as illustrated in the accompanying drawings and discussed in detail below , the present invention is directed to an improved set of iron - type golf clubs , wherein the clubs have a center of gravity distribution that enables the player to hit more precise shots than conventional clubs . referring to fig1 , a long iron club in the set includes a club head 10 attached to a shaft ( not shown ) in any manner known in the art , at a hosel 20 . the long irons of the present invention have a loft of between about 15 and 25 degrees as is well known in the art . club head 10 includes , generally , the hosel 20 , a striking or hitting face and a back portion that can be cavity backed or muscle backed as is well known in the art . the club head also has a heel 12 , a toe 14 , a top line 22 and a sole 24 . as is well know in the art , the club head 10 and hosel 20 are designed such that the club has a center of gravity cg that is located between the toe 14 and heel 12 and between the top line 22 and the sole 24 , which will be discussed in more detail below . in an embodiment of the invention , the long iron shown in fig1 also includes a plurality of weight members 32 and 34 . the weight members may be embedded into a lower chamber or cavity as set forth in detail in u . s . pat . no . 8 , 157 , 673 , which is incorporated by reference in its entirety herein since the patent is entirely directed to the weight members used in the preferred type of construction , as set forth in fig1 - 13 and 25 - 40 , and the frequencies of the preferred irons that can be made thereby , as set forth in fig1 - 24 . also , as shown in fig1 herein , the heel weight 34 can be preferably inserted into an aperture machined into the sole 24 adjacent the heel 12 . as shown , the weight aperture can be formed to be coextensive with the shaft axis such that the weight 34 is located such that it is intersected by shaft axis sa . alternatively , the weight aperture can be formed into the heel 12 adjacent the sole 24 , but would still be intersected by the shaft axis sa . in the preferred embodiment , the weight members 32 and 34 have a greater density than the material used to form the iron head 10 and preferably a density of greater than 2 times the density of the iron head 10 . most preferably the weight members 32 and 34 have a density of about 17 g / cc . in the iron head construction , the weight members 32 and 34 are sized and positioned to optimize the irons moment of inertia ( moi ) about the vertical axis ( va ) and the moi about the shaft axis ( sa ). preferably , the long iron weight members 32 and 34 are each between about 10 g and 40 g . combined , the weight members 32 and 34 should comprise greater than about 10 % of the total body weight . preferably , the weight members 32 and 34 for the long irons are located such that the weight cgw is located below the club cg in the vertical direction . more preferably , the weight members 32 and 34 each have a cgw 1 and cgw 2 , respectively , that is between about 30 % and 75 % of the cg - yg of the club . still further , the cgw 1 and cgw 2 are preferably located a distance apart that is greater than 50 % of the blade length of the club . more preferably , the cgw 1 and cgw 2 are located at least about 75 % of the blade length away from each other to maximize moi - y . the iron head 10 , including the weight members 32 and 34 , is constructed such that the cg is also allocated in an optimal position relative to the face center and the shaft axis . the details of the cg locations of the irons within the set will be discussed in more detail below . as shown in fig2 , a mid - iron 110 according to the present invention has a loft of between about 26 and 36 degrees and includes , generally , the hosel 120 , a striking or hitting face and a back portion that can be cavity backed or muscle backed as is well known in the art . the club head also has a heel 112 , a toe 114 , a top line 122 and a sole 124 . as is well known in the art , the club head 110 and hosel 120 are designed such that the club has a center of gravity cg that is located between the toe 114 and heel 112 and between the top line 122 and the sole 124 , which will be discussed in more detail below . in an embodiment of the present invention , the mid - iron shown in fig2 also includes a plurality of weight members 132 and 134 . the weight members may be embedded into a lower chamber or cavity as set forth in detail in u . s . pat . no . 8 , 157 , 673 , which is incorporated by reference in its entirety herein since the patent is entirely directed to the weight members used in the preferred type of construction , as set forth in fig1 - 13 and 25 - 40 , and the frequencies of the preferred irons that can be made thereby , as set forth in fig1 - 24 . also , as shown in fig2 herein , the heel weight 134 can be preferably inserted into an aperture machined into the sole 124 adjacent the heel 112 . as shown , the weight aperture can be formed to be coextensive with the shaft axis such that the weight 134 is located in a location where it is intersected by shaft axis sa . alternatively , the weight aperture can be formed into the heel 112 adjacent the sole 124 , but would still be intersected by the shaft axis sa . in the preferred embodiment , the weight members 132 and 134 have a greater density than the material used to form the iron head 110 and preferably a density of greater than 2 times the density of the iron head 110 . more preferably the weight members 132 and 14 have a density of about 14 to 17 g / cc . most preferably the weight members 132 and 134 have different densities , wherein the density of the heel weight 134 is less than the density of the toe weight 132 . preferably , the density of the heel weight 134 and the density of the toe weight 132 are about 14 g / cc and 17 g / cc , respectively . in the iron head construction , the weight members 132 and 134 are sized and positioned to optimize the irons moment of inertia ( moi ) about the vertical axis ( va ) and the moi about the shaft axis ( sa ). preferably , the mid - iron weight members 132 and 134 are each between about 20 g and 50 g . combined , the weight members 132 and 134 should comprise greater than about 15 % of the total body weight . preferably , the weight members 132 and 134 for the mid - irons are located such that at least one of the weight cgw is located below the club cg in the vertical direction . more preferably , the weight member 132 preferably has a cgw 3 that is between about 50 % and 90 % of the cg - yg of the club and the weight member 134 has a cgw 4 that is approximate or greater than cg - yg . still further , the cgw 3 and cgw 4 are preferably located a distance apart that is greater than 50 % of the blade length of the club . more preferably , the cgw 3 and cgw 4 are located at least about 50 % and less than 80 % of the blade length away from each other to optimize moi - y . the iron head 110 , including the weight members 132 and 134 , is constructed such that the cg is allocated in an optimal position relative to the face center and the shaft axis . the details of the cg locations of the irons within the set will be discussed in more detail below . fig3 and 4 depict alternate embodiments of short irons according to the present invention 210 and 310 , respectively . the iron short iron according to the present invention has a loft of between 37 and 47 degrees . the iron 210 includes a hosel 220 , toe 214 , heel 212 , topline 222 and sole 224 . the iron 210 is constructed such that it has a center of gravity cg as discussed in more detail below . the iron 310 includes a hosel 320 , toe 314 , heel 312 , topline 322 and sole 324 . the iron 310 may have a heel weight member 334 located in the bottom portion of the hosel 320 such that it is intersected by the shaft axis sa . preferably , the heel weight 334 has a specific gravity greater than the iron material , and more preferably , greater than about 2 times the specific gravity of the iron material . preferably , the density of the heel weight is about 17 g / cc . still further , the weight member 334 has a center of gravity cgw 5 that is located approximate or above the club cg in the vertical direction and is located a distance from the club cg that is greater than about 40 % of the club blade length . also , it is preferred that there is only a single high density weight member or no high density weight members such that the short irons 210 and 310 are constructed in a manner that they have a center of gravity cg as discussed in more detail below . in accordance with an aspect of the present invention , the inventive iron golf clubs are designed to have progressive centers of gravity as set forth in fig5 , for example and which is merely illustrative of a preferred embodiment of the present invention set of golf clubs , and is not to be construed as limiting the invention , the scope of which is defined by the appended claims . each inventive iron golf club is designed to hit golf balls a prescribed distance in the air , and to stop on the green or fairway in a predictable manner . tables i and ii provide exemplary , non - limiting dimensions for the various measurements of clubs according to the prior art and to the example of the invention , respectively . it is fully intended that all of the dimensions set forth below can be adjusted such that the overall objective of the individual irons is met . as a non - limiting example , a 3 iron according to the invention can be made with a loft of 20 - 22 degrees to adjust the angle of descent and remain within the scope of the present invention . referring to the data above and the graph in fig5 , it is clear that in the irons according to the present invention the center of gravity is located a distance away from the face center cg - xfc in a manner that is significantly different than with the prior art clubs . the face center is defined as the location that is in the middle of the scorelines and half way between the leading edge and the topline of the club . in the prior art clubs , the cg - xfc remains substantially constant through the set . in general , the cg - xfc in the prior art clubs is located between about 2 to 2 . 5 mm away from the face center towards the heel of the club ( about 0 . 1 inch ). in the irons according to the present invention , the cg - xfc for the short irons range from about 40 % to 60 % further away from the face center than the long irons . more particularly , in the inventive example above and as shown in fig5 , the cg - xfc remains approximately constant at about 2 . 4 mm from the face center through the long irons and the mid - irons . all of the long irons ( 3 and 4 ) have a cg - xfc that is within 15 % of each other . all of the mid - irons ( 5 , 6 and 7 ) have a cg - xfc that is within 15 % of each other . further , all of the long irons ( 3 and 4 ) have a cg - xfc that is within 15 % of all of the mid - irons ( 5 , 6 and 7 ). however , the short irons ( 8 - w ) have cgs that are substantially closer to the hosel or , in other words , substantially further away from the face center in the x ( horizontal ) direction . in fact , all of the example short irons have a cg - xfc that is at least 40 % greater than the cg - xfc for the long irons . preferably , all of the short irons according to the invention have a cg - xfc that is at least 30 % greater than the long irons and the mid - irons . more preferably , all of the short irons of the present invention have a cg - xfc that is between 35 % and 70 % greater than the long irons and the mid - irons . moreover , as shown in fig5 , the cg - xfc of the irons according to the present invention varies through the set according to an exponential curve when plotted versus loft angle . as shown , in the irons according to the prior art , the cg - xfc remains substantially constant , and thus , the cg - xfc is substantially linear with no slope . conversely , in the irons according to the present invention , the cg - xfc remains substantially constant for long irons and mid - irons and then significantly increases for the short irons . thus , the best fit equation to describe the relationship of the cg - xfc according to loft is a second order polynomial . preferably , the irons according to the present invention have a cg - xfc for the short irons that are greater than the values defined by the line cg - xfc = 0 . 02 ( la )+ 2 . still further , the distance of the center of gravity to the ground cg - yg remains similar for the clubs in the prior art and in the set according to the present invention . however , for the example set according to the present invention , the cg - xfc is greater than 15 % of cg - yg for the short irons . for this example , the cg - xfc ranges from about 15 % to 20 % of the cg - yg for the short irons . thus , the relationship of cg - xfc to cg - yg is substantially different than in the prior art clubs . referring to table i and table ii above , the relationship of the moment of inertia about the shaft axis ( moi_sa ) is substantially different between the prior art and the inventive clubs . in the very short irons , irons having a loft of between 45 and 52 degrees , the moi - sa in the prior art is greater than 600 kg * mm 2 and closer to about 625 kg * mm 2 . however , in the inventive irons set forth herein , the moi - sa for the very short irons is less than 600 kg * mm 2 and more preferably between 575 kg * mm 2 and 600 kg * mm 2 . as set forth in fig6 , the moi - sa for the prior art is best represented by a linear equation which is approximately moi - sa = 4 . 6la + 400 . on the other hand , the moi - sa for the irons according to the present invention are best represented by a second degree polynomial equation . as shown , the moi - sa for the short irons , including the very short irons , all fall below the linear equation of the prior art . as set forth in table ii , the center of gravity distance from the ground cg - yg within the set should be set to assist with the creation of the preferred flight paths . options can include , for example , lowering the center of gravity of the long irons through the use of inserts formed from a material having a specific gravity of greater than 10 g / cc such as tungsten or a tungsten alloy . additionally , the hosel of the long irons can be comprised of a material having a specific gravity of less than 7 g / cc such as titanium , aluminum or alloys thereof . conversely , high specific gravity materials may be employed within the topline portion of the short irons to raise the center of gravity . referring to fig7 , the short irons 310 according to the present invention , may employ a heel weight member 334 located in the bottom portion of the hosel 320 that is treaded in using a threaded section 336 , such that it is intersected by the shaft axis sa . preferably , the heel weight 334 has a specific gravity greater than the iron material , and more preferably , greater than about 2 times the specific gravity of the iron material . preferably , the density of the heel weight is about 17 g / cc . the iron 310 may also include a low weight insert 332 or an aperture that is formed from the toe section 314 so that the cg - xfc is formed closer to the shaft axis . preferably , the low weight insert 332 would have a specific gravity of less than the specific gravity of the iron material , and more preferably , about half of the specific gravity of the iron material or less . the low weight insert may be formed from a low specific gravity metal such as aluminum or an elastomeric material . fig8 is an exploded view of the components forming the long iron 10 as shown in fig1 . the long iron can be formed by forging the body 10 , including a weight pocket 18 adjacent the toe section 14 . after the body 10 is formed , an aperture can be formed in the sole 24 , near the heel 12 , such that a weight insert 32 can be securely fastened therein by a press fit , welding or adhesive . after the toe weight 32 is attached in the weight pocket 18 , a back panel 16 can be secured to the body 10 . preferably , the back panel and the body are formed from the same materials such that they can be welded together . referring to fig9 and 10 , the short irons according to the present invention may be formed by forging the body 310 . the body may include a back panel welded to the body as set forth in fig8 , but may be solid . the weight member 334 is preferably formed with a threaded portion 336 and is threaded into the bottom of the hosel 320 . alternatively , as shown in fig1 , a weight member 334 may be inserted into the hosel 320 and then a compressive force can be applied to the perimeter of the hosel 320 to form a crimped section 338 that retains the weight member securely in the hosel 320 . the diameter of the crimped section 338 of the hosel 320 should be greater than 80 % of the hosel diameter and more preferably between 90 and 95 % of the hosel diameter . referring to fig1 , irons according to the present invention may be formed by forging or casting the body 310 from titanium or steel . the body may include a back panel welded to the body as set forth in fig8 , but may be solid . a weight member 334 is preferably formed as a weighted sleeve member that can be inserted into the hosel 320 and has an inner diameter that is equal to the outer diameter of a golf shaft such that the golf shaft can be inserted and adhesively affixed therein . preferably , the weighted sleeve member 334 is formed of tungsten with a density of 15 - 17 g / cc such that the density is at least 75 % greater than the iron and is press fit into the hosel . in one embodiment , the hosel can be heated and the weighted sleeve member cooled so that it can be easily inserted into the hosel . once the hosel 320 and the weighted sleeve member 334 are at equal temperatures , the weighted sleeve member 334 is press fit into place . alternatively , as shown in fig1 , a weighted sleeve member 334 may be a spring loaded sleeve with a longitudinal slit such that the weighted sleeve member can be compressed and then released to be securely held in the hosel 320 . the outer diameter of the weighted sleeve member 334 should be greater than about 100 % of the hosel inner diameter and more preferably between 100 and 120 % of the hosel inner diameter . the weighted sleeve member 334 has a length that is less than the hosel length . preferably , the weighted sleeve member 334 has a length that is between about 30 and 80 % of the hosel bore length . in one embodiment , the weighted sleeve member can be adhesively affixed into the hosel and the golf shaft can be adhesively affixed to the weighted sleeve member 334 and the lower portion of the hosel bore . in one embodiment of the invention , the weighted sleeve member 334 is added to the hosel bore in an iron having a loft of less than about 25 degrees along with a weight member 32 in the toe such as that disclosed in fig1 . preferably , the center - to - center distance between the weighted sleeve 334 and the weight member 32 is at least 75 % of the blade length and the iron has a moiy of greater than 240 kg * mm 2 . by using the weighted sleeve member , the distance between the weight members is maximized and the moiy is maximized while maintaining a high moix . preferably , the moix is greater than about 50 or 2 times the loft angle . preferably , the weight members both have a specific gravity of greater than 15 g / cc , however if a 10 g / cc tungsten is used , the weight members 334 and / or 32 can be welded into place . referring to fig1 , in an alternate embodiment of the present invention , the club head 10 can be formed by forging the body with weight pads 32 . thus , in this embodiment , the weight members 32 are integrally formed with and attached to the back portion of the face . the back panel 16 as set forth above can then be welded over the weight member 32 . this construction method may be preferred for the long irons , mid irons or short irons of the present invention . however , referring to fig1 and 12 , if the long irons and mid irons are formed according to this method , it is preferred that the weight member 32 for the mid irons is located adjacent the face stabilizing bar 38 for the mid - irons and adjacent the sole 24 for the long irons . in this manner , the cg - yg is designed to be relatively lower in the long irons than in the mid - irons . also , as shown in fig1 , the weight member 32 can be formed into multiple portions 32 a and 32 b that are preferably located on opposite sides of the cg to provide a relatively high moi - y . the cg location through the set can also be adjusted by providing for a variable face thickness above the stabilizing bar 38 . the upper back wall 48 can be designed a depth from the front face such that the upper face thickness through the set increases with loft . for example , the long irons can be designed with an upper face thickness of about 2 . 1 mm , the mid irons can have an upper face thickness of about 2 . 4 mm to 2 . 7 mm and the short irons can have an upper face thickness of about 2 . 7 mm to 3 . 5 mm . the perimeter of the upper face 50 can be about 0 . 05 to 0 . 25 mm thicker than the center portion 48 . preferably , the upper face thickness is as thick as or thicker than the next club in the set with a lower loft and the upper face thickness of a short iron is at least 50 % greater than the upper face thickness of a long iron . yet another way to design an iron having the cg according to the present invention is to form a body 10 as shown in fig1 . the head body 10 can be formed by forging the body with a topline 22 , sole portion 24 , toe portion 14 , heel portion 12 , a weight pocket 18 and a face stabilizing bar 38 . if the member is forged , an aperture 40 can be formed in the face stabilizing bar 38 prior to the attachment of the back panel 16 . preferably , the aperture is machined into at least a portion of the face stabilizing bar 38 . if the body is cast , the aperture 40 can be formed in the casting and machining can be avoided . referring to fig1 , more than one aperture 40 may be desired . thus , the club 10 may include one or more apertures formed into the face stabilizing bar 38 . preferably , the apertures are located on the sole side of the face stabilizing bar 38 and are covered by a back panel 16 . in yet another embodiment as set forth in fig1 , the aperture 40 can extend longitudinally from the heel 12 to the toe 14 a distance of greater than about 25 % and less than about 50 % of the length of the face stabilizing bar 38 . preferably , the aperture 40 extends through the face stabilizing bar 38 toward the topline by about 50 % to about 90 %. by forming the aperture 40 such that is extends on both sides of the cg as shown in fig1 , the moi - y can be optimized . although not shown , similar apertures can be form in the bottom surface of the topline 22 . another way to accomplish the progression of the center of gravity cg - yg through the set is to employ a low weight face insert as shown in fig1 and 17 . referring to fig1 , the face 16 can be made of different materials throughout the set . for example , the long irons could employ a titanium alloy insert such as ti 6 - 4 , which has a specific gravity of 4 . 4 g / cc and the mid - irons and short irons could employ steel faces having a specific gravity of about 7 . 9 g / cc . by using higher strength steel in the mid - irons , such as 17 - 4 stainless steel , the faces can be designed thin to reduce weight and by using a softer steel , such as 431 stainless steel , in the short irons , the feel of the short irons can be improved . also , as shown in fig1 , a composite insert 42 comprised of multiple layers of prepreg layups 44 may be used . preferably , a face insert 42 can be located in a thin cavity behind the face material 16 that can be the same material as the body 10 . the insert 42 should extend longitudinally at least about 50 % between the heel 12 and the toe 14 . the height of the insert can be varied , but is preferably between at least 10 % and 90 % of the height of the iron between the sole 24 and the topline 22 . another aspect of the preferred embodiment is to have a consistent feel within the set . thus , the swing weights of the irons may be constant through the set . furthermore , the distance from the center of gravity to the shaft axis can be approximately constant through the set or progress through the set inversely to the loft . while it is apparent that the illustrative embodiments of the invention disclosed herein fulfill the objectives stated above , it is appreciated that numerous modifications and other embodiments may be devised by those skilled in the art . therefore , it will be understood that the appended claims are intended to cover all modifications and embodiments which would come within the spirit and scope of the present invention .
the present invention is direct to a set of golf clubs comprising long irons , mid - irons and short irons . the long irons have a first center of gravity positioned horizontally from the face center toward the hosel by a first distance . the mid - irons have a second center of gravity positioned horizontally from the face center toward the hosel by a second distance . the short irons have a third center of gravity positioned horizontally from the face center toward the hosel by a third distance that is greater than the first and second distances .