# Patent

## Title
Hollow golf club head

## Abstract

A hollow golf club head allowing to increase the repulsion and the degree of design freedom, while maintaining the durability. A hollow golf club head having a head body formed from a metal, wherein a hole is formed in the vicinity of the middle of a crown portion in the head body, the hole is closed with a cover member made of a material different from the head body, the ratio (ρm/ρf) of the specific gravity ρm of the head body and the specific gravity ρf of the cover member is set equal or superior to 1.3, and the ratio of the face area of a portion where the reinforcement and the head body are combined to the face area of the head body is set equal or superior to 20%.

## Background
RELATED APPLICATION
This application is a divisional of U.S. application Ser. No. 10/329,434 filed Dec. 27, 2002 now U.S. Pat. No. 6,945,876, the contents of which is incorporated herein by reference.


BACKGROUND OF THE INVENTION
The present invention concerns a golf club head having a hollow structure, and more particularly, a hollow golf club head having a higher repulsion and a greater degree of design freedom, compared to a case of composing solely with a single metallic material, by combining different kinds of materials, while keeping the durability.
For the metallic hollow golf club heads, it is planned to lower the center of gravity generally by broadening the weight distribution to the sole portion. However, in recent years, the weight margin for lowering the center of gravity is reduced and the degree of design freedom is reduced, because it is required to increase the head volume as much as possible in a limited head volume. There, if the center of gravity is not lowered in the head sufficiently, there is a problem that the repulsion of the face surface can not be utilized maximally.
By the way, Japanese Patent No. 2764883, Japanese patent application Kokai publication No. 2000-229135, and Japanese Patent No. 2773009 disclose a golf club head made by combining different materials. However, even in these golf club heads where characteristics of different kinds of materials are combined, the degree of design freedom or the repulsion has been still insufficient.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a hollow golf club head having a higher repulsion and a greater degree of design freedom, compared to a case of composing solely with a single metallic material, by combining different kinds of materials, while keeping the durability.
The hollow golf club head of the present invention for attaining the aforementioned object is a hollow golf club head having a head body formed from a metal, wherein a hole is formed in the vicinity of the middle of a crown portion in the head body, the hole is closed with a cover member made of a material different from the head body, the ratio (ρm/ρf) of the specific gravity ρm of the head body and the specific gravity ρf of the cover member is set equal or superior to 1.3, the face portion of the head body is laminated with a reinforcement made of fiber reinforced plastic, and the ratio of the face area of a portion where the reinforcement and the head body are combined to the face area of the head body is set equal or superior to 20%.
Also, the hollow golf club head of the present invention for attaining the aforementioned object is a hollow golf club head having a head body formed from a metal, wherein holes are formed respectively in the vicinity of the middle of a crown portion and a sole portion in the head body, each of the holes is closed with a cover member made of a material different from the head body, the ratio (ρm/ρf) of the specific gravity ρm of the head body and the specific gravity ρf of the cover member is set equal or superior to 1.3, the face portion of the head body is laminated with a reinforcement made of fiber reinforced plastic, and the ratio of the face area of a portion where the reinforcement and the head body are combined to the face area of the head body is set equal or superior to 20%.
A greater weight margin can be secured, by providing a hole in the crown portion or both crown portion and sole portion of the head body, closing the hole with a cover member made of a material different from the head body, and setting the ratio (ρm/ρf) of the specific gravity ρm of the head body and the specific gravity ρf of the cover member equal or superior to 1.3 in this manner. Consequently, the degree of design freedom is increased compared to a case of composing solely with a single metallic material and, as a result, it becomes possible to increase the repulsion by lowering the center of gravity. Also, it becomes possible to further improve the repulsion by thinning as much as possible the metallic part of the face portion, while maintaining the durability, as the face portion of the head body is laminated with a reinforcement made of fiber reinforced plastic, and the ratio of the face area of a portion where the reinforcement and the head body are combined to the face area of the head body is set equal or superior to 20%.
In the present invention, it is preferable to laminate the reinforcement inside the face portion of the head body, in order not to deteriorate the repulsion based on the metallic material of the face portion and the resistance to external damage of the face surface. Besides, it is good to thin the metallic part of the face portion depending on the kinds of metal, in order to obtain a more excellent repulsion. For instance, in case where the material composing the face portion of the head body is a titanium alloy, the thickness of the titanium alloy of the face portion is preferably equal or less than 3.0 mm. In case where the material composing the face portion of the head body is an iron alloy, the thickness of the iron alloy of the face portion is preferably equal or less than 2.5 mm. In case where the material composing the face portion of the head body is an aluminum alloy, the thickness of the aluminum alloy of the face portion is preferably equal or less than 4.0 mm.
On the other hand, the coefficient of elasticity of the fiber of the fiber reinforced plastic composing the reinforcement is preferably equal or less than 35 tons/mm2, in order to secure a sufficient resistance to an impact.
The material of the cover member is not particularly limited, provided that it meets the aforementioned specific gravity relationship; however, it is preferable to make the total mass Wf of the cover member closing the hole lighter than the total mass Wx of the removed portion of the head body determined from the product of the virtual area of the hole, the virtual thickness of the hole and the specific gravity ρm of the head body, independently of the material to be used, in order to secure the weight margin.
In the present invention, though it is preferable to close the hole provided in the head body with a cover member, it is also possible to compose a hollow golf club head having the other composite structure. In short, the hollow golf club head of the present invention is characterized by that at least a part of the crown portion and a part of the face portion of the head body is composed of a material different from the head body, and the ratio (ρb/ρc) of the specific gravity ρb of a portion of the head body excluding the crown portion and the face portion and the specific gravity ρc of the crown portion is set equal or superior to 1.3.
Thus, it becomes possible to secure a greater weight margin, by the fact that at least a part of the crown portion and a part of the face portion of the head body are composed of a material different from the head body, and the ratio (ρb/ρc) of the specific gravity ρb of a portion of the head body excluding the crown portion and the face portion and the specific gravity ρc of the crown portion is set equal or superior to 1.3. Consequently, the degree of design freedom is increased compared to a case of composing solely with a single metallic material and, as a result, it becomes possible to increase the repulsion by lowering the center of gravity.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(a) to FIG. 1(c) show a golf club head made of a first embodiment of the present invention; FIG. 1(a) is a plan view, FIG. 1(b) a front view, and FIG. 1(c) is a cross sectional view taken along line I—I in FIG. 1(a);
FIG. 2(a) to FIG. 2(c) show a golf club head made of a second embodiment of the present invention; FIG. 2(a) is a plan view, FIG. 2(b) a front view, and FIG. 2(c) is a cross sectional view taken along line II—II in FIG. 2(a);
FIG. 3(a) to FIG. 3(o) are plan views showing respectively variants of golf club head of the present invention;
FIG. 4(a) to FIG. 4(d) are plan view showing respectively variants of golf club head of the present invention;
FIG. 5(a) to FIG. 5(e) are cross sectional view showing respectively variants of golf club head of the present invention;
FIG. 6(a) and FIG. 6(b) show variants of golf club head of the present invention; FIG. 6(a) is a plan view, and FIG. 6(b) is a cross sectional view taken along line VI—VI in FIG. 6(a);
FIG. 7(a) and FIG. 7(b) show variants of golf club head of the present invention; FIG. 7(a) is a plan view, and FIG. 7(b) is a cross sectional view taken along line VII—VII in FIG. 7(a);
FIG. 8(a) to FIG. 8(h) are cross sectional view showing respectively variants of golf club head of the present invention;
FIG. 9(a) to FIG. 9(f) are front view showing respectively variants of golf club head of the present invention; and
FIG. 10(a) to FIG. 10(c) show a golf club head made of a third embodiment of the present invention; FIG. 10(a) is a plan view, FIG. 10(b) a front view, and FIG. 10(c) is a cross sectional view taken along line X—X in FIG. 10(a).



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Now, the composition of the present invention shall be described in detail referring to attached drawings.
FIG. 1(a) to FIG. 1(c) show a golf club head made of a first embodiment of the present invention. As shown in FIG. 1(a) to FIG. 1(c), the golf club head of the present embodiment has a hollow structure, and a head body 1 thereof is composed of a metal such as titanium alloy, aluminum alloy, stainless steel or the like. The head body 1 has a crown portion 2, a face portion 3, a sole portion 4 and neck portion 5. While a hole 2a is formed in the vicinity of the middle of the crown portion 2, a flange portion 2b spreading over a hollow space is formed around the peripheral edge of the crown portion 2. There, the hole 2a is closed with a cover member 6 made of a material of a specific gravity lower than the head body 1. Besides, the inside of the face portion 3 of the head body 1 is laminated with a reinforcement 7 made of a fiber reinforced plastic.
FIG. 2(a) to FIG. 2(c) show a golf club head made of a second embodiment of the present invention. As shown in FIG. 2(a) to FIG. 2(c), the golf club head of the present embodiment has a hollow structure, the head body 1 thereof is composed of a metal such as titanium alloy, aluminum alloy, stainless steel or the like. The head body 1 comprises a crown portion 2, a face portion 3, a sole portion 4 and a neck portion 5. While holes 2a, 4a are formed respectively in the vicinity of the middle of the crown portion 2 and the sole portion 4 in the head body, a flange 2b spreading over a hollow space is formed around the peripheral edge of the crown portion 2, and another flange 4b spreading under the hollow space is formed around the peripheral edge of the sole portion 4. And, the holes 2a, 4a are closed respectively with a cover member 6 made of a material lower than the head body 1 in specific gravity. Besides, the inside of the face portion 3 of the head body 1 is laminated with a reinforcement 7 made of a fiber reinforced plastic.
Thus, it becomes possible to secure a larger weight margin, and increase the degree of design freedom, by providing the hole 2a in the crown portion 2 of the head body 1 and further by providing a hole 4a in the sole portion 4 as necessary, and by closing the holes 2a, 4a with a cover member 6 made of a material lower than the head body 1 in specific gravity. Especially, when the crown portion 2 is provided with the cover member 6, it becomes possible to increase the moment of inertia and to lower the center of gravity by enlarging the weight margin, while in case of providing the cover member 6 in the sole portion 4, it becomes possible to increase the moment of inertia by enlarging the weight margin. Here, it is necessary to set the ratio (ρm/ρf) of the specific gravity ρm of the head body 1 and the specific gravity of the cover member 6 equal or superior to 1.3. If this ratio (ρm/ρf) is lower than 1.3, it becomes impossible to secure a sufficient weight margin. The upper limit of the ratio (ρm/ρf) is not particularly determined, but it is of the order of 16 from the combination of materials to be applied to now existing golf club heads.
Furthermore, it becomes possible to further improve the repulsion by thinning as much as possible the metallic part of the face portion 3, while maintaining the durability, as the face portion 3 of the head body 1 is laminated with a reinforcement 7 made of fiber reinforced plastic. Here, it is necessary to make the ratio (called, face combined area ratio, hereinafter) of the face area of the portion where the reinforcement 7 and the head body 1 are combined to the face area (area of hitting surface) of the head body 1 equal or superior to 20%. Here, the face area is the surface area of the portion surrounded by the edge of the face portion. Further, if the edge of is not clear, the edge of the face portion can also be known, by cutting the golf club head, and examining the bonded portion of the inner face. A good balance of the durability and the repulsion of a golf club head can be obtained by making the face combined area ratio equal or superior to 20%.
In case where the material composing the face portion 3 of the head body 1 is titanium alloy, the thickness of titanium alloy of the face portion 3 is preferably equal or inferior to 3.0 mm, more preferably equal or inferior to 2.5 mm, and still more preferably equal or inferior to −2.0 mm, setting the lower limit value thereof to 1.5 mm. In case where the material composing the face portion 3 of the head body 1 is iron alloy, the thickness of iron alloy of the face portion 3 is preferably equal or inferior to 2.5 mm, more preferably equal or inferior to 2.0 mm, and still more preferably equal or inferior to 1.5 mm, setting the lower limit value thereof to 1.0 mm. In case where the material composing the face portion 3 of the head body 1 is aluminum alloy, the thickness of aluminum alloy of the face portion 3 is preferably equal or inferior to 4.0 mm, more preferably equal or inferior to 3.5 mm, and still more preferably equal or inferior to 3.0 mm, setting the lower limit value thereof to 2.0 mm.
Thus, it becomes possible to further improve the repulsion by thinning the metallic portion of the face portion 3. Besides, a sufficient durability can be secured by the presence of the reinforcement 7, even when the metallic portion of the face portion 3 is thinned as mentioned above.
For the aforementioned holes 2a, 4a, the shape thereof is not particularly determined. For instance, the hole 2a of the crown portion 2 can take various shapes as shown in FIG. 3(a) to FIG. 3(o). However, in the head plan view, it is preferable to set the area of the hole 2a in a range of 40 to 90% of the area of the crown portion 2. It becomes possible to improve further the degree of design freedom by setting within the range. Here, the area of the crown portion is the surface area of a portion surrounded by the edge of the side portion, the edge of the face portion and the edge of the neck portion. Similarly, in the head bottom view, it is preferable to set the area of the hole 4a in a range of 40 to 90% of the area of the sole portion 4. It becomes possible to improve further the degree of design freedom by setting within the range. Here, the area of the sole portion is the surface area of a portion surrounded by the edge of the side portion and the edge of the face portion. Besides, in case where the edge is not clear, it is also possible to know edges of the side portion, face portion, crown portion and sole portion, by cutting the golf club head and examining the bonding portion of the inner face. Further, if the edge of the crown portion is not clear, the projected area of a golf club head excluding the hitting surface looked down from vertically upward in respect to a plane may be used while the golf club is put on the plane with the hitting surface adjusted to the loft angle of the golf club head.
Though the aforementioned flanges 2b, 4a are not necessarily required, preferably it exists at least on a part of the peripheral edge of the crown portion 2 and the sole portion 4, more preferably it exists at least on the face side of the head body 1, and still more preferably all around the peripheral edge. In short, the disposition of the flanges 2b, 4b in appropriate areas of the head body 1 permits to secure the durability of the head body 1 and, at the same time, to use the flanges 2b, 4b as overlap margin for the cover member 6.
It is preferable that the aforementioned cover member 6 is superimposed on the flanges 2b, 4b at least on the face side of the head body 1. In this case, the cover member 6 may cover a part of the flange 2b on the face side, as shown in FIG. 4(a) and FIG. 4(b), or, the cover member 6 may cover all over the flange 2b on the face side, as shown in FIG. 4(c) and FIG. 4(d). The width t1 of the overlap margin for the flange 2b on the face side of the cover member 6 is preferably equal or superior to 5 mm. It becomes possible to secure a sufficient durability by setting the width t1 of the overlap margin equal or superior to 5 mm. Besides, the width t2 of flange 2b on the face side is satisfactory if it is equal or superior to 5 mm and less than 30 mm. In case where the cover member 6 is superimposed on the flange 4b of the sole portion 4, it is preferable to select a laminating mode and dimensions similar to the aforementioned.
The affixation mode of the cover member 6 to the head body 1 is not especially determined. For instance, in FIG. 1(c) and FIG. 2(c), the peripheral edge portion of the cover portion 6 is divided into two layers, and the flanges 2b, 4b are pinched from inside and outside in this portion; however, the cover member 6 may be affixed to one face of the flange 2b as shown in FIG. 5(a) to FIG. 5(d), or, the cover member 6 may be affixed to the end face of the face portion 3 without intermediate of flange as shown in FIG. 5(e). Otherwise, the flange 2b may be provided in a way to receive the cover member 6, in a portion of the head body 1 to be bonded with the cover member 6, as shown in FIG. 6(a) and FIG. 6(b) or FIG. 7(a) and FIG. 7(b).
As for the material of the cover member 6, it is preferable to use a fiber reinforced plastic. As the fiber reinforced plastic, a fiber reinforced plastic made by impregnating carbon fiber, glass fiber, alamido fiber or other reinforced fiber with epoxy resin, unsaturated polyester resin, vinyl ester resin or other matrix resin, can be cited and, especially, those having carbon fiber as reinforced fiber are preferable.
It can also be made to use magnesium alloy, aluminum alloy or titanium alloy, as material of the cover member 6 and, in certain cases, in addition to the hard rubber, nylon resin, ionomer resin, polycarbonate resin, PET resin, ABS resin or the like may also be used.
Whichever material should be used, it is preferable to make the total mass Wf of the cover member 6 closing the holes 2a, 4a lighter than the total mass Wx of the removed portion of the head body 1 determined from the product of the virtual area and the virtual thickness of the holes 2a, 4a and the specific gravity ρm of the head body 1, namely Wx >Wf , in order to secure the weight margin.
For instance, in case where the hole 2a is formed in the vicinity of the middle of the crown portion 2, the flange 2b is formed all around the peripheral edge of the crown portion 2, and a cover member 6 is affixed to the outside of the crown portion 2, the total mass Wf of the cover member 6 will be Wf=Af·ρf·tf from the area Af, specific gravity ρf and thickness tf of the cover member 6. On the other hand, the total mass Wx of the removed portion will be Wx=(Af−Am)·ρm·tm from the area Am of the overlapped portion of the cover member 6 and the head body 1, specific gravity ρm of the head body 1 and average thickness tm of the crown portion and sole portion of the head body 1 in the overlalpped portion. In short, the virtual area of the holes 2a, 4a can be determined from the difference of the area Af of the cover member 6 and the area Am of the overlapped portion, and the virtual thickness of the holes 2a, 4a can be considered as equivalent to the average thickness tm of the crown portion and sole portion of the head body 1 in the overlapped portion.
The affixation mode of the reinforcement 7 to the head body 1 is not especially determined. For instance, in FIG. 1(c) and FIG. 2(c), though the reinforcement 7 is disposed all over the face area, the reinforcement 7 may also be disposed all over the face area and, at the same time, the thickness thereof may be reduced gradually from the face central portion to the peripheral edge portion, as shown in FIG. 8(a) to FIG. 8(d), or, the reinforcement 7 may also be disposed only in the face central portion as shown in FIG. 8(e) to FIG. 8(h). In addition, FIG. 9(a) to FIG. 9(f) illustrate the disposition of the reinforcement viewed from the head front face, and the reinforcement 7 may also be disposed only around the peripheral edge portion as shown in FIG. 9(c) to FIG. 9(f).
As for fiber reinforced plastic composing the reinforcement 7, a fiber reinforced plastic made by impregnating carbon fiber, glass fiber, alamido fiber or other reinforced fiber with epoxy resin, unsaturated polyester resin, vinyl ester resin or other matrix resin, can be cited and, especially, those having carbon fiber as reinforced fiber are preferable. However, the coefficient of elasticity of the fiber of the fiber reinforced plastic composing the reinforcement 7 is preferably equal or less than 35 tons/mm2, and more preferably equal or less than 24 tons/mm2. It becomes possible to secure a more satisfactory durability, by setting the coefficient of elasticity within the aforementioned range.
FIG. 10(a) to FIG. 10(c) show a golf club head made of a third embodiment of the present invention. As shown in FIG. 10(a) to FIG. 10(c), the golf club head of the present embodiment has a hollow structure, the head body 1 thereof is composed of a metal such as titanium alloy, aluminum alloy, stainless steel or the like. The head body 1 comprises a crown portion 2, a face portion 3, a sole portion 4 and a neck portion 5, and a cover member 6 is laminated on the outer surface of the crown portion 2. Besides, the inside of the face portion 3 of the head body 1 is laminated with a reinforcement 7 made of a fiber reinforced plastic. In short, a part of the crown portion 2 and a part of the face portion 3 are made of a material different from the head body 1.
There, the ratio (ρb/ρc) of the specific gravity ρb of a portion of the head body 1 excluding the crown portion 2 and the face portion 3 and the specific gravity ρc of the crown portion 2 is set equal or superior to 1.3. Here, the area of the crown portion is the portion surrounded by the edge of the side portion, edge of the face portion and edge of the neck portion. On the other hand, the face portion is the portion surrounded by the edge of the crown portion, the edge of the side portion and the edge of the sole portion. In case where the edge is not clear, it is also possible to know edges of the side portion, face portion, crown portion and sole portion, by cutting the golf club head and examining the bonding portion of the inner face. Further, if the edge of the crown portion is not clear, the edge of the crown portion can be known by the profile line excluding the hitting surface looked down from vertically upward in respect to a plane while the golf club is put on the plane with the hitting surface adjusted to the loft angle of the golf club head.
Thus, it becomes possible to secure a satisfactory weight margin, by the fact that at least a part of the crown portion 2 and at least a part of the face portion 3 of the head body 1 are composed of a material different from the head body, and the ratio (ρb/ρc) of the specific gravity ρb of a portion of the head body 1 excluding the crown portion 2 and face portion 3 and the specific gravity ρc of the crown portion 2 is set equal or superior to 1.3. Consequently, the degree of design freedom is increased compared to a case of composing solely with a single metallic material and, as a result, it becomes possible to increase the repulsion by lowering the center of gravity.
In the present embodiment, the portion of the head body 1 excluding the crown portion 2 and face portion 3 and the crown portion 2 may be composed of a single material, or, may be composed of a composite material. In case of composing from a composite material, the specific gravity ρb and the specific gravity ρc correspond to the specific gravities of the portion of the head body 1 excluding the crown portion 2 and face portion 3 and the crown portion 2 respectively.
EXAMPLE
As for the hollow golf club head whose head body is formed from a metal, a conventional example 1 (example of the prior art) made of a single metallic material and, embodiment examples 1 to 4 and comparative examples to 1 to 3 where holes are formed in the vicinity of the middle of the crown portion and the sole portion, and the holes are closed respectively with a cover member made from different materials, and at the same time, the inside of the face portion of the head body is laminated with a reinforcement made of a fiber reinforced plastic, were prepared respectively. In Table 1, “SUS alloy” means stainless steel, “FRP” fiber reinforced plastic, “GFRP” glass fiber reinforced plastic, and “CFRP” carbon fiber reinforced plastic.
Coefficient of restitution and degree of design freedom were evaluated for these golf club heads and the results thereof are shown together in Table 1. The results of evaluation are indicated by the index by taking the conventional example 1 as 100. The repulsion coefficient means that the higher is the index value, the higher is the ball initial velocity and larger is the flight distance. The degree of design freedom is the weight margin when the head total mass is set at 190g and means that higher is the index value, higher is the weight margin. For the durability, the higher is the index value, the better is the durability.

















TABLE 1












Face











combined



Degree of




Material

area
Metallic part of
Reinforcement of

design



Material of
of cover
Position
ratio
face portion
face portion
Repulsion
freedom




















pm/pf
head body
member
of hole
(%)
Material
Thickness
Material
Thickness
coefficient
elasticity
Durability























Conventional
1.0
Ti alloy
Ti alloy
—
—
Ti
2.8 mm
—
—
100
100
100


example 1





alloy


Embodiment 1
2.9
Ti alloy
FRP
Crown
85
Ti
2.6 mm
GFRP
0.6 mm
102
111
105


Comparative
2.9
Ti alloy
FRP
Crown
12
Ti
2.6 mm
GFRP
0.6 mm
96
111
96


example 1





alloy


Comparative
0.6
Ti alloy
SUS
Crown
85
Ti
2.6 mm
GFRP
0.6 mm
93
93
96


example 2


Embodiment 2
2.9
Ti alloy
FRP
Crown,
85
Ti
2.6 mm
GFRP
0.6 mm
104
121
103






sole

alloy


Comparative
0.6
Ti alloy
SUS
Crown,
12
Ti
2.6 mm
GFRP
0.6 mm
83
83
94


example 3


alloy
sole

alloy


Embodiment 3
4.9
Fe alloy
FRP
Crown
70
Fe
2.2 mm
G + CFR
0.5 mm
109
106
104


Embodiment 4
1.7
Al alloy
FRP
Crown
65
Al
3.5 mm
G + CFR
0.5 mm
107
115
103









As it is understood from the Table 1, for every golf club head of the embodiment examples 1 to 4, compared to conventional example 1, the degree of design freedom was large, and the repulsion coefficient was large, while maintaining the durability. For the comparative examples 1 and 3 the durability was deteriorated, because the face combined area ratio was too low. For the comparative examples 2 and 3 the degree of design freedom was small, and the repulsion coefficient was low, because the specific gravity of the cover member was too high.
According to the present invention, in a hollow golf club head whose head body is made of a metal, the repulsion can be increased and the degree of design freedom can be increased compared to a case of composing solely with a single metallic material, by providing a hole in the vicinity of the middle of the crown portion of the head body, closing the hole with a cover member made of a material different from the head body, setting the ratio (ρm/ρf) of the specific gravity ρm of the head body and the specific gravity ρf of the cover member equal or superior to 1.3, the face portion of the head body is laminated with a reinforcement made of a fiber reinforced plastic, and the ratio of the face area of a portion where the reinforcement and the head body are combined to the face area of the head body is set equal or superior to 20%.
Further, in a hollow golf club head whose head body is made of a metal, the repulsion can be increased and the degree of design freedom can be increased compared to a case of composing solely with a single metallic material, by the fact that at least a part of the crown portion and a part of the face portion of the head body are composed of a material different from the head body, and the ratio (ρb/ρc) of the specific gravity ρb of the portion of the head body excluding the crown portion and face portion and the specific gravity ρc of the crown portion is set equal or superior to 1.3.
Hereinabove, preferable embodiments of the present invention have been described in detail; however, it should be understood that various modifications, replacements or substitutions can be applied to the same, to the extent not to depart from the spirit and the scope of the present invention which is defined by the attached claims.

## Claims

1. A hollow golf club head comprising a hollow head body formed from a metal and having a hole formed in the vicinity of the middle of a crown portion and a hole formed in a vicinity of the middle of a sole portion of said head body, wherein each of the holes is closed with a cover member made of a material different from said head body, a ratio (ρm/ρf) of the specific gravity ρm of said head body and the specific gravity ρf of said cover member is set equal to or greater than 1.3, the face portion of the head body is laminated with a reinforcement made of a fiber reinforced plastic, and the ratio of the face area of a portion where said reinforcement and said head body are combined to the face area of said head body is set equal to or greater than 20%.

2. The hollow golf club head of claim 1, wherein said reinforcement is laminated inside the face portion of said head body.

3. The hollow golf club head of claim 1, wherein a material of the face portion of said head body is a titanium alloy, and the thickness of the titanium alloy of said face portion is equal to or less than 3.0mm.

4. The hollow golf club head of claim 1, wherein a material of the face portion of said head body is an iron alloy, and the thickness of the iron alloy of said face portion is equal to or less than 2.5 mm.

5. The hollow golf club head of claim 1, wherein a material of the face portion of said head body is an aluminum alloy, and the thickness of the aluminum alloy of said face portion is equal to or less than 4.0 mm.

6. The hollow golf club head of claim 1, wherein a coefficient of elasticity of a fiber of said fiber reinforced plastic is not more than 35 tons/mm2.

7. The hollow golf club head of claim 1, wherein a total mass Wf of the cover member closing each hole is made lighter than the total mass Wx of the removed portion of said head body determined from the product of the virtual area of said hole, the virtual thickness of said hole and a specific gravity ρm of said head body.