Method of selecting a golf club shaft

Abstract

An optimal golf club shaft for a golfer is selected from among various golf club shafts, the following. At first, a head speed of a golf club at a point when the golf club strikes a golf ball during a golf swing and a downswing time amount from a top point in time when a downswing begins during the golf swing until a golf ball striking point in time are detected. Then a golf club shaft stiffness suited to the golf swing by using the head speed at the time the golf ball is struck and the downswing time amount is computed. Finally, a golf club shaft having the computed golf club shaft stiffness is selected. The golf club shaft stiffness is computed such that the optimal golf club shaft becomes stiffer as the head speed becomes higher and/or as the downswing time amount becomes longer.

Description

INCORPORATION BY REFERENCE

[0001] This application claims priority on Japanese patent application No. 2004-097629, the entire contents of which are hereby incorporated by reference. In addition, the entire contents of literatures cited in this specification are incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to selecting an optimum golf club shaft for a golfer.

[0003] The golf equipment market has recently reached a point where a golfer may select and purchase a golf club head suited to the individual characteristics of the golfer's swing. Further, golf equipment dealers, manufacturers, and the like have proposed a variety of golf clubs suited to a variety of golf swings, and have also proposed sales methods in which a golf club suited to the characteristics of the particular golf swing of a golfer is selected. For example, golf clubs and the like have been proposed in which the material used in a golf club shaft thereof is changed, and further, in which the material and shape of a golf club head thereof are changed depending on the movement velocity of the golf club head at a point in time when a golf ball is struck (head speed).

[0004] On the other hand, the golf equipment dealers have also proposed recommending a golf club giving golf balls a preferable initial trajectory by having a golfer test-hit golf balls with a variety of golf clubs, and measuring the initial trajectories of the golf balls, in order to increase the effectiveness of their sales.

[0005] Apparatuses that measure the initial trajectory of a golf ball photograph the golf ball immediately after being struck, measuring the initial velocity of the golf ball, the amount of spin on the golf ball, and the like. Accordingly, an expensive apparatus such as a high speed camera or the like is necessary. Further, it is also necessary for a golfer attempting to purchase a golf club to test-hit a ball using a variety of golf clubs. There is a problem with this method in that selection of the golf club can thus only be made from within the range of golf clubs that have actually been test-hit by the golfer.

[0006] In contrast, JP 3061640 B discloses a selection apparatus in which a swing time amount is detected from the start of a golf swing until a golf ball is struck, and a golf club shaft having a flex optimal to a golfer is selected based on the detected swing time.

[0007] A correspondence relationship between the swing time amount and an optimal vibration frequency for the golf club shaft is set in advance by means of a function and utilized by the selection apparatus in order to select a golf club shaft flex that the golfer will feel provided the best timing during the golf swing or is easy to hit with.

[0008] The head speed of a golf club head when striking a golf ball is known to change depending upon the type of golf club shaft used. This is because an external force develops by making a continuous golf swing starting from backswing, through to a top state, and continuing through a downswing, and the external force is transmitted to a golf club through a grip portion. Accordingly, the golf club shaft, which is in a flexed state, deforms transiently due to a centrifugal force acting on the golf club head during a period from the top state through the downswing. The rate of deformation at this point is added to the head speed of the golf club at the point when the golf club strikes the golf ball. It is thus extremely important to optimally select the golf club shaft in order to maximize the head speed, which largely influences the carry distance of the golf ball and is of high concern to the majority of golfers.

[0009] Meanwhile, golf club shafts are divided into different flexes by individual golf club shaft flex markings such as "R", "S", and "X". Further, the flexes can also be classified according to vibration frequencies measured when the grip portion of the golf club shaft is taken as a fixed end of a cantilever beam and the end opposite the fixed end is made to vibrate.

[0010] However, it is merely known that golfers having a high head speed should use a stiff golf club shaft, and therefore quantitative methods for selecting a golf club shaft are generally not known in any great detail.

[0011] The selection apparatus disclosed in JP 3061640 B detects a swing time amount from a point in time at the beginning of a backswing, through to a top state, and continuing into a downswing until a golf ball is struck. The selection apparatus then selects a golf club shaft having an optimal flex for the golfer taking the swing, based on the swing time amount. The golf club shaft having an optimal flex is nothing more than one that will provide the best timing for the golfer during the golf swing, or will be felt easy to hit with by the golfer. Consequently, the head speed, which exerts a great deal of influence on the carry distance of the golf ball, cannot be maximized.

SUMMARY OF THE INVENTION

[0012] The present invention is made in view of problems like those described above. An object of the present invention is to provide a method of selecting an optimal golf club shaft for a golfer.

[0013] Results of earnest investigations lead the applicants of the present invention to discover that, in order to achieve the object described above, a downswing time amount from a stop state of a golf swing, through the beginning of a downswing, through to impact with a golf ball is particularly important factor in selecting a golf club shaft.

[0014] A method of selecting a golf club shaft of the present invention is one which selects a golf club shaft by computing the stiffness of the golf club shaft using the head speed of a golf club head when the golf club head strikes a golf ball during a golf swing, and the downswing time amount of the golf swing. A golf club shaft that is optimal for the golfer can thus be selected.

[0015] That is, the present invention provides a method of selecting a golf club shaft, comprising: detecting a head speed of a golf club at a point when the golf club strikes a golf ball during a golf swing, and detecting a downswing time amount from a top point in time when a downswing begins during the golf swing until a golf ball striking point in time; computing a golf club shaft stiffness suited to the golf swing by using the head speed at the time the golf ball is struck and the downswing time amount; and selecting a golf club shaft having the computed golf club shaft stiffness.

[0016] In the method, the computation of the golf club shaft stiffness preferably includes computing the golf club shaft stiffness to be larger as the head speed is higher and/or as the downswing time amount is longer.

[0017] It is also preferable that a vibration frequency of the golf shaft is taken as an indicator of the golf club shaft stiffness; and the computation of a golf club shaft stiffness includes computing the vibration frequency of the golf shaft based on the following equation:

vibration frequency (cpm)=a0+a1.times.HS(m/s)+a2.times.T(second) (1)

[0018] where symbol HS denotes the head speed, symbol T denotes the downswing time amount, and symbols a0, a1, and a2 denote constants. Preferably, The constant a0 is within a numerical range of 21.+-.5; the constant a1 is within a numerical range of 4.3.+-.1; and a2 is within a numerical range of 157.+-.40.

[0019] Alternatively, it is preferable that a vibration frequency of the golf shaft is taken as an indicator of the golf club shaft stiffness; and the computation of a golf club shaft stiffness includes computing the vibration frequency of the golf shaft based on the following equation:

Vibration frequency (cpm)=a0+a1.times.HS(m/s)+a2.times.HS(m/s)/T(second) (2)

[0020] where symbol HS denotes the head speed, symbol T denotes the downswing time amount, and symbols a0, a1, and a2 denote constants. Preferably, the constant a0 is within a numerical range of 74.+-.20; the constant a1 is within a numerical range of 5.5.+-.1.4; and a2 is within a numerical range of -0.38.+-.0.1.

[0021] In the method, it is also preferable that the golf club includes a grip portion; a three-dimensional magnetic sensor that is sensitive to magnetism within a magnetic field and outputs position and direction information, is fixed to the grip portion of the golf club head; a predetermined magnetic field is formed during the golf swing of the golf club; the top point in time when a downswing begins during the golf swing and the golf ball striking point in time are determined using the position and direction information output from the three-dimensional magnetic sensor; and the downswing time amount is found from the top point in time and the golf ball striking point in time.

[0022] A method of selecting a golf club shaft of the present invention selects a golf club shaft that should be used by a golfer in order to maximize the golfer's head speed. The selection is made by computing the stiffness of the golf club shaft using the head speed of a golf club (standard golf club) at a point when the standard golf club strikes a golf ball, and a downswing time amount for the standard golf club. A golf club shaft that is optimal for the golfer can thus be selected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the accompanying drawings:

[0024] FIG. 1 is a schematic view of a selection apparatus that implements a method of determining the optimal vibration frequency of a golf club shaft, and a method of selecting a golf club shaft of the present invention;

[0025] FIG. 2 is a diagram that explains a configuration of main element portions of a measuring device shown in FIG. 1;

[0026] FIG. 3 is a diagram that explains a backswing time amount and a downswing time amount used in a method of determining the optimal vibration frequency of a golf club shaft of the present invention;

[0027] FIGS. 4A to 4C are diagrams that show examples of three-dimensional time sequence data obtained by the measuring device shown in FIG. 1;

[0028] FIG. 5 is a diagram that shows a relationship between a head speed and the optimal vibration frequency;

[0029] FIG. 6 is a diagram that shows a relationship between a vibration frequency computed by using Eq. (1) and the optimal vibration frequency; and

[0030] FIG. 7 is a diagram that shows a relationship between a vibration frequency computed by using Eq. (2) and the optimal vibration frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] A method of selecting a golf club shaft of the present invention is explained in detail below based on preferred embodiments shown in the appended drawings.

[0032] FIG. 1 is a schematic view of a selection apparatus 10 that determines a vibration frequency of a golf club shaft, used as an indicator of the stiffness of the golf club shaft, and selects an optimal golf club from among a plurality of golf clubs based on the vibration frequency. A state where a golfer is gripping a golf club 30 and taking a golf swing is shown in FIG. 1.

[0033] The selection apparatus 10 includes a head speed detecting device 12, a measuring device 20, and an analysis device 24. The analysis device 24 optimally determines the vibration frequency of a golf club shaft occurring when a grip side is taken as a fixed end. This has heretofore been used as an indicator for characterizing the stiffness of the golf club shaft. The determination is made by using three-dimensional time sequence data supplied from the measuring device 20. The term three-dimensional time sequence data as used herein denotes data that expresses movement of a grip portion 36 of a golf club according to the position and the orientation of the grip portion 36.

[0034] The head speed detecting device 12, which is a well known device, has a light beam emitting unit 14 emitting two light beams in a parallel each other along the pass of the golf club head, a light beam receiving unit 16 and a measuring unit 18.

[0035] The measuring unit 18 calculates the head speed of the golf club head by measuring a time difference between the two parallel light beams when each light beam reception of the light beam receiving unit 16 is interrupted by the golf club head passing by. The head speed detecting device 12 is positioned near the pass of the golf club head just before the golf ball is struck. Preferably, the device 12 is positioned such that the parallel light beams cross the pass of the golf club head within a range of 100 mm from the back point 40 mm away from the center of the golf ball in the backward direction of the golf ball hitting direction. The calculated head speed is supplied to the analysis device 24.

[0036] Referring to FIG. 1, the measuring device 20 generates three types of predetermined magnetic fields from a transmitter 20a fixedly disposed behind the golfer taking a golf swing. On the other hand, a receiver 20b fixed to the grip portion 36, which undergoes translation and rotation, detects magnetism corresponding to the position and the orientation within the three types of magnetic fields formed by the transmitter 20a. The receiver 20b outputs a total of 9 output voltages. Data processing is performed in a controller 20c using the output voltages. Data for the three-dimensional position and orientation (Euler angles) of the receiver 20b is thus obtained.

[0037] Referring to FIG. 2, the measuring device 20 is explained in detail. The measuring device 20 includes the transmitter 20a that forms the predetermined magnetic fields, the receiver 20b that generates output voltages in three axial directions corresponding to the strength and orientation of the magnetic fields, and the controller 20c.

[0038] The controller 20c generates a driver signal to make the transmitter 20a emit the three types of predetermined magnetic fields in sequence, detects signals output from the receiver 20b, performs data processing from the detected signals, and computes time sequence data. The term time sequence data denotes time sequence data of position coordinates (x.sub.m, y.sub.m, z.sub.m) in a three-dimensional coordinate system having three mutually orthogonal axes, in which a predetermined position, the position of the transmitter 20a, for example, is taken as a reference position. The term time sequence data also denotes time sequence data of an orientation angle that expresses the orientation of the receiver 20b with respect to a predetermined reference direction, in X, Y, and Z coordinate axis directions, for example. In other words, the orientation angle is shown by a yaw angle, a pitch angle, and a roll angle (hereinafter expressed as Euler angles (.theta..sub.y, .theta..sub.p, .theta..sub.r)).

[0039] Referring to FIG. 2, the transmitter 20a and the receiver 20b are each configured by three coils wound in a loop shape in three mutually orthogonal axes. The transmitter 20a is fixedly disposed behind the golfer taking a golf swing, while the receiver 20b is fixed to an end portion of the grip portion 36 of the golf club 30.

[0040] The controller 20c is configured by a driver circuit 20d that produces driver signals to generate three types of magnetic fields in sequence, a detector circuit 20e that detects signals output from the receiver 20b, and a control unit 20f that controls the driver circuit 20d and the detector circuit 20e. The control unit 20f finds the three-dimensional position and the Euler angles of the receiver 20b from the signals sent by the detector circuit 20e. The transmitter 20a is connected to the driver circuit 20d, and the receiver 20b is connected to the detector circuit 20e.

[0041] It should be noted that the time sequence data for the three-dimensional position coordinates (x.sub.m, y.sub.m, z.sub.m) with respect to the reference position of the receiver 20b fixed to the grip portion 36, and the time sequence data for the Euler angles (.theta..sub.y, .theta..sub.p, .theta..sub.r) with respect to the reference direction are obtained as described hereinafter.

[0042] Referring to FIG. 2, the driver circuit 20d outputs identical signals having fixed frequency and phase according to a command signal from the control unit 20f, thus exciting the three loop shape coils, wound in three axial directions, of the transmitter 20a in sequence. Each of the loop shape coils generates a different magnetic field when excited, and independent voltages are generated in the respective loop shape coils of the receiver 20b based on the magnetic fields. The voltages are independently generated by the three loop shape coils of the receiver 20b for each of the magnetic fields excited by the three loop shape coils of the transmitter 20a. Accordingly, a total of 9 (3.times.3) voltages are obtained.

[0043] On the other hand, the transmitter 20a that generates the magnetic fields is fixedly disposed in a predetermined position. Distributions of the magnetic field strengths and directions are thus known in advance with respect to the reference position and reference direction in which the transmitter 20a is installed. The three-dimensional position coordinates (x.sub.m, y.sub.m, z.sub.m) of the receiver 20b with respect to the reference position and the Euler angles (.theta..sub.y, .theta..sub.p, .theta..sub.r) of the receiver 20b with respect to the reference direction, making a total of six unknown values, can thus be found by using the nine voltages generated by the magnetic fields formed.

[0044] The control unit 20f of the controller 20c computes the three-dimensional position coordinates (x.sub.m, y.sub.m, x.sub.m) and the Euler angles (.theta..sub.y, .theta..sub.p, .theta..sub.r) using the nine voltages sent from the detector circuit 20e. The three-dimensional position coordinates (x.sub.m, y.sub.m, z.sub.m) and the Euler angles (.theta..sub.y, .theta..sub.p, .theta..sub.r) are then supplied to the analysis device 24.

[0045] A 3SPACE FASTRAK device (manufactured by Polhemus Inc.), for example, can be used as the measuring device 20.

[0046] The analysis device 24 includes a data extraction portion 24a that extracts the three-dimensional time sequence data based on the data supplied from the computing device 20, a time detector portion 24b that detects a downswing time amount T, from a point when a downswing begins in a top state until a point when a golf ball is struck, using the three-dimensional time sequence data, a vibration frequency computing portion 24c that computes a vibration frequency using the detected downswing time amount T, and a selector portion 24d that selects an optimal golf club based on the computed vibration frequency. A monitor 25 and operation devices (mouse and keyboard) 26 are connected to the analysis device 24.

[0047] Each portion of the analysis device 24 described above is a computer configured to exhibit the functions described by implementing a modularized subroutine program. However, in this embodiment each of the portions may also be a dedicated device containing dedicated circuits.

[0048] The processes implemented by the analysis device 24 are also shown in FIG. 1.

[0049] The data extraction portion 24a computes three-dimensional time sequence data for the three-dimensional position of the grip portion 36 taking a predetermined position as a reference, and time sequence data for the orientation of the grip portion 36 in three directions in an orthogonal coordinate system (XYZ coordinate system) from the time sequence data of the three-dimensional position coordinates (x.sub.m, y.sub.m, z.sub.m) and the Euler angles (.theta..sub.y, .theta..sub.p, .theta..sub.r) supplied from the computing device 20 described above (step S10).

[0050] For example, the data extraction portion 24a may find three-dimensional time sequence data for the three-dimensional position coordinates (X,Y,Z), and for an azimuth angle, an elevation angle, and axial rotation of the golf club shaft 32 in an orthogonal coordinate system. The time sequence data for the three-dimensional position of the grip portion 36 and for the three-dimensional orientation of the grip portion 36 in an orthogonal coordinate system is then stored in a memory (not shown) of the analysis device 24.

[0051] The three-dimensional time sequence data for the three-dimensional position of the grip portion 36 and the three-dimensional orientation of the grip portion 36 may then be converted into trajectory data for the grip portion 36 as seen from a predetermined direction. For example, the three-dimensional time sequence data may be converted into trajectory data as seen from a location in front of, and facing, the golfer. The converted data is then supplied to the monitor 25. The three-dimensional time sequence data is then displayed on the monitor 24 as a graph.

[0052] The time detector portion 24b detects the downswing time amount T shown in FIG. 3 from the three-dimensional time sequence data found (step S20). The golfer is stopped at a point where the backswing begins, and the top state. Accordingly, the point where the backswing begins and the top state can be specified by reading in positions having extreme values from the waveform of the three-dimensional time sequence data. Further, an impact point where a golf ball is struck can also be specified from fluctuations in the time sequence data occurring due to the golf ball being struck.

[0053] FIGS. 4A to 4C show examples of graphs of the three-dimensional time sequence data displayed in the monitor 25. The three-dimensional time sequence data expresses the behavior of the grip portion 36 in the XYZ coordinate system. In FIGS. 4A to 4C the amount of time from an address state to the top state is approximately 1.1 seconds, and the downswing time amount T is approximately 0.35 seconds.

[0054] The vibration frequency computing portion 24c computes a vibration frequency (cycles per minute, cpm) according to Eq. (1) or Eq. (2) shown below using the detected downswing time amount T (seconds) and the characteristic head speed of the golfer detected by the head speed detecting device 12 (step S30).

Vibration frequency(cpm)=a0+a1.times.HS(m/s)+a2.times.T(second) (1)

Vibration frequency(cpm)=a0+a1.times.HS(m/s)+a2.times.HS(m/s)/T(second) (2)

[0055] Herein, symbol HS denotes the head speed, symbol T denotes the downswing time amount, and symbols a0, a1, and a2 denote predetermined constants.

[0056] Further, in order to find the vibration frequency (cpm), the grip portion of the golf club shaft may be held fixed by a chuck, a leading end portion (tip end portion) of the golf club head side may be vibrated as a free end, and the number of times that the free end portion of the golf club shaft cuts across a light beam of photoelectric cell may be computed. The vibration frequency (cpm) is then found from the number of times the free end portion of the golf club shaft cuts across the light beam per second. Alternatively, the natural frequency of the golf club shaft may be used as the vibration frequency. A trailing end portion (butt portion) of the golf club shaft itself, to which a grip is attached, may be held fixed, an acceleration pickup may be attached to the tip end portion at which a golf club head is attached. The vibration frequency may then be found from an acceleration signal obtained by the acceleration pickup.

[0057] It is preferable here that the fixing length of the end portion be equal to or less than 200 mm when measuring the vibration frequency. A length of 7 inches (approximately 178 mm) is commonly used, and is more preferable. The fixing length may be set to an arbitrary value, provided that it is possible to hold the golf club shaft fixed during measurements and provided that the value is less than 7 inches (approximately 178 mm). It becomes possible to perform vibration frequency measurements with high accuracy by setting the fixing length to 7 inches or less.

[0058] Further, it is preferable that procedures for measuring the vibration frequency comply with operation procedures for a bending vibration meter instituted by the Japan Golf Gear Association. A club timing harmonizer manufactured by Fujikura Rubber Co., Inc. or the like may be used for the measuring device.

[0059] A vibration frequency computed according to Eq. (1) or Eq. (2) corresponds to the vibration frequency of the golf club shaft that maximizes the head speed of the golf club when the golfer strikes a golf ball.

[0060] Further, the vibration frequency is not limited to any particular unit of time. For example, a vibration frequency per minute (cpm) and a vibration frequency per second (Hz) can be measured. The golf equipment industry generally uses a vibration frequency measured in cycles per minute (cpm), and this unit is thus used with the present invention to make things easier to understand.

[0061] The selector portion 24d selects a golf club having the computed vibration frequency as an optimal golf club for the golfer (step S40). Specifically, vibration frequencies are stored in advance in a database (not shown) provided in the selector portion 24d, along with the make and model of golf club shafts having the vibration frequencies. In addition, the make and model of golf clubs having the golf club shafts are associated with the golf club shafts and stored in the database. Information such as the make and model of golf clubs having the golf club shaft corresponding to the vibration frequency computed in step S30 is then looked up, and search results are displayed in the monitor 25.

[0062] A golf club that is optimal for the golf club swing of the golfer measured using the measuring device 20 can thus be provided to the golfer. In other words, a golf club having golf club head whose head speed is substantially maximized at the point when a golf ball is struck can be provided.

[0063] The downswing time amount T is found with the selecting device 10 from the three-dimensional time sequence data obtained using the measuring device 20. The downswing time amount T is not limited to being found using the measuring device 20, however. For example, the downswing time amount T may also be found from images photographed by using a high speed video camera or a consumer level video camera. Image frames that express a top state in the golf swing and a golf ball striking (impact) state may be identified, and the downswing time amount T may be found from the number of frames between the top state and the impact state.

[0064] The present invention can select an optimal golf club for a golfer by computing the vibration frequency of the golf club shaft based on predetermined equations using the head speed at the point when the golf ball is struck, and using the downswing time amount. However, indicators other than the vibration frequency may also be used to express the stiffness of the golf club shaft.

[0065] For example, the amount of force required to flex the golf club shaft may be measured, and the measured force can be used to indicate the golf club shaft stiffness. Further, the stiffness of the golf club shaft can also be indicated by measuring the amount of flex of the golf club shaft when a fixed load is applied (golf club flex measurement and method of measuring golf club flex instituted by the Japan Golf Gear Association). In addition, indicators used in a method of measuring a vibration frequency cited in JP 2001-293109 A or JP 2003-62128 A may also be used to indicate the stiffness of a golf club shaft.

EXAMPLE

[0066] Twenty-one golfers were used to test-hit eight different golf clubs outfitted with a variety of golf club shafts having different vibration frequencies, and the optimal vibration frequency for each of the golfers was investigated. The eight varieties of golf clubs used in the tests each had a length of 45 inches, and a mass of 305.+-.5 g.

[0067] A separate standard golf club was also provided, and the head speed for each of the golfers was measured using the standard golf club. The measured head speed for each was used as a characteristic head speed for that golfer. The standard golf club used in this embodiment had a length of 45 inches, and a mass of 302 g.

[0068] The term optimal vibration frequency refers to the vibration frequency of the golf club shaft of the golf club that, from the results of each golfer test-hitting with the eight varieties of golf clubs, struck a golf ball with the fastest head speed for each golfer. The vibration frequencies were measured using a club timing harmonizer manufactured by Fujikura Rubber Co., Inc. A 178 mm portion of a butt end portion of the golf club was fixed to a vibration frequency measuring device, and the golf club head was displaced in a vertical direction by hand. Vibration ensued when the golf club head was released, and the vibration frequency (cpm) was obtained by measuring the resulting vibrations. Basic operation procedures were performed in accordance with the operation procedures for measuring the bending frequency of a golf club instituted by the Japan Golf Gear Association.

[0069] A relationship between the optimal vibration frequency and the head speed based on the standard golf club is shown in FIG. 5.

[0070] Referring to FIG. 5, it can be seen that the correlation coefficient between the optimal vibration frequency and the head speed is 0.57 (R.sup.2=0.3337), which is not a good correlation. From FIG. 5 it becomes clear that it is insufficient to select the vibration frequency of the golf club according to the head speed, which has heretofore been a problem with conventional techniques. Taking the down swing time as a supplemental factor, vibration frequencies were computed based on Eq. (1) and Eq. (2) below.

Vibration frequency(cpm)=a0+a1.times.HS(m/s)+a2.times.T(second) (1)

Vibration frequency(cpm)=a0+a1.times.HS(m/s)+a2.times.HS(m/s)/T(second) (2)

[0071] Herein, symbol HS denotes the heads speed, symbol T denotes the downswing time amount, and symbols a0, a1, and a2 denote predetermined constants. In Eq. (1), the constant a0 is set to a range of 21.+-.5, the constant a1 is set to a range of 4.3.+-.1, and the constant a2 is set to a range of 157.+-.40. In Eq.

[0072] (2), the constant a0 is set to a range of 74.+-.20, the constant a1 is set to a range of 5.5.+-.1.4, and the constant a2 is set to a range of -0.38.+-.0.1.

[0073] The downswing time amount T is computed from images of the golfer taking a test swing with the standard golf club that are photographed by a high speed video camera. Image frames of images that express a top state and a golf ball striking (impact) state during the golf swing were identified, and the number of images frames from the top state to the impact state was computed. FIGS. 6 and 7 show correlations between the vibration frequencies found by using Eq. (1) and Eq. (2), and the measurement results (optimal vibration frequencies).

[0074] All of the vibration frequencies computed based on Eq. (1) and Eq. (2) had a correlation coefficient equal to or greater than 0.86 (R.sup.2=0.75). It can thus be seen that there is a strong correlation between the vibration frequencies found from Eq. (1) and Eq. (2), and the optimal vibration frequencies found form the measurement results. The head speed and the downswing time can therefore be used to sufficiently express the optimal vibration frequency.

[0075] From the results described above, it can be seen that a golf club shaft suited to each individual golfer can be selected by using the head speed and the downswing time amount.

[0076] It should be noted that the method of selecting a golf club shaft of the present invention is not limited to the embodiments described above. A variety of improvements and changes can of course be made in a scope that does not deviate from the gist of the present invention.

Claims

What is claimed is:

1. A method of selecting a golf club shaft, comprising: detecting a head speed of a golf club at a point when the golf club strikes a golf ball during a golf swing, and detecting a downswing time amount from a top point in time when a downswing begins during the golf swing until a golf ball striking point in time; computing a golf club shaft stiffness suited to the golf swing by using the head speed at the time the golf ball is struck and the downswing time amount; and selecting a golf club shaft having the computed golf club shaft stiffness.

2. The method of selecting a golf club shaft according to claim 1, wherein said computing of a golf club shaft stiffness includes computing the golf club shaft stiffness to be larger as the head speed is higher and/or as the downswing time amount is longer.

3. The method of selecting a golf club shaft according to claim 1, wherein: a vibration frequency of the golf shaft is taken as an indicator of the golf club shaft stiffness; and said computing of a golf club shaft stiffness includes computing the vibration frequency of the golf shaft based on the following equation: vibration frequency(cpm)=a0+a1.times.HS(- m/s)+a2.times.T(second) (1) where symbol HS denotes the head speed, symbol T denotes the downswing time amount, and symbols a0, a1, and a2 denote constants.

4. The method of selecting a golf club shaft according to claim 3, wherein: the constant a0 is within a numerical range of 21.+-.5; the constant a1 is within a numerical range of 4.3.+-.1; and a2 is within a numerical range of 157.+-.40.

5. The method of selecting a golf club shaft according to claim 1, wherein: a vibration frequency of the golf shaft is taken as an indicator of the golf club shaft stiffness; and said computing of a golf club shaft stiffness includes computing the vibration frequency of the golf shaft based on the following equation: Vibration frequency(cpm)=a0+a1.times.HS(- m/s)+a2.times.HS(m/s)/T(second) (2) where symbol HS denotes the head speed, symbol T denotes the downswing time amount, and symbols a0, a1, and a2 denote constants.

6. The method of selecting a golf club shaft according to claim 5, wherein: the constant a0 is within a numerical range of 74.+-.20; the constant a1 is within a numerical range of 5.5.+-.1.4; and a2 is within a numerical range of -0.38.+-.0.1.

7. The method of selecting a golf club shaft according to claim 1, wherein: the golf club includes a grip portion; a three-dimensional magnetic sensor that is sensitive to magnetism within a magnetic field and outputs position and direction information, is fixed to the grip portion of the golf club head; a predetermined magnetic field is formed during the golf swing of the golf club; the top point in time when a downswing begins during the golf swing and the golf ball striking point in time are determined using the position and direction information output from the three-dimensional magnetic sensor; and the downswing time amount is found from the top point in time and the golf ball striking point in time.