|
|
Will it burn: Various TT Ball Halves Ignited |
Post Reply | Page <12 |
Author | ||
AgentHEX
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
> 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. |
||
Sponsored Links | ||
AgentHEX
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
For the record I recalled the T3 spec also has force measurements, and sure enough it's 50N (bit more than 10lb) with ~0.7mm give both on the seam and poles.
This is the equivalent to a certain contact velocity assuming relatively elastic bounce, using system energy: k*x^2/2 = m*v^2/2, v = sqrt( (50/0.00075)*0.00075^2 / 0.0027 ) ~= 3.7m/s Which is a fairly generous shot, also equivalent to drop from ~70cm. --- Interesting it's possible to estimate ball rebound frequency (ie dwell-time proxy) from this. For a spring harmonic oscillation freq = sqrt(spring-constant/mass) / 2pi, and thus: sqrt((50/0.00075)/0.0027)/(2*pi)~= 800, which is twice the dwell given it's a full cycle, or ~0.63ms This is consistent with video evidence and hollow sphere particle-based sims I've seen. |
||
wturber
Premier Member Joined: 10/28/2008 Location: United States Status: Offline Points: 3899 |
Post Options
Thanks(0)
|
|
Yes, the ball slows down significantly. Part of that is parasitic drag and part of that is the horizontal velocity being changed to vertical velocity due to the topspin. But there's a whole new wrinkle to the bounce consistency question. I looked at Pathfinder's video again and his method of dropping the ball appears to introduce inconsistencies all by itself. He rolls the ball through an opening in a bottle. In some shots the ball seems to drop cleanly, rolling off of the lip. That raises the question of what the drop height actually is since the ball rolls downward before experiencing full freefall. But in other drops, the ball seems to momentarily catch on the opposite lip of the opening causing the ball to "freeze" momentarily at a height of around 290 mm (precisely 292 mm in one shot). In these shots we can actually observe and measure the effective release point. But it too low and It may or may not vary. It could be that it varies less on the 40+ balls because perhaps they "catch" more consistently. But that's pure speculation. You'd have to view all the videos to know what's really happening. He only shows exemplars in the YouTube video. No, I can't control for the spin. Even robots are inconsistent in the balls they throw. I can only observe a general tendency and range in a drill where the ball is being sent with the goal of consistency. Of course there is a lot of inconsistency within the drill. But there are occasional outliers that far exceed the normal range of flight paths.
|
||
Jay Turberville
www.jayandwanda.com Hardbat: Nittaku Resist w/ Dr. Evil or Friendship 802-40 OX |
||
AgentHEX
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
I tried the DHS (nittaku version) again recently and the only perhaps odd bounces I was seeing was with side/axial spins going low, but those can be hard to predict with any ball. I also ask around with decent/serious club players and couldn't find any specific answers; most seem to think the ball was not great, but OK.
|
||
wturber
Premier Member Joined: 10/28/2008 Location: United States Status: Offline Points: 3899 |
Post Options
Thanks(0)
|
|
Talk about over generalizing. Seriously, you have one test at one bounce height. The empirical evidence is extremely limited. Saying "all empirical evidence" isn't saying much. So yes, more heights (or more velocities should be tested). Also spinning or not spinning tests should be done to see if and to what degree a ball's deviation from being round matters. |
||
Jay Turberville
www.jayandwanda.com Hardbat: Nittaku Resist w/ Dr. Evil or Friendship 802-40 OX |
||
AgentHEX
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
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.
> Also spinning or not spinning tests should be done to see if and to what degree a ball's deviation from being round matters. For eccentricity to matter much it's geometrically going to have to be more oval than the ITTF allows. Also those specs are significantly tighter for plastic than cell. |
||
Baal
Forum Moderator Joined: 01/21/2010 Location: unknown Status: Offline Points: 14336 |
Post Options
Thanks(0)
|
|
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
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
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. |
||
wturber
Premier Member Joined: 10/28/2008 Location: United States Status: Offline Points: 3899 |
Post Options
Thanks(0)
|
|
I'm inducing from my general experience with materials and their behavior. The problem with deducing from the existing data is that it is so very limited. There are problems, of course, with my induction. But like I said, it is just a working theory. I was also remembering that the ITTF made an issue out of the shell thickness when evaluating the seamless ball in 2013. Their conclusions may not be correct, but they presumably came to them by analyzing empirical data from their tests. So I went back and looked at the paper. They correlated bounce consistency with shell thickness but even more-so with veer results (due to limitation of measuring only six points for thickness on seamless ball). They also implicitly admitted that the "imperfect celluloid balls" have irregular bounces.
Perhaps. I don't know the math on that and have no intent to check on it. But it's the one thing that would be worth at least doing a napkin analysis on before dismissing it. |
||
Jay Turberville
www.jayandwanda.com Hardbat: Nittaku Resist w/ Dr. Evil or Friendship 802-40 OX |
||
wturber
Premier Member Joined: 10/28/2008 Location: United States Status: Offline Points: 3899 |
Post Options
Thanks(0)
|
|
I believe my release mechanism and impact target will work well enough to test likely conformity to the ITTF spec. I may be able to borrow some Joola balls in order test them on my apparatus. My bet is that the seamed Chinese balls will be within the design spec. when tested on my apparatus. That said, they might very well have a lower average bounce. The DHS celluloid balls I tested were on the high side of the spec. |
||
Jay Turberville
www.jayandwanda.com Hardbat: Nittaku Resist w/ Dr. Evil or Friendship 802-40 OX |
||
Baal
Forum Moderator Joined: 01/21/2010 Location: unknown Status: Offline Points: 14336 |
Post Options
Thanks(0)
|
|
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
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
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.
> But it's the one thing that would be worth at least doing a napkin analysis on before dismissing it. Sure, assuming worst case for mean eccentricity at 0.2mm, which means radius is 0.1mm longer between shortest/longest. Between these points is a point that is "off-slope" (compared to a circle) by the same amount as line connecting these points, which should be representative of the curve assuming the effect is non-local. If these points are 90deg apart, that's ~arctan( sqrt(2* 0.1^2) / sqrt(2 * 20^2) ) = 0.28 deg. It's a bit more if less than 90deg, but it's small regardless.* In any case it doesn't make any sense since the DHS balls pass veer reasonably so ostensibly the lesser cells are worse than the better plastics. Then again yet another nocebo effect is nothing new. *edit: Btw, this is non-trivial problem which probably needs particle sim to resolve, but this is best I can do. Edited by AgentHEX - 12/02/2014 at 10:44pm |
||
AgentHEX
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
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. |
||
Baal
Forum Moderator Joined: 01/21/2010 Location: unknown Status: Offline Points: 14336 |
Post Options
Thanks(0)
|
|
Based on Debater's data, if the Joola celluloid ball is actually in the middle of the approved bounce range, then the Joola plastic won't pass. If the Joola celluloid is higher, the Joola plastic will squeak by. But in any case, it won't pass on weight.
|
||
AgentHEX
Gold Member Joined: 12/14/2004 Location: Yo Mama Status: Offline Points: 1641 |
Post Options
Thanks(0)
|
|
I think Joola cell is also DHS same as what Jay tested.
|
||
Baal
Forum Moderator Joined: 01/21/2010 Location: unknown Status: Offline Points: 14336 |
Post Options
Thanks(0)
|
|
Yes. I think so too.
|
||
wturber
Premier Member Joined: 10/28/2008 Location: United States Status: Offline Points: 3899 |
Post Options
Thanks(0)
|
|
http://tabletennisengland.co.uk/wp-content/uploads/2013/04/Poly_Celluloid_Balls_Testing.pdf |
||
Jay Turberville
www.jayandwanda.com Hardbat: Nittaku Resist w/ Dr. Evil or Friendship 802-40 OX |
||
wturber
Premier Member Joined: 10/28/2008 Location: United States Status: Offline Points: 3899 |
Post Options
Thanks(0)
|
|
... and wrong effective drop height. |
||
Jay Turberville
www.jayandwanda.com Hardbat: Nittaku Resist w/ Dr. Evil or Friendship 802-40 OX |
||
Post Reply | Page <12 |
Tweet
|
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum |
Forum Home | Go to the Forums | Forum Help | Disclaimer
MyTableTennis.NET is the trading name of Alex Table Tennis Ltd. |