With the ever-increasing popularity and competitiveness of golf, substantial effort and resources are currently being expended to improve golf clubs so that increasingly more golfers can have more enjoyment and more success at playing golf. Much of this improvement activity has been in the realms of sophisticated materials and club-head engineering. For example, modern “wood-type” golf clubs (notably, “drivers” and “utility clubs”), with their sophisticated shafts and non-wooden club-heads, bear little resemblance to the “wood” drivers, low-loft long-irons, and higher numbered fairway woods used years ago. These modern wood-type clubs are generally called “metal-woods.”
An exemplary metal-wood golf club such as a fairway wood or driver typically includes a hollow shaft having a lower end to which a hollow club-head is attached. Most of these club-heads are made, at least in part, of a light-weight but strong metal such as titanium alloy. The club-head comprises a body to which a strike plate (also called a face plate) is attached or integrally formed. The body includes a hosel that extends generally upward and is connected to the shaft of the club. The body also includes a heel region situated close to the hosel, a toe region situated opposite the heel region, a sole (lower) region, and a crown (upper) region. The body bears most of the impact load imparted to the strike plate when the club-head strikes a golf ball. The strike plate defines a front surface or strike face that actually contacts the golf ball.
In contrast to wood-type clubs used years ago, the club-heads of many modern metal-woods are hollow, which has been made possible by the use of light-weight, strong metals and other materials for fabricating the club-head. Use of titanium and other light-weight metal alloys has permitted the walls of the club-head to be made very thin, which has permitted the club-heads to be made substantially-larger than their predecessors. These larger club-heads tend to provide a larger “sweet spot” on the strike plate and to have higher club-head inertia, thereby making the club-heads more “forgiving” than smaller club-heads. This “forgiveness” means that a golfer using the club who strikes the ball from a face location other than the sweet spot still produces a ball trajectory that is substantially similar to the shot that he otherwise would have made if he had struck the ball on the sweet spot. Characteristics such as size of the sweet spot are determined by many variables including the shape profile, size, and thickness of the strike plate as well as the location of the center of gravity (CG) and the moment of inertia (MOI) of the club-head.
There are practical limits to the maximum size of club-heads, based on factors such as the particular material of the club-head, the mass of the club-head, and the strength of the club-head. Generally, as club-head sizes increase, body walls and face plates are correspondingly thinner. The distribution of mass around the club-head typically is quantified by parameters such as rotational moment of inertia (MOI) and CG. Club-heads typically have multiple MOIs, each associated with a respective Cartesian reference axis (x, y, z) of the club-head. A rotational MOI is a measure of the club-head's resistance to angular acceleration (twisting or rotation) about the respective reference axis. The MOIs are related to, inter alia, the distribution of mass in the club-head with respect to the respective reference axes. Each of the MOIs desirably is maximized as much as practicable to provide the club-head with more forgiveness.
To achieve the high MOIs, the mass of the club-head typically is distributed, as much as possible, strategically around the periphery of the club-head and rearward of the face plate. As a result, the club-head's CG generally is located rearwardly from the face plate at a prescribed location, which helps the club produce a desired launch angle upon impact with a golf ball.
Another factor in modern club-head design is the face plate. Impact of the face plate with the golf ball causes some rearward deflection of the face plate. This deflection and the subsequent recoil of the face plate are expressed as the club-head's coefficient of restitution (COR). A thinner face plate generally deflects more at impact than a thicker face plate of the same material, thus providing the thinner face plate with more recoil than a thicker face plate. Consequently, a club-head having a thinner face plate potentially can impart more energy and thus a higher initial velocity (rebound velocity) to a struck golf ball than a club with a thicker, more rigid face plate. This rebound phenomenon is called the “trampoline effect” and is an important determinant of the flight distance of the struck ball. Since face-plate deflection is usually greater in the sweet spot, a ball struck by the sweet spot generally will have a greater rebound velocity than a ball struck off-center, and thus generally will travel farther. Because of the importance of the trampoline effect, the COR of clubs is limited under USGA rules.
To achieve these ends, it typically is desirable to incorporate thin walls, including the face plate, into the designed configuration of the club-head. Thin walls also allow additional leeway in distributing club-head mass strategically to achieve a desired mass distribution and a desired high COR.
The volume of club-heads of metal-woods is limited by USGA rules. Nevertheless, certain of these club-heads have become rather large, the largest having a volume of about 460 cm3. These large club-heads have a correspondingly large strike face that presents a tall face height to the ball. Consequently, with many golfers using these clubs, there is an increased probability that the ball will be struck by the strike plate at a location other than the sweet spot. With a large strike face, these off-center shots still provide good ball-launch velocity. However, currently available large-area face plates add significant mass to the front of the club-head, which reduces the amount of mass available for placement elsewhere in the club-head, and undesirably shift the CG forwardly.
Regarding the total mass of the club-head as the club-head's mass budget, it is axiomatic that at least some of the mass be dedicated to achieving the required strength and structural support of the club-head. This is termed “structural” mass. Any mass remaining in the budget is called “discretionary” or “performance” mass, which can be distributed within the club-head to maximize performance. Much of the current research and development activity concerning golf clubs is directed to various ways of distributing the discretionary mass. For example, some club-heads include one or more weights placed relative to the heel-toe (x) axis and in-line with the percussion axis of the club-head.
As club-head engineering converges on certain basic arrangements of discretionary mass in a club-head, particularly in metal-woods, obtaining a maximal amount of any remaining discretionary mass is becoming increasingly important, especially with larger club-heads. Conventional ways of removing mass from the face plate are not always successful; if too much mass is removed from the face plate, the structural mass of the strike plate may be excessively compromised, which can result in the strike plate being too fragile and/or its COR being too high.
Another conventional approach involves using alternative materials for fabricating the club-head. Whereas the bodies and face plates of most metal-woods currently on the market are made of titanium alloy, several “hybrid” club-heads are available that are made, at least in part, of graphite-composite or another composite material. In one group of these club-heads the body is made of composite, but titanium alloy or steel is used as the primary face-plate material.
Other hybrid club-heads are made entirely of composite material (notably graphite composite). But, for several reasons, these club-heads tend to be limited to smaller face areas. First, with a conventional face plate made of composite, it has heretofore been difficult to provide the face plate with sufficient structural strength while still conforming to USGA and R&A rules for the “spring-like effect” (COR≦0.830, CT≦257 μsec). (“CT” is the “characteristic time” standard.) Second, whereas smaller club-heads made of composite can be mass-efficient, potentially even more so than similarly sized all-metal club-heads, scaling up the composite technology to produce desired larger face areas results in less mass-efficiency. One cause of this decreased mass efficiency is the required large thickness of the “sole lip” and “crown lip” at which the face plate transitions to the body. Joining a composite face plate to a composite body by current technology requires not only careful overlap of face plies with body plies in the transition zones, but also substantially thicker transition zones, which tend to negate the potential mass savings of replacing titanium alloy (density=4.5 g/cm3) with composite (density=1.5 g/cm3 for graphite composite). Thus, this technique is simply not mass-efficient (and may actually pose a mass-penalty) for club-head configurations having large face areas. There is also a general consensus that all-composite club-heads produce a disagreeable impact sound during play, mainly due to the overall stiffness of the composite structure and the damped nature of composite material compared to metal.
In view of the above, a need exists for improved metal-wood golf clubs and club-heads that have low-mass face plates, especially large-area face plates, that have sufficient mechanical strength for their intended use, and that conform to USGA and R&A restrictions on the “Spring-Like Effect.”