1. The reason why the evaluation by measurement is needed
There are so many blades in the market, and it is always very difficult for us to select proper ones. We sometimes just select famous blades with signatures of top players, but it doesn’t always bring us good result. That is because table tennis is the sport of individuality, and there are various range of the individuality of player. The signature of player doesn’t guarantee that the blade will fit for everybody. Even, in many cases the signature blades are offered as various models with one signature. For example Butterfly Timo Boll ZLC, Timo Boll ALC, Timo Boll W7 and so on. Even though we like the signature, we have to select one among various models again.
Sometimes we try to select our blade by the name of artificial material for example ALC. However, the name of artificial material doesn’t express everything of that blade. That is because blade is not compose of one artificial material. Artificial material works as ‘reinforcement’ of glue layer, and there are many ‘wood’ layers in the construction of a blade. If there are two blades with the same artificial material but with different wood, those two blades are different from each other. For example, Butterfly Timo Boll ZLC and Butterfly Mizutani Jun ZLC are quite different from each other. So we have to avoid easily predicting the characteristics of a blade just by the name of artificial material.
We can also depend on the performance indices offered by equipment makers. Usually there are two kinds of performance indices – speed and feeling. However, one big problem is that there aren’t common criteria for those indices. We can’t compare an index of Stiga blade with another index of Tibhar blade because of the difference of criterion. ’90’ of Stiga is different from ’90’ of Tibhar. The performance indices are meaningful only when we compare between blades from the same maker.
There are different way of expression that is simple and looks common. For example we can see speed rating expressed as OFF+, OFF, OFF-, ALL+, ALL, ALL- and DEF. It is sometimes expressed just as OFFENSIVE, ALLROUND and DEFENSIVE. Feeling can also be simply expressed as HARD, MEDIUM and SOFT.
However, even this kind of expression is sometimes far from understandable. A ‘MEDIUM’ blade of one maker may be dealt as ‘HARD’ blade by another maker. An ‘OFFENSIVE’ blade of one maker can be ‘ALL+’ blade of another maker. The reason why this kind of difference occurs is that those criteria are usually decided by players’ experience. Simply speaking those are biased data. If we try to use those criteria just as brief information, it will be still all right. However, as soon as we try to precisely compare for correct selection, we will soon notice that it is chaos. We can find many very slow OFFENSIVE blades and very fast ALLROUND blades.
If there are common and understandable criteria, the comparison between blades will be easier. Also the bias by the individuality of human should be avoided. So the blades should be measured. The measurement is different from testing by human. Human sometimes evaluate blade just by his/her experience, or sometimes ‘adjust’ his/her technic to be accustomed to given blade. However, ‘measurement’ just gives us values those are not biased and are not based on human experience. We can always get consistent data by measurement. It will make our understanding on a blade or comparison between blades much easier.
2. The process of measurement and calculation of performance indices
The measurement starts from the common knowledge of sound & vibration. Everything existing in this world has its own pattern of vibration. So we can understand the fundamental characteristics of a thing if we just measure the pattern of its vibration.
The pattern of vibration is usually expressed as FRF (Frequency Response Function).
\We can find some peaks of the graph of FRF. The frequency at the point of a peak is called as natural frequency. There are many natural frequencies, and each natural frequency is concerned with a specific behavior of that thing. For example, the first natural frequency of a blade is concerned with its longitudinal bending which is the primary movement of table tennis blade, and the second natural frequency of a blade is concerned with torsional movement. (In fact the movement patterns can be known by more complex process. But, I will skip it because it will make this article too long and too complex. I want to just the result of that analysis.)
So the measurement is basically the process for finding natural frequencies. A table tennis blade is stimulated, and the movement – correctly speaking, acceleration – of that blade is recorded. Stimulator and accelerator are connected to data logger, and data logger send the data to computer for the calculation of FRF. Then finally we get the natural frequencies of that blade. I will skip the detail of the equipment for measurement because it is so common in the industry or among researchers. In fact, measurement system itself is nothing. What is important is how to use measured FRF data to explain table tennis blade.
Due to the characteristics of wood materials used for table tennis blades, we can only get four or five natural frequencies. But, those are sufficient for converting the frequencies to performance indices.
From the natural frequencies and the other data of blade, performance indices are calculated. For this calculation I assumed the blade as a model which is composed of 3 masses (center of head, other part of head, and handle) and 2 springs (primary bending spring and center deflection spring).
Please note that this simple model is not as exact as very complex model such as FEM model. However, this simple model is based on the knowledge of blade behavior which has been acquired by more complex model. And, for that reason the error by the simplicity of model is not significant. Thanks to this simple model I can get the performance indices of various blades for comparison in short time. It will be much more meaningful than using many days just for getting very exact result of one blade. Through the experience during many years I noticed that simple model is sufficient for getting performance indices those will be explained in next chapter.
3. Four performance indices for the evaluation of blade
Four indices will be used for the evaluation of blade or comparison between blades in this blog. The four indices are two ‘elasticity’ indices and two ‘vibration’ indices as follows :
- Ep : Primary Elasticity Index
- Ec : Central Elasticity Index
- Vp : Primary Vibration Index
- Vl : Lateral Vibration Index
Fig.03 helps the understanding on ‘Primary’, ‘Central’ and ‘Lateral’ movement.
As the name itself expresses elasticity index means the elasticity (= stiffness) of blade. So it is directly concerned with speed of blade. Especially Ep is similar to speed. Ec becomes meaningful only when we hit blade very hard. The higher value of Ec means that the blade can give more ‘kick’ when we hit the blade very hard.
Vibration index means the level of vibration. Vp is directly concerned with longitudinal bending vibration, and it is mainly transferred to player’s palm. Vl is concerned with lateral vibration of blade head, and player can feel it at fingertip of index finger. So Vp and Vl can be translated as ‘feeling’. The higher value of Vp or Vl means sharper feeling.
Also the indices are normalized values. That is for easier comparison. The reference is Stiga Allround Classic. All 4 indices of Stiga Allround Classic are 1.00. If the Ep value of a blade is 3.00, we can understand that the primary stiffness of that blade is three times bigger than that of Stiga Allround Classic. The reason why I selected Stiga Allround Classic as the reference of performance is that it is very slow blade and at the same time very traditional and well-known blade. So Stiga Allround Classic can be very good ‘basis’ or ‘standard’ of the evaluation of blades. Once I tried to find a ‘neutral’ blade and set the value of that neutral blade to 0.0. Then plus value means that the blade is faster or harder than the neutral blade, and minus value means that the blade is slower or softer than the neutral blade. Before the age of poly ball, it seems to be proper. However, after the introduction of poly ball in 2014, I noticed that the blade that I had thought it is neutral is not neutral any more. That is because the rebound speed of blade was significantly reduced and the feeling becomes relatively softer when the same blade is tested with poly ball. For that reason I couldn’t help thinking the way of normalization again from the start point, and as a result I decided to go with the new way shown in this article. I gave up finding the neutral blade of this new age, and decided to just express everything as the comparison with reference blade – Stiga Allround Classic.
NOTE : Please note that the higher value of a performance index doesn’t mean higher performance. For example, if Ep (Primary Elasticity Index) is high, it can be interpreted to higher rebound or higher speed of that blade. But, higher speed is no more than higher speed. Higher speed doesn’t mean higher performance. That is because table tennis is the sport of arc trajectory that is made by spin. If we can control the ball, higher speed is good. However, if we can’t control the ball, higher speed is nothing, and it will just cause many mistakes. So, the indices that is concerned with speed is no more than the tool for helping us to find the blades with proper speed range that fits for our playing style.
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This is fantastic work. Would you be willing to write a technical article describing precisely how you calculate the performance indices?
Thank you for reading my article. By the way, the detail of calculation is just composed of some formulas. For that reason it is meaningless to open it unless I announce it at conference on Sound & Vibration.
Hello, blade characteristics can vary from one model to the next, being mostly wood. Could you report the weight of your tested samples in your reviews? It can help identify potential outliers: when a particular model is unusually light or heavy, it’s often also a bit slower or faster. Thank you.
As for technical details, I think you’ll find that a number of people interested in your reports are science-minded, so they won’t easily be put off by a few formulas. I’m also for explaining the calculations. This can also help suggest improvements.
Thank you for comment. I’m considering to write a posting on the influence of weight and kind of wood. Of course I have that kind of data although the data isn’t in arranged form.
In fact, the biggest problem is that I don’t have much time to write it.
I want to echo Bran’s comment. Many of us would find the formulas interesting and might even contribute to a database.
What aspect of vibrations at the handle and the wings do Vp and Vl represent? I mean, “level” of vibrations can be understood in different ways: if the stimulus is discrete, like contact with the ball, it could mean the max amplitude of the vibrations, it could mean their average, or even something like their sum over the duration of the resonance (after the discrete stimulus.)
I’m asking because in practice, it seems to me that people have different preferences with respect to: (1) the amplitude of the vibration right after the impact (sharp feel); (2) the rate of decay of the vibration over time – after the impact. (Easy to illustrate with sound: a snare drum has a high (1) (it’s loud around the impact) but a very fast (2) (the vibrations get quieter much, much faster than a piano, for instance.))
For instance, a lot of people find classic 5 ply wood blades to “vibrate too much”. It is doubtful that their problem is due to (1): those blades tend to feel quite soft around impact. But it seems plausible to me that what’s going on is that those people have an issue with (2): they prefer blades that noticeably vibrate for a shorter time after impact.
To put it in other terms: are Vp and Vl significant for (1)? (2)? Neither?
(I modified this reply because my former reply seems to be somewhat vague.)
It is very good and important question. The answer is neither (1) nor (2). The word ‘level’ may cause confusion. But, Vp and Vl are directly concerned with frequency.
In fact, I have also considered (1) amplitude of vibration and (2) decay time. However, both of those differ by point of measurement because the vibration of blade is the combination of some different vibrations. Especially at handle or at wing, the result of measurement of (1) or (2) varies significantly by the point of measurement. So I can’t get meaningful date for comparison.
Of course I can get even data if I just measure (1) and (2) at the center of blade head. However, the problem is that our hands don’t touch the center of blade head. We can’t feel the vibration of the center of blade head directly. What we can feel is just the vibration (of course including (1) and (2)) that is transferred through inside of blade. Therefore making indices concerned with (1) and (2) of center of blade head is not meaningful. That is the reason why I just use frequency data.
I’m also measuring damping constant of each vibration mode because damping is concerned with energy efficiency. I’m considering to introduce additional indices concerned with damping. However, in table tennis, covering (= rubber) plays much more important role in energy efficiency, than blade. Those indices may just cause complexity. I don’t want to write technical paper in this home page. I want to keep the data in this home page simple. That is for most of readers who don’t have knowledge base on dynamics.
Blade speed, hardness and control ratings as stand alone measures are interesting but its there performance relative to various rubber combinations move it from abstract to real time.
Yes. It is the reason why combination of blade and rubber is so difficult. I recommend you to consider three factors : (1) the surface material of blade, (2) Ec/Ep, (3) Vl/Vp.
Finally someone using objective measurements to compare blades! I love that way of thinking.
I think you are missing one important variable, vibration intensity/duration.
First generation carbon blades had horrible control because of little vibration. It takes a while to get used to different pitch but it is impossible to adapt to almost no vibration, the control will aways be compromised. In general faster blades will have less vibration intensity but not aways. One ply hinoki for example has a stronger and shorter vibration then standard 5ply wood.
I wish you also compared famous affordable alternatives like Friendship C3, Friendship Bomb, Galaxy N9, 896, T11 and W6. It would help those that can’t afford and those who want an upgrade in material quality.
Just read your comment about inconsistency in vibration intensity. Maybe you could measure the points that are reliable even if they are not grabbing points, or maybe an average of several points in the handle. Even if these methods are not perfect they should give a general idea of how much the blade vibrates. If the method is able to rate traditional 5 ply as great vibration and first generation hard carbon as very poor vibration it would be usefull to preview how a composite will behave.
Thank you for comment! And, your comment on the vibration of some blades is right although my Performance Indices are not perfect for explain what you mentioned.
And, I want to try as many blades as possible, including discontinued blades.
Hi, I’m looking to create my own blades with the help of my brother who hand-makes top of the range acoustic guitars. I’d like to create my own Hurricane Long 3.
Is quality of wood grain the biggest contributing factor to the large ‘sweet-spot’ DHS claim?
Why is Hurricane Long 3 $300 and PG7 is $25 despite having same ply thickness and composition?
I assume quality and direction of the wood grain.
I’d be interested to hear your ansaw, see if you can shed any light on this question
Any advise would be much appreciated, thanks!
I have uploaded a video on youtube on the same subject on november 2017. —– https://www.youtube.com/watch?v=QKwVp_jCqa8&t=1s , it refers in the comment to a manuscript where the procedure applied is explained (written in November 2017 ) — it is a strange coincidence that also the model of the blade (stiga allround classic) that you describe is the same that I have used in the video and in the manuscript …
Thank you for comment!
Of course there should be coincidence because my approach started at the same way as your youtube video.
At the beginning I tried to just check the frequency of sound.
Also some table tennis companies use that way when they distinguish the quality of Hinoki wood.
I have readden your article just few days ago: a friend of mine told me about your article after I showed him the procedure that I use, it has really been a coincidence.
My idea is to make the test easily with a smartphone, so that anyone can do it by himself and have the information on that blade (each blade has its own features, two blades of the same model can be really different).
My question is the proprortion between Ec and Ep components:
Obviously the sound produced is not a pure sinusoid, but for what I Have seen the main component of the spectrum (the primary) is much bigger than the others, then I am asking myself how much other components do effect the feeling for the player of the ball bouncing on the blade.
Thank you for comment! I thin that your idea will be wonderful if it can be realized.
But, the measurement in this home page is somewhat complex to be applied to smartphone app. That is because it is more that just checking sound, although the start line of this measurement is gathering sound. And, sometimes the surface of blade is broken during the measurement. That problem should be solved.
hello, i am reading this article and like it a lot. Your measurement is making reviews of blades very objective. People easily get lost in tons of subjective reviews on internet. Meanwhile can i make a translation to Chinese and publish in China Zhihu.com website? It is a huge chinese forum. If permmited, I would of cource add link to origin english pages to this website definitly at first line. And since picture link to foreign addresses may not work in Chinese website, I would also like use your pictures too. I won’t publish anything without permission.
Thank you for comment! Regarding the translation issue, could you send an e-mail to email@example.com for more detailed discussion?
Great site, thank you very much! I have a general question concerning performance indices. There’s a well-known property of some blades, that of being (say) “slow on slow shots and very fast on fast shots”. How does this property relates with your performance indices (assuming it does)?
Thank you for comment. Probably it is concerned with Ec/Ep. And, if Ep itself is low and Ec/Ep is very high, that blade behaves as you mentioned. It is slow on slow shots. But, because Ec/Ep is very high, it becomes quite fast on fast shots.
What does *sharpness* exactly mean when you say feeling is sharper?
If the frequency of vibration is high, we feel that it is ‘hard’ or ‘sharp’. Maybe there will be better term than sharpness. But, I think that it is the term that explains what we feel quite well. (High/low frequency is also related with sharp/dull sound.)