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PostPosted: 27 Aug 2015, 01:33 
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Lhet wrote:
Uh. I'll post too because I slightly disagree.


Throw is something you can feel, and is almost all in the rubber. If you try somebody else's setup and find you really have to close your bat angle to land it on the table, then it's higher throw. This is probably somehow proportional to the sideways deformation and rebound of the rubber: If the rubber absorbs spin by deforming sideways and the ball leaves before it rebounds, then you have a somewhat spin killing rubber with low throw. If the rubber absorbs spin and almost immediately rebounds while the ball is still on it, then you have a super spinny rubber with high throw. The blade can matter a bit, but the term is most useful for comparing rubbers.

Catapult is mostly in the sponge. It's probably some sort of relationship between how soft the rubber is and how much energy it returns (which can by measured by dropping a ball and seeing how many times it bounces). It's basically how much more power there is in a returned shot vs. a shot that you hit out of your hand. Even though this is largely in the sponge, a soft, stiff blade (e.g. Hinoki) can also add to this.


If you search among the arguments that were going on this last year, you'll find a certain person arguing that rubber sheets and blades don't bounce back fast enough to impart any "catapult" to the ball, that all they could do was TAKE AWAY from the impact, that all the "catapult" was in the deformation and "bounce back" of the ball, that rubbers (and blades) with high catapult were those that absorbed LESS energy than others did. I don't quite believe this, I didn't see any actual evidence presented (other than assertions that contact times were in the order of milliseconds, which, even if they were, it hadn't been shown that the "bounce back times" of the rubber and blade were slower than the contact times). It's also recently occurred to me that, if this were true, why does the ball bounce back so much faster than the blades and rubber - if it's the material that matters, then squash or racketball balls wouldn't bounce off rackets at all, since they're made of rubber and can only (as he asserts is true with table tennis rubbers) absorb energy.

Anyhow... he did point out that table tennis balls bounce higher off hardbats than off inverted sandwich. Which is true.. but those are low speed impacts to begin with. The sponge hasn't even begun to deform yet.

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PostPosted: 27 Aug 2015, 11:24 
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iskandar taib wrote:
why does the ball bounce back so much faster than the blades and rubber - if it's the material that matters, then squash or racketball balls wouldn't bounce off rackets at all, since they're made of rubber and can only (as he asserts is true with table tennis rubbers) absorb energy.
I'm sorry but this is completely misleading. You cannot compare the dynamics of a hard plastic (or celluloid) ball bouncing off a rubber covered piece of wood against a soft rubber ball bouncing off a racket with strings. The two processes are totally different. Probably the only thing they have in common is that they both require someone to swing the racket to provide the necessary force to make something happen.


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PostPosted: 27 Aug 2015, 12:40 
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iskandar taib wrote:
If you search among the arguments that were going on this last year, you'll find a certain person arguing that rubber sheets and blades don't bounce back fast enough to impart any "catapult" to the ball, that all they could do was TAKE AWAY from the impact, that all the "catapult" was in the deformation and "bounce back" of the ball, that rubbers (and blades) with high catapult were those that absorbed LESS energy than others did. I don't quite believe this, I didn't see any actual evidence presented (other than assertions that contact times were in the order of milliseconds, which, even if they were, it hadn't been shown that the "bounce back times" of the rubber and blade were slower than the contact times). It's also recently occurred to me that, if this were true, why does the ball bounce back so much faster than the blades and rubber - if it's the material that matters, then squash or racketball balls wouldn't bounce off rackets at all, since they're made of rubber and can only (as he asserts is true with table tennis rubbers) absorb energy.

Anyhow... he did point out that table tennis balls bounce higher off hardbats than off inverted sandwich. Which is true.. but those are low speed impacts to begin with. The sponge hasn't even begun to deform yet.

Hmm. Just thought about this a bit it's probably actually true. I'll try to explain it as I assume it works. A ball has very good energy retention. It doesn't deform much, but the deformation that occurs is rebounded very quickly, without losing too much energy (that's why it'll bounce so many times on a hard surface; it certainly isn't the table that's doing all the rebounding).
To put it a weird way, basically the ball wants to be a perfect ball, and any time it isn't a perfect ball (due to being deformed) it'll be trying to reform into a perfect ball. When it bounces on a table, it'll be deformed by having a slightly flatter bottom. It'll then reform by unflattening the bottom, and this unflattening energy is the bounce.
Rubber can totally deform and rebound, however, it's way way way slower in this process than the plastic of the ball. By the time the rubber stops deforming in (meaning the ball has stopped pressing hard enough to deform both of them), then they both rebound. Course the ball deforms and reforms way way way faster than the rubber; it's impossible for the much slower rubber to exert rebounding force, as the ball is gone by then.

So all that matters is how the rubber deforms the ball. Two things are important. First, the rubber and the ball are both rebounding or deforming together. If the rubber is deforming, so is the ball. On a topspin, the rubber catches the ball, and they both spend some time deforming. A spinny, high-throw rubber will spend a lot more time more deforming sideways than outwards, which will give the ball a lot of time to also deform it's top. A low throw rubber will spend more time deforming outwards, so the ball won't build up as much spinning energy and will bounce off in a classic sort of way.

iskandar taib wrote:
if it's the material that matters, then squash or racketball balls wouldn't bounce off rackets at all, since they're made of rubber and can only (as he asserts is true with table tennis rubbers) absorb energy.

nah. It's just that TT rubbers don't manage to release energy fast enough to keep up with the plastic ball. If a squash ball is used then maybe the TT rubber won't be completely outpaced and will rebound faster than the ball, though it's likely that even if the rubber is faster, it wouldn't have enough stored power to overcome the squash ball's deformation, and the squash ball would still end up mainly using it's own rebound to take off.

e. Hm. I suppose the reason soft (e.g. hinoki) blades add power is because they give the ball even more time to deform and dig into the sponge.


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PostPosted: 27 Aug 2015, 14:49 
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birchamboi wrote:
I'm sorry but this is completely misleading. You cannot compare the dynamics of a hard plastic (or celluloid) ball bouncing off a rubber covered piece of wood against a soft rubber ball bouncing off a racket with strings. The two processes are totally different. Probably the only thing they have in common is that they both require someone to swing the racket to provide the necessary force to make something happen.


It is entirely possible to hit squash/racketballs with wooden paddles - there's even an organized game with rules played on handball courts called "paddleball". If rubber isn't capable of "bouncing back", then this wouldn't work at all.

Not to mention said balls do bounce off walls and floors just fine (and so do solid rubber "superballs"). Even hard wooden balls bounce.

As I've often pointed out - there's a great deal of "theory" (or "mumbo jumbo" if you're not feeling charitable) in table tennis but little of it is supported by actual physical evidence (as may be collected using high speed cameras, strain gauges and the like). Until someone actually publishes such evidence we'll all forever be in the dark, using terms like "dwell time" and "soft" to describe what we feel when we hit the ball.

Iskandar


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PostPosted: 27 Aug 2015, 23:21 
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This is always a fun, repeated, discussion on TT forums.

I think much of the confusion comes from the fact that there are way more variables lending itself to "Speed" than we really consider.

It's not just about the macro elements of the equipment, rubber/sponge/paddle. It's not even solely about the composition of those materials (which adds a layer of complexity). The structure of the material (long pips, short pips, narrow pips, fat pips, density of pips, rubber density, sponge density, sponge structure) further confuses the issue of catapult. Engineers will all tell you, structure is everything. Just look at memory foam beds. There is no way one would ever say that the sponge used to create that material INCREASES catapult. It's sole purpose was to help astronauts absorb enormous G-forces at liftoff and re-entry. Absorption, based on a combination of structure and composition.

Last, how do these variables then react to the biggest variable of them all, the person. Everyone's stroke is different. What are they doing? Passive blocking? Chopping? Looping? Is the bat coming forward? How fast? Is the bat "catching" the ball? Does the user have soft hands at the time of impact, or is he solidly gripping it. All of those further confound the situation because they affect the perception of "Catapult and speed."

After all of the considerations and variables on material, I believe there is a pretty linear relationship between absorption and catapult based on racket speed. I'm guessing here, but I'd theorize that at zero forward motion the sponge has more properties of absorption than catapult. Then as the bat increases in forward speed, this ratio on a gradient, changes from absorption to catapult, where at the highest racket speeds we perceive rubbers with max sponge and elastic rubbers to be fast.

Of course, then you have to consider the speed of the ball, which is yet ANOTHER variable.

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PostPosted: 28 Aug 2015, 01:14 
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It's also entirely possible that there isn't really all that much in the way of differences between, say, blades, other than weight and actual speed (read: bounciness) (and things like handle shape). All the other (intangible, hard to define and agree upon) differences - "hard" vs. "soft", "throw", "stiff" vs. "flexible", "dwell time", etc. are all in our minds, and are influenced by how things FEEL (and perhaps how impacts sound). And this feel would be related to the vibrations we perceive through the handle.

Iskandar


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PostPosted: 28 Aug 2015, 01:56 
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This entire reply is to Japsican:

I've talked before about my rationale for choosing a favorite player and just trusting their judgement about all of these matters. (And if I get bored with that player I should choose another player, not start measuring pips etc.)

Suppose I have chosen such a player. Having made that choice, it does not matter whether or not he/she has any conscious understanding of the physics, properties of materials, etc., as the rubber and blade they're using *is* *the* *exactly appropriate* material for *that* game, with video documentation that I can study. How could I say that better? (Assuming the rubber and blade can be identified), it's just a logical fact. That's the rubber! Nothing to analyze!

Once I've come that far (acknowledging that it makes no difference whether or not my favorite player understands these things consciously), then I can easily accept, and set my mind at ease, that it is not necessary that I understand them either. And then I can accept that all of this verbiage (though it can be seductively interesting (but less seductive if you keep in mind how much of it is total nonsense)) is a terrible, terrible waste of time. (But we all already know that.)

(Still talking to Japsican:) It's funny. I'm deeply involved in another hobby, with an associated forum, and I've had to accept that the reading and posting I've done there (over many more years than here) has also been almost entirely a waste of time, as I should have been really studying and practicing instead of talking about it. There are a lot of smart people there, and a lot of strong players here, but we should get a coach and start drilling, just as Carbonman and others have told us. I would be sweating, right now, if I really cared about this game. What to conclude from that, and what to do about it? Just a rhetorical question. :-) See you later. :-)


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PostPosted: 28 Aug 2015, 02:21 
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Ha, nice to hear from you Zhao. Not sure if I'm reading your post correctly. But to be clear, my post is specifically speaking to the fact that...it is true folly to really attempt to understand the speed and catapult of any piece of equipment as the variables are too great.

So...I think you were replying by saying you agree? Or maybe, forget about puttingforth energy towards thinking about this stuff.

Personally, I enjoy trying to understand things, although I'm my not an engineer. My science is with healthcare and trying to connect it with technology...blood and guts with plastic and metal! Talk about variables!

Either way, I am still Emulation Junkying these days, so there's that. :rofl: :lol:

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PostPosted: 28 Aug 2015, 02:42 
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Yes, I'm agreeing.
Byebye for now!


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