Patent Publication Number: US-2022233921-A1

Title: Golf club head

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Japanese Patent Application No. 2021-010666 filed on Jan. 26, 2021. The entire contents of this Japanese Patent Application are hereby incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a golf club head. 
     Description of the Related Art 
     Rebound performance is known as one of performances required for a golf club head. A high rebound performance can increase flight distance. JP2016-47183 A discloses a head that includes a hollow protruding portion provided spaced apart from its face portion toward the back side. This protruding portion enhances rebound performance. 
     SUMMARY 
     There has been demand for a head having a higher rebound performance. The head preferably has a larger area having a high rebound performance. On the other hand, it is preferable that other performances are not sacrificed by structures for enhancing the rebound performance. A head that has a high rebound performance and is also excellent in other performances is preferable. 
     One of the objects of the present disclosure is to provide a golf club head having a high rebound performance. 
     In one aspect, the present disclosure provides a golf club head including a face portion that forms a striking face, a crown portion, a sole portion, and a hosel portion. The striking face has a face center. The sole portion includes a toe protruding portion that is a protruding portion formed on a toe side with respect to the face center and is hollow. The toe protruding portion is not present on a heel side with respect to the face center. The toe protruding portion is formed so as to be continuous with the face portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a golf club according to a first embodiment; 
         FIG. 2A  is a front view of a head of the first embodiment as viewed from a face side, and  FIG. 2B  shows a cross section taken along line E 1  in  FIG. 2A ,  FIG. 2B  showing only a cross-sectional contour line of the outer surface of the head; 
         FIG. 3  is a plan view of the head of the first embodiment as viewed from a crown side; 
         FIG. 4  is a bottom view of the head of the first embodiment as viewed from a sole side; 
         FIG. 5  is a perspective view of the head of the first embodiment as viewed from the sole side; 
         FIG. 6A  is a side view of the head of the first embodiment as viewed from a toe side, and  FIG. 6B  is a side view of the head as viewed from a heel side; 
         FIG. 7A  is a cross-sectional view taken along line A-A in  FIG. 2A ,  FIG. 7B  a cross-sectional view taken along line B-B in  FIG. 2A , and  FIG. 7C  a cross-sectional view taken along line C-C in  FIG. 2A , each of these cross-sectional views showing only a cross-sectional contour line of the outer surface of the head; 
         FIG. 8  is a cross-sectional view taken along line D-D in  FIG. 2A , the cross-sectional view showing only a cross-sectional contour line of the outer surface of the head; 
         FIG. 9  is an enlarged view of a portion surrounded by a circle indicated with a two-dot chain line in  FIG. 7A ; 
         FIG. 10  is a cross-sectional view taken along line A-A in  FIG. 2A , this cross-sectional view including the cross section (wall thickness) of the head; 
         FIG. 11A  is a bottom view of a head according to a second embodiment as viewed from a sole side, and  FIG. 11B  is a cross-sectional view taken along line A-A in  FIG. 11A ,  FIG. 11B  showing only a cross-sectional contour line of the outer surface of the head; and 
         FIG. 12  is a conceptual diagram for illustrating a reference state. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present disclosure will be described in detail according to the preferred embodiments with appropriate references to the accompanying drawings. 
     In the present disclosure, a reference state, a reference perpendicular plane, a toe-heel direction, a face-back direction, an up-down direction, a face center, a vertical cross section, a radial cross section, an evaluation cross section, and a radius of curvature are defined as follows. 
     The reference state is defined as a state where a head is placed at a predetermined lie angle on a ground plane HP. As shown in  FIG. 12 , in the reference state, a shaft axis line Z is contained in a plane VP that is perpendicular to the ground plane HP. The shaft axis line Z is the center line of a shaft. The plane VP is defined as the reference perpendicular plane. The predetermined lie angle is shown in a product catalog, for example. 
     In the reference state, a face angle is 0°. That is, in a planer view of the head as viewed from above, a tangent line to the head at its face center on a striking face is set to be parallel to the toe-heel direction. The definitions of the face center and the toe-heel direction are as explained below. 
     In the present disclosure, the toe-heel direction is the direction of an intersection line NL between the reference perpendicular plane VP and the ground plane HP (see  FIG. 12 ). A toe side in the toe-heel direction is also simply referred to as “toe side”. A heel side in the toe-heel direction is also simply referred to as “heel side”. 
     In the present disclosure, the face-back direction is a direction that is perpendicular to the toe-heel direction and is parallel to the ground plane HP. A face side in the face-back direction is also simply referred to as “face side”. A back side in the face-back direction is also simply referred to as “back side”. 
     In the present disclosure, the up-down direction is a direction that is perpendicular to the toe-heel direction and is perpendicular to the face-back direction. In other words, the up-down direction in the present disclosure is a direction perpendicular to the ground plane HP. 
     In the present disclosure, the face center is determined in the following manner. First, a point Pr is selected roughly at the center of a striking face in the up-down direction and the toe-heel direction. Next, a plane that passes through the point Pr, extends in the direction of a line normal to the striking face at the point Pr, and is parallel to the toe-heel direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Px of this intersection line is determined. Next, a plane that passes through the midpoint Px, extends in the direction of a line normal to the striking face at the midpoint Px, and is parallel to the up-down direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Py of this intersection line is determined. Next, a plane that passes through the midpoint Py, extends in the direction of a line normal to the striking face at the midpoint Py, and is parallel to the toe-heel direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Px of this intersection line is newly determined. Next, a plane that passes through this newly-determined midpoint Px, extends in the direction of a line normal to the striking face at this midpoint Px, and is parallel to the up-down direction is determined. An intersection line between this plane and the striking face is drawn, and a midpoint Py of this intersection line is newly determined. By repeating the above-described steps, points Px and Py are sequentially determined. In the course of repeating these steps, when the distance between a newly-determined midpoint Py and a midpoint Py determined in the immediately preceding step first becomes less than or equal to 0.5 mm, the newly-determined midpoint Py (the midpoint Py determined last) is defined as the face center. 
     In the present disclosure, the vertical cross section is defined as each of cross sections taken along respective planes perpendicular to the toe-heel direction. A contour line shown in the vertical cross section is also referred to as a vertical cross-sectional contour line. 
     In the present disclosure, a radial cross section is defined. With reference to  FIG. 2A , a horizontal plane X 1  that passes through the face center Fc and a vertical plane Y 1  that is located on the toe side with respect to the face center Fc are set. The horizontal plane X 1  is a plane parallel to the ground plane HP. The vertical plane Y 1  is a plane perpendicular to the ground plane HP and parallel to the face-back direction. A double-pointed arrow Dy in  FIG. 2A  shows a distance in the toe-heel direction between the vertical plane Y 1  and the face center Fc. A double-pointed arrow Dt in  FIG. 2A  shows a distance in the toe-heel direction between a toe-most point of the head  4  and the face center Fc. The distance Dy is 50% of the distance Dt. An intersection line N 2  between the horizontal plane X 1  and the vertical plane Y 1  is determined. The radial cross section is defined as each of cross sections that includes the intersection line N 2 , for example, cross sections J 1 , J 2 , J 3  and J 4  shown in  FIG. 2A . The radial cross sections are set for evaluating a toe protruding portion that extends from a toe-lower portion to a toe-upper portion of the head. A contour line shown in the radial cross section is also referred to as a radial cross-sectional contour line. 
     The toe protruding portion of the present disclosure is evaluated based on the vertical cross sections and the radial cross sections. The vertical cross sections are used in the evaluation of the toe protruding portion on the heel side with respect to the vertical plane Y 1 . The radial cross sections are used in the evaluation of the toe protruding portion on the toe side with respect to the vertical plane Y 1 . The vertical cross sections and the radial cross sections are used for the evaluation of the toe protruding portion. Each of the vertical cross sections and the radial cross sections is also referred to as an evaluation cross section. A contour line shown in the evaluation cross section is also referred to as an evaluation cross-sectional contour line. Hereinafter, the evaluation cross-sectional contour line is also simply referred to as “cross-sectional contour line”. Since there are countless number of evaluation cross sections, there are also countless number of cross-sectional contour lines. Unless a specific cross-sectional contour line is selected, descriptions using a cross-sectional contour line can hold true in all cross-sectional contour lines. 
     A radius of curvature of a curved surface is measured using the cross-sectional contour line. The radius of curvature is determined at each point located on the cross-sectional contour line. The radius of curvature is determined by specifying three points on the cross-sectional contour line: a point to be measured (hereinafter referred to as measurement point), a point located 0.5 mm apart from the measurement point toward one side, and a point located 0.5 mm apart from the measurement point toward the other side. A radius of a circle that passes through these three points is defined as the radius of curvature at the measurement point. “0.5 mm” for these points is a route length measured along the cross-sectional contour line. This “0.5 mm” is a sufficiently small distance for evaluating the radius of curvature at a measurement point. In addition, by setting two points located 0.5 mm apart from the measurement point, the radius of curvature at each point on a free-form curve can be determined without the need to solve the differential equation of a cross-sectional contour line. 
       FIG. 1  is an overall view of a golf club  2  that includes a head  4  according to an embodiment of the present disclosure.  FIG. 2A  is a front view of the head  4 , and  FIG. 2B  is a cross-sectional view taken along line E 1  in  FIG. 2A .  FIG. 3  is a plan view of the head  4  as viewed from a crown side.  FIG. 4  is a bottom view of the head  4  as viewed from a sole side.  FIG. 5  is a perspective view of the head  4  as viewed from the sole side.  FIG. 6A  is a side view of the head  4  as viewed from the toe side, and  FIG. 6B  is a side view of the head  4  as viewed from the heel side. In  FIG. 6A  and  FIG. 6B , the sole portion is positioned on the upper side. 
     As shown in  FIG. 1 , the golf club  2  includes the golf club head  4 , a shaft  6 , and a grip  8 . The shaft  6  has a tip end Tp and a butt end Bt. The head  4  is attached to a tip end portion of the shaft  6 . The grip  8  is attached to a butt end portion of the shaft  6 . 
     The golf club  2  is a driver (No. 1 wood). Typically, the club as a driver has a length of greater than or equal to 43 inches and less than or equal to 48 inches. Preferably, the golf club  2  is a wood-type golf club or a hybrid-type golf club. 
     The shaft  6  is in a tubular form. The shaft  6  is hollow. The material of the shaft  6  is a carbon fiber reinforced resin. From the viewpoint of reducing the weight, a carbon fiber reinforced resin is preferable as the material for the shaft  6 . The shaft  6  is a so-called carbon shaft. Preferably, the shaft  6  is formed with a cured prepreg sheet. In the prepreg sheet, fibers are substantially oriented in one direction. Such a prepreg in which fibers are substantially oriented in one direction is also referred to as UD prepreg. “UD” is an abbreviation of “unidirectional”. A prepreg other than the UD prepreg may be used. For example, fibers contained in the prepreg sheet may be woven. The shaft  6  may include a metal wire. The material of the shaft  6  is not limited, and may be a metal, for example. 
     The grip  8  is a part that a golfer grips during a swing. Examples of the material of the grip  8  include rubber compositions and resin compositions. The rubber composition for the grip  8  may contain air bubbles. 
     The head  4  is hollow. In the present embodiment, the head  4  is a wood type head. The head  4  may be a hybrid type head. The head  4  may be an iron type head. The head  4  may be a putter type head. The head  4  is preferably a wood type head or a hybrid type head, and more preferably a wood type head. Examples of a preferable material for the head  4  include metals and fiber reinforced plastics. Examples of the metals include titanium alloys, pure titanium, stainless steel, maraging steel, and soft iron. Examples of the fiber reinforced plastics include carbon fiber reinforced plastics. The head  4  may be a composite head including a portion made of a metal and a portion made of a fiber reinforced plastic. 
     As shown in  FIG. 2A  to  FIG. 6B , the head  4  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The striking face  10   a  is also simply referred to as a face. The striking face  10   a  has a face center Fc as defined above. The hosel portion  16  has a shaft hole  16   a . As shown in  FIG. 3 , the head  4  has an outer contour line CL 2  in the planer view. 
     An outer edge of the striking face  10   a  can be defined as follows. As shown in  FIG. 2A  and  FIG. 2B , there are cross sections each of which includes a straight line N 1  that connects a center of gravity CG of the head  4  and a sweet spot SS, for example, cross sections E 1 , E 2 , and E 3  in  FIG. 2A . In each of the cross sections, such as the cross section E 1 , when a radius of curvature r of the cross-sectional contour line of the outer surface of the head is sequentially observed from the sweet spot SS toward the outside of the striking face  10   a , a point at which the radius of curvature r becomes 200 mm for the first time is defined as a position Fe. This position Fe is defined as the outer edge of the striking face  10   a . Note that the sweet spot SS is a foot of a perpendicular drawn from the center of gravity CG of the head  4  to the striking face  10   a.    
     The crown portion  12  includes a crown protruding portion  20  on a crown outer surface  12   a . The crown protruding portion  20  is hollow. That is, the crown protruding portion  20  forms a projection on the crown outer surface  12   a  and forms a recess on a crown inner surface. The crown protruding portion  20  includes a contour line CL 20 , an upper surface  22 , and a sidewall surface  24 . In the plan view of the head  4  ( FIG. 3 ), the crown protruding portion  20  has a substantially quadrilateral shape (substantially trapezoidal shape). The crown protruding portion  20  is provided on the heel side with respect to the face center Fc. 
       FIG. 7A  is a cross-sectional view taken along line A-A in  FIG. 2A . The position of line A-A coincides with the vertical plane Y 1 .  FIG. 7A  is the cross-sectional view of the radial cross section (evaluation cross section) J 1  (see  FIG. 2A ).  FIG. 7B  is a cross-sectional view taken along line B-B in  FIG. 2A .  FIG. 7C  is a cross-sectional view taken along line C-C in  FIG. 2A . Each of  FIG. 7A ,  FIG. 7B  and  FIG. 7C  shows only the cross-sectional contour line of the outer surface of the head.  FIG. 8  is a cross-sectional view taken along line D-D in  FIG. 2A . The position of line D-D coincides with the position of the face center Fc. The position of line D-D coincides with the horizontal plane X 1 .  FIG. 8  is the cross-sectional view of the radial cross section (evaluation cross section) J 3  (see  FIG. 2A ). 
     The head  4  includes a toe protruding portion  30 . The toe protruding portion  30  is provided on the sole portion  14 . The toe protruding portion  30  is hollow. The toe protruding portion  30  protrudes toward the outside of the head  4 . The toe protruding portion  30  forms a projection on a sole outer surface  14   a . The toe protruding portion  30  forms a recess on a sole inner surface. 
     In the head  4 , the sole portion  14  includes the entirety of the toe protruding portion  30 . The head  4  does not have a side portion (skirt portion). As shown in  FIG. 2A  and  FIG. 8 , the sole portion  14  extends to an outer peripheral edge of the crown portion  12 . On the toe side of the head  4 , the toe protruding portion  30  extends to a position close to the crown portion  12 . As is clear from the fact that the toe protruding portion  30  is shown in  FIG. 8 , the toe protruding portion  30  extends to reach a position located on the upper side with respect to the face center Fc. The toe protruding portion  30  does not reach the outer contour line CL 2  in the plan view of the head  4  (see  FIG. 3 ). The toe protruding portion  30  does not influence the outer contour line CL 2 . 
     Note that the sole portion  14  may have only a part of the toe protruding portion  30 . The head  4  may include a side portion (skirt portion). When the head  4  includes a side portion (skirt portion), the toe protruding portion  30  may be provided from the sole portion  14  to the side portion (skirt portion). 
     The toe protruding portion  30  is provided on the toe side with respect to the face center Fc. The entirety of the toe protruding portion  30  is located on the toe side with respect to the face center Fc. The toe protruding portion  30  is not present on the heel side with respect to the face center Fc. As shown in  FIG. 7B , the toe protruding portion  30  is not present in the vertical cross section that passes through the face center Fc. As shown in  FIG. 7C , the toe protruding portion  30  is not present in any vertical cross sections taken at positions on the heel side with respect to the face center Fc. 
     The sole outer surface  14   a  which is the outer surface of the sole portion  14  includes a sole base surface  14   d . The sole base surface  14   d  is a curved surface and/or a flat surface that is/are (smoothly) continuous with no step. The sole base surface  14   d  is adjacent to the toe protruding portion  30 . The sole base surface  14   d  extends from a back side position of the toe protruding portion  30  toward the toe and upper side, and reaches an outer edge  14   c  of the sole outer surface  14   a  (see  FIG. 5 ). 
     The sole base surface  14   d  has a boundary line  32  that defines the boundary between the sole base surface  14   d  and the toe protruding portion  30 . The boundary line  32  is a back-side starting line of the toe protruding portion  30 . The toe protruding portion  30  smoothly and continuously extends from the boundary line  32  that is the starting line to the outer edge Fe of the striking face  10   a . The toe protruding portion  30  is smoothly connected to the striking face  10   a  at the outer edge Fe. It should be noted that, for example, if the second order derivative of a function of a cross-sectional contour line is continuous, the curved surface can be considered as smoothly continuous. 
       FIG. 9  is an enlarged view of a portion surrounded by a circle indicated with a two-dot chain line in  FIG. 7A . When the boundary line  32  is not clearly seen from the appearance of the head  4 , the boundary line  32  may be determined based on the cross-sectional contour line. As shown in  FIG. 8  and  FIG. 9 , the sole base surface  14   d  is a straight line or a curved line in the cross-sectional contour line. When the cross-sectional contour line of the sole base surface  14   d  is a curved line, the radius of curvature of the curved line can be set to greater than or equal to 20 mm, further greater than or equal to 40 mm, and still further greater than or equal to 60 mm. When the cross-sectional contour line of the sole base surface  14   d  is a curved line, the radius of curvature of the curved line is larger than the radius of curvature of the cross-sectional contour line of the toe protruding portion  30 . In the cross-sectional contour line, a starting point P 1  that forms the boundary line  32  can be recognized as an inflection point, a point having a small radius of curvature, or a vertex of an angle. The point having a small radius of curvature can be defined as a point having a radius of curvature of less than or equal to 5 mm. When points each having a radius of curvature of less than or equal to 5 mm are continuously present, the point having a small radius of curvature can be defined as the middle point of a region in which those points are present. 
     In the cross-sectional contour line, a virtually extended line L 1  of the sole base surface  14   d  can be defined. As shown in  FIG. 9 , a point P 2  located on the back side of the starting point P 1  and a point P 3  located on the back side of the point P 2  are determined. The distance between the starting point P 1  and the point P 2  is 0.5 mm, and the distance between the point P 2  and the point P 3  is 0.5 mm. “0.5 mm” for these points is a route length measured along the cross-sectional contour line. A circle or a straight line that passes through the point P 1 , the point P 2 , and the point P 3  is defined as the virtually extended line L 1 . In  FIG. 8  and  FIG. 9 , the virtually extended line L 1  is indicated by a two-dot chain line. 
     The toe protruding portion  30  protrudes further outward (toward the outside of the head  4 ) than the virtually extended line L 1 . An intersection point P 4  between the cross-sectional contour line of the outer surface of the head  4  and the virtually extended line L 1  is present. The intersection point P 4  is located between the starting point P 1  and a point P 5  that forms the outer edge Fe of the striking face  10   a . The intersection point P 4  can be considered as an end point of the toe protruding portion  30 . In other words, the intersection point P 4  can be considered as a face-side starting point of the toe protruding portion  30 . However, the toe protruding portion  30  and a transition portion  34  are connected to each other so as to form a continuous convex curved surface, and thus the face-side starting point P 4  of the toe protruding portion  30  is not recognized from the appearance of the head  4 . 
     In the cross-sectional contour line, a virtually extended line L 2  of the striking face  10   a  can be defined. As shown in  FIG. 9 , a point P 6  located on the upper side of the point P 5  forming the outer edge Fe, and a point P 7  located on the upper side of the point P 6  are determined. The distance between the point P 5  and the point P 6  is 0.5 mm, and the distance between the point P 6  and the point P 7  is 0.5 mm. “0.5 mm” for these points is a route length measured along the cross-sectional contour line. A circle or a straight line that passes through the point P 5 , the point P 6 , and the point P 7  is defined as the virtually extended line L 2 . When the striking face  10   a  has a roll, the points P 5 , P 6 , and P 7  are not located on a single straight line, normally. In this case, a circle passing through the point P 5 , the point P 6 , and the point P 7  is used as the virtually extended line L 2 . In  FIG. 8  and  FIG. 9 , the virtually extended line L 2  is indicated by a two-dot chain line. 
     In the cross-sectional contour line, the toe protruding portion  30  has an apex  30   a . The apex  30   a  is a point farthest from the virtually extended line L 1 . That is, in the cross-sectional contour line, the apex  30   a  is a point at which a height H 1  is at the maximum. The height H 1  will be detailed later. 
     The virtually extended line L 1  and the virtually extended line L 2  intersect with each other at one point.  FIG. 8  and  FIG. 9  show the intersection point as a point P 8 . In the cross-sectional contour line of  FIG. 9 , the toe protruding portion  30  protrudes further downward than the intersection point P 8 . In the cross-sectional contour line of  FIG. 8 , the toe protruding portion  30  protrudes further toward the toe side than the intersection point P 8 . As described above, in the cross-sectional contour line, the toe protruding portion  30  protrudes further outward (toward the outside of the head  4 ) than the intersection point P 8 . 
     The toe protruding portion  30  is formed so as to be continuous with the face portion  10 . The toe protruding portion  30  is formed on the back side of the face portion  10 . The toe protruding portion  30  is provided on the immediate back side of the face portion  10  with no gap between the toe protruding portion  30  and the face portion  10 . As well shown in  FIG. 9 , the toe protruding portion  30  is connected to the striking face  10   a  with a smooth convex curved surface. As well shown in  FIG. 9 , in the cross-sectional contour line, the toe protruding portion  30  is connected to the striking face  10   a  with a curved line that is convex toward the outside. 
     By providing the toe protruding portion  30  formed continuously with the face portion  10 , a bending point of the head when the head is deformed at impact can be shifted to the apex  30   a  of the toe protruding portion  30 . In conventional golf club heads, this bending point is located at a boundary (corner part) between the face portion  10  and the sole portion  14 . By shifting the bending point to the apex of the toe protruding portion  30 , a region to be deformed (hereinafter, also referred to as “deformation region”) in the striking face  10   a  can be enlarged. In addition, the amount of deformation of the striking face  10   a  can be increased. Accordingly, the rebound performance of the head can be improved. 
     The center of gravity CG of the head (hereinafter also referred to as head center of gravity CG) can be shifted to a lower-side position by the presence of the toe protruding portion  30  that protrudes downward. The head center of gravity CG can be shifted to a face-side position by the presence of the toe protruding portion  30  located at a face-most position of the sole portion  14 . Since the head center of gravity CG is shifted to a lower-side and face-side position, the position of the center of gravity of the head  4  is lowered, and the position of the sweet spot is also lowered. 
     The mass of the head is distributed to the peripheral side of the head by the toe protruding portion  30  protruding toward the outside of the head. Accordingly, the moment of inertia of the head can be increased. 
     By providing the toe protruding portion  30  formed continuously with the face portion  10 , a convex curved surface that has a large radius of curvature and a large area is formed at the boundary between the face portion  10  and the sole portion  14 . More specifically, the transition portion  34  and a convex curved surface region  36  which are detailed later are formed by the toe protruding portion  30 . Normally, the convex curved surface can effectively reduce turf resistance at and/or near impact. This convex curved surface improves the sliding motion of the sole portion  14 . 
     As shown in  FIG. 4  and  FIG. 5 , the toe protruding portion  30  has a heel-side end  30   h . The heel-side end  30   h  is located on the toe side with respect to the face center Fc. The entirety of the toe protruding portion  30  is formed on the toe side with respect to the face center Fc. The toe protruding portion  30  is not present on the lower side of the face center Fc. The toe protruding portion  30  is not present on the heel side with respect to the face center Fc. Any projecting portion that is continuous with the face portion  10  is not present on the lower side of the face center Fc. Any projecting portion that is continuous with the face portion  10  is not present on the heel side with respect to the face center Fc. 
     It is known that golfers usually address a golf ball in a state where a club is held at a flatter lie angle than the original lie angle of the club. That is, golfers usually address a golf ball in a state where an angle formed between the ground surface and the golf club shaft is smaller than the original lie angle of the club. Accordingly, only a heel-side part of the sole portion  14  is brought into contact with the ground surface at address. Since the toe protruding portion  30  is provided only on the toe side with respect to the face center Fc, the toe protruding portion  30  is not brought into contact with the ground surface at address. For this reason, the toe protruding portion  30  does not obstruct the address motion, and the posture of the head at address is stabilized. 
     At the position of the face center Fc, the width of the face portion  10  in the up-down direction is large, and thus the face portion  10  is easily bent. On the other hand, in a toe-side part with respect to the face center Fc, the width of the face portion  10  in the up-down direction is relatively small. By providing the toe protruding portion  30  on the toe side, the coefficient of restitution in the toe-side part of the face portion  10  can be made closer to the coefficient of restitution at the face center Fc. That is, an area having a high rebound performance (hereinafter referred to as high rebound performance area) can be enlarged toward the toe side. 
     Due to the presence of the hosel portion  16 , the sweet spot SS is likely to be positioned on the heel side and upper side with respect to the face center Fc (see  FIG. 2A ). By providing the toe protruding portion  30  only on the toe-side part of the sole portion  14 , the position of the sweet spot SS can be shifted to the toe side and lower side. The toe protruding portion  30  can contribute to positioning the sweet spot SS closer to the face center Fc. 
     A reference sign CP in  FIG. 3  and  FIG. 4  shows a plane that passes through the face center Fc, is perpendicular to the ground plane HP, and is parallel to the face-back direction. The toe protruding portion  30  is located on the toe side with respect to the plane CP. 
     A double-pointed arrow W 1  in  FIG. 4  shows a distance in the toe-heel direction between the heel-side end  30   h  of the toe protruding portion  30  and the face center Fc. A double-pointed arrow W 2  in  FIG. 4  shows a width of the head  4  in the toe-heel direction. A heel-side starting point for the measurement of the width W 2  is a point having a height from the ground plane HP of 0.875 inches (22.23 mm) in the reference state. That is, in the reference state, a point that has a height from the ground plane HP of 0.875 inches and is located at a heel-most position is the heel-side starting point of the width W 2 . 
     There is no limitation on a ratio (W 1 /w 2 ). From the viewpoint of stability of the posture of the head at address, the ratio (W 1 /w 2 ) is preferably greater than or equal to 0.05, more preferably greater than or equal to 0.10, and still more preferably greater than or equal to 0.15. In order to enhance advantageous effects brought by the toe protruding portion  30 , such as improvement in rebound performance, the toe protruding portion  30  preferably has a longer length. From this viewpoint, the ratio (W 1 /w 2 ) is preferably less than or equal to 0.35, more preferably less than or equal to 0.30, and still more preferably less than or equal to 0.25. 
     As shown in  FIG. 5  and  FIG. 6 , the toe protruding portion  30  has a toe-side end  30   t . The toe-side end  30   t  is located on the upper side with respect to the face center Fc. That is, in the toe-side part of the head, the toe protruding portion  30  extends to reach a position located on the upper side with respect to the face center Fc. As shown in  FIG. 3 , the toe protruding portion  30  does not reach the outer contour line CL 2  in the planer view (plan view in  FIG. 3 ) of the head  4  which is in the reference state. In this planer view, the toe protruding portion  30  does not form any part of the outer contour line CL 2  of the head  4 . Note that this planer view is a projected figure obtained by projecting the head which is in the reference state onto a plane parallel to the ground plane HP. 
     When the toe protruding portion  30  forms a part of the outer contour line CL 2  of the head  4  in the planer view, a recess can be formed on the outer contour line CL 2  of the head. A head having such a recess may violate golf rules. The head  4  does not violate the rules. 
     A double-pointed arrow W 3  in  FIG. 6A  shows a distance in the up-down direction between the ground plane HP and the toe-side end  30   t . A double-pointed arrow W 4  in  FIG. 6A  shows a width of the head  4  in the up-down direction. In the measurement of the width W 4 , the hosel portion  16  is excluded. 
     There is no limitation on a ratio (W 3 /W 4 ). In order to enhance advantageous effects brought by the toe protruding portion  30 , such as improvement in rebound performance, the toe protruding portion  30  preferably has a longer length. From this viewpoint, the ratio (W 3 /W 4 ) is preferably greater than or equal to 0.60, more preferably greater than or equal to 0.65, and still more preferably greater than or equal to 0.70. When the toe protruding portion  30  is located closer to the outer contour line CL 2  (see  FIG. 3 ), a recess tends to be formed on the outer contour line CL 2  of the head. From the viewpoint of the golf rules, the ratio (W 3 /W 4 ) is preferably less than or equal to 0.95, more preferably less than or equal to 0.90, and still more preferably less than or equal to 0.85. 
     The toe protruding portion  30  and the striking face  10   a  are made continuous with each other by a curved surface that is convex toward the outside of the head  4 . In the cross-sectional contour line, a region extending from the point P 5  that is the face outer edge to the intersection point P 4  can be the transition portion  34  that connects the toe protruding portion  30  and the striking face  10   a . The transition portion  34  is a curved surface that is convex toward the outside of the head. The radius of curvature of the transition portion  34  influences the deformation of the striking face  10   a  at impact. By increasing the radius of curvature of the transition portion  34 , a bending deformation in the transition portion  34  is suppressed, and the starting point of bending at impact can be shifted to the apex  30   a  located on further back side than the transition portion  34 . As a result, the deformation region of the striking face  10   a  at impact can be enlarged, which can improve the rebound performance. From this viewpoint, the radius of curvature (a minimum value of radii of curvature at respective points on the transition portion  34 ) of the transition portion  34  is preferably greater than or equal to 7 mm, more preferably greater than or equal to 8 mm, and still more preferably greater than or equal to 9 mm. When the radius of curvature is excessive large, a distance between the outer edge Fe of the striking face  10   a  and the sole surface becomes excessively large, whereby the width of the striking face  10   a  in the up-down direction can become excessively small. From this viewpoint, the radius of curvature (a maximum value of the radii of curvature at respective points on the transition portion  34 ) of the transition portion  34  is preferably less than or equal to 25 mm, more preferably less than or equal to 23 mm, and still more preferably less than or equal to 20 mm. 
     The transition portion  34  and the toe protruding portion  30  form a convex curved surface that is continuous from the outer edge Fe of the striking face  10   a  to the apex  30   a  of the toe protruding portion  30 . In the cross-sectional contour line, the region extending from the point P 5  that is the face outer edge to the apex  30   a  of the toe protruding portion  30  is a convex curved surface region  36  that is formed by the transition portion  34  and a part of the toe protruding portion  30 . The convex curved surface region  36  is a curved surface that is convex toward the outside of the head. The radius of curvature of the convex curved surface region  36  influences the deformation of the striking face  10   a  at impact. By increasing the radius of curvature of the convex curved surface region  36 , a bending deformation in the convex curved surface region  36  is suppressed, and the starting point of bending at impact can be shifted to the apex  30   a  located on the further back side than the convex curved surface region  36 . As a result, the deformation region of the striking face  10   a  at impact can be enlarged, which can improve the rebound performance. From this viewpoint, the radius of curvature (a minimum value of radii of curvature at respective points on the convex curved surface region  36 ) of the convex curved surface region  36  is preferably greater than or equal to 7 mm, more preferably greater than or equal to 8 mm, and still more preferably greater than or equal to 9 mm. When the radius of curvature is excessively large, the distance between the outer edge Fe of the striking face  10   a  and the sole surface becomes excessively large, or the height of the toe protruding portion  30  becomes excessively large. From this viewpoint, the radius of curvature (a maximum value of the radii of curvature at respective points on the convex curved surface region  36 ) of the convex curved surface region  36  is preferably less than or equal to 35 mm, more preferably less than or equal to 33 mm, and still more preferably less than or equal to 30 mm. 
     A double-pointed arrow H 1  in  FIG. 9  shows the height of the toe protruding portion  30 . The height H 1  is a height from the virtually extended line L 1 . The height H 1  can be measured in the cross-sectional contour line. The height H 1  can be the length of a perpendicular drawn to the virtually extended line L 1 . 
     From the viewpoint of enlarging the deformation region of the striking face  10   a , the height H 1  at the apex  30   a  is preferably greater than or equal to 1.5 mm, more preferably greater than or equal to 2 mm, and still more preferably greater than or equal to 2.5 mm. An excessively large height H 1  increases the weight of the toe protruding portion  30 , which can reduce the degree of freedom in the design of the head  4 . From this viewpoint, the height H 1  at the apex  30   a  is preferably less than or equal to 8 mm, more preferably less than or equal to 6 mm, and still more preferably less than or equal to 4 mm. 
     A double-pointed arrow D 1  in  FIG. 9  shows a distance between the lower edge Fe (point P 5 ) of the striking face  10   a  and the back-side starting point P 1  of the toe protruding portion  30 . The distance D 1  is measured in the face-back direction. From the viewpoint of enhancing the rebound performance, the toe protruding portion  30  is provided so as to be continuous with the face portion  10 . From this viewpoint, at each position in the toe-heel direction, the distance D 1  is preferably less than or equal to 25 mm, more preferably less than or equal to 23 mm, and still more preferably less than or equal to 20 mm. From the viewpoint of increasing the radius of curvature of the convex curved surface region  36  to enhance the rebound performance, the distance D 1  is preferably greater than or equal to 8 mm, more preferably greater than or equal to 10 mm, and still more preferably greater than or equal to 12 mm. 
     In a cross section taken along the face-back direction, a minimum radius of curvature of the outer surface of the toe protruding portion  30  is determined. In other words, the minimum radius of curvature of the toe protruding portion  30  is determined in the cross-sectional contour line shown in  FIG. 9 . It is preferable that a portion having the minimum radius of curvature is located at or in the vicinity of the apex  30   a . The route distance from the apex  30   a  to the vicinity of the apex  30   a  is preferably less than or equal to 2 mm, more preferably less than or equal to 1.5 mm, and still more preferably less than or equal to 1 mm. By positioning the portion having the minimum radius of curvature, this portion having the minimum radius of curvature can be the bending point at impact, and thus the deformation region of the face portion  10  can be enlarged. 
     From the viewpoint of bendability, the minimum radius of curvature of the outer surface of the toe protruding portion  30  is preferably less than or equal to 12 mm, more preferably less than or equal to 10 mm, still more preferably less than or equal to 8 mm, and yet still more preferably less than or equal to 6 mm. Considering easiness of surface polishing process and forming process in manufacturing the head, the minimum radius of curvature is preferably greater than or equal to 0.5 mm, more preferably greater than or equal to 0.8 mm, and still more preferably greater than or equal to 1 mm. 
       FIG. 10  is a cross-sectional view taken along line A-A in  FIG. 2A . This cross-sectional view is a normal cross-sectional view showing the wall thickness of the head  4 . As shown in  FIG. 10 , the face portion  10  includes a face outer surface  10   a  and a face inner surface  10   b . The face outer surface  10   a  is the striking face. The face inner surface  10   b  faces an internal space (hollow portion) of the head  4 . The crown portion  12  includes the crown outer surface  12   a  and a crown inner surface  12   b . The crown inner surface  12   b  faces the internal space (hollow portion) of the head  4 . The sole portion  14  includes the sole outer surface  14   a  and a sole inner surface  14   b . The sole inner surface  14   b  faces the internal space (hollow portion) of the head  4 . 
     As described above, the toe protruding portion  30  is hollow. The toe protruding portion  30  forms a projection on the sole outer surface  14   a  and forms a recess on the sole inner surface  14   b . The recess on the sole inner surface  14   b  can be also described as a recess on the head inner surface and a projection that protrudes toward the outside of the head. 
     An inner surface  50  of the toe protruding portion  30  and the face inner surface  10   b  are made continuous with each other by a concave curved surface. The concave curved surface is an inner surface transition portion  54 . The inner surface transition portion  54  is an inner surface of the transition portion  34 . For the same reason as discussed above for the transition portion  34 , the radius of curvature of the inner surface transition portion  54  is preferably greater than or equal to 7 mm, more preferably greater than or equal to 8 mm, and still more preferably greater than or equal to 9 mm. For the same reason as discussed above for the transition portion  34 , the radius of curvature of the inner surface transition portion  54  is preferably less than or equal to 25 mm, more preferably less than or equal to 23 mm, and still more preferably less than or equal to 20 mm. 
     The inner surface of the convex curved surface region  36  is a concave curved surface region  56 . The inner surface transition portion  54  and the inner surface  50  form a continuous concave curved surface. For the same reason as discussed above for the convex curved surface region  36 , the radius of curvature of the concave curved surface region  56  is preferably greater than or equal to 7 mm, more preferably greater than or equal to 8 mm, and still more preferably greater than or equal to 9 mm. For the same reason as discussed above for the convex curved surface region  36 , the radius of curvature of the concave curved surface region  56  is preferably less than or equal to 35 mm, more preferably less than or equal to 33 mm, and still more preferably less than or equal to 30 mm. 
     Note that correspondence between the outer surface and the inner surface of the head is determined based on a line normal to the outer surface. Accordingly, for example, the inner surface transition portion  54  is formed by a set of intersection points between the inner surface of the head and lines normal to the transition portion  34  at respective points on the transition portion  34 . 
     From the viewpoint of shifting the starting point of bending at impact toward the back side, the wall thickness of the convex curved surface region  36  is preferably greater than or equal to 0.6 mm, more preferably greater than or equal to 0.65 mm, and still more preferably greater than or equal to 0.7 mm. From the viewpoint of bendability at and in the vicinity of the apex  30   a  of the toe protruding portion  30 , in each evaluation cross section, the wall thickness of the convex curved surface region  36  is preferably greater than or equal to the minimum wall thickness of the toe protruding portion  30  at or in the vicinity of the apex  30   a . From the viewpoint of shifting the starting point of bending at impact toward the back side, the minimum wall thickness of the convex curved surface region  36  is preferably equal to the minimum wall thickness of the toe protruding portion  30  at and in the vicinity of the apex  30   a . From the viewpoint of shifting the starting point of bending at impact toward the back side, it is preferable that the wall thickness of the convex curved surface region  36  decreases continuously or stepwise toward the apex  30   a . From the viewpoint of bendability at and in the vicinity of the apex  30   a , the minimum wall thickness of the toe protruding portion  30  at and in the vicinity of the apex  30   a  is preferably less than or equal to 2.0 mm, more preferably less than or equal to 1.8 mm, and still more preferably less than or equal to 1.6 mm. From the viewpoint of strength, the minimum wall thickness of the toe protruding portion  30  at and in the vicinity of the apex  30   a  is preferably greater than or equal to 0.6 mm, more preferably greater than or equal to 0.65 mm, and still more preferably greater than or equal to 0.7 mm. These wall thicknesses are measured along respective lines normal to the outer surface of the head. The vicinity of the apex  30   a  can mean a region that extends from the apex  30   a  to a position having a distance from the apex  30   a  of less than or equal to 2 mm. This distance (2 mm) is a route length measured along the cross-sectional contour line of the outer surface of the toe protruding portion  30 . 
       FIG. 11A  is a bottom view of a head  40  according to a second embodiment as viewed from the sole side.  FIG. 11B  is a cross-sectional view taken along line A-A in  FIG. 11A . In  FIG. 11B , only the cross-sectional contour line of the head outer surface is shown. 
     The head  40  includes a face portion  10 , a crown portion  12 , a sole portion  14 , and a hosel portion  16 . The face portion  10  includes a striking face  10   a . The striking face  10   a  is the outer surface of the face portion  10 . The striking face  10   a  has a face center Fc (not shown) as defined above. The crown portion  12  forms a crown outer surface  12   a . The sole portion  14  forms a sole outer surface  14   a . The sole portion  14  includes a toe protruding portion  30 . A reference sign CP shows a plane that passes through the face center Fc, is perpendicular to the ground plane HP, and is parallel to the face-back direction. The toe protruding portion  30  is located on the toe side with respect to the plane CP. 
     The sole portion  14  includes a heel groove  42 . The difference between the head  40  of the second embodiment and the head  4  of the first embodiment is only the presence or absence of the heel groove  42 . 
     The heel groove  42  is provided on the heel side with respect to the toe protruding portion  30 . The heel groove  42  includes a portion located on the heel side with respect to the face center Fc. The heel groove  42  extends from a position located on the toe side with respect to the face center Fc to a position located on the heel side with respect to the face center Fc. The heel groove  42  is present at a position located on the lower side (at the position of the plane CP) of the face center Fc. The entirety of the heel groove  42  may be located on the heel side with respect to the face center Fc. 
     The vertical cross-sectional contour line of the outer surface of the heel groove  42  forms a recess. The heel groove  42  forms a recess on the outer surface  14   a  of the sole portion  14 , and forms a projection on the inner surface of the sole portion  14 . The heel groove  42  is easily deformed at impact. The rebound performance can be further improved by a combination of the toe protruding portion  30  and the heel groove  42 . The portion in which the heel groove  42  is provided can be brought into contact with the ground surface at address. However, unlike a projection, the heel groove  42  does not impair the stability of the posture of the head at address. 
     The heel groove  42  has a back-side contour line (starting line)  44 . A double-pointed arrow D 2  in  FIG. 11B  shows a distance between the lower edge Fe of the striking face  10   a  and the contour line  44 . The distance D 2  is measured in the face-back direction. From the viewpoint of facilitating the deformation of the heel groove  42  to enhance the rebound performance, the distance D 2  is preferably less than or equal to 30 mm, more preferably less than or equal to 28 mm, and still more preferably less than or equal to 25 mm at each position in the toe-heel direction. An excessively small distance D 2  can cause a restriction on the manufacturing method of the head. From this viewpoint, the distance D 2  is greater than or equal to preferably 10 mm, more preferably greater than or equal to 12 mm, and still more preferably greater than or equal to 14 mm. 
     The heel groove  42  has a face-side contour line (starting line)  46 . A double-pointed arrow D 3  in  FIG. 11B  shows a distance between the lower edge Fe of the striking face  10   a  and the contour line  46 . The distance D 3  is measured in the face-back direction. From the viewpoint of facilitating the deformation of the heel groove  42  to enhance the rebound performance, the distance D 3  is preferably less than or equal to 20 mm, more preferably less than or equal to 18 mm, and still more preferably less than or equal to 15 mm at each position in the toe-heel direction. An excessively small distance D 3  can cause a restriction on the manufacturing method of the head. From this viewpoint, the distance D 3  is preferably greater than or equal to 6 mm, more preferably greater than or equal to 8 mm, and still more preferably greater than or equal to 10 mm. 
     Regarding the above-described embodiments, the following clauses are disclosed. 
     [Clause 1] 
     A golf club head including:
         a face portion that forms a striking face;   a crown portion;   a sole portion; and   a hosel portion, wherein   the striking face has a face center,   the sole portion includes a toe protruding portion that is a protruding portion formed on a toe side with respect to the face center and is hollow,   the toe protruding portion is not present on a heel side with respect to the face center, and   the toe protruding portion is formed so as to be continuous with the face portion.       

     [Clause 2] 
     The golf club head according to clause 1, wherein a height of an apex of the toe protruding portion is greater than or equal to 1.5 mm and less than or equal to 8 mm. 
     [Clause 3] 
     The golf club head according to clause 1 or 2, wherein the toe protruding portion is provided in a region that extends from a lower edge of the striking face to a position having a distance from the lower edge toward a back side of less than or equal to 25 mm. 
     [Clause 4] 
     The golf club head according to any one of clauses 1 to 3, wherein a minimum radius of curvature of a cross-sectional contour line of an outer surface of the toe protruding portion is less than or equal to 10 mm. 
     [Clause 5] 
     The golf club head according to any one of clauses 1 to 4, wherein
         the sole portion further includes a heel groove that is formed on the heel side with respect to the toe protruding portion, and   the heel groove includes a portion that is located on the heel side with respect to the face center.       

     [Clause 6] 
     The golf club head according to any one of clauses 1 to 5, wherein
         the toe protruding portion extends, on its toe side, to reach a position located on an upper side with respect to the face center, and   the toe protruding portion does not form any part of an outer contour line of the golf club head in a planer view of the golf club head which is in a reference state.       

     LIST OF REFERENCE NUMERALS 
     
         
         
           
               2  Golf club 
               4 ,  40  Head 
               6  Shaft 
               8  Grip 
             Face portion 
               10   a  Striking face 
               12  Crown portion 
               12   a  Crown outer surface 
               14  Sole portion 
               14   a  Sole outer surface 
               14   d  Sole base surface 
               30  Toe protruding portion 
               32  Boundary line between the sole base surface and the toe protruding portion 
               42  Heel groove 
             L 1  Virtually extended line of the sole base surface 
             L 2  Virtually extended line of the striking face 
             Fe Outer edge of the striking face 
             Z Shaft axis line 
             CG Head center of gravity 
             SS Sweet spot 
           
         
       
    
     The above descriptions are merely illustrative and various modifications can be made without departing from the principles of the present disclosure. 
     The terminology used in the description of the various described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. The use of the terms “a”, “an”, “the”, and similar referents in the context of throughout this disclosure (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. As used throughout this disclosure, the word “may” is used in a permissive sense (i.e., meaning “having the potential to”), rather than the mandatory sense (i.e., meaning “must”). Similarly, as used throughout this disclosure, the terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted.