OOAK Table Tennis Forum

Baal,

How do you reconcile some of the XSF results in this pdf with yours?

http://newgy.com/docs/White_Paper_On_Using_40_Plus_Balls_Public.pdf

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"We don't rise to the level of our expectations, we fall to the level of our training" - Archilochus

It's 305mm not cm

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 (and thereby drop distance) 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

Equivalent to drop from ~70cm., vs the ~2.5m/s of the 30cm drop test.

3.7m/s is a fairly hard shot. 10m/s+ at 15-20deg.

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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.

Ah.. Amazing the difference an order of magnitude makes!

Iskandar

I looked at some video and found hard loops from international level players that were probably traveling vertically toward the table at speeds in excess of 4.5 m/sec on impact. I made the estimate by using the ball's blur length (assuming 1/30 shutter speed since the head of previous frame's blur seemed to match the tail of the subsequent frame's blur and the frame rate was 30 fps) and the vertical blur distance. The vertical blur was approximately 150mm (based on comparison to the net height). That time and distance results in an average velocity for that interval of about 4.5m/sec. The impact speed could be greater since the ball is accelerating downward during this time period due to gravity as well as the aerodynamic affects of the topspin.

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As mentioned before, can the ITTF try measuring for >50N on their existing apparatus?

It seems to take little effort to produce a force/deformation graph, compare against cell, and I'd be surprised if they haven't done so already.

To test for viscoelastic diffs a higher bounce test is necessary. ~1m(~4.4m/s) should be enough to extrapolate.

I would expect the Executive Committee (the "bosses") to decide that we shall not publish any detailed results. As far as I can see, ITTF never did. Amongst some reasons which I can imagine is, that even if all balls were well in, a comparison of values could lead to comments about different qualities; but first this is not ITTF's intention, because we say a ball is OK if and only if it meets all specs; and second, if ITTF is the source of prolonged quality comparisons, then ITTF might be in trouble for interfering unduly with business issues.

My assumption is that we might at most be publishing a qualitative result such as "x out of y brands tested in November passed". If so, that would be on the ITTF website's ball section.

If there comes up any explicit decision about this issue, I will post it here on the forum.

I don't quite get the point of this, i.e. why should we do so? What would the process gain?
Anyway, yes the Zwick apparatus can execute an "unlimited" force, it is a USD 150,000 equipment, almost 2 meters tall and used for testing of many materials.
But the shell of the ball breaks somewhere around 75 Newtons, and nondestructive testing is a requirement. So ?

If you've been following the forum chatter, the popular opinion is that the DHS ball sometime bounces weird or inconsistent, with claims that it should be boycotted if approved in its current form. I've been trying to figure out why that might be so we can have an idea if it's fixable.

One possible technical reason for such an effect is that the ball bounces in a non-linear manner (vs force of impact). This can happen in a way that can be statically determined like the existing force test (ie. compare stress/strain graphs between balls). It can also happen due to viscoelastic (not time-invariant) effect which requires a more dynamic method. This be done with varying impact velocity like the bounce test, or from at least some Zwick literature such dynamic testing mode is supported: http://www.zwickusa.com/en/applications ... ction.html. I have no idea if that's the case with what the ITTF owns, but if so the ITTF can help determine the source of player displeasure with the largest manufacturer of plastic balls.

The other realistic possibility is that the surface friction is non-uniform, which isn't trivial to test for (vs ball deformity which can be tested w/ veer) and the ITTF currently doesn't test for friction anyway.

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Also, 75N is only like 15lb force, and if ball is spring-like elastic only corresponds to moderate impact speed (hitting against a wall for example isn't good for ball but doesn't immediatelybreak it). Is there something about the probe/pin or whatever that makes the test more destructive than against a table?

Thank you Torsten, much appreciated!

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Thanks for the update Torsten. Can you confirm when the second round of tests will be done please?

I find the reasons you give for not releasing the results in terms of which balls passed which tests strange.

EuroNCAP is a car safety site and they test cars for safety and publish their findings without worrying about upsetting anyone from a commercial standpoint or secrecy and the car industry is worth far more in terms of sales and employment than the table tennis market and there is commercially far more at stake/affected by the test results. Surely the ITTF's tests are an opportunity for ITTF and the manufacturers to show how good their balls are in terms of their quality and ability to pass the T3 specifications and not something to be kept secret - unless of course.....

If there's nothing to hide, either the ITTF or manufacturers should publish the results for their balls and put an end to the speculation.

Here here!

Stop the pseudo commercial-political speak and coming across like FIFA, it always sounds like there is something being hidden.

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Thanks for posting. I was wondering about the same thing and ready to post it...

As to the last three postings, perhaps I expressed myself in a misunderstandable way:

Of course the information if a brand passed or failed will ultimately be public: By means of being included, retained or removed on the list of approved balls. So there will be no speculation left whether or not any ball brand meets the ITTF specifications. And perhaps we can include the same information already after the first random test.

I was mainly talking about the detailed lab results. What the majority of ITTF deciders will probably want to "hide" (openly and without denying it by "pseudo speak") is that part of data which will only lead to "quality comparisons" that in ITTF specs simply do not exist: IMHO, it's not like in the car industry, where the grade of crash test damage is a key indicator. In TT, if a ball is within the range, it's OK. No one will be obliged to change anything then.

Apart from what I already considered as reasons not to publish those data: What does it help us players if we know that ball #1 bounces 241mm and #2 bounces 256mm in the lab? These will not be the values on a TT table, and as the forum said: We are playing on a table, not in the lab.

The common perception among players is that the ITTF has no accountability nor transparency and none of your replies seem to dispel that.

I thought this was supposed to be someone who knew what they're talking about but is no better than speaking to any other PR spokesperson, or a wall.