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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