as shown in fig1 , the preferred embodiment of this invention is a tapered rigid tubular shaft assembly 19 comprising an insert 16 , of 12 to 24 inches in length , coated with a compressible friction agent 18 , such as silicone . alternatively , the rigid tubular shaft may have internal female threads at one end allowing for extraction with an insert adjuster 17 , as shown in fig3 . the insert assembly 19 , comprised of insert 16 and friction agent 18 , are inserted into a golf club shaft 10 , as shown in fig2 , for the purpose of adjusting the stiffness of the golf club shaft . as shown in fig2 , a conventional golf dub comprises a golf club shaft 10 , usually about 34 to 47 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 poly - carbonate 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 . 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 . the development of graphite and other composites in shaft construction over the last 30 years has reduced driver shaft weight from over 100 grams for steel to around 50 grams for graphite , allowing a redistribution of weight favoring increased head weight for the same swing weight for the finished club . this evolution permits increased swing speeds or heavier heads or some combination of both for added distance . adding back weight to the shaft must be approached carefully or the gain in shaft weight reduction will be nullified . as fig7 shows , adding weight under the grip has the least effect on swing speed and can even increase it due to the optimizing effect of adjusting shaft flex , which is one of the objects of this invention . the best compromise of added weight versus flex adjustment for the shaft material used in this embodiment is a preferred length of assembly of 13 to 16 inches : lengths smaller than 12 inches have little range of flex adjustment ( see fig6 ) and longer lengths become too heavy and the benefit of added flex is contravened by the decreased speed contribution of excess weight ( see fig7 ). if lighter materials are used to manufacture the shaft insert , the range of insert lengths could be increased from 16 to perhaps 24 inches . common shaft manufacturing techniques for both steel and graphite 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 rigid shaft insert assembly within the cavity as shown in fig2 and in detail in subsequent figures . refer now to the preferred embodiment of the invention shown in fig3 . the hollow rigid tubular shaft insert assembly coated with the cured compressible friction agent 18 is positioned inside the hollow cavity of the golf club shaft 10 . a silicone adhesive , such as ge silicone ™, is a suitable material for the compressible friction agent to insure that the silicone adheres to the insert and not to the golf shaft itself , the silicone should cure while not in contact with the golf club shaft . it has the desirable properties of bonding to the shaft insert 16 after curing , and after insertion , holding the assembly in place by providing friction to the golf dub shaft 10 , a large degree of compressibility enabling a wide range of insertion positions , and long life expectancy . by adjusting the insertion depth p , of a shaft insert assembly that is 12 to 24 inches long , from about 1 to 10 inches will change the overall flex of the shaft of the golf club without altering its accepted cosmetic look . the assembly can be positioned most easily if the inside surface of the assembly is threaded so that an insert adjuster 17 , which has the complimenting thread , can be joined to the assembly to easily decrease its penetration depth p . once positioned , the shaft insert assembly is held by friction at the desired penetration depth p such that there is no reasonable likelihood of it working loose during a round of golf . the range of insertion possible , before the compressible friction agent will compress no more , is best accomplished if the shaft insert is tapered with a pitch roughly equal to the shaft into which it is to be placed . however , a cylindrical or non - tapered insert can be used for this purpose by using a thicker coating of compressible friction agent such that the overall shape of the shaft insert assembly is somewhat tapered . a few shafts sold today are cylindrical under the grip having the taper begin some distance toward the shaft end attached to the head these shafts are more suitable to use with this invention , but the more common fully tapered shafts are found to have a suitable range of adjustment . while a tubular shaft insert configuration is preferred due to its strength - to - weight ratio advantage , solid cross - section configurations can be employed and may enjoy a cost advantage . the shaft insert assembly can be inserted before or after a grip has been installed on a shaft or club . the grip must have an opening at the butt end about 0 . 55 inches which is the approximate inner diameter of the shaft a special grip with an opening of this size can be installed or a standard grip modified to have a hole at the butt end of this size . this size hole will allow insertion of the shaft insert assembly as well as an insert adjuster 17 , to make penetration adjustments or to completely remove the shaft insert assembly . in general the extent of penetration p of the shaft insert assembly into the shaft will range from about 1 to about 10 inches . adjustment of the extent of penetration p of the shaft insert assembly determines 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 a 14 - inch tubular graphite shaft insert assembly is varied over a seven inch range , the overall shaft flex , as measured in industry standard terms of frequency , changes approximately 7 cycles per minute ( see fig6 ). if a greater range is desired , an 18 inch assembly can be substituted providing a combined range of 12 cycles per minute , almost a complete flex as measured by industry standards . alternatively , a stiffer shaft can be used as the starting point and the 14 inch assembly used with it to increase its flex by another 7 cpm . in this manner , a range of 30 cpm can be covered by three or four starting shafts and one 14 inch insert assembly . this cpm range covers 90 percent 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 . it should be obvious but for the sake of complete disclosure , an insert can only increase the overall stiffness of a golf shaft and cannot decrease the stiffness below the stiffness of the original golf shaft into which it is placed . as the amount of penetration p of the shaft insert assembly is increased , the overall flex of the golf club shaft increases due to increased stiffness caused by the presence of the insert as it moves from a position under the grip 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 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 insert an opportunity to change the overall stiffness of the shaft an alternative embodiment of the invention , shown in fig5 , employs small pockets of said compressible friction agent 18 spaced over the length of the shaft insert 16 instead of covering the entire length of the insert as shown in fig1 . the holding power of each setting may be somewhat diminished compared to the previous alternative , but performance is comparable , and it is less costly to manufacture . in practice , as few as three pockets have been tested and found to be satisfactory . 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 .
a moveable shaft insert assembly about 13 to 18 inches long weighing less than 50 grams is inserted into a hollow golf dub shaft wherein the depth of insertion of the shaft insert assembly may vary from about 1 to 10 inches . changing the location of the shaft insert assembly allows a player to change the flex of the shaft and thereby optimize the performance of the club dynamics for that player for that day . the shaft insert is held in place by friction between the shaft and the shaft insert assembly . so shaft flex fitting can be administered by a player with or without coaching , and can be revisited at any time by a simple adjustment . the shaft inserts are useful on all hollow shaft clubs , and can be retrofitted to existing clubs without removing the grip .