DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0080] When stiffness, mass, and internal damping characteristics of the face have suitable values, the result is commonly called the “spring effect” or the “trampoline effect”, and this can add 5 to 20 yards to a golfer's hit.

[0081] Off-center hits have basic differences from center hits. They must be studied by much more complicated analysis and/or evaluated experimentally. Off-center hits ordinarily give much less distance and altered launch direction of the ball for drives as compared to center hits. The design problem is generally to minimize this change of launch direction and this loss of distance. The stiffness, moment of inertia of the head, its face curvature, the amount of the off-center distance, the shear coefficient of restitution (or an equivalent), and other factors must be considered in addition to the concept of NCR.

[0082] The present inventors used computer-based analysis to study the face stiffness problem with a model of club head-ball impact which is illustrated in FIG. 4 for center hits. This is like a model described by Johnson and Hubble, page 495 of the book, SCIENCE AND GOLF III, © 1999, The World Scientific Congress of Golf Trust, except for the addition of damper c3. Provision was made by the inventors for all three springs to be linear or nonlinear as desired and as was done in this reference. This model is also similar to models used by Cochran, page 488 and by Yamaguchi et al, page 501 in the same book; and by Lieberman and Johnson in the book SCIENCE AND GOLF II, © 1994. Elastic and damping constants for the ball were the same as used in an article by Lieberman and Johnson on page 307 of that book.

[0083] FIG. 5 illustrates some of the results of the analysis conducted by computer with the model of FIG. 4. It shows how the NCR varies for various values of k3, at approach speed of 160 feet per second (109 miles per hour). It is for the case where c3=0 and the spring k3 is linear. The three curves are for three values of the parameter R. Parameter R is the ratio of mass of the front part of the model, Mhf, which moves with the ball surface, to the total mass of the head, Mt, where Mt is the sum of Mhf and Mhb in FIG. 4. That is, R=Mhf/Mt. The curve, R=0.005 is essentially the same as for R=0 and shows how NCR varies with stiffness k3 when R is nearly zero, which means the mass Mhf is relatively near to zero. For k3 less than about 1 million pounds per foot, when R=0.10, there are marked changes, and when R=0.20, there is a profound change. The horizontal dashed line represents an NCR=0.83 which is the upper limit as specified by the United States Golf Association rules. This graph shows that for these conditions, stiffness k3 should be between about 0.4 million and 0.7 million pounds per foot (33,333 and 58,333 pounds per inch) if the NCR is to be near to 0.83, and shows that R should not be much greater than 0.10.

[0084] Other values for nonlinearity of k3 and for damping c3 alter results, but FIG. 4 gives a useful general picture.

[0085] For ideally designed club heads, and with head-ball approach speed for center hits at 109 miles per hour (160 feet per second); with a club head weighing about 200 grams; and with appropriate values for R, k3, and c3 causes maximum rearward deflection of the k3 spring to be typically 0.05 to 0.08 inch and may be even more in some good designs. For the novel present designs off-center hits even as far as the edge of the face, cause deflections of 0.02 to as much as 0.10 inch. Such deflections are much lower at or near the edge for conventional spring effect designs. Lower speed reduces such deflection and higher speed increases them.

[0086] Other characteristics are easily found from the computer based calculation using the model of FIG. 4.

[0087] It is clear that for k3 below about 1 million pounds per foot, large changes can appear if the value of R is much more than 0.10. This is because of a tendency for resonance effects and attendant phase shifts. Another effect becomes significant for k3 less than about 300,000 pounds per foot: Analysis shows that the front of the face may bounce away from the ball, then return so that there are actually two hits very closely separated in time and more than two hits are possible.

[0088] When k3 is nonlinear, and when c3 is not zero, the curves differ considerably from those of FIG. 4, if k3 is less than about 1 million pounds per foot. For larger k3 values, the differences are generally smaller.

[0089] A workable conclusion is that R should be less than 0.10, stiffness k3 should be well below 1 million pounds per foot, and damping c3 should be small.

[0090] This model of FIG. 4 is not sufficiently detailed to replace the need to do testing of actual samples, but provides useful general guidance for the trends of changes in the parameters. More detail and other conclusions regarding club head design are given in 2 books written by the inventors, “HOW GOLF CLUBS REALLY WORK AND HOW TO OPTIMIZE THEIR DESIGNS” © 2000 and “BETTER GOLF FROM NEW RESEARCH” © 2001, available from Origin Inc of Jackson Wyoming.

[0091] FIG. 2 and 3 illustrate one embodiment of the invention wherein the elastic structure 6 is in the form of an elastic ring, preferably an elastomeric ring, having the same outline shape as the perimeter of the basic face plate structure. The face plate 7 is joined near its periphery to the edges of a shell 7A, only by elastic ring 6. At impact on the face plate, this ring can compress, giving less stiffness for rearward movement of the basic face plate structure. There is a small gap 8 between the basic face plate structure and the rear body of the club head, to allow rearward movement of the basic face plate structure upon impact with a ball.

[0092] This alternate has an advantage over prior art clubs in that there is a useful degree of control over the elasticity near the edge of the club face plate. Note that in U.S. Pat. No. 5,505,453, belville washers provided a similar edge support, but as seen in FIG. 6 of patent '453, numeral 58 of patent '453 is the face plate structure and numeral 60 of patent '453 is part of stiff shell 20 of patent '453. Thus, the structure of patent '453 suffers from defects 2 and 3 (defined above) because a hit near the perimeter of the face structure encounters the stiff shell.

[0093] As was pointed out earlier, most prior art basic face plate structures give little or no control over the stiffness at or near the perimeter of the face. There is a disadvantage for the embodiment of FIG. 2 in that the ratio R tends to be high unless the face is thin and then the face is weak and stiffness is very low. As indicated above, R=[mass of the face which moves with the ball surface] divided by [total mass of the head].

[0094] FIG. 6 and the cross sectional view in FIG. 7 show a modified elastic structure of the present invention which is preferred over that of FIG. 2 and FIG. 3. In FIGS. 6 and 7 there is a central rod 17 which is firmly joined to the rear surface of the basic face plate structure 3, for example, by a weldment 23. The basic face plate structure is supported at its periphery by a hollow shell in a conventional manner and such shell has moderate stiffness as discussed later, wherein such shell allows significant deformation for hits at or near the edge of the face. When rod 17 is loaded in compression upon impact it slightly compresses or reduces in length and has a defined spring constant (meaning the ratio of force divided by deflection). It thus forms a spring member. When made of most metals, in order to achieve the values desired for stiffness, such a central compression rod must be 10 or more inches long. For a shorter configuration, it may be joined at 20 (welded or brazed) to a concentric tube 18 which will be in tension at impact, and will elongate. In turn, tube 18 is joined to tube 19 at 22 and tube 19 is in compression. Tube 19 is joined to the rear end of the shell at 21. Plug 25 may be used to fill the hole.

[0095] The spring member shown in cross section in FIG. 7 need not be round. Square or other cross sectional shapes can be used. Furthermore, more than the 3 concentric spring elements shown in FIGS. 6 and 7 may be used when more deflection is needed without exceeding desired material stresses.

[0096] In a specific example for 17-4 ph stainless steel construction, the concentric cylinders or members 17, 18, and 19 may be of equal cross sectional area of 0.0163 square inches (which means that central rod 17 is 0.144 inch in diameter). The sum of the lengths of 17, 18, and 19 may be about 11 inches but the actual length is only about 4 inches because of the telescoping of the members. In this case, the combination of 17, 18, and 19 has a stiffness resisting rearward movement of the face plate 3 of 533,000 pounds per foot. If the face plate 3 is relatively much less stiff, the center of the face plate 3 may exhibit a combined stiffness in the vicinity of 600,000 to 700,000 pounds per foot. Reference to FIG. 5 shows that this is in the range desired to give an NCR of about .82 provided R is in the range of 0.1 or less.

[0097] Long, slender columns may buckle to one side and fail as a long column, as is well known in structural theory. The three concentric cylinders providing the elastic structure in this example provide mutual lateral support and buckling failure is not likely. In cases where buckling may be of concern, the central part of the column can easily be supported against significant lateral deformation by use of light weight support members.

[0098] Instead of one such multi-element column, two or more such columns may be used, suitably distributed over the basic face plate structure 3.

[0099] The type of spring of FIGS. 6 and 7 is unusual in that essentially all of the material is uniformly stressed in tension or compression, causing it to have less mass than a coil spring or leaf spring having the same stiffness and same maximum load capability. In the latter types of springs, the fibers near the neutral axis are lightly stressed and contribute extra mass but little strength or stiffness. Minimum mass which moves with the golf ball at impact with the face (Mhf in FIG. 4) tends to be an important consideration for good spring effect club head designs. Minimal mass of the springs helps to achieve this.

[0100] The basic face plate structure 3 may be thickened in an annular location spaced toward the periphery from the elastic structure as shown at 24 in FIG. 6. This allows the basic face plate structure 3 to have less weight and may also be designed to broaden the central area where desired values of NCR are obtained. A disadvantage for FIG. 6 in the form described is that impacts near the edge have little or no advantageous spring effect when shell 3A is made of metal or composites (defect 3). This may be avoided by making shell 3A of less stiff design such as using polycarbonate, and joining face structure 3 to said shell.

[0101] FIG. 8 is a horizontal cross section of a club head 26 showing a different form of elastic structure 27 comprising a layer which is bonded to the front side of a relatively stiff basic face plate structure 26A. The basic face plate structure is supported at its periphery by a shell 26B.

[0102] The elastic structure 27 is preferably made of rubber or other elastomer such as polyurethane, PPDI, TPE, EPDM, or ECH. Here, “elastomer” refers to an elastic material which is rubber-like in its nature and has a very low value of E. In this case, most of the deformation upon impact takes place in the elastic structure 27. That part of the mass of the outer face surface 27 which moves (deflects) with the surface of the ball at impact is quite small. This means that the mass ratio R is very small, corresponding approximately to the curve in FIG. 5 labeled R=0.005. This curve is well removed from the area of large R for stiffness in the vicinity of 400,000 pounds per foot (in this region, large R values correspond to low NCR values.) To serve well, most elastomers have too much internal damping when stressed. Though not shown in FIG. 5, when damping constant c3 is not small, the NCR value tends to be lower than desired.

[0103] FIG. 9 is a horizontal cross section of a club head 28 having a shell 28B and a basic face plate structure 28A. The basic face plate structure 28A is supported at its periphery by the shell 28B in a normal manner. An elastomeric layer 30, corresponding to the elastomeric layer 27 in FIG. 8 is bonded to the outer surface of face plate 28A. A thin metal layer 31 is bonded to the outer face of elastomeric layer 30, and thus the outer surface of thin metal layer 31 forms the ball strike face. (It is also possible to use other materials such as composite materials or polycarbonate for such face layer 31). Upon impact, the metal layer 31 deforms under the ball and is subject to tensile and bending stresses which may advantageously be designed to provide an important part of the stiffness of the club head 28. In that case, since the elastomeric layer 30 need not supply the bulk of the stiffness, its internal damping characteristics are less important. For an elastic structure made in this way, the part of the mass or weight of the club head which moves (deflects) with the ball during impact is slightly higher than for FIG. 8, but is still quite small compared with prior art basic face plate structures which have been designed to increase NCR.

[0104] Shell structures 26B in FIG. 8 and 28B in FIG. 9 may be made of material such as polycarbonate, which is much less stiff than metal or composites, to provide more spring effect for hits at or near the edge of the face structure as was discussed above in connection with FIGS. 2 and 3. In the design of FIGS. 8 and 9 layers 27 or 30 also provide a spring effect near the edge of the face structure.

[0105] An advantage for the embodiments of FIGS. 8 and 9 is that most of the mass of the head is far from the center of gravity and this causes the moment of inertia of the head about all axes to be large. In turn, that causes the head to have less rotation for an off-center hit with the result of more distance and less scatter for said off-center hits. Large inertia values are quite important for all designs.

[0106] FIG. 10 is an enlarged cross sectional view of a portion circled at D in FIG. 8 or at E in FIG. 9. It shows a useful variation for the elastic or elastomeric layer 27 in FIG. 8 and/or elastomeric layer 30 in FIG. 9. When elastomeric layer 27 or elastomeric layer 30 is solid, at impact it may behave as if it is rather stiff because the elastomeric material cannot easily deform away from the center of impact. In FIG. 10, there is shown a plurality of holes 32 created in the elastomeric material, also shown as 34 in FIG. 12 which is a view in the direction 12-12 in FIG. 10. In this case, at impact, the elastomeric material layer may move toward the central axes of holes 32 and thus allow the face to deform (so as to reduce its thickness) more easily under the ball at impact.

[0107] FIG. 11 is essentially identical with FIG. 10 except that the holes 33 extend completely through the elastomer layer. Accordingly, FIG. 11 is more suitable for use with the construction of FIG. 9, which has the metal layer 31 on the outer face.

[0108] FIG. 12 is a view in direction 12-12 of FIG. 10 or FIG. 11. At 34, FIG. 12 shows the holes or cavities 32 and 33. They need not be round as shown, but could be square, rectangular, or even triangular if desired, and may be tapered to help realize the desired elastic characteristics. Further, their centers may be located at apexes of a hexagonal pattern rather than of a square pattern as shown or may be otherwise distributed.

[0109] The relief areas of removed material or cavities of FIGS. 10 and 11 may take the form of grooves or slots rather than holes. This configuration is shown in FIG. 13 which shows a view of FIG. 10 or 11 in direction 12-12, for the case where grooves or slots 35 are formed, rather than holes. The grooves or slots leave elastomeric ribs 40 for support of metal layer 31 of FIG. 9. This is simply another option for causing the elastomer layer to have the desired elastic properties at impact.

[0110] A further refinement is to vary the cavity hole or slot size and/or spacing so as to permit design control over the local values of stiffness in compression. For example, a hit at (or near) the edge of the face may give best results if the local stiffness is changed. In this vicinity holes 32 or 33 may be more or less widely spaced, be larger or smaller holes, or even be omitted. Stated otherwise, in case a different stiffness is desired, no matter where on the face, the holes may be made larger or more closely spaced or smaller and more widely spaced. In the case where slots 35 or ribs 40 are used rather than holes, they may be varied as desired by being wider or narrower or may be positioned at narrower or wider spacing or may be absent. In either case, the thickness of the elastomeric layer may also be varied to achieve the stiffness variation desired.

[0111] In addition to varying the nature of the cavities formed as holes, ribs, or slots, the thickness and/or modulus of elasticity of metal layer 31 in FIG. 9 can be varied to provide control of local values of stiffness for the same purpose as discussed in connection with varying the stiffness of the layers 27 or 30. Furthermore, a combination of such changes in the metal layer and the elastomeric layer may be used.

[0112] FIG. 14 is a further embodiment in which coil springs are made to function as elastic members in place of the use of the above described elastomeric material. A long spring wire is wrapped in a spiral to form a coil spring to support the strike face or cover sheet 38. The coil springs may be wound of flat wire rather than round, which will result in less weight for the same spring performance. The coils 37 are bonded to the basic face plate structure 36 and to the face sheet 38 across the diameter of the spring by bonding material indicated at 39. The bonding material may be epoxy or other adhesives, brazing, solder or other selected bonding materials.

[0113] It is also possible to combine one or more of the various disclosed elastic structures with a shell structure having small or moderate stiffness to modify the elastic properties of the club head.

[0114] FIGS. 15 and 16 show an embodiment basically like those described above, but markedly different in appearance.

[0115] In FIGS. 15 and 16, 41 is a shell structure. There is a basic face structure 40 which is joined around its perimeter to shell 41 and there are one or more internal supports shown at 42 which serve to provide compression spring characteristics. Thicknesses of sections 41 and 42 are preferably less than what is suggested by the drawings of FIGS. 15 and 16. These parts have much less stiffness than typical wood-type clubs of current designs, primarily by being made of material having much lower modulus of elasticity, E. Whereas E is typically 10 to 30 million pounds per square inch (psi) for metals, and composites are 1 to 40 million psi, materials for items 40, 41, and 42 preferably have values of E less than 0.5 million psi and consequently, tend to be much less stiff. Polycarbonate plastic is a favored material for these parts and has a value, E=0.34 million psi.

[0116] Shell 41 may have a cavity or other provision to which concentrated weights such as 44 may be bonded wherever desired in or on said shell. A shaft socket 43 is provided for the shaft. The parts are joined by bonding, plastic welding or the like.

[0117] The face structure's thickness may be varied as shown at 45 and 46 to help maintain the desired NCR for off-center hits and reduce the mass R which moves with the impact.

[0118] While only one internal support is shown in FIGS. 15 and 16, a number of similar supports are possible. One possibility is to have a plurality of tubular supports that are similar in construction to internal support 42, of varying dimensions such that they can be bonded in place with one inside another. They may also be smaller tubes like 42 but located side-by-side rather than concentric. They may also be simple posts, plates, or columns spaced as desired. These embodiments will be discussed below in a somewhat different form in connection with FIGS. 17 through 21.

[0119] This embodiment makes it possible to minimize all 5 of the defects which were defined above.

[0120] FIG. 17 shows the general nature of a preferred embodiment in which all or most of the structure concerned with elasticity and spring effect at impact is of moderate stiffness such that in addition to center hits on the face, off-center hits and hits at or near the perimeter of the face have benefits of the spring effect. Such benefits are present for prior art designs only for hits at or near the center of the face. While the other novel embodiments previously described also have this characteristic, the embodiment of FIGS. 17 through 21 represents a preferred form and combines features from the earlier embodiments.

[0121] In FIG. 17, 50 is the shell structure made of material of suitable stiffness and strength and having low internal damping. Polycarbonate plastic is satisfactory. Polyester and other strong, resilient plastics are possibilities.

[0122] This shell structure may have a rear portion of greater strength and/or stiffness if desired. The location of such transition, if used, is shown as a bonded joint at 50A. If preferred, it may be located more forwardly or more rearwardly or may be absent if one material is used for the entire shell structure.

[0123] A shaft socket 51 is provided on the shell. The illustrated elements 54 may be much more closely spaced than shown in FIG. 17 (and also in FIGS. 18-21). In such case, the internal central part elastic structure 54 may have a ball strike surface 53 at a forward end of the internal central elastic structure. The central elastic support for the ball impact is primarily supplied by the central structure 54 consisting of various forms of elastic structural elements that are described in detail below in connection with FIGS. 18 through 21. As shown, the internal central elastic structure is supported by a plate 55. In turn, plate 55 is supported by shell 50, using a shoulder or ledge 57. Central structure 54 may alternatively be supported at its rear end on the shell in which case plate 55 is not used. Internal masses may be added as desired, and bonded in place, as shown at 56 or in other places in or on the head.

[0124] An alternate layer 52 forming a ball strike or impact surface can be supported on the internal central elastic structure for providing a ball striking surface. This is a preferred arrangement.

[0125] FIG. 18 is a cross sectional view in the direction indicated by 18-18 in FIG. 17. It shows one form for central part structure or member 54. This form for central part structure or member 54 has walls 58 which are compressible in the fore-aft direction upon impact on the ball impact surface. Such walls are joined as at 59 and may also be joined to the shell so as to form a one-piece honeycomb structure having square open elements. Alternately, the walls of the honeycomb may be free of the shell, depending on the needs for stiffness of the structure. The open elements need not be square and could be circular, triangular, hexagonal, or other shape as desired.

[0126] When a compressive force is applied by impact of the ball on the ball impact surface of the face structure, the stress in walls 58 spreads outwardly toward the shell 50 along a distance rearward from the face. The result is that the wall structure 58 shows greater stiffness in compression than would be the case if only the wall portion 58 directly rearward of the impact carried the entire compression load. This spreading of the stress tends to make it desirable for walls 58 to be thicker toward the front and thinner toward the rear when the compressive stiffness under an impact has suitable value for the usual case where there is also a desire for minimum mass of central member 54.

[0127] For such cases which involve spreading of the stress, plate 55 may be eliminated so that the stress is caused to spread to the shell walls 50. Alternately, plate 55 can be made less thick such that it provides a part of the elastic behavior.

[0128] FIG. 19 shows how the honeycomb structure of FIG. 18 could be replaced by an assembly of individual columns 60 forming the central part structure and extending along the length “L” in FIG. 17. Such columns may have a cross sectional shape which is rectangular, circular, triangular, X-shaped, or other shape or they could be hollow tubes. Because they are individual, independent columns, they act to keep the stress of the impact load from spreading to such neighboring columns as are located outside of the impact area (toward the toe, heel, up, or down, relative to the impact area). This allows more flexible design control over compressive stiffness at various parts of the face.

[0129] If very long, the columns of FIG. 19 have a tendency to buckle as Euler columns. FIG. 20 shows how columns 60 may be replaced by columns 61 at least selected ones of which have a means of support against buckling which support being provided by thin walls 62 that connect columns 61 from one to another. Some columns 61 can be left unconnected. The result is that the center part structure can be designed so as to behave very nearly as individual columns with respect to stress spreading, or very nearly as a homogeneous structure as approximated by the honeycomb of FIG. 18.

[0130] The inventors have found that suitable materials for the honeycomb and shell include polycarbonate plastic, ABS, and polyester resin plastic and similar materials.

[0131] These materials dictate a minimum column length L indicated in FIG. 17 of about 1.6 to 2. If shorter, A when designed for the desired compressive stiffness, the columns must have less cross sectional area, which raises the maximum compressive stress and thus provides a minimum length L for such materials. That is, under these conditions the compressive strain at impact exceeds the failure limit for such materials if such columns are too short.

[0132] The columns, honeycomb elements, or plates of FIGS. 18 through 21 may be varied in thickness and proximity to each other as desired, so as to optimize compressive stiffness over the face which may be needed to minimize distance variations of off-center hits. Such variations of stiffness over the face are of great concern for current designs for off-center hits. Variation of the shell thickness and material also provides an important degree of design control over said variations of stiffness. In the case of FIG. 20, the thin walls 62 may be varied in thickness or even eliminated in some areas as further means to allow design control over compressive stiffness at various locations of off-center hits.

[0133] There is another alternate for central part structure or member 54 in FIG. 17. This is to use elastomers. They have the ability to survive far greater strain and have far lower values of E as compared to the plastic materials discussed above. In turn, this means that that dimension L in FIG. 17 will be shorter. Candidate elastomers are the elastomeric compounds used for the interior of so-called “solid” golf balls; natural rubber; and perhaps a few others. Most synthetic rubbers have excessive internal damping. That means that they do not return promptly and nearly completely to their original shape after being loaded in compression or tension and would not be suitable for this use.

[0134] With such elastomers, a solid form can be considered for central part structure or member 54, having no holes or fewer and smaller holes. With very low values for E, and particularly if there are holes, length L in FIG. 17 may be much shorter, such as 0.3 or less. If fore-aft holes are chosen they allow a degree of design control over the coefficient of restitution at various locations on the face. In this case the elastic structure 54 can become essentially the same as was described at D and E in FIGS. 8 and 9 and in detail in FIGS. 10-14.

[0135] FIG. 21 is another variation of the honeycomb center part structure of FIG. 18. In this form, the honeycomb structure 58 is replaced by spaced walls or plates 65 that extend along the length L in FIG. 17. Design and performance characteristics are similar. The advantage is that molding and assembly of this design is easier than for FIG. 18.

[0136] While all of the center part structure elements described in FIGS. 10, 11, and 17 through 21 are discussed and illustrated as being generally perpendicular to the face, they may advantageously be as much as 45 degrees from perpendicular. This allows them to provide reduced stiffness for impacts on the face, while not becoming too short.

[0137] There are preferred dimensional ranges for the designs of FIGS. 17 through 21 which are subject to variation, depending strongly on the modulus of elasticity E for the material chosen. The following dimensional ranges are for polycarbonate material and similar materials such as polyester. Shell thickness 50B in FIGS. 17, 18, and 21 may range from .04 to 0.10 inch and may vary at various locations on the shell as desired for best performance. Dimension 50C in FIG. 18, 63 in FIG. 20, and 68 in FIG. 21 represent the spacing of elastic elements and may range from 0.05 or less to about 0.50 inch and may vary in various locations. In FIG. 20, dimensions 63 and 64 may vary from 0.1 to 0.6 inch and this also applies to the corresponding dimensions of FIG. 19 and dimensions 63 and 64 need not be equal. Dimension 50D in FIG. 18 and dimension 67 in FIG. 21 may vary from 20% to 60% of dimension 50C or dimension 68. In FIGS. 19 and 20, the cross sectional area of columns may be large or small compared to the open areas, and may range from 20% to 80% of the total area.

[0138] It should be noted that FIGS. 17 through 21 are an extension of the form and concepts described in connection with FIGS. 8 to 13 and if elastomeric material is used, the similarity is close.

[0139] At the hitting surface, the front end of the central elastic member 54 of FIG. 17 may need no face cover if its elements (58, 60, 61 or 65) are closely spaced. The surface serves best with a face cover 52 such as shown in dash lines. Where durability and abrasion resistance is important polyurethane rubber or thin polycarbonate are good choices for thin cover 52, contributing negligibly to stiffness, mass, and internal damping while still serving as a durable face hitting surface. Various other plastic materials are suitable.

[0140] Face cover 52 may be made of thin metal. In that case, the thin face cover may be a significant part of the stiffness of the club face and of the mass which moves at impact. Such cover may also cause the load to spread more broadly over the central elastic member 54 and alter its optimum design.

[0141] Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.