Performance indices : the way to evaluate blade by measurement

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.

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.

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 :

Fig.03 helps the understanding on ‘Primary’, ‘Central’ and ‘Lateral’ movement.