Abstract:
A hollow golf club shaft and a moveable solid shaft insert about 6 to 24 inches long wherein the depth of insertion of the shaft insert into the shaft may vary from about 1 to 10 inches. Changing the location of the shaft insert allows a player to change the flex of the shaft and thereby optimize the performance of the club dynamics to the player. The shaft insert is initially held in place by friction between the shaft and the shaft insert, optionally with a joining agent located therebetween, until a final setting of the shaft insert is determined. Then the shaft insert is permanently located within the shaft to make the club legal for play under U.S.G.A. Rules. Shaft fitting can be administered by a player with or without coaching and can be revisited at any time prior to permanent attachment by a simple adjustment. The shaft inserts are useful on all hollow shaft clubs, from putter to driver, and can be retrofitted to existing clubs without removing the grip.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of U.S. application Ser. No. 10/366,854, filed Feb. 14, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to golf clubs, golf club shafts, a rigid tubular insert placed within a golf club shaft so as to be free-floating, and to a method of adjusting the overall shaft flex, wherein the resulting golf clubs fully comply with the rules of the U.S. Golf Association for play.  
         [0003]     Several technical specifications are commonly used to describe golf clubs: total weight, swing weight, length, loft angle, lie angle, head size and weight, grip diameter, and shaft flex. But the proper selection of the latter is generally acknowledged to be most important when fitting clubs to a golfer. Moreover, recent studies have shown that it is very important for custom golf club fitters to find the optimum flex for each player for improved shot control and greater distance. But finding the optimum flex has perplexed most teachers, club fitters and golfers.  
         [0004]     Three methods for finding the best flex before club assembly presently dominate the market for custom clubs. In the first method, the player swings one club several times and the average club head speed measured and used to calculate the flex for the entire club set. The second approach relies on matching one club or a set of clubs to a player&#39;s favorite club. In the third method, a player hits several shots with calibrated test clubs, finally selecting the best of the lot to serve as a model for his set, which can be matched to it. Once the clubs are constructed, they can only be used with minor adjustment until another set is purchased, prompted in many cases by the golfer&#39;s belief that a better fit is probable.  
         [0005]     Most misfits of clubs to players, the rule rather than the exception, are mostly due to the methods available rather than to poor club fitters. U.S. Pat. No. 5,821,417 to T. Naruo et al (Oct. 13, 1998), concludes that many factors other than club head speed, such as shaft strain at impact, swing time, and acceleration, effect the choice of optimum flex for each player. Yet the most popular flex fitting method in use today calculates shaft flex solely from club head speed. This same study concedes that there is a best flex for every player, but points out the difficulty in determining it. If this conclusion is correct, it would suggest that a method for altering the flex after the club is constructed, through trial and error to reveal the best flex, might be a benefit.  
         [0006]     Heretofore, there have been several attempts to modify the flexural characteristics of golf shafts after club assembly by adding either fixed or adjustable materials to either the outer or the inner surface of the shaft. Almost all of these attempts which add material to the outer surface of the shaft are accompanied by adverse cosmetic results. Those which attempt to change the shaft frequency from inside the shaft have been too heavy, too expensive, violate the present rules of golf, or cannot achieve the results promised.  
         [0007]     Thus there remains a need for a system which will allow accurate adjustment of the flexural characteristics while not adversely effecting the club weight and while still conforming to the rules of the U.S. Golf Association so that the club is “tournament-qualified” for use in competitive play, i.e. during rounds used to establish a golfer&#39;s handicap and golf tournaments. The USGA rules specifically require that (1) the club shaft exhibit uniform properties in all directions from the axis of the shaft and (2) the club contain no movable parts.  
         [0008]     The prior art advocates of adjusting the flex of a golf club in practice or play have focused on trying to maximize the transfer of energy stored in the flexing of the shaft by somehow arranging to have the shaft straight at impact. Negating this theory, many studies have found that all swings, for all golfers, with all golf clubs tested, feature the shaft bent toward the target just before impact, well past the point where energy stored in a shaft could be transferred to the ball. This means that the job of the shaft is, evidently, not to store energy for later use. But the shaft does bend and rotate during a golf swing and clearly it influences the result. Many granted patents have been alleged to adjust shaft dynamics and the most important ones are listed below.  
         [0009]     Several attempts have been made to adjust shaft flex by adding assemblies to a conventional shaft of a club. One of the oldest shaft flex adjuster employs an external wire and bridge seen in U.S. Pat. No. 4,685,682 of J. Isabell (Aug. 11, 1987), which has several limitations including a poor cosmetic look.  
         [0010]     An attempt to improve the cosmetic appearance of a flex adjuster is seen in U.S. Pat. No. 6,113,508 of M. Locarno et al (Sep. 05, 2000), which employs an internal eccentric stiffening rod having a different, lateral stiffness at right angles. The clubs produced by this method violate the USGA rules of golf because the patent deliberately causes the shaft flex to be radially unequal in shape as well as, specifically, in flex. Moreover, the method is flawed by the fact that stiffness can be transferred from one plane to the other, but both cannot be adjusted independently. This can lead to mixed results in use.  
         [0011]     Another method for modifying flex entails adding a stiffening rod to the inside of a shaft, e.g. U.S. Pat. No. 3,833,233 of R. Shulkin (Sep. 03, 1974). Varying lengths of shaft elements are inserted into clubs used specifically for fitting flex to a player. The inserted shaft elements are not to be adjusted in their position once in place, but only exchanged and are not intended to be present in a set of clubs during play. Rather, the elements are only to be used for fitting.  
         [0012]     In U.S. patent application Ser. No. 2001/0005696 of M. Hendrick (Jun. 28, 2001), a short, generally 1-3 inch long, hollow shaft insert is used to change the swing weight of a club. It can be readjusted at any time, but does not, of itself, have any impact on the swing characteristics of the club other than swing weight. The patent specifically excludes changing the shaft flex using this design.  
         [0013]     U.S. Pat. No. 5,478,075 of C. R. Saia et al (Dec. 26, 1995) describes a method of changing shaft flex using an insert with radially expandable rubber discs that can be expanded by turning a threaded energizing rod. The rubber discs are stationary as they expand and do not move in or out of the outer shaft.  
         [0014]     U.S. Pat. No. 6,361,451 of B. Masters et al (Mar. 26, 2002), U.S. Pat. No. 6,241,623 to C. Liabangyang (Jun. 05, 2001), and U.S. Pat. No 6,394,909 to C. Laibangyang (May 28, 2002) utilize a wire strung down the center of a golf shaft the tension of which is adjustable to exert varying compressive forces on the shaft thereby seeking to change its flex. The three inventions allow players to adjust flex in order to deliver more energy stored in the flex at ball impact, a near impossibility in practice, as mentioned earlier.  
         [0015]     Another attempt to change the overall flex and also dampen the shock effect of a ball strike from the club head to the hands is referred to in U.S. Pat. No. 5,083,780 of T. C. Walton et al (Jan. 28, 1992). In this patent, the grip end is reinforced by a cylinder placed between the grip and the shaft under the grip thereby lowering the flex point, increasing the flex and dampening vibrations from the club head to the hands. Once set, it is not adjustable in practice or play.  
         [0016]     U.S. Pat. No. 6,045,457 of T. Soong (Apr. 04, 2000) discloses a method of inserting a second shaft inside a bulged outer shaft to increase the flex of the resultant shaft combination. The patent claims that the resultant fixed amount of increase in flex and lowering of the flex point increases club head speed. The bulged outer shaft serves to ensure that the second shaft only contact the main shaft at the two ends of the second shaft, i.e. the middle section of the second shaft does not contact the main shaft. The increased flex occurs when the end of the second shaft closest to the clubhead makes contact with the outer (main) shaft. The force exerted by the second shaft is due only to rigidity of the insert because the insert is anchored at its butt end at the grip and only touches the outer shaft at the opposite end. This differs from the present invention wherein at least three locations of the shaft insert must contact the outer shaft during a swing.  
         [0017]     U.S. Pat. No. 5,054,781 of T. Soong (Oct. 08, 1991) discloses a method of building a shaft with a fold back shaft that is inserted and contacts the inner wall of the outer shaft only after some degree of shaft bending. The claim of increased energy storage and release at ball contact is dubious, and while it employs an insert to change the flex of the shaft, once set, it is not adjustable.  
         [0018]     U.S. Pat. No. 6,056,646 of T. Soong (May 02, 2000) employs a fixed insert to stiffen the flex of the outer golf shaft but is non-adjustable once installed. It is intended to stiffen the flex only when the shaft is flexed beyond a certain point. When the tip of the insert is in contact with the outer shaft, the flex is increased.  
         [0019]     Many other patents feature shaft inserts that are primarily concerned with damping high frequency vibrations transmitted from the club head to the hands.  
         [0020]     None of the prior art has succeeded in creating a practice golf club in which a player can repeatedly adjust the flex and play with that club until such time as the player (or some third party) determines that the club performance is maximized and then convert that practice golf club into a golf club which is 100% legal under USGA Rules for tournament play.  
         [0021]     Accordingly, it is the object of this invention to provide a means which will enable a player to adjust the shaft flex of a golf club during practice and non-regulation play in order to discover the best shaft flex for that player.  
         [0022]     It is the further object of this invention to provide means of eliminating further shaft flex adjustments during play in competition in order to conform the Rules of Golf of the USGA which require a golf club shaft to “bend in such a way that the deflection is the same regardless of how the shaft is rotated about its longitudinal axis.” (Appendix II, Section 2b(i))  
         [0023]     It is the further object of this invention to provide the means for a fixed insert to be used to increase the flex of a golf club shaft by predetermined amounts.  
         [0024]     It is the further object of this invention to provide the best golf club shaft flex for each individual player and to provide a means to do so in the form of a multiplicity of alternative adjustable embodiments.  
         [0025]     Still further objects and advantages will become apparent from a consideration of the ensuing description and accompanying drawings 
     
    
     DRAWINGS—BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a plan view of a golf club with a shaft insert in place.  
         [0027]      FIG. 2   a  is an expanded view of section cc in  FIG. 1 .  
         [0028]      FIG. 2   b  is an expanded view showing the section of  FIG. 2   a  during a golf swing.  
         [0029]      FIG. 2   c  is a graph showing the changing direction of bending of a golf club during a swing.  
         [0030]      FIG. 3  is a plan view of the preferred embodiment.  
         [0031]      FIG. 4  is an end view of section  4 - 4  of  FIG. 3 .  
         [0032]      FIG. 5   a  is an end view of a shaft insert of this invention (legal for play under U.S. Golf Association Rules, Appendix II, 2b.  
         [0033]      FIG. 5   b  is not an example of this invention. It is an end view of a shaft insert which is illegal under U.S. Golf Association Rules.  
         [0034]      FIG. 5   c  is not an example of this invention. It is an end view of a shaft insert which is illegal under U.S. Golf Association Rules.  
         [0035]      FIG. 6  is a plan view of a shaft insert adjuster in place.  
         [0036]      FIG. 7  is a plan view of a first alternative joining agent embodiment  
         [0037]      FIG. 8  is and end view of section  8 - 8  of  FIG. 7 .  
         [0038]      FIG. 9  is a plan view of a second joining agent alternative embodiment.  
         [0039]      FIG. 10  is an end view of the section  10 - 10  of  FIG. 9 .  
         [0040]      FIG. 11  is a plan view of a third joining agent alternative embodiment.  
         [0041]      FIG. 12  is and end view of section  12 - 12  of  FIG. 11 .  
         [0042]      FIG. 13  is a plan view of a fourth joining agent alternative embodiment  
         [0043]      FIG. 14  is an end view of the section  14 - 14  of  FIG. 13 .  
         [0044]      FIG. 15  is a plan view of a fifth joining agent alternative embodiment.  
         [0045]      FIG. 16  is a plan view of a thread holding agent alternative embodiment.  
         [0046]      FIG. 17  is an end view of the section  17 - 17  of  FIG. 16 .  
         [0047]      FIG. 18  is a plan view of the threaded adjuster in place.  
         [0048]      FIG. 19  is a plan view of a universal shaft insert embodiment.  
         [0049]      FIG. 20  is a graph of the results of Example 1.  FIG. 20   a  is a graph showing the performance of a player using a club not containing a shaft insert of this invention.  FIG. 20   b  is a second graph showing the effect of inserting a shaft insert into the club of  FIG. 20   a  and then changing the location of that shaft insert within the club shaft upon the same player. 
     
    
     DRAWINGS—REFERENCE NUMERALS  
       [0050]    
       
           10  golf club shaft  
           12  grip  
           14  club head  
           16  shaft insert  
           17  insert adjuster  
           18  joining agent  
           20  grip plug  
           22  threaded adjuster  
           24  stop  
           26  self-tapping threads  
           28  thread holding agent  
           30  torquer  
           31  grip end force  
           32  central shaft insert force  
           33  club end force  
       
     
       SUMMARY OF THE INVENTION  
       [0065]     The object of the invention is achieved by placing a moveable rigid shaft insert in the hollow portion of the grip end of a golf club shaft or a finished golf club. While shafts might be specially constructed to facilitate the deployment of the present invention, the invention is best used with standard shafts. A suitable insert is a uniform piece of rigid material about 6 to 24 inches, preferably about 10 to 18 inches, and most preferably about 12 to 16 inches long, having (i) the same deflection regardless of how the shaft insert is rotated about its longitudinal axis and (ii) a smaller diameter than the golf club shaft into which it is inserted. By changing the depth of penetration P of the shaft insert between about 1 to 10 inches, the flex of a golf club can be adjusted to a particular player&#39;s swing dynamics to achieve better, i.e. more uniform, performance. The insert is firmly fixed in place within the shaft and there is little to no likelihood of it working loose during a round of golf. However, it can be intentionally unfixed and thus the fitting of the club to the player can be revisited at any time by a simple re-adjustment process.  
         [0066]     This invention can be used on any clubs with hollow shafts from putters to drivers. It can also be retrofitted into existing clubs by enlarging the hole in the butt end of the grip using a simple tool to cut out a plug, inserting an adjustable shaft insert, determining the preferred location of the shaft insert within the shaft, fixedly attaching the shaft insert to the shaft, and if desired replacing the plug, thereby rendering the shaft insert hidden and un-adjustable during play.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0067]     As shown in  FIG. 1 , a conventional golf club comprises a golf club shaft  10 , usually about 34 to 46 inches in length, having a grip  12  at the butt end and a club head  14  at the tip end of said golf club shaft. A club head may be a “wood” head or an “iron” club head, both or which can be manufactured from a variety of materials including metals, wood, composites, graphite and polycarbonate or combinations of materials. Shafts are constructed from a variety of materials, mostly steel, aluminum, graphite, or titanium. They are usually tapered but must be of homogeneous circular cross section and have equal stiffness in all orthogonal directions in order to conform to the current rules governing competition enforced by the U.S. Golf Association.  
         [0068]     Shaft materials have a high strength to weight ratio to minimize the overall weight of the club while providing the necessary rigidity for desired performance. To accomplish this, common shaft manufacturing techniques require a thin wall construction with a circular cavity in the center of the golf club shaft  10 . The existence of this cavity creates the opportunity for this invention, namely, the placement of a shaft insert  16  within the cavity as shown in  FIG. 1  and in detail in subsequent figures.  
         [0069]     Refer now to the preferred embodiment of the invention shown in  FIG. 2   a  (with the grip  12  and the grip plug  20 ) and  FIG. 3  (without the grip), which are expanded drawings of section cc of  FIG. 1 . In both figures, the shaft insert  16  is positioned inside the hollow cavity of the golf club shaft  10 . The outer surface of the shaft insert  16  may be grooved to provide friction fitting between the insert and the shaft.  
         [0070]     Adjusting the insertion depth P of an about 6 to 24 inch long shaft insert from about 1 to 10 inches will change the overall flex of the shaft of the golf club without altering its accepted cosmetic look. Once positioned, the shaft insert  16  is firmly fixed at the desired penetration depth P such that there is no reasonable likelihood of them working loose during a round of golf. This may be accomplished by friction alone if the shaft insert has the same shape but a smaller diameter as the shaft into which it is to be placed. However, use of a joining agent as explained further below is preferred.  
         [0071]     Golf shafts with no taper or minimal taper will allow maximum range of insertion for shaft insert materials, especially if the shaft insert material offers minimal compressibility. Reduction in the outer shaft diameter due to tapering may eventually limit further insertion of the shaft insert.  
         [0072]      FIG. 5   a  shows the cross-sectional shape of a golf club  10  with a solid shaft insert  16  in place. While a tubular shaft insert configuration is preferred (as shown in  FIGS. 1-4 ) due to its strength-to-weight ratio advantage, solid cross-section configurations as shown may enjoy a cost advantage.  
         [0073]      FIGS. 5   b  and  5   c  show two comparison shaft insert configurations which are not within the scope of this invention because they are do not comply with U.S. Golf Association Rule, Appendix II, 2b. which requires that “along its length, a shaft shall: 
        “(i) bend in such a way that the deflection is the same regardless of how the shaft is rotated about its longitudinal axis; and     “(ii) twist the same amount in both directions.”
 
 These comparison inserts fail to satisfy both requirements. 
         
         [0077]     As best seen shown in  FIG. 6 , the extent of penetration P of a shaft insert within a shaft can be adjusted with insert adjuster  17 . It can be used to adjust penetration P either by pushing the butt end of the insert farther into the shaft or by hooking the tip end of the shaft insert  16  and pulling the shaft insert out of the shaft. Alternatively, a threaded adjuster  22  as shown in  FIG. 18  can be used. Numbered gradations or color-coded stripes may be placed along the length of either adjuster as an aid in making repeat settings. This is particularly useful when using the threaded adjuster  22  which is inserted until it contacts the stop  24 .  
         [0078]     The shaft insert  16  of this invention can be inserted before or after a grip has been installed on a shaft or club. If a grip is in place, e.g. when retrofitting existing clubs, as shown in  FIG. 2   a,  the air hole in the standard size grip, which is normally about 0.15 inches in diameter, needs to be enlarged to about 0.55 inches. This may be accomplished by use of a plug cutter (not shown). The larger hole will allow insertion of the shaft insert  16  itself as well as an insert adjuster  17  or  22 , to make penetration adjustments or to remove the shaft insert  16 . Although not necessary when a bonding agent has been used to attach the shaft insert within a shaft, the grip plug  20  can be replaced and bonded into its original position to assure that the club has been rendered non-adjustable during play and thereby conforming to the present Rules of Golf.  
         [0079]     The maximum range of said penetration P possible with this embodiment will depend in large part on the profile of the inner surface of the golf shaft  10  and the size and shape of the shaft insert  16 . Materials hard enough to provide the range of stiffness required for suitable shaft inserts will allow minimal compression, so that retrofitting standard golf clubs may constrain the range of penetration P due to the taper of the shaft. The range of penetration can be increased with the use of a compressible material placed between the two shafts as described in the alternative embodiments.  
         [0080]     In general the extent of penetration P of the shaft insert into the shaft will range from about 1 to about 10 inches, preferably about 2 to about 6 inches. Adjustment of the extent of penetration P of the shaft insert changes the overall flex of the shaft in the Toe and Swing Planes of the golf club and therefore the club&#39;s dynamic swing parameters. For instance, as the penetration of an 18-inch tubular aluminum shaft insert is varied over a four-inch range, the overall shaft flex, as measured in industry standard terms of frequency, changes approximately 7 CPM (cycles per minute). The range of adjustment can be increased by an additional 7 CPM by replacing the 18-inch insert cited above with a 24-inch insert. Thus starting with a golf club which has a 255 CPM natural flex without any insert, and using either the 18-inch aluminum insert or the 24-inch insert, a combined range of flexes from 255 to 269 CPM can be spanned. This CPM range covers 90% of the hundreds of golfers tested by the inventor using the traditional trial and error fitting methods to find the best flex for each player. Other higher strength-to-weight ratio materials can be used to form the shaft inserts, e.g. graphite, aluminum, or titanium. These materials will increase the range of a single insert length and are within the scope of this invention. An insert can only increase the overall stiffness of a golf shaft. It cannot decrease the stiffness below the original stiffness of the shaft.  
         [0081]     As the amount of penetration P of the shaft insert  16  is increased, the overall flex of the golf club shaft increases due to increased stiffness caused by the presence of the shaft insert  16  as it moves from an initial position mostly under the grip  12  farther into the middle portion of the golf club shaft  10  where bending of the shaft increases during a swing. When the golf club shaft  10  is not bent (as shown in  FIG. 2   a ), there is little effect from the presence of the shaft insert. But during the swing of a club, the shaft typically bends a total of about three inches over its entire length, which affords the shaft insert  16  an opportunity to change the overall stiffness of the shaft. As best seen in  FIG. 2   b,  during a swing three sections of the shaft insert contact the main shaft. The stiffness changes incrementally proportional to the extent of bending of the golf club shaft  10 . Three forces are applied in different locations within the club shaft—a grip end force  31 , a central shaft insert force  32 , and a club end force  33 . The central shaft insert force  32  is exerted in the direction of the club bending. The grip end force  31  and club end force  33  are exerted in the opposite direction.  
         [0082]     The forces are constantly changing planes during a golf swing due to the changing direction of bending of the club as shown in  FIG. 2   c.  At the address, very little bending occurs, but as soon as the club is drawn back, the shaft bends away from the target but remains relatively straight in the toe plane. At the top of a swing, most of the bending is in the toe plane and the swing plane bending is minimal. A result of the constantly changing positions of the forces  31 ,  32 , and  33  and their absolute values result in a non-linear stiffening of the overall flex. The changing flex is not as important, however, as the average value of the flex since the flex is what determines the phase angle of flexing at the time of ball contact.  
         [0083]     Although there is little agreement over the role played by the flexing of the shaft during a swing of a golf club, most experts agree that there is one best flex for each player. This inventor believes that one flex works best for each player, but is a different numerical value for each shaft type, grip, club head design and swing weight. The enormous number of combinations of these variables is so great that it is nearly impossible for club fitters to build optimum clubs no matter what calculations are performed before construction. Thus the inventor advocates finding the best flex for each player for each individual club. This results in each shaft delivering the club head to the ball at the best attitude and phase angle of the various oscillations occurring in the Swing Plane, Toe Plane and Torque Axis of the specific player. Thus the ball is struck solidly or at least in a manner to compensate for the errors in that player&#39;s average swing path and timing. Most preferably, the adjustment of the flex of a club is performed after the club has been assembled as this technique provides an opportunity to compensate for all the variables in a practical manner.  
         [0084]     For maximum energy transfer and therefore the longest shots, the center of gravity of the club head should strike the ball. But for straighter shots for a golfer with an average swing, it may be better for ball contact to occur slightly off-center to compensate for player-induced errors in the swing plane or torque deflection errors caused by club head and shaft characteristics. Although hard to fix in practice, player-induced errors are easy to identify using photography. Errors in directional control due to shaft oscillation phase angle are much more difficult to measure. When shaft flex oscillation in the Swing, Toe and Torque Planes are measured during player swings using strain gauges mechanically attached to the shaft and electrically connected to a computer, and the waveforms recorded. During a typical swing, the shaft bends around 3 inches in all directions through 1.25 cycles of its flex frequency, in both the Swing and Toe Plane, both axes having the same flex frequency. The Torque Plane features an oscillation that is independent of the other two flex frequencies and is three to four times higher, 3.75 to 5 cycles, depending on the shaft model, with an amplitude of a few degrees of angle. All three axes can have different phase angles at ball contact, varying from swing to swing for the same player, and varying more from player to player. The Toe Plane phase angle predominantly determines where on the club head the ball is struck, and the Swing Plane and Torque Plane phase angles determine the aiming direction of the club head at contact.  
         [0085]     For example, if the club head is slightly open at ball contact due to player-induced Swing Plane error and Torque oscillation combined, which would cause the ball to go to the right of the target, then striking the ball off-center near the toe of the club face is best, since that will bring the ball back toward the target line. This is known as the gear effect due to right-to-left spin imparted from such contact, the latter of which being well known in the art. The net result of all these errors, without identifying the extent of any of them, is that some errors will cancel each other and cannot be predicted in their net effect, which is required to build optimized clubs with fixed parameters. The best compromise of each player&#39;s swing idiosyncrasies and club design parameters can only be found by adjusting the flex after club manufacture in order to adjust shaft deflection incrementally in the Toe Plane. This has the effect of determining where the ball strikes the club head, which the inventor believes allows fine tuning of directional control for most players. This concept runs counter to almost all that is written and most likely believed by experts who teach golf and design golf equipment, but is the crux of this invention.  
         [0086]     It is not necessary to know anything about a player&#39;s ability to equip him or her with clubs that the player can adjust to optimize his ability to hit shots directed at a target. This is particularly true with the driver, but extends through the irons and surprisingly, includes the putter. Good players know that there is an optimum flex for a putter and have usually found it experimentally by trying hundreds of putters. But the average player is not aware of this, and if he were, would have only limited opportunity to experiment. This invention allows a player to try a large number of flexes un-supervised, to discover the best flex for each of his clubs individually, and then to stop adjusting them whenever he sees fit, and attach the shaft inserts in proper place to make the club legal under USGA Rules. ps Use of Added Joining Agent  
         [0087]     Several alternative joining agent embodiments are shown in  FIG. 7  through  FIG. 15 , each with the addition of a compressible material which provides friction and holding power between the two shafts while allowing greater depth of penetration for tapered shafts. The joining agents are shown as being added to a tubular shaft insert embodiment but are equally useful with the solid cross section shaft insert embodiment of  FIG. 5   a.    
         [0088]     The maximum possible penetration for a shaft insert depends on the inside diameter of the golf club shaft  10 , the outer diameter and length of the shaft insert  16 , the relative tapers of both shafts, and the compressibility of the added joining agent  18 . These factors also limit the range of flex adjustments possible for any single insert. To increase the range of flexes, shaft inserts of different materials, flexes, and lengths can also be used. Once an adjustment is made, it will remain for many rounds of golf, particularly when there is an added joining agent  18  present in the system.  
         [0089]     A first joining agent configuration is shown in  FIGS. 7 and 8 . In  FIG. 7 , the joining agent  18  is a sleeve which fits over the shaft insert  16  while still having an outer diameter small enough to fit within the cavity of the golf club shaft  10 . The sleeve is a compressible material that fits between the shaft and the shaft insert. It is not joined to either and is shown in the drawings as a scalloped surface.  
         [0090]     A second variation of joining agent  18  is shown in  FIGS. 9 and 10 . This is the most preferred joining agent, particularly for finally locating a shaft insert within a shaft. It entails placing a coating of a compressible adhesive material around a portion of the shaft insert  16  so that upon curing of the adhesive material, the shaft insert becomes bonded into place. A silicone adhesive, such as GE Silicone™, is a suitable material. It has the desirable properties of bonding to the shaft insert  16 , providing friction to the golf club shaft  10 , a large degree of compressibility, and long life expectancy. Since it is bonded to one surface, it may be superior to the previous alternative in terms of handling convenience.  
         [0091]     A third alternative embodiment reverses the location of the bonding of said joining agent  18  from the outer surface of said insert  16  to the inner surface of said golf club shaft  10  shown in  FIGS. 11 and 12 . There may be manufacturing disadvantages to this arrangement.  
         [0092]     A fourth alternative embodiment of the invention adds the joining agent to two surfaces: a thin coating of a compressible material adhering to both the insert and the outer shaft as shown in  FIGS. 13 and 14 . The insert is held in place by friction between the two joining agents. This configuration may provide a maximum range of penetration and maximum friction.  
         [0093]     The fifth alternative embodiment of the invention, shown in  FIG. 15 , employs small pockets of said joining agent  18  spaced over the length of the insert instead of covering the entire length. The holding power of each setting may be somewhat diminished compared to the previous alternatives, but performance is comparable to the other choices, and it is the least costly to manufacture.  
         [0094]     A useful variation of this embodiment, shown in  FIG. 19 , employs small pockets of joining agent, of ever increasing diameters progressing from the tip to the butt end of the insert. The largest diameter to fit inside the golf club shaft would be the last left on the insert, the other larger diameters of the joining agent to be removed by the club fitter or player before final assembly. This configuration provides a universal insert size for use in golf shafts of varying diameters.  
         [0095]     The sixth alternative method of ad Listing and holding the shaft insert  16  in place is shown in  FIG. 16  and  17 . Shaft insert  16  has a spiral length of wire wrapped around it and bonded to it to form self-tapping threads  26 . Female threads are impressed in the thread holding agent  28  by rotation of said self-tapping threads  26 . Torquer  30  provides a means to twist the shaft insert using, for instance, a standard Philips head screwdriver. Said thread holding agent  28  is applied to the inner surface of said golf club shaft  10  and must be pliable enough to allow the self-tapping threads to penetrate it slightly and obtain a purchase. Alternatively, but not shown, the position of the threads and holding agents could be reversed with the threads bonded to said golf club shaft  10  and said thread holding agent  28  bonded to said shaft insert  16 .  
         [0096]     Once in place, the threaded shaft insert is held in place until a new position is obtained by rotation of the insert, the amount of rotation determining the degree of insertion like any standard screw-driven device. The operation of a golf club fitted with this alternative embodiment is similar to the embodiment described earlier.  
         [heading-0097]     Fixed Flex Configuration  
         [0098]     This alternative method for finding and adjusting the flex of a golf club, not shown in any of the figures, uses one of the embodiments described or any other suitable method to determine the optimum flex setting for a specific player for the specific club parts to be used to build a custom club for that player. After the flex setting is measured numerically on a frequency analyzer common in the golf club industry, the adjustable shaft insert can be replaced by a non-adjustable insert that is permanently bonded in whatever position that yields the same flex reading.  
         [0099]     Alternatively, a shaft insert bonded in place could be used to stiffen any shaft to any target flex, however that target flex is determined, by the method described above or by any other flex-fitting methods, of which there are many in use by custom club fitters and amateurs alike.  
         [0100]     Once inserted and bonded in place, a shaft insert will forever change the original flex of the club to the target flex. The balance of a set of clubs can then be “tuned” to that target flex by proper placement of a shaft insert of this invention. For the average golfer this will enable all of the clubs in his set to feel and swing the same. This will also insure that the adjustment would not drift from its predetermined setting. Moreover, a standard grip can be used to hide the existence of the shaft insert.  
         [0101]     For a professional golfer and those with very low handicaps, the ability to individually control the flex of each club using the adjustable shaft inserts will generally be preferred.  
         [0102]     The present invention allows the building of partially finished clubs with a variety of flexes starting with only a few flexes and then altering them with non-adjustable inserts. This would lower the cost of manufacturing and stocking many different flexes, as is the present industry practice. Clubs could be pre-assembled with fewer flexes and later adjusted, perhaps even while the customer waits, by using quick setting bonding agents.  
         [0103]     Accordingly, it can be seen that locating a movable stiffening device inside a golf shaft is the most effective and cosmetically desirable method for adjusting the flex of a golf club during experiments, practice, and play. The shaft insert can be made a permanent part of a club and thereby legal for play under U.S.G.A. Rules.  
       EXAMPLE 1  
       [0104]     The distance and scatter of shots from a single player using a driver with no shaft insert is shown in  FIG. 20   a.  The distance and scatter of shots from the same player using the same driver but with a 14 inch shaft insert in accordance with  FIG. 1  but varying the amount of penetration P in the club is shown in  FIG. 20   b.  Each dot represents a single shot measured in distance with a radar gun with estimated 2% accuracy and left or right track by less accurate visual estimation from the tee. The diameters drawn are the weighted average of 80% of shots. The crosses at the center represent average distance of each sample. As shown, with no shaft insert ( FIG. 20   a ) the player aver-aged 240 yards, but with a broad scatter of 30 yards. With the addition of the insert at the very beginning of the butt end of the shaft, the yardage dropped about 8 yards, but the scatter became less—thus a more repetitive performance from shot to shot with the same club. And as the shaft insert was moved farther into the shaft, the yardage increased and the scatter was reduced until a maximum of performance was reached for this player and this shaft insert when the shaft insert depth was 3″. When the depth was increased to 4″, performance (both distance and scatter) worsened.  
         [0105]     These results demonstrate that the presence of a shaft insert at the ideal location for this player reduced distance by only 2 yards (less than 1%). However, the shot cluster improvement (reduction in scatter) was 100%, going from an initial 30 yards to only 15 yards.  
         [0106]     While these results were obtained using a radar gun, they are identical to those obtained for the same player on a trial and error basis without a radar gun. Thus players can find their own best setting while practicing and then once the setting is found, the shaft insert can be permanently bonded in place to produce a club which is approved for use by the U.S. Golf Association.  
         [0107]     Although the description above contains much specificity, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently envisioned embodiments of this invention. Various other embodiments and ramifications that would occur to a workman in the field are possible within its scope. The scope of this invention is determined by the appended claims and their legal equivalents, rather than by the description and example given.