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Blade Performance Vs Wood Type and Design

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jrscatman View Drop Down
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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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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.
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= 184.8g
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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
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+ 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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