The complexities of golf club design are well known. The specifications for each component of the club (i.e., the club head, shaft, hosel, grip, and sub-components thereof) directly impact the performance of the club. Thus, by varying the design specifications, a golf club can be tailored to have specific performance characteristics.
The design of club heads has long been studied. Among the more prominent considerations in club head design are loft, lie, face angle, horizontal face bulge, vertical face roll, face progression, face size, sole curvature, center of gravity, material selection, and overall head weight. While this basic set of criteria is generally the focus of golf club engineering, several other design aspects must also be addressed. The interior design of the club head may be tailored to achieve particular characteristics, such as the inclusion of hosel or shaft attachment means, perimeter weights on the face or body of the club head, and fillers within hollow club heads.
Golf club heads must also be strong to withstand the repeated impacts that occur during collisions between the golf club and the golf ball. The loading that occurs during this transient event can accelerate the golf ball to several orders of magnitude greater than gravity. Thus, the club face and body should be designed to resist permanent deformation or catastrophic failure by material yield or fracture. Conventional hollow metal wood drivers made from titanium typically have a uniform face thickness exceeding 0.10 inch to ensure structural integrity of the club head.
Players generally seek a metal wood driver and golf ball combination that delivers maximum distance and landing accuracy. The distance a ball travels after impact may be dictated by variables including: the magnitude and direction of the ball's translational velocity; and, the ball's rotational velocity or spin. Environmental conditions, including atmospheric pressure, humidity, temperature, and wind speed, further influence the ball's flight. However, these environmental effects are beyond the control of the golf equipment manufacturer. Golf ball landing accuracy is driven by a number of factors as well. Some of these factors are attributed to club head design, such as center of gravity and club face flexibility.
The United States Golf Association (USGA), the governing body for the rules of golf in the United States, has specifications for the performance of golf balls. These performance specifications dictate the size and weight of a conforming golf ball. One USGA rule limits the golf ball's initial velocity after a prescribed impact to 250 feet per second ±2% (or 255 feet per second maximum initial velocity). To achieve greater golf ball travel distance, ball velocity after impact and the coefficient of restitution of the ball-club impact must be maximized while remaining within this rule.
Generally, golf ball travel distance is a function of the total kinetic energy imparted to the ball during impact with the club head, neglecting environmental effects. During impact, kinetic energy is transferred from the club and stored as elastic strain energy in the club head and as viscoelastic strain energy in the ball. After impact, the stored energy in the ball and in the club is transformed back into kinetic energy in the form of translational and rotational velocity of the ball, as well as the club. Since the collision is not perfectly elastic, a portion of energy is dissipated in club head vibration and in viscoelastic relaxation of the ball. Viscoelastic relaxation is a material property of the polymeric materials used in all manufactured golf balls.
Viscoelastic relaxation of the ball is a parasitic energy source, which is dependent upon the rate of deformation. To minimize this effect, the rate of deformation must be reduced. This may be accomplished by allowing more club face deformation during impact. Since metallic deformation may be purely elastic, the strain energy stored in the club face is returned to the ball after impact thereby increasing the ball's outbound velocity after impact.
A variety of techniques may be utilized to vary the allowable deformation of the club face, including uniform face thinning, thinned faces with ribbed stiffeners and varying thickness, among others. These designs should have sufficient structural integrity to withstand repeated impacts without permanent deformation of the club face. In general, conventional club heads also exhibit wide variations in the coefficient of restitution depending on the impact location on the face of the club. Furthermore, the accuracy of conventional clubs is highly dependent on impact location.
It has been reported in F. Werner and R. Greig, “How Golf Clubs Really Works and How to Optimize Their Designs”, Ch. 4, pp. 17-21 (2000) that a typical distribution of golf ball hits on the face of a driver club follows an elliptical pattern with its major axis orientating in a direction from high toe to low heel. The size of the hit distribution depends on the handicap of the golfer. Players with low handicap have smaller elliptical distribution and players with high handicap have larger elliptical distribution. These authors also patented golf clubs that have an elliptical outer hitting face that aligns in the direction of high toe to low heel. See U.S. Pat. No. 5,366,233, entitled “Golf Club Face for Drivers,” issued on Nov. 22, 1994. However, there is no teaching to align the coefficient of restitution of the golf club head to the ball impact pattern.