Will it burn: Various TT Ball Halves Ignited

AgentHEX wrote: Hopefully you have a link since I don't know what you're saying. The DHS balls really don't have a bounce problem at least in ITTF's tests.

http://tabletennisengland.co.uk/wp-content/uploads/2013/04/Poly_Celluloid_Balls_Testing.pdf

I think Joola cell is also DHS same as what Jay tested.

The reply above is to Jay.

> Why do you think it is PVC?  I am not a materials expert so that is an honest question. 

It's just an example, all such plastics I checked are better Young's modulus than celluloid. I arbitrarily picked PVC because I recall it has almost same density.

> About the rest, if Debater's data are close to accurate, they will not pass Jan 2016 tests,

Why not? Jay's test shows ~255mm for cell and Debater's test you mentioned was ~1cm lower (I didn't bother to check again). The cutoff is 240, the temp cuttoff is 237. Debater's actual results per se aren't accurate (but precise) because he bounced off wrong plate, but the relative delta is about right.

AgentHEX wrote: The base plastic materials like PVC these balls are made of tend to be significantly more elastic than celluloid so there's little reason for inherent low bounce, esp given other balls made of similar stuff. There's also zero evidence I've seen that they don't conform to 2016 bounce specs already. The difference to amended spec is only 3mm or ~1% anyway.

There may be little obvious reason but there must be something because they clearly bounce low.  I have no idea what they are made of, maybe you do.  Why do you think it is PVC?  I am not a materials expert so that is an honest question.  About the rest, if Debater's data are close to accurate, they will not pass Jan 2016 tests, but they might squeeze by on the low end under ITTF conditions.  Still, they are always lower than every celluloid ball and the other plastic balls too.  Always.  There are all sorts of not to fancy ways this can be shown.   The larger problem they face in the future, though, is that DHS/Jooola plastic are quite far from meeting weight specs.  I have not yet seen a single one that would pass!  My laboratory balances are very accurate (I don't normally use them for ping pong balls).  Personally I don't think the weight is why they suck.  It is the low bounce.

AgentHEX wrote: I'm only deducing from reproducible data for reasons you can be sympathetic to; otherwise the new ball might have more gears but less catapult and/or throw. Hopefully the direct hardness test (across both axis types) and its corresponding speed equivalency point can put your mind more at ease.

I am no engineer but the static bounce height strongly predicts whether people generally like to play with a plastic ball or not.  It is the single best predictor and I have actually have done some blind tests.  Of course, it is certainly possible that one could collect more sophisticated data with all sorts of variations in bounce tests and other stuff too.  The important thing is that the static bounce test suffices.  Probably other properties co-vary with this one and it doesn't mean that knowing all those other physical features of the ball are not in some way interesting.  If somebody wants to do it, great.  But the static bounce height is the single most obvious deficiency of Chinese seamed balls. Anybody can see it at their club or on any hard flat surface.  They are not at the ITTF standard coming in Jan. 2016, and it has a big effect on how Chinese seamed balls sometimes behave in live play (and why I do not plan to ever buy another one).  I also suspect the underlying reason may also explain their fragility too, but that is just guessing.

AgentHEX wrote: > > All the more reason to think that a ball with a more consistent structure might produce a more consistent bounce.  Except it doesn't at all empirically. Should we do a bounce test from 2x, 3x the height and see if it matters?

I think we'd have to do slow motion video studies of some ball flights to settle this.  While practicing chop against an over 70 hardbat player, his forehand drives frequently bounced well over my shoulder height - well more than 30cm and possibly over 50cm.  I'm pretty sure that high level players regularly loop well more than six inches above net height.

> The forces involved are much less that we typically see in even a low level match.

I recall a study of college level players (>>2k) showed ~20m/s top loop speeds. The ball drops significantly in speed (maybe ~1/3) at the other end of the table. This is for top end scenario, not typical club level shots. Most such fast shots taken are taken at near table level and don't clear the net by much, so the ~15deg angle isn't off by much.

Also, I'm pretty sure 2x speed isn't 4x force unless maybe you assume same stopping distance, and we can at least agree the ball is elastic and not magical.

> I see the errant bounces in my chopping drills against loops and forehand drives.

In your shots or their topspins? Can you experimentally/observationally control for their spin?

> All the more reason to think that a ball with a more consistent structure might produce a more consistent bounce. 

Except it doesn't at all empirically. Should we do a bounce test from 2x, 3x the height and see if it matters?

> Small cracks can have very minor (essentially not noticeable in play) affect on the bounce.

I'm talking about balls substantively cracked. Most of the time they're not landing on the crack yet don't bounce right at all since the structural integrity of the whole is affected.

> Players seem to be very much prejudiced by the sound that a ball makes.

I've mentioned this before somewhere as a possible source (I also think one of the main reason "bad" tables "bounce weird" even if they're uniformly rigid). Maybe it just sometimes sound, not actually bounce, different.

I've explained why I didn't think it was case. If you found it unconvincing, that's fine, but do note that ITTF's 30cm drop contacts at ~2.5m/s and a good 10m/s hit at 15deg is also ~2.5m/s on the table at contact. Empirical evidence shows the DHS bounces more consistently regardless of edge or not. This is why I ask about the specific shots it supposedly affects for clues of where the elusive "weirdness" lies. [/QUOTE]

305mm is about twice the net height.  So a ball dropped from 305mm (as in the ITTF test) hits at about the same velocity as would a ball in a typical pushing drill.  The forces involved are much less that we typically see in even a low level match. 

10m/s is not a very high velocity for a table tennis shot.  Even club amateurs will routinely hit at twice that speed.  Further, such balls are  usually hit with topspin.  We learn to hit with topspin early on as a way to keep the ball on the table.  The topspin accelerates the ball downward and toward the table.  The greater the horizontal speed and speed of the spin, the greater the force as the ball is driven into the table. Furthermore, gravity is acting on the ball.  So that's adding to the balls velocity.  In short, any topspun ball that traveled more than one foot above the table before striking it surely strikes it at a greater vertical velocity than tested in per the ITTF T3 pamphlet. 

Given that loop drives can surely be 40mph (nearly 20m/s)or more, clearing the net by well more than six inches (150mm) and loaded with topspin, its seems to me that we can easily expect balls to hit the table at twice the t3 test velocity or more.  My bet is that it is sometimes much more.  Twice the velocity equates to four times the force.

So, like I said before.  The t3 test is suspect as a test for consistency of bounce in play - on the velocity of impact alone.

AgentHEX wrote: It also seems different "experts" have differing opinions (such as on the DF vs cell, etc) and I'm not a fan of coincidental statistical consensus given its history of reliability.

Sure.  I understand the hesitation.  But then, I'm not calling for conclusions. Also, I'm observing the consistency of bounce independently.

As far as the specific shots where the inconsistency shows up, I see the errant bounces in my chopping drills against loops and forehand drives.  I don't recall seeing them in over-the-table low velocity play. 

LOOKING FOR A BETTER PLASTIC? THEN YOU SHOULD TAKE TO THE NOVELTY BRANDS......

No more modification/improvement to the existing plastic is expected.
REASON (1) Players will not accept any further amendments to the current ball, for not liking to repeatedly readjust their playmode to a ball over and over again.

(2) Similarly, the major china manufacs will not going to re-pay the heavy approval fees so many times. Not they !!

The current plastic balls we have now got on market are most likely to stay just the same for a most prolonged time.   
It is the newly listed ball brands that will sure vary their perfomance and material composition.

So, if you want to have a different (better?) ball you always need to try a fresh ball brand.

Hopefully,, Mr, Baal is the very person to do all the job for us. He is strongly required to collect all the plastic novelties to come up on market onwards and to make a thorough instrumental examination on each one new brand. Hopefully ...

I've explained why I didn't think it was case. If you found it unconvincing, that's fine, but do note that ITTF's 30cm drop contacts at ~2.5m/s and a good 10m/s hit at 15deg is also ~2.5m/s on the table at contact. Empirical evidence shows the DHS bounces more consistently regardless of edge or not. This is why I ask about the specific shots it supposedly affects for clues of where the elusive "weirdness" lies.

It also seems different "experts" have differing opinions (such as on the DF vs cell, etc) and I'm not a fan of coincidental statistical consensus given its history of reliability.

The celluloid ball also had the fat seam and it seemed ok.

Sometimes we tend to place too much faith in generalizations from sparse data. Remember when seamed-type plastic balls were considered a dead end? And the opposing seamless ones before that? Yeah, good times.

> Anyway, my point is that I believe players observe "weird" results all the time and that this could very well be an indication of inconsistent ball performance.  I don't think we can conclude that we are not experiencing and seeing inconsistent ball rebounds from rackets.

That is reasonable, which is why I asked for more specific details to isolate the effect. Hopefully you can help remedy the current dearth of feedback.

AgentHEX wrote: > But you forget that the racket has two to four millimeters of cushioning sponge that makes that collision quite different than the collision with the table. I considered it but didn't think it was significant. It's worth pointing out that contrary to popular perception it's mostly the ball doing the rebounding in TT interactions. The viscoelastic time-variable properties of rubber are far greater. IOW, the ball is in a similar state before bouncing off either. So some due diligence was done, but it's possible there's something else I'm not seeing. > And players report weird shots all the time in my experience.  I've thought about this to some length roundabouts here: http://ooakforum.com/viewtopic.php?p=285955#p285955. IMO "weird" as commonly used is not synonymous with "inconsistent".

Edited by wturber - 11/25/2014 at 5:31pm