[0041] Coefficient of Restitution (COR) is a measurement of the energy lost when two objects collide, such as when a golf club head impacts a golf ball. This measurement is expressed as a number between 0 and 1, wherein 0 represents a collision in which all energy is lost and 1 represents a perfectly elastic collision in which no energy is lost and all energy is transferred from one object to another.

[0042] The COR of a ball striking face of a golf club head can effect the distance a golf ball will travel upon being stuck by the golf club head. The higher the COR of the ball striking face of the golf club head, the more energy it will transfer to the golf ball upon impact. Therefore, if two golf club heads with different CORs are swung at the same speed, the golf club head with the higher COR will transfer more energy and, as a result, the golf ball will travel a further distance. For example, if two golf clubs with golf club heads that have different CORs of 0.82 and 0.83, respectively, are each swung with a swing speed of 100 mph, the difference in the distance the golf ball will travel after impact would be around 3 to 4 yards.

[0043] Therefore, having a golf club head with the highest possible COR would be advantageous as it would provide the longest possible distance. However, the USGA has placed a maximum limit on the COR of a golf club head. The Rules of Golf, as approved by the USGA [see e.g., the 2008 edition], includes a rule at Appendix II, Section 4(c) entitled, Design of Golf Clubs; Clubheads; Spring Effect and Dynamic Properties. The first subsection of this rule states that the design, material and/or construction of, or any treatment to, the club head (which includes the club face) must not have the effect of a spring which exceeds the limit set forth in the Pendulum Test Protocol on file with the USGA. Currently the protocol sets the maximum limit on the COR of a golf club head at 0.830 when the impacting the golf ball at 160 ft/sec. Hence, any golf club head which provides a higher COR when the golf ball is impacted at 160 ft/sec. is illegal and not useable in competitions governed by the USGA. The rationale behind this rule was to protect the integrity of the game and prevent technology from allowing golfers to drive the golf ball "too" far.

[0045] However, the USGA's rule limiting the maximum COR of a club head only specifies an impact at 160 ft/sec. Therefore, if the impact is at a speed less than 160 ft/sec., then the maximum COR may be higher than 0.830. For example, if a golf club is swung at a speed of only 140 ft/sec., then the maximum COR of the golf club head at impact could be more than 0.830. Because most of the energy lost during the impact of club head body with the golf ball comes from the golf ball, if the face of the golf club head body is made so that it is highly flexible and resilient, then the golf ball will lose less energy during the impact and the COR will be increased (e.g., above 0.830).

[0046] Therefore, aspects of this disclosure relate to the double wall structure described above which allows a golf club head to have a ball striking face that provides a high COR (in some cases higher than 0.830), even when swung at slow speeds. In other words, the ball striking face can be made so that is highly flexible and resilient (e.g., more flexible and resilient than a conventional golf club head face that would provide a COR of 0.83 when swung at 160 ft/sec). For example, the ball striking face may be "ultra" thin (e.g., thinner than conventional ball striking faces that provide a COR or 0.83). With the ball striking face being more flexible, deformable, resilient, etc., the golf ball will not lose as much energy at impact and, therefore, the value of the COR of the ball striking face will be increased. As a result, the velocity of the golf ball at impact will be increased and the golf ball will carry farther. As mentioned above, such a ball striking face could provide a COR of 0.83 or higher when swung at typical amateur speeds (e.g., 140 ft/sec).

[0047] However, in order to prevent such a ball striking face from breaking the USGA's COR limit of 0.83 when it is swung at 160 ft/sec., the second wall portion 105 is positioned close enough to the ball striking face 103 to halt the deflection/deformation of the ball striking face 103 and, thereby, limit the maximum obtainable COR if the golf club head is swung at 160 ft/sec or faster. In other words, if such a club is swung at a slow speed, then the deflection of the club face will not be halted by the second wall 105 and, hence, the COR can be as high as the characteristics of the ball striking face 103 (e.g., the dimensions of the ball striking face, such as the thickness; the material used to the make ball striking face, etc.) will allow. On the other hand, if the club is swung at a high speed, the positioning of second wall 105 ensures that the ball striking face cannot deflect fully (i.e., deflect as much as the characteristics of the ball striking face (e.g., the dimensions of the ball striking face, such as the thickness; the material used to the make ball striking face, etc.) will allow) and, hence, cannot achieve the full COR that the characteristics of the ball striking face would have provided. Thereby, the second wall 105 is positioned from the ball striking face 103, based on the characteristics of the ball striking face such that only a maximum COR value of 0.83 is obtainable if the golf club head is swung at 160 ft/sec or faster. Therefore, it is appreciated that the double wall structure allows slow swing speed golfers to achieve the same (or even greater) amount of "trampoline" or "spring-like" effect from the ball striking face 103 as high speed golfers and, hence, slow swing speed golfers are not "penalized" by the USGA's rules regarding COR.

[0048] In view of the above discussion, it is understood that it would be advantageous for a golfer to obtain the highest possible COR allowed by the rules. Further, as described above, the highest possible COR that a particular golfer can achieve would depend, at least in part, on the particular golfer's swing speed and the particular characteristics of the ball striking face 103. Hence, it is appreciated that a particular ball striking face may not provide the maximum COR for golfers with different swing speeds.

[0049] Therefore, aspects of this disclosure relate to selectively removable and interchangeable portions of golf club head bodies that include a double wall structure. Specifically, some aspects of the disclosure relate to ball striking face portions 102 that may be configured to be selectively disengaged (i.e., removed) from the rear portion of the golf club head body. Further, some aspects of the disclosure relate to ball striking face portions 102 are interchangeable with other ball striking face portions that can be attached to the rear portion of the club head body. By providing such ball striking face portions 102, the ball striking face 103 can be customized to the particular golfer.

[0050] Further, in a double wall structure, the positioning of the second wall may also effect the highest possible COR that a particular golfer can achieve. Therefore, aspects of this disclosure relate to second walls of double wall golf club head bodies that may be selectively disengaged (i.e., removed) from the golf club head body 101. Further, some aspects of the disclosure relate to second walls that are interchangeable and adjustable. By providing such second walls, the double wall golf club head body can be customized to the particular golfer.