Prestressed Shaft and Impact Deceleration
 

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.:dontknow:

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.

pres-stressed
 

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.

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.

Speed of Sound, Springs, and Things
 

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).

prestress
 

Yes.

Yes.

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....

prestress
 

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

6-c-2-a
 

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."

Stressful Situations
 

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?

That's it
 

Thanks Tong- That's what I was looking for.

Wallop of Centrifugal Force
 

Thanks Lynn! Great addition to this thread!

Just for my curiosity can you or someone else reference where he says the "wallop of centrifugal force" in the book- I believe it's there but am blanking out where to look.

Actually, never mind- I found it in 6-F-1 "Right" Timing...."Acceleration ceases when the speed it has produced equals that of the Thrust, and though the thrust is still present and able to maintain Velocity, it loses the flexed, stressed Clubshaft (Hitters) and the wallop of the Centrifugal Force (Swingers)."

Toe Talk
 

I used the phrase "Toe of the Club" because that is exactly the phrase Homer Kelley used. I wondered the same thing as I wrote the post and here would be my answer (not his):

Remember, during Release, both the Clubshaft and the Toe of the Club are rotating around the Sweetspot. And the Toe of the Swinger's Club is rotating counter-clockwise trying desperately to get 'in line' with the Sweet Spot and #3 Pressure Point for Horizontal Hinging through Impact. Hence, Homer's reference to the Toe causing the Release Bend.

Interesting sidelight:

That's why those old-timey wooden golf clubs were made with such long Toes. They were virtually impossible to Angled Hinge!

If it is only the stress of the clubshaft (which stiffens the shaft) that helps to resist impact deceleration, why does it matter in which direction it is stressed?

Clubshaft Stress And Loading Action
 

Centrifugal Force stiffens the Swinger's Clubshaft, and that Thrust (and Loading) is Longitudinal. And Body Momentum Transfer (via the Pivot) offers resistance to Impact Deceleration.

Muscular Thrust stiffens the Hitter's Clubshaft, and that Thrust (and Loading) is Radial. And Body Momentum Transfer is not available. Hence, only Pre-Stress (via the Right Triceps Muscle Power) offers resistance to Impact Deceleration.

Would Body Momentum Transfer be available for the Four Barrel Hitter?

No Momentum Transfer For The Hitter
 

No, the Hitter forfeits Body Momentum and its Centrifugal Left Arm Flying Wedge Throw-Out Action. Instead, he uses his Right Shoulder as a backstop for his Muscular Right Forearm Flying Wedge Drive-Out Action.

Nonetheless, he is not exempt from executing the Pivot in Full Strokes. Hitting or Swinging, the Golf Stroke is a Gyroscopic, Rotational Motion. And the rotation of the Pivot is necessary to Transport the Power Package from the Top to the Finish and to maintain its essential Alignments.

It's interesting that he mentioned both of these bending features because some of the guys doing advanced shaft fitting take into account the bending characteristics at different points (zones) in the shaft. Specifically, the swing tempo may be considered for "Acceleration Bend" (using your terminology) and release quality for "Release Bend".

As I mentioned earlier the "Release Bend" is generally toward the target for almost all golfers (for full swings) and is caused by the centrifugal force due to the clubhead speed pulling outward on the shaft through it's center of gravity - thus, CoG wants to line up with the shaft axis bending the shaft forward. That's why offset heads tend to hit the ball higher and could slightly correct a fade because the CofG is further behind the shaft and the shaft will tend to bend more forward with these heads. Ok - maybe too much information there.:redface:

Still a little dubious on the impact deceleration concept - but it may be a "real vs feel" thing in that it helps to think about accelerating through impact to avoid quitting, throwaway, etc.

Getting 'Smarter'...Day By Day
 

"'Release Bend' is generally toward the target for almost all golfers."

Thanks, jmessner. Your insights help us all!

Pictures
 

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Some pictures showing these bends would be nice... or are they too subtle to see?

Pictures
 

We have some footage of players at the Zurich taken with a very high speed camera. We have exactly the type of pictures you want to see. And they will blow your mind when you see how much the shaft is stressed during these points during the swing. I had no idea the shaft actually bends this much. If you can tell me how to post some sings from V1 I can show what I am talking about. Incredible.

Oooooo!!!! Ooooooooo!!!!! I know!!!!! I know!!!!!
 

Mike-O-the-Psycho,

6-F-1 “RIGHT” TIMING Maximum Force is delivered by maximum Thrust (muscular and/or centrifugal) near – but prior to – full extension.

Acceleration ceases when the speed it has produced equals that of the Thrust, and though the Thrust is still present and able to maintain Velocity, it loses the flexed, stressed Clubshaft (Hitters) and the wallop of the Centrifugal Force (Swingers).

How's the rash by the way?

Bucket

Mission Not-So-Impossible
 

Chris needs help to help us, Bambam. Go get'em!

I'll show a couple, but I'm looking forward to the high-speed images from Rhythm. Most video cameras take 60 fields (or 30 frames per second) and it's hard to get a good field at just before impact. I've taken a couple of images from my CSwing video analysis program and yes the bends are kind of subtle but they are there. See what you think. Scott Verplank and Zach Johnson.

Lynn, it would be great to see you in front of a high speed camera executing both Swinging and Hitting procedures to capture the shaft bend for each.

I know you have no problem with typing "a thousand words" to describe it, but wouldn't a picture be easier? :)

This thread has me particularly concerned about what I'm reading. Maybe I'm confused. (I just got a headache and took three Advil) It has always been and still is my understanding that one of the most important concepts in TGM, and it may be what everything boils down to, is a Pre-Stressed Clubshaft at impact for both Swingers and Hitters. The Bend is in the shape of a crescent, with the clubhead trailing the bent shaft, not leading it as the pictures above illustrate.

The purpose of a pre-bent clubshaft is to guard against impact deceleration. It's been my understanding that if you fail to pre-stress (bend) the shaft, the impact certainly will, with a loss of clubhead mass and speed at separation. Without pre-stressing the clubshaft then all the pre-impact work was for almost nothing.

Lag pressure, including swingers, is needed not only to trace the plane line but to maintain the pre-stressed clubshaft through impact and even till follow through. One should be able to feel the bend, at all times throughout the swing, and especially between release and impact. That's what the yellow book is all about (the other 241 pages are important too).

If centrifugal force stiffens the clubshaft and replaces the need for a pre-stressed clubshaft, then I'm going to stop swinging and become a Hitter. (as a Switter, I'm half way there anyway) Clear the lesson book Ted. :)

Titlelist Performance Institute To Meet The Golfing Machine
 

There is an extremely good chance we will be doing something soon. The folks at the Titlelist Performance Institute in Oceanside, California, have invited a short list of us out to do just this kind of thing. Their camera runs at 10,000 frames per second.

That should do it!

Do y'all think we're seeing Sequenced Release here? Or are these cats playing with a 10-2-D grip?

Does the Clubface 'match' the back of the left hand?

Timing the Flip?
 

It looks like both are timing the flip a little to me. Like maybe the left wrist is in the process of bending through impact. That might explain the forward bend as the response to the lost lag pressure. Sproing!!!

Or maybe I am full of shi....pping containers on my door step. It is the buying season, isn't it? :D

Food for thought - how would a more flexible shaft with more offset (hook face) alter the way we see this forward bending?

How much does axis tilt play into the equation, if at all?

As long as thrust (lag pressure) is maintained and Rhythm and aiming point is correct, I see no reason that there should be forward shaft bend until AFTER seperation - indeed, ideally after both arms straight if at all.

The fact that so many people have this forward bend demonstrates how rare proper aiming point and Rhythm are - that people are not paying attention to 1-L-15, and that over acceleration happens with even the best of players.

Which Way?
 

Are you pondering whether a more flexible shaft will show forward bend easier or pre-stressing easier? I would say yes. A softer spring can be flexed easier either way. Of course our tour boys are very likely cranking X-flexes so the bend will be minimal no matter direction, especially since neither Janzen or Johnson are known for maximum length off the tee.

I don't think that swing speed is the deciding factor to sustain the pre-stressed clubshaft bend at Start Down. Sustain the Lag and you sustain the bend. Look at Lee Trevino in "Pros at Impact". That amount of Bend is not caused by speed.

I do think that regular or more flexible shafts can show more flex at impact but thats because they aren't stiff enough to rebound after the initial Start Down Shaft Bend.

Arms alone are nowhere close to the force developed by Axis Tilt and #4 Acc against the #3 PP to initiate the Clubshaft Bend and/or sustain it.

Not Necessarily What I Meant to Imply
 

Sustaining lag and hence bend is not a function of swing speed, but of technique (agree), however, magnitude of bend is a function of acceleration as well as shaft flex. Greater acceleration produces greater swing speed unless a countering deceleration occurs prior to impact (loss of lag).

Easier to maintain lag pressure without the forward kick - assuming proper technique, but certainly exposes improper technique easier as well.

Hmm? Swing speed is a function of Lag Pressure. The greater the pressure the greater the speed. Therefore the degree of bend is a function of Lag Pressure. 6-C-2-0 "Your "normal" pressure will move lighter Clubs faster and heavier Clubs slower...".

But, when in doubt, "high thrust, low speed" especially on shorter shots. High ratio of bend degree to speed.

Great Thread!
 

I am really enjoying this thread. Great stuff, everybody. Keep those cards and letters comin'! :)

What a post:laughing9 :laughing1 :toothy:

I see this so much in old books but rarely in Tour Pros video ,is this a deliberate use of an overstiff shaft in order to enhance direction control?,or too soft a shaft to enhance effective loft.Or is it good loading of the shaft PRIOR TO IMPACT to be certain of keeping it loaded through impact ?Is this what actually happens as opposed to what we feel.I would like to see the same swing with the shaft at impact-THE TRUTH!

It's hard to say with my uneducated eye. I'd need to see the left wrist at lowpoint. Man, these guys do so-much right.