Golf club

A golf club shaft assembly for attaching to a club head, the assembly comprising:

• a shaft having a lower end portion;
• and
• a sleeve mounted on the lower end portion of the shaft and configured to be inserted into a hosel opening of the club head, the sleeve having an upper portion defining an upper opening that receives the lower end portion of the shaft and a lower portion having a plurality of longitudinally extending, angularly spaced external splines located below the shaft and adapted to mate with complimentary splines in the hosel opening, the lower portion defining a longitudinally extending, internally threaded opening adapted to receive a screw having a nominal diameter of about 4 mm for securing the shaft assembly to the club head when the sleeve is inserted in the hosel opening,
• wherein each spline has two radially extending sidewalls that diverge from each other moving in a direction from the base of the spline to its outer surface, and each spline has a mid-span width to height ratio of about 1 to about 22, and a mid-span width to average diameter ratio of about 0.1 to about 0.5;
• and
• wherein the upper portion of the sleeve has an upper flange surface that is adapted to engage a hosel of the club head when the sleeve is inserted into the hosel opening, and the sleeve and the shaft have a combined axial stiffness from the upper flange surface to a lower end of the sleeve of about 1.86×108 N/m or less.