Badminton racket
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2024-10-30
- Publication Date
- 2025-06-26
AI Technical Summary
Existing badminton rackets face challenges in achieving both weight reduction and high rigidity of the frame while improving productivity and ensuring grommet durability against high string tension.
The badminton racket features an annular frame with a groove that has a substantially V-shaped cross section, allowing for improved rigidity and weight reduction, enhanced productivity through easier molding, and increased grommet durability by increasing the thickness and volume of the grommets.
The solution achieves weight reduction and high rigidity of the frame, improves productivity in manufacturing, and enhances the durability of the grommets, leading to improved play performance and longevity of the racket.
Abstract
Description
Badminton racket
[0001] The present invention relates to a badminton racket having an annular frame.
[0002] Badminton is played by a player swinging a racket to hit a shuttlecock. Patent Document 1 discloses a badminton racket with a loop-shaped frame. In Patent Document 1, grooves are formed on the outer surface of the frame, and grommets with holes for inserting strings are arranged in the grooves.
[0003] Utility Model Registration No. 3223277
[0004] Here, badminton racket frames are required to be both lightweight and highly rigid in order to improve playing performance, and there is also a demand for improved productivity in the manufacture of frames using molding dies.
[0005] Furthermore, the grommets are required to be durable against the loads of stringing at high tension, etc. In other words, the shape of the grooves in the frame is required to be formed so that durable grommets can be placed therein.
[0006] The present invention has been made in consideration of these points, and aims to provide a badminton racket that can be easily produced while achieving both a lightweight and highly rigid frame, and that allows for the placement of durable grommets in the grooves of the frame.
[0007] The badminton racket of the present invention is a badminton racket comprising, in order in the longitudinal direction, an annular frame, a shaft, and a grip, wherein the frame comprises an outer forming body that forms the outside in the outward-inward direction, an inner forming body that forms the inside in the outward-inward direction, and a groove that is recessed inward in the outer forming body and forms an opening on the outside, the groove comprises a bottom, a first inclined surface connecting the bottom to the edge forming the front side of the opening, and a second inclined surface connecting the bottom to the edge forming the back side of the opening, the first inclined surface and the second inclined surface extend in a substantially linear direction when viewed in cross section on a plane perpendicular to the extension direction of the frame, and the width of the groove in the front-to-back direction is set to be 4.1 times or more the width of the bottom in the front-to-back direction.
[0008] According to the present invention, the relatively small bottom portion allows each inclined surface to be relatively large, and the inclined surfaces can form the groove in a generally V-shaped cross section. This improves the rigidity of the frame, and the increased rigidity allows the frame base material to be reduced, thereby reducing its weight. Furthermore, the generally V-shaped cross-sectional groove can improve productivity by facilitating the flow of the base material during molding. Furthermore, the generally V-shaped cross-sectional groove can ensure a large groove depth, and when a grommet is placed in the groove, the thickness and volume of the grommet can be increased, improving durability.
[0009]
[0023] Figures 1A and 1B are external views of a badminton racket according to an embodiment, with Figure 1A being a front view and Figure 1B being a side view of the badminton racket. Figure 2A is a schematic cross-sectional view taken along line A-A in Figure 1A, and Figure 2B is an exploded view of Figure 2A. Figure 2A is an enlarged front view of a first grommet. Figure 2B is a schematic perspective view of the first grommet. Figure 5A is an enlarged front view of a second grommet, Figure 5B is an enlarged view of the second grommet from the inside, and Figure 5C is a schematic perspective view of the second grommet. Figure 6A is a schematic cross-sectional view taken along line B-B in Figure 5A, and Figure 6B is an exploded view of Figure 6A. Figure 7A is a cross-sectional view similar to Figure 2A of a comparative structure, and Figure 7B is an exploded view of Figure 7A.
[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is an external view of a badminton racket according to an embodiment of the present invention, Fig. 1A is a front view of the badminton racket, and Fig. 1B is a side view of the badminton racket. Note that for the sake of convenience, some components have been omitted from the following drawings.
[0011] As shown in Figure 1, a badminton racket (hereinafter referred to as "racket") 10 includes a grip 11 that is held by a player, a shaft 12 that extends in a straight line and is connected at one end to the grip 11, and an oval, annular frame 13 that is connected at the other end to the shaft 12. In other words, the racket 10 includes, in order in the longitudinal direction, the annular frame 13, the shaft 12, and the grip 11. Strings 14 are stretched inside the frame 13, and the strings 14 form a striking surface 15 against which a shuttlecock is struck.
[0012] Unless otherwise specified, in the claims and the description of this specification, the side of the racket 10 in the longitudinal direction where the frame 13 is located will be referred to as the upper side, and the side where the grip 11 is located will be referred to as the lower side, as shown by the arrows in Figure 1. The direction perpendicular to the striking surface 15 of the racket 10 will be referred to as the front-to-back direction. Furthermore, the direction perpendicular to the longitudinal direction on the striking surface 15 (i.e., on a plane along the striking surface 15) will be referred to as the left-to-right direction, and the left and right sides are based on the view of the racket 10 when viewed with the front side facing forward, as shown in Figure 1A.
[0013] The frame 13 has regions that are, from top to bottom in the longitudinal direction, a top portion 13a, a middle portion 13b, and a sleeve portion 13c. The middle portion 13b is a region of a predetermined width that includes the position where the left-to-right width of the frame 13 is at its maximum, the top portion 13a is a region above the middle portion 13b, and the sleeve portion 13c is a region that extends from the middle portion 13b to the connection position with the shaft 12 below. The top portion 13a, the middle portion 13b, and the sleeve portion 13c are, although not particularly limited, regions that occupy approximately one-third of the longitudinal length of the frame 13.
[0014] A groove 17 is provided on the outer peripheral surface of the frame 13, and a plurality of holes 13d (not shown in FIG. 1B) are formed around the circumference of the frame 13. Of the plurality of holes 13d, a first grommet 30 and a plurality of second grommets 50 (grommets, not shown in FIG. 1B) are attached to holes 13d that overlap with the grooves 17. Each of the grommets 30, 50 is attached so as to protrude from the inner periphery of the frame 13, and the string 14 is folded back and passed through each of the grommets 30, 50, thereby tensioning the frame 13.
[0015] In this embodiment, one first grommet 30 is provided on the upper part of the frame 13, and second grommets 50 are attached to all of the holes 13d formed by overlapping the grooves 17, to which no first grommets 30 are attached. Note that this configuration is one example and may be changed, and other examples will be described later.
[0016] Next, the cross-sectional shape of the frame 13 will be described below with reference to Figures 2A and 2B. Figure 2A is a schematic cross-sectional view taken along line A-A in Figure 1A, and Figure 2B is an exploded view of Figure 2A. Figures 2A and 2B are cross-sectional views of the frame 13 taken along a plane perpendicular to the extension direction of the frame 13.
[0017] 2A and 2B, the frame 13 is formed by a hollow cylindrical body 18. The thickness of the cylindrical body 18 is set to 0.8 mm or more and 2.0 mm or less, taking into consideration the molding, weight, and rigidity of the frame 13. Although not shown, the cylindrical body 18 may be formed by filling the interior thereof with a predetermined foam material (not shown). Filling of the foam material may be omitted in part or all of the frame 13. Examples of the foam material include urethane and acrylic.
[0018] 2, the frame 13 has a peripheral shape that is roughly elliptical with the major axis extending in the front-to-back direction and the minor axis extending in the outward-to-inward direction, excluding the area where the grooves 17 are formed, and both the outward-to-inward and front-to-back surfaces are curved except for the area where the grooves 17 are formed. Each hole 13d of the frame 13 is formed to penetrate in the outward-to-inward direction and allow the string 14 to pass through.
[0019] In the area where the groove 17 of the frame 13 is formed, the frame 13 is formed into a cross-sectional shape shown in Fig. 2 or a cross-sectional shape that is substantially the same as that shown in Fig. 2, although the depth of the groove 17 is slightly different from that shown in Fig. 2. In addition, in the area where the groove 17 is not formed in the sleeve portion 13c (see Fig. 1), the frame 13 is formed into a cylindrical shape with a cross-section that bulges outward slightly without recessing the area where the groove 17 is formed.
[0020] 2 is the outer-inner direction of the frame 13, with the upper side in the figure being the outer side of the frame 13 and the lower side in the figure being the inner side of the frame 13, unless otherwise specified. The outer side of the frame 13 is formed by the outer forming body 21, and the inner side of the frame 13 is formed by the inner forming body 22, with the middle part of the frame 13 in the outer-inner direction as the boundary. The boundary position between the outer forming body 21 and the inner forming body 22 may be changed as appropriate within the width of the frame 13 in the outer-inner direction.
[0021] Groove 17 is formed inwardly by outer forming body 21 and has opening 17a on the outside. Groove 17 has a bottom 17b and a first inclined surface 17c and a second inclined surface 17d that sandwich bottom 17b, and is formed in a substantially V-shape when viewed in cross section as shown in Figure 2.
[0022] The bottom 17b of the groove 17 is formed in an arc shape with the center recessed from both sides in the front-to-back direction in the cross-sectional view shown in Fig. 2. The bottom 17b is formed in the central region of the groove 17 in the front-to-back direction.
[0023] The first inclined surface 17c connects the bottom 17b to the edge of the opening 17a on the front side, and is inclined outward from the bottom 17b toward the front side. The second inclined surface 17d connects the bottom 17b to the edge of the opening 17a on the back side, and is inclined outward from the bottom 17b toward the back side.
[0024] The groove 17 is formed symmetrically on the front and back sides, so that the first inclined surface 17c and the second inclined surface 17d are formed symmetrically with respect to a plane that is parallel to the outside-inside direction and passes through the center of the frame 13 in the front-back direction. The first inclined surface 17c and the second inclined surface 17d extend in a substantially linear direction when viewed in cross section as shown in FIG.
[0025] Here, the "substantially linear direction" of the first inclined surface 17c and the second inclined surface 17d means not only a direction extending in a straight line without bending or curving, but also a direction that is slightly different from the linear direction due to processing precision, tolerance, likelihood, etc., even though the direction is intentionally manufactured to be linear. The "substantially linear direction" also means a direction extending in an arc, but also includes a state in which the curvature of the arc is significantly smaller than the curvature of the arc that forms the bottom 17b.
[0026] The width W2 of the groove 17 in the outer-inner direction is set to 0.17 times or more the width W1 of the groove 17 in the front-to-back direction. By setting the width W2 in this range, the depth of the groove 17 can be made greater than that of grooves of a conventional structure described later.
[0027] The widths of the first inclined surface 17c and the second inclined surface 17d in the front-to-back direction are set to be the same. The width W2 of the groove 17 in the front-to-back direction is set to be 10 times or more the width W3 of the bottom 17b in the front-to-back direction. The width W1 of the groove 17 in the front-to-back direction is set to be 4.1 times or more the width W4 of the bottom 17b in the front-to-back direction. By setting the widths W3 and W4 of the bottom 17b within this range, the range of the bottom 17b is reduced, and the ranges of the inclined surfaces 17c and 17d extending in a substantially linear direction can be relatively increased.
[0028] Next, the first grommet (grommet) 30 attached to the frame 13 will be described with reference to FIGS. 3 and 4 in addition to FIG. 2. FIG. 3 is an enlarged front view of the first grommet. FIG. 4 is a schematic perspective view of the first grommet. The first grommet 30, which is disposed in the left-right center of the upper part of the frame 13 (shown by the two-dot chain line) shown in FIG. 3, will be described below. In this embodiment, eight strings 14 (eight rows) that pass through the left-right center region of the ball-striking face 15 (see FIG. 1A) of the strings 14 extending in the longitudinal direction (see FIG. 1A) are inserted into the first grommet 30.
[0029] 4, grommet 30 includes a plurality of (eight in this embodiment) generally cylindrical tubular portions 31 and support portions 32 that extend in the circumferential direction, which is the extension direction of frame 13, and connect and support each tubular portion 31. For example, grommet 30 can be formed as an integrally molded product using a synthetic resin material such as nylon.
[0030] 2A , the cylindrical portion 31 has a base on the support portion 32 side, and a tip end on the opposite side to the base that is penetrated through hole 13d from the outside of frame 13 and positioned inside frame 13. This penetration allows first grommet 30 to be attached to frame 13, and in this state, the tip end of cylindrical portion 31 is disposed so as to protrude inward from the inner peripheral surface of frame 13.
[0031] An insertion hole 33 is formed in the cylindrical portion 31 at a position along the central axis and in the support portion 32 on an extension of that position. The string 14 passes through the insertion hole 33, and the string 14 is folded back at the opening of the insertion hole 33 on the outer surface side of the support portion 32 so as to pass through the interior of the adjacent cylindrical portion 31.
[0032] The support portion 32 is disposed inside and along the groove 17 as part of the first grommet 30. The support portion 32 is continuous with the base portions of all the tubular portions 31 and is formed in a strip shape extending in the extension direction of the frame 13. The width of the support portion 32 in the front-to-back direction is formed to be larger than the diameter of the hole 13d.
[0033] The support portion 32 includes a first inner forming surface 35 that is in surface contact with the first inclined surface 17c of the groove 17, a second inner forming surface 36 that is in surface contact with the second inclined surface 17d, and a central forming surface 37 that connects the first inner forming surface 35 and the second inner forming surface 36 and runs along the bottom 17b. In other words, the inner forming surface of the support portion 32 that faces the groove 17 is formed in a substantially V-shape along the groove 17 when viewed in cross section as shown in Figure 2. It is preferable that the central forming surface 37 be in surface contact with the bottom 17b, but a gap may be formed partially or entirely between the central forming surface 37 and the bottom 17b.
[0034] Furthermore, the support portion 32 has an outer forming surface 38 that is disposed on the opening 17a side of the groove 17 and is exposed to the outside of the frame 13. The outer forming surface 38 is formed in a curved shape with the center portion in the front-to-back direction bulging outward when viewed in cross section as shown in Figure 2. As a result, the outer forming surface 38 is formed parallel to the curved outer peripheral surface of the outer forming body 21 of the frame 13. Note that the outer forming surface 38 may also have a bulging shape that smoothly connects to the outer peripheral surface of the outer forming body 21.
[0035] Passage grooves 39 (see also FIG. 3 ) are formed in the outer forming surface 38 of the support portion 32 in a region where the strings 14 run along the outer peripheral surface. In other words, the region of the outer forming surface 38 where the strings 14 do not run along the outer peripheral surface is a non-forming region where no passage grooves 39 are formed.
[0036] Next, the second grommet 50 will be described with reference to Figures 5 and 6. Figure 5A is an enlarged front view of the second grommet, Figure 5B is an enlarged view of the second grommet from the inside, and Figure 5C is a schematic perspective view of the second grommet. Figure 6A is a schematic cross-sectional view taken along line B-B in Figure 5A, and Figure 6B is an exploded view of Figure 6A.
[0037] Here, as shown in Fig. 5A, the second grommet 50 disposed on the right side of the frame 13 (shown by the two-dot chain line) will be described as an example. As shown in Figs. 5A to 5C, the second grommet 50 is modified compared to the first grommet 30 (see Fig. 3) mainly by reducing the number of tubular portions 31 by one and shortening the length of the support portions 32, 52 in the circumferential direction of the frame 13. Therefore, in the second grommet 50, components common to the first grommet 30 are designated by the same reference numerals, and descriptions thereof may be omitted.
[0038] In the second grommet 50, the length of the support portion 52 in the circumferential direction of the frame 13 is shorter than the distance between adjacent holes 13d (see FIG. 1A) in the circumferential direction and slightly larger than the width in the front-to-back direction. In this embodiment, the support portion 52 is formed in a substantially rectangular shape when viewed in the axial direction of the cylindrical portion 31 (see FIG. 5B).
[0039] 6A and 6B, the outer forming surface 53 of the support portion 52 of the second grommet 50, which is disposed on the opening 17a side of the groove 17, is formed in a shape that extends linearly in parallel to the front-to-back direction when viewed in cross section as shown in the figures. The outer forming surface 53 is disposed slightly inward from the opening 17a and is positioned so as not to protrude outward from the opening 17a.
[0040] Here, the functions of the groove 17 and the support portions 32, 52 of each grommet 30, 50 in this embodiment will be described in comparison with the comparative structure in Fig. 7. Fig. 7A is a cross-sectional view of the comparative structure similar to Fig. 2A, and Fig. 7B is an exploded view of Fig. 7A. The comparative structure, which is a conventional example, is configured by modifying the shapes of the groove 17 and the support portions 32, 52 in this embodiment, and in Fig. 7, the same reference numerals are used to designate the same components as in the embodiment.
[0041] In the comparative structure of Fig. 7, a bottom 101b of a groove 101 formed in a frame 100 is formed so as to extend parallel to the front-to-back direction when viewed in cross section as shown in Fig. 7. In addition, in the groove 101, a first inclined surface 101c and a second inclined surface 101d formed on either side of the bottom 101b are formed so as to have a smaller width in the front-to-back direction than the bottom 101b. Therefore, the groove 101 is formed in a substantially trapezoidal shape that is longer in the front-to-back direction when viewed in cross section as shown in Fig. 7.
[0042] In the comparative structure of Fig. 7, the inner surface of the support portion 106 of the grommet 105, which faces the groove 101, is formed in a substantially trapezoidal shape along the groove 101 when viewed in cross section as shown in Fig. 7. In addition, tapered surfaces are formed on both the front and back sides of the outer surface of the support portion 106.
[0043] Comparing the above embodiment with a conventional structure, the groove 17 of the embodiment can be made deeper (width in the outward and inward directions) than the groove 101 of the conventional structure. As a result, a comparison using a structural analysis application shows that the frame 13 of the embodiment can have greater flexural rigidity and bending rigidity than the frame 100 of the conventional structure. Here, the flexural rigidity refers to the rigidity when a force is applied in a direction pulling the frames 13, 100 via the strings 14 when the striking surface 15 is deformed to concave when hitting with a shuttlecock. The bending rigidity refers to the rigidity when a force is applied in a front-to-back direction to the frames 13, 100 when the striking surface 15 is hit with a shuttlecock with the grip 11 side of the racket 10 fixed.
[0044] In the embodiment, the rigidity of the frame 13 can be increased, which can provide a lighter feel when hitting the shuttlecock and improve resilience performance. Furthermore, by improving the rigidity of the frame 13 in the embodiment, it is possible to reduce the amount of base material, such as resin, used in the frame 13 while slightly increasing the rigidity, thereby achieving weight reduction. Therefore, the frame 13 in the embodiment can achieve both a lightweight frame 13 with high rigidity, thereby improving playing performance. The reduced base material in the frame 13 can be relocated to other parts of the racket 10, which can also improve performance.
[0045] Furthermore, by increasing the depth of the groove 17 in the embodiment, it is possible to ensure a large thickness and volume for the support portions 32, 52 of each grommet 30, 50. This makes it possible to suppress deformation of the support portions 32, 52 and improve durability, even when the string 14 is strung with high tension or when the string 14 is replaced multiple times. Furthermore, by increasing the thickness and volume of the support portions 32, 52, it is possible to disperse stress generated by the string 14 being strung, thereby also improving durability.
[0046] Furthermore, in the first grommet 30, the thickness of the support portion 32 is increased, so that the depth of the passage groove 39 can be ensured, and the string 14 can be well protected by preventing the string 14 from protruding from the passage groove 39 or by reducing the amount of protrusion.
[0047] Furthermore, by increasing the thickness of the support portion 32 of the first grommet 30, it is possible to locally increase the weight of the top portion 13a of the frame 13. This allows for an increase in power when striking the shuttlecock while minimizing the increase in overall weight. In addition, the outer surface 38 of the first grommet 30 can be formed into a curved shape that bulges outward, reducing air resistance during a swing and improving swing speed.
[0048] Another comparative structure is a structure in which the grooves of the frame are curved and recessed in a circular arc shape in cross section. Comparing this structure with the embodiment, in the embodiment, each of the inclined surfaces 17c, 17d extends in a substantially linear direction, so that the outer forming body 21 can ensure a width in the front-to-back direction at the portion 17e above the first inclined surface 17c and the portion 17f below the second inclined surface 17d (see FIG. 2). By ensuring such a width, the rigidity of the outer forming body 21 of the frame 13 in the embodiment can be increased.
[0049] As yet another example of a structure, a rectangular groove having a reduced width in the front-to-back direction and a greater depth than the above-described groove 17 is formed to ensure the durability of the grommet. Compared to such a structure, the groove 17 of the embodiment has a generally V-shaped cross section, which makes it easier for the base material of the frame 13 to flow during molding of the frame 13 using a molding die, and also makes it easier to release the frame 13 from the mold. Therefore, the embodiment can achieve better productivity than the above-described rectangular groove.
[0050] The present invention is not limited to the above-described embodiment, and various modifications can be made to the embodiment. In the above-described embodiment, the size, shape, direction, etc. shown in the accompanying drawings are not limited to these, and can be modified as appropriate within the scope of the effects of the present invention. In addition, the present invention can be modified as appropriate without departing from the scope of the object of the present invention.
[0051] The mounting positions and mounting ranges of the two types of grommets 30, 50 on the frame 13 in the above embodiment are merely examples, and various modifications are possible. For example, the first grommet 30 may be omitted, and the second grommets 50 may be mounted in all of the holes 13d formed in the groove 17. As another example, the number of tubular portions 31 of the first grommet 30 may be increased or decreased to a number other than eight (e.g., four, six, or ten), or the second grommet 50 may be omitted, and the number of tubular portions 31 of the first grommet 30 may be increased so that the tubular portions 31 are mounted in all of the holes 13d formed in the groove 17. In such a configuration, the first grommet 30 may have a single support portion 32, or the support portion 32 may be divided into multiple portions in the extension direction of the frame 13.
[0052] In comparison with the above embodiment, by omitting the first grommets 30 or reducing the number of tubular portions 31 and increasing the number of second grommets 50, it is possible to reduce the total weight of the grommets 30, 50 and achieve weight reduction around the frame 13. Also, in comparison with the above embodiment, by increasing the number of tubular portions 31 of the first grommets 30 and lengthening the support portions 32, it is possible to increase the rigidity of the mounting portion of the frame 13 to which the first grommets 30 are attached.
[0053] Furthermore, the formation range of the groove 17 on the frame 13 may be changed as long as the above-described groove 17 is formed at the mounting position of each grommet 30, 50. For example, in the above embodiment, the second grommet 50 may be omitted, and the groove 101 of the comparative structure shown in Figure 7 may be formed in the frame 13 except for the mounting position of the first grommet 30, and a grommet 105 of the comparative structure may be mounted in each hole 13d. Even with this configuration, the string 14 stretched in the sweet spot of the ball-striking surface 15 is inserted through the tubular portion 31 of the first grommet 30, thereby improving the resilience performance by increasing the flexural rigidity.
[0054] Furthermore, when viewed from the axial direction of the tubular portion 31 as shown in Figure 5B, the shape of the support portion 52 of the second grommet 50 is not limited to a square shape, and various modifications are possible, such as a circle, an ellipse, a polygon, or a square with arc-shaped corners.
[0055] Furthermore, when viewed in cross section as in Figures 2 and 6, the peripheral surface of the frame 13 may have other shapes, such as a curved surface that bulges outward in an arc shape on both the outer and inner sides and on both the front and back sides, so that the area other than the area where the groove 17 is formed is a continuous, smooth ring shape.
[0056] In addition, the bottom 17b of the groove 17 that is recessed in an arc shape in cross section may be formed by an arc of a single curvature, or may be formed by combining multiple arcs of different curvatures. Furthermore, the bottom 17b may have a shape in which at least a portion is linear, for example, the connecting portion with the first inclined surface 17c and the connecting portion with the second inclined surface 17d may both be formed in an arc shape, and the portion between these arc-shaped portions may be formed in a linear shape.
[0057] The present invention relates to a badminton racket that can be easily produced while achieving both a lightweight and highly rigid frame, and that can accommodate grommets with enhanced durability in grooves in the frame.
[0058] This application is based on Japanese Patent Application No. 2023-214728, filed December 20, 2023, the contents of which are incorporated herein in their entirety.
Claims
1. A badminton racket comprising, in order in the longitudinal direction, an annular frame, a shaft, and a grip, wherein the frame comprises an outer forming body which forms the outside in the outward-inward direction, an inner forming body which forms the inside in the outward-inward direction, and a groove which is recessed inward in the outer forming body and forms an opening on the outside, wherein the groove comprises a bottom, a first inclined surface which connects the bottom to the edge forming the front side of the opening, and a second inclined surface which connects the bottom to the edge forming the back side of the opening, wherein the first inclined surface and the second inclined surface extend in an approximately straight direction when viewed in cross section on a plane perpendicular to the extension direction of the frame, and wherein the width of the groove in the front-to-back direction is set to be 4.1 times or more the width of the bottom in the front-to-back direction.
2. A badminton racket as described in claim 1, characterized in that, when viewed in cross section on a plane perpendicular to the extension direction of the frame, the bottom is formed in an arc shape, and the width of the groove in the outward and inward directions is set to be 10 times or more greater than the width of the bottom in the outward and inward directions.
3. The badminton racket according to claim 1, characterized in that the width of said groove in the outer-inner direction is set to be 0.17 or more times the width in the front-back direction.
4. The badminton racket according to claim 1, characterized in that the frame is formed by a hollow tubular body, the wall thickness of which is set to be 0.8 mm or more and 2.0 mm or less.
5. The badminton racket of claim 1, further comprising a grommet a portion of which is disposed inside the groove, the grommet comprising at least one tubular portion and a support portion connected to the tubular portion and disposed along the groove, the support portion comprising a first inner forming surface in face-to-face contact with the first inclined surface, a second inner forming surface in face-to-face contact with the second inclined surface, a central forming surface connecting the first inner forming surface and the second inner forming surface and extending along the bottom, and an outer forming surface disposed on the opening side of the groove.
6. A badminton racket as described in claim 5, characterized in that the grommet has a plurality of tubular portions, and the support portion extends in the extension direction of the frame while connecting with all of the tubular portions.
7. The badminton racket according to claim 6, wherein the grommet is disposed at least at the center in the left-right direction of the upper portion of the frame.