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PostPosted: 25 Nov 2014, 03:53 
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What better way to impress and tick off the crowd than some finite element analysis backed by real experimental data of an actual table tennis blade?

Seeing is believing - the second coming!

Image

Up above are two charts illustrating the various modes of vibration of the blade Cornilleau HINOTEC ALL+. Sand(blue areas in the sim) is used to visualize the modes of vibration, namely the modal lines which are areas with no movement. Notice the picture for Mode 1 of the clamped blade is missing, as the sand moves off the entire blade head, meaning the modal line is at the clamped end for that mode, which is commonly referred to as the "diving board" mode.

Of note here is that there is an established view in baseball, tennis and cricket that a bat or racquet when hand-held can never reach the grip strength of a vice, nor does the mass and grip strength of the hand or the vice have any effect on the impact as the ball leaves well before the vibration can travel down, reach the barrel end or handle and make it back to the ball in time.

Hence, it follows that table tennis blades would behave in a similar fashion as if freely-supported but with additional damping from the hand. However, JRSDallas suspects otherwise given the size difference.

Anyhow. Please pay attention to where the bend is at on the blade head.

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PostPosted: 25 Nov 2014, 04:18 
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I'm sure anyone following the technical discussion can figure out what's going on, but I've also been accused of being unnecessarily mean so for those along for the social drama:

Imagine it was said no way this guy's ever set foot in a decent educational institute, and the reply was that they have indeed worked as a sanitary-engineer at a prestigious school, plus dictionary and idiomatic definitions of "set", "foot", and "set foot in", thus rebutting such a claim.

Imagine the thread going for pages as various co-morons (to be fair I was one the first time, http://www.urbandictionary.com/define.php?term=Co-moron) attempt to explain the relationship between "set foot in", physical presence vs. attending a school, with zero resulting comprehension.

This is IME the average conversation involving zeio.


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PostPosted: 28 Nov 2014, 22:32 
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Blade: Joola CWX
FH: S&T Secret Flow 2.1
BH: Troublemaker ox
This is cheapest possible "Walmart" blade I borrowed from sport center. It's bad, thin and slow as hell. Seems to be 3 ply construction.

It flexes noticeably, when pressed with fingers.

Image

Image

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PostPosted: 29 Nov 2014, 00:31 
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Roy wrote:
This is cheapest possible "Walmart" blade I borrowed from sport center. It's bad, thin and slow as hell. Seems to be 3 ply construction.

It flexes noticeably, when pressed with fingers.


Now we're talking! Is it a good looping blade? ;) Note also where it bends - it's not along the entire length, it bends at the handle joint, just where it ought to.

Iskandar


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PostPosted: 29 Nov 2014, 03:09 
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That's pretty much an exaggeration of how thin ALL blades flex, which is expected for its even thinner profile. Even seemingly minor diffs here can significant given the polynomial thickness-stiffness relationship.


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PostPosted: 02 Dec 2014, 15:34 
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iskandar taib wrote:
The blade is wood. If it doesn't deform or move it's behaving just like that block of steel. It obviously isn't, since different blades behave differently. I can accept that it's not springing back and adding acceleration to the ball, but if it's causing energy loss I see no way for this to happen except that the surface is moving in some manner. The fact that you can hear it bears that out - the ball causes the sound, the sound is the wood vibrating. Vibration is movement. I remember in the past you brought up cardboard. Why is cardboard so "slow"? Because when the ball hits it, it gives. The wood in the blade is doing the same thing, just less of it.


Wouldn't an easy test of this be to FIRMLY clamp a variety of blades into clamps and drop from varying heights a TT ball several times, and check out the resulting bounce height? Enough balls and enough drops from enough heights to be statistically significant. No rubbers or sponges, just the bare blades. That could speak to the "flex" in a blade.

A second test would be to rest the blade flat on a solid block of steel or something - that would help to determine whether any differences were due to the surface (material) of the blade or any flexing from the neck or something. That could speak to whether anything seen in the first example was somehow related to the surface.

No?

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PostPosted: 02 Dec 2014, 16:16 
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The former would build a COR profile (% rebound vs impact speed) itself, and the realistic limit would be the ~80% ittf spec. It would more or less fully characterize the blade. These are usually start reasonably flat and taper off on (the equivalent of) harder hits.

The latter experiment might be interesting because if it's secured down well enough it should be that ~80% minus the surface + "mush" effect of the blade, because the rest of the blade would have ~infinite rigidity. It should be close to that limit in general but materials like balsa will have more leeway on softer-moderate impacts (just as they should also roll off a bit on the left side on the COR graph).


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PostPosted: 02 Dec 2014, 19:14 
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iacas wrote:
iskandar taib wrote:
The blade is wood. If it doesn't deform or move it's behaving just like that block of steel. It obviously isn't, since different blades behave differently. I can accept that it's not springing back and adding acceleration to the ball, but if it's causing energy loss I see no way for this to happen except that the surface is moving in some manner. The fact that you can hear it bears that out - the ball causes the sound, the sound is the wood vibrating. Vibration is movement. I remember in the past you brought up cardboard. Why is cardboard so "slow"? Because when the ball hits it, it gives. The wood in the blade is doing the same thing, just less of it.


Wouldn't an easy test of this be to FIRMLY clamp a variety of blades into clamps and drop from varying heights a TT ball several times, and check out the resulting bounce height? Enough balls and enough drops from enough heights to be statistically significant. No rubbers or sponges, just the bare blades. That could speak to the "flex" in a blade.


Not the "flex" (as I defined it), but it would test the "speed" of the blade. I suppose you'd also want something that could fire a ball at (various repeatable) high speeds towads the blade, and then measure the resulting speed of the ball coming off the blade. Since we do use sandwich rubber in the game, doing this experiment with (a standardized sheet) of rubber would also be interesting.

But this won't actually measure blade deformation, you'd need photographs and strain gauges. For most blades the movements due to vibration will be tiny - maybe in the tens of micrometers, maybe 100 microns (0.1mm) max. If the blade is really FLEXING (i.e. bending along its length) to any great degree this should be visible when the ball hits, especially if projected at high speeds.

Quote:
A second test would be to rest the blade flat on a solid block of steel or something - that would help to determine whether any differences were due to the surface (material) of the blade or any flexing from the neck or something. That could speak to whether anything seen in the first example was somehow related to the surface.

No?


You still have deformation that propogates through the blade, though. Imagine a drumskin. A block of steel under a drumskin will prevent that sort of deformation as well as any sort of flex.

Iskandar


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PostPosted: 02 Dec 2014, 19:17 
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agenthex wrote:
The former would build a COR profile (% rebound vs impact speed) itself, and the realistic limit would be the ~80% ittf spec. It would more or less fully characterize the blade. These are usually start reasonably flat and taper off on (the equivalent of) harder hits.


So there's a ITTF mandated spec for maximum restitution? Didn't realize that - where can I look this up? This would mean there's a limit to how fast you're ALLOWED to make a blade.

Iskandar


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PostPosted: 03 Dec 2014, 03:31 
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That's for the ball per t3 spec, not blade. Recall that conceptually both take away from the ideal of 1, and it's already at best ~250mm out of a ~300mm bounce by fiat.


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PostPosted: 04 Dec 2014, 20:12 
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Had a look at t3. Yeah, there's a range of bounce tests on a standard steel plate (which would behave like a rigid substance, I suppose). Restitution COULD be higher if you bounce the ball, instead, off something elastic, such as sponge rubber. If your blade DID behave like a drumskin (or sheet of rubber), then you could also theoretically exceed the bounce height off steel.

Iskandar


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PostPosted: 04 Dec 2014, 20:29 
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Rigid isn't the opposite of elastic but instead correlated. So the blade is more elastic than the rubber.

Some form of a resonant bounce like jumping on a trampoline is possible but the reason why it doesn't happen is the ball itself has a frequency higher than the blade or rubber. Same as analogy before of the ball jumping way too fast on the racket trampoline; it gets nothing from the trampoline's rebound.

The dwell is also the least (in time, highest in freq) of these, which of the ball is ~1500hz or ~6-7ms. I show some rudimentary calcs here: viewtopic.php?f=69&t=26354&p=286427#p286427

The blade's main bending freq is a few hundred hz, and rubber is even less.


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PostPosted: 04 Dec 2014, 20:42 
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So you're saying the rubbber/sponge does NOT add to the bounce?? It doesn't store energy and release it to the ball when it leaves the bat?? If that's the case a flat hit with hard rubber - or with a bare blade - would be faster than a flat hit (in the strict sense) with sandwich. I don't think that's the case at all.

I think someone really needs to perform some experiments and settle this - all this theorizing isn't getting anywhere.

Iskandar


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PostPosted: 04 Dec 2014, 21:25 
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> So you're saying the rubbber/sponge does NOT add to the bounce?? It doesn't store energy and release it to the ball when it leaves the bat??

Yes, this is what must be like to grasp the earth goes around sun and not other way around.

This is probably the core misconception of let's call it naive TT theory that's dominated forums to this point.


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PostPosted: 13 Dec 2014, 14:00 
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iskandar taib wrote:
So you're saying the rubbber/sponge does NOT add to the bounce?? It doesn't store energy and release it to the ball when it leaves the bat?? If that's the case a flat hit with hard rubber - or with a bare blade - would be faster than a flat hit (in the strict sense) with sandwich. I don't think that's the case at all.

I think someone really needs to perform some experiments and settle this - all this theorizing isn't getting anywhere.


I don't believe experiments are necessary. Anytime you move away from a perfectly elastic collision energy is lost to deformation.

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