[ThinkingPlus]

What does HK mean in 2-E and 2-M-1 regarding a "prestressed" shaft? He seems to imply it's bent but I'm not sure in which direction. Also, the way I read it he is saying this condition will reduce Impact Deceleration(?).

The reason I ask is I've been into clubmaking a lot longer than TGM and on the face seems to conflict with my understanding of the physics of impact. For example, Cochran & Stobbs in "The Search for the Perfect Swing" showed that there was basically no influence by the player or shaft during impact and that the exiting ball speed is determined solely by the clubhead speed just prior to impact (assuming square contact etc.). This doesn't jive with the idea of "resisting impact deceleration" so I'm missing something.

Maybe I'm reading it too literally - he could be just trying to emphasize not quitting and reducing throwaway and such but he uses rather specific scientific terminology so I'm not so sure.

I have never read SftPS and don't know how C&S did their analysis. Basically, as far as I can tell, hitting the ball with a pre-stressed shaft should allow more momentum transfer to occur during impact. The shaft is just a spring. When a spring is compressed as far as it will go, it becomes effectively stiffer (up to the yield point of the material in question).

In addition, how a players hands brace the club at impact may provide a stiffer interface as well. That is one of the important aspects of the bent right and flat left wrists at and through impact.

Now for the kicker. All of this may not matter, from a momentum transfer point of view, if the speed of sound in the shaft is small enough such that the vibration or feel of impact occurs after the ball has left the face. At least in the case of steel rods (not exactly a shaft, but maybe a good approximation???), the speed of sound would allow the feel of impact to reach the hands roughly halfway through impact. As you can see, it is a very complicated interaction and highly dependent on shaft material properties, ball material properties, and clubhead speed at impact.

FWIW, regardless of how this impact momentum transfer stuff works out, in the book "The Physics of Golf" by Theodore Jorgensen, his model (matched against the swing of a tour player) showed that the most efficient transfer of clubhead speed to ball speed occurred when the club was at a 15* angle (ahead) relative to the ball at impact. That is essentially what one gets with a bent right wrist and flat left (more or less).