 216 kg*m. These values of the fracture angle of torsion and the torsional strength were higher than predetermined levels. It was also confirmed that each of the primary and secondary bottom bridge prototypes could give the rider of the two-wheeled vehicle a feel according to predetermined standards. As can be understood from FIG. 1, the fracture angle of torsion and torsional strength of the secondary bottom bridge prototype represented by D3 are substantially the same as those of the conventional forged bottom bridge and the secondary bottom bridge prototype represented by D3 can give the rider of the two-wheeled vehicle substantially the same feel during steering operation as that which is provided by the conventional forged bottom bridge under nor¬mal two-wheeled vehicle running conditions in which tor¬sional torques are applied up to 50 kg»m. As a consequence, the primary bottom bridge pro¬totype was more preferable than tho secondary bottom bridge prototype with respect to the weight and the mechanical strength. FIG. 9 shows a cast bottom bridge 10 accordinq to another embodiment of the present invention. As shown in FIG. 9, the cast bottom bridge 10, which is made of ductile 1 cast iron, has central hole 11 defined in a central region thereof and a pair of arms 12 extending away from each other from the central region. The arms 12 have respective holes 13 defined therein for receiving respective front fork mem¬bers by which a front wheel of the two-wheeled vehicle is rotatably supported. The central hole 11 receives therein the lower end of a stem pipe 14. The stem pipe 14 is made of soft steel such as steel STAM40G. To join the stem pipe 14 to the bottom bridge 10, the lower end of the stem pipe 14 is press-fitted into the central hole 11 until the tip of the lower end of the stem pipe 14 projects slightly from the lower end of the central hole 11. Then, a flange-shaped weld bead 15 is formed on and around a projecting corner of the lower end of the stem pipe 14 by arc welding. The weld bead 15 is so ductile that it will not be made brittle by carbon entering from the bottom bridge 10 and hence will not develop bead cracking. For example, when the projecting corner of the lower end of the stem pipe 14 t is joined by arc brazing using a copper-based welding wire having a lower welding point than the bottom bridge 10, the ductile weld bead 15 is formed because no carbon is intro¬duced from the bottom bridge 14. Alternatively, when the projecting corner of the lower end of the stem pipe 14 is joined by arc welding using a nickel-based welding wire, the ductile weld bead 15 is formed because nickel is ductile though carbon is introduced from the Jiottom bridge 14 into the weld bead 15, and the ductile weld bead 15 will not suf¬fer bead cracking due to brittlenees. The bottom bridge according to the present inven¬ tion is not limited to use in two- wheelm} vehicles, but may be incorporated in three-wheeled vehicles or the like which have similar steering assemblies. Although there have been doscribed what are at present considered to be the preferred embodiments of the invention, it will be understood that the invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustra¬tive, and not restrictive. The scope of the invention is \ indicated by the appended claims rather than by the forego¬ing description. WE CLAIM : 1. A cast bottom bridge for use in a steering assembly having a stem pipe and a pair of front fork members, comprising a central region having a central hole defined therein for receiving the stem pipe therein; a pair of arms extending away from each other from said central region, said arms having respective holes defined therein for receiving the front fork members, respectively; each of said arms having a cavity defined in a lower surface thereof between the central hole and one of said holes by front, rear and upper walls of the arm, the upper wall being thinner than the front and rear walls, wherein the upper wall has a thickness in a range of about 40 to 80% of the thickness of the front and rear walls. 2. The cast bottom bridge according to claim 1, wherein said central region and said arms are made of ductile cast iron equivalent to FCD380, FCD450orFCD550. 3. The cast bottom bridge according to claim 1, comprises a stem pipe of soft steel having an end inserted through said central hole and having a corner projecting from the bottom bridge and joined thereto by a ductile weld bead formed by arc welding. 4. The cast bottom bridge according to claim 4, wherein said corner is joined to said bottom bridge by the ductile weld bead which is formed by arc brazing using a copper-based welding wire having a lower welding point than the bottom bridge. 5. The cast bottom bridge according to claim 4, wherein said corner is joined to said bottom bridge by the ductile weld bead which is formed by arc welding using a nickel-based welding wire. 6. A cast bottom bridge for use in a steering assembly substantially as herein described with reference to the accompanying drawings.

Documents:

1271-mas-96 abstract.pdf

1271-mas-96 claims.pdf

1271-mas-96 correspondence-others.pdf

1271-mas-96 correspondence-po.pdf

1271-mas-96 description (complete).pdf

1271-mas-96 drawings.pdf

1271-mas-96 form-1.pdf

1271-mas-96 form-26.pdf

1271-mas-96 form-4.pdf

1271-mas-96 petition.pdf