Ask the Experts No. 193: Stefan Feth – Sponge Thickness Explained
Butterfly Table Tennis Question and Answer

Question:

Blade: Mizutani ZLC
Forehand Rubber: Tenergy 05 2.1mm
Backhand Rubber: Tenergy 05 2.1mm

Hi,
I would like to know practical and technical information about the sponge thickness on Tenergy rubbers.

In general, I would assume thicker sponge is good for top spin looping, and thinner sponge is good for flat hard hit. Is this correct or not?
More importantly however, we pay for different type of blades because of their specifications. For example, ALC, ZLC and etc with different number and types of wood layers. So the question is, if the rubber is attached to the wood with the sponge in between, then there would be no meaning for the feeling of ALC, ZLC and wood itself because sponge damps almost everything…
Does a thinner sponge brings more feeling about the blade itself?

Please explain it from different aspects in detail.
Thank you.

Alex Bradarani

Answer: 

This Ask the Experts question was answered by Stefan Feth. He is one of the coaches at the World Champions Table Tennis Academy in Santa Clara, CA. Coach Feth’s response is as follows:

Hi Alex,

Thanks for your question and interest in Butterfly equipment.

First, let’s start by talking about what the sponge and blade do during different types of shots, so we understand the effect they have.

On soft shots (any kinds of shots…  looping or hitting, or even lobbing!), the ball sinks a little bit into the top sheet, and then the pimples on the back of the top sheet push down into the sponge a little bit, causing it to compress (and store energy from the ball). After reaching some maximum amount of compression for that shot (based on how fast the ball is moving, and how hard you’re swinging), the ball doesn’t have any more energy to give to compressing the sponge, and the sponge springs back and releases all of the energy stored in its compression, pushing on the ball, and kicking it off the racquet.

If you have a more energetic shot, the ball will sink in further, causing the sponge to compress more, and store more energy, before it springs the ball back out.

Now we get to the first place where the thickness of the sponge matters. For any thickness and hardness of sponge, there’s some maximum amount of energy it can store during compression.

To understand that, it might be easier to think of what happens if you have a very, very thin (or thick!) sponge.

If the sponge is very, very thin, maybe as thin as a single sheet of paper, then it basically wouldn’t have any thickness to compress! Without having any thickness to compress, our paper-thin sponge won’t be able to store much energy at all, and it’ll “bottom out” on every hit. This sponge would be terrible for table tennis.

On the other side, imagine a sponge that was a foot (about 30cm) thick. A sponge that thick would have huge room for compression. Heck, you could probably use it to hit a baseball, or even a steel shot put, without bottoming out the sponge. Unfortunately, that’s way more compression than we need for any shot in table tennis, and this much sponge would be super heavy, so it would also be terrible for table tennis.

Instead, we have sponge thicknesses somewhere in-between. In particular, Tenergy comes in sponge thicknesses between 1.7mm and 2.1mm.

So, what happens if you do hit hard enough to bottom out the sponge?

First of all, since the sponge is a huge part of returning energy to the ball, and you’re hitting hard enough to exceed the amount of energy that the sponge can store and return, you’re going to lose energy on the shot. This is the main reason why professional players play with very thick sponges. The thicker sponge simply allows them to hit harder and return more energy to the ball on those stronger shots.

Now, what about the blade?

The most important aspect of the blade to this discussion is its stiffness. Even though they don’t look like it, all blades have some amount of flexibility, and they bend slightly back (and spring back forward again) during every shot. And once again, the stronger the shot, the more the deflection in the blade.

This deflection, again, stores and returns some energy, and much like the compression of the sponge, it also has a limit to how much energy it can store and return. The difference here is that while we make the sponge thicker to allow it to store and return more energy, we achieve a similar effect with the blade by making it stiffer.

Now we can start putting all the pieces together.

On a soft shot, the sponge doesn’t compress very much, and the blade also doesn’t deflect very much. However, since the sponge is usually softer than the blade is stiff, most of the energy storage and return happens because of the sponge, and so the sponge/top sheet combo dominate the character of the shot from the player’s perspective. Note, however, that even on a soft shot, the blade matters! After all, ultimately, something has to transmit the force of the impact back to your hand, and that is always the blade, so you’ll absolutely feel it.

As the shot gets stronger, the sponge will compress more, and the blade will deflect more. We still expect the sponge/top sheet combo to dominate the character of the shot, but the blade will definitely start to have a larger effect.

As the shot gets even stronger, the sponge will start to bottom out. Some of the energy of the shot will be dissipated, and the blade will begin to play a much larger effect in the character of the shot. At this strength of shot, you’ll also start to hear a different sound during the shot, hearing “the wood” more than the rubber.

Eventually, as the shot gets even stronger, the sponge will be bottomed out and the blade will reach its maximum deflection. Much of the energy after this point is lost, and this is also, really, an unusual strength of shot to play. Maybe you can make a shot this strong when you’re absolutely killer smashing an easy ball. Maybe.

Finally, there’s one last thing to consider in this energy-centric analysis of the sponge and blade, and that’s the angle of contact.

Consider the following diagram, showing one ball moving straight towards a flat paddle, and another ball moving towards a paddle with a closed racquet angle.

In the case of the flat paddle, we’ve marked in red the maximum amount of thickness that the sponge has to compress.

In the case of the closed racquet angle, we’ve put another red line, showing how the more closed racquet angle gives a greater effective maximum compression distance.

This means that if you’re hitting with a more closed racquet angle, you can make stronger shots before you bottom out the sponge. You can try this at home by seeing how easy it is to “hear the wood” when you flat hit a shot vs looping a shot. It’s very difficult to “hear the wood” when you loop!

From this, more athletic players who make stronger shots will benefit more from thicker rubber and stiffer blades because that combination will let them store and return more energy to the ball without excessive energy loss.

In my opinion, thicker sponges are more important for flat hitters than loopers, because the closed racquet angle used for looping gives a greater effective maximum compression distance on the sponge, whereas flat hitting a ball will bottom out the sponge sooner.

At the same time, the extra energy return that thicker sponges and stiffer blades give you comes with a cost: less control.

One way to think about this is again with extremes. Imagine a racquet covered in glue, so it returns zero energy to the ball, and every time you hit the ball with that racquet, the ball just stops and sticks to the paddle. In this case, you’d always have perfect control (because the ball was stuck to the paddle!) but you’d have zero energy on each shot.

On the other hand, think about a racquet that always hits the ball out at a thousand miles (or km) an hour no matter how you swing! This racquet would give you lots of energy on every shot, but you’d have a very hard time controlling it, because slight changes in the angle of your stroke would make a huge difference on where the ball goes.

Luckily we live in the world in between these two extremes, but the general trend holds: the more energy the racquet returns to the ball, the less control.

Now, that was the energy-storage perspective on sponges and blades, but there’s another direction we can approach this problem from, and that’s dwell time.

Simply put, dwell time is how long the ball is physically in contact with the rubber’s surface. The longer the dwell time, the more control the player will have, because they’ll simply be in contact with the ball for longer, and thereby have more ability to influence its trajectory.

Dwell time works a little differently than energy storage.

As long as we’re not bottoming out the sponge, we maximize dwell time with thick (and soft) sponges and flexible blades. We can also increase dwell time by making stronger shots (though this can result in less control).

If you think about the effect of dwell time, we find something curious: thick sponges that maximize the energy storage and return also help maximize the dwell time (as long as we’re not bottoming them out). For blades, though, we see something different: stiff blades that maximize energy storage and return will minimize dwell time!

Now, it’s important to notice that something a little strange here. We said that thick sponges give you more energy, and thus less control. But we’ve also said that thick sponges give you longer dwell time, and more control! So what wins? Generally the loss of control from the extra energy is a bigger deal than the extra control from the longer dwell time, and so overall, thicker sponges have less control.

When you put these competing factors about the sponge together, you find that if you keep the thickness of the sponge the same, we expect more control the softer that sponge is, but the easier it is to bottom out the sponge and start hearing the wood of the racquet in the shot. Meanwhile, if you keep the hardness of the sponge the same, the thinner that sponge is, the more control you’ll have because the sponge returns less energy to the ball, making it overall easier to control, despite the lower dwell time because of the sponge’s reduced thickness.

In the end, it’s important to find the right combination for you. The most athletic players hitting the strongest shots will care the most about maximizing the energy storage and return, and will want to look towards thick sponges and stiff blades.

Defenders, however, may actually want thin rubber and flexible blades to take advantage of the energy loss associated with those things, and also gain even more control from the increased dwell time.

And for everyone else, there’s probably a good setup in between.

Best,

Stefan Feth
US National Team & Olympic Coach
Coach @ WCTTA
www.tt-champions.com