Golf club shaft fitting method

1. A fitting method, of a shaft of a golf club, for selecting a shaft having a torque matching a golfer based on a swing of the golfer, the method including the steps of:

obtaining measurement values from a sensor attached to a grip of the golf club and capable of measuring angular velocities about three axes when a golf ball is hit by the golf club;

obtaining a predetermined index value for the angular velocities quantified from the measurement values; and

selecting a shaft matching the golfer from among multiple shafts whose torques have been measured in advance, based on a relationship established through test-hitting performed in advance between the index value and a torque of a shaft,

wherein, when x-axis is oriented in a direction along toe-heel direction of a golf club head, when y-axis is oriented in a direction in which a ball is hit, and when z-axis is oriented to match an axial direction of a shaft, the fitting method further includes the steps of:

determining address, top, and impact of a swing from the measurement values;

calculating t_ ωy _ max˜imp , which is a time from when a grip angular velocity about the y-axis becomes maximum during a downswing to when the impact occurs;

calculating ωx_ ωy _ max˜ , which is an average value of a grip angular velocity about the x-axis from when the grip angular velocity about the y-axis becomes maximum to when the impact occurs;

calculating, as the index value, a magnitude of a change amount of the grip angular velocity about the x-axis per unit of time by dividing the calculated ωx_ ωy _ max˜imp by t_ ωy _ max˜imp ; and

calculating the torque of the shaft from the calculated magnitude of the change amount of the grip angular velocity about the x-axis per unit of time, using an approximate formula that is prepared in advance through test-hitting and that expresses a relationship between a torque of the shaft and the magnitude of the change amount of the grip angular velocity about the x-axis.

2. The fitting method of claim 1 , wherein the sensor is attached to a grip end.

3. The fitting method of claim 1 , wherein, as the approximate formula used for calculating the torque, one formula among multiple approximate formulae prepared in accordance with a magnitude of ωz —top which is a change amount of a grip angular velocity about the z-axis around the top is used.

4. The fitting method of claim 3 , wherein, among the multiple approximate formulae, a first approximate formula is used when ωz —top is not lower than 20 (deg/s), and a second approximate formula is used when ωz 13 top is lower than 20 (deg/s).

5. The fitting method of claim 1 , wherein the fitting method further includes the steps of:

calculating, as the index value, ωz —top , which is a change amount of a grip angular velocity about the z-axis around the top; and

calculating the torque of the shaft from the calculated change amount of the grip angular velocity about the z-axis, using an approximate formula that is prepared in advance through test-hitting and that expresses a relationship between a torque of the shaft and the change amount of the grip angular velocity about the z-axis.

6. The fitting method of claim 5 , wherein, as the approximate formula used for calculating the torque, one formula among multiple approximate formulae prepared in accordance with a magnitude of the change amount of the grip angular velocity about the z-axis from when the top is reached to when a grip angular velocity about the y-axis becomes maximum during a downswing is used; and

wherein a magnitude of the change amount of the grip angular velocity about the z-axis is a value obtained by dividing ωz_ top˜ωy _ max, which is an average value of the grip angular velocity about the z-axis from when the top is reached to when the grip angular velocity about the y-axis becomes maximum during the downswing, by t_ top˜ωy _ max , which is a time from when the top is reached to when the grip angular velocity about the y-axis becomes maximum during the downswing.

7. The fitting method of claim 6 , wherein, among the multiple approximate formulae, a third approximate formula is used when ωz_ top˜ωy _ max / t_ top˜ωy _ max , which is a value obtained from the dividing, is not higher than −500 (deg/s 2 ), and a fourth approximate formula is used when ωz_ top˜ωy _ max/t _ top˜ωy _ max is higher than −500 (deg/s 2 ).