Abstract:
A club head for a golf club comprises a strike face and an outer shell. The strike face and the outer shell define a head volume of the club head. The club head has a first axis that extends generally horizontally and parallel to the strike face, a first moment of inertia about the first axis, a second axis that lies generally vertically and perpendicular to the first axis, a second moment of inertia about the second axis, and a center of gravity lying below a horizontal centerline of the club head. The first moment of inertia in units of kg-mm 2  is greater than or equal to approximately 77 plus 0.46 times the head volume in units of cm 3 .

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuation of U.S. application Ser. No. 11/214,559, filed Aug. 31, 2005, which is a continuation of U.S. application Ser. No. 09/821,370, filed Mar. 29, 2001, now U.S. Pat. No. 6,991,558, which are herein incorporated by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to golf clubs, and, in particular, to a golf club head with a designated relationship between the volume of the club head and the rotational inertia of the club head about a particular axis.  
         [0004]     2. Description of the Related Art  
         [0005]     A wood-type golf club typically includes a hollow shaft with a golf club head attached to the lower end of the shaft. The club head typically includes a load-bearing outer shell with an integral or attached strike plate. The strike plate defines a front surface or strike face adapted for striking a golf ball.  
         [0006]     The mass of a club head is limited by various practical considerations, such as the desire to keep the swing weight of the golf club close to a conventional value. Accordingly, most club heads have a mass between 180-250 grams. A certain portion of the club head&#39;s mass is reserved for components that provide structural support, such as the load bearing outer shell. The remaining mass, which is referred to as performance mass, can be distributed within the club head to optimize performance.  
         [0007]     For some time, golf club manufacturers have searched for ways to best distribute the performance mass so as to improve club head performance. Recently, golf club manufacturers have attempted to position most of the performance mass along the perimeter of the club head so as to increase the rotational moment of inertia (“MOI”) of the club head about the club head center of gravity (“CG”). In particular, many club heads include two or more weights spaced along the heel/toe axis (i.e., an axis that extends through the club head CG generally parallel to the strike face in a generally horizontal direction relative to the ground when the club head is at address position). Such perimeter weighting increases the MOI of the club head about the vertical axis (i.e., an axis that extends through the club head CG in a generally vertical direction relative to the ground when the club head is at address position). This tends to make the club head more resistant to twisting during off-center hits. However, as will be explained below, such perimeter weighting represents an inefficient use of the performance mass.  
         [0008]     An exception to the general trend of heel/toe weighting is U.S. Pat. No. 5,176,383, which discloses a club head with a weight positioned at the rear of a support. The support and the weight are in-line with the center of percussion of the club head. This patent claims that this arrangement concentrates the inertial energy of the club head along the center of percussion, which, in turn, maximizes the amount of energy that is imparted to the golf ball. However, a golf club according to this patent disadvantageously has a CG that is above the horizontal centerline of the golf club.  
       SUMMARY OF THE INVENTION  
       [0009]     In one embodiment of the invention, a golf club head comprises a strike face and an outer shell. The strike face and the outer shell define a head volume of the club head. The club head has a first axis that extends generally horizontally and parallel to the strike face, a first moment of inertia about the first axis, a second axis that lies generally vertically and perpendicular to the first axis, a second moment of inertia about the second axis, and a center of gravity lying below a horizontal centerline of the club head. The first moment of inertia in units of kg-mm 2  is greater than or equal to approximately 77 plus 0.46 times the head volume in units of cm 3 .  
         [0010]     In another embodiment of the invention, the first moment of inertia in units of kg-mm 2  is greater than or equal to approximately 107 plus 0.46 times the head volume in units of cm 3 .  
         [0011]     In another embodiment of the invention, the center of gravity lies more than 1 mm below the horizontal centerline. In yet another embodiment of the invention, the center of gravity lies more than 2 mm below the horizontal centerline.  
         [0012]     In another embodiment of the invention, the club head has a mass of less than 250 grams. In yet another embodiment of the invention, the club head has a mass of less than 230 grams. In yet another embodiment of the invention, the club head has a mass of less than 210 grams.  
         [0013]     In another embodiment of the invention, the club head has a volume greater than 300 cm 3 . In yet another embodiment of the invention, the club head has a volume less than 200 cm 3 .  
         [0014]     In another embodiment of the invention, the second moment of inertia is greater than 250 kg-mm 2 . In yet another embodiment of the invention, the second moment of inertia is greater than 300 kg-mm 2 .  
         [0015]     In another embodiment of the invention, the club head includes a plurality of weights.  
         [0016]     For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.  
         [0017]     All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     These and other features of the invention will now be described with reference to the drawings of a preferred embodiment, which are intended to illustrate and not to limit the invention, and in which:  
         [0019]      FIG. 1  is a front view of a golf club head centered about a coordinate system;  
         [0020]      FIG. 2  is a top plan view of a golf club striking a golf ball;  
         [0021]      FIG. 3  is a side view of a golf club striking a golf ball;  
         [0022]      FIG. 4  is another side view of a golf club illustrating the location of the center of gravity;  
         [0023]      FIG. 5  is a front perspective view of the golf club head having certain features and advantages according to the present invention;  
         [0024]      FIG. 6  is a front view of the golf club head of  FIG. 5 ;  
         [0025]      FIG. 7  is a cross-sectional view of the golf club head of  FIG. 4 ;  
         [0026]      FIG. 8  is a bottom perspective view of the golf club head of  FIG. 4 ; 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]      FIG. 1  is a perspective view of a club head  10  located about a coordinate system  12 . The coordinate system  12  is centered about the center of gravity (“CG”) of the club head. As is typical in the art, the club head  10  comprises a strike plate  14 , which defines a front surface or strike face  16  for impacting a golf ball. A hosel  18  extends upwardly from the strike plate  14 . The hosel  18  is used to attach the club head  10  to a golf club shaft (not shown) as is well known in the art. The club head  10  also includes a load bearing outer shell  20  that is either integrally made with or attached to the strike plate  14 . A heel region  22  of the club head  10  is located close to the hosel  18  while the toe region  24  of the club head is located opposite the heel region  22 .  
         [0028]     The coordinate system  12  comprises three axes: (i) a vertical axis  26  that extends through the CG generally parallel to the strike face  16  in a generally vertical direction relative to the ground when the club head  10  is at address position, (ii) a heel/toe axis  28  that extends through the CG generally parallel to the strike face  16  and generally perpendicular to the vertical axis  26 , and (iii) a front/back axis  30  that extends through the CG generally perpendicular to the vertical axis  26  and the heel/toe axis  28 . Heel/toe axis  28  and front/back axis  30  both extend in a generally horizontal direction relative to the ground when the club head  10  is at address position.  
         [0029]     The club head  10  has a rotational moment of inertia (i.e., a resistance to twisting) about each of the three axes. Specifically, the club head  10  has a moment of inertia (“Izz”) about the vertical axis  26 , a moment of inertia (“Ixx”) about the heel/toe axis  28 , and a moment of inertia (“Iyy”) about the front/back axis  30 . The methods for determining these moments of inertia for any particular club head are well known to those skilled in the art.  
         [0030]     An aspect of Applicant&#39;s invention is the realization that preferably most or more preferably all of the performance mass of the club head  10  should be arranged so as to increase the moment of inertia Ixx about the heel/toe axis  28  and the moment of inertia Izz about the vertical axis  26 .  FIG. 2  is a top plan view of a golf ball  32  hitting the strike face  16  of a club head  10 . As is not unusual in golf, the club head  10  is shown striking the golf ball  32  “off-center”. In this case, the golfball  32  has hit the club head  10  near the toe  24  of the club head (i.e., a “side off-center hit”). The side off-center hit causes the club head  10  to twist about the vertical axis  26  as shown by arrow  27 A. This tends to produce an inaccurate shot.  
         [0031]     To mitigate the twisting about the vertical axis  26  during such side off-center hits, golf club manufacturers have typically sought to increase the golf club&#39;s moment of inertia Izz about the vertical axis  26  by concentrating at least some of the performance weight along the heel/toe axis  28 . For example, heel/toe weights, which are indicated by the reference number  25 , can be added to the club head  10  to increase the club head&#39;s moment of inertia Izz about the vertical axis  26 . This produces more accurate shots.  
         [0032]     However, such heel/toe weights  25  do not necessarily improve performance during all off-center hits. For example,  FIG. 3  is a side view of the club head  10  striking a golf ball  32 . As with  FIG. 2 , the club head  10  has struck the golf ball  32  off-center. However, in this case, the golf ball  32  has hit the club head  10  below the center of the club head (i.e., a “vertical off-center hit”). This type of off-center hit causes the club head  10  to twist about the heel/toe axis  28  as indicated by arrow  27 B. However, heel/toe weights  25  do not increase the club head&#39;s moment of inertia Ixx about the heel/toe axis  28 . Thus, they do not reduce the tendency of the club head  10  to twist about the heel/toe axis  28 . Accordingly, heel/toe weights  25  do not improve the golf club&#39;s performance during vertical off-center hits. Heel/toe weights  25  do increase the club head&#39;s moment of inertia Iyy about the front/back axis  30 . However, it has been determined that during off-center hits the club head  10  tends not to rotate about this axis. Accordingly, the moment of inertia Iyy about the front/back axis  30  is not as effective in improving club head performance.  
         [0033]     In contrast, front/back weights  29 , which are spaced substantially about the front/back axis  30 , increase the club head&#39;s moment of inertia Ixx about the heel/toe axis  28 . Thus, front/back weights  29  improve the golf club&#39;s performance during vertical off-center hits. Moreover, as shown in  FIG. 2 , such front/back weights  29  also increase the club head&#39;s moment of inertia Izz about the vertical axis  26 . Therefore, front/back weights  29  improve the club head&#39;s performance during side off-center hits and vertical off-center hits.  
         [0034]     Another aspect of the invention is the recognition that the performance mass of the club head  10  should also be arranged such that the club head has a low CG. More specifically, as shown in  FIG. 4 , the CG of the club head  10  is preferably located below a horizontal centerline  31  of the club head (i.e., the line  31  that extends through the geometric center of the strike face  16  and bisects a vertical line  33 , which extends perpendicularly from the ground  35  to the top of the strike face  16  when the club head  10  is in the normal address position). Consequently, in some embodiments the performance mass is concentrated below the physical center of the club head. In contrast, most golf clubs have a CG above the horizontal centerline  31 .  
         [0035]     The vertical distance between the CG and the horizontal centerline  31  will be referred to as CGz. As mentioned above, a club head  10  desirably has CG that lies below the horizontal centerline  31 , which extends through the geometric center of the strike face  16 . Preferably, the CG lies at least 1 millimeter below the horizontal centerline  31  (i.e., CGz is at least 1 mm). More preferably, CGz is at least 2 millimeters. It is difficult to design wood-type clubs with a CG below the horizontal centerline  31 . Accordingly, the front/back weights  29  of the club head  10  preferably are located entirely below the horizontal center line  31  of the club head. Moreover, moving the CG even a small distance below the horizontal centerline  31  has a large effect on the golf shot. For example, failure to get the golf ball air borne results in drastically reduced shot distance. A low CG helps the golfer get a golf ball air borne. Specifically, a lower CG increases the launch angle of a golf shot because when the CG is below the point of impact the strike face  16  rotates in such away that it increases the loft of the golf club.  
         [0036]     The club head  10  preferably should also be arranged such that the CG is located not too far back from a shaft or hosel axis  37  of the club head (i.e., a line that extends axially through the center of the shaft and the hosel). The horizontal distance measured in a direction back from the strike face  16  between the CG and the hosel axis  37  will be referred to as Delta  1 . Preferably, Delta  1  is in the range of 12-25 millimeters. More preferably, Delta  1  is in the range of 16-20 millimeters. Most preferably, Delta  1  is in the range of 17-18 millimeters. Delta  1  can be manipulated by varying the mass in front of the CG (i.e., closer to the face) with respect to the mass behind the CG. That is, by increasing the mass behind the CG with respect to the mass in front of the CG, Delta  1  can be increased. In a similar manner, by increasing the mass in front of the CG with respect to the mass behind the CG Delta  1  can be decreased. The above ranges for Delta  1  are preferred for several reasons. If Delta  1  is too far forward, the trajectory of the golf ball tends to be too low and to the right, especially in large club heads (e.g., club heads having a head volume greater than 300 cm 3 ). Conversely, if Delta  1  is too far back the trajectory of the golf ball tends to be too high and the golf ball tends to have too much spin.  
         [0037]     With reference now to  FIGS. 5-8  a preferred construction of a golf club head  50  with certain features and advantages according to the present invention will now be described. As shown in  FIG. 5 , the club head  50  is comprised of a strike plate  58 . The strike plate  58  defines a front surface or strike face  60  for impacting a golf ball. A hosel  62  extends upwardly from the strike plate  58 . The hosel  62  is configured to be coupled to a golf club shaft (not shown) in a well known manner. The strike plate  58  and hosel  62  are preferably made of a strong yet light weight metal, such as titanium or a composite material. Of course, other suitable materials can be used.  
         [0038]     The club head  50  further comprises a load bearing outer shell  64  that is preferably attached to the strike plate  58 . As with the strike plate  58 , the outer shell is preferably made of a strong yet light weight metal, such as, for example) titanium or a composite material. Of course, other suitable materials can be used. The outer shell  64  preferably defines an interior cavity  65  (see  FIG. 7 ) within the club head  50 . Together the strike plate  58  and the outer shell  64  define a head volume (i.e., “HV”) of the club head  50 . The head volume HV represents the volume occupied by the club head  50  and is traditionally measured in cm 3 . Head volume is an important design parameter. Other things being equal, it is easier to achieve a higher rotational moment of inertia about the CG in a club head that defines a larger head volume as compared to a club head that defines a smaller head volume. This is because the performance weight can be distributed farther from the CG in a club head with a large head volume. Conversely, other things being equal, it is easier to achieve a lower CG in a club head with a small head volume as compared to a club head with a large head volume. Accordingly, a design compromise must be made between desired inertial characteristics of the club head and the location of the CG. Moreover, golfers generally do not like the look and feel of unusually large or small club heads. Thus, the head volume of the club head  50  preferably is between 200-450 cm 3 .  
         [0039]     With reference to  FIG. 6 , the club head  50  includes a toe region  66  and a heel region  68 , as will be known to those of skill in the art. The bottom of the club head  50  is delimited in part by a sole  70  and the top of the club head is delimited by a crown  72 . The features of the club head  50  described up to this point can be considered conventional.  
         [0040]     Golfers prefer a driver type golf club to have a total mass of less than 250 grams. Therefore, the club head  50  preferably has a total mass of less than 250 grams. More preferably, the club head  50  has a total mass of less than 230 grams. Most preferably, the club head  50  has a total mass of less than 210 grams. A lighter club head  50  is preferred because it reduces the swing weight of the golf club. However, a lighter club head  50  also has less performance mass available to increase the rotational inertia of the club head  50  about the club head CG. Thus, a design compromise must be made between the total mass of the club head  50  and the desired rotational inertia characteristics of the club head.  
         [0041]     The structural members (i.e., the outer shell  64  and the strike plate  58 ) comprise approximately 60%-90% of the total mass of the club head  50 . The remaining 40%-10% of the club head mass constitutes the performance mass, which is preferably distributed in weight plugs or weights  74  described below.  
         [0042]      FIGS. 7 and 8  show cross-sectional side and bottom views, respectively, of the club head  50 . In the preferred embodiment, the golf club head  50  includes two or more weights or plugs  74   a ,  74   b  that are situated within corresponding recesses  76   a ,  76   b  formed in the outer shell  64 . In the illustrated embodiment, the weights  74   a ,  74   b  are removably coupled to the sole  70  of the club head  50  by screws  78 . However, it should be appreciated that the weights  74   a ,  74   b  can be coupled to the club head  50  by using an adhesive, brazing, etc., or the weights may be integrally formed with the sole  70 . The weights  74   a ,  74   b  preferably are made of a material, such as, for example, tungsten, that is denser than the material(s) that form the outer shell  64  and the strike plate  58 .  
         [0043]     As best seen in  FIG. 8 , the weights  74   a ,  74   b  are preferably located along a front/back axis  80  that extends generally perpendicularly away from the strike face  60  of the club head  50 . More preferably, one of the weights  74   a  is located along the front back axis  80  near the strike plate  58  and the other weight  74   b  is also located along the front back axis  80  near a rear end  81  of the club head  50 .  
         [0044]     In addition, as best seen in  FIG. 7 , both of the weights  74   a ,  74   b  are preferably located below the horizontal centerline  82  of the club head  50 . This arrangement is preferred because it moves the CG of the club head  50  to a position below the horizontal centerline  82 .  
         [0045]     The club head  50  described above preferably has a moment of inertia Ixx about the heel/toe axis  28  that is significantly greater than conventional club heads (i.e., interior volumes between 200-350 cm 3  and a mass between 180-250 grams). As mentioned above, the inertial properties of a club head are dependent upon the head volume. Accordingly, the club head  50  preferably has a moment of inertia Ixx about the heel/toe axis  28  as set forth below in equation 1.
 
 Ixx≧ 0.46* HV+ 77  (1)
        where: HV is club head volume in units of cm 3  
            Ixx is in units of kg-mm 2  
 
 More preferably, the club head  50  has a moment of inertia Ixx about the heel/toe axis  28  as set forth below in equation 2.
 
 Ixx≧ 0.46* HV+ 107  (2)
   
            where: HV is club head volume in units of cm 3  
            Ixx is in units of kg-mm 2  
 
 The higher moments of inertia Ixx of equation 2 can be achieved by reducing or holding constant the mass of the shell  64  and/or the strike plate  58  while increasing or holding constant the mass of the weights  74   a ,  74   b  while also giving due consideration to the structural integrity of the club head  50 . 
   
               
 
         [0050]     In addition, the CG of the club head  50  preferably lies below the horizontal centerline  82  of the club head  50 . More preferably, the CG is more than 1 mm below the horizontal centerline  82  of the club head  50 . The lower CG can be achieved by increasing the mass of the weights  74   a ,  74   b  while reducing or holding constant the mass of the shell  64  and strike plate  58 . The CG can also be reduced by decreasing the thickness of the weights  74   a ,  74   b  and/or decreasing the density of the weights  74   a ,  74   b.    
         [0051]     Preferably, the club head  50  also has a moment of inertia Izz about the vertical axis  26  that is at least 250 kg-mm 2 . More preferably, the club head has a moment of inertia Izz about the vertical axis  26  of at least 300 kg-mm 2 . As with the moment of inertia Ixx about the heel/toe axis  28 , the moment of inertia Izz about the vertical axis  26  can be increased by reducing or holding constant the mass of the shell  64  and/or the strike plate  58  while increasing or holding constant the mass of the weights  74  while also giving due consideration to the structural integrity of the club head  50 .  
         [0052]     As mentioned above, the Delta  1  of the club head  50  preferably is less than 30 mm. Preferably, Delta  1  is in the range of 12-25 mm. More preferably, Delta  1  is in the range of 16-20 mm. Most preferably, Delta  1  is in the range of 17-18 mm.  
         [0053]     The club head  50  described above has generally traditional dimensions as a driver-type wood (i.e., the head volume is between 200 and 300 cm 3 ). However, some golfers prefer a “large” club head. That is, some golfers prefer a club head that defines an interior volume greater than 300 cm 3  and a mass between about 180-210 grams. If such a club head is desired, it can be constructed as described above by enlarging the size of the strike plate  58  and the outer shell  64 .  
         [0054]     As with the club head  50  described above, the club head preferably has a moment of inertia Ixx about the heel/toe axis  28  as set forth above in equation 1. More preferably, the club head  50  has a moment of inertia Ixx about the heel/toe axis  28  as set forth in equation 2. The CG of the club head  50  also preferably lies below the horizontal centerline  82  of the club head. More preferably, the CG is more than 1 mm below the horizontal centerline  82  of the club head  50 . Preferably, the club head  50  also has a moment of inertia Izz about the vertical axis  26  that is at least 250 kg-mm 2 . More preferably, the club head has a moment of inertia Izz about the vertical axis  26  of at least 300 kg-mm 2 . Preferably, Delta  1  is in the range of 12-25 mm. More preferably, Delta  1  is in the range of 16-20 mm. Most preferably, Delta  1  is in the range of 17-18 mm.  
         [0055]     In a modified arrangement, the club head  50  may comprise a smaller driver or a fairway wood club head. This smaller club head defines a head volume of less than 200 cm 3  and a mass between about 200-250 grams. If such a club head  50  is desired, it also can be constructed as described above by adjusting the shape and size of the strike plate  58  and the outer shell  64 . As with the club bead  50  described above, a smaller driver or fairway wood type club head preferably has a moment of inertia Ixx about the heel/toe axis  28  as set forth above in equation 1. More preferably, the club head  50  has a moment of inertia Ixx about the heel/toe axis  28  as set forth in equation 2. The CG of the club head  50  also preferably lies at least 1 mm below the horizontal centerline  82  of the club head  50 . More preferably, the CG is more than 2 mm below the horizontal centerline  82  of the club head  50 . Preferably, the club head  50  also has a moment of inertia Izz about the vertical axis  26  that is at least 200 kg-mm 2 . More preferably, the club head  50  has a moment of inertia Izz about the vertical axis  26  of at least 250 kg-mm 2 . Delta  1  preferably is in the range of 12-25 mm. More preferably, Delta  1  is in the range of 16-20 mm. Most preferably, Delta  1  is in the range of 17-18 mm.  
         [0056]     For purposes of describing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.  
         [0057]     Moreover, although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combination or subcombinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combine with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.