Golf club head

Abstract

A golf club having an increased moment of inertia and improved ball-hitting directionality is provided. A metal hollow golf club head comprises: a face portion; a crown portion; and a sole portion, and when the golf club has a lie angle of 60° with its club head volume being within 470 cm 3 , a moment of inertia about the axial line centered on the plumb line passing through the golf club head center of gravity is 5000 to 6000 g-cm 2 . In order to increase the moment of inertia, the thickness of the center portion of the crown portion is reduced by chemical etching, and a mass, including the portion of mass reduction, is positioned in the sole portion on the side of the toe portion; moreover, the separation distance from the center of gravity to the mass is increased.

What is claimed is: 
     
         1 . A golf club, comprising: a face portion positioned on the front surface of a metal hollow golf club head and having a striking face to strike a golf ball; a crown portion forming an upper surface of the club, and a sole portion forming a lower surface of the club; wherein
 a mass of the metal hollow golf club head is 210 g or less,   characteristic time (CT value) of the metal hollow golf club head, relating to a restitution characteristic, is 257 μs or less,   the volume of the metal hollow golf club head is 400 to 470 cm 3  when a lie angle of the metal hollow golf club head is 60 degrees,   the metal is made of titanium alloy entirely and formed through welding to be an integrated body   the crown portion and sole portion constituting the body has an area of thick portion and an area of thinned portion, the area of the thinned portion amounts to more than 60% in both of the crown portion and the sole portion, and the thickness difference between the thinned portion and the thick portion is more than 0.2 mm and the removed mass amounts to more than 10 g in both of the crown portion and the sole portion,   a weight is located and fixed at a position of a rotation radius most distant from the plumb line and at toe-side and back-side sites of the crown portion and/or the sole portion,   the metal hollow golf club head has:   a length (X) from a heel portion to a toe portion longer than a length (Y) from a face portion to a rear surface;   a length from a heel to a toe of 127 mm (5 inches) or less;   a length from a sole to a crown of 71.12 mm (2.8 inches) or less; and   a moment of inertia (MOI) calculated using the following approximating equation (1):
     MOI= ( aY   2   +bY+c )*( dX+e ) /f    (1)
 
   
       where, in the approximating equation (1), the second order equation of Y is determined by approximating the MOI value with a second order equation of variable Y as a head width under a constant value X as a toe-heel length and the constants a, b and c are coefficients of the terms in the second order equation respectively, the first order equation of X is determined by approximating the MOI value with a first order equation of a variable X as a toe-heel length under a constant value Y as a head width and d and e are constants of terms in the first order equation respectively, and f is a value of MOI with a certain selected value of X satisfying the required condition;
 a position of the center of gravity (u, v), as seen from the plumb direction, exists within a range encompassed by the following two approximating equations:
     v=gu   2   +hu+i    (2)
 
     v=ju   2   +ku+ 1   (3)
 
 
 
       where u is a position of center of gravity in the direction from the heel portion to the toe portion, v is a position of center of gravity in the direction from the face portion to the rear surface, u has an origin at the center of the distance from the heel portion to the toe portion and v has an origin in the face surface; and the approximating equation (2) is determined by approximating the relation between u and v with a second order equation when MOI is the maximum, g, h and i are constants of terms of the equation respectively, and the approximating equation (3) is determined by approximating the relation between u and v with a second order equation when MI is 5000 gcm 2 , j, k and l are constants of terms of the equation respectively, and
 the moment of inertia about an axial line, which is the plumb line passing through the center of gravity of the metal hollow golf club head, is in a range of 5000 to 6000 g-cm 2 . 
 
     
     
         2 . The golf club according to  claim 1 , wherein the metal hollow golf club head is formed by welding to join the face portion, the sole portion, the crown portion and a hosel portion to which the shaft is connected. 
     
     
         3 . The golf club according to  claim 1 , wherein the weight is 20 g or more. 
     
     
         4 . The golf club according to  claim 1 , wherein toe-side and back-side sites of the sole portion are formed in shapes protruding outward relative to the crown portion, and the weight is positioned in this protruding sole portion. 
     
     
         5 . The golf club according to  claim 1 , wherein the area of the thinned portion is larger in the crown portion than in the sole portion. 
     
     
         6 . The golf club according to  claim 1 , wherein the thinned portions in the crown portion and the sole portion are formed by chemical etching.