Patent Publication Number: US-9409067-B2

Title: Golf club head with flexure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/618,963, filed on Sep. 14, 2012, which is currently pending, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an improved golf club head. More particularly, the present invention relates to a golf club head having a compliant portion adjacent to its face. 
     BACKGROUND 
     The complexities of golf club design are well known. The specifications for each component of the club (i.e., the club head, shaft, grip, and subcomponents thereof) directly impact the performance of the club. Thus, by varying the design specifications, a golf club can be tailored to have specific performance characteristics. 
     The design of club heads has long been studied. Among the more prominent considerations in club head design are loft, lie, face angle, horizontal face bulge, vertical face roll, center of gravity, inertia, material selection, and overall head weight. While this basic set of criteria is generally the focus of golf club engineering, several other design aspects must also be addressed. The interior design of the club head may be tailored to achieve particular characteristics, such as the inclusion of hosel or shaft attachment means, perimeter weights on the club head, and fillers within hollow club heads. 
     Golf club heads must also be strong to withstand the repeated impacts that occur during collisions between the golf club and the golf ball. The loading that occurs during this transient event can create a peak force of over 2,000 lbs. Thus, a major challenge is designing the club face and body to resist permanent deformation or failure by material yield or fracture. Conventional hollow metal wood drivers made from titanium typically have a face thickness exceeding 2.5 mm to ensure structural integrity of the club head. 
     Players generally seek a metal wood driver and golf ball combination that delivers maximum distance and landing accuracy. The distance a ball travels after impact is dictated by the magnitude and direction of the ball&#39;s translational velocity and the ball&#39;s rotational velocity or spin. Environmental conditions, including atmospheric pressure, humidity, temperature, and wind speed, further influence the ball&#39;s flight. However, these environmental effects are beyond the control of the golf equipment manufacturer. Golf ball landing accuracy is driven by a number of factors as well. Some of these factors are attributed to club head design, such as center of gravity and club face flexibility. 
     The United States Golf Association (USGA), the governing body for the rules of golf in the United States, has specifications for the performance of golf balls. These performance specifications dictate the size and weight of a conforming golf ball. One USGA rule limits the golf ball&#39;s initial velocity after a prescribed impact to 250 feet per second+2% (or 255 feet per second maximum initial velocity). To achieve greater golf ball travel distance, ball velocity after impact and the coefficient of restitution of the ball-club impact must be maximized while remaining within this rule. 
     Generally, golf ball travel distance is a function of the total kinetic energy imparted to the ball during impact with the club head, neglecting environmental effects. During impact, kinetic energy is transferred from the club and stored as elastic strain energy in the club head and as viscoelastic strain energy in the ball. After impact, the stored energy in the ball and in the club is transformed back into kinetic energy in the form of translational and rotational velocity of the ball, as well as the club. Since the collision is not perfectly elastic, a portion of energy is dissipated in club head vibration and in viscoelastic relaxation of the ball. Viscoelastic relaxation is a material property of the polymeric materials used in all manufactured golf balls. 
     Viscoelastic relaxation of the ball is a parasitic energy source, which is dependent upon the rate of deformation. To minimize this effect, the rate of deformation must be reduced. This may be accomplished by allowing more club face deformation during impact. Since metallic deformation may be purely elastic, the strain energy stored in the club face is returned to the ball after impact thereby increasing the ball&#39;s outbound velocity after impact. 
     A variety of techniques may be utilized to vary the deformation of the club face, including uniform face thinning, thinned faces with ribbed stiffeners and varying thickness, among others. These designs should have sufficient structural integrity to withstand repeated impacts without permanently deforming the club face. In general, conventional club heads also exhibit wide variations in initial ball speed after impact, depending on the impact location on the face of the club. Hence, there remains a need in the art for a club head that has a larger “sweet zone” or zone of substantially uniform high initial ball speed. 
     Technological breakthroughs in recent years provide the average golfer with more distance, such as making larger head clubs while keeping the weight constant or even lighter, by casting consistently thinner shell thickness and going to lighter materials such as titanium. Also, the faces of clubs have been steadily becoming extremely thin. The thinner face maximizes the coefficient of restitution (COR). The more a face rebounds upon impact, the more energy that may be imparted to the ball, thereby increasing distance. In order to make the faces thinner, manufacturers have moved to forged, stamped or machined metal faces which are generally stronger than cast faces. Common practice is to attach the forged or stamped metal face by welding them to the body or sole. The thinner faces are more vulnerable to failure. The present invention provides a novel manner for providing the face of the club with the desired flex and rebound at impact thereby maximizing COR. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a golf club head including a flexure that alters the compliance characteristics as compared to known golf club heads. 
     In an embodiment, a golf club head includes a crown, a sole, a side wall, a hosel, a face and a flexure. The crown defines an upper surface of the golf club head, the sole defines a lower surface of the golf club head, and the side wall extends between the crown and sole. The hosel extends from the crown and includes a shaft bore. The face defines a ball-striking surface and intersects the lower surface at a leading edge. The flexure is elongate and recessed into the sole, extending in a generally heel-to-toe direction and parallel to the leading edge of the golf club head, and intersecting the side wall of the golf club head. The flexure is defined by a first portion and a second portion that join at an apex to form a generally sharktooth cross-sectional shape. The height of the flexure is between about 5.0 mm and 15.0 mm, and the width of the flexure across the recess at the lower surface, is between about 5.0 mm and about 12.0 mm, and the flexure is tuned so that the width across the flexure in a face-to-aft direction varies sinusoidally, immediately after impact, at a frequency of about 2900 Hz to about 4000 Hz. 
     In another embodiment, the golf club head includes a crown, a sole, a side wall, a hosel, a face and a flexure. The crown defines an upper surface of the golf club head, the sole defines a lower surface of the golf club head, and the side wall extends between the crown and sole. The hosel extends from the crown and includes a shaft bore. The face defines a ball-striking surface and intersects the lower surface at a leading edge, and a perimeter of the face is coupled to the crown and the sole. The flexure is elongate and recessed into the sole, and defined by a first portion and a second portion. The length of the first portion is different than the length of the second portion so that the flexure has a generally sharktooth cross-sectional shape. The first portion extends from the sole toward the interior of the golf club head and the second portion extends from the sole toward the interior of the golf club head, and the first portion interfaces the second portion at an apex. The flexure extends across the body in a generally heel-to-toe direction within about 5.0 mm and about 20.0 mm from the leading edge of the golf club head and intersects at least a portion of the side wall of the golf club head. 
     In a further embodiment, a golf club head includes a crown, a sole, a side wall, a hosel, a face and a flexure. The crown defines an upper surface of the golf club head, the sole defines a lower surface of the golf club head, and the side wall extends between the crown and sole. The hosel extends from the crown and includes a shaft bore. The face defines a ball-striking surface and intersecting the lower surface at a leading edge, wherein a perimeter of the face is coupled to the crown and the sole. The flexure is elongate and recessed into the sole and defined by a first portion and a second portion. The length of the first portion is different than the length of the second portion so that the flexure has a generally sharktooth cross-sectional shape. The first portion extends from the sole toward the interior of the golf club head and the second portion extends from the sole toward the interior of the golf club head, and the first portion interfaces the second portion at an apex. A cover that extends across a width of the elongate flexure across the recess. The flexure extends across the body in a generally heel-to-toe direction within about 5.0 and about 20.0 mm from the leading edge of the golf club head and intersects at least a portion of the side wall of the golf club head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein: 
         FIG. 1  is a side view of an embodiment of a club head of the present invention; 
         FIG. 2  is bottom plan view of an embodiment of a club head of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view, corresponding to line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a portion, shown in  FIG. 3  as detail A, of the golf club head of  FIG. 1 ; 
         FIG. 5  is a perspective view of a portion of another embodiment of a club head of the present invention; 
         FIG. 6  is a cross-sectional view, corresponding to line  6 - 6  of  FIG. 5 . 
         FIG. 7  is a side view of another embodiment of a golf club head of the present invention; 
         FIG. 8  is a another side view of the golf club head of  FIG. 7 ; 
         FIG. 9  is a side view of another embodiment of a golf club head of the present invention; 
         FIG. 10  is a another side view of the golf club head of  FIG. 9 ; 
         FIG. 11  is a side view of another embodiment of a golf club head of the present invention; 
         FIG. 12  is a bottom plan view of the golf club head of  FIG. 11 ; 
         FIG. 13  is a cross-sectional view, corresponding to line  13 - 13  of  FIG. 12 ; 
         FIG. 14  is a side view of another embodiment of a golf club head of the present invention; and 
         FIG. 15  is a bottom plan view of the golf club head of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Other than in the operating examples, or unless otherwise expressly specified, all of the numerical ranges, amounts, values and percentages such as those for amounts of materials, moments of inertias, center of gravity locations, loft and draft angles, and others in the following portion of the specification may be read as if prefaced by the word “about” even though the term “about” may not expressly appear with the value, amount, or range. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. 
     Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Furthermore, when numerical ranges of varying scope are set forth herein, it is contemplated that any combination of these values inclusive of the recited values may be used. 
     Coefficient of restitution, or “COR”, is a measure of collision efficiency. COR is the ratio of the velocity of separation to the velocity of approach. As an example, such as for a golf ball struck off of a golf tee, COR may be determined using the following formula:
 
( M   ball ( V   ball-post   −V   ball-pre )+ M   club ( V   ball-post   −V   club-pre ))/ M   club ( V   club-pre   −V   ball-pre )
 
     where,
         V club-post  represents the velocity of the club after impact;   V ball-post  represents the velocity of the ball after impact;   V club-pre  represents the velocity of the club before impact (a value of zero for USGA COR conditions); and   V ball-pre  represents the velocity of the ball before impact.
 
Because the initial velocity of the ball is 0.0 during the collision, because it is stationary on a golf tee, the formula reduces to the following:
 
( M   ball   V   ball-post   +M   club ( V   ball-post   −V   club-pre ))/ M   club ( V   club-pre )
 
COR, in general, depends on the shape and material properties of the colliding bodies. A perfectly elastic impact has a COR of one (1.0), indicating that no energy is lost, while a perfectly inelastic or perfectly plastic impact has a COR of zero (0.0), indicating that the colliding bodies did not separate after impact resulting in a maximum loss of energy. Consequently, high COR values are indicative of greater ball velocity and distance.
       

     Referring to  FIGS. 1-4 , an embodiment of a golf club head  10  of the present invention is shown. Club head  10  includes a construction that improves behavior of the club when struck by a golf ball, particularly when a lower portion of the face is struck. Club head  10  is a hollow body that includes a crown  12 , a sole  14 , a skirt  16 , or side wall, that extends between crown  12  and sole  14 , a face  18  that provides a ball striking surface  20 , and a hosel  22 . It should be understood that skirt  16  may comprise perimeter portions of crown  12  and sole  14  that curve towards each other to form the transition between an upper surface and a lower surface of the golf club head. The hollow body defines an inner cavity  24  that may be left empty or may be partially filled. If it is filled, it is preferable that inner cavity  24  be filled with foam or another low specific gravity material. 
     When club head  10  is in the address position, crown  12  provides an upper surface and sole  14  provides a lower surface of the golf club head. Skirt  16  extends between crown  12  and sole  14  and forms a perimeter of the club head. Face  18  provides a forward-most ball-striking surface  20  and includes a perimeter that is coupled to crown  12 , sole  14  and skirt  16  to enclose cavity  24 . Face  18  includes a toe portion  26  and a heel portion  28  on opposite sides of a geometric center of face  18 . Hosel  22  extends outward from crown  12  and skirt  16  adjacent heel portion  28  of face  18  and provides an attachment structure for a golf club shaft (not shown). 
     Hosel  22  may have a through-bore or a blind hosel construction. In particular, hosel  22  is generally a tubular member and it may extend through cavity  24  from crown  12  to the bottom of the club head  10  at sole  14  or it may terminate at a location between crown  12  and sole  14 . Furthermore, a proximal end of hosel  22  may terminate flush with crown  12 , rather than extending outward from the club head away from crown  12  as shown in  FIGS. 1 and 2 . 
     Inner cavity  24  may have any volume, but is preferably greater than 100 cubic centimeters, and the golf club head may have a hybrid, fairway or driver type constructions. Preferably, the mass of the inventive club head  10  is greater than about 150 grams, but less than about 220 grams, although the club head may have any suitable weight for a given length to provide a desired overall weight and swing weight. The body may be formed of stamped, forged, cast and/or molded components that are welded, brazed and/or adhered together. Golf club head  10  may be constructed from a titanium alloy, any other suitable material or combinations of different materials. Further, weight members constructed of high density mater, such as tungsten, may be coupled to any portion of the golf club head, such as the sole. 
     Face  18  may include a face insert  30  that is coupled to a face perimeter  32 , such as a face flange. The face perimeter  32  defines an opening for receiving the face insert  30 . The face insert  30  is preferably connected to the perimeter  32  by welding. For example, a plurality of chads or tabs (not shown) may be provided to form supports for locating the face insert  30  or a face insert may be tack welded into position, and then the face insert  30  and perimeter  32  may be integrally connected by laser or plasma welding. The face insert  30  may be made by milling, casting, forging or stamping and forming from any suitable material, such as, for example, titanium, titanium alloy, carbon steel, stainless steel, beryllium copper, and carbon fiber composites and combinations thereof. Additionally, crown  12  or sole  14  may be formed separately and coupled to the remainder of the body. 
     The thickness of the face insert  30  is preferably between about 0.5 mm and about 4.0 mm. Additionally, the insert  30  may be of a uniform thickness or a variable thickness. For example, the face insert  30  may have a thicker center section and thinner outer section. In another embodiment, the face insert  30  may have two or more different thicknesses and the transition between thicknesses may be radiused or stepped. Alternatively, the face insert  30  may increase or decrease in thickness towards toe portion  26 , heel portion  28 , crown  12  and/or sole  14 . It will be appreciated that one or both of the ball-striking surface or the rear surface of face  18  may have at least a portion that is curved, stepped or flat to vary the thickness of the face insert  30 . 
     As mentioned above, club head  10  includes a construction that improves behavior of the club when it strikes a golf ball, particularly when a lower portion of the face impacts a golf ball. A flexure  36  is formed in a forward portion of the crown, sole and/or skirt. Flexure  36  is an elongate corrugation that extends in a generally heel to toe direction and that is formed in a forward portion of sole  14 . 
     Flexure  36  is generally flexible in a fore/aft direction and provides a flexible portion in the club head  10  away from face  18  so that it allows at least a portion of face  18  to translate and rotate as a unit, in addition to flexing locally, when face  18  impacts a golf ball. The golf club head is designed to have two distinct vibration modes of the face between about 3000 Hz and about 6000 Hz, and the flexure is generally constructed to add the second distinct vibration mode of the face. The first face vibration mode primarily includes the local deflection of the face during center face impacts with a golf ball. The deflection profile of the second face vibration mode generally includes the entire face deflecting similar to an accordion and provides improved performance for off-center impacts between the face and a golf ball. 
     Flexure  36  is also configured to generally maintain the stiffness of sole  14  in a crown/sole direction so that the sound of the golf club head is not significantly affected. A lower stiffness of the sole in the crown/sole direction will generally lower the pitch of the sound that the club head produces, and the lower pitch is generally undesirable. 
     Flexure  36  allows the front portion of the club, including face  18 , to flex differently than would otherwise be possible without altering the size and/or shape of face  18 . In particular, a portion of the golf club head body adjacent the face is designed to elastically flex during impact. That flexibility reduces the reduction in ball speed, and reduces the backspin, that would otherwise be experienced for ball impacts located below the ideal impact location. The ideal impact location is a location on the ball-striking surface that intersects an axis that is normal to the ball-striking surface and that extends through the center of gravity of the golf club head, and as a result the ideal impact location is generally located above the geometric face center by a distance between about 0.5 mm and 5.0 mm. By providing flexure  36  in sole  14 , close to face  18 , the club head provides less of a reduction in ball speed, and lower back spin, when face  18  impacts a golf ball at a location below the ideal impact location. Thus, ball impacts at the ideal impact location and lower on the club face of the inventive club head will go farther than the same impact location on a conventional club head for the same swing characteristics. Locating flexure  36  in sole  14  is especially beneficial because the ideal impact location is generally located higher than the geometric face center in metal wood-type golf clubs. Therefore, a large portion of the face area is generally located below the ideal impact location. Additionally, there is a general tendency of golfers to experience golf ball impacts low on the face. Similar results, however, may be found for a club head  10  with flexures provided on other portions of the club head  10  for impacts located toward the flexure from the geometric face center. For example, a club having a flexure disposed in the crown may improve performance for ball impacts that are between the crown and the geometric face center. 
     In an embodiment, flexure  36  is provided such that it is substantially parallel to at least a portion of a leading edge  38  of the club head  10 , so that it is generally curved with the leading edge, and is provided within a selected distance D from ball-striking surface  20 . Preferably, flexure  36  is provided a distance D within 30 mm of ball-striking surface  20 , more preferably within 20 mm of ball-striking surface  20 , and more preferably between about 5.0 mm and 20.0 mm. For smaller golf club heads, such as those with fairway wood or hybrid constructions, it is preferable that the flexure  36  is provided within 10 mm of ball striking surface  20 . 
     Flexure  36  is constructed from a first member  40  and a second member  42 . First member  40  is coupled to a rearward edge of a forward transmittal portion  46  of sole  14  and curves into inner cavity  24  from sole  14 . Second member  42  is coupled to a forward edge of a rearward portion of sole  14  and also curves into inner cavity  24  from sole  14 . The ends of first member  40  and second member  42  that are spaced away from sole  14  are coupled to each other at an apex  44 . Preferably, the flexure is elongate and extends in a generally heel to toe direction. 
     The dimensions of flexure  36  are selected to provide a desired flexibility during a ball impact. Flexure  36  has a height H, a width W, and a curl length C, as shown in  FIG. 4 . Height H extends in the direction of the Y-axis between apex  44  and an outer surface of sole  14 . Width W is the width of an opening in the sole that is created by flexure  36  and extends in the direction of the Z-axis between the junctions of flexure  36  with sole  14 . Curl length C extends in the direction of the Z-axis and extends between the forward junction of flexure  36  with sole  14  and apex  44 . Preferably, flexure  36  has a height that is greater than 4.0 mm, preferably about 5.0 mm to about 15.0 mm, more preferably about 6.0 mm to about 11.0 mm. Further, flexure  36  preferably has a width that is greater than 4.0 mm, preferably about 5.0 mm to about 12.0 mm, more preferably about 7.0 to about 11.0 mm. The flexure also has a wall thickness between about 0.8 mm and about 2.0 mm, and those dimensions preferably extend over a length that is at least 25% of the overall club head length along the X-axis. Further, first member  40  is curved inward, into the inner cavity, from the sole and preferably has a radius of curvature between about 20.0 mm and about 45.0 mm. Table 1, below, illustrates dimensions for inventive examples that provide a more efficient energy transfer, and therefore higher COR, for ball impacts that are below the ideal impact location of the golf club head. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Flexure Dimensions 
               
            
           
           
               
               
               
               
            
               
                   
                 Height 
                 Width 
                 Curl Length 
               
               
                   
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Inv. Example 1 
                 10.0 
                 10 
                 13 
               
               
                   
                 Inv. Example 2 
                 6.5 
                 10 
                 13 
               
               
                   
                 Inv. Example 3 
                 10.0 
                 8 
                 13 
               
               
                   
                 Inv. Example 4 
                 6.5 
                 8 
                 13 
               
               
                   
                 Inv. Example 5 
                 5.0 
                 8 
                 13 
               
               
                   
                   
               
            
           
         
       
     
     The inventive examples described above were analyzed using finite element analysis to determine the effect on COR and vibration response of the golf club head. In particular, a club head lacking a flexure (i.e., Baseline) was compared to the inventive examples. Table 2 summarizes the comparison. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Comparison 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Weight 
                 Ball 
                 Extra 
                   
                   
                   
               
               
                   
                 Penalty 
                 Speed 
                 Mode 
                 Mode 2 
                 Mode 3 
                 Mode 4 
               
               
                   
                 [g] 
                 [mph] 
                 [Hz] 
                 [Hz] 
                 [Hz] 
                 [Hz] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Baseline 
                 N/A 
                 160.67 
                 N/A 
                 3409 
                 3538 
                 3928 
               
               
                 Inv. Example 1 
                 7.0 
                 157.16 
                 2157 
                 3608 
                 3767 
                 3907 
               
               
                 Inv. Example 2 
                 5.4 
                 161.28 
                 3196 
                 3639 
                 3840 
                 4002 
               
               
                 Inv. Example 3 
                 7.6 
                 No data 
                 2186 
                 3559 
                 3706 
                 3895 
               
               
                 Inv. Example 4 
                 5.6 
                 161.28 
                 3406 
                 3603 
                 3796 
                 4019 
               
               
                 Inv. Example 5 
                 4.1 
                 160.87 
                 N/A 
                 3540 
                 3675 
                 4163 
               
               
                   
               
            
           
         
       
     
     In the above table, “extra mode” refers to a mode shape, or a natural mode of vibration that does not exist unless a flexure is present. The extra mode generally presents itself as a the face portion rotating and flexing relative to the remainder of the golf club body. In particular, the inventive examples include a flexure that extends across a portion of the sole and the extra mode includes the face rotating about the interface between the face and crown so that the flexure flexes. The flexure is tuned so that that extra mode takes place in a range of frequencies from about 2900 Hz to about 4000 Hz, and more preferably at approximately 3600 Hz, which has been analyzed to be most effective in increasing the ball speed after impact. Practically speaking, that tuning results in the width W of the flexure varying sinusoidally, immediately after impact, at a frequency of about 2900 Hz to about 4000 Hz. If the extra mode takes place at a frequency that is higher or lower than that range, the ball speed can actually be lower compared to the baseline example that does not include a flexure. It has been determined using FEA analysis of inventive example 1 that a flexure that is tuned to provide an extra mode with a frequency below 2900 Hz, particularly approximately 2157 Hz, the ball speed is reduced below the baseline golf club head that does not include a flexure. Additionally, including a flexure that is too rigid provides a golf club head that does not include the extra mode, as shown by inventive example 5, and only provides minimal increase in ball speed after impact. 
     Transmittal portion  46  of sole  14  extends between flexure  36  and leading edge  38 . Transmittal portion  46  is preferably constructed so that the force of a golf ball impact is transmitted to flexure  18  without transmittal portion  46  flexing significantly. For example, transmittal portion is oriented so that it is less inclined to bend. In particular, a transmittal plane that is tangent to the center of transmittal portion  46  (in both fore/aft and heel/toe directions) of sole  14  is angled relative to the ground plane by an angle α. Angle α is preferably less than, or equal to, the loft angle of the golf club head at address, so that the angle between the transmittal plane and the ball striking surface is generally equal to, or less than, 90° so that transmittal portion  46  is less likely to bend during a ball impact. 
     Flexure  36  may be formed by any suitable manner. For example, flexure  36  may be cast as an integral part of sole  14 . Alternatively, flexure  36  may be stamped or forged into a sole component. Additionally, the flexure may be formed by including a thickened region and machining a recess in that thickened region to form the flexure. For example, a spin-milling process may be used to provide a desired recess, the spin-milling process is generally described in U.S. Pat. No. 8,240,021 issued Aug. 14, 2012 as applied to face grooves, but a flexure with a desired profile may be machined using that process by increasing the size of the spin mill tool and altering the profile of the cutter. In general, that process utilizes a tool having an axis of rotation that is parallel to the sole and perpendicular to the leading edge of the golf club head and a cutting end that is profiled to create the desired profile of the flexure. The tool is then moved along a cutting path that is generally parallel to the leading edge. As a further alternative described in greater detail below, a separate flexure component may be added to a flexure on the sole to further tune the flexure of the sole, as shown in  FIGS. 5 and 6 . 
     As shown in the embodiment of  FIG. 1 , the face of the golf club head may include a face insert that is stamped, forged and/or machined separately and coupled to the body of the golf club head. Alternatively, the entire face may be stamped, forged or cast as part of a homogeneous shell, as shown in  FIGS. 5 and 6 , thereby eliminating the need to bond or otherwise permanently secure a separate face insert to the body. As a still further alternative, the face may be part of a stamped or forged face component, such as a face cup, that includes portions of the sole, crown and/or skirt, as shown in  FIG. 12 . In such an embodiment, the face component is coupled to the remainder of the club head body away from the face plane by a distance from about 0.2 inches to about 1.5 inches. Preferably, the face component includes a transmittal portion of the sole that extends to a flexure or the face component includes both the transmittal portion and the flexure. 
     In another embodiment, illustrated in  FIGS. 5 and 6 , a golf club head  60  is a hollow body that includes a crown  62 , a sole  64 , a skirt  66  that extends between crown  62  and sole  64 , a face  68  that provides a ball striking surface  70 , and a hosel  69 . The hollow body defines an inner cavity  74  that may be left empty or it may be fully or partially filled. 
     A flexure  76  is formed in a forward portion of the sole, but it may alternatively be formed in the crown and/or skirt. Preferably, flexure  76  is an elongate corrugation that extends in a generally heel to toe direction and is formed in a forward portion of sole  64  of the body of golf club head  60 . Flexure  76  provides a flexible portion in the club head  60  rearward from face  68  so that it allows at least a portion of face  68  to translate or rotate as a unit, in addition to flexing locally, when face  68  impacts a golf ball. 
     Flexure  76  allows the front portion of the club, including face  68 , to flex differently than would otherwise be possible without altering the size and/or shape of face  68 . That flexibility provides less reduction in ball speed that would otherwise be experienced for mis-hits, i.e., ball impacts located away from the ideal impact location, and less spin for impacts below the ideal impact location. For example, by providing flexure  76  in sole  64 , close to face  68 , the club head provides less of a reduction in ball speed when ball impact is located below the ideal impact location. Thus, during use, ball impacts that occur lower on the club face of the inventive club head will go farther than when compared with the same impact location on a club face of a conventional club head, for common swing characteristics. 
     In an embodiment, flexure  76  is provided such that it is substantially parallel to at least a portion of a leading edge  78  of the club head  60  and is provided within a certain distance D from ball-striking surface  70 . Preferably, flexure  76  is provided a distance D within 30 mm of ball-striking surface  70 , more preferably within 20 mm of ball-striking surface  70 , and most preferably within 10 mm. 
     In the present embodiment, flexure  76  is constructed from a first member  80 , a second member  82  and a third member  83  and is generally constructed as a separate component that is coupled to sole  64 . First member  80  is coupled to a rearward edge of a forward transmittal portion  65  of sole  64  and curves into inner cavity  74  from the transmittal portion  65 . Second member  82  is coupled to a forward edge of a rearward portion of sole  64  and also curves into inner cavity  74  from sole  64 . The ends of first member  80  and second member  82  that are spaced away from sole  64  are coupled to each other at an apex  84 . Preferably, the flexure is elongate and extends in a generally heel to toe direction. 
     Similar to previous embodiments, the dimensions of flexure  76  are selected to provide a desired elastic flex in response to a ball impact. Flexure  76  defines a height H, a width W, and a curl length C. Preferably, flexure  76  has a height that is greater than 4 mm, preferably about 5 mm to about 15 mm, and a width that is greater than 4 mm, preferably about 5 mm to about 10 mm, and a wall thickness between about 0.8 mm and about 2.0 mm, and those dimensions preferably extend over a length that is at least 25% of the overall club head length along the X-axis. 
     Flexure  76  includes third member  83  that may be used to tune the flexibility of flexure  76 . Third member  83  may be coupled to an inner surface (as shown) or an outer surface of flexure  76  and locally increases the rigidity of flexure  76 . Third member  83  is preferably constructed from a material that has a lower specific gravity than the material of at least one of first member  80  and second member  82 . Third member  83  may be bonded, such as by using an adhesive, or mechanically coupled, such as by fasteners, welding or brazing, to first member  80  and second member  82 . The third member may be constructed from any metallic, such as aluminum, or non-metallic material, such as a carbon fiber composite material or polyurethane. 
     The location, dimensions and number of flexures in a golf club head may be selected to provide desired behavior. For example, a plurality of flexures may be included as shown in golf club head  90  of  FIGS. 7 and 8 . Golf club head  90  has a hollow body construction generally defined by a sole  92 , a crown  94 , a skirt  96 , a face  98 , and a hosel  100 . A crown flexure  102  is disposed in a forward portion of crown  94  and a sole flexure  104  is disposed in a forward portion of sole  92 . Each of the flexures  102 ,  104  is preferably shaped and dimensioned as the previously described flexures. 
     In other embodiments, flexures may be included that wrap around a portion of the golf club head body or entirely around the golf club head body. As shown in  FIGS. 9 and 10 , a golf club head  110  has a hollow body construction that is defined by a sole  112 , a crown  114 , a skirt  116 , a face  118  and a hosel  120 . A flexure  122  is formed in a forward portion of the golf club head and wraps around the perimeter of the golf club head. Flexure  122  is generally formed in a plane that is parallel to a face plane of golf club head  110 . The distance between flexure  122  and face  118  may vary along its length to tune the local effect that flexure  122  provides to flexibility of the golf club head. For example, portions of flexure  122  may be spaced further from face  118  as compared to other portions. As illustrated, in an embodiment, heel and toe portions of flexure  122  are spaced further from face  118  than sole and crown portions of flexure  122 . Additionally, the dimensions of flexure  122  may also be altered to tune the local effect that flexure  122  provides to the flexibility of the golf club head. As illustrated, portions of flexure  122  may have different height, width, and/or curl length to alter the behavior of the portions of flexure  122 . 
     In additional embodiments, a compliant flexure may be combined with a multi-material, light density cover member, as shown in  FIGS. 11-13 . For example, golf club head  130  generally has a hollow body construction that is defined by a sole  132 , a crown  134 , a skirt  136 , a face  138  and a hosel  140 . Golf club head  130  also includes a flexure  142  that is formed in a forward portion of sole  132  of golf club head  130 . A cover  144  is also included in golf club head  130  and is configured to cover the outer surface of the flexure. 
     Cover  144  is generally a strip of material that is disposed across flexure  142  to generally enclose flexure  142 . Cover  144  may be dimensioned so that it covers a portion or all of flexure  142 , and it may extend into portions of golf club head  130  that do not include flexure. For example, and as shown in  FIGS. 11 and 12 , cover  144  extends across, and covers flexure  142  that is disposed on sole  132 . Further, cover  144  forms a portion of skirt  136  and crown  134 . Preferably, cover  144  is constructed of a material that is different than the materials of sole  132 , crown  134  and skirt  136 . Cover  144  is coupled to the adjacent portions of golf club head  130  by welding, brazing or adhering to those adjacent portions. 
     The cover may be included to both assist in the control of the address position of the golf club head when the sole is placed on the playing surface and to eliminate undesirable aesthetics of the flexure. In particular, the cover may be included to tune the visual face angle of the golf club head when the head is placed on the playing surface by altering the contact surface of the golf club head. The cover may be configured to wrap around a perimeter of the golf club head to the crown and may replace a portion of the material of the perimeter to create a lower density body structure to provide additional discretionary mass, a lower and/or deeper center of gravity location and a higher moment of inertia, thus improving performance and distance potential. 
     Referring now to  FIGS. 14 and 15 , a golf club head  150  including a flexure  162  having a varied spatial relationship to the face plane along its heel to toe length will be described. Due to the geometry of a golf club head face coupled with the circular shape of the stress imparted to the face during ball impact, the lower portion of the face generally experiences different magnitudes of stress at different heel-to-toe locations. Generally the portions of the golf club head at the heel and toe ends experience lower stresses than the portion of the golf club directly below the geometric center of the face and that stress gradient translates to the stress on the sole in the region of flexure  162 . The distance of the flexure relative to the face plane and/or the leading edge of the face/sole intersection is altered to correspond to the relative amount of stress at the various portions. For example, the heel and toe portions of the flexure are preferably located closer to the face plane and leading edge of the golf club head so that those portions will be more likely to experience flexing even under the lower stress conditions, and especially during off-center ball impacts. 
     Golf club head  150  has a hollow body construction that is defined by a sole  152 , a crown  154 , a skirt  156 , a face  158  and a hosel  160 . Flexure  162  is formed in a forward portion of the golf club head and extends generally across the golf club head in a heel to toe direction through the sole and skirt. Flexure  162  generally includes a central portion  164 , a toe portion  166  and a heel portion  168 . As described above, the portions of flexure  162  are disposed at varied spatial relationships relative to the face plane so that central portion  164  is further aftward from the face plane compared to toe portion  166  and heel portion  168 . Further, flexure  162  includes heel and toe extensions  170 ,  172  that extend from the heel and toe portions  168 ,  166 , respectively along skirt  156  aftward. Heel and toe extensions  170 ,  172  may also extend aftward and meet at a location on the skirt or sole. 
     As described above, the flexure of the present invention provides lower stiffness locally in a portion of the golf club head. Generally the lower stiffness may be achieved by selecting the geometry of the flexure, such as by altering the shape and/or cross-sectional thickness, and/or by selecting the material of portions of the flexure. Materials that may be selected to provide the lower stiffness flexure include low Young&#39;s modulus beta (β), or near beta (near-β), titanium alloys. 
     Beta titanium alloys are preferable because they provide a material with relatively low Young&#39;s modulus. The deflection of a plate supported at its perimeter under an applied stress is a function of the stiffness of the plate. The stiffness of the plate is directly proportional to the Young&#39;s modulus and the cube of the thickness (i.e., t 3 ). Therefore, when comparing two material samples that have the same thickness and differing Young&#39;s moduli, the material having the lower Young&#39;s modulus will deflect more under the same applied force. The energy stored in the plate is directly proportional to the deflection of the plate as long as the material is behaving elastically and that stored energy is released as soon as the applied stress is removed. Thus, it is desirable to use materials that are able to deflect more and consequently store more elastic energy. 
     Additionally, it is preferable to match the frequency of vibration of a golf club face with the frequency of vibration of a golf ball to maximize the golf ball speed off the face after an impact. The frequency of vibration of the face depends on the face parameters, such as the material&#39;s Young&#39;s modulus and Poisson&#39;s ratio, and the face geometry. The alpha-beta (α-β) Ti alloys typically have a modulus in the range of 105-120 GPa. In contrast, current β-Ti alloys have a Young&#39;s modulus in the range of 48-100 GPa. 
     The material selection for a golf club head must also account for the durability of the golf club head through many impacts with golf balls. As a result, the fatigue life of the face must be considered, and the fatigue life is dependent on the strength of the selected material. Therefore, materials for the golf club head must be selected that provide the maximum ball speed from a face impact and adequate strength to provide an acceptable fatigue life. 
     The β-Ti alloys generally provide low Young&#39;s modulus, but are also usually accompanied by low material strength. The β-Ti alloys can generally be heat treated to achieve increases in strength, but the heat treatment also generally causes an increase in Young&#39;s modulus. However, β-ti alloys can be cold worked to increase the strength without significantly increasing the Young&#39;s modulus, and because the alloys generally have a body centered cubic crystal structure they can generally be cold worked extensively. 
     Preferably, a material having strength in a range of about 900-1200 MPa and a Young&#39;s modulus in a range of about 48-100 GPa is utilized for portions of the golf club head. For example, it would be preferably to use such a material for the face and/or flexure and/or flexure cover of the golf club head. Materials exhibiting characteristics in those ranges include titanium alloys that have generally been referred to as Gum Metals. 
     Although less preferable, heat treatment may be used on β-Ti to achieve an acceptable balance of strength and Young&#39;s modulus in the material. Previous applications of β-titanium alloys generally required heat treating to maximize the strength of the material without controlling Young&#39;s modulus. Titanium alloys go through a phase transition from hexagonal close packed crystal structure α phase to a body centered cubic β phase when heated. The temperature at which this transformation occurs is called the β-transus temperature. Alloying elements added to titanium generally show either a preference to stabilize the α phase or the β phase, and are therefore referred to as α stabilizers or β stabilizers. It is possible to stabilize the β phase even at room temperature by alloying titanium with a certain amount of β stabilizers. However, if such an alloy is re-heated to elevated temperature, below the β-transus temperature, the β phase decomposes and transforms into α phase as dictated by the thermodynamic rules. Those alloys are referred to as metastable β titanium alloys. 
     While the thermodynamic laws only predict the formation of α phase, in reality a number of non-equilibrium phases appear on the decomposition of the β phase. These non-equilibrium phases are denoted by α′, α″, and ω. It has been reported that each of these phases has different Young&#39;s moduli and that the magnitude of the Young&#39;s modulus generally conforms with β&lt;α″&lt;α&lt;ω. Thus, it is speculated that if one desires to increase the strength of β-titanium through heat treatment, it would be advantageous to do it in such a manner that the material includes α″ phase as a preferred decomposition product and we eliminate, or minimize the formation of α and ω phases. The formation of α″ phase is facilitated by quenching from the α+β region on the material phase diagram, which means the alloy should be quenched from below the β-transus temperature. Therefore, preferably a β-Ti alloy that has been heat treated to maximize the formation of α″ phase from the β phase is used for a portion of the golf club head. 
     The heat treatment process is selected to provide the desired phase transformation. Heat treatment variables such as maximum temperature, time of hold, heating rate, quench rate are selected to create the desired material composition. Further, the heat treatment process may be specific to the alloy selected, because the effect of different β stabilizing elements is not the same. For example, a Ti—Mo alloy would behave differently than Ti—Nb alloy, or a Ti—V alloy, or a Ti—Cr alloy; Mo, Nb, V and Cr are all β stabilizers but have an effect of varying degree. The β-transus temperature range for metastable β-Ti alloys is about 700° C. to about 800° C. Therefore, for such alloys the solution treating temperature range would be about 25-50 Celsius degrees below the β-transus temperature, in practical terms the alloys would be solution treated in the range of about 650° C. to about 750° C. Following water quenching, it is possible to age the β-Ti alloys at low temperature to further increase strength. Strength of the solution treated material was measured to be about 650 MPa, while the heat treated alloy had a strength of 1050 MPa. 
     Examples of suitable beta titanium alloys include: Ti-15Mo-3Al, Ti-15Mo-3Nb-0.3O, Ti-15Mo-5Zr-3Al, Ti-13Mo-7Zr-3Fe, Ti-13Mo, Ti-12Mo-6Zr-2Fe, Ti—Mo, Ti-35Nb-5Ta-7Zr, Ti-34Nb-9Zr-8Ta, Ti-29Nb-13Zr2Cr, Ti-29Nb-15Zr-1.5Fe, Ti-29Nb-10Zr-0.5Si, Ti-29Nb-10Zr-0.5Fe-0.5Cr, Ti-29Nb-18Zr—Cr-0.5Si, Ti-29Nb-13Ta-4.6Zr, Ti—Nb, Ti-22V-4Al, Ti-15V-6Cr-4Al, Ti-15V-3Cr-3Al-3Sn, Ti-13V-11Cr, Ti-10V-2Fe-3Al, Ti-5Al-5V-5Mo-3Cr, Ti-3Al-8V-6Cr-4Mo-4-Zr, Ti-1.5Al-5.5Fe-6.8Mo, Ti-13Cr-1Fe-3Al, Ti-6.3Cr-5.5Mo-4.0Al-0.2Si, Ti—Cr, Ti—Ta alloys, the Gum Metal family of alloys represented by Ti+25 mol % (Ta, Nb, V)+(Zr, Hf, O), for example, Ti-36Nb-2Ta-3Zr-0.35O, etc (by weight percent). Near beta titanium alloys may include: SP-700, TIMET 18, etc. 
     In general, it is preferred that a face cup or face insert of the inventive golf club head be constructed from α−β or near-β titanium alloys due to their high strength, such as Ti-64, Ti-17, ATI425, TIMET 54, Ti-9, TIMET 639, VL-Ti, KS ELF, SP-700, etc. Further the rear portion of the golf club body, i.e., the portion other than the face cup, face insert, flexure and flexure cover, is preferably made from α, α−β, or β titanium alloys, such as Ti-8Al-1V-1Mo, Ti-8Al-1Fe, Ti-5Al-1Sn-1Zr-1V-0.8Mo, Ti-3Al-2.5Sn, Ti-3Al-2V, etc. 
     Various manufacturing methods may be used to construct the various components of the golf club head of the present invention. Preferably all of the components are joined by welding. The welding processes may be manual, such as TIG or MIG welding, or they may be automated, such as laser, plasma, e-beam, ion beam, or combinations thereof. Other joining processes may also be utilized if desired or required due to the material selections, such as brazing and adhesive bonding. 
     The components may be created using stamping and forming processes, casting processes, molding processes and/or forging processes. The following are examples of material selections for the portions of the golf club head utilizing stamping and forming processes:
         a) α−β face member+β flexure+α−β rear body   b) β face member+α−β face insert+β flexure+α−β rear body   c) β face member+α−β face insert+β flexure+β rear body   d) β face member+α−β face insert+β flexure+α−β rear body (Heat Treated)
 
The following are examples of material selections for the portions of the golf club head utilizing cast components:
   a) Cast α−β face member+Cast β flexure+Cast α−β rear body   b) Formed α−β face member+Cast β flexure+Cast α−β rear body   c) Formed α−β face member+Cast β flexure+Formed α−β rear body   d) Cast α−β face member+Cast β flexure+Formed α−β rear body
 
The following are examples of material selections for the portions of the golf club head utilizing forged components:
   a) Forged α−β face member+Cast β flexure+Cast α−β rear body   b) Forged α−β face member+Cast β flexure+Formed α−β rear body       

     The density of β alloys is generally greater than the density of α−β or α alloys. As a result, the use of β alloys in various portions of the golf club head will result in those portions having a greater mass. Light weight alloys may be used in the rear portion of the body so that the overall golf club head mass may be maintained in a desired range, such as between about 170 g and 210 g for driver-type golf club heads. Materials such as aluminum alloys, magnesium alloys, carbon fiber composites, carbon nano-tube composites, glass fiber composites, reinforced plastics and combinations of those materials may be utilized. 
     While various descriptions of the present invention are described above, it should be understood that the various features of each embodiment could be used alone or in any combination thereof. Therefore, this invention is not to be limited to only the specifically preferred embodiments depicted herein. Further, it should be understood that variations and modifications within the spirit and scope of the invention might occur to those skilled in the art to which the invention pertains. For example, the face insert may have thickness variations in a step-wise continuous fashion. In addition, the shapes and locations of the slots are not limited to those disclosed herein. Accordingly, all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is accordingly defined as set forth in the appended claims.