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Blade Performance Vs Wood Type and Design

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AgentHEX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 01/01/2015 at 8:42pm
Ok, I see what's going on and I'm wrong on that point.

> There is also the issue of your gloat post that followed your careful explanation but was removed by the moderators. 

You mean this "removed" post?: http://mytabletennis.net/forum/forum_posts.asp?TID=60725&PID=845339&title=blade-performance-vs-wood-type-and-design#845339

> Both can be true if you use the right equation in the right circumstance. 

No, a given object has an inherent vibrational frequency, even using your own solution type. So which side is this frequency/"speed" for in an asymmetrical blade?

> Many people tried to explain this to you on the One of a Kind Forum as well, but to no avail. 

You have me confused with someone else, so please argue whatever this point is with them and not me.

> Uh oh, it looks like the rebound speed of the ball vball’ approaches infinitiy when the blade speed approaches infinity! 

Yes, that's why we're using a ratio for speed. The COR ratio also happens to be the correct one for measuring blade "speed" since it cannot excel the limit without violating physics.

Honestly I expected better from someone who can at least look up the equation above. For example, it was explained why a sim should be considered instead of just static calcs (to establish a correct timeline which is easy to forget when using time-invariant equations). Or what the shape (ie. order) of an equation means, which is a pretty fundamental consideration whenever math is put to science. None of this is remotely difficult to grasp for the technically adept.



Edited by AgentHEX - 01/01/2015 at 9:56pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 01/01/2015 at 8:51pm
Originally posted by Baal Baal wrote:

Originally posted by AgentHEX AgentHEX wrote:


With a musical instrument the notes required are standardized frequencies which human hearing can be well attuned to.


I can't figure out what point this comment was supposed to be making since some stringed instruments are clearly better than others and there is a lot of variability (especially when, as with table tennis blades, they are mostly made of wood).  And of course, musical instruments don't produce pure sine waves, which is why a lute and a violin sound different even when they play the same note.


The original comment was that they stop when it sounds right. The point is going much further than the basic well defined vibrational tendencies of the instrument is just going to sound wrong.

Technically all sound is a composition of pure sine waves, which is why a freq-codec + speaker can reproduce all sounds within reason.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JRSDallas Quote  Post ReplyReply Direct Link To This Post Posted: 01/01/2015 at 10:33pm
Originally posted by AgentHEX AgentHEX wrote:

So your axis of rotation is through the center of each slice while integrating across the beam? The slices that move up and down in y? Are you expecting partial credit for this?  No sorry, full credit is due since as the slices are attached they can't slide past one another and so they cannot move up and down without also rotating about each of their respective axes of rotation.   I think its only fair to also say you get no points again since you done no work at all.

I'm not sure how this can be explained any easier. As a matter of first principles, a rotational inertia only has meaning if the object of interest actually rotates around the axis (instead of, say, the correct axis around the end that the book already assumes for the beam). From basis of basic movement how exactly does each slice of that solid object rotate with respect to these new axis's you use?   Well no one can claim you are not willing to go down with your ship.  The beam is not infinitely rigid so as it vibrates, each slice within it both translates and rotates according to its role in the a particular harmonic oscillation bending mode (Eigen mode) solution.   Now you may not have gone beyond the motion of rigid bodies, so this is not a fair fight, but then again you did start it, so tough titties.   How about you show us your math for the correct moment of area of either a free or cantilevered beam?    Lets see you make a positive and well defined statement for once. 

> There is also the issue of your gloat post that followed your careful explanation but was removed by the moderators. 

You mean this "removed" post?: http://mytabletennis.net/forum/forum_posts.asp?TID=60725&PID=845339&title=blade-performance-vs-wood-type-and-design#845339.   Obviously that one isn't removed.  Lets just agree that you might want to put some clothes on.  Something conservative and less clashy, clashy.

> Both can be true if you use the right equation in the right circumstance. 

No, a given object has an inherent vibrational frequency, (OK tell us all exactly what is an object's "inherent" vibrational frequency?  I know what it is but it has nothing to do with this topic.  Still you'll get partial credit if you can explain it with a formula.) even using your own solution type.  I cannot claim that a solution to a 300 year old equation is my own solution type.  I can only claim that I independently re-solved a well worn problem.  So which side is this frequency/"speed" for in an asymmetrical blade?   Your question is poorly written, are you asking what is the quantitative value of frequency divided by "speed" for different sides of an assymmetrical blade?   Please rewrite your question in a well defined manner, I don't want to interpret your gaps.      

> Many people tried to explain this to you on the One of a Kind Forum as well, but to no avail. 

You have me confused with someone else, so please argue whatever this point is with them and not me.  Please accept my apologies, I had you confused with someone that also goes by the name AgentHEX on the One of a Kind Forum.  I read a long thread that you pointed me to and it had this other fellow who is also called AgentHEX in it.  Lots of argument in that long thread.

> Uh oh, it looks like the rebound speed of the ball vball’ approaches infinitiy when the blade speed approaches infinity! 

Yes, that's why we're using a ratio for speed. The COR ratio also happens to be the correct one for measuring blade "speed" since it cannot excel the limit without violating physics.  So please explain how does moving a blade faster and faster violate physics?

Honestly I expected better from someone who can at least look up the equation above. For example, it was explained why a sim should be considered instead of just static calcs (to establish a correct timeline which is easy to forget when using time-invariant equations). Or what the shape (ie. order) of an equation means, which is a pretty fundamental consideration whenever math is put to science. None of this is remotely difficult to grasp for the technically adept.  Agree but you have shown yourself to be technically ill-adept at understanding or presenting any bit of it as a written or math argument to even first order.   You just misuse words to create gibberish such as your question above.   You never answer a single question with a firm fixed answer or even the simplest equation.   Just words that allude to every one else having conceptual shortcomings relative to your own ODIN like but objectively given the written record, gibberish filled grasp.  

Now perhaps English is a second language for you (and that is an important point so if it is I owe you slack on your ability to communicate), and your ego may also be a handicap (and this may be culturally based since I don't know what your background is) but ego is your own problem and I don't have to cut you slack on this.   Still is is possible to assert that you are being misjudged, and it is clear that you are enthusiastic and that is always a huge positive.   So you have published technical articles?  Do you have patents granted?   Can you just stop trying to say I am wrong on something that I am not wrong on so that we can talk constructively?  I understand every one of your points but I am not going to accept having points accompanied with an insult.  Not going to happen.    
 



Edited by JRSDallas - 01/01/2015 at 10:34pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baal Quote  Post ReplyReply Direct Link To This Post Posted: 01/01/2015 at 11:04pm
Originally posted by AgentHEX AgentHEX wrote:


The original comment was that they stop when it sounds right. The point is going much further than the basic well defined vibrational tendencies of the instrument is just going to sound wrong.



I still have no idea what you are talking about.  Confused  Oh well.  In any case, with table tennis blades, they are mass produced in a factory and if you are lucky, you get a really good one.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 01/01/2015 at 11:10pm
I see my misconception about your equation, and I edited the post like an hour ago.

> (OK tell us all exactly what is an object's "inherent" vibrational frequency?  I know what it is but it has nothing to do with this topic. 

There's one freq per your own formula and many possible speeds depending on place of impact. This is quite relevant to the topic of whether freq is representative of speed: this is clearly a way it's not.

> Please accept my apologies, I had you confused with someone that also goes by the name AgentHEX on the One of a Kind Forum. 

Can you point to where this happens?: "Still your language shows that you still confuse frequency and blade stiffness as being explicitly linked to blade speed or ball impact COR when in fact the linkage one of reducing energy losses which effects COR due to transfer of impact energy to vibration modes of the blade"

I thought I was quite explicit about energy loss above.

> So please explain how does moving a blade faster and faster violate physics?

A ball which rebounds faster than it impacts the blade violates physics.

Look, if you just said that the inertia's for the component slices of beam stiffness and not blade as a whole this would've been a lot easier. The way the integral's calculated for this specifically is not obvious from looking at the result (nor is it mentioned in your prior post). You should be able to see how someone isn't aware of this detail would think the way inertial is being used is wrong. It's plainly obvious the points of contention here are largely conceptual so it's unclear how greater technical detail of, say, how to make freq calc slightly better helps in any case. How exactly does actually taking the enormous amount of time to create a sim advance the point about timelines?

Also, if you're familiar with academia this sort of contentious disagreement shouldn't even be unusual. The only difference is I personally didn't care to be passive aggressive about it.



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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 01/01/2015 at 11:16pm
Originally posted by Baal Baal wrote:

Originally posted by AgentHEX AgentHEX wrote:


The original comment was that they stop when it sounds right. The point is going much further than the basic well defined vibrational tendencies of the instrument is just going to sound wrong.



I still have no idea what you are talking about.  Confused  Oh well.  In any case, with table tennis blades, they are mass produced in a factory and if you are lucky, you get a really good one.


The main point is that if you sand/plane to the point where it's already the right speed/freq/thickness/etc, going past it isn't going to help. For the instrument makers, they're mostly slowly going to the right point, which is a lot more subtle in their case since it's along one dimension.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote arg0 Quote  Post ReplyReply Direct Link To This Post Posted: 10/27/2017 at 4:09pm
I noticed the images on the first page have disappeared due to licencing of the hosting service.
But I had saved them, so here they are again:






BTW, is JRSDallas still around?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zeio Quote  Post ReplyReply Direct Link To This Post Posted: 10/27/2017 at 4:43pm
Last Visit:     01/12/2016 at 5:44am

I actually have a question or two for him.
Viscaria FL - 91g
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baal Quote  Post ReplyReply Direct Link To This Post Posted: 10/27/2017 at 5:00pm
The guy knows his stuff!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote wanhao Quote  Post ReplyReply Direct Link To This Post Posted: 10/27/2017 at 11:34pm
So with all this MIT STANDARD thesis..how can we improve our skills ?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote zeio Quote  Post ReplyReply Direct Link To This Post Posted: 10/28/2017 at 1:57am
So that EJs alike would understand when their skills are the problems, no equipment could ever remedy that.
Viscaria FL - 91g
+ Neo H3 2.15 Blk - 44.5g(55.3g uncut bare)
+ Hexer HD 2.1 Red - 49.3g(68.5g 〃 〃)
= 184.8g
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Post Options Post Options   Thanks (0) Thanks(0)   Quote adyy Quote  Post ReplyReply Direct Link To This Post Posted: 11/22/2018 at 8:40am
@ JRSDallas - I tried to replicate your XLS table from this picture with formulas embedded




for the column Moment of Area Ik if I try to calculate it via this formula:

Now the moment of inertia I for a beam with rectangular cross section of width and height h, is given by I = b h3/12.   

 

When this cross section is not at the center of the beam (such as will be the case with all but the center ply in our laminate table tennis blade), the Parallel Axis Theorem lets us calculate the moment of Inertia of each ply.  Thus the moment of inertia for the kth ply is then:

 

k = b h3/12 +  bh d 2 

 

where is the distance from the midplane to the center of the kth ply.


the only way I can match your values from Layer Dist dk and Tickness hk column is using 254 for b.
Can you share your XLS? Or was this the formula you used?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 24prozent Quote  Post ReplyReply Direct Link To This Post Posted: 11/06/2022 at 12:26pm
Hi JRSDallas!

I enjoyed your deep treatment of the physics of the blade, and I also want to thank HEX for his contributions; even though the discussion between the two had taken on at some instances a highly competitive and confrontational nature. 

I have a somewhat naive question and a comment.

Can you make it plausible to me why the blade has a very noticable impact on the playing properties, given that it is covered by a rather thick rubber?
Seeing the very different rebound velocities of a ball from just wood and from rubber coated wood, I would expect that the rubber has a much bigger impact than the blade itself. And my "prejudice-soaked intuition" would conclude the same, i.e. that the blade properties are "buried" under the rubber properties. And yet I do know from own experience how a blade changes the playing properties. 
Can you enlighten this a bit?

You had posted an excerpt from an article that concluded that the racquet/ball contact time would be 4-5 milliseconds, while in your argument with HEX, he claimed that the interaction time is rather 1ms. 
Before I had read your discussion I had studied this paper:
https://www.researchgate.net/publication/305080758_Non_Linearity_of_the_BallRubber_Impact_in_Table_Tennis_Experiments_and_Modeling 
and from analyzing the stroboscope pictures I came to the conclusion that the contact time is indeed just about 1 ms. 
How certain are you about the result in the paper that you cited? 
Would you mind looking at the stroboscope images and giving your feedback on this with respect to ball interaction duration?

Ah, and one more thought/question:
In an article by Daniel A. Russell on vibro-acustical properties of a table tennis racquet ( https://www.tandfonline.com/doi/full/10.1080/02640414.2018.1462578 ), or his video ( "Acoustics of Ping-Pong" - Dr. Daniel Russell, Penn State University - YouTube https://www.youtube.com/watch?v=ekalYkdrwI8 ) I think the conclusion in a passing comment was that the ball playing (or rebounce) properties did not depend notably whether or not the ball was bouncing off of locations on the blade that were vibrational nodes. I would conclude, that vibration cannot really be the determinator of the rebounce. What -if anything- is wrong about the notion?

Ha! And here is one more question:
My reading about balsa properties, and the fact that it is used e.g. by TSP (now Victas) in a "cross cut" configuration, where the grain direction is perpendicular to the plane of the verneer, gave me the impression that there might be a compression/vibration/elasticity component of the blade that comes into play. Somewhere else it was described as a spring force by the compression of the blade material, which was mentioned to be particularly effective/elastic in balsa in the direction of the grain/fibres, ...and hence the "cross cut" / "endgrain" configuration.
Do you have any thoughts on that?

Sorry, as I am writing, more topics come into my head: 
What are your thoughts about the blade impacting the spin-generation potential of a racquet?
The company Re-Impact ( www.re-impact.de ), whose racquets I really like, makes such claims. I have always discarded them as marketing hokus-pokus; but the deeper I dig into the physics of the ball/blade interaction the more I realize how little I understand.
Any thoughts on this? 
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