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Blade Performance Vs Wood Type and Design

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Giangt Quote  Post ReplyReply Direct Link To This Post Posted: 12/23/2014 at 6:49pm
I did not mind reading all in this thread but who made all the engineering?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/23/2014 at 7:00pm
Originally posted by Baal Baal wrote:

AgenetHEX you're treading a pretty fine line.


Sorry I got carried away there with reasoning over appeasing the right people. My bad.

Originally posted by Giangt Giangt wrote:

I did not mind reading all in this thread but who made all the engineering?


For the most part TT companies do very little engineering, and we can now see why they don't need to given lack of intricate coupling between the nature of each component rubber/blade/ball.

However there's significant social prestige in online TT circles built on the basis that the system needs to be finely tuned, with sway given to those who supposedly hold the keys to "understanding" the interplay. In reality things don't necessarily turn out well for the guy who points out the emperor has no cloths.


Edited by AgentHEX - 12/23/2014 at 7:07pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JRSDallas Quote  Post ReplyReply Direct Link To This Post Posted: 12/24/2014 at 2:27am
Glant -

I did the engineering.  It was a fun project for me in 2008 to see what I could understand about the issues by thinking it through. Spare time at home, learned CAD and then ANSYS to do FEA, built a test platform, captured sound data did the Fourier analysis on my PC.  Fairly entertaining and I learned something real.

I had the background that let me work this problem out myself.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JRSDallas Quote  Post ReplyReply Direct Link To This Post Posted: 12/24/2014 at 2:54am

Originally posted by AgentHEX AgentHEX wrote:

Please try to understand what is being said before going off. I did not make the statements above nor below without careful consideration and any possible error will not be trivial. OTOH the error in that frequency equation rather is.

> Well since you have by omission left me with my choice of what frequency is correlated with, I hereby claim that frequency is directly correlated with frequency

Your general assertion through this thread and reinforced just above is that frequency is a good first order approximation of speed. This is not in dispute.  Then why is there any further discussion?.  Its useful. OK subject closed.  Apparently not.  What is in contention is the quality of approximation and its exclusivity to frequency, namely that other simple "indicators" are similarly valid (and flawed in their individual ways). I don't believe you're disputing this, or at least haven't done so above.

I agree that I have never said frequency is an "exclusive" indicator nor that no other “indicators” exist or are useful.  I also agree that you have adopted the language “what is in contention” in the paragraph above as an innuendo so you can argue that you have a greater perspective that separates you from a view that was never expressed.  You then close, by saying that you don’t believe I am disputing your position or at least haven’t done so.   You make a specious claim to establish a Potemkin controversy to look good against. 

What is actually in contention is your position on insisting that better players should use your paddle so that it can learn from them how to play better, and on how race and religious preference are “indicators” of table tennis success – when other simple “indicators” like visual learning, cumulative training, and opportunity are similarly valid and less flawed by the influence of cultural and sampling frame biases.  I don’t believe you’re disputing this, or at least haven’t done so I would hope.


> Your parenthetic statement (they certainly aren’t with respect to size, and who knows what else) says you claim that (frequency) is not correlated with blade size or other physical parameters.   
 
given that statement since I claimed the exact opposite, and this claim is exactly the reason why frequency is particularly flawed as mentioned at least twice above: speed will not vary significantly with size, as better predicted by, say, stiffness rather than frequency. I noted this to punctuated these particular flaws.

I purposefully misunderstood you with the thought that you might see that I was imitating you.

> yes the contribution of plies that are displaced from the plywood mid-plane do correctly does show up as a distance squared weighting factor per the Parallel Axis theorem
> Now you admitted as much earlier in this thread, you said when you did not understand how I was using the parallel axis theorem to calculate the area moment of inertia I of the laminate beam in the first place. 

First of all, please consider that I was trying to be polite. In hindsight this was a poor idea. The error is actually pretty obvious and the reason it evaded capture only slightly less so.

I've detailed above exactly why the use of Parallel Axis theorem here is wrong. More briefly, note that it is by definition calculated from the center of mass to the center of rotation. By your own assumptions for the beam equation, the center of rotation is around the "end" of the blade (ie the fixed neck point), which is certainly reasonable. But note that the distance from that point/axis to blade center of mass is quite a bit further and in any case perpendicular to the few mm the plies are shifted.

Before being dismissive, draw out the diagram with both axis specified, it's not hard to see the problem and I would expect any first year physics student to be able to do it. The way Parallel Axis is being used in your freq equation is a fundamental error in the basic geometry involved. The specific numeric illustration a few posts back should provide additional illumination. The reason why this clear error is not evident (ie blows up the results) is that the terms involved are rather small as mentioned.

Thank you for being explicitly clear so that I and others can understand. You are right that it is not hard to see the problem.   It is absolutely true that when a free body is rotating, its moment of area is calculated with respect to the principal axis of rotation and that it passes through the center of mass (COM) of the body.  Similarly, if using the Parallel Axis Theorem to account for a segment of that body, that distance should also be from the COM of the segment to the principal axis of rotation that passes through the COM of the body.

However, vibration analysis solutions address the internal degrees of freedom of the body, and are independent of the 3 translation and 3 rotation degrees of motion freedom of the body.   Free vibration does not cause rotation around a rigid body’s center of mass, and our cantilever constraint is just a boundary condition on the differential equation for the free vibration of the beam.  Applying boundary conditions will shift the theta values of the modes which shifts the final frequencies but they don’t make the beam translate or rotate.      

 So….when looking at the vibration modes of a beam with uniform cross section (like ours), the moment of area is taken with respect to the center of mass of the beam's cross section.   For a beam with rectangular cross section, if it was a single material, its moment of area would be I= (bH^3)/12.  However since our beam is a symmetric multi-layer stack of rectangles (of different material), and having correctly applied the Parallel Axis Theorem, the correct moment for each layer is as I previously presented.  In conclusion then, the effect of plywood layer stiffness and placement within the plywood stack as described by the equation is correct, and the size of the effect are exactly are originally presented. 

A page from Stokey Chapter 7 Vibration of Systems Having Distributed Mass and Elasticity shows that I have used the correct axis of rotation and have correctly calculated the moment for a cantilever.  By extension, this means my equation is correct and that the effects I calculated based on layer placement are correct.  



However, also as mentioned it's visible how this came to be in the first place: you're trying to account for the intuition that the outer plies have more impact than inner ones, and way how parallel axis seems to work out (with its wrong application) assuages that concern since bigger numbers are multiplied for plies further out. This intuition is not in dispute given you just repeated it in another post above. The weighing is however completely wrong and certainly not the weighing in actually plate equations I was referring to.



To tie this all together, consider a blade with a hard surface on only one side (and the other missing or whatever). Note your equation is agnostic to which side the impact is on; surely they differ if surface layers matter, which rather ruins that "greater surface effect" illusion the equation is supposed to illustrate.   Of course it is agnostic, it solves for a symmetrically laminate beam.   I can extend it but I don’t need to prove anything here.

Again, to provide some perspective, a careful particle sim of the whole collision would probably show some surface effect, but this has to do with the peculiarities of how internals of solid materials work, and nothing to do with rotational inertia. Analysis of such matters are well beyond what can be provided by simple clean equations.

You were very careful in explaining your objection to my moment of area calculation on the beam.  I carefully answered and I provided 3rd party support to corroborate my position, I assume that you now accept my calculation.   My equation describes how the plywood construction leads to its vibration frequencies and how selecting what materials and where in the stack they are placed creates a calculable effective stiffness.   In particular, my statement that moving the stiffest plies to the outside has the greatest increase on moment and therefor on system stiffness and therefor on frequency as per the equation.


You have coined concept you call “A GREATER SURFACE EFFECT” and are using it in regards to the collision of a ball on that surface but this has nothing to do with your objection to the equation I derived.   You are mixing apples and oranges.   I agree that oranges are interesting but you not having an orange does not mean I do not have an apple.  

Now you recommend a careful particle sim be performed of the total collision.  I agree, please perform the careful sim you recommend and get your orange.  Show us your results because it is an interesting topic. I don't have access to the tools to do a collision FEA with sufficient node count to be useful. 

 

My work in this thread has been on the engineering properties of woods, selection of woods, the engineering of plywood and vibration modes of a cantilevered plywood, how plywood design effect plywood vibration frequencies, example calculations of multiple different plywood designs, FEA modeling of a single ply table tennis blade (Amultart dimensions), measured sound spectrums from struck similar size blades of differing plywood constructions, and qualitative correlation between frequency and speed feel of these blades.  Energy loss considerations support a connection with increased COR but no quantitative measurement of COR, nor full FEA collision modeling have been performed.

Reader’s should note that I do not agree or disagree with the conclusions Hex is espousing regarding the magnitude of blade or rubber selection has.  For me, these issues are outside the topic of this thread and I myself have not worked through them to the point that I can prove a clarifying insight –  There is obvious merit to Hex’s saying it’s the player and not the equipment –Yep. At the same time there is obvious merit in saying that players do feel the differences in equipment-Yep.  And finally there is merit in noting the obvious that in all sports, that as the best players are very careful in selecting their equipment, there may be causal reasons for doing so – Yep.        

I look forward to seeing any efforts that can replace the arguments about what also floats on water besides witches.

> Please also refrain from using Wolfram Alpha to pull up some non-physical function (as you did earlier in this thread) to argue that it is a better predictor.  We all know that the declining number of pirates is strongly negatively correlated with the increase in global temperatures. 

I'm not sure how this is supposed to make any sense. My Wolfram Alpha plot is just how any realistic speed vs (thickness, freq, etc) plot must necessary look like. If that's not clear, please consider what bounded vs unbounded means again. Any unbounded method of determining it is necessarily wrong without some attempt to massage the math into a sane shape. This is basic physical modeling 101.

So when I followed your link it did not lead to a plot, just an equation with no discussion of causal parameters.  If you had Wolfram Alpha derive that equation based on a physical model you set up and then have it create a plot then please walk us through those details by dropping screen captures into a post.  Show your work.       

Still I believe you made this argument to show that my equation was not physical or something. 

I've shown my equation was properly generated so this contention is now irrelevant. 


Wrap up:

I invite and look forward to seeing any work from anyone on the topic of collision.

As for myself, I’m going to move on now and spend more of my vacation time with my wife. 



Edited by JRSDallas - 12/24/2014 at 3:07am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Giangt Quote  Post ReplyReply Direct Link To This Post Posted: 12/24/2014 at 7:04am
Originally posted by AgentHEX AgentHEX wrote:


For the most part TT companies do very little engineering, and we can now see why they don't need to given lack of intricate coupling between the nature of each component rubber/blade/ball.

However there's significant social prestige in online TT circles built on the basis that the system needs to be finely tuned, with sway given to those who supposedly hold the keys to "understanding" the interplay. In reality things don't necessarily turn out well for the guy who points out the emperor has no cloths.

It is not obvious that the companies do engineering on their products at all. The only thing we see (us consumers) are the result of a marketing video where the companies show some flashy graphics to sell their product. On the other most people do not understand all physics that lies behind.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Giangt Quote  Post ReplyReply Direct Link To This Post Posted: 12/24/2014 at 7:33am
Originally posted by fatt fatt wrote:

What agenthex means is most of us will analyse table tennis items without really understanding what's going on; the companies play their marketing game knowing well about this. so we have an irrational dance featuring ignorant people who pretend not to know they buy from companies making fun of them. 
<span style="line-height: 1.4;">I think we all know about that; in golf and tennis it is about the same; those are just 2 sports that come first in mind (because somehow I find their mind game close to tt's) but in any sport the same argument can be developed successfully.</span>
<span style="line-height: 1.4;">It is fun to pretend we understand the game as a whole and then from an expensive setup drill down a mine of ideas hoping we find nuggets here and there; and we do, sometimes with luck; sometimes with logic; most of the time with too much money and time invested in material and enjoyable babbling. It is harder to be the one starting with an allround classic + moon and discover with the same setup overtime new sides of the game as they get </span><span style="line-height: 1.4;">slowly </span><span style="line-height: 1.4;">better. I do understand the latter gets annoyed with the former, especially when the amount of time playing becomes a drop in the ocean of talking on forums. oops, the last sentence is hitting me hard so I will make a pause and get ready for my new resolutions.</span>
<span style="line-height: 1.4;"> </span>


Hi fatt,
There will always be different types of people that will have an opinion of how a new product is released. Some people will eat all the marketing up to the last bit, some will be more sceptic about their statements etc. The former will of cause buy the new products.
There is a saying in Denmark which sounds like: never go down on your equipment! The meaning of this quote is that you have to buy the best equipment to be like the best. Unfortunately if you lose a match you can only blame it on yourself and not the equipment ;)
IMO it is constructive to discuss TT and hear other experiences from people. That is one of the reasons that forums like MYTT is existing, but unfortunately sometimes people have more in their mouths than their TT abilities.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Giangt Quote  Post ReplyReply Direct Link To This Post Posted: 12/24/2014 at 7:44am
Originally posted by JRSDallas JRSDallas wrote:

Glant -

I did the engineering.  It was a fun project for me in 2008 to see what I could understand about the issues by thinking it through. Spare time at home, learned CAD and then ANSYS to do FEA, built a test platform, captured sound data did the Fourier analysis on my PC.  Fairly entertaining and I learned something real.

I had the background that let me work this problem out myself.
Hi JRSDallas,
First of all I want to thank you for all the effort you have put into this. Great job! I have the background as well, but have not taking the time to do any engineering on TT yet. Of cause the difficult part in your calculations would be to determine the right E-modulus of a laminate, but also to verify models by testing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pnachtwey Quote  Post ReplyReply Direct Link To This Post Posted: 12/24/2014 at 7:08pm
A couple of points.
Through all this thread there has been talk of frequency but not amplitude. If one works through the math you will see the amplitudes are very small. Most likely too small for any contribution to the catapult effect.

JRS Dallas also pointed out that the ball and blade restoring to the normal shape must be in phase or energy will be lost. I tried to use the analogy of a spring board diver on another forum but apparently no one dives. The same affect occurs when jumping on a trampoline. You must jump at just the right time. In TT the ball and blade properties are fixed so even if the vibration or flexing of the ball are significant enough the chance that it would do it in phase with the ball is small.

Another issue is damping. A blade can vibrate at a frequency but what about the decay of the amplitude at that frequency. Blades with greater damping will also tend to be slower but for the most part the ball leave the paddle so quickly the damping probably doesn't affect the speed after impact much because the ball is gone.

As far as the dwell time. We have been over this before on mecuur's very long thread. The dwell time is in the range that AgentHex says it is. Baal once did a "napkin" calculation and he came up with an answer closer to 1 millisecond than 3 or 4. In addition I have the high speed videos taken at 2000 FPS.

If you remember my Toxic 5 hard bat vibrated or flexed a lot whereas my Firewall Plus didn't appear to flex or vibrate at all but obviously it vibrates because I can hear it. It is just that the amplitude is small.

I think it is safe to say that in general blades that vibrate at a higher frequency are faster. I don't have any blades that violate that general rule. I think JRS Dallas did a good job of showing that the position of the ply of wood relative to the center makes a big difference.

I just think JRS Dallas should have examined the impact of the ball and paddle a little more. I don't find the higher harmonics that useful. The primary mode of vibration and the frequency of the TT ball is think could be useful for further research because this gets back to the ball and blade restoring their shape in phase.

I think it would be interesting to put an accelerometer on the back side of the paddle. When I find one that can handle 20gs and has bandwidth of 10Khz I will start to get interested. The accelerometer, CPU and battery can probably be put together so it weighs about the same amount as a sheet of rubber.











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Post Options Post Options   Thanks (0) Thanks(0)   Quote pnachtwey Quote  Post ReplyReply Direct Link To This Post Posted: 12/24/2014 at 10:00pm
Merry Christmas. It has been a long time. I still had private messages in my in box. They are obviously old now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 6:45am
> Then why is there any further discussion?.  Its useful. OK subject closed.  Apparently not. 

I mean what you said is clear and the meaning not in dispute, and it seems you understood what I meant, too; the combination of which is a christmas miracle compared to most of these discussions. For example no one is disputing that frequency is some indicator, only how good it is. This is not a trivial question given the nature of the problem as mentioned, and thus not conducive to easy answers.


> However since our beam is a symmetric multi-layer stack of rectangles (of different material), and having correctly applied the Parallel Axis Theorem, the correct moment for each layer is as I previously presented....A page from Stokey Chapter 7 Vibration of Systems Having Distributed Mass and Elasticity shows that I have used the correct axis of rotation and have correctly calculated the moment for a cantilever.  By extension, this means my equation is correct and that the effects I calculated based on layer placement are correct. 

Note that page doesn't contain your specific sub-equation for inertia of the sublayers. You know, the part that uses the PA theorem. I pointed out specifically why just that part is wrong, in very explicit detail. To be clear, I'm not contending the rest of it.

You added that part, which is why I called it a corrective factor (to the original equation such as in the book), because it tries to "correct" the original inertia var. Again, the contention is that the bit you added is done incorrectly, specifically because you don't use the same axis of rotation with that PA theorem subequation.

Again, it would really help if you just drew out where the axis around which the inertial is calculated for the original equation and your two PA axis's (you know, the ones mm's apart). I'd do it but this is one of those things which is more illustrative if you did yourself.

> Of course it is agnostic, it solves for a symmetrically laminate beam.   I can extend it but I don’t need to prove anything here.

Let's go through the logical steps one by one:

1. Your claim is that frequency is a good indicator for speed ostensibly because it accounts for ply depth (and you certainly made sure of that in your equation modification).

2. However, it also gives the same answer for two sides of a blade with complete different material on each. A vibration frequency is inherent to the object and can't be "extended" away.

3. Clearly 1 and 2 cannot be simultaneously true.

I've only pointed out a sim to speak of how a solid collision actually happens, and how it's nothing like how your frequency "extension" attempts to correct for it. Usually when two things are correlated in a relevant way in the natural world they work through similar mechanisms. That's for example one way we can tell piracy and global warming aren't.

> So when I followed your link it did not lead to a plot, just an equation with no discussion of causal parameters. 

Just FYI, but Wolfram Alpha by default plots any sane equation as it d, which is why I used it for illustration. It's not meant to be causal, just a point that the right answer for speed vs anything is that general shape thus anything quite dissimilar is well off. The only reason why stiffness or freq even work at all is because COR for a TT blade is somewhere in the ~0.5 ballpark where all these indicators have slopes close enough to ~1 to not matter too much.

The other related point that was probably missed is that if you simply transform whatever other "kind of correlated" indicator into that shape it'll give much better estimates. This can be done with any number of methods just as subbing x for y in a plot mirrors along the y=x axis.





Edited by AgentHEX - 12/27/2014 at 6:49am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jrscatman Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 7:12am
Hmm AgentHEX seems to be making some points and JRSDallas has equations and graphs ..... will someone be explaining what's going on in plain English - so I can follow along?
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Post Options Post Options   Thanks (1) Thanks(1)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 7:42am
Originally posted by jrscatman jrscatman wrote:

Hmm AgentHEX seems to be making some points and JRSDallas has equations and graphs ..... will someone be explaining what's going on in plain English - so I can follow along?
Thanks


JRS modified a relatively straightforward book equation to correct for offset plies of a TT blade, and I believe the way the modification was done is wrong. It's done in a way which "makes sense", which I guess make it harder to figure out for the person it made sense for.

In ELI5 terms, an object has greater rotational inertia when it's moved further away from the axis of rotation. In physics there's a certain way to calculate this for the distance between the axis of rotation and the center of gravity of the object. With a TT blade that rotational axis is nearer the end of the blade (where you presumably hold it) and the line to its center is along the length of the blade. But JRS is calculating that line from the center outward towards the face. From first principles when you move the object (plies in this case) in same direction as the one the object swings in, it doesn't really change inertia at all which so the correction was unnecessary.

Those few mm are not very consequential in term of results since the correction is small anyway compared to the other blade dimensions (parameters used elsewhere), but the conceptual error is not the greatest.

---

In his post above JRS just says the original book equation is right, which it probably is, but that's not what I'm talking about.


Edited by AgentHEX - 12/27/2014 at 7:54am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 8:25am
There was also IMO an overemphasis on freq as the indicator of merit, which is what likely caused mistake. IOW, there's an overriding belief that freq explains everything, such as speed differences between blade sizes. In that case different blade sizes don't differ much in speed so freq predicting it does clearly doesn't explain it. In this case freq doesn't differ for outward displacement of plies, so it doesn't explain blades being bit faster with harder plies on the outside rather than inside either. The internal motivation to explain that phenomenon with frequency is what probably led to the modification.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Krantz Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 8:48am

Originally posted by AgentHEX AgentHEX wrote:

(...)


 With a TT blade that rotational axis is nearer the end of the blade (where you presumably hold it) and the line to its center is along the length of the blade. But JRS is calculating that line from the center outward towards the face. From first principles when you move the object (plies in this case) in same direction as the one the object swings in, it doesn't really change inertia at all which so the correction was unnecessary.
(...)

Calculating the distance from the center towards the face is crucial for measuring the stiffness of the blade and I am surprised that you seem to ignore this fact - properties of such composite constructions are being deeply studied in modern technology and manufacturing in things like rocket fuel tanks, super-light sport yachts and even cars bodies. If you mean that these calculations are wrong then please say exactly where the mistake is, because so far you are only presenting puzzles to readers and giving some homework to OP and personally I wouldn't even expect to get a serious answer for such nonconstructive critique.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote pnachtwey Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 1:48pm
Originally posted by AgentHEX AgentHEX wrote:


Let's go through the logical steps one by one:

1. Your claim is that frequency is a good indicator for speed ostensibly because it accounts for ply depth (and you certainly made sure of that in your equation modification).

2. However, it also gives the same answer for two sides of a blade with complete different material on each. A vibration frequency is inherent to the object and can't be "extended" away.

3. Clearly 1 and 2 cannot be simultaneously true.
Good one AgentHEX.  I love it.

Originally posted by AgentHex AgentHex wrote:

For example no one is disputing that frequency is some indicator, only how good it is.
Obviously frequency isn't the only indicator.  I have mentioned damping before but this topic always seems to be ignored.  A few pages back you posted a series of wave equations but there was no damping term in those equations.  If there were there would be a exp(-t/τ) where the tau in the denominator is the time constant of decay.  Anything that vibrates has a damping factor or it would vibrate forever.

In practical TT player terms, hands provide damping.  A lose grip (more damping ) results in a slower return than a tight grip ( less damping).

My Toxic 5 video shows the blade vibrating like crazy but it is in a vice.  I doubt it would vibrate like that in my hand.

Here is an example of what I am talking about
http://ldf.mendelu.cz/und/sites/default/files/soubory_akustika/acoust_lect_damping.pdf
There is more.  Those that want to can find the rest of the document.




   


Edited by pnachtwey - 12/27/2014 at 2:02pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 5:00pm
Originally posted by Krantz Krantz wrote:

Originally posted by AgentHEX AgentHEX wrote:

(...)


 With a TT blade that rotational axis is nearer the end of the blade (where you presumably hold it) and the line to its center is along the length of the blade. But JRS is calculating that line from the center outward towards the face. From first principles when you move the object (plies in this case) in same direction as the one the object swings in, it doesn't really change inertia at all which so the correction was unnecessary.
(...)

Calculating the distance from the center towards the face is crucial for measuring the stiffness of the blade and I am surprised that you seem to ignore this fact - properties of such composite constructions are being deeply studied in modern technology and manufacturing in things like rocket fuel tanks, super-light sport yachts and even cars bodies. If you mean that these calculations are wrong then please say exactly where the mistake is, because so far you are only presenting puzzles to readers and giving some homework to OP and personally I wouldn't even expect to get a serious answer for such nonconstructive critique.



Someone asked for a simple summary to help grasp what's going on. Please at least try to understand the easy explanation before criticizing the original arguments. The basic physics of beams and inertia have nothing specific do with composite constructions and applies to any and all physical matter.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 5:13pm
Originally posted by pnachtwey pnachtwey wrote:

Obviously frequency isn't the only indicator.  I have mentioned damping before but this topic always seems to be ignored.  A few pages back you posted a series of wave equations but there was no damping term in those equations.  If there were there would be a exp(-t/τ) where the tau in the denominator is the time constant of decay.  Anything that vibrates has a damping factor or it would vibrate forever.

In practical TT player terms, hands provide damping.  A lose grip (more damping ) results in a slower return than a tight grip ( less damping).

My Toxic 5 video shows the blade vibrating like crazy but it is in a vice.  I doubt it would vibrate like that in my hand.

Here is an example of what I am talking about
http://ldf.mendelu.cz/und/sites/default/files/soubory_akustika/acoust_lect_damping.pdf
There is more.  Those that want to can find the rest of the document.


The long term effects of dampening (ie on vibrations) wouldn't affect the collision since the ball is well and off the blade as you're aware of. However the immediate dampening/inelastic effect of material, mostly the rubber and wood, would effect the energy lost during collision. That's why I mentioned thinking of this like a simulation rather than modeled equations, mostly to consider the details of what's actually going on during the relevant timeframe.

In this case what's going on is the ball is largely the object doing the bouncing (deforming and rebounding with its own dampening), and everything else is better framed as a modifier to this behavior (in this case the dampening providing some "cushioning"). That's actually part of why these loosely affiliated indicators sort of work out: incidentally because nobody thought to change the main body of relevant factors. That and they're being used in the "slopey" part of COR, where any positive slope we're comparing against close enough to 1 has the appearance of sane results.

-----

It's also worth stepping back and considering why these indicator happen to work. For frequency it appear for an object of given size, higher vibrational freq are correlated with lower energy loss. Something like elastic stiffness is similarly correlated with kinetic energy loss (of the ball here) in inelastic collisions. Note both of these are inverse correlations with the ball's collision COR. For visual thinkers, their inverse correlate to the loss, in a COR plot the space between that wolfram function and y=1.


Edited by AgentHEX - 12/27/2014 at 5:58pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jrscatman Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 6:25pm
This is quite interesting, so from a blade design point of view: what are the most important things a designer should focus on when building the ideal wood blade. 

Also thought a short film interlude might help everyone to relax a bit: enjoy voilin wood resonance.


...




Edited by jrscatman - 12/27/2014 at 6:26pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 6:34pm
Originally posted by fatt fatt wrote:

It is harder to be the one starting with an allround classic + moon and discover with the same setup overtime new sides of the game as they get slowly better.



I think it's easier to learn a slower setup since it provides a larger range of physical motion to work with. A racket that doesn't allow the user to manipulate the ball as much least they miss tends to nudge players down a more tactical path. But this is still a sport where physical motion is more foundational
given that better players can easily beat lesser ones with simply better shots, whereas the same can't be said for strategy as anyone who's ever been beaten by a kid with a proper loopdrive can attest to. That's why effective development programs mostly focus on getting the quality shots down first, and it's just easier for the less than athletically gifted (perfect timing, perfect swing control, etc) to do so with slow control equipment.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 6:41pm
Originally posted by jrscatman jrscatman wrote:

This is quite interesting, so from a blade design point of view: what are the most important things a designer should focus on when building the ideal wood blade. 

Also thought a short film interlude might help everyone to relax a bit: enjoy voilin wood resonance.


The thing is it's pretty easy to make a TT blade because the ball doesn't stick around for the vibration/resonance or other wood properties. It does change how you feel/perceive the equipment, but not so much the shot.

It's understandable that's not what anyone least a community heavily invested in equipment wants to hear. But to be fair, I also have a rather large bag of TT blades and rubber (really they wouldn't all fit in any bag).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jrscatman Quote  Post ReplyReply Direct Link To This Post Posted: 12/27/2014 at 7:05pm
Originally posted by AgentHEX AgentHEX wrote:

Originally posted by jrscatman jrscatman wrote:

This is quite interesting, so from a blade design point of view: what are the most important things a designer should focus on when building the ideal wood blade. 

Also thought a short film interlude might help everyone to relax a bit: enjoy voilin wood resonance.


The thing is it's pretty easy to make a TT blade because the ball doesn't stick around for the vibration/resonance or other wood properties. It does change how you feel/perceive the equipment, but not so much the shot.

It's understandable that's not what anyone least a community heavily invested in equipment wants to hear. But to be fair, I also have a rather large bag of TT blades and rubber (really they wouldn't all fit in any bag).
But you must agree, each blade plays differently, how would you account for the difference? JRSDallas suggested frequency, is there some other property or properties we can use to measure blade performance?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/28/2014 at 6:27am
It's worth taking another step back for a meta-discussion of this topic. The holy grail we're after is the ratio of the ball's speed coming off the blade divided by its speed going in for all such speeds. This can be done by a machine which controls for and measures ball speed, or equivalently by drop the ball from ever greater heights.

Since such a contraption is a pain to build, and meticulously dropping and measuring heights is also bit annoying (the ITTF does this for one height on the sanctioned balls against a hard platform, effectively placing a ceiling of ~.85 on the ratio above). Thus all these other numbers advanced are just somewhat more amenable proxies for this value. They're useful enough to indicate this blade is likely faster than that blade, but even in a more limited capacity they each have advantages and drawback. Personally I just look at the thickness with some adjustment for harder or softer plies (esp composite ones), and it's ~80% effective for <<20% of the work.

As to playing differently, other than this speed metric it's just a subjective feel of what happens after the ball's left the blade. Some people for whatever reason don't like certain sounds or vibrations; for example I've been told ~ALL wood blades feel hollow, and a hard sponged Yinhe Moon I just tried on a Sweden Classic sounded broken/cracked etc.  This has a significant internalized effect on confidence to swing for shots, etc; and because of that discomfort we're not likely to put in time to get used to a setup (compare this to folks expending considerable effort adjusting to pro setups obviously too fast for them, for similar psychological causes).

Of course there's no social currency in presenting this as a largely subjective evaluation, so there's a tendency to attach "reasons" via peudo-technical terminology to support a personal bias. Blade X is good because of this and that nebulous aspect. As mentioned manufacturers are more than happy to play this game of attributing near-magical properties to a very easy to make piece of wood/fiber.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote JacekGM Quote  Post ReplyReply Direct Link To This Post Posted: 12/28/2014 at 1:55pm
This is a fantastic thread. Some people well familiar with Newtonian physics take on the aspect of how blade design will affect it's performance. Great, and thank you, and please continue... 

However, the problem I have here is that the OP title is - to me - actually misleading. For I do not care that much about blade performance (does anyone?) Rather, I care about racket performance. Although the influence of the rubber sheet(s) has been, shyly, mentioned here and there above, there is this formidable component conveniently dropped from this discussion altogether: the quality and quantity of the glue layer(s). Oh, and the various types of handles, too, and also how the type of rubber on the other side affects the performance of a given racket side...
Oh, boy... I do have a problem, I guess because I play with a racket and not with a blade...
(1) Juic SBA (Fl, 85 g) with Bluefire JP3 (red max) on FH and 0.6 mm DR N Desperado on BH; (2) Yinhe T7 (Fl, 87 g) with Bluefire M3 (red 2.0) on FH and 0.6 mm 755 on BH.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pnachtwey Quote  Post ReplyReply Direct Link To This Post Posted: 12/28/2014 at 3:24pm
This thread is interesting if you are going to make your own blades.  Other than that most people will find a blade that feels good and play with it.  If one puts identical rubbers on two similar blades they won't play that differently and there is no impulse ( trajectory ) that one paddle can generate than another can't.   I don't feel there is much difference between my Samsonov Alpha with H3 Neo than my TBS with S2 on it.  Yes they feel different but I can play the same way with both.  People are very adaptable.

I find the blade makes more of a difference when playing hard bat or with long pips 0X than with inverted because there is no sponge to mask the performance of the blade.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote jrscatman Quote  Post ReplyReply Direct Link To This Post Posted: 12/28/2014 at 6:44pm
I am also very interested in the feel of the blade, but don't know how to measure it or describe it. I have been watching lot videos about luthiers on youtube. They say every instrument they make is unique. A lot of what they do is by feel. One mentioned, he doesn't measure the thickness of the wood - he just feels for the correct stiffness - when gets there he stops. 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote AgentHEX Quote  Post ReplyReply Direct Link To This Post Posted: 12/29/2014 at 2:50am
> However, the problem I have here is that the OP title is - to me - actually misleading. For I do not care that much about blade performance (does anyone?) Rather, I care about racket performance.

The same aforementioned restitution ratio exists for rubber, except since the ball grips the surface the input parameters of spin and angle need to be characterized in addition to speed alone.

> I have been watching lot videos about luthiers on youtube. They say every instrument they make is unique. A lot of what they do is by feel.

With a musical instrument the notes required are standardized frequencies which human hearing can be well attuned to.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baal Quote  Post ReplyReply Direct Link To This Post Posted: 12/29/2014 at 10:11am
Originally posted by jrscatman jrscatman wrote:

I am also very interested in the feel of the blade, but don't know how to measure it or describe it. They say every instrument they make is unique. A lot of what they do is by feel. One mentioned, he doesn't measure the thickness of the wood - he just feels for the correct stiffness - when gets there he stops. 




This is surprisingly true for blades too considering they are fairly mass produced. I have a bunch of Viscarias and two feel much h better than the others. Its not just weight. I guess that is what happens when you make things out of wood.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jrscatman Quote  Post ReplyReply Direct Link To This Post Posted: 12/29/2014 at 5:41pm
Originally posted by Baal Baal wrote:

Originally posted by jrscatman jrscatman wrote:

I am also very interested in the feel of the blade, but don't know how to measure it or describe it. They say every instrument they make is unique. A lot of what they do is by feel. One mentioned, he doesn't measure the thickness of the wood - he just feels for the correct stiffness - when gets there he stops. 




This is surprisingly true for blades too considering they are fairly mass produced. I have a bunch of Viscarias and two feel much h better than the others. Its not just weight. I guess that is what happens when you make things out of wood.
It's very much an art rather than science. 
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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 6:15pm

Originally posted by AgentHEX AgentHEX wrote:

>
I mean what you said is clear and the meaning not in dispute, and it seems you understood what I meant, too; Yes I have shown that your point-by-point critique of frequency as an indicator has been wrong point by point, the combination of which is a christmas miracle compared to most of these discussions.   Nice put down, but as my equation has not changed…the accurate statement is that you have stopped disagreeing with me.        

For example no one is disputing that frequency is some indicator, only how good it is.   You originally disagreed that frequency was even an indicator of blade speed and you still don’t understand why it is.   You are now just frustrated with having your arguments picked apart by someone who is both unwilling to be trolled and is capable of defending themselves.     

This is not a trivial question given the nature of the problem as mentioned, and thus not conducive to easy answers.   Yes it is not trivial but it is possible to solve for an idealized situation in order to gain useful insight.      

>JRSDallas said:   However since our beam is a symmetric multi-layer stack of rectangles (of different material), and having correctly applied the Parallel Axis Theorem, the correct moment for each layer is as I previously presented....A page from Stokey Chapter 7 Vibration of Systems Having Distributed Mass and Elasticity shows that I have used the correct axis of rotation and have correctly calculated the moment for a cantilever.  By extension, this means my equation is correct and that the effects I calculated based on layer placement are correct.   

HEX said:   Note that page doesn't contain your specific sub-equation for inertia of the sublayers. You know, the part that uses the PA theorem. I pointed out specifically why just that part is wrong, in very explicit detail. To be clear, I'm not contending the rest of it.   Let me just note that I am underlining what exactly what it is you claim that I have done wrong.   I took the time earlier to ask you for a clear explanation, you provided one, and I have thanked you for it.   I underline the above again to make sure it is real hard for you to deny your position and claim I am have twisted your words.    Sidebar: There is also the issue of your gloat post that followed your careful explanation but was removed by the moderators.  Removing it was the right thing to do, but as I had already read it, and it is now insanely funny.    

I’ll explain more in an ELI5, but for now I just say that the page from the textbook shows the moment of a simple rectangular beam and that the calculated moment for that beam is only possible when the axis of rotation for the PA theorem is where I have shown it to be.  If we use your explanation, we cannot reach that same answer for even the text book beam.   

You added that part, which is why I called it a corrective factor (to the original equation such as in the book), because it tries to "correct" the original inertia var. Again, the contention is that the bit you added is done incorrectly, specifically because you don't use the same axis of rotation with that PA theorem subequation.

Again, it would really help if you just drew out where the axis around which the inertial is calculated for the original equation and your two PA axis's (you know, the ones mm's apart). I'd do it but this is one of those things which is more illustrative if you did yourself.   So what you are actually saying in your coded language is: “Alright, I HEX am still confused, but will never admit it.  Please show me a diagram while I pretend that I am giving you a lesson.”  

OK here is an ELI5 response:



 



Let's go through the logical steps one by one: OK but only because you’ve asked me to.

1. Your claim is that frequency is a good indicator for speed [Yes], “ostensibly because it accounts for ply depth [Incomplete statement = Gibberish] (and you certainly made sure of that in your equation modification)” False statement since I did not modify the equation, I only showed how to apply it.  The above ELI5 shows this in even more detail, but the fact remains that the original equation is not changed so your statement is FALSE.    

2. However, it also gives the same answer for two sides of a blade with complete different material on each. [Misuse of the equation that only proves you don’t understand it. Solve the Extra Credit problem from the ELI5 and then use it to explore asymmetric plywood construction.]  A vibration frequency is inherent to the object and can't be "extended" away.  [Gibberish to confuse readers from the fact that you are actually saying gibberish.]  

3. Clearly 1 and 2 cannot be simultaneously true.     Both can be true if you use the right equation in the right circumstance.  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.  Many people tried to explain this to you on the One of a Kind Forum as well, but to no avail.  

What you nor anyone else have discussed, is that COR and the efficiency of collision energy transfer to any particular blade vibration mode will vary as the point of impact of the ball on the blade changes.  

Furthermore, even when we impact the ball at the same spot on the blade every time, the COR of the entire racket assembly will decline as the energy of impact increases due to increasing inelastic deformation losses in the ball, topsheet with pips, sponge, blade and hand.   Ball on racket impact physics that includes the effects of the normal and tangential COR’s of the topsheet with pips and normal and tangential COR's of the sponge and the normal COR of the blade is a far more complicated topic than my simple equation (that has caused you so much grief) so any exploration of it would only be more painful. 

Still the interesting question raised by Krantz on if a loop drive could impart more total energy to the ball than a smash can would be a cool one to actually solve.  Others have noted that it would be good to understand collision and if it can be if done to the level of providing a useful insight, then it might be worth the trouble.

I've only pointed out a sim to speak of how a solid collision actually happens [So how are you coming with that sim approach? When can we expect you to have results?  How are you linking it to conservation of energy and momentum?  What does the speed of sound in wood tell you about how big the particle count in your sim has to be?  Have you estimated a sim calculation time?  How big is your particle size in your sim?  Since you were so taken with the low count hard sphere particle model of a bouncing ball, is your model based on hard sphere and a repulsion force in the manner of a computer game?  Are you using different particles for different materials?  Do you consider an FEA to be equivalent to a particle sim or are FEAs for quitters since they are not the end all in physical modeling?], and how it's nothing like how your frequency "extension" attempts to correct for it. [A two for one: (1) Gibberish and (2) proof that you were lying at the beginning of your post when you said you were not disagreeing!] Usually when two things are correlated in a relevant way in the natural world they work through similar mechanisms. [Gibberish with a glimmer of hope that you might be trying to say we should consider the physics behind an issue.] That's for example one way we can tell piracy and global warming aren't.  [Whoo hoo – Recognition that causality and correlation are not the same!]

> So when I followed your link it did not lead to a plot, just an equation with no discussion of causal parameters. 

Just FYI, but Wolfram Alpha by default plots any sane equation [You’ll have to post an equation before claiming it is sane.] as it d, which is why I used it for illustration. [You’ll have to post an illustration before claiming it shows something] It's not meant to be causal [So you've just admited that you make no causal physics claims and instead are using gibberish that is no more illuminating than how the number of pirates effects global temperatures], just a point that the right answer for speed vs anything is that general shape thus anything quite dissimilar is well off. 

The only reason why stiffness or freq even work at all is because COR for a TT blade is somewhere in the ~0.5 ballpark where all these indicators have slopes close enough to ~1 to not matter too much.   [All talk, no physics.  Let do just a bit of physics.  Hmmm….the equations for simple head on elastic collision between two masses mblade, mball with converging velocities vblade and vball =0, (ball at rest) shows that the ball rebound velocity  vball’ = 2* mblade * vblade / (mblade + mball).    Uh oh, it looks like the rebound speed of the ball vball’ approaches infinitiy when the blade speed approaches infinity!  The rebound speed of a ball is unbounded!  How can this be when both HEX and COR says nothing about the possibility of unbounded rebound speed!  It gets worse, since for any COR value greater than zero, ball rebound speed still approaches infinity as blade speed approaches infinity.   Clearly COR is no indicator.  COR is bounded between 0 and 1, and without additional information it doesn’t seem to explain anything about actual ball rebound or blade speed.  But hey wait a minute, frequency is unbounded also, oh and so is blade velocity, oh and so is the length of an imaginary line, oh and so is the happiness in a child’s smile, and …..]      

The other related point that was probably missed is that if you simply transform whatever other "kind of correlated" indicator into that shape it'll give much better estimates. This can be done with any number of methods just as subbing x for y in a plot mirrors along the y=x axis.  [The correct statement is that I have not missed the point that physics is tied to physical parameters and that I unlike you, have not relied on as you say “kind of correlated” indicators such as pirates versus global temperatures or your admitted reliance on “it’s not meant to be causal” gibberish.]



Edited by JRSDallas - 01/01/2015 at 8:22pm
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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 8:14pm
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.
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