Golf club head having an internal undercut

Abstract

Described herein is a hollow-body iron-type golf club head having a sole and ballast configured to relieve stress within a forward portion of the sole. In a first configuration, the golf club head includes a ballast undercut for stress relief. In another configuration, the ballast undercut is combined with additional stress relief features, such as a cascade sole, near the face-sole junction to obtain a further reduction in face thickness.

Description

Technical Field

[0001] (Related Application) This application claims the benefit of U.S. Provisional Patent Application No. 63 / 013,341, filed Apr. 21, 2020, which is hereby incorporated by reference in its entirety.

[0002] The present disclosure generally relates to golf equipment, and more particularly, to a flexure structure for obtaining improved performance characteristics of a hollow body iron and a method of manufacturing a hollow body iron having the flexure structure.

Background Art

[0003] A hollow body iron ideally acts as a trampoline and flexes rearwardly during impact. In club design, the extent to which a hollow body iron behaves as a trampoline or spring is constrained by the peak stress value. To ensure that conventional golf clubs do not exceed their maximum stress limits, the face and sole are thickened to make the club more rigid. The rigidity of conventional golf clubs has, as a result, reduced the behavior of the club head as a trampoline or spring.

[0004] Accordingly, there is a need in the art to produce a golf club head having a structure that extends the limits of modification to the face and improves the energy transfer from the club to the ball during impact.

Brief Description of the Drawings

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[0019] For purposes of simplicity and clarity of illustration, these drawings illustrate a general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring this disclosure. Additionally, the elements in the drawings are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to assist in improving the understanding of the embodiments of this disclosure. The same reference numerals in different figures indicate the same elements.

[0020] If so, the terms "first", "second", "third", "fourth", "fifth", etc. in this specification and the claims are used to distinguish similar elements and are not necessarily used to describe a particular order or chronological order. The terms so used are interchangeable under appropriate circumstances. For example, it should be understood that the embodiments described herein can operate in a sequence other than the sequence illustrated or otherwise described herein. Further, the terms "comprising", "having", and any variations thereof are intended to cover non-exclusive inclusions such that a process, method, system, article, device, or apparatus comprising a list of elements is not necessarily limited to those elements and may include other elements not expressly listed in such process, method, system, article, device, or apparatus.

[0021] If so, the terms "left", "right", "front", "rear", "upper", "lower", "above", "below", etc. in this specification and the claims are used for illustrative purposes and are not necessarily for describing a permanent relative position. It should be understood that the terms so used are replaceable in appropriate circumstances such that the embodiments described herein can operate in an orientation other than, for example, that illustrated or otherwise described herein.

[0022] The terms "couple", "coupled", "couples", and "coupling" etc. should be understood broadly and refer to connecting two or more elements or signals electrically, mechanically, and / or in other manners. As used herein, the term "loft" or "loft angle" of a hollow body golf club (hereinafter "hollow body", "hollow body iron", "iron type golf club head", or "golf club head") refers to the angle formed between the club face and the shaft as measured by any suitable loft and lie machine. The loft plane is positioned tangent to the striking face at the geometric center. The loft angle is measured between the ground plane and the loft plane. The loft angle is measured between the ground plane and the loft plane. In many embodiments, the loft angle of the club head is less than about 50 degrees, less than about 49 degrees, less than about 48 degrees, less than about 47 degrees, less than about 46 degrees, less than about 45 degrees, less than about 44 degrees, less than about 43 degrees, less than about 42 degrees, less than about 41 degrees, less than about 40 degrees, less than about 39 degrees, less than about 38 degrees, less than about 37 degrees, less than about 36 degrees, less than about 35 degrees, less than about 34 degrees, less than about 33 degrees, less than about 32 degrees, less than about 31 degrees, less than about 30 degrees, less than about 29 degrees, less than about 28 degrees, less than about 27 degrees, less than about 26 degrees, less than about 25 degrees, less than about 24 degrees, less than about 23 degrees, less than about 22 degrees, less than about 21 degrees, less than about 20 degrees, less than about 19 degrees, less than about 18 degrees, or less than about 17 degrees. Further, in many embodiments, the loft angle of the club head is greater than about 16 degrees, greater than about 17 degrees, greater than about 18 degrees, greater than about 19 degrees, greater than about 20 degrees, greater than about 21 degrees, greater than about 22 degrees, greater than about 23 degrees, greater than about 24 degrees, greater than about 25 degrees, greater than about 26 degrees, greater than about 27 degrees, greater than about 28 degrees, greater than about 29 degrees, greater than about 30 degrees, greater than about 31 degrees, greater than about 32 degrees, greater than about 33 degrees, greater than about 34 degrees, greater than about 35 degrees, greater than about 36 degrees, greater than about 37 degrees, or greater than about 38 degrees. **DETAILED DESCRIPTION OF THE INVENTION**

[0023] The present disclosure describes a technique for an improved, hollow-body iron-type golf club head (hereinafter, "hollow body", "hollow body iron", "iron-type golf club head", or "golf club head") having a sole and a ballast configured to relieve stress within a front portion of the sole. In a first configuration, the golf club head includes a ballast undercut for relieving stress. In other configurations, the ballast undercut is combined with additional stress relief features, such as a cascading sole, in the vicinity of the face-sole junction to obtain further reduction in face thickness.

[0024] The hollow body can include a striking face, a rear portion opposite the striking face, a heel portion, a toe portion opposite the heel, a sole, and a topline, and define an inner void. The rear portion can further include a ballast extending forwardly from the rear portion and into the inner void. In many embodiments, the ballast is an internal component that is not visible from the outside of the golf club. The ballast can further have a geometry configured to increase the inner surface area of the sole. For example, the ballast can include a top surface, a front surface, and a bottom surface defined as an undercut region. The undercut is formed by a concave geometry of the bottom surface with respect to the face when viewed from a toe cross-section. The undercut allows a thinner front portion of the sole to extend under the ballast. The ballast with the bottom undercut surface, as opposed to the front surface that meets the inner surface of the sole at a right angle, (1) prevents stress from concentrating between the face and the ballast along the sole, and (2) increases the portion of the sole capable of accumulating strain energy. Thus, the hollow body iron with an undercut has a geometry of the sole and the face with a greater range of thinning compared to a hollow iron without an undercut.

[0025] The sole of the hollow body iron can be divided into two regions, namely, a front portion and a rear portion. The front portion defines a thin region of the sole adjacent to the striking face, and the thin region can accumulate strain energy. The rear portion of the sole describes the region of the sole adjacent to the rear portion of the body, and the region of the sole does not accumulate strain energy. In other words, the front portion 132 of the sole is the portion of the sole 110 that behaves as a spring. As a result of the weight relief undercut, a hollow body iron having a thinner face and an extended front sole portion accumulates more strain energy (i.e., potential energy) than the face and front sole portion of a club without an undercut. Consequently, the undercut improves the spring-like energy transfer between the club body and the golf ball (compared to a golf club without an undercut). This energy transfer can be further improved in the hollow body iron if the front sole portion also has a cascade in addition to the undercut. The cascade sole improves the flow of stress within the front portion of the sole near the face-sole junction, while the undercut improves the flow of stress near the weight. Therefore, the application of the undercut and / or the combined application of the undercut and the cascade sole can result in a golf club head that can tolerate a face that is 3 to 8% thinner. Thus, a thinner face, which was previously unattainable, results in an improved flight trajectory and distance. I. Undercut

[0026] Of the drawings, FIG. 1 depicts a perspective view of the outside of an iron-type golf club head 100 having an internal stress relief sole 110 and a ballast 114 having an undercut 102 shown in FIG. 2A. The golf club head 100 has a hollow body structure having an internal void 104. The hollow body structure of the golf club head 100 is further defined by a striking face 106, a rear portion 108 opposite the striking face 106, a heel portion 103, a toe portion 105 opposite the heel portion 103, a sole 110, and a top rail 112 opposite the sole 110.

[0027] FIG. 2A illustrates a heel breakaway view of the golf club head 100 of FIG. 1 along the cross-section line I-I. FIG. 2A shows the internal void 104 and the stress relief features of the golf club head 100. The rear portion 108 further includes a ballast 114 positioned within the internal void 104. As shown in FIG. 2, the ballast 114 is a monolithic weight element necessary for optimal CG (center of gravity) positioning in the golf club head 100. The ballast 114 projects vertically from the sole 110 and forward from the rear portion 108 and has a solid structure extending in the heel-to-toe direction along the sole 110. The front portion 132 of the sole is defined between the striking face 106 and the ballast 114.

[0028] Continuing to refer to FIG. 2B, the weight 114 includes a top surface 116, a front surface 118, and a bottom surface 120. As illustrated, the bottom surface 120 is contoured to create undulations that define an undercut region 128 having an undercut 102. The undercut region 128 of the weight 114 can be considered as the undercut region 128 of the material removed from the weight 114 adjacent to the inner surface 122 of the sole 110. The undercut region 128 includes an undercut 102 and an undercut transition 141. The undercut region 128 extends transversely in the heel-to-toe direction across the heel-to-toe length 124 of the weight 114. In the illustrated embodiment, the undercut 102 is generally centered within the club head 100 between the heel portion 103 and the toe portion of the golf club 100. As shown in FIG. 2B, the undercut 102 extends under the weight 114 such that the front portion 132 of the sole 110 abuts the boundary between the face and the undercut 102 / bottom surface 120 of the weight 114. The front portion 132 of the sole 110 is effectively lengthened as compared to a golf club head without an undercut (i.e., the front portion defined between the striking face and the front surface of the weight). Thus, the undercut 102 not only reduces stress in the front portion 132 of the sole, but also creates a larger spring (i.e., the front portion of the sole) for transferring and returning energy to the ball upon impact.

[0029] Figure 2B depicts an enlarged view of the ballast 114 and the undercut 102 shown in the cross-section of Figure 2A. As shown in Figure 2B, the ballast 114 includes a top surface 116, a front surface 118, and a bottom surface 120. The ballast 114 projects vertically from the inner surface of the sole 106 along the inner surface of the rear portion 108. The bottom surface 124 has a contoured geometric shape that extends inwardly from the front surface 118 towards the rear portion 108 and defines the undercut 102, which extends in the heel-to-toe direction. Continuing with reference to the cross-section of Figure 2B, the ballast bottom surface 120 further includes an undercut joint 130 defined as the joint between the ballast bottom surface 120 and the inner surface 122 of the sole 110. The undercut joint 130 is the trailing point of the ballast bottom surface 120 that defines the undercut 102. As shown, the front portion 132 of the sole is defined between the striking face 106 and the undercut joint 130 rather than between the striking face 106 and the front surface 118 in a hollow body iron that does not have the undercut 102.

[0030] Referring to FIG. 2B, the undercut 102 is defined by four parameters, namely, the undercut depth 134, the undercut height 136, the undercut length 138, and the undercut sole thickness 123. Further, the ballast bottom surface 120 can be curved such that the undercut 102 is defined between the undercut bottom edge 139 and the undercut top edge 137. The undercut depth is measured as the vertical distance between the ballast front face 20 and the undercut joint 130 (i.e., the trailing point of the undercut). The undercut height 136 is defined as the vertical distance between the undercut top edge 137 and the undercut bottom edge 139. The undercut length is measured parallel to the ground surface 10 between the undercut toe end 133 and the undercut heel end 135. Finally, the undercut sole thickness 123 is measured as the vertical distance between the outer surface 121 of the sole and the inner surface 121 of the sole. In the first embodiment, the undercut 102 has a depth 134 of 0.065 inches, a height 136 of 0.083 inches, and a length of 1.16 inches.

[0031] The undercut depth 134 between the front side 20 of the ballast and the undercut joint 130 has a range from 0.010 inches to 0.100 inches. For example, the undercut depth 134 can be 0.010 inches, 0.015 inches, 0.020 inches, 0.025 inches, 0.030 inches, 0.035 inches, 0.040 inches, 0.045 inches, 0.050 inches, 0.055 inches, 0.060 inches, 0.065 inches, 0.070 inches, 0.075 inches, 0.080 inches, 0.085 inches, 0.090 inches, 0.095 inches, or 0.100 inches. Alternatively, the undercut face depth 131 can be measured as the vertical distance between the striking face 106 and the undercut joint 130. In some embodiments, the undercut depth from the face ranges from 0.200 inches to 0.500 inches. For example, the undercut depth from the face can be 0.200 inches, 0.220 inches, 0.240 inches, 0.260 inches, 0.280 inches, 0.300 inches, 0.320 inches, 0.340 inches, 0.360 inches, 0.380 inches, 0.400 inches, 0.420 inches, 0.440 inches, 0.460 inches, 0.480 inches, or 0.500 inches.

[0032] The undercut height 136 measured between the undercut bottom edge 137 and the undercut top edge 139 can range from 0.030 inches to 0.200 inches. For example, the undercut height 136 can range from 0.030 inches to 0.040 inches, 0.040 inches to 0.050 inches, 0.050 inches to 0.060 inches, 0.060 inches to 0.070 inches, 0.070 inches to 0.080 inches, 0.080 inches to 0.090 inches, 0.090 inches to 0.100 inches, 0.100 inches to 0.110 inches, 0.110 inches to 0.120 inches, 0.120 inches to 0.130 inches, 0.130 inches to 0.140 inches, 0.140 inches to 0.150 inches, 0.150 inches to 0.160 inches, 0.160 inches to 0.170 inches, 0.170 inches to 0.180 inches, 0.180 inches to 0.190 inches, or 0.190 inches to 0.200 inches.

[0033] Figure 3 shows a front view of golf club 100 with the striking face 106 removed, exposing the undercut length 138 that extends from the undercut heel end 135 to the undercut toe end. In some embodiments, the undercut length 138 ranges from 0.5 inches to 3.0 inches. In other embodiments, the undercut length ranges from 0.050 inches to 0.075 inches, 0.075 inches to 0.100 inches, 0.100 inches to 0.125 inches, 0.125 inches to 0.150 inches, 0.150 inches to 0.175 inches, 0.175 inches to 0.200 inches, 0.200 inches to 0.225 inches, 0.225 inches to 0.250 inches, 0.250 inches to 0.275 inches, or 0.275 inches to 0.300 inches. Figure 3 further shows the balance length 124 that can be measured from the balance heel end 125 to the balance toe end 127. In some embodiments, the balance length 124 ranges from 1.0 inches to 3.0 inches. In other embodiments, the balance length 124 is 1.2 inches, 1.4 inches, 1.6 inches, 1.8 inches, 2.0 inches, 2.2 inches, 2.4 inches, 2.6 inches, 2.8 inches, or 3.0 inches.

[0034] The undercut length 138, measured as the distance between the undercut heel end and the undercut toe end, may further define a ratio of the balance length 124 that describes the portion of the balance 114 that includes the undercut 102. In embodiments of an iron-type golf club head that includes an undercut 102, the undercut 102 can increase the surface area that receives impact loads. The balance length ratio can be calculated by dividing the undercut length 138 by the balance length 124. In some embodiments, the undercut ratio of the balance length ranges from 20% to 100%. The length of the undercut can range from 10% of the balance length to a maximum of the same length as the balance length (i.e., 100%). For example, the balance length ratio is 20%, 25%, 30%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%.

[0035] In addition, as shown in FIG. 2B, the undercut transition height 142 is defined as the vertical distance between the inner surface of the sole 122 and the lower edge of the front surface 140. In some embodiments, the transition height 142 can range from 0.150 inches to 0.300 inches. It can range from 0.150 inches to 0.160 inches, 0.160 inches to 0.170 inches, 0.170 inches to 0.180 inches, 0.180 inches to 0.190 inches, 0.190 inches to 0.200 inches, 0.200 inches to 0.210 inches, 0.210 inches to 0.220 inches, 0.220 inches to 0.230 inches, 0.230 inches to 0.240 inches, 0.240 inches to 0.250 inches, 0.250 inches to 0.260 inches, 0.260 inches to 0.270 inches, 0.270 inches to 0.280 inches, 0.280 inches to 0.290 inches, or 0.290 inches to 0.300 inches. In the first embodiment described above, the transition height is 0.185 inches. The undercut 102 smoothly transitions to the ballast front surface 118 by the undercut transition 141 having the transition height 142 and the contour profile. This smooth transition promotes a uniform flow of stress through the undercut 102 and the ballast 114.

[0036] As described above, the undercut 102 and the undercut region 128 can be considered as regions where the ballast material has been removed, when compared to an iron-type golf club head without an undercut. The undercut volume 146 is defined by the surface 146 of the undercut region 128 and the ballast front face 20. For example, in one embodiment, the surface 146 of the undercut region and the ballast front face 20 define an undercut volume 146 of 0.018 cubic inches. In other embodiments, the undercut volume ranges from 0.018 cubic inches to 0.050 cubic inches. For example, the undercut volume 146 can be 0.018 cubic inches, 0.020 cubic inches, 0.022 cubic inches, 0.024 cubic inches, 0.026 cubic inches, 0.028 cubic inches, 0.030 cubic inches, 0.032 cubic inches, 0.034 cubic inches, 0.036 cubic inches, 0.038 cubic inches, 0.040 cubic inches, 0.042 cubic inches, 0.044 cubic inches, 0.046 cubic inches, 0.048 cubic inches, or 0.050 cubic inches. The undercut volume 146 can be utilized to form the mass removed from the ballast 114 by the undercut region 128. The mass is calculated by multiplying the undercut volume 146 by the material density of the ballast 114. For example, the undercut volume ranges from 0.018 cubic inches to 0.030 cubic inches. The undercut volume can be 0.018 cubic inches, 0.020 cubic inches, 0.021 cubic inches, 0.024 cubic inches, 0.026 cubic inches, 0.028 cubic inches, or 0.030 cubic inches. The material removed from the ballast to form the undercut has a material density in the range of 6.0 g / cm 3 to 7.75 g / cm 3 and thus has a mass of from 1.75 grams to 2.40 grams. The material removed from the ballast to form the undercut is 6.0 g / cm 3 6.5 g / cm 3 7.0 g / cm 3 or 7.5 g / cm 3The material density, i.e., the mass is from 1.75 grams, 2.0 grams, 2.20 grams, 2.32 grams, or 2.40 grams from the ballast 114.

[0037] The front portion 132 of the sole 110 that extends from the striking face 106 to the ballast 114 affects the impact response of the golf club head 100 with a golf ball. As shown in FIG. 2, the undercut joint 130 is spaced further rearward from the striking face 106 than the front surface 118 of the ballast. What the additional distance of the undercut joint from the striking face 106 means is that the thinner front portion 132 of the sole 110 is effectively lengthened (compared to a conventional golf club head lacking the undercut 102) such that a portion of the front sole portion extends under the ballast 114 (with respect to the overall front-to-back sole width). The front sole length can be measured as the vertical distance between the undercut joint 130 and the face plane 130. In some embodiments, the effective increase in length ranges from 6% to 12%. For example, the undercut 102 can increase the length of the front sole portion 132 by 6% to 7%, 7% to 8%, 8% to 9%, 9% to 10%, and 11% to 12%. By increasing the length of the thinned front portion 132 of the sole 110, the peak stress value of the golf club head 100 is reduced. The undercut 102 allows the front portion 132 of the sole 110 between the striking face 106 and the ballast 114 to flex to a greater extent under impact loading, rather than behaving as a rigid connection, thereby creating stress relief at the face-sole transition 126. The effective increase in the length of the front sole 132 due to the undercut results in an increase in the total surface area over which the impact load is distributed, resulting in a stress reduction of 1000 psi to 2000 psi within the front portion 132 of the sole. The undercut 102 non-uniformly reduces stress concentration within the front sole portion 132 and increases the bending / spring effect of the front sole portion 132. Additionally, the undercut 102 reduces the peak stress value within the striking face 106 by 2000 psi to 3500 psi.For example, the undercut can reduce the peak stress value in the hitting face to be between 2000 psi and 2100 psi, between 2100 psi and 2200 psi, between 2200 psi and 2300 psi, between 2300 psi and 2400 psi, between 2400 psi and 2500 psi, between 2500 psi and 2600 psi, between 2600 psi and 2700 psi, between 2700 psi and 2800 psi, between 2800 psi and 2900 psi, between 2900 psi and 3000 psi, between 3100 psi and 3200 psi, between 3200 psi and 3300 psi, between 3300 psi and 3400 psi, or between 3400 psi and 3500 psi.

[0038] Even just the above-described reduction in stress within the sole 110 and the hitting face 106 can lead to an improved wear life of the golf club head 100. In other words, the golf club head 100 provided with the weight 114 having the undercut 102 can be hit more times and played for a longer period than a conventional golf club head without an undercut. For example, a hollow body golf club provided with the undercut 102 can have an increase in the failure count of 50 hits, 100 hits, 150 hits, 200 hits, 250 hits, or 300 hits. Fatigue failure of a golf club subjected to repeated loads occurs over time at the location of the peak stress where small cracks have occurred in the material. The cracks then amplify the stress. Therefore, the golf club head 100 with reduced peak stress experiences crack growth and the resulting fatigue failure at a slower rate.

[0039] Alternatively, the stress reduction achieved by the above-mentioned weight 114 and undercut 102 can be utilized to improve club performance and ball speed. In some embodiments, the weight 114 having the undercut 102 can be provided in combination with the thinned striking face 106. The striking face of a golf club head without the undercut 102 is constrained by the peak stress level at the face-sole transition. In other words, it is impossible to improve the performance of a conventional golf club using a thinner face because the additional stress from the thinner face results in peak stresses that exceed the critical K value. As discussed above, the golf club head 100 includes the weight 114 having the undercut 102 for stress reduction. Thus, in some embodiments, the striking face 106 can be thinned without causing the peak stress value to rise above the critical K value at the sole-face transition.

[0040] Thinning can be applied to the entire face. For example, at the geometric center of the face of an undercut club, the thickness of this area of the face can range from 0.080 inches to 0.150 inches. The thickness of the face at the geometric center of the face can be 0.150 inches, 0.140 inches, 0.130 inches, 0.120 inches, 0.110 inches, 0.100 inches, 0.090 inches, or 0.080 inches. In the peripheral toe area of the face of an undercut club, the thickness of the face can range from 0.050 inches to 0.090 inches. The thickness of the face in the peripheral toe area can be 0.0500 inches, 0.060 inches, 0.065 inches, 0.070 inches, 0.071 inches, 0.074 inches, 0.076 inches, 0.077 inches, 0.079 inches, 0.080 inches, 0.082 inches, 0.084 inches, 0.086 inches, 0.088 inches, or 0.090 inches. The thickness of the face in the peripheral heel area of an undercut club can range from 0.045 inches to 0.090 inches. The thickness of the face in the peripheral heel area can be 0.045 inches, 0.050 inches, 0.055 inches, 0.060 inches, 0.065 inches, 0.070 inches, 0.075 inches, 0.080 inches, 0.085 inches, or 0.090 inches.

[0041] In some embodiments, the weight 114 having the undercut 102 reduces the face thickness by 0.003 inches. In other examples, the striking face 106 can be thinned by 0.004 inches, 0.005 inches, 0.006 inches, 0.007 inches, 0.008 inches, 0.009 inches, or 0.010 inches by the undercut 102. In a thin striking face 106, this reduction is equivalent to a thinning of approximately 6%, i.e., increasing the ball speed by 0.5 mph to 0.7 mph. In some embodiments, the striking face is 3 to 8% thinner by the undercut 102 than the striking face of a golf club head without an undercut. For example, the striking face 106 can be thinned by 3%, 4%, 5%, 6%, 7%, or 8%.

[0042] As discussed above, the undercut region 128 has a volume 146 that represents the mass removed from the ballast 114. The ballast 114 functions as a mass pad for controlling the center of gravity (CG) for the golf club head 100, thereby enabling the undercut 102 to change the club head CG. The CG can be defined relative to the geometric center 126 of the striking face 106. The geometric center 126 of the striking face 118 can be identified in accordance with Section 6.1 of the USGA's procedure for measuring the flexibility of a golf club head (USGA-TPX3004, Revision 1.0.0, May 1, 2008) (available at http: / / www.usga.org / eqipment / testing / protocols / Procedure-For-Measuring-The-Flexibility-Of-A-Golf-Club-Head / ) ("Flexibility Procedure"). The front-to-back CG depth 144 can be defined as the horizontal distance between the geometric center 126 and the CG. For example, the front-to-back CG depth 144 can range from 0.080 inches to 0.110 inches. The front-to-back CG depth can be 0.080 inches, 0.082 inches, 0.084 inches, 0.086 inches, 0.088 inches, 0.090 inches, 0.092 inches, 0.094 inches, 0.096 inches, 0.098 inches, 0.100 inches, 0.105 inches, or 0.110 inches.

[0043] The ratio of the undercut face depth 146 to the front-to-back CG position is constrained to be between 3.0 and 5.5. For example, the face depth ratio can be 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, or 5.0. Within this range, the undercut 102 improves the peak stress within the front sole portion 132 without removing material from the ballast to the extent that the CG position is impaired.

[0044] Furthermore, due to the CG position, the undercut does not affect the overall MOI of the club. For the purpose of specifying the moment of inertia of the club head, at the CG, a coordinate system can be defined via axes (i.e., the x-axis, y-axis, and z-axis) (not shown in the figure) that are orthogonal to each other. The y-axis extends perpendicular to the ground plane 10, through the head CG, from the topline 112 to the sole 110 when the club head is in the address position. The x-axis extends perpendicular to the y-axis, through the head CG, from the heel 103 to the toe 105. The z-axis extends perpendicular to the x-axis and y-axis, through the head CG, from the striking face 106 to the rear portion 108.

[0045] The moments of inertia exist about the x-axis Ixx (i.e., the topline-to-sole moment of inertia), about the y-axis Iyy (i.e., the heel-to-toe moment of inertia), and about the z-axis (i.e., the striking face-to-rear). In many embodiments, a golf club head having an undercut has a topline-to-sole moment of inertia, Ixx, of from 96 g·inch 2 to 130 g·inch 2 In many embodiments, a golf club head having an undercut has a topline-to-sole moment of inertia, Ixx, greater than about 95 g·inch 2 greater than about 98 g·inch 2 greater than about 100 g·inch 2 greater than about 102 g·inch 2 greater than about 103 g·inch 2 greater than about 104 g·inch 2 greater than about 105 g·inch 2 greater than about 106 g·inch 2 greater than about 110 g·inch 2 greater than about 115 g·inch 2 greater than about 125 g·inch 2 greater than about 135 g·inch 2 greater than about 140 g·inch 2 greater than about 145 g·inch 2It has a top rail to sole moment of inertia, Ixx, that is greater than. Further, in many embodiments, a golf club head having an undercut has an Ixx of about 350 g·inch 2 greater than about 360 g·inch 2 greater than about 370 g·inch 2 greater than about 380 g·inch 2 greater than about 390 g·inch 2 greater than about 400 g·inch 2 greater than about 410 g·inch 2 greater than about 420 g·inch 2 greater than or about 430 g·inch 2 It has a heel to toe moment of inertia, Iyy, that is greater than. In many embodiments, a golf club head having an undercut has an Iyy in the range of 350 g·inch 2 to 420 g·inch 2 Furthermore, a club head having an undercut has an Iyy of about 400 g·inch 2 greater than about 410 g·inch 2 greater than about 420 g·inch 2 greater than about 430 g·inch 2 greater than about 440 g·inch 2 greater than about 450 g·inch 2 greater than about 460 g·inch 2 greater than about 470 g·inch 2 greater than or about 480 g·inch 2 It has a face to back moment of inertia, Izz, that is greater than. In many embodiments, a golf club head having an undercut has an Izz that can be in the range of 400 g·inch 2 to 450 g·inch 2 The undercut of the golf club head does not significantly alter the moments of inertia Ixx, Iyy, and Izz with respect to a golf club head without an undercut. II. Undercut and Cascading Sole

[0046] Figure 4A illustrates another embodiment of a golf club head 200 having a ballast 214, an undercut 202, and a cascaded front portion 232 of the sole 210. FIG. 4 depicts a cross-sectional view of the golf club head 200. The golf club head 200 is substantially similar to the golf club head 100 and includes a thin front portion 232 of the sole 210 that is effectively lengthened via the undercut 202. The golf club head 200 further includes a striking face 206, a rear portion 208 opposite the striking face 206, a heel portion 203, a toe portion 205 opposite the heel portion 203, a sole 210, and a topline 212 opposite the sole 210. Together, these components define a hollow body club having an inner void 204. The rear portion 206 further includes a ballast 214 positioned within the inner void 204. As shown in FIG. 4, the ballast 214 includes a top surface 216, a front surface 218, and a bottom surface 220. The ballast bottom surface 220 is similar to the ballast bottom surface 120. The contoured bottom surface 220 is recessed toward the rear portion 208 to create the undercut 202.

[0047] Figure 4B provides an enlarged view of the ballast 214 and sole 210 illustrated in FIG. 4. As shown, the front portion 232 of the sole 210 extends from the undercut 202 in the ballast 214 to the striking face 206. The front portion 232 of the sole further includes an inner region 260 and a cascaded region 262. The cascaded region 262 can include an inner radius transition 264 between the inner surface of the striking face 206 and the inner surface of the sole 210. The cascaded region 262 can include at least two thickness steps or levels. The stepped structure results in a progressive thinning of the front sole portion 132. In some embodiments, the cascaded region 262 can include an inner radius transition 264 having 2, 3, 4, 5, 6, or 7 levels.

[0048] Continuing to refer to FIG. 4B, the cascade region 262 includes a first step 266, a second step 268, and a step transition portion 270 between the first step 266 and the second step 268. The cascade region 262 of the front sole portion 232 can have a thickness measured as the vertical distance between the outer surface 221 of the sole and the inner surface 222 of the sole. This thickness can decrease in the front-rear direction across the cascade region 262. The first step 266 can have a first thickness 272 defined as the vertical distance between the outer surface 221 and the inner surface 222 of the sole within the first step 266. The second step 268 can have a second thickness 274 defined as the vertical distance between the outer surface 221 and the inner surface 222 of the sole within the second step 268. In some embodiments, the first thickness 272 is greater than the second thickness 274, thereby causing the overall thickness of the cascade region 262 to decrease in the front-rear direction. The first thickness 272 and / or the second thickness 274 can have a constant thickness across the step length in the front-rear direction. In other embodiments, the first thickness 272 and / or the second thickness 274 can be tapered such that the thickness decreases across the step length in the front-rear direction.

[0049] The cascade region can include a first step 266, a second step 268, a third step (not shown), a first step transition portion 270 between the first step 266 and the second step 268, and a second step transition portion between the second step and the third step. As described above, the cascade region of the front sole region having three steps can have a thickness measured as the vertical distance between the outer surface and the inner surface of the sole. This thickness again decreases in the front-rear direction across the cascade region. As described above, the first step can have a first thickness. The second step can have a second thickness. The third step can have a third thickness, and the third step thickness is (like the first step thickness and the second step thickness) measured as the vertical distance between the outer surface and the inner surface of the sole. In some embodiments, the first thickness is greater than the second thickness, and then the third thickness is greater than the second thickness, such that the overall thickness of the cascade region 262 decreases in the front-rear direction. The first thickness and / or the second thickness and / or the third thickness can have a constant thickness across the step length in the front-rear direction. In other embodiments, the first thickness and / or the second thickness and / or the third thickness can be sloped such that the thickness decreases across the step length in the front-rear direction.

[0050] The stage transition part 270 is located between the trailing edge of the first stage and the leading edge of the second stage, and can be inclined in the front-rear direction so that the cascade region thickness decreases uniformly between the first thickness 272 and the second thickness 274. Alternatively, in a cascade region having two stage transition parts (i.e., the first transition part between the first stage and the second stage, and the second transition part between the second stage and the third stage), these transition parts can be inclined in the front-rear direction to uniformly decrease the cascade region thickness among the first thickness, the second thickness, and the third thickness (or the first stage, the second stage, and the third stage). In some embodiments such as FIG. 4B, the stage transition part 270 is linearly inclined at an angle less than 45 degrees adjacent to and between the first stage 266 and the second stage 268. In some embodiments, the stage transition part 270 is linearly inclined at an angle in the range between 10 degrees and less than 45 degrees. The linear inclination can be gradual between 5 degrees and 10 degrees, between 10 degrees and 15 degrees, between 15 degrees and 20 degrees, between 20 degrees and 25 degrees, between 25 degrees and 30 degrees, between 30 degrees and 40 degrees, or between 40 degrees and 45 degrees. Although not shown, in other embodiments, the stage transition part 270 can be steeper and more step-like. For example, the stage transition part 270 can be between 45 degrees and 50 degrees, between 50 degrees and 55 degrees, between 55 degrees and 60 degrees, between 60 degrees and 65 degrees, or between 65 degrees and 70 degrees.

[0051] As described above, the front sole part 232 further includes an inner region 260 between the cascade region 262 and the ballast undercut 202. The uniform inner region 260 also has an inner thickness 276 defined as the vertical distance between the outer surface 221 of the sole and the inner surface 222 of the sole. The inner thickness 276 is less than the thickness of the adjacent stage or the last stage in the cascade region 262. As shown in FIG. 4B, the inner thickness 276 is less than the second thickness 274.

[0052] Continuing to refer to FIG. 4B, the inner region 260 of the front sole portion 232 can be effectively lengthened by the ballast 214 having the undercut 202. The ballast 214 is substantially similar in geometric shape to the ballast 114. The ballast bottom surface defines an undercut region 228 having an undercut 202, an undercut transition portion 241, and an undercut joint portion 230. As shown in FIG. 4B, the inner region 260 is positioned adjacent to the undercut 202. The undercut region 228 functions in substantially the same manner as the undercut 202 and the undercut region 228. Specifically, the undercut region 228 reduces stress concentration within the front sole portion 232 and also increases the bending / spring effect of the front sole portion 232.

[0053] In many embodiments, the performance improvements resulting from the cascade sole 262 and the undercut 202 are even greater. In other words, a golf club head having both a cascade region and an undercut 202, such as the hollow body club 202, has a significantly reduced peak stress compared to a golf club head having only one of the cascade region or the undercut. The reduction in peak stress within the front sole portion 232 increases the tolerance of the region to modifications for improving ball speed. Specifically, the hollow body club 200, which includes the front sole region 232 defined by the undercut 202 and also includes a cascade region, can have a thinner face (compared to a hollow body club lacking either or both of the undercut and the cascade sole). This results in better ball speed and carry distance. In some embodiments, the undercut 202 and the cascade sole 242 enable the front portion 232 of the sole to be more responsive. The front portion 232 can be made thinner rather than remaining rigid, causing the front portion 232 to behave as a spring under impact loads. This means that the golf club head 200 is more efficient in transferring swing energy to the golf ball. The ultimate increase in ball speed through the reduction in the average thickness of the front portion 232 of the sole is a result of stress reduction at the face-sole transition 226. The undercut and the cascade sole act together to improve the stress flow within the front portion 232, thereby reducing the stress concentration level at impact.

Example

[0054] [Example 1: Consideration of Undercut in a Hollow Body Iron] As described in detail above, the sole and undercut can be applied to the golf club head, either alone or in combination with other features such as a cascading sole, to improve club performance. In the following examples, the performance improvement resulting from the undercut 102 was examined by comparing a golf club head without an undercut (golf club A, hereinafter referred to as "club A"), a golf club head with an undercut (golf club B, hereinafter referred to as "club B"), a golf club head without an undercut and with a cascading sole (golf club C, hereinafter referred to as "club C"), and a golf club head with an undercut and with a cascading sole (golf club D, hereinafter referred to as "club D"). The performance improvement was measured and analyzed using finite element analysis (FEA). Specifically, FEA was used to measure the peak stress values within the front portion. The average peak stress, along with the measured surface area experiencing the peak stress, was used to identify the potential of each club to efficiently transfer and return impact energy to the ball. The reduction in average peak stress serves as an indicator for improved durability and potential performance improvement through face and sole thinning.

[0055] Each of clubs A, B, C, and D of the examples had the same overall mass, material construction, and loft angle and was substantially similar. The impact load on each club was simulated at 105 mph. Each of the clubs of the examples had the above-described unique internal void configuration. The average peak stress between the striking face and the ballast within the front portion of the sole was calculated for each example. Similarly, the area of the average peak stress was calculated for each example. Finally, the average peak stress within the striking face was calculated for each example. Table 1 below shows the peak face stress, the peak stress of the front sole portion, and the peak stress area within the front portion of the sole for each of the clubs of the examples discussed below. Stress values were used to identify the effect on club performance through thinning of the face and sole of the undercut. Club A of the examples was compared to club B. Club C of the examples was compared to club D. The control club head was similar to the club heads of the examples but did not have any stress-relieving features. [Table 1] [Club A]

[0056] Club A represented a prior art golf club head lacking all stress-relieving features and was similar to FIG. 5. Club A represented a conventional hollow body golf club head, included a ballast without an undercut, and did not have a cascade sole. Without an undercut, the front portion of the sole and the ballast met at substantially a right angle. Similarly, without a cascade sole, the striking face transitioned smoothly to the front portion of the sole.

[0057] As shown in Table 1, using FEA analysis, the value for the peak stress within the striking face of club A was calculated. Under an impact load of 105 mph, the peak stress of the striking face was 218,469 psi. Under the same impact load, the front portion of the striking face had a peak stress of 157,440 psi. [Club B]

[0058] Club B represented a hollow body golf club head having undercut stress relief features. The hollow body club B was similar to club A, but club B included an undercut as a stress relief feature. The undercut enabled the front portion of the sole to extend under the weight rather than meet at a right angle. The undercut of Example 1 had a depth of 0.065 inches, a height of 0.083 inches, an undercut transition height of 0.185 inches, and a length of 1.16 inches.

[0059] Values for peak face stress, peak front sole stress, and peak stress area were determined using FEA analysis and simulated impact with a golf ball at 105 mph. The peak face stress was 217,117 psi and the peak front sole stress was 156,257 psi. Compared to club A, the undercut reduced the peak stress in the striking face by 1,352 psi and the peak stress in the front portion of the sole by 1,183 psi. This club demonstrated that both the striking face and the front portion of the sole were able to accumulate more strain energy due to the weight and undercut. This means that club B demonstrated improved durability and improved spring response to impact loads. [Club C]

[0060] The hollow body club C represented a club head having only the front portion of the sole with a cascade. Club C was similar to clubs A and B but had a cascade sole as a single form of stress relief. The transition portion from the face to the sole had a first step, a second step, and a step transition portion between the first step and the second step. The first step had a first step thickness, and the second step had a second step thickness that was less than the first step thickness. The step transition portion was inclined so as to gradually transition from the first step thickness to the second step thickness. This embodiment did not have an undercut, and the front portion of the sole and the ballast met at a substantially right angle.

[0061] Referring again to Table 1, the hollow body golf club head of Example 2 had a peak face stress of 213311 psi, or the peak stress in the striking face was reduced by 5158 psi. The club of Example 2 had a peak front sole stress of 154742 psi (pounds per square inch), or the peak front sole stress was reduced by 2698 psi. This example showed that the cascade sole reduced stress through an increase in the accumulation of strain energy, resulting in improved durability and spring response under impact loads. [Club D]

[0062] Club D (shown as FIG. 6) had an undercut and a cascade sole as two forms of stress relief for the striking face and the front sole portion. The ballast had an undercut, and this undercut effectively lengthened the front sole portion under the ballast. The cascade sole had a first step, a second step, and a step transition portion between the first step and the second step. The first step had a first step thickness, and the second step had a second step thickness that was less than the first step thickness. The step transition portion was inclined so as to gradually transition from the first step thickness to the second step thickness.

[0063] Club D was also subjected to FEA analysis under simulated ball impact at 105 mph. With a reduction in peak stress in the hitting face of 8618 psi, the peak face stress was 209851 psi. In other words, Club D had a 4% reduction in peak stress within the hitting face. The peak stress in the front sole portion was 154689 psi. The front sole portion had a peak stress reduction of 3480 psi, or a 2.2% reduction from Club A. This example showed that both the undercut and the cascaded sole acted together to reduce the peak stress. Further, this example showed that the front portion of the sole could tolerate additional loads without reaching fatigue failure. This example showed that it was possible to improve ball speed by thinning the face and sole to match the load-bearing capacity of the front sole portion.

[0064] The peak stress in the front sole portion for each of these club heads specifically showed the potential to adjust the thickness of the sole and face and the resulting change to ball speed. The peak stress in the front sole portion was compared to the limiting K yield stress value of the front sole portion. The stress that indicated that the hitting face and sole had to be thickened informed that the internal void configuration would have reduced the ball speed. The stress that indicated that the hitting face and sole could be thinned informed that the internal void configuration would have increased the ball speed.

[0065] Club A and Club B were compared to each other with respect to a limiting K value of 156 ksi. The peak stress of Club A, which had no undercut, was 158,169 psi. This peak stress value suggested that the sole and face would have needed to be thickened by approximately 2.5% in order to achieve stress values not exceeding 156 ksi. The thickened face and sole indicated that the internal void configuration would reduce the ball speed. Club B had an undercut and improved the peak stress in the front sole portion. Club B had a peak stress of 156,868 psi. The lower peak stress of Club B indicated that Club B did not require as much thickening of the sole and face as Club A's sole and face. These results showed that after modification, Club B and the undercut exhibited better ball speed than Club A, which had no undercut.

[0066] Similarly, Club C and Club D were compared to each other with respect to the same limiting K value of 156 ksi. The peak stress of Club C, which had a cascade sole and no undercut, was 155,416 psi. Club C had a peak stress slightly below the limiting K stress, indicating that no modification would have been necessary to improve or reduce the ball speed. The slightly lower peak stress suggested that the cascade sole in Club C would have increased durability. Club D had an undercut in addition to the cascade sole and had a peak stress of 154,689 psi. Club D showed that the undercut resulted in a further reduction in peak stress. This reduction in stress indicated that Club D had a face and sole that would allow for thinning to improve the ball speed.

[0067] The comparison between Club A and Club B, and the comparison between Club C and Club D showed that the undercut reduced the peak stress within the front portion of the sole. These results further showed that by applying an undercut to a hollow body golf club head and leveraging the stress reduction to thin the face and sole, it is possible to improve ball speed. [Example 2: Club Performance with Undercut]

[0068] In the second example, player testing of actual clubs was used to examine the performance benefits of the undercut. In this example, a 7-iron with an undercut was compared to a structurally similar 7-iron lacking an undercut. The sole and face of the 7-iron with an undercut were optimized and had reduced thickness. For each golf club, over 700 shots were hit and the ball speed, launch angle, and spin rate were analyzed.

[0069] Figure 7 compares the average ball speeds of a 7-iron with an undercut and a 7-iron without an undercut. The average ball speed of the iron with an undercut was 119.7 mph. The average ball speed of the iron without an undercut was 118.7 mph. Figure 8 compares the average vertical launch angles of a 7-iron with an undercut and a 7-iron without an undercut. This data showed that the 7-iron with an undercut and the 7-iron without an undercut had substantially similar launch angles. Figure 9 compares the average spin rates of the same 7-iron with an undercut and a 7-iron without an undercut. The 7-iron without an undercut had an average spin rate of 6079.9 rpm. The 7-iron with an undercut had a reduced average spin and was 5990.6 rpm.

[0070] Finally, the statistical area (data not shown) of the 7-iron with an undercut was compared with that of the 7-iron without an undercut. By using the data of the statistical area to plot the shot distance according to the left and right deviations from the straight shot, the consistency of each golf club head was identified. The 7-iron without an undercut had a distance deviation of 20 m, while the 7-iron with an undercut had a distance deviation of 14 m. This data indicated that the undercut 7-iron produced shots with a more consistent distance.

[0071] The player results of Example 2 highlighted the performance benefits of the undercut. Specifically, the data showed that the undercut reduced spin on low-loft golf club heads such as the 7-iron and improved ball speed to obtain improved distances. The reduction of spin on low-loft golf clubs is preferred due to distance requirements and the expectation for longer and lower-loft golf clubs. This example also highlighted that the statistical area was narrower for the iron with an undercut, indicating that the undercut iron operated more consistently with respect to distance. [Example 3: Performance in Wet Conditions and Dry Conditions by Undercut]

[0072] In the third example, actual club player tests were used to examine the performance benefits of the undercut during variations in turf conditions. In this example, a pitching wedge with an undercut was compared with a structurally similar pitching without an undercut. Each golf club was hit in wet and dry conditions, and values for the average launch angle, spin amount, and ball speed were measured.

[0073] Figure 10 compares the ejection angles of wedges with undercuts and wedges without undercuts in both wet and dry conditions. The wedge with an undercut had an average ejection angle of 24.0 degrees in the dry condition and 24.5 degrees in the wet condition. The wedge without an undercut had an average ejection angle of 23.6 degrees in the dry condition and 25.1 degrees in the wet condition. Therefore, the ejection angles of the wedge with an undercut and the wedge without an undercut were comparable under wet conditions.

[0074] Figure 11 compares the spin amounts of the same wedges with undercuts and wedges without undercuts in both wet and dry conditions. The wedge with an undercut had an average spin amount of 8617 rpm (revolutions per minute) in the dry condition and 8031 rpm in the wet condition. The wedge without an undercut had an average spin amount of 8310 rpm and a spin amount of 7144 rpm in the wet condition. Therefore, the wedge with an undercut increased the spin amount and showed better turf interaction in both wet and dry conditions for the undercut wedge.

[0075] Figure 12 compares the ball speeds of wedges with undercuts and wedges without undercuts. The wedge with an undercut had an average ball speed of 97.3 mph (miles per hour) in dry conditions and 96.9 mph in wet conditions. The wedge without an undercut had an average ball speed of 97.4 mph in dry conditions and 96.9 mph in wet conditions. The ball speeds for wedges with and without undercuts were comparable in both wet and dry conditions.

[0076] The above data showed that a pitching wedge with an undercut operates more consistently than a wedge without an undercut under variable turf conditions. The launch angle of the wedge with an undercut varied only 0.5 degrees between wet and dry conditions, while the wedge without an undercut had a launch angle variation of 1.5 degrees. This data showed that the launch angle of the wedge without an undercut varied more than three times that of the wedge with an undercut. Similarly, the amount of ball spin generated by the wedge with an undercut was more consistent than that of the wedge without an undercut. The amount of spin varied only 586 rpm between dry and wet conditions for the wedge with an undercut, while it varied 1166 rpm between dry and wet conditions for the wedge without an undercut. The undercut wedge preferably has a consistent amount of spin for wet and dry conditions because the purpose of a wedge-type golf club is to provide consistent ball delivery on the green regardless of weather conditions. The ball speeds of the wedge with an undercut and the wedge without an undercut were substantially similar.

[0077] The rules for golf are sometimes changed (new rules may be applied by golf standard organizations and / or governing bodies, or old rules may be repealed or changed), so golf equipment related to the methods, devices, and / or products described herein may or may not conform to the rules of golf at any given time. Accordingly, golf equipment related to the methods, devices, and / or products described herein may be advertised, offered for sale, and / or sold as conforming or non-conforming golf equipment. The methods, devices, and / or products described herein are not limited in this regard.

[0078] The specific order of the acts has been described above, but these acts may be performed in other time series. For example, the plurality of acts described above may be performed continuously, in parallel, or simultaneously. Alternatively, the plurality of acts may be performed in the reverse order. Further, one or more of the acts described above may not be performed at all. The manufacturing apparatus, method, and article described in this specification are not limited in this regard.

[0079] Although the invention has been described in various aspects, it will be understood that the present invention is further modifiable. This application covers any variations, uses, or adaptations of the following invention, generally including the principles of the present invention and including departures from the present disclosure within the known and customary practice of the art to which the present invention pertains. ​

Claims

1. A golf club head, comprising a hollow body that defines a sealed internal cavity, wherein the hollow body has a striking face, a heel portion, a toe portion opposite the heel portion, a sole having a front sole portion and a rear sole portion, a solid ballast that extends substantially from the heel portion to the toe portion, the solid ballast having a ballast top surface, a ballast front surface, and a ballast bottom surface, the solid ballast, a top rail, a rear portion that extends opposite the striking face between the top rail and the sole, and is provided with wherein the ballast bottom surface is contoured toward the rear portion so as to define an undercut, the undercut having a bottom edge, a top edge, an undercut joint, a heel end, a toe end, a face depth measured as the vertical distance between the face surface and the undercut joint, which is between 0.200 inches and 0.500 inches, an undercut height measured as the distance between the bottom edge and the top edge of the undercut, which is between 0.080 inches and 0.090 inches, an undercut volume between 0.018 cubic inches and 0.050 cubic inches, and has wherein the solid ballast further has a contoured undercut transition portion that creates a smooth transition between the ballast front surface and the ballast bottom surface, the undercut is configured to reduce stress between 1000 psi and 2000 psi in the front sole portion, a golf club head.

2. The undercut further has an undercut length measured as the distance between the heel end and the toe end, and the undercut length is between 1.00 inches and 1.25 inches, the golf club head according to claim 1.

3. The face depth is 0.300 inches, the golf club head according to claim 1.

4. The undercut volume is between 0.018 cubic inches and 0.030 cubic inches, the golf club head according to claim 1. **Claim 5**: The golf club head according to claim 1 or 2, wherein the undercut has a depth of 0.065 inches and extends the length of the front sole portion between 7% and 8%. **Claim 6** Further comprising a cascade sole, the cascade sole comprising an internal transition region from the striking face to the sole, the internal transition region comprising a first step having a first thickness, a second step having a second thickness different from the first thickness, and a step transition region between the first step and the second step, the golf club head according to any one of claims 1 to 5. **Claim 7** The golf club head according to claim 6, wherein the internal transition region further comprises a third step. **Claim 8** The golf club head according to claim 7, wherein the step transition region linearly slopes at an angle less than 45 degrees. **Claim 9** The golf club head according to claim 7, wherein the step transition region linearly slopes at an angle in the range between 10 degrees and less than 45 degrees. **Claim 10** The golf club head according to any one of claims 1 to 9, wherein the undercut depth between the ballast front face and the undercut joint is in the range of 0.010 inches to 0.100 inches. **Claim 11** The hollow body further has a CG depth of 0.096 inches, a top rail to sole moment of inertia in the range of 95 g·inches ２ to 130 g·inches ２ and a heel to toe moment of inertia in the range of 350 g·inches ２ to 420 g·inches ２ The golf club head according to any one of claims 1 to 10, comprising the above. **Claim 12**: Further comprising a ratio of the face depth of the undercut to the CG depth, The golf club head according to claim 11, wherein the ratio is between 3.0 and 3.

5. **Claim 13** A golf club head, Comprising a hollow body defining a sealed internal void, The hollow body, A striking face, A heel portion, A toe portion opposite the heel portion, A sole, A solid ballast substantially extending from the heel portion to the toe portion, the solid ballast having a ballast top surface, a ballast front surface, and a ballast bottom surface, the solid ballast, A top rail, A rear portion extending opposite the striking face between the top rail and the sole, Comprising, The sole having a front portion and a rear portion, The ballast bottom surface is curved toward the rear portion so as to define an undercut, The undercut, A bottom edge, A top edge, An undercut joint, A heel end, A toe end, A face depth measured as the vertical distance between the face surface and the undercut joint, being between 0.200 inches and 0.500 inches, and An undercut height measured as the distance between the bottom edge of the undercut and the top edge of the undercut, being between 0.080 inches and 0.090 inches, and An undercut volume, and Having, The solid ballast further has a contoured undercut transition portion that creates a smooth transition between the front surface of the ballast and the bottom surface of the ballast, The front portion of the sole has a length defined between the rear surface of the striking face and the undercut joint such that a part of the front portion extends under the solid ballast, The undercut is configured to reduce stress between 1000 psi and 2000 psi in the front portion of the sole, a golf club head.

14. The undercut has an undercut depth defined between the front face of the ballast and the undercut joint, The undercut depth ranges from 0.065 inches to 0.100 inches, and increases the length of the front portion of the sole between 6% and 12% with respect to the entire front and rear sole width, the golf club head according to claim 13.

15. The face depth is 0.300 inches, the golf club head according to claim 13.

16. The undercut has an undercut volume defined by the surface of the undercut and the front surface of the ballast, The undercut volume ranges from 0.018 cubic inches to 0.050 cubic inches, the golf club head according to claim 13 or 14.

17. The undercut volume is between 0.018 cubic inches and 0.030 cubic inches, the golf club head according to claim 16.

18. The front portion of the sole further has a cascade, the cascade includes an internal transition region from the striking face to the sole, and the internal transition region includes a first step having a first thickness, a second step having a second thickness different from the first thickness, and a step transition region between the first step and the second step, the golf club head according to any one of claims 13 to 17.

19. The golf club head according to claim 18, wherein the internal transition region further includes a third stage.

20. A golf club head, comprising: a hollow body defining a sealed internal cavity; wherein the hollow body includes: a striking face; a heel portion; a toe portion opposite to the heel portion; a sole; a solid ballast substantially extending from the heel portion to the toe portion, the solid ballast having a ballast top surface, a ballast front surface, and a ballast bottom surface; a top rail; a rear portion extending opposite to the striking face between the top rail and the sole; and wherein the sole has a spring portion for accumulating strain energy and a rear portion; wherein the ballast bottom surface is contoured toward the rear portion so as to define an undercut; wherein the undercut has: a bottom edge; a top edge; an undercut joint; a heel end; a toe end; an undercut height measured as the distance between the bottom edge and the top edge of the undercut, which is between 0.080 inches and 0.090 inches; an undercut face depth in the range of 0.200 inches to 0.500 inches measured as the vertical distance between the inner surface of the striking face and the undercut joint; an undercut length between the heel end and the toe end; and wherein the spring portion of the sole extends between the striking face and the undercut joint to define a spring portion length and has a thickness between the outer surface and the inner surface of the sole; wherein the spring portion is thinner than the rear portion of the sole; wherein the undercut is configured to reduce stress between 1000 psi and 2000 psi in the spring portion of the sole; wherein the undercut and the spring portion are configured to reduce stress between 2000 psi and 3500 psi in the striking face.

21. The striking face has a central region, a peripheral toe region, and a peripheral heel region; the central region has a first thickness between 0.080 inches and 0.150 inches; the peripheral toe region has a second thickness between 0.050 inches and 0.090 inches. ​ ​ The golf club head according to claim 20, wherein the peripheral heel region has a third thickness between 0.045 inches and 0.090 inches.

22. The undercut further has an undercut depth of 0.065 inches, measured as the vertical distance from the ballast front face to the undercut joint, The golf club head according to claim 20 or 21, wherein the undercut depth is configured to increase the length of the spring portion between 6% and 12% of the overall front and back sole width.

23. The spring portion further has a cascade region, the cascade region includes an internal transition region from the striking face to the sole, and the internal transition region has a first step having a first thickness, a second step having a second thickness different from the first thickness of the first step, and a third step having a third thickness different from the first thickness of the first step and the second thickness of the second step. The golf club head according to any one of claims 20 to 22.

24. The golf club head according to claim 23, wherein the second thickness of the second step is smaller than the first thickness of the first step and larger than the third thickness of the third step, such that the overall thickness of the cascade region decreases in the front-rear direction.

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