Patent Publication Number: US-9839820-B2

Title: Golf club head with flexure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 14/685,266, filed Apr. 13, 2015, which is a continuation-in-part of U.S. patent application Ser. No. 14/584,822, filed on Dec. 29, 2014, which is a continuation-in-part of U.S. patent application Ser. No. 13/844,954, filed on Mar. 16, 2013, now U.S. Pat. No. 8,986,133, which is a continuation-in-part of U.S. patent application Ser. No. 13/720,885, filed on Dec. 19, 2012, now U.S. Pat. No. 8,834,290, which is a continuation-in-part of U.S. patent application Ser. No. 13/618,963, filed on Sep. 14, 2012, now U.S. Pat. No. 8,834,289, the disclosures of which are hereby incorporated by reference in their entireties. 
    
    
     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. 
     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 (CG), 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 head 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 face, a sole, a side wall, a hosel 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 comprises a face transition portion, a rear portion and an aperture interposed between the face transition portion and the rear portion. The face transition portion extends between the face and the aperture. The aperture is defined by a front wall and a rear wall in a central portion of the aperture, and the front wall extends from the face transition portion toward a crown of the golf club head. The face defines a ball striking surface intersecting the sole at a leading edge. The side wall extends between the crown and the sole. The hosel extends from the crown and includes a shaft bore. The flexure is constructed as a separate component and disposed in the aperture. The flexure comprises a front wall, a rear wall and a base that extends between the front and rear walls, and the flexure defines a cavity that is opened to the interior of the golf club head. The aperture and flexure define a flexure ratio (h 1 /h 4 ) of a vertical height of a lower edge of the cavity relative to the ground plane (h 1 ) to the vertical height of a free end of the front wall relative to the ground plane (h 4 ) that is between about 0.25 and about 0.75. 
     In another embodiment, a golf club head comprises a crown, a face, a sole, a side wall, a hosel, and a curved flexure. The crown defines an upper surface of the golf club head. The sole defines a lower surface of the golf club head and comprises a face transition portion, a rear portion and an aperture interposed between the face transition portion and the rear portion. The face transition portion extends between the face and the aperture, and the aperture is defined by a front wall and a rear wall in a central portion of the aperture. The front wall extends from the face transition portion toward a crown of the golf club head by a height between about 5.0 mm and about 8.0 mm. The face defines a ball striking surface intersecting the sole at a leading edge. The side wall extends between the crown and the sole. The hosel extends from the crown and includes a shaft bore. The flexure is constructed as a separate component and disposed in the aperture. The flexure is curved so that it contacts the walls of the aperture at a plurality of contact locations that are spaced from each other by portions of the aperture wall where there is no contact between the curved flexure and the aperture wall. 
     In another embodiment, a golf club head comprises a crown, a face, a sole, a side wall, a hosel and a compliant plate. The crown defines an upper surface of the golf club head. The face defines a ball striking surface. The sole defines a lower surface of the golf club head and comprises a face transition portion, a rear portion and an aperture interposed between the face transition portion and the rear portion. The face transition portion extends between the face and the aperture, and the aperture includes an inner portion and an outer portion that form a shoulder. The ball striking surface intersects the sole at a leading edge. The side wall extends between the crown and the sole. The hosel extends from the crown and includes a shaft bore. The compliant plate is disposed in the outer portion of the aperture and on the shoulder, and includes a plurality of voids. 
    
    
     
       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 golf club head of the present invention; 
         FIG. 2  is bottom plan view of the golf 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 golf 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 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 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; 
         FIG. 15  is a bottom plan view of the golf club head of  FIG. 14 ; 
         FIG. 16  is a perspective view of another embodiment of a golf club head of the present invention; 
         FIG. 17  is an exploded view of the golf club of  FIG. 16 ; 
         FIG. 18  is a cross-sectional view of the golf club of  FIG. 16 ; 
         FIG. 19  is a cross-sectional view of an alternative construction of the golf club head of  FIG. 16 ; 
         FIG. 20  is a perspective view of another embodiment of a golf club head of the present invention; 
         FIG. 21  is an exploded view of the golf club head of  FIG. 20 ; 
         FIG. 22  is a perspective view of an embodiment of a golf club head of the present invention; 
         FIG. 23  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 24  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 25  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 26  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 27  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 28  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 29  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 30  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 31  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 32  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 33  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 34  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 35  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 36  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 37  is a cross-sectional view of a portion of another embodiment of a golf club head of the present invention; 
         FIG. 38  is a bottom view of another embodiment of a golf club head of the present invention; 
         FIG. 39  is a side view of the golf club head of  FIG. 38 ; 
         FIG. 40  is a cross-sectional view of the golf club head of  FIG. 38 , taken along line  40 - 40 ; 
         FIG. 41  is a front view of an embodiment of a golf club head of the present invention; 
         FIG. 42  is a side view of the golf club head of  FIG. 41 ; 
         FIG. 43  is a cross-sectional view of the golf club head of  FIG. 41 , taken along line  43 - 43 ; 
         FIG. 44  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 45  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 46  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 47  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 48  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 49  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 50  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 51  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 52  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 53  is a cross-sectional view of a portion of an embodiment of a golf club head of the present invention; 
         FIG. 54  is a cross-sectional view of a portion of another embodiment of a golf club head of the present invention; 
         FIG. 55  is a cross-sectional view of an embodiment of a golf club head of the present invention; 
         FIG. 56  is a bottom view of the golf club head of  FIG. 55 ; 
         FIG. 57  is a bottom view of another embodiment of a golf club head of the present invention; 
         FIG. 58  is a front view of a golf club head illustrating dimensional characteristics and a coordinate system used herein; 
         FIG. 59  is a top view of the golf club of  FIG. 58 ; 
         FIG. 60  is a cross-sectional view of a portion of the golf club head of  FIG. 58 ; 
         FIG. 61  is bottom plan view of an embodiment of a golf club head according to the present invention; 
         FIG. 62  is a cross-sectional view, corresponding to line  62 - 62  of  FIG. 61 ; 
         FIG. 63  is a cross-sectional view of an alternative embodiment, showing a cross-section generally corresponding to line  62 - 62  of  FIG. 61 , 
         FIG. 64  is a sole view of a golf club head in accordance with an alternative embodiment of the present invention; 
         FIG. 65  is a sole perspective view of the golf club head shown in  FIG. 64 ; 
         FIG. 66  is an exploded view of a golf club head shown in  FIG. 64  from a sole perspective view; 
         FIG. 67  is a cross-sectional view of the golf club head shown in  FIG. 64  taken along cross-sectional line  67 - 67 ; 
         FIG. 68  is an enlarged cross-sectional view of the detail B portion of the golf club head shown in  FIG. 67 ; 
         FIG. 69  is an enlarged cross-sectional view of the detail B portion of a golf club head in accordance with an alternative embodiment of the present invention; 
         FIG. 70  is a cross-sectional view of a golf club head in accordance with a further alternative embodiment of the present invention; 
         FIG. 71  is a sole view of a golf club head in accordance with another alternative embodiment of the present invention; 
         FIG. 72  is an exploded view of a golf club head shown in  FIG. 71  from a sole perspective view; 
         FIG. 73  is a cross-sectional view of a golf club head of the golf club head shown in  FIG. 71  along cross-sectional line  73 - 73 ; 
         FIG. 74  is an exploded cross-sectional view of the golf club head shown in  FIG. 71 ; 
         FIG. 75  is an enlarged cross-sectional view of detail C portion of the golf club head shown in  FIG. 73 ; 
         FIG. 76  is an enlarged cross-sectional view of detail C portion of a golf club head in accordance with an alternative embodiment of the present invention; 
         FIG. 77  shows a perspective view of a golf club head in accordance with a further alternative embodiment of the present invention; 
         FIG. 78  shows a perspective view of a golf club head in accordance with another further alternative embodiment of the present invention; 
         FIG. 79  is bottom plan view of a golf club head of the present invention; 
         FIG. 80  is a cross-sectional view, corresponding to line  80 - 80  of  FIG. 79 ; 
         FIG. 81  is a cross-sectional view of a portion, shown in  FIG. 80  as detail C, of the golf club head of  FIG. 79 , with the cross-hatching removed for clarity; 
         FIG. 82  is another cross-sectional view of a portion, shown in  FIG. 80  as detail C, of the golf club head of  FIG. 79 , with the cross-hatching removed for clarity; 
         FIG. 83  is a side view of a golf club head of the present invention; 
         FIG. 84  is a cross-sectional view, corresponding to line  84 - 84  of  FIG. 83 ; 
         FIG. 85  is a cross-sectional view of a portion, corresponding to line  85 - 85  of  FIG. 84 , with the cross-hatching removed for clarity; 
         FIG. 86  is a cross-sectional view of a portion, corresponding to line  85 - 85  of  FIG. 84 , with the cross-hatching removed for clarity; 
         FIG. 87  is a bottom plan view of a golf club head of the present invention; 
         FIG. 88  is a cross-sectional view, corresponding to line  88 - 88  of  FIG. 87 ; 
         FIG. 89  is a bottom plan view of a golf club head of the present invention; 
         FIG. 90  is a cross-sectional view, corresponding to line  90 - 90  of  FIG. 89 ; and 
         FIG. 91  is bottom plan view of another embodiment of a golf club head of the present invention. 
     
    
    
     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. Additionally, golf club head  10  includes at least one weight mounting feature  34  so that the overall weight of the golf club head can be altered and/or so the location of the center-of-gravity may be altered, and any number of weight mounting features may be included anywhere on the golf club head. 
     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 matter, 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 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. 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. Flexure  76  may be bonded, welded or coupled to sole  64  using mechanical fasteners and the material of flexure  76  may be selected from materials having a plurality of densities, Young&#39;s moduli and dimensions to provide a plurality of flexures having different masses and stiffnesses. Furthermore, constructing the flexure as a separate component allows the repair of a broken flexure by replacing the flexure, and it allows the flexure to be constructed from different processes compared to the remainder of the golf club head such as by forging the flexure and casting the remainder of the golf club head. 
     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 material, 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. Preferably, the flexure and cover are constructed from titanium alloys, such as beta-titanium alloys, and have widths between about 2.0 mm and about 20.0 mm, and thicknesses between about 0.35 mm to 2.0 mm. 
     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. 
     In effect, cover provides crown compliance and the flexure provides sole compliance. As a further alternative, the cover may be removed from the flexure so that it only provides compliance in portions of the golf club head that are away from the sole. In such an example, the dimensions of the components are preferably in the ranges described with regard to  FIGS. 11-13 . 
     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. 
     In additional embodiments, the flexure is provided primarily by a multi-material construction. Referring to  FIGS. 16-18 , a golf club head  180  generally has a hollow body construction that is defined by a sole  182 , a crown  184 , a skirt  186 , a face  188  and a hosel  190 , and includes a flexure  192 . Flexure  192  is included in a forward portion of golf club head  180  and may be constructed as a tubular member, as shown, that is interposed between a face portion  194  and a rear body portion  196  so that it forms an intermediate ring. The ring has a selected stiffness to allow the face to deflect globally in concert with the deflection that occurs locally at the impact point. Similar to previous embodiments, flexure  192  is tuned so the impact imparts a frequency of vibration across the flexure that is about 2900 Hz to about 4000 Hz. The properties of the ring are selected as an additional means of controlling and optimizing the COR, and corresponding characteristic time (CT), values across the face, especially for ball impacts that are away from the ideal impact location. 
     Flexure  192  is constructed of a material that provides a lower Young&#39;s Modulus than the adjacent portions of face portion  194  and rear body portion  196 . Preferably, flexure  192 , face portion  194 , and rear body portion  196  are constructed from materials that can be easily coupled, such as by welding. For example, face portion  194  and rear body portion  196  are preferably constructed from a first titanium alloy and flexure  192  is constructed from a beta-titanium alloy as described in greater detail below. Flexure  192  may be constructed so that it has a thickness that is about equal to the thickness of the adjacent portions and so that the outer surface of flexure is flush with the outer surface of the adjacent portions, as shown in  FIG. 18 . Alternatively, as shown in  FIG. 19 , a flexure  192   a  may be constructed so that the thickness is different than the adjacent portions and so that the outer surface of flexure  192   a  is recessed compared to the adjacent portions. As further alternatives, the flexure may be constructed so that the outer surface of the flexure is proud, or raised, compared to the adjacent portions. 
     Alternatively, a carbon composite ring may be incorporated for flexure  192  that provides a lower stiffness. The joint configuration, ring geometry (such as the ring width and thickness which may vary with the location in the ring), ring position, fiber orientation, resin type and percentage resin content are all parameters that are selected to optimize the flexibility of flexure  192  so that the outgoing ball speed is improved across the face of the driver while the durability of the golf club head is maintained. Preferably, a carbon composite flexure is bonded to an adjacent metallic face portion and an adjacent metallic rear body portion. As an example, the flexure may be a ring having a width in a range of about 12.0 mm to about 20.0 mm and a thickness of about 0.5 mm to about 3.0 mm and the thickness may vary depending on the location around the perimeter. 
     A multi-material flexure is incorporated into the golf club head of  FIGS. 20 and 21 . A golf club head  200  includes a flexure  202  that primarily relies upon the material properties to alter the stiffness, similar to flexure  192 , but incorporates a multi-material construction. Golf club head  200  is generally constructed as a hollow body that is defined by a face portion  204 , flexure  202  and rear body portion  206 . When face portion  204 , flexure  202  and rear body portion  206  are coupled, they generally form a face  208 , a crown  210 , a sole  212 , a skirt  214  and a hosel  216 . 
     Flexure  202  includes a front member  218 , a central member  220 , and an aft member  222 . Preferably, the materials are chosen so that front member  218  and aft member  222  are easily coupled to face portion  204  and rear body portion  206  and so that central member  220  is thin and flexible enough to provide an extra vibration mode having a frequency in a range of about 2900 Hz to about 4000 Hz. In an embodiment, front member  218  and aft member  222  are metallic, and central member  220  is interposed between front member  218  and aft member  222  and is constructed of a carbon fiber composite. Preferably, aft member  222  is spaced from an interface between face  208  and front member  218  by at least 6.0 mm and more preferably, at least 12.0 mm. Hosel  216  may be constructed of metallic and/or non-metallic materials. In an embodiment, face portion  204  and rear body portion  206  are constructed of a titanium alloy, front member  218  and aft member  222  are constructed of a lower density, and preferably lower modulus, material than titanium, such as an aluminum or magnesium alloy, and central member  220  is constructed of a carbon fiber composite that is thin and flexible enough to provide the desired frequency response. Additionally, the front member and/or the aft member may be co-molded with the composite central member. Generally, the materials are selected to provide adequate bonding strength between the components using common practices, such as adhesive bonding. 
     Golf club heads of the present invention may also include a flexure that extends across the interface between the rear portion of the golf club head and the face, as shown in  FIGS. 22 and 23 . A golf club head  230  generally has a hollow body construction that is defined by a sole  232 , a crown  234 , a skirt  236 , a face  238  and a hosel  240 , and includes a flexure  242 . Flexure  242  is included in a forward portion of golf club head  230  and is interposed between face  238  and sole  232 , crown  234  and skirt  236 . 
     The flexure has a selected stiffness to allow the face to deflect globally in concert with the deflection that occurs locally at the impact point. Similar to previous embodiments, flexure  242  is tuned so impact imparts a frequency of vibration across the flexure that is about 2900 Hz to about 4000 Hz. The properties of the ring are selected as an additional means of controlling and optimizing the COR, and corresponding characteristic time (CT), values across the face, especially for ball impacts that are away from the ideal impact location. 
     Flexure  242  is located generally around the perimeter of face  238  and so that it extends across the transitional curvature from the face of golf club head  230  to the rear portion of the golf club head, e.g., sole  232 , crown  234  and skirt  236 . Flexure  242  may be discontinuous, as shown, so that it is interrupted by the hosel portion of the golf club head. Flexure  242  terminates at flanges that provide coupling features for mounting flexure  242  in golf club head  230 . It should be appreciated that coupling features may be surfaces provided to form butt joints, lap joints, tongue and groove joints, etc. Flexure  242  includes a face flange  244  and a rear flange  246 . Face flange  244  is coupled to a perimeter edge  248  of face  238 . Portions of rear flange  246  are coupled to portions of perimeter edges of sole  232 , crown  234  and skirt  236 , such as by being coupled to a crown flange  250  and a sole flange  252 . Preferably, the face and rear flanges are between about 2.0 mm and about 12.0 mm. 
     Flexure  242  is preferably constructed of a material that provides a lower Young&#39;s modulus than the adjacent portions of the golf club head. Preferably, flexure  242 , face  238 , and the rear portion of golf club head  230  are constructed from materials that can be easily coupled, such as by welding. For example, face  238  and the rear portion are preferably constructed from a first titanium alloy and flexure  242  is constructed from a beta-titanium alloy as described in greater detail below. 
     Alternatively, flexure  242  may be constructed from a carbon fiber composite ring that provides a lower stiffness. The joint configuration, ring geometry, ring position, fiber orientation, resin type and percentage resin content are all parameters that are selected to optimize the flexibility of flexure  242  so that the outgoing ball speed is improved across the face of the driver while the durability of the golf club head is maintained. Preferably, a carbon composite flexure is bonded to an adjacent metallic face and an adjacent metallic rear body portion. 
     In another embodiment, shown in  FIG. 24 , a flexure is coupled to a face member at the transition between the face and the rear portion of the golf club head. For example, a golf club head  260  generally has a hollow body construction that is defined by a sole  262 , a crown  264 , a skirt  266 , a face  268 , a hosel, and a flexure  272 . Flexure  272  is included in a forward portion of golf club head  260  and is generally constructed as an annular member that is interposed between face  268 , and sole  262 , crown  264  and skirt  266 . 
     Similar to previous embodiments, flexure  272  is tuned so impact imparts a frequency of vibration across the flexure that is about 2900 Hz to about 4000 Hz. Flexure  272  is located around the perimeter of face  268  and so that it extends across the transitional curvature from the face of golf club head  260  to the rear portion of the golf club head, e.g., sole  262 , crown  264  and skirt  266 . Flexure  272  terminates at flanges that provide examples of coupling features for mounting flexure  272  in golf club head  260 . In particular, flexure  272  includes a face flange  274  and a rear flange  276 . Face flange  274  is coupled to a perimeter flange  278  of face  268 . Portions of rear flange  276  are coupled to portions of perimeter edges of sole  262 , crown  264  and skirt  266 , such as by being coupled to a crown flange  280  and a sole flange  282 . 
     Flexure  272  is preferably constructed of a material that provides a lower Young&#39;s modulus than the adjacent portions of the golf club head. Preferably, flexure  272 , face  268 , and the rear portion of golf club head  260  are constructed from materials that can be easily coupled, such as by welding. For example, face  268  and the rear portion are preferably constructed from a first titanium alloy and flexure  272  is constructed from a beta-titanium alloy as described in greater detail below. 
     In another embodiment, shown in  FIG. 25 , a golf club head  290  includes interface members that are included that are used to couple a flexure  292  to adjacent portions of golf club head  290 . A front interface member  294  is interposed between flexure  292  and a face member  296 . Similarly, an aft interface member  298  is interposed between flexure  292  and an aft body member  300 . 
     In the present embodiment, front interface member  294  and aft interface member  298  are both constructed as annular members that are interposed between the adjacent components. Front interface member  294  includes a face flange  302  that is coupled to face member  296  with a lap joint, and a flexure flange  304  that is coupled to flexure  292  with a lap joint. A portion of front interface member  294  is exposed and forms a portion of the front surface of golf club head  290 . Interface member  294  spaces a forward edge of flexure  292  from a perimeter edge of face member  296 . Aft interface member  298  includes a rear body flange  306  that is coupled to aft body member  300  and a flexure flange  308  that is coupled to flexure  292 . Aft interface member  298  space aft body member  300  and flexure  292 . 
     Golf club head  290  has a multi-material construction. In an example, aft body member  300  and face member  296  are constructed of titanium alloys, and may be constructed of the same titanium alloy, such as Ti6-4. Front interface member  294  and aft interface member  298  are constructed of a material selected to be coupled to the materials of face member  296 , flexure  292  and aft body member  300 . In an example, the interface members are constructed of an aluminum alloy and flexure is constructed from a carbon fiber composite. It should further be appreciated, that the interface member  298  need not be constructed with a constant cross-sectional shape. 
     A golf club head  320 , shown in  FIG. 26 , includes interface members that are used to couple a flexure  322  to adjacent portions of golf club head  320 . A front interface member  324  is interposed between flexure  322  and a face member  326 . Similarly, an aft interface member  328  is interposed between flexure  322  and an aft body member  330 . 
     Front interface member  324  and aft interface member  328  are both constructed as annular members that are interposed between the adjacent components. Front interface member  324  includes a face flange  332  that is coupled to face member  326  with a lap joint. Front interface member  324  also includes a flexure flange  334  that is coupled to a front flange  340  of flexure  322 . A portion of front interface member  324  is exposed and forms a portion of the front surface of golf club head  320 . Interface member  324  spaces a forward edge of flexure  322  from a perimeter edge of face member  326 . Aft interface member  328  includes a rear body flange  336  that is coupled to aft body member  330  and a flexure flange  338  that is coupled to flexure  322 . Aft interface member  328  spaces aft body member  330  and flexure  322 . 
     Golf club head  320  has a multi-material construction. In an example, aft body member  330  and face member  326  are constructed of titanium alloys, and may be constructed of the same titanium alloy, such as Ti6-4. Front interface member  324  and aft interface member  328  are constructed of a material selected to be coupled to the materials of face member  326 , flexure  322  and aft body member  330 . In an example, the interface members are constructed of an aluminum alloy and flexure is constructed from a carbon fiber composite. 
     Referring to  FIG. 27 , a golf club head  350  includes a flexure  352  that is spaced from the transition between the rear portion of the golf club and a face  354 . Generally, golf club head  350  has a hollow body construction that is defined by a sole  356 , a crown  358 , a skirt  360 , face  354 , a hosel, and flexure  352 . 
     Flexure  352  is interposed between face  354  and a rear portion of golf club head  350 . Flexure  352  is generally an annular member that has a U-shaped cross-sectional shape so that it includes a forward flange  362  and an aft flange  364 . Forward flange  362  is coupled to a face flange  366  of face  354 , and aft flange  364  is coupled to a flange of the rear portion of the golf club that includes a crown flange  368  and a sole flange  370 . 
     Embodiments are illustrated in  FIGS. 28 and 29  that are similar to that of  FIG. 27 , but include alternative flange configurations. As shown in  FIG. 28 , a golf club head  380  has a hollow body construction that is defined by a sole  382 , a crown  384 , a skirt  386 , face  388 , a hosel, and flexure  390 . Flexure  390  is interposed between face  388  and the rear portion of the golf club head that includes sole  382  and crown  384 . Flexure  390  is a generally annular member that includes a forward coupling portion  392  and an aft flange  394 . Forward coupling portion  392  is a portion of flexure  390  that wraps around and is coupled to a face flange  396 , so that it receives at least a portion of face flange  396 . Portions of aft flange  394  abut and are coupled to a sole flange  398  and a crown flange  400 . 
     As shown in  FIG. 29 , a golf club head  410  has a hollow body construction that is defined by a sole  412 , a crown  414 , a skirt  416 , face  418 , a hosel, and flexure  420 . Flexure  420  is interposed between face  418  and the rear portion of the golf club head that includes sole  412  and crown  414 . Flexure  420  is a generally annular member that includes a forward flange  422  and an aft flange  424 . Forward flange  422  abuts, and is coupled to, a face flange  426 . Portions of aft flange  424  abut and are coupled to a sole flange  428  and a crown flange  430 . 
     The configuration of the flexure of each of the embodiments may be selected from many different alternatives to provide a tuned behavior during impact with a golf ball.  FIGS. 30-34  illustrate various alternative multi-piece constructions of a flexure. In particular, the illustrated flexures include flexure components that have various alternative geometries. For example, a flexure  440  of  FIG. 30 , includes an angular cross-sectional shape that includes a flexure component  442  that is generally formed as an L-shaped member. Flexure component  442  is coupled to a forward flange  444  and an aft flange  446  of a golf club body  448 . As shown, forward flange  444  and aft flange  446  are convergent flanges that are angled toward each other. Forward flange  444  and aft flange  446  are integrated into a sole  450  of golf club head body  448  generally in a location near a face  452  of the golf club head. As mentioned previously, flexure  440  is preferably located within about 20 mm of the ball-striking surface of face  452 , and more preferably between about 5.0 mm and about 20.0 mm. Flexure component  442  may be coupled to forward flange  444  and aft flange  446  by any mechanical coupling process, such as welding, brazing, mechanical fasteners, diffusion bonding, liquid interface diffusion bonding, super plastic forming and diffusion bonding, and/or using an adhesive. A construction that allows for access to the internal cavity of the golf club head during manufacture, such as a crown pull construction or a face pull construction, so that the coupling process may be easily accomplished. 
     In another embodiment, shown in  FIG. 31 , a flexure  440  that has a wavy, or corrugated, cross-sectional shape is included in a golf club head  462 . Flexure  440  is constructed from a flexure component  464  that is coupled to a forward flange  466  and an aft flange  468  of golf club head  462 . Forward flange  466  and aft flange  468  are integrated into a sole  472  of golf club head body  462  generally in a location near a face  470  of the golf club head. As mentioned previously, flexure  460  is preferably located within about 20 mm of the ball-striking surface of face  470 , and more preferably between about 5.0 mm and about 20.0 mm. Flexure component  464  may be coupled to forward flange  466  and aft flange  468  by any mechanical coupling process, such as welding, brazing, mechanical fasteners and/or using an adhesive. 
     In additional embodiments, a flexure is formed from flanges and a generally channel-shaped flexure component. Referring to  FIG. 32 , a golf club head  480  includes a flexure  482  that is formed by a flexure component  484  that is coupled to flanges of a sole  492  of golf club head  480 , such as by welding, brazing and/or an adhesive. Flexure  482  is preferably located within about 20 mm of the ball-striking surface of a face  494 , and more preferably between about 5.0 mm and about 20.0 mm. In particular, flexure component  484  is a generally channel-shaped member that includes recesses  486  that receive portions of a forward flange  488  and an aft flange  490 . Recesses  486  are spaced by a portion of flexure component  484  that is selected to provide a desired spacing between forward flange  488  and aft flange  490 . 
     In a similar embodiment, illustrated in  FIG. 33 , a golf club head  500  includes a flexure  502  that is formed by a flexure component  504  that has a channel-shaped cross section. Flexure component  504  is coupled to flanges formed on a sole  506  of golf club head  500 , such as by welding, brazing and/or an adhesive. Flexure  502  is preferably located within about 20 mm of the ball-striking surface of a face  508 , and more preferably between about 5.0 mm and about 20.0 mm. In particular, flexure component  504  is a generally channel-shaped member that defines a slot that receives portions of a forward flange  510  and an aft flange  512 . 
     In another embodiment, illustrated in  FIG. 34 , a golf club head  520  includes a flexure  522  that is formed by a flexure component  524  that has a channel-shaped cross section. Flexure component  524  is constructed having a generally sharktooth-shaped cross section, and in particular includes a first curved portion and a generally planar portion that meet at an apex. Flexure component  524  is coupled to flanges formed on a sole  526  of golf club head  520 , such as by welding, brazing and/or an adhesive. Flexure  522  is preferably located within about 20 mm of the ball-striking surface of a face  528 , and more preferably between about 5.0 mm and about 20.0 mm. In particular, flexure component  524  is a generally channel-shaped member that defines a slot that receives portions of a forward flange  530  and an aft flange  532 . 
     Referring to  FIG. 35 , another embodiment of a golf club head  540  includes a flexure  542  that is similar in shape to the embodiment illustrated in  FIG. 34 , but flexure  542  extends outward from a sole  546  of the golf club head. Flexure  542  is formed by a flexure component  544  that has a cross section that forms a channel. Flexure component  544  is constructed having a generally sharktooth-shaped cross-sectional shape, and in particular includes a first curved portion and a generally planar portion that meet at an apex. Flexure component  544  is coupled to flanges formed on sole  546  of golf club head  540 , such as by welding, brazing and/or an adhesive. Flexure  542  is preferably located within about 20.0 mm of the ball-striking surface of a face  548 , and more preferably between about 5.0 mm and about 20.0 mm. 
     In another embodiment, illustrated in  FIG. 36 , a golf club head  560  includes a flexure  562 . Flexure  562  is formed by a flexure component  564  that has a generally tubular cross-section. Flexure component  564  is constructed having a generally tubular cross-sectional shape, and although it is illustrated as having an annular cross-sectional shape, it should be appreciated that it may have any cross-sectional shape. Flexure component  564  is coupled to flanges  568  formed on sole  566  of golf club head  560 , such as by welding, brazing and/or an adhesive. Flexure component  564  has an exterior shape that complements flanges  568  and provides a coupling surface so that flexure component  564  may be coupled to flanges  568 . Flexure  562  is preferably located within about 20.0 mm of the ball-striking surface of a face  570 , and more preferably between about 5.0 mm and about 20.0 mm. 
     Referring to  FIG. 37 , in an additional embodiment, a golf club head  580  includes a flexure  582 . Flexure  582  is similar in shape to the embodiment illustrated in  FIG. 34 , but flexure  582  is oriented so that the generally sharktooth-shaped cross-section is reversed. In particular, the curved portion of flexure  582  is further rearward than in other illustrated embodiments. As shown, flexure  582  is formed by a flexure component  584  that has a cross section that forms a channel, but it should be appreciated that flexure  582  may be formed as a monolithic structure with a sole  586  of golf club head  580 . By altering the orientation of the flexure relative to the remainder of the golf club head, the stress exerted on the flexure is applied in an alternative direction and the behavior of the flexure is different so that the flexure is effectively stiffer. As a result, the flexure may be tuned for the golf club head by altering the orientation. Flexure component  584  is coupled to flanges formed on sole  586  of golf club head  580 , such as by welding, brazing and/or an adhesive. Flexure  582  is preferably located within about 20.0 mm of the ball-striking surface of a face  588 , and more preferably between about 5.0 mm and about 20.0 mm, and has a thickness that is preferably between about 0.35 mm and 2.0 mm. 
     Referring to  FIGS. 38-40 , a golf club head  600  includes an elongate cavity that provides a flexure  602  that may be tuned to provide a desired compliance. For example, the golf club head includes a compliant tube that may be filled, or partially filled, with a compliant material, to adjust sound, feel and compliance, or left empty. Golf club head  600  includes a crown  604 , a sole  606 , a skirt  608 , a face  610  that defines a ball-striking surface  611 , and a hosel  612  that combine to form hollow-bodied golf club head construction that defines an interior cavity  614 . Flexure  602  is an elongate tubular structure that extends generally in a heel-to-toe direction, and defines a flexure cavity  613 . In an embodiment, flexure  602  extends across golf club head  600  so that it intersects a vertical, fore-aft plane extending through the geometric center of the face of golf club head  600  when the golf club head is in the address position. 
     An aperture  616  is included that provides access to the interior of flexure  602  and may be closed with a cover  618  that is preferably removeably coupled to flexure  602  in aperture  616 . As an example, aperture  616  may be threaded and cover  618  is threaded into aperture  616  and includes a tool engagement feature that allows cover  618  to be installed and removed. 
     As a further alternative, flexure  602  may be completely or partially filled with an insert  603 , such as a high density elastomeric insert. For example, an elastomeric material that is infused with a high density material, such as Tungsten, to create a high density flexible insert with is inserted into the tubular flexure, or into one of the other embodiments described herein including open slots, behind the face. The insert may be used to fill, or partially fill, the flexure to alter the acoustic behavior of the golf club head. A plurality of inserts constructed from materials with different densities and/or having different weight distributions may be provided to create inserts that fit into the flexure with different masses and weight distributions so that the final weight and mass distribution of the golf club head may be selected. Further, the flexure may include an opening that extends into the interior cavity and the insert may be used to plug the opening so that the interior cavity is not exposed to the environment so debris and water are not able to enter the interior cavity. Exemplary suitable materials include polyurethane, rubber, thermoset polymers, thermoplastic polymers, epoxy, foam, and neoprene. The selected material has a hardness that is selected to combine with the flexure to provide a combined flexibility. Preferably, the selected material has a hardness generally in a Durometer A range of 30-95 or a Durometer D range of 45-85. 
     Referring to  FIGS. 41-43 , another embodiment of a golf club head  620  including a flexure  622  that extends outward from a sole  624  of the golf club head will be described. Golf club head  620  is constructed with a crown  626 , sole  624 , a skirt  628 , a face  630  that defines a ball-striking surface  631 , and a hosel  632  that combine to form a hollow-body construction and to define an interior cavity  634 . In the present embodiment, flexure  622  extends across sole  624 , across skirt  628 , and across crown  626  continuously so that it wraps over the toe portion of skirt  628  of golf club head  620 . 
     In additional embodiments, a sole plate is integrated into the golf club head and is at least partially integrated into a flexure. As illustrated in  FIG. 44 , a golf club head  640  includes a crown  642 , a sole  644 , a face  646 , a skirt  648  and a sole plate  650  that combine to form a hollow body defining an inner cavity  651 . Sole  644  and sole plate  650  combine to form a flexure  652 . Flexure  652  is a channel-shaped feature that extends in a generally heel-to-toe direction and is formed from a first member  654 , a second member  656 , and sole plate  650 . First member  654  is coupled to a rearward edge of a forward transmittal portion  658  of sole  644  and curves into inner cavity  651  from sole  644 . Second member  656  is coupled to a forward edge of a rearward portion of sole  644  and also curves into inner cavity  651  from sole  644 . The ends of first member  654  and second member  656  that are spaced away from sole  644  are coupled to each other at an apex  660 . A second, lower, end of second member  656  is joined with a forward portion of sole plate  650  to complete the rear portion of flexure  652  that extends from apex  660  to a lower, outer sole surface of golf club head  640 , so that the depth of flexure  652  is greater than the thickness of sole plate  650 . 
     In fairway wood or hybrid embodiments, which are generally constructed to provide a ground-contacting surface, sole  644  has a generally stepped configuration so that only the forward transmittal portion  658  of sole  644  provides a ground surface contacting surface, and the remainder of the ground contacting surface is provided by a lower surface of sole plate  650 . Preferably, the flexure is elongate and extends in a generally heel to toe direction. 
     Additionally, in this embodiment and following examples, the material of the sole plate is selected to provide a desired mass distribution in the golf club head, and the material may have a higher or lower density than the remainder of the body material. For example, because the sole plate is generally integral with a flexure that is relatively close to the face of the golf club head, it may be beneficial to utilize a high density material for fairway and hybrid embodiments to maintain the center of gravity of the golf club head low, while a lower density material may be beneficial in driver embodiments so that material mass that would otherwise be dedicated to the sole structure may be distributed to the perimeter of the golf club head. The sole plate material is preferably selected from aluminum, titanium, magnesium, zirconium, steel, tungsten, and the sole plate may be coupled to the golf club head body by fasteners, brazing, welding, adhesives or any other suitable attachment method. In an example, a fairway wood may be constructed using titanium for the majority of the body while a steel or tungsten sole plate is brazed to the titanium body. 
     In another embodiment, shown in  FIG. 45 , a golf club head  670  is constructed similar to that of  FIG. 44  so that it includes a sole plate  672  that forms a portion of a flexure  674 , but in the present embodiment, sole plate  672  is received in a recessed portion of a sole  676  of golf club head  670 . Golf club head  670  is generally hollow and is constructed from a crown  678 , sole  676 , a face  680 , a skirt  682  and sole plate  672  that combine to form a hollow body defining an inner cavity  684 . 
     Flexure  674  is generally formed from a first member  686 , a second member  688 , and sole plate  672 . First member  686  is coupled to a rearward edge of a forward transmittal portion  690  of sole  676  and curves into inner cavity  684  from sole  676 . Second member  688  is coupled to a forward edge of a rearward portion of sole  676  and also curves into inner cavity  684  from sole  676 . The ends of first member  686  and second member  688  that are spaced away from sole  676  are coupled to each other at an apex  692 . A second, lower, end of second member  688  is joined with a forward portion of sole plate  672  to complete the rear portion of flexure  674  that extends from apex  692  to a lower, outer sole surface of golf club head  670 . 
     Sole  676  and second member  688  combine to form a recess in the lower wall of golf club head  670  that receives sole plate  672 . In particular the lower end of second member  688  extends below the junction between second member  688  and sole  676  to form a shoulder, such as tab  689 , which extends below the adjacent lower surface of sole  676 . As a result, in fairway wood and hybrid embodiments that utilize the lower surface for ground contact, the forward transmittal portion  690 , sole plate  672 , and a rear portion of sole  676  provide the ground-contacting lower surface of golf club head  670 . 
     Referring to  FIG. 46 , another embodiment of a golf club head is illustrated that includes a sole plate. Golf club head  700  includes a sole plate  702  that is coupled to a sole  704  and that forms a portion of a flexure  706 . Flexure  706  is constructed from a first member  708 , a second member  710  and a portion of sole plate  702 . First member  708  and second member  710  extend into an interior cavity of golf club head  700  and meet at an apex  712 . The lower end of second member  710  extends below the junction between second member  710  and sole  704  to form a shoulder, or tab  714 , that complements and engages a shoulder  716  of sole plate  702 . Sole  704  has a stepped configuration so that sole plate  702  provides the lowest surface of golf club head  700 . 
     In another embodiment, shown in  FIG. 47 , a golf club head  720  includes a sole plate  722  that covers an aperture  724  included in a sole  726  of golf club head  720  and forms a portion of a flexure  730 . Aperture  724  may be used to provide access to an interior cavity of the golf club head, to locate sole plate  722 , and/or to allow for greater adjustment in the mass of sole plate  722  while maintaining the overall outer shape of golf club head  720 . For example, sole plate  722  may include a projection  728  that increases the mass of sole plate  722  and that extends into aperture  724  and/or into the interior cavity. 
     In another embodiment, illustrated in  FIG. 48 , a golf club head  740  includes a sole plate  742  that covers an aperture  744  included in a sole  746  of golf club head  740  and provides a weight port for coupling a weight member  748  to the golf club head. Preferably, the weigh port is located so that changing, or removing, weight member  748  does not alter the location of the center of gravity of the combined sole plate  742  and weight member  748  to provide a more effective mechanism to alter the swingweight of a golf club including golf club head  740 . In particular, sole plate  742  includes a mounting feature, such as a threaded bore, that is coupled to a removable weight member  748 . 
     As a further alternative, any of the open flexures described herein may be completely or partially filled with an insert, such as insert  743 , which may be a high density elastomeric insert. For example, an elastomeric material that is infused with a high density material, such as Tungsten, to create a high density flexible insert with is inserted into the tubular flexure, or into one of the other embodiments described herein including open slots, behind the face. The insert may be used to fill, or partially fill, the flexure to alter the acoustic behavior of the golf club head. A plurality of inserts constructed from materials with different densities and/or having different weight distributions may be provided to create inserts that fit into the flexure with different masses and weight distributions so that the final weight and mass distribution of the golf club head may be selected. Further, the flexure may include an opening that extends into the interior cavity and the insert may be used to plug the opening so that the interior cavity is not exposed to the environment so debris and water are not able to enter the interior cavity. Exemplary suitable materials include polyurethane, rubber, thermoset polymers, thermoplastic polymers, epoxy, foam, and neoprene. The selected material has a hardness that is selected to combine with the flexure to provide a combined flexibility. Preferably, the selected material has a hardness generally in a Durometer A range of 30-95 or a Durometer D range of 45-85. 
     Referring to  FIG. 49 , an embodiment of a golf club head including a sole plate and a flexure will be described. Golf club head  750  includes a crown  752 , a sole  754 , a skirt  756 , a face  758 , and a sole plate  760 . A recess  762  is included in sole  754  that receives sole plate  760 , but is shaped so that a gap is formed between a forward wall  764  of recess  762  and a forward end of sole plate  760 , when sole plate  760  is installed. As a result, the gap forms a flexure  766  in the lower portion of the golf club head close to face  758 . 
     In another embodiment, shown in  FIG. 50 , a golf club head  770  includes a stepped sole  772  and a sole plate  774  that combine to form a flexure  775 . Sole  772  includes a front transmittal portion  778  that extends from a face  776  rearward toward a transition wall  780  of sole  772  that forms a forward wall of flexure  775 . Sole plate  774  is coupled to sole  772  so that it is spaced from transition wall  780  to form flexure  775 . Sole plate  774  extends rearward from transition wall  780  and desired distance as indicated by the dashed line. 
     Another embodiment of a golf club head includes a recessed sole and a sole plate that combine to form a flexure, and a portion of the golf club is shown in  FIG. 51 . Golf club head  790  includes a sole  792  that defines a recess  794  that receives a sole plate  796  and the sole and the sole plate combine to define a flexure  800 . In particular, sole  792  includes a forward transmittal portion  798  that extends between a face  802  of the golf club head and a transition wall  804  that extends inward from the forward transmittal portion  798  and forms a portion of recess  794 . Sole plate  796  is received in recess  794  and coupled to sole  792  so that the forward portion of sole plate  796  is spaced from transmittal portion  798  so that a generally V-shaped gap is formed at flexure  800 . 
     Referring to  FIG. 52 , an embodiment of a golf club head  810  that includes a flexure  812  and flexure tuning features. Golf club head  810  includes a crown  814 , a sole  816 , a skirt  818 , and a face  820  that defines a ball-striking surface  822 . Sole  816  includes a front transmittal portion  824  that extends rearward from face  820  toward a front wall  826  of flexure  812 . Front wall  826  is coupled to a rear wall  828  at an apex  830  to form flexure  812 . A rear portion of sole  816  extends rearward from rear wall  828  and forms the remainder of sole  816 . As illustrated, the rear portion of sole  816  may have a thickness that varies, such as by including a thickened region  832  spaced rearward from flexure  812  by an isolation portion  834 . 
     Flexure  812  is elongate and extends in a heel-to-toe direction and forms an exterior channel in sole  816 . The thickness of transmittal portion  824 , front wall  826 , apex  830 , rear wall  828 , and isolation portion  834  are selected to tune the flexure  812  to a desired frequency of vibration during impact with a golf ball. Thicknesses t 1 -t 7  are defined having a specific relationship so that transmittal portion  824  transitions from a first thickness t 1  adjacent the face to a second thickness t 2  adjacent front wall  826 . Front wall  826  varies in thickness from approximately t 2  where it is coupled to transmittal portion  824  to a central thickness t 3  and to a thickness approximately equal to a thickness t 4  of apex  830 . Similarly, rear wall  828  varies in thickness from approximately t 4  where it joins apex  830  to a central thickness t 5  and to a thickness approximately equal to a thickness t 6  of isolation portion  834 . Rearward of isolation portion  834 , the thickness of sole  816  varies from thickness t 6  of isolation portion to thickness t 7 . 
     As described above, the flexibility added to golf club heads of the present invention having flexures located in the sole reduces the backspin for ball impacts located below the ideal impact location. Because of that reduction in backspin, the curvature of the ball-striking surface of the golf club head is different above and below the ideal impact location so that the launch of the golf ball may be tuned to the amount of backspin reduction. The curvature of the ball-striking surface of a golf club between the top edge of the face and the leading edge of the golf club is defined as the “roll” of the face. The golf club heads of the present invention preferably have a roll radius above the ideal impact location that is different than the roll radius below the ideal impact location. Alternatively, the roll radius above the geometric face center of the golf club face is different than the roll radius below the geometric face center of the golf club face. As a further alternative, the upper ⅔ of the face of the golf club head has a roll radius that is different than the lower ⅓ of the face. Preferably, the roll radius of the portion of the ball-striking surface closer to the flexure is greater than the portion of the face further from the flexure so that the portion of the ball-striking surface closer to the flexure is flatter than the other portion. For example, in golf club head  810 , flexure  812  is located in the lower surface of the golf club head and a portion of the ball-striking surface below the ideal impact location has a roll radius R 1  that is greater than the roll radius R 2  of the portion of the ball-striking surface above the ideal impact location. Preferably the portion of the ball-striking surface closest to the flexure has a roll radius that is greater than about 12.0 inches, and more preferably greater than 12.5 inches. 
     Similarly, the curvature of the ball-striking surface of a golf club between the heel and toe of the face is defined as the “bulge” of the face. Golf club heads of the present invention that include a flexure that extends to the skirt of the golf club head provide a similar reduction in sidespin of a struck golf ball for off-center impacts and therefore have a bulge radius that is greater than a golf club head without a flexure on the skirt. Increasing the bulge radius creates a flatter face increasing the hot spot area of the golf club face by reducing the obliqueness of impact for off-center hits to provide a more efficient transfer of energy between the golf club head and the ball. Preferably, the portion of the ball striking surface closest to a flexure in the skirt of the golf club head has a bulge radius that is greater than about 12.0 inches, and more preferably greater than 12.5 inches. 
     Alternative embodiments of the thickness transitions are illustrated in  FIGS. 52-54 . The thickness relationships used herein are utilized to provide a desired distribution of flexing throughout the flexure and the portions of the golf club head adjacent the flexure. In an embodiment shown in  FIG. 52 , the thickness in the transmittal portion t 1  and t 2  are at least 50% of the minimum face thickness, and more preferably at least 60% of the minimum face thickness, and preferably thickness t 1  is greater than t 2  (t 1 &gt;t 2 ). Additionally, the thickness of the front wall t 3  and the thickness of the rear wall t 5  of the flexure are different by less than 40%, more preferably by less than 30%, and even more preferably by less than 20%. Furthermore, the thicknesses of the front wall t 3  and rear wall t 5  of the flexure are preferably less than 90% of the minimum thickness of the face, and the thicknesses of the walls of the flexure are preferably less than or equal to the thickness of the transmittal portion t 1 , t 2 . The apex of the flexure preferably has a thickness that is preferably greater than or equal to the minimum thickness of the front wall t 3  and the thickness of the rear wall t 5  of flexure. Additionally, the thickness of the apex t 4  is preferably within 30% of the larger of the thickness of front wall t 3  and the thickness of the rear wall t 5 , and more preferably within 15% of the larger of those thicknesses. 
     The thickness of the sole adjacent the rear wall of the flexure is preferably reduced if a portion of the sole within about 30.0 mm of the rear wall of the flexure has a thickness that is greater than the thickness of the transmittal portion forward of the front wall of the flexure. For example, if sole thickness t 7  is greater than the minimum thickness of the transmittal portion within 30.0 mm of the rear wall of the flexure, then thickness t 6  of the portion of the sole immediately rearward of the flexure is preferably less than the minimum thickness of the transmittal portion and less than the minimum face thickness. Preferably, thickness t 6  is less than 70% of the minimum thickness of the transmittal portion, and more preferably less than 60% of the minimum thickness of the transmittal portion. Additionally, thickness t 6  is less than 60% of the minimum face thickness, and more preferably less than 50% of the minimum face thickness. 
     In another embodiment, shown in  FIG. 53 , the transmittal portion is modified to include a thickness that changes over the length L of the transmittal portion. The thickness relationships for the other portions of the flexure and sole described above are the same as the previous embodiment and will not be repeated. In the transmittal portion the thickness of the transmittal is about constant over at least 60% of the length L of the transmittal portion, and more preferably over at least 70% of the length L of the transmittal portion. Additionally, the maximum thickness of the transmittal portion is closer to the face of the golf club head than the front wall of the flexure. The maximum thickness is generally located at thickness t 1  and the minimum thickness of the transmittal portion is generally located at thickness t 2 , shown in  FIG. 53 . Preferably, the minimum thickness of the transmittal portion is greater than or equal to the minimum thickness of the sole of the golf club head. The minimum thickness of the transmittal portion is preferably less than 70% of the maximum thickness of the transmittal portion, and more preferably less than 60% of the maximum thickness of the transmittal portion. 
     In another embodiment, shown in  FIG. 54 , the transmittal portion is modified to include a thickness that changes over the length L of the transmittal portion, the apex thickness is illustrated greater than the minimum thickness of the front wall t 3  and the thickness of the rear wall t 5  of flexure, and the thicknesses of the sole rearward of the flexure are illustrated as about constant and generally less than the maximum thickness of the transmittal portion. In this embodiment, the thickness of the transmittal portion has a generally linear taper from adjacent the face to the front wall of the flexure. The linear taper, or linear reduction in thickness, is preferably greater than about 4% (i.e., 0.4 mm reduction in thickness over 10.0 mm length), and more preferably greater than about 5%, from the adjacent the face to the flexure. In the present embodiment, the thickness of the portion of the sole adjacent the rear wall of the flexure t 6  and the sole thickness t 7  further rearward from the flexure are about equal and are less than the maximum thickness of the transmittal portion. 
     In embodiments of golf clubs according to the present invention having loft angle in a range of about 13°-30°, such as in fairway wood and hybrid type golf club heads, the thicknesses are generally in the following ranges: t 1 ) 1.4-2.0 mm; t 2 ) 1.2-1.6 mm; t3) 1.2-1.7 mm; t 4 ) 1.2-2.0 mm; t 5 ) 1.2-1.7 mm; t 6 ) 0.6-1.2 mm; and t 7 ) 0.6-4.0 mm. Similarly, in embodiments of golf clubs according to the present invention having loft angle in a range of about 6°-12°, such as in driver type golf club heads, the thicknesses are generally in the following ranges: t 1 ) 1.4-2.0 mm; t 2 ) 0.6-1.6 mm; t 3 ) 0.5-1.7 mm; t 4 ) 0.5-2.0 mm; t 5 ) 0.5-1.7 mm; t 6 ) 0.5-1.2 mm; and t 7 ) 0.5-3.0 mm. 
     Referring now to  FIGS. 55 and 56 , a golf club head  840  includes a flexure  842  that is at least partially covered by a removable member  844 . Golf club head  840  includes a crown  846 , a sole  848 , a skirt  850 , a face  852  that defines a ball-striking surface  854 , and a hosel  856  that is attached to an elongate golf club shaft and grip in an assembled golf club. 
     Flexure  842  is located in a forward portion of sole  848 , generally adjacent to face  852 , and includes a mounting portion for removable member  844 . Flexure  842  includes a front wall  858  that is joined with a rear wall  860  at an apex  862 . Rear wall  860  extends between apex  862  and the mount  864  for removable member  844 . Mount  864  includes a recessed support portion  866  that receives removable member  844  and positions it so that, when it is mounted, the lower surface of removable member  844  is flush or recessed relative to the adjacent exterior surface of sole  848 . A coupling feature  868  is included so that removable member  844  may be removably attached to golf club head  840 . For example, coupling feature  868  may be a threaded bore and removable member  844  may be a weighted sole plate that is coupled to the threaded bore using a threaded fastener. 
     Removable member  844  is sized to fit within the recessed mount  864  so that it is spaced from front wall  858  of flexure  842  to form a gap  870 . Gap  870  provides an opening into flexure  842  and the opening provides a pathway into a cavity  872  defined by removable member  844  and flexure  842 . Gap  870  provides a space so that during a golf ball impact, flexure  842  is able to flex and gap  870  allows front wall  858  to move relative to removable member  844  in a fore-aft direction. 
     Referring to  FIG. 57 , a golf club head  880  includes a flexure  882  that intersects a removable member  884  mount and an interchangeable shaft system  886 . In the present embodiment, golf club head  880  includes a hollow-body construction that is formed by a crown, a sole  888 , a skirt, and a hosel  890 . Golf club head  880  includes a removable member  884 , such as a weight member and a portion of sole  888  includes a mounting feature for the weight member. In the present embodiment the mounting feature includes a generally cylindrical receiver  892  that extends from an outer surface of sole to the interior of golf club head  880 . 
     Golf club head  880  also includes flexure  882  extending in a generally heel to toe direction across a forward portion of sole  888 . Flexure  882  may have any of the specific constructions described with regard to the other embodiments described herein. 
     Golf club head  880  includes an interchangeable shaft system that includes a fastener  894  that is engaged with the head from the sole side. An access bore  896  is included that receives fastener  894  and extends toward hosel  890  from sole  888 . 
     The sole structures of receiver  892 , flexure  882  and access bore  896  intersect so that the structures are created by common portions. In particular, a side wall of receiver  892  intersects a side wall of flexure  882  so that the structures are combined in a toe portion of golf club head  880 . Similarly, a side wall of access bore  896  intersects a side wall of flexure  882  so that the structures are combined in a heel portion of golf club head  880 . The intersection of the structures of receiver  892 , flexure  882  and access bore  896 , reduces the amount of mass that is dedicated to the extra structures by combining the structures. 
     Referring to  FIGS. 61 and 62 , another embodiment including a replaceable flexure component will be described. In the present embodiment, similar to flexure  76  of the embodiment of  FIG. 6 , a golf club head  900  includes a flexure  902  that is generally constructed as a separate component and is coupled to a sole  904 . Golf club head  900  is a hollow body that includes a crown  901 , sole  904 , a skirt  903  that extends between crown  901  and sole  904 , a face  905  that provides a ball striking surface  907 , and a hosel  909 . The hollow body defines an inner cavity  910  that may be left empty or it may be fully or partially filled. 
     Flexure  902  may be constructed as a partial sole plate and may form any portion of the sole of the golf club head. In the present embodiment, flexure  902  replaces a forward portion of the sole surface of the golf club head  900 . Flexure  902  includes a first member  906  that extends from a rearward edge of a forward flange portion  908  and curves into inner cavity  910  of the golf club head. A second member  912  extends from a rearward flange portion  914  of flexure  902  and curves into inner cavity  910 . The ends of first member  906  and second member  912  that extend into inner cavity  910  are joined to each other. Preferably, the flexure is elongate and extends in a generally heel to toe direction. 
     As shown, flexure  902  fits into an aperture defined by sole  904  and skirt  903  and may be mechanically coupled to sole  904  using a plurality of fasteners. In particular, a plurality of fasteners  916  extend through fastener bores included in the forward and rearward flange portions  908 ,  914  of flexure  902  and extend into bosses  918  of sole  904 . Alternatively, or in addition, the flexure may be bonded, brazed or welded to sole  904 . The edge of the aperture may be provided with a recessed flange  917  on all or a portion of the perimeter of the aperture that may be bonded to a perimeter edge of flexure  902  in addition to the plurality of fasteners  916 . 
     The material of flexure  902  may be selected from materials having different densities, Young&#39;s moduli and dimensions to provide a plurality of flexures having different masses and stiffness. For example, the flexure may be constructed from a material that is different than the sole of the golf club, such as including a carbon composite flexure in a titanium sole. Furthermore, constructing the flexure as a separate component allows the repair of a broken flexure by replacing the flexure or tuning the flexure to a particular design club head speed. It also allows the flexure to be constructed from different processes compared to the remainder of the golf club head such as by forging the flexure and casting the remainder of the golf club head, which may also provide better material properties of the flexure, such as by being able to remove an oxidized layer, known as alpha case, that can form on the material. Still further, the weight of flexure  902  may be selected to allow control over the final head weight. 
     In an alternative embodiment, a golf club head  1000  is shown in  FIG. 63  including a flexure component  1002  that reinforces and tunes a portion of a sole  1004 . Similar to previous embodiments, golf club head  1000  is a hollow body that includes a crown  1001 , sole  1004 , a skirt that extends between crown  1001  and sole  1004 , a face  1005  that provides a ball striking surface  1007 , and a hosel. In the present embodiment, the flexure component is coupled to an outer surface of sole  1004  and combines with a flexure  1003  included in sole  1004  that forms a recessed channel. In particular, flexure  1003  may be constructed so that without flexure component  1002  flexure  1003  would fail under the stresses produced during impact between a golf ball and the golf club head  1000 . 
     Flexure component  1002  includes a first member  1006  that extends from a rearward edge of a forward flange portion  1008  and curves toward inner cavity  1010  of the golf club head, but in the present embodiment, the flexure component  1002  is not exposed to the inner cavity  1010 . A second member  1012  extends from a rearward flange portion  1014  of flexure  1002  and curves toward inner cavity  1010 . The ends of first member  1006  and second member  1012  that extend toward inner cavity  1010  are joined to each other. Preferably, the flexure is elongate and extends in a generally heel to toe direction, and is constructed as a single monolithic body. 
     As shown, sole  1004  includes a recess that receives flexure component  1002 , and the recess and flexure component  1002  have complementary geometries so that flexure component  1002  abuts and supports the flexure  1003  of sole. Flexure component  1002  may be mechanically coupled to sole  1004  using a plurality of fasteners. In particular, a plurality of fasteners  1016  extend through fastener bores included in the forward and rearward flange portions  1008 ,  1014  of flexure component  1002  and extend into threaded bores  1018  of sole  1004 . Alternatively, or in addition, the flexure may be bonded, brazed or welded to sole  1004 . 
     The physical attributes of golf club heads are generally controlled to provide desired behavior during an impact with a golf club head. In metalwood golf club heads, the mass distribution is controlled to provide a desired location of the center of gravity and a desired moment of inertia. As illustrated in  FIGS. 58-60 , the center of gravity of a golf club head may be dimensionally related to any number of features on the golf club head. Desired dimensional ranges for golf clubs of the present invention are presented in the table below, with negative values denoted by parenthesis to indicate the direction relative to the reference feature (e.g., fc-face center; g-ground). 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 CG-Neutral 
               
               
                 Golf Club 
                 CG-C-sa 
                 CG-X-fc 
                 CG-Y-fc 
                 CG-Z-fc 
                 CG-Y-g 
                 Axis 
               
               
                 Type 
                 [mm] 
                 [mm] 
                 [mm] 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
               
             
            
               
                 Driver 
                 13.5-28.0 
                 (1.6)-7.8 
                 (7.8)-1.2 
                 (43.0)-(29.0) 
                 26.3-32.7 
                 (5.3)-7.0 
               
               
                 Preferred 
                 18-22 
                 (1.3)-3.5 
                 (5.4)-0.0 
                 (38.0)-(30.0) 
                 26.9-29.0 
                 (1.0)-6.3 
               
               
                 Driver 
               
               
                 Fairway 
                  5.8-21.9 
                 (0.9)-5.3 
                 (4.8)-0.9 
                 (33.3)-(18.2) 
                 13.8-18.9 
                 (2.8)-7.8 
               
               
                 Preferred 
                  8.0-15.9 
                     0.3-2.5 
                     (4.8)-(0.6) 
                 (29.5)-(22.0) 
                 14.1-18.8 
                 (2.5)-6.8 
               
               
                 Fairway 
               
               
                   
               
            
           
         
       
     
     The flexures of the present invention are also sized relative to the location of the center of gravity of the golf club head to provide desired behavior. It should also be appreciated that the width W, height H and distance to ball striking-surface D may be measured on all of the embodiments described herein as illustrated in  FIGS. 1 and 4 . Preferably the distance D from the ball-striking surface to the flexure is less than or equal to 30.0 mm, more preferably less than or equal to 20.0 mm, and more preferably between 5.0 mm and 20.0 mm. Additionally, the distance D is preferably between 20% and 50% of the CG-Z-fc distance, and more preferably between 25% and 45% of the CG-Z-fc distance. Additionally, the sum of the height and width of the flexure is preferably within +/−30% of the CG-Y-g distance, and more preferably within +/−20% of the CG-Y-g distance. 
     The reduction in backspin provided by the flexure of the present invention also more flexibility in mass distribution to increase the moment-of-inertia of a golf club head. In particular, the incorporation of a flexure of the present invention into the sole of a golf club head provides ball impacts that emulate launch conditions of a golf club head without a flexure that has a low center of gravity. Analysis has shown that the incorporation of a flexure of the present invention provides the same effect as lowering the center of gravity of a golf club without the flexure by as much as 3.0 mm. However, lowering the center of gravity requires that mass is placed lower in the golf club head and because of the shape of the golf club head it limits the amount of mass that can be placed at the perimeter to increase moment-of-inertia. Therefore, the flexure of the present invention may be used to provide the behavior of a golf club head with a lower center-of-gravity while additional mass is placed at the perimeter of the golf club head to increase moment-of-inertia and moving the center-of-gravity rearward. 
     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. 
     The construction of the flexure generally results in material extending into the cavity of the golf club, which generally raises the CG when the flexure is located in the sole or the crown of the golf club head. The increase in CG height is more substantial when a flexure is included in the crown. Preferably, in embodiments utilizing a crown flexure, the portion of the crown rearward of the flexure is lowered relative to the portion of the crown forward of the flexure to lower the overall CG of the golf club head. In particular, the height of the forward edge of the crown flexure is greater than the height of the rearward edge of the crown flexure. Preferably, the difference in height is greater than 1.0 mm, and more preferably greater than 2.0 mm, and the location of the crown having a maximum height from the ground surface is between the face of the golf club head and the flexure. 
     As shown in previous embodiments, a golf club head may be constructed with one or more mounting features for removable weights to alter the overall golf club head weight and/or the location of the CG, in addition to a flexure. In an embodiment, a golf club head including a flexure in the sole of the golf club head has a CG-C-sa value that is greater than 18.0 mm behind the shaft axis, and preferably a CG-Z-fc value greater than 33.0 mm rearward of face center, and/or a moment-of-inertia value about the Y-axis of the golf club head of at least 450 kg-mm 2 . Additionally, the golf club head has at least one weight mounting feature and at least one removable weight that allows the CG of the golf club head to be altered by at least 2.0 mm in a direction. 
     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 a 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 a 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 a 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-0.30, 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-13Zr-2Cr, 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-0.350, 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, Ti-64, etc. 
     As described previously, the flexure may be constructed as a separate component and attached to the remainder of a golf club head body. For example, the flexure component may be stamped and formed from wrought sheet material and the remainder of the body constructed as one or more cast components. Stamping a flexure component may be preferable over casting the flexure because casting can introduce mechanical shortcomings. For example, cast materials often suffer from lower mechanical properties as compared to the same material in a wrought form. As an example, Ti-64 in cast form has mechanical properties about 10%-20% lower as compared to wrought Ti-64. This is because the grain size in castings is significantly larger as compared to the wrought forms, and generally finer grain size results in higher mechanical properties in metallic materials. 
     Further, titanium castings also develop a surface layer called “alpha case”, a region at the surface that has predominantly alpha phase of titanium that results from titanium that is enriched with interstitial oxygen. The alpha phase in and of itself is not detrimental, but it tends to be very hard and brittle so in fatigue applications, such as repeated golf ball impacts that cause repeated flexing, the alpha case can compromise the durability of the component. 
     Most titanium alloys are almost impossible to form at room temperature. Thus, the titanium alloys have to be heated to an elevated temperature to form them. The temperature necessary to form the alloy will depend on the alloy&#39;s composition, and alloys that have higher beta transus temperature typically require higher forming temperatures. Exposure to elevated temperature results in lowered mechanical properties when the material is cooled down to ambient temperature. Additionally, the exposure to elevated temperature results in the formation of an oxide layer at the surface. This oxide layer is almost like the “alpha case” discussed above except that it typically does not extend as deep into the material. Thus, it is beneficial if the forming temperature can be lowered. 
     Generally, if using Ti-64 as a baseline since it is commonly used in the construction of metal wood type golf club heads, alloys that have beta transus temperatures that are lower than that of Ti-64 can provide a significant benefit. For example, one such alloy is ATI 425, which has a beta transus temperature in the range of about 957°-971° C., while Ti-64 has a beta transus temperature of about 995° C. Thus, it can be expected that ATI 425 can be formed at a lower temperature as compared to Ti-64. Since ATI 425 has mechanical properties comparable to Ti-64 at room temperature, it is expected that a sole fabricated from ATI 425 alloy will be stronger as compared to a sole made from Ti-64. In addition, ATI 425 generally has better formability as compared to Ti-64, so in an example, a flexure is formed of ATI 425 sheet material and will experience less cross-sectional thinning than a flexure formed of a Ti-64 sheet material. Further, ATI 425 may be cold formable which would further result in a stronger component. 
     In an example, a multi-material golf club head is constructed from components constructed of Ti-64 and ATI 425. A body including a crown, a sole or partial sole, a skirt, a hosel and a face flange may be cast of Ti-64. Then a portion of the sole may be formed by a flexure component that is constructed from ATI 425 sheet material and welded to the cast Ti-64 body, such as in a slot or recess, such as in the configuration shown in  FIGS. 5 and 6 . A forged face insert is then welded to the face flange of the cast Ti-64 to complete the head. 
     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. As used herein, forging is a process that causes a substantial change to the shape of a specimen, such as starting with a bar and transforming it into a sheet, that characteristically includes both dimensional and shape changes. Additionally, forging generally is performed at higher temperature and may include a change in the microstructure of the material, such as a change in the grain shape. Forming is generally used to describe a process in which a material is shaped while generally retaining the dimension of the material, such as by starting with a sheet material and shaping the sheet without significantly changing the thickness. 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. 
       FIG. 64  of the accompanying drawings shows a sole view of a golf club head  1100  in accordance with a further alternative embodiment of the present invention. In this embodiment of the present invention, the flexure  1104 , similar to the flexure  902  shown in  FIG. 61-62 , can be a completely separate and detachable piece. The flexure  1104  in the current exemplary embodiment, may fit into an aperture  1102  that spans lengthwise along the sole of the golf club head  1100  in a heel and toe direction. This separate and detachable piece can be made out of the same titanium material as the remainder of the golf club head  1100  or even be made out of a different material with different densities, Young&#39;s moduli and dimensions to provide different flexures without departing from the scope and content of the present invention. Finally,  FIG. 64  also shows a cross-sectional line  67 - 67 , allowing a cross-sectional view of the golf club head to be shown in later in  FIG. 67 . 
       FIG. 65  of the accompanying drawings shows a sole perspective view of the golf club head  1100  in accordance with the embodiment shown in  FIG. 64 . The sole perspective view of the golf club head  1100  allows a better view into the flexure  1104  and how it fits into the aperture  1102 . Moreover the perspective view of the golf club head  1100  shown in  FIG. 65  allows the depth of the flexure  1104  to be shown more clearly. 
       FIG. 66  of the accompanying drawings shows an exploded view of the golf club head  1100  with the flexure  1104  being exploded from its position inside the aperture  1102 . Based on this exploded view it can be seen that the flexure  1104  may be attached to the aperture  1102  utilizing a press fit type of connection. This type of connection utilizes one or more ribs  1106  on the flexure  1104  to engage one or more flanges  1108  in the aperture  1102 . This type of press fit connection allows the flexure  1102  to deflect and move independently of the remainder of the golf club head  1100 , further improving the performance of the golf club head  1100 . Visually, it can be said that the rib  1106  circumferentially surrounds a bottom portion of the flexure  1104  and the flange  1108  circumferentially surrounds an upper portion of the aperture  1102 . It should be noted that although the flexure  1104  is not connected to the golf club  1100  within the aperture  1102  directly, the movement of the flexure  1104  is dependent on the shift in size of the aperture  1102  when the golf club head  1100  impacts a golf ball. Finally, this exploded view of the golf club head  1100  shown in  FIG. 66  illustrates that the aperture  1102  resembles a cutout from the sole  1114  portion of the golf club head  1100  that opens into the interior cavity of the golf club head  1100 . In one embodiment of the present invention the flexure  1104  can be held in place within the aperture  1102  using merely the flange  1108  and rib  1106  combination alone; however, in alternative embodiments of the present invention an additional bonding agent can be used to help the bond without departing from the scope and content of the present invention. 
     It is worth noting here that decoupling the flexure  1104  from the golf club head  1100  is critical to the improvement to the performance of the golf club head  1100 . Removing the constraint between the flexure  1104  and the aperture  1102  allows an increase in performance of the golf club head because it allows the striking face portion to move and deflect when it impacts a golf ball. Without the connection constraint, the face is allowed to deflect in a translational direction as well as a radial direction, further improving the compliance of the striking face portion. However, a cross-sectional view of the golf club head  1100  provided in subsequent figures will provide a clearer illustration of the relationship between the components. 
       FIG. 67  of the accompanying drawings shows a cross-sectional view of the golf club head  1100  in accordance with the current exemplary embodiment of the present invention. The cross-sectional view of the golf club head  1100  allows the interface between the flexure  1104  and the striking face  18  as well as the sole  14  of the golf club head  1100  to be shown more clearly. More specifically, it can be seen that the removable flexure  1104  may generally be placed at a position that is close to the frontal striking face  18  of the golf club head  1100 . The placement of the removable flexure at a location that is close to the frontal striking  18  of the golf club head  1100  may help improve the performance of the golf club head  1100  by reducing the amount of stress that is experienced by the striking face  18  at the bottom portion. 
     To help better illustrate the relationship established between the striking face  18  and the flexure  1104 , several dimensions are illustrated by  FIG. 67  and  FIG. 68 .  FIG. 68  provided an enlarged view of Detail B, as shown in  FIG. 67 . First and foremost, it is worth noting that FIGS.  67  and  68  illustrates a distance D′, which measures the thickness of the striking face portion of the golf club head  1100 . In a golf club head that has a constant face thickness, distance D′ may be easily determined from a frontal plane that is tangent to the face center  30  of the striking face  18 , also known as the striking surface, together with a backing plane that is tangent at the rear surface of face center  30  of the striking face  18  serving as a rear plane. In an alternative embodiment where the striking face  18  may have a variable thickness, the definition of D′ is the same, with the tangent line to the front of the striking face  18  at the face center  30  serving as the frontal plane and the tangent line at the rear surface of the face center  30  serving as the rear plane. Once the thickness of the face D′ is determined, that distance D′ can be used to determine the distance of the flexure  1104  away from the striking face  18  as a function of the thickness D′ of the face  18 . In the current exemplary embodiment of the present invention, the location of the flexure  1104  as a function of D and D′ is preferably governed by the equation D≦2.0*D′, more preferably by the equation D≦1.75*D′, and most preferably by the equation D≦1.5*D′. In an extreme case scenario, distance D can be defined by the equation D≦1.3*D′ without departing from the scope and content of the present invention. It should be noted that the measurement of the location of the flexure may generally exclude any protrusions, ribs, or any other ancillary features that are used for securing the flexure  1104  in the aperture  1102 . 
     In one exemplary embodiment of the present invention wherein the golf club head  1100  is a fairway wood, the thickness D′ of the striking face may be constant at a thickness of 2.1 mm. In that scenario, the beginning of the flexure  1104 , which coincides with the thickness of the striking face  18  at the junction with the flexure  1104 , may generally be at a distance D of less than about 3.15 mm, more preferably less than about 2.94 mm, and most preferably less than about 2.73 mm. 
     In another embodiment of the present invention wherein the golf club head  1100  is a driver type club, the thickness D′ of the striking face may be variable between about 2.3 mm to about 4.3 mm. Because the thickness D′ of a driver type golf club head is also based off the face center  30 , it can generally be said that the thickness D′, defined using the definition above may generally be about 4.3 mm. In that scenario, the flexure  1104  may generally be at a distance D of less than about 6.45 mm, more preferably less than about 6.02 mm, and most preferably less than about 5.59 mm. 
     In the current exemplary embodiment of the present invention, the flexure  1104  and the body of the golf club head  1100  may both be made out of a steel type material due to its inherent material properties. However, in alternative embodiments the material of the flexure  1104  and the body of the golf club head  1100  may both be made out of titanium type material also without departing from the scope and content of the present invention. In fact, in a further alternative embodiment of the present invention, flexure  1104  could be made out of a completely different material from golf club head  1100  also without departing from the scope and content of the present invention. 
       FIG. 69  provides an enlarged cross-sectional view of a golf club head  1100  and the flexure  1104  in accordance with an alternative embodiment of the present invention. In this alternative embodiment of the present invention, the frontal portion of the flexure  1104  may still be comprised out of a rib  1106  while the striking face  18  may be comprised out of a flange  1108 . However, the rear portion of the flexure  1104  in this embodiment may differ from prior embodiments in that it utilizes a lap joint type construction further comprising of a cutout  1112  and a flap  1110  that supports the flexure  1104 . 
       FIG. 70  of the accompanying drawings shows a cross-sectional view of a golf club head in accordance with a further alternative embodiment of the present invention. In this alternative embodiment of the present invention, the golf club head  1100  may have a flexure  1104  at the bottom sole  14  portion of the golf club head  1100  as well as another flexure  1105  at the top crown  12  portion of the golf club head  1100 . In this embodiment of the present invention, the flexure  1104  at the bottom of the sole uses a rib  1106  and flange  1108  interface while the flexure  1105  at the crown portion of the club head uses a lap joint type of an interface with a cutout  1112  and a flap  1110 . However, in an alternative embodiment of the present invention the interface selected could be flipped, all rib  1106  and flange  1108 , or all cutout  1112  and flap  1110  without departing from the scope and content of the present invention. 
       FIG. 71  of the accompanying drawings shows a sole view of a golf club head  1100  in accordance with a further alternative embodiment of the present invention wherein the flexure  1104  is another separate and detachable piece of material that fits within an aperture  1102 . To illustrate the separate and independent characteristic of the flexure  1104 , a perspective view of the golf club head  1100  is shown in  FIG. 72 . Finally,  FIG. 71  also shows a cross-sectional line  73 - 73  to allow for the interface of the components to be shown more clearly in  FIG. 73 . 
       FIG. 72  of the accompanying drawing shows an exploded view of the golf club head  1100  wherein the flexure  1104  is exploded out of the aperture  1102  where it sits in. In the exploded view provided by  FIG. 72 , it can be seen that the flexure  1104  has an elongated C shape with an opening towards the rear of the golf club head while the aperture  1102  compliments the shape of the flexure  1104  to help improve the performance of the golf club head  1100 . 
       FIG. 73  of the accompanying drawings shows a cross-sectional view of the golf club head  1100 , allowing interaction between the flexure  1104  and the aperture  1102  to be shown more clearly. In this embodiment of the present invention, it can be seen that the opening of the aperture is not directly downward from the sole  14  portion of the golf club head  1100 . In fact, the opening of the aperture  1102  is created in an orientation that points forward toward the striking face  18  of the golf club head  1100 . To complement the aperture  1102 , the flexure  1104  in this embodiment may appear in a u-shaped geometry, and engage the aperture  1102  by overlapping some of the surfaces. 
       FIG. 74  of the accompanying drawings shows an exploded cross-sectional view of a golf club head  1100  in accordance with the embodiment of the present invention described above with  FIGS. 71, 72, and 73 . In this exploded view, it can be seen that the flexure  1104  may engage the aperture  1102  in a unique orientation, and this orientation may further increase the performance of the golf club head  1100  without departing from the scope and content of the present invention. The interface between the flexure  1104  and the aperture  1102  may rely on overlapping of surfaces to help retain the flexure  1104  within the aperture  1102 . The details of the interface will be shown more clearly in the enlarged view of the flexure  1104  in  FIG. 75 . 
       FIG. 75  of the accompanying drawings shows an enlarged cross-sectional view of the flexure  1104  and the aperture  1102  of the golf club head in accordance with an alternative embodiment of the present invention. In this enlarged view, it can be seen that the flexure  1104  fits in the aperture  1102  snugly with surface overlap near the front of the flexure  1104  as well as a surface overlap near the back of the flexure  1104 . The flexure  1104  is allowed to move and deflect when the striking face  18  comes in contact with a golf ball, and the elasticity of the material used for the flexure  1104  contributes to that deformation. Finally, it is worth noting that the flexure  1104  may have a leading edge  1111  that is rounded to help with the deflection and shifting of the flexure  1104 , however, in alternative embodiments wherein the stress level is higher, more material could be added to the leading edge  1111  without departing from the scope and content of the present invention. 
       FIG. 76  shows an enlarged cross-sectional view of the flexure  1104  and the aperture  1102  in accordance with an alternative embodiment of the present invention wherein the leading edge  1111  contains more material to create a more square edge when compared to prior embodiments. The square edge shaped leading edge  1111  increases the material thickness at that portion of the flexure, resulting in a more durable flexure  1104 . 
       FIGS. 77 and 78  show alternative embodiments of the present invention wherein the flexure  1104  could extend entirely from the heel portion of the sole of the golf club head  1100  to the sole portion of the golf club head  1100 . Having this more expansive flexure  1104  that spans wider than earlier embodiments will be preferred in certain embodiments, as it allows for more ease of machining of the aperture  1102 . The difference between  FIG. 77  and  FIG. 78  is shown in the interface between the flexure  1104  and the bottom of the hosel portion. In the embodiment of the present invention shown in  FIG. 77  the flexure  1104  has an opening at the rear portion of the flexure  1104  where the flexure  1104  intersects the bottom of the hosel bore. Alternatively,  FIG. 78  of the accompanying drawings shows an alternative embodiment of the present invention wherein the flexure  1104  contains a wall that separates the flexure  1104  from the bottom of the hosel bore. 
     In another embodiment, shown in  FIGS. 79-82 , a golf club head  1120  includes an aperture and a flexure similar to the embodiments described with regard to  FIGS. 71-76 . Golf club head  1120  is illustrated having a hollow-bodied construction including a crown  1122 , a sole  1124 , a skirt  1126 , and a hosel  1128 . Skirt  1126  generally extends between crown  1122  and sole  1124  and generally forming a side wall of the golf club head, and a hosel  1128  extending from a heel portion of the golf club head. Hosel  1128  may extend through the golf club head from crown  1122  to sole  1124  or it may provide a blind bore configuration. Furthermore, the hosel  1128  may be configured to include an interchangeable and/or adjustable shaft mechanism. As illustrated, hosel  1128  extends through the head and provides access at a sole side of the hosel  1128  that receives a fastener to couple a shaft to golf club head  1120 . 
     A flexure  1130  is disposed in an aperture included in sole  1124  of golf club head  1120 . Flexure  1130  is a separate piece of material that fits within an aperture  1132  and tunes the deflection of the side walls of the aperture  1132  during a golf ball impact. The height and thickness of the side walls of aperture  1132  vary around the perimeter of the aperture. In the illustrated embodiment, the front side wall  1144  that is closest to the face of golf club head  1120  is taller and thinner than the rear side wall  1146  of aperture  1132 . In particular, the front side wall  1144  extends from a face transition  1148  upward generally toward crown  1122 . In the current embodiment, front side wall  1144  extends both upward and rearward as shown in the cross-section, however it should be appreciated that the front side wall may alternatively extend vertically upward or angled upward and forward toward the face, and would still be within the scope of the present invention. 
     The flexure  1130  and the walls of aperture  1132  are shaped to tune the deflection of the golf club head. The depth of cavity may be defined in its installed condition relative to the shape of aperture  1132 . In an example, as illustrated in  FIG. 81 , a cross-section is taken of the assembled golf club head  1120  so that the cutting plane is a vertical Y-Z plane that extends through the geometric face center. The aperture and flexure are defined by dimensions taken in that cross-section in relation to a ground plane when the golf club is contacting the ground with the lie set so that the scorelines are at an angle between 0° and 3° relative to the ground plane, the face center is determined and the golf club is in an orientation in which the ball-striking face is in a squared orientation in accordance with the USGA guidelines. The measurements shown in the figures are generally taken vertically in a direction perpendicular to the ground plane within the Y-Z plane at face center, or at a 45° angle with respect to the ground plane within the Y-Z plane at face center. 
     The thickness of front side wall  1144  may be constant or vary along its length from the face transition  1148  to a free end  1150  and along the front side wall  1144  from the heel to toe side of the aperture. For example, at the cross-section shown in  FIGS. 81 and 82 , the thickness of front side wall  1144  includes a minimum thickness (t 8 ) and a maximum thickness (t 9 ) between face transition  1148  and free end  1150 . Preferably, the minimum thickness t 8  is less than 3.0 mm and maximum thickness, t 9 , is not less than 2.0 mm, even more preferably, the minimum thickness is in a range of about 0.70 mm to about 1.10 mm and the maximum thickness is between about 0.80 mm and about 1.30 mm. Still further, the maximum thickness is preferably interposed between the minimum thickness location and the free end  1150  in the cross-section taken at face center as shown in  FIG. 82 . The free end  1150  is shaped so that a lower terminal end, i.e., the portion of the terminal free end  1150  closest to the flexure is rounded or chamfered. 
     In the present embodiment, flexure  1130  includes a front wall  1136 , a rear wall  1138  and a base  1140  that extends between front and rear walls  1136 ,  1138 . The front wall  1136  and the base  1140  join at a leading edge  1134  of flexure  1130 . The combination of the front wall  1136 , rear wall  1138  and base  1140  defines a cavity  1142 . Preferably, the cavity  1142  is open to the interior of the golf club head so that foreign debris is not able to enter the cavity during use. The width of cavity  1142  between front wall  1136  and rear wall is preferably between about 0.25 mm and about 4.0 mm and more preferably between about 0.5 and about 2.0 mm. Additionally, the closed end of cavity  1142  is preferably radiused and may have a radius r 1  that is about equal to ½ of the width. 
     In embodiments of the present invention, the dimensions of the aperture and the flexure are selected to provide desired behavior. As an exemplary embodiment, a vertical height (h 1 ) of the bottom of cavity  1142  relative to the ground plane ranges between about 1.9 mm and about 3.9 mm, a 45° height (h 2 ) of the bottom of cavity  1142  relative to the ground plane ranges between about 2.9 mm and about 5.8 mm, and a 45° height (h 3 ) of the bottom of cavity  1142 . In that embodiment, a vertical height (h 4 ) of the free end  1150  of the front wall  1144  of the aperture  1132  ranges between about 6.5 mm and about 8.0 mm, a 45° height (h 5 ) of the free end  1150  relative to the ground plane ranges between about 9.0 mm and about 11.0 mm. Additionally, in that embodiment, a 45° height difference (h 6 ) between the 45° height of rear side wall  1146  of aperture  1132  and free end  1150  of front side wall  1144  of aperture  1132  is between about 3.0 mm and about 4.0 mm, and a distance (h 9 ) between the axes along which heights h 5  and h 2  are measured is between about 1.0 mm and 3.0 mm. It should be appreciated that the vertical measurements are taken to a point determined at a point of contact with horizontal plane that is tangent to the feature, and similarly the 45° measurements are taken at the point of contact with a plane that is angled at 45° relative to a ground plane as illustrated in  FIGS. 81 and 82 . 
     Additionally, in an embodiment, the exposed surface of base  1140  of flexure  1130  is spaced from the ground plane by a minimum distance h 7  that is preferably between about 0.1 mm and about 2.0 mm, and the distance may vary across flexure  1130  in both face to aft and heel to toe directions. For example, the flexure  1130  may be installed to have a maximum height to ground h 8  that is closer to the face than the minimum height to ground h 7 . Preferably the maximum height h 8  is between about 1.0 and about 5.0 mm. More preferably, height h 7  is between about 0.1 mm and about 0.9 mm, and height h 8  is between about 1.0 mm and about 2.5 mm. Additionally, the leading edge  1134  of flexure  1130  is disposed in sole  1124  within a distance D of the ball-striking surface that is preferably less than about 20 mm. 
     In another aspect of the present invention, the flexure  1130  and aperture  1132  are tuned by maintaining length relationships between the walls of the aperture and the walls of the flexure. In a first relationship, a first flexure ratio (h 1 /h 4 ) of the vertical height, h 1 , of bottom edge of the cavity  1142  to the vertical height, h 4 , free end  1150  of front wall  1144  of the aperture  1132  is preferably between about 0.25 and about 0.75, and more preferably between about 0.45 and about 0.55. In another relationship, an aperture ratio (h 6 /h 5 ) of the 45° height difference h 6  and the 45° height h 5  of the free end  1150  of front side wall  1144  is maintained between about 0.25 and about 0.45, and more preferably between about 0.30 and about 0.40. In a still further relationship, a second flexure ratio (h 2 /h 5 ) of the 45° height h 2  of the bottom of cavity  1142  relative to the ground plane to the 45° height h 5  of the free end  1150  is maintained between about 0.45 and about 0.65, and more preferably between about 0.50 to about 0.60. In another aspect of the present invention, the sum of the distance between the axes and the 45° height of the bottom of the cavity  1142  (h 9 +h 3 ) is preferably less than 70% of the 45° height h 5  of free end  1150 , and more preferably less than 60%. Additionally the sum h 9 +h 3  is preferably in a range between about 4.0 mm and about 8.0 mm, and more preferably between about 5.0 mm and about 7.0 mm. 
     Another embodiment is illustrated in  FIGS. 83-86 , a golf club head has a similar configuration of the flexure of golf club head  1120 , however the thickness of a front side wall of the aperture has a thickness that varies in a heel to toe direction as described below in further detail. Golf club head  1170  has a hollow construction and includes a sole  1172  that defines an aperture  1174  that receives a flexure  1176 . The aperture  1174  is partially defined by a front side wall  1178  which includes variable thickness. In particular, front side wall  1178  includes a first thickness pad  1180  having a first thickness that is about constant over a distance of about 6.0-26.0 mm in the first thickness pad  1180 . Additionally, a second thickness pad  1182  may be included that has a second thickness that is about constant over a distance of about 6.0-26.0 mm in the second thickness pad  1182 . The thicknesses of the first thickness pad  1180 , second thickness region  1182  and surrounding portions of front side wall  1178  are selected to tune portions of aperture  1174  and flexure. For example, to balance the frequency response of the structure, the first thickness pad  1180 , which is generally located at a central portion of the structure, may be thicker than the surrounding portions of front side wall  1178  while the second thickness pad  1182  may be thinner than the surrounding portions of the front side wall  1178 . The transitions between the thickness pads and the adjacent portions of front side wall  1178  may be stepped or gradual. Additionally, the dimensional attributes of aperture  1174  and flexure  1176  are in the same ranges as described above with respect to golf club head  1120 . It should be appreciated that the number of thickness pads is selected to tune the frequency response of the golf club head. As shown, the golf club head may include two thickness pads, but the club head may include a single thickness pad or more than two thickness pads as required to create the desired behavior. 
     Referring now to  FIGS. 87 and 88 , a golf club head  1190  has a hollow body construction and is generally defined by a crown  1192 , a sole  1194  and a skirt  1196  extending between the crown  1192  and sole  1194 . Similar to the previous embodiments, sole  1194  defines an aperture  1198  that receives a flexure  1200 . The deformable aperture  1198  and flexure  1200  are tuned to improve ball speed performance, to reduce the stress in the golf club head, and to reduce the backspin imparted to a golf ball during impact. In the illustrated embodiment, aperture  1198  includes a front side wall  1202  and a rear side wall  1204  that circumscribe aperture  1198 . 
     In the illustrated embodiment, front side wall  1202  and rear side wall  1204  extend from the edges of aperture  1198  generally vertically toward the interior of golf club head  1190  so that they form an angle β that is about 90°, but the angle β may vary from about 1° to 90°. The walls preferably have a height h 10  that is generally between about 0.1 mm and about 15.0 mm, and a thickness t 10  is generally between about 0.1 mm and about 5.0 mm. Additionally, a width W of aperture  1198  is preferably in a range of 0.1 mm to about 20.0 mm and the aperture is spaced by a distance D less than about 25 mm from a leading edge of the golf club head. 
     In an example, the golf club head includes an aperture and a C-shaped flexure. The walls of the aperture  1198  have a height that is generally between about 5.0 mm and about 8.0 mm, and more preferably between about 6.0 mm to about 7.0 mm, and the flexure  1200  has a height that is generally between about 4.5 mm and about 5.5 mm, and a thickness that is about 1.2 mm to about 1.6 mm. The C-shaped flexure contacts a rearward wall, i.e., a wall furthest from the face of the golf club head, of the aperture at two locations that are spaced about 50.0 mm to about 52.0 mm apart and the flexure contacts the forward wall, i.e., the wall closest to the face of the golf club head, at a single location. As an alternative, the flexure need not contact the forward wall, but may be spaced from the forward wall by a gap that is preferably less than 1.0 mm wide. Additionally, in a spaced embodiment, the gap is preferably filled by a material, such as a polymer, etc., that at least partially fills the void between the flexure and the forward wall. The aperture is positioned at a distance D of about 14.0 mm to about 16.0 mm rearward from the leading edge of the golf club head in a vertical, fore-aft plane extending through a geometric face center of the face of the golf club head. Additionally, the aperture extends in a heel to toe direction about 60.0 mm to about 75.0 mm and has a fore-aft width W of about 9.0 mm to about 10.0 mm. 
     Flexure  1200  is generally a C-shaped, curved member that may be cast, welded or mechanically locked, such as by press or shrink fit or mechanical fasteners, in place in aperture  1198 . As shown, the flexure  1200  contacts the side walls of the aperture at a plurality of spaced contact locations  1206  that include forward and rearward contact locations. The contact locations are generally spaced so that portions of the side wall of the aperture extend between the contact locations that have no contact with the flexure. In the illustrated embodiment, the flexure contacts the front side wall  1202  at a single contact location  1206  and the rear side wall  1204  at two contact locations  1206 . The flexure  1200  is preferably constructed of steel, titanium, composite including fiber reinforced polymers, tantalum, zirconium or a combination thereof. The thickness, length, height, modulus and strength of flexure  1200  are selected to tune the combined flexibility of aperture  1198  and flexure  1200  while maintaining the durability of the golf club head. Additionally, the void remaining in aperture  1198  after flexure  1200  is installed may be filled with urethane, other polymeric materials, ABS plastic, ionomer, thermoplastic, thermoset plastic or combinations thereof. The fill material may be recessed from the plane of the sole so that the fill material is spaced from a ground plane when the golf club head is resting on the ground plane. As an additional aspect of the present invention, multiple flexures may be included in a single aperture and the flexures may be in contact with each other. For example, the flexures could be constructed in the form of a series of angled struts or the flexure may be constructed with one or more inflection points so that the flexure is generally S-shaped. As a further embodiment, the walls forming the aperture and the flexure may be combined and formed as a component and coupled to the golf club head. The flexure may be coupled to the golf club head metallurgically, such as by welding or brazing, adhesively, such as by adhering the flexure with an epoxy or another adhesive material, or mechanically, such as by using mechanical fasteners. 
     In additional embodiments, a golf club head  1220  includes a compliant plate  1222  that is disposed in an aperture and the plate  1222  is structured to tune the compliance of the gap formed by the aperture, as shown in  FIGS. 89-90 . The compliant plate includes a plurality of stiffness reducing voids to allow slight deflection during impact between the golf club head and a golf ball to increase ball speed, launch angle and/or spin of the golf ball. The plate is preferably welded, brazed or bonded into a slot defined by the golf club head. In general, golf club head  1220  includes a crown  1224 , a sole  1226  and a skirt  1228  extending between the crown and sole and forming a side wall of golf club head  1220 . An aperture  1230  is formed in sole  1226  and receives the compliant plate  1222 . Aperture  1230  has a stepped configuration that includes a first, outer, portion  1232  and a second, inner, portion  1234  and that forms a shoulder between. As shown, the outer portion  1232  is shaped to complement the perimeter shape of the compliant plate  1222  which is received in that portion and attached to the sole  1226  of golf club head  1220 . 
     Compliant plate  1222  includes a body  1236  and voids  1238 . The body  1264  may be formed as a flat or curved plate and voids  1238  are included to tune the compliance of the body  1236 . As an alternative, or in addition, to voids  1238 , the body  1236  may include regions having different thicknesses, or different curvature, to tune the compliance of the plate  1222 . As an example, compliant plate  1222  includes a plurality of voids  1238  with curved shape in the form of oval apertures that extend through the entire thickness of body  1236 . The number, size and shape of voids  1238  may be selected to provide a desired compliance of plate  1222 . Preferably, the number of voids is between about 1 and 20, and more preferably between about 5 and 15. As an example, the compliant plate may be constructed of titanium and the voids may be filled with a plastic material that is molded in the plate. 
     Golf club head  1220  may include an alternative compliant plate  1223 , as shown in  FIG. 91 . As an example, compliant plate  1223  includes a plurality of voids  1239  with polygonal shape in the form of polygonal apertures. In the illustrated embodiment, the orientations of the apertures vary through the plate  1223  so that the flexibility of the compliant plate  1223  may be tuned to a desired frequency. 
     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.