Golf club

Abstract

A golf club includes a shaft and a head. The golf club has a shaft mass Ms of 60 g or less and defines a head mass Mh(g) and the shaft mass Ms(g) to have a relation to satisfy the following expressions (A) and (B): Mh≧Ms+155  (A) Mh≦−0.14·Ms+240.9  (B)

Description

FIELD OF THE INVENTION

The present invention relates to a golf club having an excellent flight distance performance.

DESCRIPTION OF THE PRIOR ART

In general, the flight distance performance is most valued in the golf clubs. In recent years, therefore, various attempts have been made to increase head speed by reducing the weight of the golf club, thereby to improve the flight distance performance. As a measure to realize a lightweight golf club, the weight reduction of a shaft or a head is pursued.

As to the weight reduction of the shaft, the current trend is to employ a carbon shaft having a weight of 40g to 80g, whereas the use of a steel shaft having a weight in excess of 100g has been predominant in the conventional practice. The weight reduction of the head has also been pursued. As an example, a so-called driver head has been reduced in weight to nearly 180g in the state of the art, while the conventional driver head has a weight of about 200g.

When reduced in weight, the golf club allows a higher swing speed. Hence, the weight reduction of the club can contribute to the improvement of the flight distance performance in that a head speed upon impact is increased. However, the reduction of a head mass, in particular, may constitute a causative factor of lowering the inertial energy (momentum) of the head upon impact, thus leading to the decrease of the restitution coefficient thereof. Therefore, it is impossible to fully improve the flight distance performance by merely reducing the weight of the head or of the golf club. Japanese Unexamined Patent Publication No. 2002-65906 discloses a wood golf club directed to the improvement of the fight distance performance by defining a relation between the head mass and the length of the club or the like.

OBJECT AND SUMMARY OF THE INVENTION

The present inventors studied the relation between the shaft mass and the head mass based on an absolutely different technical concept from that of the aforementioned prior art, thereby discovering a possibility of further improving the flight distance performance. It is therefore an object of the invention to provide a golf club achieving a superior flight distance performance by optimizing the relation between the shaft mass and the head mass.

A golf club according to the invention has a shaft mass Ms of 60g or less and defines a head mass Mh(g) and the shaft mass Ms(g) to have a relation to satisfy the following expressions (A) and (B): Mh≧Ms+155  (A) Mh≦−0.14·Ms+240.9  (B)

Such an arrangement is adapted to obviate a lowered swing speed due to an excessively heavy shaft, because the shaft mass is limited up to 60g. The arrangement adequately contemplates a flexing behavior of the shaft during golf swing as well as time taken to accomplish a swing motion (a period of time it takes for the shaft to move from “top of swing” to an impact position). Thus, balance between the head mass Mh and the head speed is optimized so that the golf club may provide an increased flight distance. In this respect, a detailed description will be made hereinlater.

The aforesaid head may also define the head mass Mh(g) and the shaft mass Ms(g) to have a relation to satisfy the following expression (C): Mh≧−0.1Ms+212  (C)

In this case, as well, the balance between the head mass Mh and the head speed is optimized so that the golf club may provide the increased flight distance. In this respect, a detailed description will be made hereinlater.

In addition, the durability of the shaft is assured by defining the shaft mass Ms to be 30g or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a golf club 1 according to one embodiment of the invention and the conditions of the golf club during golf swing;

FIG. 2A is an exploded perspective view showing a head used in examples of the invention and comparative examples;

FIG. 2B is a sectional view of the above head;

FIG. 3 is a developed view showing a prepreg structure of shafts employed by the examples and comparative examples; and

FIG. 4 is a graph wherein the shaft mass Ms(g) and the head mass Mh(g) of each of the examples and the comparative examples are plotted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the invention will hereinbelow be described with reference to the accompanying drawings.

FIG. 1 shows the conditions of a wood golf club 1 according to one embodiment of the invention, which is swung. The golf club 1 includes a bar-like shaft 2 which is assembled with a wood head 3 at one end thereof and with a grip 4 at the other end thereof. FIG. 1 is a diagram showing how the golf club 1 is swung by a golfer P as viewed from a front side of the golfer P. The figure depicts a golf club ‘k’ at a moment that the golf club, swung up from an address position to “top of swing” (hereinafter, referred to as “top”) by the golfer executing a backswing, is shifted to a downswing motion (hereinafter, referred to as “swing shift time”). The figure also depicts a golf club ‘m’ reaching an impact position through the downswing motion. The figure depicts in phantom only a posture of the golfer P at the moment of impacting a ball. The golf club 1 is swung down from the top toward the impact position along a direction of an arrow FIG. 1. Hereinafter, the direction of the downswing (the direction of the arrow in FIG. 1) will be referred to as “forward swing direction”, whereas a direction opposite to the downswing direction (the opposite direction to the direction of the arrow in FIG. 1) will be referred to as “backward swing direction”.

By the way, a flight distance of a golf ball gb given by hitting the golf ball is principally dependent upon three initial ball conditions, provided that influences such as of the wind are set aside. The initial ball conditions include: an initial speed of the ball just having been launched (hereinafter, referred to as “initial ball speed”); an amount of ball spin; and a launch angle (elevation angle). Provided that the other conditions than the initial ball speed are constant, the flight distance is increased as the initial ball speed is increased.

As effective measures for increasing the initial ball speed, it may be contemplated to increase a head speed Vh0 just prior to impact, and to increase a head mass Mh. The reason is that since a head-on-ball impact phenomenon wherein the head 3 impacts the golf ball gb substantially reserves the momentum as it is, the following equation (1) is approximately established. Accordingly, an initial ball speed Vbl is approximately expressed by the following equation (2): Mh·Vh1+Mb·Vb1=Mh·Vh0  (1) Vb1=Mh(Vh0−Vh1)/Mb  (2), where Vh1 indicates a head speed just after impact, and Mb indicates a ball mass.

A mere increase of the head mass Mh does necessarily lead to the increase of the initial ball speed Vb1 expressed by the equation (2) because of the following tendency. With an increased head mass, the golf club is increased in inertial moment (inertial moment about a grip end) so that the swing speed is lowered to decrease the head speed Vh0 just prior to impact. Conversely, if the head mass Mh is decreased, the head speed Vh0 just prior to impact is increased. However, just as in the above case, the initial ball speed Vb1 expressed by the equation (2) is not necessarily increased.

In a case where the aim is a mere increase of the head speed Vh0 just prior to impact, what to do is to reduce the head mass Mh and the shaft mass Ms. In the prior art, therefore, most attempts to increase the flight distance have resorted to the reduction of both the head mass Mh and the shaft mass Ms. However, the present study has revealed that an adequate increase of the flight distance cannot be achieved simply by decreasing the head mass Mh and the shaft mass Ms. Based on this finding, the invention has been accomplished.

The golf club 1 according to the embodiment of the invention defines the shaft mass Ms to be 60g or less and also defines the head mass Mh(g) and the shaft mass Ms(g) to have a relation to satisfy the following expressions (A) and (B): Mh≧−Ms+155  (A), and Mh<−0.14·Ms+240.9  (B).

FIG. 4 is a graph wherein the abscissa is the shaft mass Ms(g) whereas the ordinate is the head mass Mh(g). In this graph, a region satisfying the expression (A) is above a line expressed by a mathematical expression Mh=Ms+155. On the other hand, a region satisfying the expression (B) is below a line expressed by a mathematical expression Mh=−0.14·Ms+240.9.

Now, description is made on a flexing behavior of the shaft of the golf club 1 during the golf swing by way of explanation of the expressions (A) and (B).

As shown in FIG. 1, the shaft 2 of the golf club ‘k’ at the swing shift time is bent as flexed in the backward swing direction due to the inertia of the head 3. Specifically, the golf club ‘k’ assumes a position where the head 3 is located backward from an extension ‘z’ of a shaft axis (hereinafter, simply referred to as “shaft axis z”) of a portion fitted with the grip 4 with respect to the backward swing direction (hereinafter, the position will be referred to as “the position with the head falling behind”).

On the other hand, positions assumed by the golf club ‘m’ at the moment of impact are roughly classified into: a position α (indicated by a solid line in FIG. 1) where the shaft 2 is substantially straight; a position β where the shaft 2 is flexed in the backward swing direction so that the head falls behind; and a position y where the shaft 2 is conversely flexed in the forward swing direction so that the head 3 is advanced from the shaft axis ‘z’ in the forward swing direction (hereinafter, the position will be referred to as “the position with the head advanced forward”).

The position α is a state where the shaft 2 flexed or elastically deformed in the backward swing direction in conjunction with the swing shift of the golf club ‘k’ is substantially returned to its initial position during a swing motion from the top to the impact position (hereinafter, also referred to as “downswing motion”) so that the shaft 2 becomes substantially straight.

The position β is a state where the shaft 2 flexed or elastically deformed in the backward swing direction in conjunction with the swing shift of the golf club ‘k’ fails to return to its initial position during the downswing motion so that the golf club reaches the impact position with the head remaining in the behind position.

The position γ is a state where the shaft 2 flexed or elastically deformed in the backward swing direction in conjunction with the swing shift of the golf club ‘k’ overreaches the substantially straight position to be flexed in the forward swing direction during the downswing motion, so that the golf club reaches the impact position with the shaft flexed in the forward swing direction.

By the way, it may be thought that the head speed Vh0 just prior to impact is decided by the sum of two kinds of elements.

The first element is a moving speed of the head 3 just prior to impact, which is given by the movement of the whole body of the golf club 1. The moving speed of the head will hereinafter be referred to as “swing speed sp” because this speed is dependent upon the speed of the swing made by the golfer P. The swing speed ‘sp’ is principally associated with an angular speed of the golf club 1 during the downswing motion, a club length, a length of the arm of the golfer P, and the like.

The second element is a moving speed of the head 3 just prior to impact, which is given by flexural vibrations of the shaft 2 itself of the golf club 1. Specifically, as shown in FIG. 1, the golf club ‘k’ at the swing shift time is elastically deformed as flexed in the backward swing direction due to the inertia of the head 3. During the downswing motion, the inertia of the head 3 and an elastic restoring force of the shaft 2 act to return the shaft 2 elastically deformed at the swing shift time to its initial position. Thus, the shaft 2 is vibrated in the forward/backward swing directions during the downswing motion, thereby imparting the moving speed to the head 3. The moving speed is a head speed relative to the shaft axis ‘z’ which is produced by the inertia of the head 3. Hence, the moving speed will hereinafter be referred to as “head speed ‘zs’ relative to the shaft axis”.

Accordingly, the head speed Vh0 just prior to impact may be considered as the sum of the first element and the second element described above. That is, the head speed Vh0 just prior to impact may be expressed by the following equation: Vh0=(swing speed sp)+(head speed zs relative to the shaft axis)

As described above, the head speed ‘zs’ relative to the shaft axis is produced by the vibrations of the shaft 2 in the forward/backward swing directions. Therefore, the head speed ‘zs’ is at maximum when the golf club assumes the position a where the shaft 2 becomes substantially straight. In the position β or γ where the shaft 2 is flexed in the forward swing direction or in the backward swing direction, the head speed ‘zs’ is lower than the head speed when the shaft 2 becomes substantially straight.

In a case where the head mass Mh and the shaft mass Ms are in good balance, the golf club assumes the position a upon impact, the position where the shaft 2 is substantially straight (see FIG. 1). Hence, the head speed ‘zs’ relative to the shaft axis is increased, so that the head speed Vh0 just prior to impact is also increased. Thus, the flight distance is increased. In a case where the head mass Mh and the shaft mass Ms are in poor balance, on the other hand, the golf club assumes the aforesaid position β or γ upon impact. Therefore, the head speed ‘zs’ relative to the shaft axis is relatively low and hence, the flight distance is decreased.

In addition, the aforesaid positions β and γ involve the following problem. In these positions, a relative positional relation among the shaft 2, the head 3 and the grip 4 differs from that at address and hence, an impact point is deviated to result in a decreased flight distance.

The flexure of the shaft upon impact may be optimized by contemplating the balance between the shaft mass Ms and the head mass Mh.

In a case where the head mass Mh is relatively small, the golf club ‘k’ at the swing shift time has a relatively small amount of flexure of the shaft 2. This results in a small amount of displacement ‘kh’ of the head 3 from the shaft axis ‘z’ at the swing shift time (see FIG. 1). In this case, therefore, the head speed ‘zs’ relative to the shaft axis is relatively low (even though the golf club nearly in the aforesaid position a reaches the impact position), so that the head speed Vh0 just prior to impact is also low.

Because of the small amount of displacement ‘kh’ of the head, as described above, the shaft 2 so flexed at the swing shift time takes less time to return to its initial position. This leads to a higher probability that the golf club reaches the impact position with the shaft 2 overreaching the straight position to be flexed in the forward swing direction during the downswing motion (that is, the aforesaid position (y)). This also constitutes a causative factor of lowering the aforesaid head speed Vh0. Furthermore, the relative positional relation among the shaft 2, the head 3 and the grip 4 differs from that at address.

Therefore, the following points (1) and (2) have great significance for increasing the flight distance with the golf club having a relatively small head mass Mh.

(1) In accordance with the decrease of the head mass Mh, the shaft mass Ms is also drastically decreased so as to allow the swing speed ‘sp’ to be increased. This not only provides for the increase of the head speed Vh0 just prior to impact but also compensates for the decrease of the flight distance associated with the decreased head mass.

(2) The swing speed ‘sp’ is increased for quickly accomplishing the downswing motion or reducing time taken to accomplish the downswing motion, thereby minimizing the possibility of the shaft being impacted as assuming the aforesaid position y. That is, the shaft is allowed to assume substantially the aforesaid position a when impacted.

The head speed Vh0 just prior to impact can be maximized by satisfying the above points (1) and (2). This aim is achieved by defining the head mass Mh and the shaft mass Ms to have a relation to satisfy the following expression (A): Mh≧Ms+155  (A) and, by further defining the head mass Mh and the shaft mass Ms to have a relation to satisfy by the following expression (C): Mh≧0.1·Ms+212  (C).

In a case where the head mass Mh is relatively great, the golf club ‘k’ at swing shift time presents a relatively great amount of flexure of the shaft 2, so that the head 3 at the swing shift time is displaced from the shaft axis ‘z’ by a greater amount ‘kh’ (see FIG. 1). In this case, therefore, the shaft 2 flexed at the swing shift time takes a longer time to return to its initial position. Hence, the downswing motion involves a higher probability that the shaft 2 is impacted before it is substantially returned to its initial position, or as assuming the position β. Accordingly, it is more likely that the head speed ‘zs’ relative to the shaft axis is lowered to decrease the head speed Vh0. Furthermore, the shaft is prone to be impacted as assuming the position β so that the relative positional relation among the shaft 2, the head 3 and the grip 4 is more likely to differ from that at address.

Furthermore, having a relatively great head mass Mh, the golf club has a relatively great total weight and a relatively heavy club balance, so that an effect to decrease the swing speed ‘sp’ is increased. In the aforementioned case where the head mass Mh is relatively small, the shaft mass Ms is increased in accordance with the increase of the head mass Mh. If the golf club having the relatively great head mass Mh defines the relation between the head mass and the shaft mass the same way, the total weight of the golf club or the weight of the club balance thereof is increased so much that the golf club will suffer a relatively great effect to decrease the swing speed ‘sp’.

Therefore, the following points (3) to (5) have great significance for increasing the flight distance with the golf club having a relatively great head mass Mh.

(3) The shaft mass Ms is reduced to some extent in correspondence to the increase of the head mass Mh, so as to minimize the effect to decrease the swing speed ‘sp’, the effect resulting from the increased total weight of the club or the increased weight of the swing balance.

(4) The relatively small head mass Mh leads to the following problem. That is, if the shaft mass Ms is reduced too much, time taken to accomplish the downswing motion is too short. This results in a higher probability that the shaft 2 may be impacted before it is substantially returned to the straight position or while the shaft is in the position β. Therefore, the shaft is designed to have a certain magnitude of shaft mass Ms for allowing an adequate length of time to accomplish the downswing motion. It is thus ensured that the shaft nearly in the aforesaid position a is impacted. This approach provides an increased head speed ‘zs’ relative to the shaft axis and also permits the relative positional relation among the shaft 2, the head 3 and the grip 4 to coincide with that at address as much as possible.

(5) A measure to satisfy both the points (3) and (4) is contemplated. To satisfy the above point (3), the shaft mass Ms is reduced in accordance with the increase of the head mass Mh. In view of the point (4), however, the reduction of the shaft mass Ms in conjunction with the increase of the head mass Mh is limited to a relatively low degree. Thus, the golf club is designed to have a certain magnitude of shaft mass Ms thereby allowing the substantial length of time to accomplish the downswing motion. It is thus ensured that the golf club nearly in the aforesaid position a reaches the impact position.

The aforesaid aims (3) to (5) may be achieved by defining the shaft mass Ms and the head mass Mh to have a relation to satisfy the following expression (B): Mh≦−0.14·Ms+240.9  (B). This ensures the maximum head speed Vh0 just prior to impact under the condition where the golf club has a relatively great head mass Mh. As a result, the golf club provides the increased flight distance.

Having too small a shaft mass Ms, the shaft 2 is incapable of ensuring the durability thereof. Therefore, the shaft 2 may preferably have a mass of at least 20g, more preferably of at least 30g, or particularly preferably of at least 35g. An upper limit of the shaft mass Ms is defined to be up to 60g. However, if the shaft is too heavy, the golf club 1 may be inferior in handling characteristic such as portability, or may not be suited to a golfer who does not have strong arms. Therefore, the shaft may preferably have a mass of 55g or less, more preferably of 50g or less, and particularly preferably of 45g or less.

A material for the head 3 is not particularly limited. Examples of a usable material include: stainless materials such as SUS630 and maraging steel; pure titanium; titanium alloys; thermoplastic resins; thermosetting resins; and fiber reinforced resins such as CFRP. The titanium alloys are preferred when importance is placed on the flight distance performance. Examples of a suitable titanium alloy include 6Al-4V titanium, 15V-3Cr-3Al-3Sn titanium, 15Mo-5Zr-3Al titanium, 13V-11Cr-3Al titanium, 15V-6Cr-4Al titanium and the like. Particularly preferred as a material for a face portion are β-type titaniums such as 15V-3Cr-3Al-3Sn titanium, 15Mo-5Zr-3Al titanium, 13V-11Cr-3Al titanium, and 15V-6Cr-4Al titanium which have high strength and can be formed thin.

The structure of the head 3 may be selected from common head structures which include: four-piece structure wherein four members including a crown member, a face member, a sole member and a neck member are joined together; two-piece structure wherein two members including the face member and the other member (a body member) are joined together; three-piece structure wherein a face-neck member unifying the face member and the neck member, the crown member and the sole member are joined together; and such. However, the two-piece structure is particularly preferred from the viewpoint of productivity.

The head 3 too small in volume may present a small inertial moment and may also have a small sweet area. Accordingly, the head 3 may preferably have a volume of at least 300 cc, more preferably of at least 320 cc and particularly preferably of at least 340 cc. If, on the other hand, the head 3 has an excessive volume, the golfer may feel awkward with the club at address, or the head may be decreased in durability. Therefore, the head 3 may preferably have a volume of 470 cc or less, and more preferably of 450 cc or less. As required, a weight member (a high specific gravity material) such as a tungsten alloy may be disposed on the sole portion or the like for the purpose of controlling the center of gravity of the head such as lowering the center of gravity. The design to lower the center of gravity makes it easier to achieve initial striking conditions of high launch and low spin, thus contributing to the increase of the flight distance.

The shaft 2 may employ a carbon shaft or a steel shaft. The carbon shaft is preferred because it allows lightweight and high-strength characteristics to be attained and also provides a high degree of freedom of shaft weight design. A usable carbon shaft may be fabricated by winding a prepreg sheet into a roll, the prepreg sheet formed from a carbon fiber reinforced resin (CFRP). A usable material for the grip 4 includes rubber commonly used in the art.

A club too short in length may encounter a problem that the shaft is flexed insufficiently during the downswing motion so as to decrease the aforementioned effects of the invention, or to decrease the head speed Vh0 just prior to impact. In consequence, the flight distance may be decreased. Therefore, the golf club may preferably have a length of at least 42 inches, more preferably of at least 43 inches, or even more preferably of at least 43.5 inches. On the other hand, an excessively long club is inferior in operability so that the strike point is varied greatly. This may result in the decrease of flight distance. Therefore, the golf club may preferably have a length of 48 inches or less, more preferably of 47 inches or less, and even more preferably of 46 inches or less.

A golf club having an excessive head mass Mh may suffer an excessively heavy club balance so that the swing speed ‘sp’ may be lowered too much. Therefore, the head may preferably have a mass of 240g or less, and more preferably of 230g or less. On the other hand, a golf club having too small a head mass Mh may be decreased in the momentum of the head just prior to impact so that the head may exhibit a lowered restitution coefficient. Therefore, the head may preferably have a mass of at least 180g, or more preferably of at least 190g.

The invention may preferably be applied to a so-called wood club which normally has a club length in the preferred range as described above and which is required of the greatest flight distance. It is particularly preferred to apply the invention to a driver (W#1). It is noted that the driver normally has a loft angle (real loft angle) in the range of 6 degrees to 15 degrees.

The golf club may preferably have a total weight of 340g or less, because an excessively heavy club may suffer an excessive decrease of the swing speed ‘sp’.

In a case where the total weight of the club is relatively great, the swing speed ‘sp’ is lowered so that an increased length of time is taken to accomplish the downswing motion. Hence, the shaft is prone to be impacted as flexed in the forward swing direction to present the head forwardly. Therefore, the club balance may preferably be set to a relatively heavy level of D3 to D5 (the club balance of the 14-inch system, the same balance system is also used hereinafter) thereby to ensure that the shaft is impacted as substantially assuming the aforesaid position a where the head is not advanced forwardly. In a case where the total weight of the club is relatively small, on the other hand, the swing speed ‘sp’ is increased so that a decreased length of time is taken to accomplish the downswing motion. Hence, the shaft is prone to be impacted with the head falling behind. Therefore, the club balance may preferably be set to a relatively light level of D0 to D3 thereby to ensure that the shaft is impacted as substantially assuming the aforesaid position α where the head does not fall behind. For the aforementioned reasons, the invention may preferably define the club balance to range from D0 to D5.

The effects of the invention were confirmed by comparing the examples of the invention with comparative examples. Specifically, 14 types of clubs of Examples 1 to 14 and 7 types of clubs of Comparative Examples 1 to 7 were fabricated and subjected to a comparison test. The specifications and the test results of each of the clubs are listed in the following table 1.

TABLE 1
			Club			Total
	Shaft	Head	total	Club	Club	flight	Prepreg
	weight	weight	weight	Length	bal-	distance	struc-
	(g)	(g)	(g)	(inch)	ance	(yard)	ture
Ex. 1	50	205.0	303.0	44.0	D2	248.4	S2
Ex. 2	40	195.0	283.0	45.0	D0	248.2	S3
Ex. 3	30	185.0	263.0	46.5	D1	248.0	S4
Ex. 4	40	205.0	293.0	44.0	D1	248.9	S3
Ex. 5	30	195.0	273.0	45.5	D2	248.8	S4
Ex. 6	30	215.0	313.0	44.0	D3	251.1	S4
Ex. 7	60	215.0	333.0	43.0	D2	248.6	S1
Ex. 8	50	225.0	333.0	42.5	D2.5	252.2	S2
Ex. 9	50	233.9	351.9	42.0	D5	249.9	S2
Ex. 10	40	235.3	343.3	42.0	D5	249.7	S3
Ex. 11	30	236.7	334.7	42.0	D5	249.7	S4
Ex. 12	60	232.5	360.5	42.0	D5	251.0	S1
Ex. 13	60	218.0	336.0	43.0	D3	251.4	S1
Ex. 14	50	217.0	324.0	44.0	D3	251.2	S2
CEx. 1	45	195.0	288.0	45.0	D0.5	246.2	S6
CEx. 2	35	180.0	263.0	47.0	D2	245.2	S5
CEx. 3	60	205.0	333.0	45.0	D2	245.9	S1
CEx. 4	60	244.0	372.0	42.0	E1	246.1	S1
CEx. 5	50	244.0	362.0	42.0	E0	246.1	S2
CEx. 6	40	244.0	352.0	42.0	D9	246.5	S3
CEx. 7	30	246.0	342.0	42.0	D8	244.9	S4
Note:
“S” means “structure”

The heads 3 of all the examples and comparative examples have the two-piece structure wherein, as shown in FIG. 2, a face member fb and a body member (a portion of the head 3 that excludes the face member fb) bd are joined together by TIG welding. The face member fb was produced by blanking a plate material, commercially available as DAT55G from Daido Steel Co.,Ltd., into a predetermined shape, followed by press working. The body member bd was produced by precision casting 6Al-4V titanium. As required, a weight member J formed from tungsten-nickel (specific gravity: 14.5) was bonded to an inside surface of the sole portion of the body member bd for adjustment of the head weight.

The shafts of all the examples and comparative examples were fabricated as follows. A prepreg sheet formed from a carbon fiber reinforced resin was laminated by winding the sheet about a mandrel (core bar) and then, was thermally cured. Used as the prepreg sheet was MR350C commercially available from MITSUBISHI RAYON CO.,LTD. The sheet was composed of an epoxy resin and PAN carbon fiber.

The lamination structure of the shaft 2 is shown in FIG. 3 and TABLE 1. As shown in FIG. 3, the shaft of each of the examples and comparative examples includes: a bias layer 10; a straight layer 12; a first tip reinforcing layer 11 interposed between the bias layer 10 and the straight layer 12; and a second tip reinforcing layer 13 laminated on the outermost layer. It is noted that the number of sheets of the straight layer 12 is not limited to four as shown in FIG. 3 but is varied depending upon the individual examples and comparative examples (a detailed description thereof will be made hereinlater). In FIG. 3, the orientation of the carbon fiber of each sheet is indicated by alternate long and short dash lines. The bias layer 10 includes: a first bias layer 10a having the carbon fiber oriented substantially at +45° relative to a longitudinal direction of the shaft; and a second bias layer 10b having the carbon fiber oriented substantially at −45° relative to the longitudinal direction of the shaft. The second bias layer 10b is bonded to the first bias layer as turned inside out, and the resultant bias layer 10 is wound about the mandrel, whereby the first bias layer 10a and the second bias layer 10b have mutually opposite orientation angles of the fiber. The first tip reinforcing layer 11, the straight layer 12 and the second tip reinforcing layer 13 each have the fiber oriented substantially parallel to the longitudinal direction of the shaft (at an orientation angle of 0). The lamination of the shaft 2 is completed by winding the first tip reinforcing layer 11 on an outside of the bias layer 10, followed by winding the straight layer 12 and then the second tip reinforcing layer 13. The resultant lamination is thermally cured to form the shaft 2.

Prepreg lamination structures of the examples and comparative examples are listed in TABLE 1. In structure 1 to structure 6 shown in TABLE 1, the bias layers 10 each include the same number of sheets; the first tip reinforcing layers 11 each include the same number of sheet; and the second tip reinforcing layers 13 each include the same number of sheet. However, the respective straight layers 12 of the structures 1 to 6 include different numbers of sheets, which are listed as below. Structure 1: the straight layer 12 includes four sheets; Structure 2: the straight layer 12 includes three sheets; Structure 3: the straight layer 12 includes two sheets; Structure 4: the straight layer 12 includes one sheet; Structure 5: the straight layer 12 includes two sheets; Structure 6: the straight layer 12 includes three sheets.

FIG. 3 shows only the structure 1 as a typical representative.

While both the straight layers 12 of the structure 3 and the structure 5 include two sheets, the straight layer 12 of the structure 5 has a smaller base weight (per-unit-area weight of the prepreg sheet) than that of the structure 3, whereby the shafts of these structures are adjusted for the weight. Likewise, both the straight layers 12 of the structure 2 and the structure 6 include three sheets, but the straight layer 12 of the structure 6 has a smaller base weight than that of the structure 2, whereby the shafts of these structures are adjusted for the weight. In all the examples and comparative examples, the prepreg sheet is properly adjusted for the resin content and the base weight so that all the shafts of the examples and comparative examples may have substantially the same shaft flexure.

FIG. 4 is a graph wherein the shaft mass Ms(g) is plotted on the abscissa whereas the head mass Mh(g) is plotted on the ordinate, and wherein solid squares represent the values of the individual examples whereas solid triangles represent the values of the individual comparative examples. The figure also shows each line defining a boundary of each region satisfying each of the expressions (A), (B) and (C). As shown in FIG. 4, all the examples satisfy the expressions (A) and (B) and have the shaft masses in a region defined by Ms≦60. The examples 1, 2, 3 and 7 are individually represented by the dots on a line (Mh=Ms+155); the examples 9, 10, 11 and 12 are individually represented by the dots on the line (Mh=-0.14.Ms+240.9); and the examples 6, 13 and 14 are individually represented by the dots on the line (Mh=0.1·Ms+212). The examples 7, 12 and 13 are individually represented by the dots on a line (Ms=60). The examples 3, 5, 6 and 1 1 are individually represented by the dots on a line (Ms=30).

The “total flight distance” in TABLE 1 means the average of flight distances of 100 balls hit by 10 low handicap (1-9) golfers, who each hit 10 balls. The flight distance is determined at the final reach point, including the carry distance and the run distance. The golf ball used in the test was TOURSPECIAL (tradename) commercially available from SRI group. All the examples achieve greater total flight distances than the comparative examples.

Claims

1. A golf club having a shaft mass Ms of 60g or less, and defining a head mass Mh(g) and the shaft mass Ms(g) to have a relation to satisfy the following expressions (A) and (B):

2. A golf club according to claim 1, wherein the head mass Mh(g) and the shaft mass Ms(g) are defined to have a relation to satisfy the following expression (C):

3. A golf club according to claim 1, wherein the shaft mass Ms is 30g or more.

4. A golf club according to claim 2, wherein the shaft mass Ms is 30g or more.