Abstract:
This invention provides a wood type golf club including a shaft, a grip at one end of the shaft, and a head at the other end of the shaft, wherein a mass m (g) of the golf club and a length L (cm) from a grip side end of the golf club to a barycentric position of the golf club satisfy m×L 2 &gt;2.280×10 6  (g·cm 2 ) and m×L 2 &gt;140×m×L−(5.95×m−417)×10 3 .

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a golf club and, more particularly, to a wood type golf club such as a driver or fairway wood.  
       BACKGROUND OF THE INVENTION  
       [0002]     Factors that influence the performance of a golf club include the primary moment and secondary moment (moment of inertia) of the golf club. The primary and secondary moments are respectively expressed as m×L and m×L 2  where m is the mass of the golf club and L is the length from the grip side end of the golf club to the barycentric position of the golf club.  
         [0003]     Japanese Patent Laid-Open Nos. 8-173577 and 2005-198816 disclose a golf club improved based on its primary moment. These references point out that when the primary moment is decreased, the golfer can easily increase the head speed and can expect that the traveling distance of a ball will increase. On the other hand, the golf club becomes lighter and the swing orbit of the golf club becomes unstable thus degrading the direction of the ball.  
         [0004]     Japanese Patent Laid-Open No. 2001-46563 discloses a golf club improved by considering the secondary moment. Generally, it is known that the secondary moment of a golf club influences the feel at impact. When the secondary moment is large, the feel at impact is enhanced, and the golfer can easily swing the golf club fully. Also, the direction of the ball improves.  
         [0005]     When the secondary moment is increased, however, the primary moment also increases accordingly. Consequently, the head speed decreases and reduces the traveling distance of the ball. Conventional golf clubs have been improved based on only either the primary or secondary moment and is difficult to increase the traveling distance of the ball and at the same time enhance the feel at impact.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention has been made in order to overcome the deficits of prior art.  
         [0007]     According to the aspects of the present invention, it is provided a wood type golf club comprising a shaft, a grip at one end of the shaft, and a head at the other end of the shaft, wherein a mass m (g) of the golf club and a length L (cm) from a grip side end of the golf club to a barycentric position of the golf club satisfy m×L 2 &gt;2.280×10 6  (g·cm 2 ) and m×L 2 &gt;140×m×L−(5.95×m−417)×10 3 .  
         [0008]     The golf club has a comparatively small primary moment (m×L) and a comparatively large secondary moment (m×L 2 ) within ranges where the practical limitations on the lengths and masses of the golf club and its constituent components are satisfied. As a result, both an increase in traveling distance of the ball and enhanced feel at impact are achieved.  
         [0009]     Other features and advantages of the present invention will be apparent from the following descriptions taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention.  
         [0011]      FIG. 1A  is a view for explaining a golf club A according to an embodiment of the present invention;  
         [0012]      FIG. 1B  is a view for explaining a shaft  1 ;  
         [0013]      FIG. 2A  is a table showing the specifications of examples of the present invention and comparative examples;  
         [0014]      FIG. 2B  is a table showing the evaluation results of the examples of the present invention and the comparative examples; and  
         [0015]      FIG. 3  is a graph showing the ranges of a primary moment M and secondary moment I, respectively, expressed by equations (1) and (2). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     A preferred embodiment of the present invention will now be described in detail in accordance with the accompanying drawings.  
         [0017]      FIG. 1A  is a view for explaining a wood type golf club A according to an embodiment of the present invention. The golf club A has a shaft  1 , a grip  2  attached to one end of the shaft  1 , and a head  3  attached to the other end of the shaft  1 . The head  3  has a face portion  3   a  which serves as a golf ball hitting surface and a sole portion  3   b  which forms the bottom surface of the head  3 .  
         [0018]     In  FIG. 1A , reference symbol Lw denotes the total length of the golf club A, which indicates the length between the grip  2  side end of the golf club A and the intersection of an axis La of the shaft  1  and the sole portion  3   b.  A point P 1  indicates the barycentric position of the golf club A, where the golf club A balances with its shaft  1  being in a substantially horizontal state. A point P 2  indicates a position  14  inches from the grip  2  side end of the golf club A in the direction of the axis La.  
         [0019]     A primary moment M (g·cm) and secondary moment I (g·cm 2 ) of the golf club A are respectively expressed as: 
 
 M=m×L  
 
 I=m×L   2 
 
 where m (g) is the mass (total mass) of the golf club A and L (cm) is the length (to be referred to as the barycentric length hereinafter) from the grip  2  side end to the barycentric position P 1  of the golf club A. 
 
         [0020]     This embodiment is aimed at maximizing the secondary moment while minimizing the primary moment to achieve both an increase in traveling distance of the ball and enhanced feel at impact. The secondary moment I is a value obtained by multiplying the primary moment M by the barycentric length L. Accordingly, the primary moment M and secondary moment I tend to change basically in the same manner. Generally, a certain practical limitation is imposed on the lengths and masses of the golf club and its constituent components. In the case of a wood type golf club, for example, the mass of the grip is about 40 g to 55 g, the mass of the shaft is about 40 g to 65 g, and the mass of the head is about 180 g to 200 g. The total mass of the golf club is about 260 g to 340 g.  
         [0021]     In this embodiment, the mass m and barycentric length L are set to satisfy: 
 
 m×L   2 &gt;2.280×10 6  (g·cm 2 )  (1) 
 
 m×L   2 &gt;140× m×L− (5.95× m− 417)×10 3   (2)
 
 By using the primary moment M and secondary moment I, equations (1) and (2) are respectively rewritten as: 
 
 I&gt; 2.280×10 6  (g·cm 2 ) 
 
 I&gt; 140× M− (5.95× m− 417)×10 3 
 
         [0022]     Preferably, the secondary moment I is set in the following range: 
 
2.280×10 6  (g·cm 2 )&lt; I≦ 2.500×10 6  (g·cm 2 )
 
         [0023]     Equations (1) and (2) are formulated by adjusting the masses and barycentric positions of the shaft  1 , grip  2 , and head  3  which are constituent components of the golf club A.  FIG. 3  is a graph showing the ranges of the primary moment M and secondary moment I, respectively, expressed by equations (1) and (2). If equations (1) and (2) are satisfied, the primary moment M and secondary moment I are set within the range, of the ranges divided by lines in  FIG. 3 , indicated by an arrow. In the range indicated by the arrow, the primary moment M and second moment I are comparatively small and large, respectively. This range is dependant on the mass m of the golf club A (m 1 , m 2 , and m 3  in  FIG. 3 )  
         [0024]     The golf club A according to this embodiment provides a golf club which has a comparatively small primary moment M and a comparatively large secondary moment I within ranges where the practical limitations on the lengths and masses of the golf club A and its constituent components are satisfied. As a result, both an increase in traveling distance of the ball and enhanced feel at impact are achieved.  
         [0025]     The mass m of the golf club A is preferably within the range of 260 (g) to 300 (g) (both inclusive). If the golf club A is excessively lightweight, the golfer may feel it difficult to swing the golf club A. If the mass m falls within this range, the above effect can be obtained while solving the swing difficulty of the golf club A.  
         [0026]     The total length Lw of the golf club A can be set within a range of 41 inches to 50 inches (both inclusive), and the loft angle of the face portion  3   a  of the head  3  can be set to 25° or less. To set the total length and loft angle of the golf club within these ranges is preferable for a wood type golf club such as a driver or fairway wood for which an increase in traveling distance of the ball is regarded significant. Furthermore, when the golf club A according to this embodiment is a driver, the total length Lw is preferably set within a range of 44 inches to 48 inches (both inclusive) and the loft angle is set within a range of 5 degrees to 15 degrees (both inclusive).  
         [0027]     The golf club A according to this embodiment preferably has a club balance of C7 to D5 according to the 14-inch method. The 14-inch method is a known method of evaluating the club balance of a golf club. This method will be described with reference to  FIG. 1A . According to the 14-inch method, the load of the grip  2  side end of the golf club A when the shaft  1  is maintained substantially horizontal with the point P 2  being as the fulcrum is converted to be used as an evaluation index. If the club balance of the golf club A is within the above range, the above effect can be obtained within the range of the club balance of a general golf club. With this golf club, address and swing can be performed naturally when compared to a general golf club. It is further preferable if the club balance of the golf club A is C8 to D2 according to the 14-inch method.  
         [0028]     To further decrease and increase the primary moment M and secondary moment I, respectively, of the golf club A, the barycentric length L is significant. More specifically, if the barycentric length L is increased, the secondary moment I can be increased more. Even when the barycentric length L is large, if the mass m is decreased, the primary moment M decreases.  
         [0029]     For this reason, for example, it is preferable to set the mass of the grip  2  to 40 (g) or less. When the weight of the grip  2  side of the golf club A is decreased, a larger barycentric length L can be more easily ensured, and the primary moment M and secondary moment I of the golf club A can easily be set within the above numerical ranges. If the weight of the grip  2  is excessively decreased, the wall thickness of the grip  2  may be decreased to impair the grip feel of the golfer. Therefore, more preferably, the mass of the grip  2  is set within a range of 25 (g) to 40 (g) (both inclusive). Within this range, the above effect can be obtained without largely impairing the grip feel of the grip  2 .  
         [0030]     To further increase the barycentric length L, specifications of the shaft  1  may be contrived.  FIG. 1B  is a view for explaining the shaft  1 . In  FIG. 1B , reference symbol Ls denotes the total length of the shaft  1 . A point P 3  indicates the barycentric position of the shaft  1  alone. When the shaft  1  is substantially horizontal, the shaft  1  balances at the point P 3 . Reference symbol L′ denotes the length from the head  3  side (tip side) end of the shaft  1  to the barycentric position P 3 .  
         [0031]     For example, preferably, the mass of the shaft  1  is 50 (g) or less, and the barycentric position P 3  of the shaft  1  falls within the range of 50%. the total length Ls of the shaft  1  from the head  3  side (tip side) end of the shaft  1 , that is, satisfies L′≦0.5×Ls. When the weight of the head  3  side of the golf club A is increased, a larger barycentric length L can be ensured more easily, and equations (1) and (2) can be satisfied more easily.  
         [0032]     When the weight of the shaft  1  is decreased, the mass m of the golf club A can be decreased more to decrease the primary moment M. If the weight of the shaft  1  is excessively decreased, however, the strength of the shaft  1  may decrease. Therefore, the mass of the shaft  1  is preferably within the range of 30 (g) to 50 (g) (both inclusive). If the barycentric position P 3  of the shaft  1  is extremely closer to the head  3  side, the strength of the shaft  1  may decrease. Therefore, preferably, the barycentric position P 3  of the shaft  1  falls within the range of 45% to 49% the total length Ls of the shaft  1  from the head  3  side end of the shaft  1 , that is, satisfies 0.45×Ls≦L′≦0.49×Ls.  
       EXAMPLE  
       [0033]     Performance evaluation was performed using the examples of the present invention and commercially available golf clubs as comparative examples.  FIG. 2A  is a table showing the specifications of the examples (Examples 1 and 2) of the present invention and comparative examples (commercially available products A to C), each of which is a driver. The symbols (e.g., m, M, I) of the respective parameters correspond to the reference symbols described above. “X” is the right-hand side of equation (2), which is (140×M−(5.95×m−417)×10 3 ). “I−X” is a value obtained by subtracting “X” from a secondary moment I. The requirement of equation (2) is not satisfied if “I−X” is a negative value. Equation (2) is satisfied if “I−X” is a positive value.  
         [0034]     The commercially available product A satisfies neither equation (1) nor (2). The commercially available products B and C satisfy equation (1) but do not satisfy equation (2). Both Examples 1 and 2 satisfy equations (1) and (2). Among Examples 1 and 2 and the commercially available products A to C, no substantial difference exists concerning the total length Lw and the loft angle that mainly influence the traveling distance of the ball. However, Example 2 has the largest total length Lw.  
         [0035]      FIG. 2B  is a table showing the evaluation results of Examples 1 and 2 and commercially available products A to C. The performance evaluation was conducted in the following manner. Five testers hit the golf ball as test hitting using the golf clubs of Examples 1 and 2 and commercially available products A to C. Of the trial hits, three hits with good traveling distances and directions were selected as the evaluation targets. The respective parameters shown in  FIG. 2B  of the evaluation targets were averaged. Of the parameters shown in  FIG. 2B , “deviation amount” indicates the leftward or rightward distance from the target traveling direction of the ball to the actual falling point of the ball, which serves as the index of the directivity of the ball. The organoleptic evaluation is the comment formed by the five testers through exchanging opinions on the relative swing easiness and the like of each club after the test hitting.  
         [0036]     From the results of the performance evaluation of  FIG. 2B , Examples 1 and 2 are superior to the commercially available products A to C in head speed. Each of Examples 1 and 2 has a smaller deviation amount than that of either one of the commercially available products A to C, and has a comparatively better enhanced feel at impact as the organoleptic evaluation. This may be because Examples 1 and 2 satisfy equations (1) and (2), therefore the secondary moments I are comparatively large while the primary moments M are comparatively small.  
         [0037]     As described above, the commercially available products B and C satisfy equation (1), and the secondary moments I are comparatively large. However, the commercially available products B and C are inferior to either of Examples 1 and 2 in terms of the organoleptic evaluation. This may be due to the following reason. The primary moments M of the commercially available products B and C are large and inferior to that of either one of Examples 1 and 2. Thus, in spite that the tester swung the golf club of each of the commercially available products B and C strongly, the head speed was not high enough and the tester could not swing the golf club easily.  
         [0038]     In spite that Example 2 has the largest total length Lw, it was evaluated as easiest to swing. Generally, it is more difficult to swing a golf club as its length increases. The reasons why Example 2 was evaluated as easy to swing may be because the secondary moment I of Example 2 is comparatively large while the primary moment M is comparatively small.  
         [0039]     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.  
       CLAIM OF PRIORITY  
       [0040]     This application claims priority from Japanese Patent Application No. 2005-324017 filed on Nov. 8, 2005, the entire contents of which are hereby incorporated by reference herein.