other than in the operating examples , or unless otherwise expressly specified , all of the numerical ranges , amounts , values and percentages such as those for amounts of materials , moments of inertias , center of gravity locations , loft and draft angles , and others in the following portion of the specification may be read as if prefaced by the word “ about ” even though the term “ about ” may not expressly appear with the value , amount , or range . accordingly , unless indicated to the contrary , the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention . at the very least , and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims , each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques . notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations , the numerical values set forth in the specific examples are reported as precisely as possible . any numerical value , however , inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements . furthermore , when numerical ranges of varying scope are set forth herein , it is contemplated that any combination of these values inclusive of the recited values may be used . fig1 shows a golf club head 1 of the present invention . the club head 1 includes a body 10 having a strike face 11 , a sole 12 , a crown 13 , a skirt 14 , and a hosel 15 . the body 10 defines a hollow , interior volume 16 . foam or other material may partially or completely fill the interior volume 16 . weights may optionally be included within the interior volume 16 . the face 11 may be provided with grooves or score lines therein of varying design . the club head 1 has a toe t and a heel h . the club head 1 is comprised of a plurality of body members that cooperatively define the interior volume 16 . a first body member 101 includes a sole portion and a face portion . the first body member may include a complete face 11 and sole 12 . alternatively , either or both the face 11 and the sole 12 can be inserts coupled to the first body member 101 . the club head 1 also includes at least one second body member 102 coupled to the first body member 101 along the skirt 14 in known fashion . the crown 13 can be unitarily a portion of either body member 101 , 102 or it may be an insert coupled to either of the body members 101 , 102 . the second body member 102 includes a concave portion 20 that , when the body members 101 , 102 are coupled together , extends inward into the interior volume 16 . fig2 shows an isolated view of an exemplary second body member 102 . the first body member 101 preferably is formed of a metallic material such as stainless steel , aluminum , or titanium . the material of the first body member 101 is chosen such that it can withstand the stresses and strains incurred during a golf swing , including those generated through striking a golf ball or the ground . the club head 1 can be engineered to create a primary load bearing structure that can repeatedly withstand such forces . other portions of the club head 1 , such as the skirt 14 , experience a reduced level of stress and strain and advantageously can be replaced with a lighter , weight - efficient secondary material . lighter weight materials , such as low density metal alloys , plastic , composite , and the like , which have a lower density or equivalent density than the previously mentioned metallic materials , can be used in these areas , beneficially allowing the club head designer to redistribute the “ saved ” weight or mass to other , more beneficial locations of the club head 1 . these portions of the club head 1 can also be made thinner , enhancing the weight savings . exemplary uses for this redistributed weight include increasing the overall size of the club head 1 , expanding the size of the club head “ sweet spot ,” which is a term that refers to the area of the face 11 that results in a desirable golf shot upon striking a golf ball , repositioning the club head 1 center of gravity , and / or produce a greater moment of inertia ( mod . inertia is a property of matter by which a body remains at rest or in uniform motion unless acted upon by some external force . moi is a measure of the resistance of a body to angular acceleration about a given axis , and is equal to the sum of the products of each element of mass in the body and the square of the element &# 39 ; s distance from the axis . thus , as the distance from the axis increases , the moi increases , making the club more forgiving for off - center hits since less energy is lost during impact from club head twisting . moving or rearranging mass to the club head perimeter enlarges the sweet spot and produces a more forgiving club . increasing the club head size and moving as much mass as possible to the extreme outermost areas of the club head 1 , such as the heel h , the toe t , or the sole 12 , maximizes the opportunity to enlarge the sweet spot or produce a greater moi , making the golf club hotter and more forgiving . the second body member 102 is light - weight , which gives the opportunity to displace the club head center of gravity downward and to free weight for more beneficial placement elsewhere without increasing the overall weight of the club head 1 . when the wall thickness of the second body member 102 is at the minimum range of the preferred thickness , a reinforcing body layer can be added in the critical areas in case the member shows deformations . these benefits can be further enhanced by making the second body member 102 thin . to ensure that the structural integrity of the club head 1 is maintained , these thin panels may preferably include a concave portion 20 . inclusion of these concave portions 20 allow the second body member 102 to withstand greater stress — both longitudinally and transversely — without sustaining permanent deformation or affecting the original cosmetic condition , ensuring the structural integrity of the club head 1 is maintained . preferred thicknesses for the first body member 101 include from 0 . 03 inch to 0 . 05 inch , while preferred thicknesses for the second body member 102 include from 0 . 015 inch to 0 . 025 inch . preferably , the concave portion 20 displaces at least 10 cubic centimeters . more preferably , the concave portion 20 displaces at least 25 cubic centimeters . while the club head 1 can be virtually any size , preferably it is a legal club head . a plurality of concave portions 20 may be used with the club head 1 . for example , concave portions 20 of uniform or varying size may be positioned in the toe , heel , back , etc . fig3 shows a cross - sectional view taken substantially perpendicular to the face 11 of a second club head 2 of the present invention , and fig4 shows a bottom view of the club head 2 . in the illustration of this embodiment , the concave portion 20 is positioned at the back of the club head 2 . the concave portion 20 preferably is not visible to the golfer at address . in addition to the concave portion 20 , the second body member 102 further includes a convex bulge 22 that extends generally away from the interior volume 16 . an insert 23 may be positioned within the convex bulge . the insert 23 is not visible from outside the club head 2 , and is thus illustrated using broken lines . in a preferred embodiment , the insert 23 is a weight insert . the convex nature of the bulge 23 allows the weight to be positioned to maximize the mechanical advantage it lends to the club head 2 . as shown in fig4 , the club head 2 may include a plurality of convex bulges 22 , such as on a heel side and on a toe side of the club head 2 . the club designer may place inserts 23 as desired within the bulges 22 . the masses of the inserts may be substantially equal . alternatively , one of the inserts may have a greater mass than the other . this may be beneficial to design the club to correct a hook swing or a slice swing . a preferred mass range for the weight insert 23 is from 1 gram to 50 grams . as shown in fig3 , the first body member 101 may comprise a majority of the sole 12 and the second body member 102 may include a majority of the crown 13 . this beneficially removes a large majority of the mass from the upper part of the club head 2 . in this embodiment the first body member 101 includes an attachment perimeter 18 that extends around its edge . the second body member 102 is coupled to the first body member 101 along the attachment perimeter 18 . the first and second body members 101 , 102 cooperatively define the interior volume 16 . the attachment perimeter 18 preferably may contain a step defining two attachment surfaces 18 a , 18 b . as illustrated , the second body member 102 may be coupled to both of these surfaces 18 a , 18 b to help ensure a strong bond between the body members 101 , 102 . while the body members 101 , 102 may be formed in a variety of manners , a preferred manner includes forming a complete club head shell ( first body member 101 ) in known manner and removing material to create openings to which the second body member 102 can be coupled . the opening may be created in any desired manner , such as with a laser . the second body member 102 may be joined to the first body member 101 in a variety of manners , such as through bonding or through a snap - fit in conjunction with bonding . if a composite material is used for the concave inserts , molding six plies of 0 / 90 / 45 /- 45 / 90 / 0 is preferred . while the preferred embodiments of the present invention have been described above , it should be understood that they have been presented by way of example only , and not of limitation . it will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention . for example , while two body members have been described above , the present invention may be embodied in a club head having more than two body members . additionally , the present invention may be embodied in any type of club in addition to the wood - type clubs shown in the illustrated embodiments . thus the present invention should not be limited by the above - described exemplary embodiments , but should be defined only in accordance with the following claims and their equivalents . furthermore , while certain advantages of the invention have been described herein , it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention . thus , for example , those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein .
a hollow golf club head with a concave portion is disclosed and claimed . the club head includes a metallic portion and a light weight portion , which may be formed of plastic , composite , or the like . the concave portion allows the club designer to make a club head having very thin portions while still maintaining the requisite structural integrity . convex bulges may optionally be provided to house weight inserts to enhance the playing characteristics of the golf club .