golf irons typically include a set of eleven irons , numbers one ( long ) through nine ( short ), a pitching wedge and a sand wedge . referring to fig1 - 7 , each iron comprises a head , indicated generally at 10 , including a hosel 12 and a shaft , a portion of which is indicated at 14 , which is attached to the head by fixing the shaft within an axial bore 12a of the hosel as by a suitable adhesive . an over - hosel connection of shaft 14 to hosel 12 could also be employed if desired . the hosel is attached to and generally formed integral with the head . the head is preferably made from a suitable metal , as by forging or casting , and includes a toe portion 16 , a heel portion 18 , a bottom sole 20 , a planar ball - striking face 22 extending between the toe and heel portions , and a rear surface or backside 24 . an upper edge or top line surface 26 is inclined upwardly relative to the sole from the heel 18 to the toe 16 and merges with the upper margin of the face 22 and with the upper margin of the rear surface or backside 24 . a plurality of parallel grooves or score lines 28 are formed in the face 22 so as to lie in a horizontal orientation when the club head is in a ball addressing position as shown in fig2 and 3 . the eleven irons of a set conventionally have varying degrees of loft angle , lie angle and face progression . the loft angle of an iron is the included angle between a vertical plane , such as represented by line 34 in fig4 and which contains or is parallel to the longitudinal axis of the shaft and parallel to the grooves 28 , and the plane of the ball - striking face of the iron . the loft angle determines how much loft is theoretically imparted to the ball when it is hit by the exact sweet spot on the face . the lie angle of an iron is the included acute angle between the axis of the shaft and a plane tangent to the bottom sole directly under the center of mass of the head , such as represented by line 36 in fig3 and which is substantially horizontal when the shaft lies in a vertical plane with the club head in a normal ball addressing position . the lie angle of the iron assures that when swung properly , the grooves or score lines 28 in the club face 22 will be disposed substantially horizontal and the sole of the iron will contact the ground evenly so that the striking face will not tend to twist or rotate about an axis generally normal to the ground . face progression is the distance between a vertical plane parallel to the score lines in the face and containing the centerline of the hosel , and a vertical plane generally tangent to the lower leading edge of the face or blade of the club head when in a ball addressing orientation . the face progression assists in getting the player &# 39 ; s hands ahead of the ball , facilitates higher flight and lessens the tendency to slice the ball . briefly , in accordance with the present invention , each iron of a set has a cavity formed in the rear surface or backside of the head . each cavity is configured to create a predetermined weight distribution about the perimeter of the club head and establish a sweet spot , or optimum ball - striking spot , at substantially the center of the planar ball - striking face . for example , and referring to fig3 the exact sweet spot for club head 10 is designated at 40 and is located mid - length of the longer length face grooves , indicated at 28a , and mid - height of the vertical height of the face 22 between the sole 20 and the upper edge 26 measured at the groove mid - length . the overall configuration of each club head of the set , and the configuration of its corresponding rear cavity , are such that the center of mass of the club head , alternatively termed the center of gravity , is aligned directly behind the corresponding sweet spot on the face of the club head . the rear cavity in each club head is also configured to establish a perimeter weight distribution such that a plurality of elliptical force lines are created in the ball - striking face concentric to the sweet spot , with the major axis of the concentric force lines being parallel to the grooves or score lines in the club face . as will be described in greater detail , these concentric elliptical force lines effectively increase the sweet spot on the ball - striking faces of the irons in a manner to significantly improve the distance , accuracy and consistency in shot trajectory even though a ball may be hit off - center from the exact sweet spot . referring to fig7 taken in conjunction with fig4 - 6 and 8 - 10 , and with the illustrated club head 10 generally representative of the club heads of a set of golf irons in accordance with the present invention , the rear surface 24 has a convexly curved envelope from the upper edge surface 26 to the bottom sole 20 . the upper edge surface 26 has equal transverse width through its length so that the upper marginal edge of the rear surface 24 is parallel to the upper marginal edge of the planar face 22 . the sole 20 has progressively greater transverse width along its length from the heel to the toe , except for a central region to be described , and has a downwardly convex transverse cross - section of generally constant radius along its length . with the toe end of the ball - striking face 22 having greater height than the heel portion 18 , considered in the place of the face , and with the rear surface of back 24 having a convex envelope , greater mass or weight is located in the toe of the club head to achieve the desired overall weight of the head and generally offset or counterbalance the combined weight of the heel portion 18 and hosel 12 while locating the center of mass of the head in rearward alignment with the centered sweet spot 40 on the face . as illustrated in fig7 a cavity 44 is formed in the rear surface or backside 24 of the head 10 . the lower margin or boundary of cavity 44 is defined by a curved surface 44a having a curvature which generally takes the form of a lower approximately one - half of an ellipse whose major axis is parallel to the grooves 28 in the face 22 . an upper boundary of cavity 44 is defined by a surface 44b which merges with the upwardly curved left - hand end of the lower boundary surface 44a and is inclined in an upward direction toward the toe area spaced from the upper edge surface 26 . the boundary surface 44b is rectilinear for the cavities 44 in irons one through seven , and is curved to extend parallel to the curvature of the upper edge surface 26 on the eight iron through sand wedge . the upper right - hand corner of cavity 44 , as viewed in fig7 is defined in part by a generally uniformly curved surface 44c that merges with the right - hand end of the lower curved surface 44a . a generally rectilinear surface 44d extends between and merges with the boundary surfaces 44c and 44b . fig1 depicts the outer peripheries of the club heads of a set of eleven irons in superimposed relation , with the corresponding rear cavities 44 in the various heads also being shown in superimposed relation to illustrate the progressive increase in cavity size as the head size increases from the longer irons to the short irons . for purposes of illustration , the top edge 26 of the number one iron is designated 26 &# 39 ;, and the top edge of the sand wedge is designated 26s . the cavities 44 both increase in size and are formed progressively higher from the sole 20 from the long irons to the short irons , including the pitching and sand wedges . while fig1 illustrates the surfaces 44c and 44d as being relatively rectilinear and intersecting the respective surfaces 44a and 44b at sharp internal corners , the various surfaces 44a - d preferably intersect at rounded or radial fillet - like internal junctions as illustrated in fig7 . the cavity 44 has a generally planar base surface area 48 which lies in a plane parallel to the front face 22 . because the sole 20 has significantly greater thickness than the upper edge surface 26 , as considered transversely of the blade length between the heel and toe ends , the depth of cavity 44 at the lower boundary surface 44a is greater than the depth adjacent the upper boundary surface 44b . the lower cavity boundary surface 44a is angularly inclined in a downward direction relative to the plane of the base surface 48 , as seen in fig8 and 9 , so as to intersect the bottom sole surface 20 and reduce its transverse thickness along an intermediate portion of its length , as indicated at 20a in fig6 . this enables a greater portion of the weight of the club head to be selectively distributed about the perimeter of the head while maintaining the overall weight of each head at a weight necessary to obtain the desired swing weight for the finished iron . referring again to fig . 7 , an elliptical shaped recess 50 is preferably formed generally centrally within each cavity 44 . the recess 50 has a relatively shallow depth , such as approximately 0 . 015 &# 34 ;, and has its major axis extending parallel to the grooves 28 in the club face 22 and preferably through a horizontal rearward projection of the exact sweet spot 40 on the face , considered with the club head in a normal ball - addressing orientation . the recess 50 has a planar base surface 50a which is parallel to the face 22 and establishes the minimum wall thickness between the face and the cavity 44 , preferably not less than 0 . 130 &# 34 ;. the weight of material equivalent to the volume of recess 50 further contributes to the mass of material which can be selectively distributed about the perimeter of the head ; that is , the mass of material theoretically removed from the cavity base surface 48 to create the elliptical recess 50 is available for perimeter weighting at other selective locations on the head . in accordance with a feature of the invention , the cavity 44 in the backside of each iron of a set , including the corresponding elliptical recess area 50 , is configured to establish a perimeter weight distribution about the head such that a plurality of elliptical force lines are created on the ball - striking face 22 of the club head concentric with the exact sweet spot 40 , as depicted schematically in fig3 by elliptical phantom lines 54a - d . each of the elliptical force lines 54a - d may be considered as a line of points any one of which , when impacted by a ball at a moment of theoretical point contact , will cause substantially the same impact energy to be imparted to the ball . thus , each ball which impacts the club head on the same elliptical force line will travel substantially the same distance , assuming the orientation of the club head and the swing speed at the moment of impact to be the same between shots . each elliptical force line imparts an impact energy to a ball different from the other elliptical force lines , with each successive outward elliptical force line from the sweet spot imparting less impact energy to a ball . for example , a ball struck by a point on the elliptical force line 54a on a three iron may travel approximately 185 yards . a ball struck by a point on the elliptical force line 54b may travel approximately 180 yards , and a ball struck by a point on the elliptical force lines 54c or 54d may travel approximately 175 or 170 yards , respectively . the size , configuration and orientation of each cavity 44 is such that the perimeter weighting of the corresponding club head orients the concentric elliptical lines of force within the ball - striking face so that the major axis of the elliptical lines lies substantially parallel to the grooves or score lines 28 and passes through the exact sweet spot . as illustrated in fig3 by orienting the concentric elliptical lines of force so that their major axis lies parallel to the grooves 28 , the effective size of the sweet spot is significantly increased in the area where the ball is hit most often , namely , along a generally rectangular relatively narrow band passing horizontally through the exact sweet spot 40 . the elliptical force lines effectively extend the sweet spot area on both sides of the center 40 parallel to the face grooves or score lines 28 . stated alternatively , the different concentric elliptical force lines are spaced horizontally apart greater distances measured along their common major axis , thereby enabling a ball to be struck at a greater distance from the exact sweet spot without having the impact energy imparted to the ball significantly reduced , as compared to the case where the major axis of the elliptical force lines is angularly inclined to the grooves or score lines in the club face . the specific configuration of the cavity 44 creates a weight mass 58 at the upper right - hand corner of the cavity in the high toe area of the club head . the weight mass 58 is bounded by the cavity boundary surfaces 44c and 44d and may be formed integral with the toe end of the club head . preferably , a shallow recess or indentation 60 is formed in the back surface 24 about the outer perimeter of the weight mass 58 to give the cavity 44 a generally triangular appearance when viewed from the back of the club head . the weight mass 58 , together with the remaining mass or weight of the head about the perimeter of the head , effects the aforedescribed orientation of the elliptical force lines so that their major axis lies parallel to the score lines on the ball - striking face 22 and passes through the exact centered sweet spot . as illustrated in fig5 and 6 , the weight mass 58 does not protrude outwardly of the convex boundary envelope of the rear surface or backside 24 of the club head . this is particularly advantageous when the club heads are viewed by the player from the normal address position prior to the golf swing because the club head does not have any obtrusive or distracting projections extending from the rear surface of the club head . by way of example , the following table sets forth dimensional characteristics of a set of irons constructed in accordance with a preferred embodiment of the present invention . in addition to the table dimensions , the hosels of the various irons have a length of approximately 21 / 4 &# 34 ;, considered as the length of the hosel centerline from the outer end of the hosel to the intersection with the sole 20 . each iron has a blade length of approximately 31 / 4 &# 34 ;, considered from the toe to the intersection of the hosel centerline with the sole 20 . in the illustrated embodiment , the score lines or grooves 28 are modified v - shaped grooves and have their outer ends spaced approximately 0 . 537 &# 34 ; from the toe , as considered in fig3 . the longer score lines 28a are approximately 23 / 8 &# 34 ; in length . the bottom score line is preferably spaced approximately 1 / 4 &# 34 ; above the lowermost point of curvature of the sole 20 . the radius of curvature of the toe 16 , as considered in the plane of the ball - striking face 22 , is approximately 3 &# 34 ;. the upper edge surface 26 is preferably substantially rectilinear for club heads one through seven , and is curved convexly upwardly at a top line radius of approximately 40 &# 34 ; for the eight iron , 30 &# 34 ; for the nine iron , 20 &# 34 ; for the pitching wedge , and 10 &# 34 ; for the sand wedge . the wall thickness between the elliptical recessed area 50 in the base surface 48 of each cavity 44 and the corresponding ball - striking face 22 is preferably maintained at a minimum thickness of 0 . 130 &# 34 ;. __________________________________________________________________________club head 1 2 3 4 5 6 7 8 9 p - 48 s - 56__________________________________________________________________________lie angle 55 ° 56 ° 57 ° 58 ° 59 ° 60 ° 61 ° 62 ° 63 ° 63 ° 63 ° toe height 2 5 / 32 &# 34 ; 2 3 / 16 &# 34 ; 2 7 / 32 &# 34 ; 2 / 4 &# 34 ; 9 / 32 &# 34 ; 2 5 / 16 &# 34 ; 2 11 / 32 &# 34 ; 2 3 / 8 &# 34 ; 2 13 / 32 &# 34 ; 2 7 / 16 &# 34 ; 2 1 / 2 &# 34 ; maxloft angle 17 ° 19 ° 21 ° 25 ° 28 ° 32 ° 36 ° 40 ° 44 ° 48 ° 56 ° face -. 053 &# 34 ; -. 053 &# 34 ; -. 021 &# 34 ; . 010 &# 34 ; . 041 &# 34 ; . 072 &# 34 ; . 104 &# 34 ; . 135 &# 34 ; . 166 &# 34 ; . 166 &# 34 ; . 166 &# 34 ; progressionsect . 8 sole 21 / 32 &# 34 ; 22 / 32 &# 34 ; 23 / 32 &# 34 ; 24 / 32 &# 34 ; 25 / 32 &# 34 ; 26 / 32 &# 34 ; 27 / 32 &# 34 ; 28 / 32 &# 34 ; 29 / 32 &# 34 ; 30 / 32 &# 34 ; 32 / 32 &# 34 ; thicknesssect . 8 blade 1 . 97 &# 34 ; 2 . 00 &# 34 ; 2 . 03 &# 34 ; 2 . 06 &# 34 ; 2 . 09 &# 34 ; 2 . 12 &# 34 ; 2 . 16 &# 34 ; 2 . 21 &# 34 ; 2 . 24 &# 34 ; 2 . 27 &# 34 ; 2 . 33 &# 34 ; heightsect . 9 sole . 50 &# 34 ; . 50 &# 34 ; . 50 &# 34 ; . 53 &# 34 ; . 56 &# 34 ; . 59 &# 34 ; . 62 &# 34 ; . 65 &# 34 ; . 68 &# 34 ; . 71 &# 34 ; . 77 &# 34 ; thickness 20asect . 9 blade 1 . 55 &# 34 ; 1 . 57 &# 34 ; 1 . 59 &# 34 ; 1 . 61 &# 34 ; 1 . 62 &# 34 ; 1 . 64 &# 34 ; 1 . 66 &# 34 ; 1 . 75 1 . 77 &# 34 ; 1 . 79 &# 34 ; 1 . 83 &# 34 ; heightsect . 10 sole . 47 &# 34 ; . 48 &# 34 ; . 49 &# 34 ; . 50 &# 34 ; 51 &# 34 ; . 52 &# 34 ; . 53 &# 34 ; . 54 &# 34 ; . 55 &# 34 ; . 56 &# 34 ; . 58 &# 34 ; thicknesssect . 10 blade 1 . 135 &# 34 ; 1 . 140 &# 34 ; 1 . 145 &# 34 ; 1 . 150 &# 34 ; 1 . 155 &# 34 ; 1 . 160 &# 34 ; 1 . 165 &# 34 ; 1 . 220 &# 34 ; 1 . 225 &# 34 ; 1 . 230 &# 34 ; 1 . 240 &# 34 ; heightsole radius -- -- -- -- -- 1 1 / 4 &# 34 ; r -- 2 &# 34 ; 2 1 / 2 &# 34 ; 3 &# 34 ; 3 1 / 2 &# 34 ; front tobackclub length 39 1 / 2 &# 34 ; 39 &# 34 ; 38 1 / 2 &# 34 ; 38 &# 34 ; 37 1 / 2 &# 34 ; 37 &# 34 ; 36 1 / 2 &# 34 ; 36 &# 34 ; 35 1 / 2 &# 34 ; 35 1 / 2 &# 34 ; 35 1 / 2 &# 34 ; finished head 224 230 237 244 251 258 265 272 279 283 287weight ( grams ) __________________________________________________________________________ while a preferred embodiment of a set of golf irons has been illustrated and described , it will be understood to those skilled in the art that changes and modifications may be made therein without departing from the invention in its broader aspects .
golf club irons of the perimeter weighted type are provided wherein each iron of a set includes a club head having a toe portion , a heel portion having a hosel , a ball - striking face having a plurality of parallel substantially horizontal grooves formed therein , and a generally convex back surface . a cavity is formed in the back surface of each club head and configured to selectively distribute the weight about the perimeter of the head to create a plurality of elliptical force lines concentric with the sweet spot on the club face and having their major axis substantially parallel to the grooves on the club face . golf irons in accordance with the present invention effectively provide larger sweet spots on the hitting faces of the irons so as to produce results from off - center shots that more closely approach the results produced when balls are struck by the exact sweet spot on the club face .