JP5860214B2 - Bicycle crank arm and method of manufacturing the same - Google Patents

Description

  The present invention relates to a bicycle crank arm and a method for manufacturing the same.

  The bicycle drive unit is configured such that left and right crank arms are attached to both ends of a crankshaft rotatably supported by a hanger part of a frame, and a pedal is attached to each tip of each crank arm. . A front gear is attached to the base end of the right crank arm.

  The crank arm includes a boss portion attached to the crankshaft and an arm portion extending in a longitudinal direction from a base end connected to the boss portion. The boss part has a cylindrical fitting part for fitting with the crankshaft. A screw hole for attaching a pedal is formed at the tip of the arm portion.

  There is a demand for weight reduction of bicycles, and there is a strong demand for weight reduction of crank arms for bicycles. In response to such a demand, a configuration in which the arm portion of the bicycle crank arm has a hollow structure to reduce the weight is known (see, for example, Patent Documents 1 and 2).

  In the bicycle crank arm disclosed in Patent Literature 1, a hollow groove is formed along the longitudinal direction of the arm portion, and a lid member is welded to the open surface of the groove to form a hollow arm portion. Is done.

  In the crank arm for a bicycle disclosed in Patent Document 2, a crank arm material corresponding to the arm portion and the boss portion is integrally formed by casting using a core. The core has a shape corresponding to extending along the longitudinal direction from the distal end side (pedal mounting side) to the proximal end side of the arm portion. In the crank arm material after casting, an opening is formed at the tip (pedal side end), and the inside of the crank arm material is occupied by core sand along the longitudinal direction from the opening. Next, a pilot hole for the pedal is formed on the distal end side of the crank arm material, and the crank arm material is formed again by forging, and the opening at the pedal side end portion is crushed. Thereafter, the core sand is taken out from the pedal lower hole. In this manner, a hollow arm portion is formed.

  However, as described in Patent Document 1, in the case where the arm portion having a hollow structure is formed by welding, there is a possibility that the strength may be reduced due to, for example, poor welding. Further, as described in Patent Document 2, when the opening on the pedal side end portion is crushed after forming the crank arm material, the manufacturing process is complicated, leading to a decrease in productivity and an increase in production cost. I was worried about the reliability and durability of the pedal mounting part.

Japanese Patent Laid-Open No. 9-136685 JP-A-10-181665

  The present invention has been conceived under such circumstances, and it is an object of the present invention to provide a bicycle crank arm that is excellent in strength and suitable for increasing production efficiency, and a method for manufacturing the same. To do.

  In order to solve the above problems, the present invention takes the following technical means.

A bicycle crank arm provided by the first aspect of the present invention includes a boss portion having a fitting portion to be fitted to a crankshaft, and an arm portion extending in a longitudinal shape from a base end connected to the boss portion to reach a distal end. The arm part has a hollow part along the longitudinal direction thereof, and the hollow part has an opening on the base end side of the arm part , In addition, the cross-sectional area is equal or gradually decreased from the opening toward the distal end side of the arm portion .

  In a preferred embodiment, the boss portion is displaced inward in the axial direction of the crankshaft with respect to the arm portion, so that the base end of the arm portion is connected to the boss portion from the outer surface of the arm portion. A step wall connected to the outer surface in the axial direction of the boss portion is formed, and the opening of the hollow portion is formed in the step wall.

  In a preferred embodiment, the hollow portion has a bottomed shape closed at an end portion on the distal end side of the arm portion.

  In a preferred embodiment, a lid for closing the opening of the hollow portion is further provided.

  In a preferred embodiment, a plurality of gear mounting portions extending radially from the boss portion in the radial direction of the crankshaft are further provided.

  In a preferred embodiment, the plurality of gear mounting portions are in positions that do not overlap with the direction in which the opening of the hollow portion faces in the axial direction of the crankshaft.

  A bicycle crank arm manufacturing method provided by the second aspect of the present invention is a method for manufacturing a bicycle crank arm according to the first aspect of the present invention, wherein the hollow portion is formed by forging or casting. A first step of integrally forming a crank arm material having a shape corresponding to the arm portion and the boss portion using a nest or a core having a corresponding shape, and the crank arm material obtained by the first step And a second step of forming a hollow portion by removing the insert or the core from the center.

  The bicycle crank arm manufacturing method provided by the third aspect of the present invention is a method of manufacturing the bicycle crank arm according to the first aspect of the present invention, wherein the bicycle crank arm is manufactured by molding using a resin material. A first step of integrally forming a crank arm material having a shape corresponding to the arm portion and the boss portion using a nest having a shape corresponding to the hollow portion, and the crank arm obtained by the first step A second step of forming a hollow portion by extracting the insert from the material.

  The bicycle crank arm manufacturing method provided by the fourth aspect of the present invention is a method of manufacturing the bicycle crank arm according to the first aspect of the present invention, wherein the arm portion is formed by forging or casting. A solid arm portion original shape having a shape corresponding to the outer shape, and a first step of integrally forming a portion corresponding to the boss portion to obtain a crank arm material, and the base portion of the arm portion original shape by machining And a second step of forming a hollow portion that opens.

  The bicycle crank arm manufacturing method provided by the fifth aspect of the present invention is a method for manufacturing the bicycle crank arm according to the first aspect of the present invention, wherein the bicycle crank arm is manufactured by molding using a resin material. A solid arm portion original shape having a shape corresponding to the outer shape of the arm portion, a first step of integrally forming a portion corresponding to the boss portion to obtain a crank arm material, and the arm portion original shape by machining. And a second step of forming a hollow portion whose base end is open.

  Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

It is sectional drawing which shows an example of the gear crank apparatus for bicycles provided with the bicycle crank arm which concerns on this invention. It is a right view of the gear crank apparatus for bicycles shown in FIG. It is a left view of the bicycle gear crank apparatus shown in FIG. It is the IV-IV arrow line view of FIG. It is sectional drawing which follows the VV line of FIG. It is sectional drawing which follows the VI-VI line of FIG. It is sectional drawing which follows the VII-VII line of FIG. It is sectional drawing which shows the one part process of the method of manufacturing the crank arm for bicycles concerning this invention. An example of a crank arm material is shown, (a) is a sectional view and (b) is an IXb-IXb arrow view of (a). It is sectional drawing which shows the one part process of the method of manufacturing the crank arm for bicycles concerning this invention. An example of a crank arm material is shown, (a) is a sectional view, and (b) is a XIb-XIb arrow view of (a).

  Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the drawings.

  FIGS. 1-7 has shown an example of the bicycle gear crank apparatus comprised provided with the crank arm for bicycles concerning this invention. As shown in FIGS. 1 to 3, the bicycle gear crank apparatus A includes a crankshaft 2 that is rotatably supported by a hanger portion 1 of the bicycle, and a crank that is attached to the right end of the crankshaft 2. An arm 3, a crank arm 4 attached to the left end of the crankshaft 2, and a gear 5 are provided. The gear 5 is integrally attached to the crank arm 3, and the crank arm 3 and the gear 5 constitute a gear crank 6. 4 to 7, the crankshaft 2 and the crank arm 4 are omitted.

  The crankshaft 2 has an intermediate portion 21 having a uniform outer diameter, a spline-like large-diameter portion 22 at the left end, and a small-diameter portion 23 at the right end, and has a hollow cylindrical shape. The small-diameter portion 23 at the right end protrudes in a tapered diameter shape toward the right, has an irregular cross-sectional shape having a spline-like outer periphery, and is formed with a screw hole 24 penetrating in the axial direction.

  The crank arm 3 includes a boss portion 31, an arm portion 32 extending longitudinally from the base end connected to the boss portion 31 and reaching the distal end, and a plurality of (this embodiment) extending radially from the boss portion 31 in the radial direction of the crankshaft 2. In this embodiment, five gear support stays 33 are integrally formed, and are made of, for example, an aluminum alloy.

  The arm portion 32 has a plurality of (three in this embodiment) hollow portions 321, 322, and 323 along the longitudinal direction (see FIGS. 1, 2, 5, and 6). The hollow portions 321, 322, and 323 have openings 321 a, 322 a, and 323 a that are open on the proximal end side of the arm portion 32. The hollow portions 321, 322, and 323 have a bottomed shape that is closed at the end of the arm portion 32 on the tip side (the side opposite to the openings 321 a, 322 a, and 323 a). As shown in FIGS. 1, 2, 6, and 7, the hollow portions 321, 322, and 323 are tapered such that the cross-sectional area decreases from the proximal end to the distal end of the arm portion 32. The hollow portion 321 is located along the center line of the arm portion 32. The hollow portions 322 and 323 are located on both sides of the hollow portion 321, and the depth in the longitudinal direction is shallower than that of the hollow portion 321 and is narrowed. A screw hole 324 for attaching a pedal is formed at the tip of the arm portion 32.

  The boss 31 protrudes inward in the axial direction of the crankshaft 2 with respect to the arm 32 and the gear support stay 33 and includes a central through hole 311. The boss part 31 is displaced inward in the axial direction with respect to the arm part 32, so that the base end of the arm part 32 has an outer surface in the axial direction of the boss part 31 (from the outer surface of the arm part 32). A step wall 325 is formed which is connected to a counterbore portion 311b described later. An opening 321 a of the hollow portion 321 is formed in the step wall 325. In the present embodiment, the hollow portions 322 and 323 are formed across the arm portion 32 or the gear support stay 33, and the openings 322 a and 323 a of the hollow portions 322 and 323 are formed in the arm portion 32 or the gear support stay 33. Is formed.

  The central through hole 311 of the boss portion 31 includes an inward flange 311a formed at an axially intermediate portion, a counterbore portion 311b outside the inward flange 311a, and a crankshaft receiving portion 311c inside the inward flange 311a. ing. The crankshaft receiving portion 311c is inwardly expanded in a tapered shape to the same extent as the small diameter portion 23 so as to receive and fit the small diameter portion 23 of the crankshaft 2 so as not to be relatively rotatable. It forms a spline-like deformed hole corresponding to the outer peripheral shape of the small diameter portion 23.

  As shown in FIG. 1, the gear support stay 33 is displaced inward in the axial direction of the crankshaft 2 from the opening 321 a of the arm portion 32, and is evenly arranged in the circumferential direction. As shown in FIG. 2, the gear support stay 33 is in a position that does not overlap with the direction in which the opening 321 a of the hollow portion 321 faces in the axial direction view of the crankshaft 2. A screw hole 331 for gear attachment is formed at the tip of each gear support stay 33. In the present embodiment, two gears 5 of different sizes are supported on the gear support stay 33.

  The crank arm 4 attached to the left end portion of the crankshaft 2 includes a boss portion 41 and an arm portion 42 extending in the longitudinal shape from the base end connected to the boss portion 41 and reaching the distal end, and these are integrally formed. For example, it is made of an aluminum alloy. The crank arm 4 extends in a direction opposite to the crank arm 3 by 180 ° in the assembled state.

  The arm part 42 has a hollow part 421 along the longitudinal direction thereof. The hollow portion 421 has an opening 421 a that opens on the proximal end side of the arm portion 42. Moreover, the hollow part 421 is made into the bottomed shape closed in the edge part on the opposite side to the opening 421a. A screw hole 422 for attaching a pedal is formed at the tip of the arm portion 42.

  The boss portion 41 protrudes inward in the axial direction of the crankshaft 2 with respect to the arm portion 42 and includes a central through hole 411. The boss portion 41 is displaced inward in the axial direction with respect to the arm portion 42, so that the base end of the arm portion 42 has an outer surface in the axial direction of the boss portion 41 (from the outer surface of the arm portion 42). A step wall 423 is formed which is connected to a counterbore portion 411b described later. An opening 421 a of the hollow portion 421 is formed in the step wall 423.

  The central through hole 411 of the boss portion 41 includes a crankshaft receiving portion 411a formed inward in the axial direction, and a counterbore portion 411b outside the crankshaft receiving portion 411a. The crankshaft receiving portion 411a is fitted to the left end portion of the crankshaft 2 so as not to be relatively rotatable. In the present embodiment, the left end portion of the crankshaft 2 is the spline-shaped large-diameter portion 22 as described above, and the spline-shaped large-diameter portion 22 is formed using the crankshaft receiving portion 411a of the crank arm 4 as a corresponding spline hole. By press fitting, the crank arm 4 is connected to the left end portion of the crankshaft 2 so as not to be relatively rotatable.

  In this embodiment, the hanger part 1 is comprised by the hanger pipe 11 itself. The crankshaft 2 is rotatably supported by the hanger pipe 11 in a state in which the gear crank 6 is integrated at one end (right end) thereof. As shown in FIG. 1, in the bicycle gear crank apparatus A according to this embodiment, the right end portion and the left end portion of the crankshaft 2 are between the outer peripheral surface of the crankshaft 2 and the inner peripheral surface of the hanger pipe 11. The bearings 7 </ b> A and 7 </ b> B are interposed in the hanger pipe 11 directly.

  The bicycle gear crank apparatus A having the above-described configuration can be assembled as follows.

  First, the bearings 7A and 7B are press-fitted and installed at predetermined positions in the axial direction of the hanger pipe 11. Next, the crankshaft 2 is inserted into the boss portion 41 of the crank arm 4 from the left side, and the crankshaft receiving portion 411a and the spline-shaped large-diameter portion 22 are fitted. In this state, the crankshaft 2 is inserted from the left side of the hanger pipe 11 and the boss portion 31 of the crank arm 3 is inserted from the right side of the hanger pipe 11 so that the small diameter portion 23 of the crankshaft 2 and the boss of the crank arm 3 are inserted. The crankshaft receiving portion 311c of the portion 31 is spline fitted as described above. At that time, the crank arm 4 and the crank arm 3 are adjusted so as to extend in directions opposite to each other by 180 °. Then, a bolt 34 inserted from the right side into the central through hole 311 of the boss portion 31 is screwed into the screw hole 24 of the small diameter portion 23 of the crankshaft 2.

  Next, an example of a method for manufacturing the crank arms 3 and 4 constituting the bicycle gear crank apparatus A will be described with reference to FIGS.

  In manufacturing the crank arm 3, first, a crank arm material is formed. The crank arm material is formed by forging (hot die forging). FIG. 8 shows a die 8 (lower die 8A and upper die 8B) for crank arm material forging. The mold 8 has a shape corresponding to the outer shape of the crank arm 3. In this method, an aluminum alloy heated to a predetermined temperature is placed in the mold 8, and the upper and lower molds 8 </ b> A and 8 </ b> B are pressed, and the insert 8 </ b> C having a shape corresponding to the hollow portions 321, 322 and 323 is molded. Is pressed by hydraulic pressure from the side of the divided surface (first step). Thereby, the crank arm material 3 ′ having a shape corresponding to the boss portion 31, the arm portion 32, and the gear support stay 33 is integrally formed. Thereafter, the insert 8C is removed, and the upper and lower molds 8A and 8B are separated to form the hollow portions 321, 322 and 323 in the crank arm material 3 '(second step). FIG. 9 shows the crank arm material 3 '.

  Next, the crank arm 3 is formed by machining the crank arm material 3 'using a drill or the like. Here, a screw hole 324 for pedal attachment, a central through hole 311, a step wall 325, and a screw hole 331 for gear attachment are formed. In this way, the crank arm 3 described above with reference to FIGS. 1 to 7 is obtained.

  In manufacturing the crank arm 4, first, a crank arm material is formed. The crank arm material is formed by forging (hot die forging). FIG. 10 shows a die 9 (lower die 9A and upper die 9B) for crank arm material forging. The mold 9 has a shape corresponding to the outer shape of the crank arm 4. In this method, an aluminum alloy heated to a predetermined temperature is placed in the mold 9, and the upper and lower molds 9A and 9B are pressed, and the insert 9C having a shape corresponding to the hollow portion 421 is formed on the divided surface of the mold 9. Pressing from the side by hydraulic pressure (first step). Thereby, the crank arm material 4 ′ having a shape corresponding to the boss portion 41 and the arm portion 42 is integrally formed. Thereafter, the insert 9C is removed, and the upper and lower molds 9A and 9B are separated to form the hollow portion 421 in the crank arm material 4 '(second step). FIG. 11 shows the crank arm material 4 '.

  Next, the crank arm 4 is formed by machining the crank arm material 4 'using a drill or the like. Here, a screw hole 422 for mounting the pedal, a central through hole 411, and a step wall 423 are formed. In this way, the crank arm 4 described above with reference to FIGS. 1 to 3 is obtained.

  Next, the operation of the crank arms 3 and 4 configured as described above will be described.

  In the crank arm 3 (4) configured as described above, the hollow portions 321, 322, 323 (421) formed in the arm portion 32 (42) are openings 321 a that are open on the base end side connected to the boss portion 31 (41). , 322a, 323a (421a). Thus, the hollow parts 321, 322, 323 (421) having the openings 321a, 322a, 323a (421a) can be formed by forging, for example. Therefore, according to the crank arm 3 (4) provided with the hollow portions 321, 322, 323 (421) having the above-described configuration, unlike the present embodiment, compared with the case where the arm portion having the hollow structure is formed by welding, the welding is poor. There is no problem such as a decrease in strength due to, and the strength is excellent. Further, according to the configuration having the openings 321a, 322a, 323a (421a) at the base end of the arm portion 32 (42), unlike the present embodiment, the arm having a hollow structure in which the tip end (pedal side end portion) of the arm portion is opened. Compared with the case of forming the portion, the step of crushing the opening after the formation of the crank arm material is unnecessary. Therefore, according to the crank arm 3 (4) having the above configuration, the manufacturing process can be simplified, which is suitable for increasing the production efficiency.

  In the crank arm 3 (4), the boss portion 31 (41) is displaced inward in the axial direction of the crankshaft 2 with respect to the arm portion 32 (42). In such a configuration, the volume of the proximal end of the arm portion 32 (42) or the boss portion 31 (41) becomes relatively large, and the outer surface of the arm portion 32 (42) and the boss portion 31 (41) A step wall 325 (423) is formed between the outer surface in the axial direction, and an opening 321a (421a) of the hollow portion 321 (421) is formed in the step wall 325 (423). As a result, the base end of the arm portion 32 (42) has a hollow pipe shape, and this pipe-like portion is connected to the boss portion 31 (41). Such a pipe-shaped configuration is superior in torsional rigidity and bending rigidity compared to a bar-shaped configuration. In a bicycle, the stress acting on the pedal is greater at the base end than at the tip of the arm portion 32 (42), but the configuration of the arm portion 32 (42) having the opening 321a (421a) on the base end side. Is suitable for reducing the weight of the crank arm 3 (4) and is advantageous in terms of strength.

  The hollow portions 321, 322, 323 (421) formed in the crank arm 3 (4) have a bottomed shape that is closed at the end of the arm portion 32 (42) on the tip side (opposite side of the opening). Yes. According to such a configuration, by appropriately setting the depth from the proximal end of the hollow portions 321, 322, 323 (421), the hollow portions 321, 322, 322 in the predetermined range of the distal end of the arm portion 32 (42). A solid structure in which H.323 (421) is not formed can be used. Therefore, according to the above-described configuration, the screw hole 324 (422) for attaching the pedal can be formed at a desired position at the tip of the arm portion 32 (42), and a change in the attachment position of the pedal can be easily handled. It is possible.

  The hollow portions 321, 322, 323 (421) formed in the arm portion 32 (42) are tapered from the base end to the tip end of the arm portion 32 (42). According to such a configuration, the insert 8C (9C) for forming the hollow portions 321, 322, 323 (421) can be appropriately extracted at the time of forging, and acts on various portions of the arm portion 32 (42). It is suitable for making a cross-sectional shape (cross-sectional area) without waste according to the stress to be applied. This is suitable for the crank arm 3 (4) in order to increase the production efficiency and reduce the weight and strength.

  The crank arm 3 has a gear support stay 33 that extends radially from the boss portion 31, and the gear support stay 33 does not overlap with the direction in which the opening 321 a of the hollow portion 321 faces in the axial direction of the crankshaft 2. In the position. According to such a configuration, even when the gear support stay 33 is provided, a sufficient processing space can be secured when forming the central through hole 311, the step wall 325, and the like. Hard to interfere with machining. This is suitable for increasing the production efficiency of the crank arm 3. Further, according to the arrangement of the gear support stay 33, which is unlikely to hinder machining, there is no need to cut the gear support stay 33 excessively, which is advantageous in terms of strength.

  While specific embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the invention. The specific shape, material, etc. of each part of the bicycle crank arm according to the present invention are not limited to the above embodiment.

  In the above embodiment, the configuration in which the openings 321a, 322a, 323a, and 421a of the hollow portions 321, 322, 323, and 421 are exposed to the outside has been described, but a lid for closing these openings may be provided. As the lid for closing the opening, one made of metal or resin can be appropriately employed. The lid for closing the opening 321a can be configured to cover the bolt 34 for connecting the crankshaft 2 and the crank arm 3, and the lid for closing the opening 421a 2 left end openings can be covered. Moreover, according to the bicycle crank arm provided with the hollow portion 321 (421) having the opening 321a (421a) as in the above-described embodiment, for example, various types for measuring the crank rotation speed and the stress acting on the crank. A detection device (such as a rotation speed sensor or a pedal force sensor) can be easily inserted into the hollow portion through the opening. When various sensors are mounted on the inner surface of the hollow portion through the opening, the above-described configuration in which the opening closing lid is provided is preferable in order to appropriately maintain the function of the sensor.

  In the above embodiment, the crank arm 4 is attached to the left end portion of the crankshaft 2 by press-fitting. However, the present invention is not limited to this, and the crank arm 4 uses bolts as in the case of the crank arm 3. It is good also as a structure attached to the left end part of the crankshaft 2.

  In the above embodiment, the fitting portion between the crankshaft 2 and the crank arms 3 and 4 is formed in a spline shape, but other fitting structures may be employed. For example, a tapered square shaft portion (for example, a square) may be provided at the end of the crankshaft, and a tapered square hole that can be fitted to the square shaft portion may be provided at the boss portion of the crank arm.

  In the above-described embodiment, the case where the gear support stay 33 (gear mounting portion) is integrally formed with the crank arm 3 has been described as an example, but instead, the separate gear mounting portion is replaced with appropriate means such as a bolt. It is good also as a structure attached to a crank arm.

  In the above embodiment, the case where the crank arm material is formed by forging has been described. However, the crank arm material can be formed by casting, for example. When the crank arm material is formed by casting, a core corresponding to the hollow portion may be used. If the core is extracted after casting, the hollow portion is formed. Further, the crank arm material can be molded by molding using a resin material. When the crank arm material is molded by resin molding, the resin material used is, for example, a synthetic resin such as a thermoplastic resin or a thermosetting resin, or a carbon-based filler or a glass-based filler in a synthetic resin base material. Examples thereof include added fiber reinforced resin. Further, in the resin mold molding, if a nest having a shape corresponding to the hollow portion is used and the nest is extracted after molding, the hollow portion is formed.

  Further, the hollow portion of the crank arm can be formed by machining. When forming a hollow part by machining, a solid arm part corresponding to the outer shape of the arm part and a part corresponding to the boss part are integrally formed by casting, forging, or resin molding to form a crank arm material. A crank arm having a predetermined shape having a hollow portion can be obtained by forming the arm portion and machining the original shape of the arm portion with a drill or the like.

A Bicycle gear crank device 1 Hanger part 2 Crankshaft 3, 4 Crank arm 5 Gear 6 Gear crank 7A, 7B Bearing 8, 9 Mold 8A, 9A Lower mold 8B, 9B Upper mold 8C, 9C Nest 11 Hanger pipe 21 Intermediate part 22 Spline-like large-diameter part 23 Small-diameter part 24 Screw hole 31 Boss part 311 Central through-hole 311a Inward flange 311b Counterbore part 311c Crankshaft receiving part 32 Arm part 321, 322, 323 Hollow part 321a, 322a, 323a Opening 324 Screw hole 325 Step wall 33 Gear support stay (gear mounting part)
331 Screw hole 41 Boss part 411 Central through hole 411a Crankshaft receiving part 411b Counterbore part 42 Arm part 421 Hollow part 421a Opening 422 Screw hole 423 Step wall

Claims (10)

A bicycle crank arm comprising a boss portion having a fitting portion to be fitted to the crankshaft, and an arm portion extending longitudinally from a proximal end connected to the boss portion and reaching the distal end,
The arm portion has a hollow portion along its longitudinal direction,
The hollow portion has an opening on the proximal end side of the arm portion, and the cross-sectional area thereof is equal or gradually decreased from the opening toward the distal end side of the arm portion. A bicycle crank arm.   The boss portion is displaced inward in the axial direction of the crankshaft with respect to the arm portion, so that the base end of the arm portion extends from the outer surface of the arm portion to the outer surface in the axial direction of the boss portion. The bicycle crank arm according to claim 1, wherein a stepped wall is formed, and the opening of the hollow portion is formed in the stepped wall.   The bicycle crank arm according to claim 2, wherein the hollow portion has a bottomed shape closed at an end portion on a distal end side of the arm portion. The bicycle crank arm according to any one of claims 1 to 3 , further comprising a lid for closing the opening of the hollow portion.   The bicycle crank arm according to any one of claims 1 to 4, further comprising a plurality of gear mounting portions extending radially from the boss portion in a radial direction of the crankshaft. 6. The bicycle crank arm according to claim 5 , wherein the plurality of gear attachment portions are in positions that do not overlap with a direction in which the opening of the hollow portion faces in the axial direction of the crankshaft. A method for manufacturing a crank arm for a bicycle according to any one of claims 1 to 6,
A first step of integrally forming a crank arm material having a shape corresponding to the arm part and the boss part by forging or casting using a nest or core having a shape corresponding to the hollow part;
And a second step of forming a hollow portion by extracting the insert or the core from the crank arm material obtained in the first step. A method for manufacturing a crank arm for a bicycle according to any one of claims 1 to 6,
A first step of integrally forming a crank arm material having a shape corresponding to the arm portion and the boss portion by using a nest having a shape corresponding to the hollow portion by molding using a resin material;
A second step of forming a hollow portion by extracting the insert from the crank arm material obtained in the first step, and a method of manufacturing a bicycle crank arm. A method for manufacturing a crank arm for a bicycle according to any one of claims 1 to 6,
A first step of obtaining a crank arm material by integrally forming a solid arm portion original shape having a shape corresponding to the outer shape of the arm portion by forging or casting, and a portion corresponding to the boss portion;
A method of manufacturing a crank arm for a bicycle, comprising: a second step of forming a hollow portion having a base end opened in the original shape of the arm portion by machining. A method for manufacturing a crank arm for a bicycle according to any one of claims 1 to 6,
A first step of obtaining a crank arm material by integrally molding a solid arm portion original shape having a shape corresponding to the outer shape of the arm portion and a portion corresponding to the boss portion by molding using a resin material; ,
A method of manufacturing a crank arm for a bicycle, comprising: a second step of forming a hollow portion having a base end opened in the original shape of the arm portion by machining.

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