[MizunoJoe]

I am going to include a link that deals with club design but it contains a paragraph about the "sweet spot" and rotation about the shaft.

http://www.calpoly.edu/~rbrown/MOI.html

Please Note that the author is/was a Prof. in Physics at Cal Tech. And Yess there are errors (such as STERNUM)but they do not effect this analysis.

Read it all but pay particular attention to the paragraph below.



But there is an interesting side story to this that involves the axis of rotation about the shaft. That is, when you place the driver behind the ball at address, the shaft represents another rotation axis that comes into play during the golf swing. As the club is released through the ball on the through-swing, the club head (as well as the shaft) rotates about that axis in order to square the clubface with respect to the intended target line. The larger the club head, the more the mass of the club head is distributed away from the shaft, hence the larger the moment of inertia of the club head about the axis of the shaft. When the club head design approaches the USGA maximum of 5900 g-cm2 for the MOI about the vertical axis through the center of gravity of the club head, the moment of inertia about the shaft itself is considerably larger than that - by a factor of two or more, since the shaft is attached near the heel of the club and is thus removed from the center of mass of the club by five centimeters or so (see the figures below). The larger that moment of inertia the more difficult it would be to rotate the club head square to the target as the club is released on the through-swing. As a consequence, making the moment of inertia about the vertical axis even larger to make the club more forgiving on mis-hits would have a deleterious effect on closing the clubface - and hence on working the ball from right-to-left (for right-handed golfers). As with most engineering design issues, it is always a compromise!

I am sure this will make it harder to understand because it deals with "sweet spot" inertia but it also bakes it clear that it is not of practical us.

I'm done for now.

HB

The professor gets it wrong right out of the gate - the axis of rotation is not the shaft, but a line from PP#3 through the sweetspot - that's a BIG error, because the clubhead does not rotate around the shaft, but rather, around the imaginary sweetspot line, which enables the sweetspot to move perfectly on plane without weaving around like a drunk driver.

So yes, it does make it harder to understand, because it's incorrect.