OOAK Table Tennis Forum

Great paper on analysis the effect of the change in material from celluloid to plastic:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5304273/

For those that don't want to read the whole thing, here is the conclusion:
Conclusion
Changes between the trajectory of celluloid balls, which had been long used in table tennis, and newly introduced plastic balls were investigated in this study. Plastic balls demonstrated a higher coefficient of restitution than celluloid balls when the initial vertical velocities were higher. Moreover, the coefficient of friction was higher for plastic balls when the initial horizontal contact point velocities were lower. As a result, for slower balls with back-spin, as in the case of a service, plastic balls are expected to experience more deceleration upon collision with the table than celluloid balls. On the other hand, for faster balls with greater amounts of top-spin, plastic balls are expected to experience greater acceleration upon collision with the table than celluloid balls. Since the behavior of the ball is largely influenced by the initial conditions, testing at various initial conditions is necessary to understand the characteristics of each type of the ball.

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Fascinating stuff...

My comments after a quick skim read of the linked article:

1. It is comparing Nittaku Premium 40 celluloid with Nittaku Premium 40+ plastic balls only. We all know that there are wide variations between the various plastic balls. So this is only valid for these balls. What ball is used by the World Tour events, World Championships, World Cup, Olympics, different national TT organizations, us mug amateurs? Interesting that the plastic balls were actually lighter than the celluloid ones on average.

2. In one section they set a robot to propel both types of balls at 100 rps (revolutions per second) and 170 rps of top spin. What would have been interesting would have been to set the machine to produce celluloid balls at 100 rps and then fire plastic balls at the same setting and measure the plastic ball rps. Or would this produce a plastic ball with 100 rps as well? It may depend on how the robot imparts the spin. The objective answer to the theory that "plastic balls have less spin" would be very useful.

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Retriever (sometimes golden, but often leaden)
Moderator, Inverted Retriever Technique sub-forum - http://ooakforum.com/viewforum.php?f=74

Only read the conclusion but...I'd be more interested in it's behaviour after 'collision' with my table tennis rubber than with the table.

Edit: Having read further I note from Discussion:

"However, since the behavior of the balls upon collision with the racket was not investigated, future research should be conducted if specific tactics are to be recommended with regard to the change in ball materials."

Yes please!

The gist of the analysis seems to be that the plastic ball grips the table more.

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To know the actual Specific Gravity (physical dencity) of plastic materials is a great assistance to all the table tennis players.
Through knowing the SG you could safely foretell how the plastic ball will play compared to celluloid.
The "non-cell" plastics with low dencity 1.10 -1.15 should play far different from celluloid, and those of high dencity 1.20 -1.35 are going to play very close to celluloid.


HOW DO I KNOW ACTUAL DENCITY OF THE PLASTIC MATERIAL.







1. Pick up a broken ball and use a precise digital scales to get Dry Weight (weight in air).
2. immerse the ball into water to get Wet Weigh (weight in water).
3. You find Material Dencity = DW / WW

Be happy.

Igor, did you actually do this yourself before posting?

This works for relatively dense objects with decent volume - coins, stones, etc. Try it with a broken ball and you'll find the experimental error so large that repeated readings will give widely divergent results, because the weight of water dispaced (mistakenly referred to here as "wet weight") is going to be miniscule, because the volume of the actual plastic is miniscule, and because you'll get varying amounts of tiny bubbles sticking to the plastic. This also assumes that the plastic doesn't FLOAT.

Interesting way of doing it, though - never thought of doing it that way - we've always gotten the wet weight by suspending the object to be weighed by string to a hook on the scientific balance. You get the weight of water displaced by the difference between dry and wet weights. And even with rocks we have to worry about bubbles - usually we soak them overnight and make sure the surfaces are smooth and don't have sharp corners.

Besides - your assertion that plastic density is a good predictor of ball performance is dubious - just from first principles, given other conditions are the same, a ball with less dense plastic and thicker walls would have exactly the same angular momentum as another ball that weighs the same, has thinner walls, but made with a denser plastic, if spinning at the same RPM. The less-dense ball might even bounce higher and be "faster" than the more-dense ball, since the walls are thicker, but this would depend on th hardness (and therefore coefficients of elasticity) of the respective materials.

Iskandar

I am no engineer, so correct me physics majors if I misunderstood the paper.


This sounds to me like the polyballs bounce higher (COR) if they are faster, relative to the celluloid. What confuses me is the term "vertical velocities." Meaning hit vertically? So they rebound more than celluloids when hit downward at greater velocities.


More kick or more resistance due to greater COF. So...as stated above, polyballs grip the table more.

One thing they didn't tackle was the spin on the ball while in the air vs. celluloid, after contact with the rubber.

Last night, I was just having a conversation with Gman about his sidespin loops. He said his sidespin loops don't curve in the air as when he watches some videos of pros, but that when his hit the table, the sidespin really kicks in and the overall curving trajectory is comparable. However, he did cite videos of pros who played with cell balls, and my reply was that cell balls curved more in the air with than poly.

That observation is very much in jive with this study, and my theory, that the polys spin less and are heavier, and therefore affected by magnus a bit less, but when they hit the table 'kick' more because they grip the table more.

So perhaps the "spin" as it is perceived is a wash (or close to) because magnus is reduced, spin is reduced, but table grip is greater. Subject to situation of course.

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Japsican wrote:


Ah but now we are getting into coefficients of friction of the balls and the coefficient of friction of the table. I believe that the ITTF is looking into establishing a standard for table - currently there is a maximum coefficient of friction for the table and the ball, but no minimum yet, so you could play on glass or something else relatively anti. They do note that different balls will make this differ, so what can they do?

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Retriever (sometimes golden, but often leaden)
Moderator, Inverted Retriever Technique sub-forum - http://ooakforum.com/viewforum.php?f=74

My non - technical take on sidespin is on serve with pure sidespin and the "right " contact, you can bend the flight of the ball!!

Sidespin kick off the table has always been massive with these new balls and on the same table that cell balls were used, it is greater and continually catches me out when setting up for a big loop and my insufficient footwork!!!

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