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Post by dubiousgolfer on Sept 20, 2020 7:10:12 GMT -5
I have sent an email to Dave Tutelman about something which seems contradictory. --------------------- I've been reading your 'Required Grip Pressure' article again and you've mentioned something that I cannot understand (not related to grip pressure). You said in the article: "Because biomechanics studies show a positive net torque on the club, applied by the hands, even at impact." Then you validated this fact by showing Dr Kwon and Dr Sasho MacKenzie's graphs below. One can see at impact that the 'Hand Couple' is negative , but the 'MOF' is larger so there will be a net positive torque applied by the hands at impact. But we know that there is forward shaft bend at impact which suggests that there must be negative torque applied by the hands (according to your own articles).These graphs must therefore be suggesting that we can have forward shaft bend at impact but with a net positive torque applied by the hands. The above doesn't make sense to me so wondering whether you know the answer? DG
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Post by imperfectgolfer on Sept 20, 2020 9:03:40 GMT -5
DG,
Good question - I look forward to DT's answer!
Jeff.
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Post by dubiousgolfer on Sept 20, 2020 10:45:38 GMT -5
Here is DT's reply:
--------------------- Great question,
Let me assume for the discussion below you know how moment of force (MOF) works.
The hand couple is a torque applied directly to the handle. It will cause a bend in the shaft, as the article asserts.
The MOF is due to a force applied at the handle (not a torque AT the handle). It turns into a torque only by its interaction with the mass of the club, assumed concentrated at the center of mass. So that is a torque on the club, but NOT a torque on the handle. In fact, if you think about it, that torque would want to bend the club forward (at least at the handle) because it is in effect pulling the center of mass (which is well toward the clubhead) forward.
It does not in fact cause that bend because, without any hand couple resisting it, the handle direction would just follow the center of mass. That would result in a straight shaft, no bend at all. It is the hand couple resisting that MOF that causes the forward bend.
I hope that was graphic enough. I know what I see in my head; I can only hope I induced that same vision in your head. Let me know. If you don't see it, I might need to come up with a diagram. But that diagram would probably be in a different article; as you point out, this isn't an essential part of the Grip Pressure article.
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DG
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Post by imperfectgolfer on Sept 20, 2020 11:26:23 GMT -5
DG, I find DT's explanation very unsatisfactory. I cannot understand why the torque due to a MoF is not applied at the level of the club handle. The MoF is due to a change in the left hand's direction of pull relative to the COM of the clubhead, and if it applies a torque then it surely must be happening at the level of the club handle. DT also wrote-: " In fact, if you think about it, that torque would want to bend the club forward (at least at the handle) because it is in effect pulling the center of mass (which is well toward the clubhead) forward." It is true that the left hand is pulling the club handle downward (forward) along the hand arc path, but the torque generated can be bi-directional - being clockwise in the early downswing and counterclockwise in the later downswing as shown in Kwon's image below. Note that direction of the torque due to the motion of the left hand down the hand arc path changes from being clockwise to counterclockwise at P5.5 (marked with a *). I believe that the MoF torque operates the level of the left wrist joint and it induces a left wrist upcocking motion between P4 => P5.5 and a left wrist uncocking (downcocking) motion between P5.5 => P7. Jeff.
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Post by dubiousgolfer on Sept 20, 2020 12:35:20 GMT -5
Dr Mann
Sometimes its difficult to interpret DT's responses.
"The MOF is due to a force applied at the handle (not a torque AT the handle)"
He means that the hands are not applying an active twisting force (ie. a torque) on the handle , just a 'linear' force.
"In fact, if you think about it, that torque would want to bend the club forward (at least at the handle) because it is in effect pulling the center of mass (which is well toward the clubhead) forward. It does not in fact cause that bend because, without any hand couple resisting it, the handle direction would just follow the center of mass.That would result in a straight shaft, no bend at all. "
I've interpreted the above as meaning the 'linear force induced MOF' will tend to cause the whole of the club (handle included) to rotate counterclockwise around the lead wrist joint or mid-hand point (ie. without any bending of the shaft).
If it wasn't for the negative couple applied by the hands while the club is being rotated counterclockwise by the MOF , then there wouldn't be any forward bending of the shaft.
If you look at an extreme analogy its similar to swinging the club (just using the MOF caused by a linear force at the handle) where the handle suddenly meets an obstruction - the handle speed decreases but the clubhead continues moving on unaffected causing the shaft to bend forward. In a real swing , the 'obstruction' is the negative torque applied by the hand couple.
I'm also in disagreement with DT's reply but for completely different reasons (which we've both agreed to no longer debate).
DG
PS. I also think 'torques/couples/moments of force' are ill-defined in physics/maths/engineering.
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Post by imperfectgolfer on Sept 20, 2020 21:21:14 GMT -5
Dr Mann Sometimes its difficult to interpret DT's responses. "The MOF is due to a force applied at the handle (not a torque AT the handle)"He means that the hands are not applying an active twisting force (ie. a torque) on the handle , just a 'linear' force. "In fact, if you think about it, that torque would want to bend the club forward (at least at the handle) because it is in effect pulling the center of mass (which is well toward the clubhead) forward."I've interpreted the above as meaning the 'linear force induced MOF' will tend to cause the whole of the club (handle included) to rotate counterclockwise around the lead wrist joint or mid-hand point (ie. without any bending of the shaft). If it wasn't for the negative couple applied by the hands while the club is being rotated counterclockwise by the MOF , then there wouldn't be any forward bending of the shaft. If you look at an extreme analogy its similar to swinging the club (just using the MOF caused by a linear force at the handle) where the handle suddenly meets an obstruction - the handle speed decreases but the clubhead continues moving on unaffected causing the shaft to bend forward. In a real swing , the 'obstruction' is the negative torque applied by the hand couple. I'm also in disagreement with DT's reply but for completely different reasons (which we've both agreed to no longer debate). DG PS. I also think 'torques/couples/moments of force' are ill-defined in physics/maths/engineering. I disagree with your bold-highlighted statement because there is also forward bend of the peripheral shaft in the late downswing in a Golf Robot's driver swing action, which operates without a hand couple phenomenon producing a torque around the mid-hand point, and the only torque in play causing the release of PA#2 is due to the MoF. Jeff.
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Post by dubiousgolfer on Sept 21, 2020 6:34:01 GMT -5
Dr Mann
That is a good point and I was coincidentally searching for ultra slow motion videos of Iron Byron and PingMan several days ago (to check forward shaft bend). Do you have any web links because I couldn't find any info via my google searches?
DG
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Post by imperfectgolfer on Sept 21, 2020 6:48:06 GMT -5
Dr Mann That is a good point and I was coincidentally searching for ultra slow motion videos of Iron Byron and PingMan several days ago (to check forward shaft bend). Do you have any web links because I couldn't find any info via my google searches? DG Unfortunately, I have not found a good quality, ultra-slow motion video of a Golf Robot's driver swing action - despite searching for one. I only have this example - but it may use a secondary motor to actively uncock the peripheral hinge joint. However, even if the PA#2 release phenomenon is actively positive, the peripheral clubshaft bend is happening in the absence of a negative hand couple phenomenon - which means that peripheral clubshaft bend can happen in the absence of a negative hand couple torque. Jeff.
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Post by dubiousgolfer on Sept 21, 2020 6:54:39 GMT -5
Dr Mann
I have sent an email to Ping Golf Director Erik Henrikson asking the question.
DG
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Post by dubiousgolfer on Sept 22, 2020 12:41:28 GMT -5
Dr Mann
I have finally received an answer from DT that I now find acceptable regarding SMK's MOF concept and am no longer arguing against its logic (from a physics perspective). I was using physics laws related to a 'rigid body' but they are not applicable to a 'flexible' golf club.
DG
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This is my email to DT:
As far as I am aware,:
1. An eccentric force at the handle of the club will cause it to translate and also cause a 'moment of a couple' around its COM.
2. A hand couple applied at the handle will also cause a 'moment of a couple ' about the clubs COM.
So I view the net 'moment of a couple' at impact being the sum of 1 and 2 about the COM (which shows on the graphs as a 'net positive moment of a couple' ).
The only difference I can visualise is the physical location of the application of the couple forces, where the 'hand couple' is near the 'stiffer ' grip end , while the 'linear force Induced moment of a couple' is interacting directly closer to the COM (where the club is more flexible).
Maybe it will all make more sense if I just wait for your new article to be published.
-------------------------------------- This is DT's reply.
It may be a long wait. It may be a forever wait, because I am not strongly motivated to do such an article right now. This just isn't being discussed; you're the only one who has asked. So let me try something simple and see if it grabs you.
First, let me explain the difference between free body analysis and flex analysis.
* Free-body analysis assumes a rigid body. All those laws of how to analyze forces and torques on a body assume rigidity.
* Flex or bending is analyzed by assuming a number of rigid bodies connected together by springs. When Sasho MacKenzie includes a rigid shaft in his analysis, he breaks the shaft into 4-6 sections and connects them together elastically. Each section is a complete rigid free-body in the analysis. When a shaft or clubhead engineer wants to look at flex, they fire up a piece of software called FEA, for "Finite Element Analysis". This assumes the shaft is an assembly of hundreds or thousands of infinitesimal pieces connected together elastically.
I point this out to emphasize that you can't apply the canonical rules of free body analysis to a flexible body, especially if the output you want is the flex. And one of the first casualties of trying to apply the same rules is the assumption that any couple applied anywhere operates through the CoM. True enough for a rigid body. But a flexible body is made of a lot of rigid bodies. A couple applied to one operates through the CoM OF THAT SEGMENT only. That segment will (may) apply torques and forces to the next segment as a result, but it does matter where that couple was applied.
Now for a demonstration of what I was trying to get at. Get an alignment stick. They are long enough and pretty flexible. This will represent a shaft. Think of one end as 'G' (grip) and the other end as 'H' (head).
Experiment #1 (E1): Hold G loosely in two fingers of your left hand. With your right hand, pull H back and forth slowly so the stick turns back and forth around G. There is no bending of the stick.
Experiment #2 (E2): Hold G tightly in your left hand, and exert whatever effort is needed to keep it pointing vertically. With your right hand, pull H back and forth slowly as you did in E1. Notice two things: (a) The stick bends markedly. (b) Your left hand has to exert a considerable couple (as well as a force, but you might not notice that) to keep G in position and vertical.
What is the difference between E1 and E2? Well, the right hand is exerting a force in both, and its distance from G ensures it is a moment of force around the stationary G. The difference is in the couple exerted at G by the left hand. The bend comes not from the moment of force at H, but by the couple that resists it at G.
E1 is a free-body problem. None of the forces or torques are enough to bend the stick enough to matter; at that level of stress it is approximately rigid. In E2, the stick is no longer a free body, but rather a large number of free bodies connected together by a large number of infinitesimal springs.
Does this make any sense to you?
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Post by dubiousgolfer on Sept 25, 2020 13:06:49 GMT -5
I've yet to receive a response from the Ping Golf director Erik Henrikson but I looked at some slo motion of Iron Byron (via Paul Wilson's you-tube series of videos) and copied and pasted the driver image below just before impact. It looks like there is no forward bend of the shaft close to impact, more like a slight backward bend . DG
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Post by dubiousgolfer on Sept 28, 2020 6:48:55 GMT -5
Reflecting on DT's email to me and these specific paragraphs: "When Sasho MacKenzie includes a rigid shaft in his analysis, he breaks the shaft into 4-6 sections and connects them together elastically. Each section is a complete rigid free-body in the analysis." "When a shaft or clubhead engineer wants to look at flex, they fire up a piece of software called FEA, for "Finite Element Analysis". This assumes the shaft is an assembly of hundreds or thousands of infinitesimal pieces connected together elastically." "A couple applied to one operates through the CoM OF THAT SEGMENT only. That segment will (may) apply torques and forces to the next segment as a result, but it does matter where that couple was applied." "In E2, the stick is no longer a free body, but rather a large number of free bodies connected together by a large number of infinitesimal springs." So shouldn't a linear force acting on a tiny segment make it move like a rigid body and behave like the image below (where black arrow is a linear force applied to a segment of a flexible rod - such as a golf shaft)? If DT is strictly correct that it's not the 'Moment Of Force' that causes shaft bend but that it's due to the hand couple ,then what is causing the shaft bend in the Iron Byron (which cannot apply a hand couple)? It must therefore be the linear force applied by the mechanical arm on the golf grip. DG
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