Manufacturer's myths. Paddle Palace's article on sponge hardness.

In the Paddle Palace catalog #307, on page 66, there is an article that was cobbled together with input from so called Andro and Tibhar experts. These "experts" are clearly just marketing people, not experts.
The article is about choosing the correct sponge hardness. There are 5 questions.
1. "why does sponge hardness matter?"
2. "What is the difference in performance from a rubber that has a soft sponge, medium sponge, or a hard sponge?"
3. "Why would I, as a player, choose a harder or softer sponge?"
4. "What is the most important consideration for which kind of rubber I should use?"
5. "How should my choice of sponge hardness be affected by my style of play, how far back I stand from the table, what type of rubber I use (smooth, short pips, longpips), and my level of expertise, or other factors?"

The answers are long but the over all theme is that soft sponges are for beginners and hard sponges are for experts. The answer attributed features like speed, spin and control to the hardness of the rubber. This is nonsense. Speed and spin is a function of the rubbers elasticity. There can be hard dead rubbers or soft dead rubbers. These types of sponges are on anti rubbers. There can be springy hard and soft rubbers too.

We all know that the hardness is measured in durometers. Although it is easy to find how much force it takes to make an indentation, there is no measure of elasticity or how long it takes for the rubber to spring back to the original shape after the indenting force is removed. Hardness or softness cannot be directly equated to springiness. This is fact.

I think the "experts" need to go back an read the story about Goldilocks and the Three Bears. Some rubbers will be too soft and may even "bottom out" and some rubbers will be too hard ( Why don't the pros use SST Pro Team, it is very hard. Why do they want to make the hard rubbers softer by boosting? ). Most of us are like Goldilocks. We want a rubber that is not to hard and not too soft, but just right.

So overall I would forget what the so called "experts" have to say. Get what works for you.

Now for the geek speak.

Harder rubbers absorb less energy than soft rubbers. If they absorb less energy then they will return less energy If you look at a cm^2 of rubber as a spring you will see the amount of energy absorbed is roughly proportional the distance the rubber is compressed squared. E = k*Δx^2. Harder rubbers will have a higher value for the spring constant k but lower values for compression distance Δx. When the rubber decompresses the rubber will push the ball back most of that compression distance. That fraction of the compression distance, where the rubber is still pushing the ball, divided by the time is the extra speed or trampoline affect that gets added to the paddle speed.

Something that must be considered it that the ball must accelerate the mass of the top sheet and sponge between the ball and sponge. When the rubber springs back it must accelerate not only the ball but also the mass of the compressed sponge. The energy that is required to accelerate the sponge is lost. So there is a trade off between hard and soft rubbers. Harder rubbers will not compress much so the trampoline effect will be small. Soft rubbers may not spring back fast enough to keep up with the ball so again a lot of the trampoline effect is lost. There is a happy medium but it will not be the same for every body and for every condition.

What is interesting is that people talk about dwell time of the rubber and seem to blindly state they want more dwell time. If the time it takes the rubber to spring back is longer then the trampoline effect is reduced. Again, there is a Goldilocks zone.

It should be clear I don't see this as a hard vs soft issue but more of a what is just right for you issue.

@SugaD. There is no by-line for the article on page 66. The article just said it came from Andro and Tibhar experts.

@Badger, i agree with Dubina's opening paragraphs about how the angle of contact determines the ratio of spin to speed. However at that point Dubina has said nothing about the hardness of rubber. Dubina does seem to echo the article on page 66. While I agree that starting out beginners with a soft sponge is good, it is only because beginners do not play far away from the table. The high arcs or slow loops are easy with Reflectoid. Dubina then goes on to say this medium rubbers to this and hard rubbers do that. He doesn't address why and I think why is import to understanding what is right for you. I think it is wrong to say soft rubbers make high arcs. This implies it can't be done with harder rubbers. What makes a high arc? Why do we even want high arcs? Do the pros hit high arcs? No!

What makes a ball move on its trajectory is called an impulse. This is the integral of force over the contact time or dwell time.
Harder rubbers will have shorter contact or dwell times but the force during that time can be higher so the same impulse can be achieved with a hard rubber as with a soft rubber. Obviously pros must be able to make high arcs if necessary. What Dubina said in the first paragraph is exactly how the pros can make these high arc shots if they want too. It is a matter of trading off speed for spin. With harder rubbers a high arc is achieved by brushing the ball more so there is less speed and more spin.
Obviously a beginner will not have the fine timing necessary to achieve this hard rubber so that is why we agree a beginner should start out with softer rubber but it is wrong to imply that the only way to achieve slow high arc loops is with soft rubber.

@Der_Echte, I don't think I mentioned anything about the elasticity or coefficient of restitution this time. You have a good memory because I have said it before. It is nice to know someone is listening and remembers. However, it is true that elasticity drops almost linearly with impact speed. What is a shame is that I have seen no modern research on this with the plastic balls.
[Detour] There is a specification for how high a ball must bounce off a steal plate from a distance of 30cm I believe. I haven't seen where this specification has changed from celluloid to plastic balls. However, what if the ball bounces from 20cm or 100 cm. Will the plastic balls bounce the fraction of the drop height as the celluloid balls? I think this question has been over lookedl
[/Detour]
While I agree there are many things that can affect the elasticity or coefficient of restitution, only the coefficient of restitution and the mass of the ball and the paddle/hand are used in the the speed after impact formulas. There is no equation for calculating the speed after impact using durometer or hardness readings. There can be fast and slow hard rubbers and fast and slow soft rubbers. The same goes for spin. It is still a optimization problem and the optimal solution will be somewhere between vary soft and very hard. After all who do you know that plays with very soft rubber or very hard rubbers like SST Pro Team God Favored ( very hard but not that fast).

@langel, what you are saying is that softer sponge will "bottom out" where harder sponges will not. That is why I don't buy 1.5mm soft sponges. I have few problems with 1.8mm and had no problem with 2mm+ thick soft sponges.

@genrel001. I like graphs too. I think about it. I have other priorities that pay real money.

There seems to be a difference in opinion between Dubina and Zeio. This is the problem with TT forums. They have too much opinion and little facts. I said above that I would like Dubina to say why he thinks the way he does. I would like to know why Zeio thinks the opposite.

It would be good if Zeio or Dubina could point to an equation that proves their point but I know there are no equations for determining the path of the ball using the hardness of the rubber. There are equations that use elasticity or coefficient of restitution. There must be more than opinion.

I know there is a wide range of rubbers that can generate the same impulse to generate the same trajectory. The stroke may be a little different from rubber to rubber but the same result can be achieve with the skill of the player. People are very adaptable.

Now for an opinion.
I think there are way too much talk about changing equipment. The Paddle Palace catalog is getting to be very big. There are probably a 20-50 rubbers in everybody's Goldilocks zone that will be just right. The same goes for blades.

I like this
"I have a trajectory in mind"