A tennis physicist says racquet company marketing claims may have little meaning in a scientific sense and may not be serving players.
By Rod Cross
Recent issues of RSI have included nice summaries of the different technologies used by various racquet companies. As a physicist, I know that the most important physical properties of a racquet are its mass, length, swing weight, head size and stiffness. Together with string type and tension, these are the things that determine how a racquet performs.
I also know that these physical properties and the performance of a racquet can be measured, and so can the differences between different racquets. I would therefore expect that the various technologies used by the racquet companies, if they are of any real benefit to a player, could be described in terms of the above properties.
But given that these properties are rarely mentioned in the advertising literature, I wonder if the tennis industry is being well-served by advances in technology, or whether it is more of a smokescreen. Some of the improvements that are quoted, such as “30% more feel,” have no meaning in an engineering or scientific sense, and leave me feeling that I am being conned.
The actual performance of a racquet can be measured in terms of ball speed, ball spin and rebound angle. The question that players should ask is how each technology affects the speed, spin and direction of the ball as it comes off the racquet. If the technology makes no difference, or if the difference is too small to measure, then there may be no real advantage.
Suppose that a manufacturer comes up with a racquet that delivers 5% more power, in the sense that the ball comes off the strings 5% faster. Will that actually benefit a player? Most players would probably agree that it will, but the physics of the situation says there are only two basic ways to increase the ball speed by 5%. One way is to swing the racquet 5% faster. The other way is to increase the swing weight of the racquet, by making the racquet head heavier. Either way, the player will need to work a bit harder to get an increase in the ball speed.
A decrease in string tension will also help to increase the ball speed off the strings, but the effect is very small and it leads to a small decrease in ball control. An increase in racquet stiffness will increase ball speed off the tip of a racquet, by reducing the energy lost to racquet vibrations, but it has no effect on ball speed off the middle of the strings.
There are certain properties of a racquet that have nothing to do with ball speed, spin, or direction, but that do affect the choice of the racquet that a player prefers. Players are sensitive to the feel of a racquet, and they are also sensitive to the sound of a racquet. If a racquet doesn’t feel right or if it doesn’t sound right, then the player won’t like it.
The main way to affect the sound is to add or remove a string dampener. Players who normally use a string dampener say that the racquet feels terrible if the dampener is removed. Players who don’t use a dampener say that the racquet feels terrible if a dampener is added. In either case, the player confuses the feel of the racquet with its sound. If the dampener is stuck to the throat of the racquet with tape, rather than being inserted in the strings, then sound will be different but the feel and performance of the racquet will be the same. In any case, dampeners are too light to have any significant effect on racquet performance.
The feel of a racquet is affected by its weight, its swing weight and its stiffness. It is also affected by the type of grip used. The stiffness affects the level of vibrations in the handle that occur after the ball leaves the strings. Excessive vibration feels uncomfortable and the player won’t like it, even if the ball comes off the strings as fast as with any other racquet. For that reason, much of the effort that goes into the design of a racquet is aimed at reducing handle vibrations, thereby improving the feel of the racquet. If the player hits the ball at the sweet spot in the middle of the strings, then the racquet won’t vibrate at all. But most recreational players hit the ball all over the string plane, in which case any technology that helps reduce racquet vibrations will feel good, even if there is no difference in racquet performance.
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See all articles by Rod Cross
About the Author
Rod Cross retired in 2003 as an honorary member of the Sydney University staff and continues to work on the physics of sport and forensic physics. He is a physicist and co-author of The Physics and Technology of Tennis available from the USRSA.
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