Prestressed Shaft and Impact Deceleration

[jmessner]

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.

[Mike O]

Jmessner,
This is one item where Homer Kelley would disagree with the Search for the Perfect Swing.

Prestressed would be the result of a Hitting motion- stressing the shaft or bending it away from the target.- as a result of the "radial acceleration" or pushing against the clubshaft. If you were looking face on to a golfer hitting a golf ball - the clubhead would be back of the hands.

The above concept is important when combined with his formula for ball speed as determined by what happens during impact- I.E. it's the ball speed at separation- not impact that is important- assuming the ball rides on the clubface/head for some period of time.

In contrast, while the swinger has some other advantages that the hitter does not have, the swinger wouldn't have the advantage of a "prestressed" clubshaft - because the nature of the physics is different- the "pulling" on the shaft in this context creates a "bowing up" of the shaft- not a "bowing forward" of the shaft that happens when one is pushing/hitting.

[tongzilla]

In contrast, while the swinger has some other advantages that the hitter does not have, the swinger wouldn't have the advantage of a "prestressed" clubshaft - because the nature of the physics is different- the "pulling" on the shaft in this context creates a "bowing up" of the shaft- not a "bowing forward" of the shaft that happens when one is pushing/hitting.

It is interesting that Homer said the Swinger's Clubshaft is also stressed, but during the change of direction at Start Down.

Even though the "bowing up" effect does bend the shaft, it is not regarded as a prestressed Clubshaft? So the term prestress refers to the stress placed on the shaft in only one dimension? There are lots of Swingers with a bent shaft at Impact, but this is obviously not what Homer was getting at.

[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).

[Mike O]

It is interesting that Homer said the Swinger's Clubshaft is also stressed, but during the change of direction at Start Down.

Even though the "bowing up" effect does bend the shaft, it is not regarded as a prestressed Clubshaft? So the term prestress refers to the stress placed on the shaft in only one dimension? There are lots of Swingers with a bent shaft at Impact, but this is obviously not what Homer was getting at.

Yes.

Yes.

[jmessner]

Thanks for the info folks-

Just to throw it out there, here's what I have gleened from my reading about impact dynamics from a clubmaking perspective. It was interesting that the "prestressed" condition is considered bent away from the target, because it is pretty well documented that for 95% or more of golfers the shaft at the head end is actually bent forward (and down) at impact. There's video, stills and deflection graphs (True Temper Shaft Lab) that have shown this.

Regarding the deceleration issue, C&S tested a clubhead on a hinge at the end of a shaft and concluded that it made no difference to the ball flight. The still impact pictures are very interesting. Basically, they conclude (as have others including Jorgensen mentioned by Steph) that the head at the point of impact basically acts as if it is disconnected from the shaft. That's why in the impact formula (conservation of momentum), the weight of the shaft is not considered - only the head. C&S also figured that the player doesn't feel the impact until the ball is well on its way.

I don't think this at all negates the imperatives of TGM - these factors in my view aren't really necessary to support the model and I'm guessing HK did not have access to some the equipment that has been used to characterize impact and shaft bending.

One area where the physics guys do basically support Homer is in the ball flight laws in which TGM is somewhat at odds with the more popular ball flight laws put forth by John Jacobs and others but that's probably another discussion....

[Mike O]

It is interesting that Homer said the Swinger's Clubshaft is also stressed, but during the change of direction at Start Down.

Even though the "bowing up" effect does bend the shaft, it is not regarded as a prestressed Clubshaft? So the term prestress refers to the stress placed on the shaft in only one dimension? There are lots of Swingers with a bent shaft at Impact, but this is obviously not what Homer was getting at.

Prestressed- means stressed before it's going to be stressed. So it's only in reference to that dimension that the ball will cause the shaft to be stressed.

When you say Homer said the Swinger's Clubshaft is also stressed- Which may be very accurate- where did he say that- where's the reference being used for that statement?

Thanks,
Mike

[tongzilla]

When you say Homer said the Swinger's Clubshaft is also stressed- Which may be very accurate- where did he say that- where's the reference being used for that statement?

Thanks,
Mike

Second paragraph of 6-C-2-A.

"The Clubshaft is stressed by the weight of the Clubhead resisting a change in its direction or velocity – which is Acceleration. Acceleration bends the Clubshaft during Radial Acceleration (10-19-A). Change of direction bends it during Longitudinal Acceleration (10-19-C) which may be, or just include, the Clubhead Lag Pressure Point in addition to its main function of Acceleration Control."

[Yoda]

It is interesting that Homer said the Swinger's Clubshaft is also stressed, but during the change of direction at Start Down.

In a 1981 GSEM Class discussion, Homer Kelley referred to two types of Clubshaft Bend for Swingers. The first -- "Acceleration Bend" -- stresses the top of the Shaft during the Start Down. During the second -- "Release Bend" -- the stress is opposite the Acceleration Bend as the toe of the Club attempts to get in line with the #3 Pressure Point.

The Hitter's stress is at the back of the Shaft and does not vary.

[armourall]

In a 1981 GSEM Class discussion, Homer Kelley referred to two types of Clubshaft Bend for Swingers. The first -- "Acceleration Bend" -- stresses the top of the Shaft during the Start Down. During the second -- "Release Bend" -- the stress is opposite the Acceleration Bend as the toe of the Club attempts to get in line with the #3 Pressure Point.

The Hitter's stress is at the back of the Shaft and does not vary.

Love it when you post these little gems, but why the "toe" of the Club, and not the Sweet Spot?