Patent Description:
Hollow body irons, ideally, operate as a diving board, flexing rearward during impact. In club design, the degree to which a hollow body iron behaves as a diving board, or spring is constrained by peak stress values. To ensure that traditional golf clubs do not exceed maximum stress limits, the face and sole are thickened such that the club is made more rigid. The rigidity of the traditional golf clubs results in a degradation to the diving board, or spring behavior of the club head. Therefore, there is a need in the art to produce a golf club head having a construction which expands the limit of modifications to the face to improve energy transfer from the club to the ball at impact. <CIT> proposes a golf club head. <CIT> proposes golf club heads with energy storage characteristics. <CIT> proposes a weighted golf club head. <CIT> proposes an iron golf club head. <CIT> proposes a golf club head.

SUMMARY OF THE INVENTION The invention is defined by the appended claims.

For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures 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 help improve understanding of embodiments of the present invention. The same reference numerals in different figures denote the same elements.

The terms "first," "second," "third," "fourth," and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms "include," and "have," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, device, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, system, article, device, or apparatus.

It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

The terms "couple," "coupled," "couples," "coupling," and the like should be broadly understood and refer to connecting two or more elements or signals, electrically, mechanically and/or otherwise.

The terms "loft" or "loft angle" of a hollow body golf club (hererafter "hollow body" or "hollow body iron" or "iron-type golf club head" or "golf club head"), as described herein, refers to the angle formed between the club face and the shaft, as measured by any suitable loft and lie machine. A loft plane lies tangent to the strikeface at the geometric center. A loft angle is measured between the ground plane and the loft plane. A 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 approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, less than approximately <NUM> degrees, or less than approximately <NUM> degrees. Further, in many embodiments, the loft angle of the club head is greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, greater than approximately <NUM> degrees, or greater than approximately <NUM> degrees.

The present disclosure describes technologies for an improved hollow body iron-type golf club head (hererafter "hollow body" or "hollow body iron" or "iron-type golf club head" or "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 comprises a ballast undercut for relieving stress. In other configurations, the ballast undercut is combined with additional stress relief features, such as a cascading sole near the face sole juncture, for further reductions to face thickness.

The hollow body can comprise a strikeface, a rear portion, opposite the strikeface, a heel portion, a toe portion, opposite the heel, a sole, and a top rail to define an interior cavity. The rear portion can further include a ballast extending forward from the rear portion and into the interior cavity. In many embodiments, the ballast is an internal component such that it is not visible from the exterior of the golf club. The ballast can further comprise a geometry configured to increase the interior surface area of the sole. For example, the ballast can comprise a top surface, a forward surface, and a bottom surface defined as an undercut region. When viewed from a toe side cross section, an undercut is formed by the bottom surface's concave geometry relative to the face. The undercut allows the thinner, forward portion of the sole to extend beneath the ballast. A ballast comprising a bottom undercut surface, as opposed to a front surface that meets the interior surface of the sole at a right angle, (<NUM>) prevents stress from concentrating along the sole between the face and the ballast and (<NUM>) increases the portion of the sole capable of storing strain energy. Hollow body irons comprising an undercut, therefore, comprise sole and face geometries with greater range of thinning, as compared to hollow irons without an undercut.

The sole of the hollow body iron can be divided into two regions, the forward portion and the rear portion. The forward portion defines the thin region of the sole adjacent the strikeface, which can store strain energy. The rear portion of the sole describes the region of the sole adjacent to the to the rear portion of the of the body, which does not store strain energy. In other words, the forward portion of the sole <NUM> is the portion of the sole <NUM> that behaves as a spring. Hollow body irons having a thinner face and extended forward sole portion, as a result of the ballast undercut, store more strain energy (i.e., potential energy) than the face and forward 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 (as compared to a golf club without and undercut). This energy transfer can be further improved in hollow body irons when the forward sole portion also comprises a cascade, in addition to the undercut. The cascading sole improves the flow of stress within the forward portion of the sole near the face sole juncture, while the undercut improves the flow of stress near the ballast. Accordingly, the application of the undercut and/or the combined application of the undercut and cascading sole can result in a golf club head, which can tolerate a <NUM>-<NUM>% thinner face. Thus, the thinner face, which had been previously unattainable, results in an improved flight trajectory and distance.

<FIG> of the drawings depicts a perspective view of an iron-type golf club head <NUM> exterior having an internal stress relieving sole <NUM> and ballast <NUM> having an undercut <NUM>, shown in <FIG>. The golf club head <NUM> comprises a hollow body structure with an internal cavity <NUM>. The hollow body structure of golf club head <NUM> is further defined by a strikeface <NUM>, a rear portion <NUM> opposite the strikeface <NUM>, a heel portion <NUM>, a toe portion <NUM> opposite the heel portion <NUM>, a sole <NUM>, and a top rail <NUM> opposite the sole <NUM>.

<FIG> illustrates a heel cut away view of the <FIG> golf club head <NUM> along cross-sectional line I-I. <FIG> shows the internal cavity <NUM> and stress relieving features of golf club head <NUM>. The rear portion <NUM> further comprises a ballast <NUM> located within the internal cavity <NUM>. As shown in <FIG>, the ballast <NUM> is an integral weighting element necessary for optimal CG (center of gravity) positioning in golf club head <NUM>. The ballast <NUM> is a solid structure protruding vertically from the sole <NUM>, forward from the rear portion <NUM>, and extending along the sole <NUM> in a heel to toe direction. A forward portion of the sole <NUM> is defined between the strikeface <NUM> and the ballast <NUM>.

Continuing to refer to <FIG>, the ballast <NUM> comprises a top surface <NUM>, a forward surface <NUM>, and a bottom surface <NUM>. As illustrated, the bottom surface <NUM> is contoured to create a relief defining an undercut region <NUM> with undercut <NUM>. The undercut region <NUM> of the ballast <NUM> can be studied as an undercut region <NUM> of material that has been removed from the ballast <NUM> adjacent an interior surface <NUM> of the sole <NUM>. The undercut region <NUM> comprises undercut <NUM> and an undercut transition <NUM>. The undercut region <NUM> extends laterally in a heel to toe direction over a heel to toe length <NUM> of the ballast <NUM>. In the illustrated embodiment, the undercut <NUM> is generally centered within the club head <NUM> between the heel portion <NUM> and toe portion of the golf club <NUM>. As shown in <FIG>, the undercut <NUM> extends beneath the ballast <NUM>, such that forward portion <NUM> of the sole <NUM> is bounded between the face and the undercut <NUM>/bottom surface <NUM> of the ballast <NUM>. The forward portion <NUM> of the sole <NUM> is effectively lengthened, as compared to golf club head without an undercut (i.e., a forward portion defined between the strikeface and the forward surface of the ballast). Therefore, the undercut <NUM> not only reduces stress in the forward portion <NUM> of the sole, but creates a larger spring (i.e., the forward portion of the sole) for transferring energy back to the ball at impact.

<FIG> depicts a zoomed-in view of the ballast <NUM> and undercut <NUM> shown the <FIG> cross section. As shown in <FIG>, the ballast <NUM> comprises top surface <NUM>, forward surface <NUM>, and bottom surface <NUM>. The ballast <NUM> protrudes vertically from an interior surface of the sole <NUM> along an interior surface of the rear portion <NUM>. The bottom surface <NUM> comprises a contoured geometry that extends inward, from the forward surface <NUM> toward the rear portion <NUM> to define the undercut <NUM>, which extends in a heel to toe direction. Continuing to refer to the <FIG> cross section, the ballast bottom surface <NUM> further comprises an undercut juncture <NUM> defined as the juncture between the ballast bottom surface <NUM> and the interior surface <NUM> of the sole <NUM>. The undercut juncture <NUM> is a rearmost point of the ballast bottom surface <NUM> that defines the undercut <NUM>. As shown, the forward portion <NUM> of the sole is defined between the strikeface <NUM> and the undercut juncture <NUM>, rather than the strikeface106 and the forward surface <NUM> in a hollow body iron without undercut <NUM>.

Referring to <FIG>, the undercut <NUM> is defined by four parameters: undercut depth <NUM>, undercut height <NUM>, undercut length <NUM>, an undercut sole thickness <NUM>. Further, the ballast bottom surface <NUM> can be curved such that the undercut <NUM> is defined between an undercut bottom edge <NUM> and an undercut top edge <NUM>. The undercut depth is measured as a perpendicular distance between a ballast forward plane <NUM> and the undercut juncture <NUM> (i.e., the rear most point of the undercut). The undercut height <NUM> is defined as the vertical distance between an undercut top edge <NUM> and an undercut bottom edge <NUM>. The undercut length is measured parallel to a ground plane <NUM> between the undercut toe end <NUM> and the undercut heel end <NUM>. Finally, the undercut sole thickness <NUM> is measured as the perpendicular distance from the exterior surface of the sole <NUM> and an interior surface of the sole <NUM>. In a first embodiment, the undercut <NUM> has a depth <NUM> of <NUM> (<NUM> inch), a height <NUM> of <NUM> (<NUM> inch), a length of <NUM> (<NUM> inches).

The undercut depth <NUM>, between the ballast forward plane <NUM> and the undercut juncture <NUM>, has a range of <NUM> (<NUM> inch) to <NUM> (<NUM> inch). For example, the undercut depth <NUM> can be <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), or <NUM> (<NUM> inch). Alternatively, an undercut face depth <NUM> can be measured as the perpendicular distance between an interior surface of the strikeface <NUM> and the undercut juncture <NUM>. In some embodiments, the undercut depth from the face ranges from <NUM> (<NUM> inch) to <NUM> (<NUM> inch). For example, the undercut depth from the face can be <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), <NUM> (<NUM> inch), or <NUM> (<NUM> inch).

According to the invention, as defined by appended independent claim <NUM>, the undercut face depth <NUM> is measured as a perpendicular distance between an interior surface of the strikeface to the undercut juncture, said undercut face depth ranging from <NUM> to <NUM> (<NUM> inches to <NUM> inches).

The undercut height <NUM>, measured between the undercut bottom edge <NUM> and undercut top edge <NUM>, can range from <NUM> (<NUM> inch) to <NUM> (<NUM> inch). For example, the undercut height <NUM> range from <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM>) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), or <NUM> (<NUM> inch) to <NUM> (<NUM> inch).

According to the invention, as defined by appended claim <NUM>, the undercut height <NUM> is measured as a distance between the bottom edge of the undercut and the top edge of the undercut and measures between <NUM> to <NUM> (<NUM> inch to <NUM> inch).

<FIG> depicts a front view of golf club <NUM> wherein the strikeface <NUM> is removed to expose the undercut length <NUM> extending from the undercut heel end <NUM> to the undercut toe end <NUM>. In some embodiments, the undercut length <NUM> ranges from <NUM> (<NUM> inch) to <NUM> (<NUM> inches). In other embodiments, the undercut length ranges from <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inches), <NUM> (<NUM> inches) to <NUM> (<NUM> inches), <NUM> (<NUM> inches) to <NUM> (<NUM> inches), <NUM> (<NUM> inches) to <NUM> (<NUM> inches), <NUM> (<NUM> inches) to <NUM> (<NUM> inches), <NUM> (<NUM> inches) to <NUM>. <NUM> (<NUM> inches), <NUM> (<NUM> inches) to <NUM> (<NUM> inches), or <NUM> (<NUM> inches) to <NUM> (<NUM> inches). <FIG> further shows the ballast length <NUM>, which can be measured from a ballast heel end <NUM> to a ballast toe end <NUM>. In some embodiments the ballast length <NUM> ranges from <NUM> (<NUM> inch) to <NUM> (<NUM> inches). In other embodiments the ballast length <NUM> is <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), or <NUM> (<NUM> inches).

The undercut length <NUM>, measured as the distance between the undercut heel and toe ends, may further define a percent of the ballast length <NUM>, to describe the portion of the ballast <NUM> comprising the undercut <NUM>. In embodiments of iron type golf club heads comprising an undercut <NUM>, the undercut <NUM> can increase the surface area experiencing impact loading. The percent ballast length can be calculated as the undercut length <NUM> divided by the ballast length <NUM>. In some embodiments, the undercut percent ballast length ranges from <NUM>% to <NUM>%. The length of the undercut can range from <NUM>% the length of the ballast length up to the same length as the ballast length (i.e., <NUM>%). For example, the percent ballast length is <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>%.

In addition, an undercut transition height <NUM>, as shown in <FIG>, is defined as the perpendicular distance between an interior surface of the sole <NUM> and forward surface lower edge <NUM>. In some embodiments, the transition height <NUM> can range from <NUM> (<NUM> inch) to <NUM> (<NUM> inch). The transition height can range from <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), or <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inches) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), <NUM> (<NUM> inch) to <NUM> (<NUM> inch), or <NUM> (<NUM> inch) to <NUM> (<NUM> inch). In the first embodiment discussed above, the transition height is <NUM> (<NUM> inch). The undercut transition <NUM> having transition height <NUM> and a contoured profile allows the undercut <NUM> to smoothly transition to the ballast forward surface <NUM>. This smooth transition promotes an even flow of stress through the undercut <NUM> and the ballast <NUM>.

As discussed above, the undercut <NUM> and undercut region <NUM> can be considered as a region of ballast material that has been removed, when compared to iron-type golf club heads lacking an undercut. An undercut volume <NUM> is defined by a surface <NUM> of the undercut region <NUM> and the ballast forward plane <NUM>. For example, in one embodiment, the surface <NUM> of the undercut region and the ballast forward plane <NUM> define an undercut volume <NUM> of <NUM> cubic mm (<NUM> cubic inches). In other embodiments, the undercut volume ranges from <NUM> cubic mm (<NUM> cubic inches) to <NUM> cubic cm (<NUM> cubic inches). For example, the undercut volume <NUM> can be <NUM> cubic mm (<NUM> cubic inches), <NUM> cubic mm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), or <NUM> cubic mm (<NUM> cubic inches). The undercut volume <NUM> can be used to calculate mass removed from the ballast <NUM> by the undercut region <NUM>. Mass is calculated by multiplying the undercut volume <NUM> by the material density of the ballast <NUM>. For example, an undercut volume ranging from <NUM> cubic mm (<NUM> cubic inches) to <NUM> cubic cm (<NUM> cubic inches). The undercut volume can be <NUM> cubic mm (<NUM> cubic inches), <NUM> cubic mm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), <NUM> cubic cm (<NUM> cubic inches), or <NUM> cubic cm (<NUM> cubic inches). The amount of material removed from the ballast to form the undercut with a material density ranging from <NUM>/cm<NUM> to <NUM>/cm<NUM> or a range of mass from <NUM> grams to <NUM> grams. The amount of material removed from the ballast to form the undercut with a material density of <NUM>/cm<NUM>, <NUM>/cm<NUM>, <NUM>/cm<NUM>, or <NUM>/cm<NUM> or a mass of <NUM> grams, <NUM> grams, <NUM> grams, <NUM> grams or <NUM> grams from the ballast <NUM>.

According to the invention, as defined by independent claim <NUM>, the undercut volume <NUM> is of <NUM> cubic mm (<NUM> cubic inches).

The forward portion <NUM> of the sole <NUM> extending from the strikeface <NUM> to the ballast <NUM> affects the impact response of golf club head <NUM> with a golf ball. As shown in <FIG>, the undercut juncture <NUM> is spaced further rearward from the strikeface <NUM> than the ballast forward surface <NUM>. The undercut juncture's additional distance from the strikeface <NUM>, means that the thinner, forward portion <NUM> of the sole <NUM> has been effectively lengthened (relative to an overall front-to-rear sole width) such that part of the forward sole portion extends beneath the ballast <NUM> (as compared to traditional golf club heads, which lack the undercut <NUM>). A forward sole length can be measured as the perpendicular distance between the undercut juncture <NUM> and face plane <NUM>. In some embodiments, the effective increase in length ranges from <NUM>% to <NUM>%. For example, the undercut <NUM> can increase length of the forward sole portion <NUM> by <NUM>% to <NUM>%, <NUM>% to <NUM>%, <NUM>% to <NUM>%, <NUM>% to <NUM>%, and <NUM>% to <NUM>%. Increasing the length of the thinned out forward portion <NUM> of the sole <NUM> reduces peak stress values in golf club head <NUM>. Rather than behaving as a rigid connection, the undercut <NUM> generates stress relief at the face-sole transition <NUM> by allowing the forward portion <NUM> of the sole <NUM>, between the strikeface <NUM> and the ballast <NUM>, to deflect to a greater extent under impact loads. The undercut's effective increase in forward sole <NUM> length increases the total surface area over which impact load is distributed for a stress reduction of <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi) within the forward portion <NUM> of the sole. Undercut <NUM> dually reduces stress concentrations within forward sole portion <NUM> and increases the bending/spring effect of the forward sole portion <NUM>. Additionally, undercut <NUM> reduces peak stress values within the strikeface <NUM> by <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi). For example, the undercut can reduce peak stress values in the strikeface between <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM>) psi to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi), or <NUM> MPa (<NUM> psi) to <NUM> MPa (<NUM> psi).

Alone, the above decrease in stress, within the sole <NUM> and strikeface <NUM>, can translate to an improved wear life of golf club head <NUM>. In other words, golf club head <NUM> comprising ballast <NUM> with undercut <NUM> can be hit more times and played longer than a traditional golf club head without an undercut. For example, a hollow body golf club comprising an undercut <NUM> can have a failure count increase of <NUM> hits, <NUM> hits, <NUM> hits, <NUM> hits, <NUM> hits, or <NUM> hits. Fatigue failure in a cyclically loaded golf club occurs over time in locations of peak stress where small cracks form in the material. Cracks, in turn, amplify stress. Therefore, golf club head <NUM>, with reduced peak stresses, experiences the crack growth and eventual fatigue failure at a slower rate.

Alternatively, the stress reduction achieved by the above ballast <NUM> and undercut <NUM> can be leveraged to improve club performance and ball speed. In some embodiments, the ballast <NUM> with undercut <NUM> can be provided in conjunction with a thinned strikeface <NUM>. The extent to which the strikeface of a golf club head without the undercut <NUM> has been constrained by peak stress levels at the face-to-sole transition. Said another way, it is not possible to improve the performance of traditional golf clubs with a thinner face because the added stress from the thinner face results in peak stresses that exceed the critical K value. Golf club head <NUM>, as discussed above, comprises ballast <NUM> with undercut <NUM> for stress reduction. Therefore, in some embodiments, strikeface <NUM> can be thinned without raising peak stress values beyond the critical K value at the sole-to-face transition.

The thinness reductions can be applied throughout the face. For example, in the geometric center of the face of the undercut club, the thickness at this region of the face can range between <NUM> (<NUM> inches) to <NUM> (<NUM> inches). The thickness of the face at the geometric center of said face can be <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), or <NUM> (<NUM> inches). In the perimeter toe region of the face of the undercut iron club, the thickness of the face can range from <NUM> (<NUM> inches) to <NUM> (<NUM> inches). The thickness of the face at the perimeter toe region can be <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM>,<NUM>) inches, or <NUM> (<NUM> inches). The thickness of the face at the heel perimeter end of the undercut iron club can range from <NUM> (<NUM> inches) to <NUM> (<NUM> inches). The thickness of the face at the heel perimeter end can be <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), or <NUM> (<NUM> inches).

In some examples, the ballast <NUM> with undercut <NUM> reduces face thickness by <NUM> (<NUM> inches). In other examples the undercut <NUM> can allow the strikeface <NUM> to be thinned by <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), or <NUM> (<NUM> inches). In an already thin strikeface <NUM>, this reduction equates to a thinning of roughly <NUM>%, or an increase in ball speed of <NUM>/s (<NUM> mph) to <NUM>/s (<NUM> mph). In some examples, the undercut <NUM> allows the strikeface to be <NUM> to <NUM>% thinner than the strikeface of a golf club head without an undercut. For example, the strikeface <NUM> can b <NUM>% thinner, <NUM>% thinner, <NUM>% thinner, <NUM>% thinner, <NUM>% thinner, or <NUM>% thinner.

As discussed above, the undercut region <NUM> has a volume <NUM> representative of mass removed from ballast <NUM>. Ballast <NUM> functions as a mass pad for controlling the center of gravity (CG) for golf club head <NUM>, such that the undercut <NUM> can alter club head CG. The CG can be defined relative to a geometric center <NUM> of the strikeface <NUM>. The geometric center <NUM> of the strikeface <NUM> can be determined in accordance with Section <NUM> of the USGA's Procedure for Measuring the Flexibility of a Golf Clubhead (USGA-TPX3004, Rev. <NUM>, May <NUM>, <NUM>) (available at http://www. org/equipment/testing/protocols/ Procedure-For-Measuring-The-Flexibility-Of-A-Golf-Club-Head/) (the "Flexibility Procedure"). A front-rear CG depth <NUM> can be defined as a horizontal distance between the geometric center <NUM> the CG. For example, the front-rear CG depth <NUM> can range from <NUM> (<NUM> inches) to <NUM> (<NUM> inches). The front-rear CG depth can be <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), <NUM> (<NUM> inches), or <NUM> (<NUM> inches).

A ratio of undercut face depth <NUM> to the front-rear CG position is constrained between <NUM> and <NUM>. For example, the face depth ratio <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. In this range, the undercut <NUM> improves peak stress within the forward sole portion <NUM> without removing material from the ballast to the extent that CG position is compromised.

Furthermore, because of the CG position, the undercut does not affect the overall MOI of the club. For the purpose of determining club head moments of inertia, a coordinate system may be defined at the CG via mutually orthogonal axes (i.e., an x-axis, a y-axis, and a z-axis) (Figure not shown). The y-axis extends through the head CG from the top rail <NUM> to the sole <NUM>, perpendicular to a ground plane <NUM> when the club head is at an address position. The x-axis extends through the head CG from the heel <NUM> to the toe <NUM> and perpendicular to the y-axis. The z-axis extends through the head CG from the strikeface <NUM> to the rear portion <NUM>, and perpendicular to the x-axis and the y-axis.

Moments of inertia then exist about the x-axis Ixx (i.e. top rail-to-sole moment of inertia), about the y-axis Iyy (i.e. heel-to-toe moment of inertia) and about the z-axis (i.e., strikeface to rear). In many embodiments, the golf club head with undercut comprises a rail-to-sole moment of inertia Ixx can range from <NUM>·cm<NUM> (<NUM>·in<NUM>) to <NUM>·cm<NUM> (<NUM>·in<NUM>). In many embodiments, the golf club head with undercut comprises a rail-to-sole moment of inertia Ixx can be greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>, greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), or greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>). Further, in many embodiments, the golf club head with undercut comprises a heel-to-toe moment of inertia lyy may be greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), or greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>). In many embodiments, the golf club head with undercut comprises a heel-to-toe moment of inertia lyy can range from <NUM>·cm<NUM> (<NUM>·in<NUM>) to <NUM>·cm<NUM> (<NUM>·in<NUM>). Further, the club head with undercut comprises a strikeface to rear moment of inertia Izz may be greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>), or greater than approximately <NUM>·cm<NUM> (<NUM>·in<NUM>). In many embodiments, the golf club head with undercut comprises a strikeface to rear moment of inertia Izz can range from <NUM>·cm<NUM> (<NUM>·in<NUM>) to <NUM>·cm<NUM> (<NUM>·in<NUM>). The undercut of the golf club head does not significantly alter the moment of inertia of the Ixx, lyy, and Izz axes over a golf club head without the undercut.

<FIG> illustrates another embodiment of a golf club head <NUM> comprising a ballast <NUM>, undercut <NUM>, and a cascading forward portion <NUM> of the sole <NUM>. <FIG> depicts a cross-sectional view of golf club head <NUM>. Golf club head <NUM> is substantially similar to golf club head <NUM> and comprises a thin forward portion <NUM> of sole <NUM> that has been effectively lengthened via the undercut <NUM>. Golf club head <NUM> is further defined by a strikeface <NUM>, a rear portion <NUM> opposite the strikeface <NUM>, a heel portion <NUM>, a toe portion <NUM> opposite the heel portion <NUM>, a sole <NUM>, and a top rail <NUM> opposite the sole <NUM>. Together, these components define a hollow body club with an interior cavity <NUM>. The rear portion <NUM> further comprises a ballast <NUM> located within the internal cavity <NUM>. As shown in <FIG>, the ballast <NUM> comprises the top surface <NUM>, the forward surface <NUM>, and the bottom surface <NUM>. Ballast bottom surface <NUM> is similar to ballast bottom surface <NUM>. The contoured bottom surface <NUM> is indented toward the rear portion <NUM> to create undercut <NUM>.

<FIG> provides a zoomed in view of the ballast <NUM> and sole <NUM> illustrated in <FIG>. As shown, the forward portion <NUM> of the sole <NUM> extends from the undercut <NUM> in ballast <NUM> to the strikeface <NUM>. The forward portion <NUM> of the sole further comprises an inner region <NUM> and a cascading region <NUM>. The cascading region <NUM> can comprise an internal radius transition <NUM> between an internal surface of the strikeface <NUM> and an internal surface of the sole <NUM>. The cascading region <NUM> can comprise at least two thickness tiers, or levels. The tiered structure creates successive thinning of the forward sole portion <NUM>. In some embodiments, the cascading region <NUM> can comprise an internal radius transition <NUM> having <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> tiers.

Continuing to refer to <FIG>, the cascading region <NUM> comprises a first tier <NUM>, second tier <NUM>, and a tier transition <NUM> between the first tier <NUM> and second tier <NUM>. The cascading region <NUM> of the forward sole portion <NUM> can have a thickness measured as the perpendicular distance between the exterior surface <NUM> of the sole and interior surface <NUM> of the sole. This thickness can decrease in a front to rear direction over the cascading region <NUM>. The first tier <NUM> can have a first thickness <NUM> defined as the perpendicular distance between the exterior surface <NUM> and interior surface <NUM> of the sole within the first tier <NUM>. The second tier <NUM> can have a second thickness <NUM> defined within the second tier <NUM> as the perpendicular distance between the exterior surface <NUM> and interior surface <NUM> of the sole. In some embodiments, the first thickness <NUM> is greater than the second thickness <NUM>, such that the overall thickness of the cascading region <NUM> decreases in the front to rear direction. The first thickness <NUM> and/or the second thickness <NUM> can have a constant thickness over a tier length in the front to rear direction. In other embodiments, the first thickness <NUM> and/or the second thickness <NUM> can be sloped to decrease in thickness over the tier length in the front to rear direction.

The cascading region can comprise a first tier <NUM>, second tier <NUM>, a third tier (not shown), and a first tier transition <NUM> between the first tier <NUM> and second tier <NUM>, and a second tier transition between the second tier and the third tier. As described above, the cascading region of the forward sole region with three tiers can have a thickness measured as the perpendicular distance between the exterior surface of the sole and interior surface of the sole. Again, the thickness decreases in a front to rear direction over the cascading region. As described above, the first tier can have a first thickness. The second tier can have a second thickness. The third tier can have a third thickness, wherein the third tier thickness (like the first and second tier thicknesses) is measured as the perpendicular distance between the exterior surface and interior surface of the sole. In some embodiments, the first thickness is greater than the second thickness, and in turn, the second thickness is greater than the third thickness, such that the overall thickness of the cascading region <NUM> decreases in the front to rear direction. The first thickness and/or the second thickness and/or third thickness can have a constant thickness over a tier length in the front to rear direction. In other embodiments, the first thickness and/or the second thickness and/or third thickness can be sloped to decrease in thickness over the tier length in the front to rear direction.

The tier transition <NUM>, between a rear edge of the first tier and a forward edge of the second tier, can be declined in a front to rear direction to steadily decrease the cascading region thickness between the first thickness <NUM> and second thickness <NUM>. Alternatively, in a cascading region with two tier transitions (i.e., a first transition between the first tier and second tier, and a second transition between the second tier and third tier), the transitions can be declined in a front to rear direct to steadily decrease the cascading region thickness between the first thickness, second thickness and third thickness (or first tier, second tier and third tier). In some embodiments, such as <FIG>, the tier transition <NUM> is linearly declined at an angle less than <NUM> degrees between adjacent first <NUM> and second tiers <NUM>. In some embodiments, the tier transition <NUM> is linearly declined at an angle ranging between <NUM> degrees and less than <NUM> degrees. The linear decline can be gradual between <NUM> degrees and <NUM> degrees, <NUM> degrees and <NUM> degrees, <NUM> degrees and <NUM> degrees, <NUM> degrees and <NUM> degrees, <NUM> degrees and <NUM> degrees, <NUM> degrees and <NUM> degrees, or <NUM> degrees and <NUM> degrees. In other embodiments, not shown, the tier transition <NUM> can be a steeper, and more like a step. For example, tier transition <NUM> can be between <NUM> and <NUM> degrees, <NUM> degrees and <NUM> degrees, <NUM> degrees and <NUM> degrees, <NUM> degrees and <NUM> degrees, or <NUM> degrees and <NUM> degrees.

As mentioned above, the forward sole portion <NUM> further comprises inner region <NUM> between the cascading region <NUM> and ballast undercut <NUM>. The uniform inner region <NUM> also comprises an inner thickness <NUM> defined as the perpendicular distance between the exterior surface of the sole <NUM> and the inner surface of the sole <NUM>. The inner thickness <NUM> is less than the thickness of an adjacent tier, or final tier within the cascading region <NUM>. As shown in <FIG>, the inner thickness <NUM> is less than the second thickness <NUM>.

Continuing to refer to <FIG>, the inner region <NUM> of forward sole portion <NUM> can be effectively lengthened by ballast <NUM> comprising undercut <NUM>. Ballast <NUM> is substantially similar to the geometry of ballast <NUM>. Ballast bottom surface defines an undercut region <NUM> comprising the undercut <NUM>, undercut transition <NUM>, and undercut juncture <NUM>. As shown in <FIG>, the inner region <NUM> is positioned adjacent undercut <NUM>. The undercut region <NUM> functions in a substantially similar manner as undercut <NUM> and undercut region <NUM>. Specifically, undercut region <NUM> also reduces stress concentrations within forward sole portion <NUM> and increases the bending/spring effect of the forward sole portion <NUM>.

In many embodiments, performance improvements from cascading sole <NUM> and undercut <NUM> are compounding. In other words, golf club heads having both a cascading region and undercut <NUM>, such as hollow body club <NUM>, have a greater reduction in peak stress than golf club heads comprising one of a cascading region or an undercut. Reduction of peak stress within forward sole portion <NUM> increases the region's tolerance to modifications for improving ball speeds. Specifically, hollow body club <NUM> comprising forward sole region <NUM>, which is defined by undercut <NUM> and comprising cascading region, can comprise a thinner face (as compared to a hollow body club lacking either or both of the undercut and cascading sole). This results in better ball speeds and flight distance. In some embodiments, the undercut <NUM> and cascading sole <NUM> allow the forward portion of the sole <NUM> to be made more reactive. Rather than remaining rigid, the forward portion <NUM> can be thinned, such that the forward portion <NUM> behaves as a spring under impact loads. This means that the golf club head <NUM> is more efficient at transferring swing energy to the golf ball. The ultimate increase in ball speed via reduction in average thickness of the forward portion <NUM> of the sole is the result of stress reduction at the face-to-sole transition <NUM>. The undercut and the cascading sole work together to improve the flow of stress within the forward portion <NUM>, thereby reducing stress concentration levels at impact.

According to the invention, as defined by appended claim <NUM>, the cascading region comprises a first tier <NUM> having a first tier thickness, a second tier <NUM> having a second tier thickness, and a third tier having a third tier thickness. Also according to the invention, as defined in claim <NUM>, the first tier thickness is greater than the second tier thickness, the second tier thickness is greater than the third tier thickness such that the overall thickness of the cascading region decreases in a direction from the forward sole portion toward the rear sole portion.

As described in detail above, the ballast and undercut can be applied to a golf club head alone and in conjunction with other features, such as a cascading sole, to improve club performance. In the example below, performance improvements generated by the undercut <NUM> were studied by comparing a golf club head without an undercut (golf club A, hereafter "Club A"), a golf club head with an undercut (golf club B, hereafter "Club B"), a golf club head without an undercut and with a cascading sole (golf club C, hereafter "Club C"), and a golf club head with an undercut and with a cascading sole (golf club D, hereafter "Club D"). Performance improvements were measured and analyzed using finite element analysis (FEA). Specifically, FEA was used to measure peak stress values within the forward portion. Average peak stress, along with a measured surface area experiencing peak stress, were used to determine the potential for each club to efficiently transfer impact energy back to the ball. Reductions in average peak stress serve as an indicator for improved durability and potential performance enhancement via face thinning and sole thinning.

Each of the example Clubs A, B, C, and D were substantially similar having the same overall mass, material construction, and loft angle. Impact loading in each club was simulated at <NUM>/s (<NUM> mph). The example clubs each comprise unique internal cavity configurations, described above. Average peak stress between the strikeface and ballast, within the forward portion of the sole, was calculated for each example. Likewise, an area of average peak stress was calculated for each example. Finally, average peak stress within the strikeface was calculated for each example. Table <NUM> below, shows the peak face stress, peak stress of the forward sole portion, and the peak stress area within the forward portion of the sole, for each of the example clubs discussed below. Stress values were used to determine the undercut's effect on club performance through face and sole thinning. Example Club A was compared to Club B. Example Club C was compared to Club D. The control club head was similar to the example club heads, but devoid of any stress relieving features.

Club A was representative of a prior art golf club head lacking all stress relieving features and was similar to <FIG>. As the representative of a traditional hollow body golf club head, Club A comprised a ballast without an undercut and without a cascading sole. Without an undercut, the forward portion of the sole and the ballast met at a substantially right angle. Likewise, without the cascading sole, the strikeface transitioned smoothly to the forward portion of the sole.

As shown in Table <NUM>, FEA analysis was used to calculate a value for peak stress within the strikeface of the Club A. Under a <NUM>/s (<NUM> mph) impact load, the peak stress of the strikeface was <NUM> MPa (<NUM> psi). Under the same impact load, the forward portion of the strikeface had a peak stress of <NUM> MPa (<NUM> psi).

Club B was representative of a hollow body golf club head with an undercut stress relieving feature. Hollow body Club B was similar to Club A, but Club B included an undercut as stress relieving feature. Rather than meeting at a right angle, the undercut allowed the forward portion of the sole to extend beneath the ballast. The undercut of Example <NUM> comprised a depth of <NUM> (<NUM> inch), a height of <NUM> (<NUM> inch), an undercut transition height of <NUM> (<NUM> inches), and <NUM> (<NUM> inches).

The values for peak face stress, peak forward sole stress, and peak stress area were determined with FEA analysis and simulated impact with a golf ball at <NUM>/s (<NUM> mph). The peak face stress was <NUM> MPa (<NUM> psi) and the peak forward sole stress <NUM> MPa (<NUM> psi). When compared to the Club A, the undercut reduced peak stress within the strikeface by <NUM> MPa (<NUM> psi) and reduced peak stress within the forward portion of the sole by <NUM> MPa (<NUM> psi). This club showed that the ballast and undercut allow the both the strikeface and forward portion of the sole to store more strain energy. This means that Club B showed improved durability and improved spring response to impact loading.

The hollow body Club C was representative of a club head comprising a forward portion of the sole with a cascade, only. Club C was similar to Club A and B, but comprised a cascading sole as a singular form of stress relief. The transition from face to sole comprised first tier, a second tier and a tier transition between the first tier and the second tier. The first tier had a first tier thickness and second tier thickness, less than the first tier thickness. The tier transition was sloped to gradually transition the first tier thickness to the second tier thickness. The example did not comprise an undercut and the forward portion of the sole and ballast met at a substantially right angle.

Referring again to Table <NUM>, the Example <NUM> hollow body golf club head had a peak face stress of <NUM> MPa (<NUM> psi), or a <NUM> MPa (<NUM> psi) reduction of peak stress within the strikeface. The Example <NUM> club had a peak forward sole stress of <NUM> MPa (<NUM> psi (pounds per square inch)), or a reduction in peak forward sole stress of <NUM> MPa (<NUM> psi) This example showed that cascading sole reduced stress through increased storage of strain energy for improved durability and spring response under impact loading.

Club D (shown as <FIG>) comprising an undercut and a cascading sole as two forms of stress relief for the strikeface and forward sole portion. The ballast comprised an undercut, which effectively lengthened the forward sole portion beneath the ballast. The cascading sole comprised a first tier, a second tier, and a tier transition between the first and second tiers. The first tier comprised a first tier thickness and the second tier comprised a second tier thickness, less than the first tier thickness. The tier transition was sloped to gradually transition the first tier thickness to the second tier thickness.

Club D was also subjected to FEA analysis under simulated ball impact at <NUM>/s (<NUM> mph). The peak face stress was <NUM> MPa (<NUM> psi), for a reduction of peak stress in the strikeface of <NUM> MPa (<NUM> psi). In other words, the Club D had a <NUM>% reduction in peak stress within the strikeface. The peak stress of the forward sole portion was <NUM> MPa (<NUM> psi). The forward sole portion had a peak stress reduction of <NUM> MPa (<NUM> psi), or a <NUM>% reduction from the Club A. This example showed that the undercut and cascading sole worked together to reduce peak stresses. Further, this example indicated that the forward portion of the sole could tolerate additional loading without reaching fatigue failure. The example showed that ball speed could be improved by thinning the face and sole to match the loading capacity of the forward sole portion.

The peak stresses of the forward sole portion in each of the club heads, specifically, indicated the potential for adjusting sole and face thickness and the resulting changes to ball speed. The peak stress of the forward sole portion was compared to the critical K yield stress value of the forward sole portion. Stresses that indicated that the strikeface and sole must be thickened, signaled that the internal cavity configuration would have reduced ball speed. Stresses that indicated that the strikeface and sole could be thinned, signaled that the internal cavity configuration would have increased ball speed.

Club A and Club B were compared to each other relative to a critical K value of <NUM> MPa (<NUM> ksi). The peak stress of Club A, without an undercut, was <NUM> MPa (<NUM> psi). This peak stress value suggested that the sole and face would have needed to be thickened by roughly <NUM>% in order to achieve stress values that did not exceed <NUM> MPa (<NUM> ksi). The thickened face and sole indicated that the internal cavity configuration that would degrade ball speed. Club B, which comprised an undercut, improved peak stress within the forward sole portion. Club B had a peak stress of <NUM> MPa (<NUM> psi). The lower peak stress of Club B indicated Club B required the sole and face to be thickened less than the sole and face of Club A. These results showed that, after modifications, Club B and the undercut indicated better ball speed over Club A, without an undercut.

Similarly, Club C and Club D were compared to each other relative to the same critical K value of <NUM> MPa (<NUM> ksi). The peak stress of Club C, with a cascading sole and without an undercut, was <NUM> MPa (<NUM> psi). Club C, with peak stress slightly less than the critical K stress, indicated that no modifications for improving or degrading ball speed would have been necessary. The slightly lower peak stress did indicate that the cascading sole in Club C would have increased durability. Club D comprised an undercut in addition to the cascading sole and had a peak stress of <NUM> MPa (<NUM> psi). Club D showed that the undercut provided further reduction to peak stress. This reduction in stress indicated that Club D had a face and sole that could tolerate thinning in order to improve ball speed.

The comparison of Club A and Club B and the comparison of Club C and Club D showed that the undercut reduced peak stress within the forward portion of the sole. These results further showed that the undercut could be applied to hollow body golf club heads to improve ball speed by leveraging stress reduction to thin the face and sole.

In a second example, player testing of physical clubs was used to study the performance benefits of the undercut. In this example, a <NUM> iron comprising an undercut was compared to a structurally similar <NUM> iron, which lacked an undercut. The sole and face of the <NUM> iron having the undercut were optimized and reduced in thickness. Over <NUM> shots were taken on each golf club to analyze ball speed, launch angle, and spin rate.

<FIG> compared the average ball speed of the <NUM> iron having an undercut and the <NUM> iron without an undercut. The average ball speed of the iron with the undercut was <NUM>/s (<NUM> mph). The average ball speed of the iron without the undercut was <NUM>/s (<NUM> mph). <FIG> compared the average vertical launch angle of the <NUM> iron with an undercut and the <NUM> iron without the undercut. The data showed that the <NUM> iron with the undercut and the <NUM> iron without the undercut had substantially similar launch angles. <FIG> compared the average spin rate of the same <NUM> iron with an undercut and <NUM> iron without an undercut. The <NUM> iron without an undercut had an average spin rate of <NUM> (<NUM> rpm). The <NUM> iron without an undercut had a reduction in average spin with <NUM> (<NUM> rpm).

Finally, the stat area (data not shown) of the <NUM> iron with the undercut was compared to the <NUM> iron without the undercut. The stat area data was used to determine the consistency of each of the golf club heads by plotting shot distance according to the left-right deviation from a straight shot. The <NUM> iron without the undercut had a distance deviation of <NUM>, while the <NUM> iron with the undercut had a distance deviation of <NUM>. The data showed that the undercut <NUM> iron produced shots that with more consistent distance.

The player results of Example <NUM> highlighted the performance benefits of the undercut. Specifically, the data showed that the undercut reduced spin on low lofted golf club heads, such as a <NUM> iron, and improved ball speed for improved distance. Reduced spin on low lofted golf clubs was preferred due to the distance requirements and expectations of longer, low lofted golf clubs. The Example also highlighted a tighter stat area for irons with an undercut and showed that the undercut irons performed more consistently for distance.

In a third example, player testing of physical clubs was used to study the performance benefits of the undercut in varying turf conditions. In this example, a pitching wedge comprising an undercut was compared to a structurally similar pitching, which lacked an undercut. Each golf club was hit in wet conditions and dry conditions and values for average launch angle, spin rate, and ball speed were measured.

<FIG> compared the launch angle of a wedge with an undercut and a wedge without an undercut in both wet conditions and dry conditions. The wedge with an undercut had an average launch angle of <NUM> degrees in dry conditions and an average launch angle of <NUM> degrees in wet conditions. The wedge without an undercut had an average launch angle of <NUM> degrees in dry conditions and an average launch angle of <NUM> degrees in wet conditions. Therefore, launch angle in wedges with an undercut and without an undercut was comparable under wet conditions.

<FIG> compared the spin rate of the same wedge with an undercut and wedge without an undercut in both wet and dry conditions. The wedge with an undercut had an average spin rate of <NUM> (<NUM> rpm (revolutions per minute)) in dry conditions and an average spin rate of <NUM> (<NUM> rpm) in wet conditions. The wedge without an undercut had an average spin rate of <NUM> (<NUM> rpm) and a spin rate of <NUM> (<NUM> rpm) in wet conditions. Therefore, the wedge with the undercut had increased spin rates to indicate better turf interaction in both wet and dry conditions for the undercut wedge.

<FIG> compared the ball speed of the wedge with the undercut and the wedge without the undercut. The wedge with the undercut had an average ball speed of <NUM>/s (<NUM> mph (mile per hour)) in dry conditions and an average ball speed of <NUM>/s (<NUM> mph) in wet conditions. The wedge without the undercut had an average ball speed of <NUM>/s (<NUM> mph) in dry conditions and an average ball speed of <NUM>/s (<NUM> mph) in wet conditions. The ball speed for the wedge with the undercut and wedge without the undercut were comparable in both wet and dry conditions.

The data above showed that the pitching wedge with the undercut performed more consistently in variable turf conditions than the wedge without an undercut. The launch angle of the wedge with the undercut varied by <NUM> degrees between wet and dry conditions, while the wedge without the undercut had a launch angle that <NUM> degrees. The data showed that the launch angle of the wedge without the undercut varied three times as much as the wedge with the undercut. Similarly, the spin rate of the ball coming off the wedge with the undercut was more consistent than the spin rate of the wedge without the undercut. The spin rate varied by just <NUM> (<NUM> rpm) between dry and wet conditions for the wedge with the undercut, while the spin rate varied by <NUM> (<NUM> rpm) between dry and wet conditions for the wedge without the undercut. Consistent spin rates for wet and dry conditions of the undercut wedge were preferred, as the purpose of wedge-type golf clubs is consistent ball delivery on the green regardless of weather conditions. The ball speed of the wedge with and without the undercut were substantially similar.

As the rules to golf may change from time to time (e.g., new regulations may be adopted or old rules may be eliminated or modified by golf standard organizations and/or governing bodies), golf equipment related to the methods, apparatus, and/or articles of manufacture described herein may be conforming or non-conforming to the rules of golf at any particular time. Accordingly, golf equipment related to the methods, apparatus, and/or articles of manufacture described herein may be advertised, offered for sale, and/or sold as conforming or non-conforming golf equipment. The methods, apparatus, and/or articles of manufacture described herein are not limited in this regard.

Claim 1:
A golf club head (<NUM>, <NUM>) comprising:
a unitary hollow body defining an enclosed internal cavity (<NUM>, <NUM>), the body comprising:
a strikeface (<NUM>, <NUM>) having a front surface and a rear surface opposite the front surface;
a heel portion (<NUM>, <NUM>);
a toe portion (<NUM>, <NUM>) opposite the heel portion;
a sole (<NUM>, <NUM>), wherein the sole comprises a forward sole portion (<NUM>, <NUM>) and a rear sole portion;
a top rail (<NUM>, <NUM>); and
a rear portion (<NUM>, <NUM>) extending between the top rail and the sole and on an opposite side of the internal cavity from the strikeface;
a solid ballast (<NUM>, <NUM>) extending substantially from the heel portion to the toe portion, wherein the solid ballast comprises a ballast top surface, a ballast forward surface, and a ballast bottom surface;
wherein the ballast bottom surface is contoured toward the rear portion to define an undercut (<NUM>, <NUM>);
wherein the undercut comprises:
a bottom edge;
a top edge;
an undercut juncture (<NUM>, <NUM>);
a heel end;
a toe end;
an undercut face depth (<NUM>) measured as a perpendicular distance between an interior surface of the strikeface to the undercut juncture, said undercut face depth ranging from <NUM> to <NUM> (<NUM> inches to <NUM> inches);
an undercut height (<NUM>) measured as a distance between the bottom edge of the undercut and the top edge of the undercut between <NUM> to <NUM> (<NUM> inch to <NUM> inch); and
an undercut volume (<NUM>) of <NUM> cubic mm (<NUM> cubic inches);
wherein the solid ballast further comprises a contoured undercut transition (<NUM>, <NUM>) between the ballast forward surface and the ballast bottom surface; and
wherein the forward portion of the sole comprises a length defined between the rear surface of the strikeface and the undercut juncture such that part of the forward portion extends beneath the ballast;
wherein the forward portion of the sole further comprises an inner region (<NUM>) and a cascading region (<NUM>);
the cascading region comprises a first tier (<NUM>) having a first tier thickness, a second tier (<NUM>) having a second tier thickness, and a third tier having a third tier thickness;
wherein the first tier thickness is greater than the second tier thickness; and the second tier thickness is greater than the third tier thickness such that the overall thickness of the cascading region decreases in a direction from the forward sole portion toward the rear sole portion.