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Weighty Decisions

An expert on the physics of tennis says to best help your customers pick the right frame, you need to consider swingweight.

By Rod Cross

Most tennis shops generally have a good selection of racquets on display. But, from a scientific point of view, the display itself may not necessarily be designed to assist the customer in making a good choice of what frame to buy. The display could even be bewildering to consumers, especially if there are lots of racquets on the wall and if the customer has not already decided which racquet he or she wants to buy.

The easiest way to display racquets would be to arrange them by brand or by price, with less expensive frames on the left or on the bottom and the more expensive models on the right or up at the top. That way, a customer who has already decided on a brand and a price range can quickly pare down the bewildering array of options available.

Alternatively, the racquets could be arranged according to the ability of the player, with beginner racquets on the left, intermediate player racquets in the middle and advanced player racquets on the right.

From a scientific point of view, the performance of a racquet has nothing to do with its price or brand or its perceived level of performance. It has to do with technical things such as weight, balance, swingweight, head size and stiffness.

Scientific Display

A more scientific way to display racquets would be to have lighter weight racquets on the left and heavier racquets on the right, arranged vertically according to one of the other physical parameters. But therein lies a problem. There are too many variables to make a logical selection of the top-to-bottom order, especially given the limited vertical and horizontal dimensions of the wall on which the racquets are displayed.

One way around this problem would be to display only a small selection of racquets around the perimeter of the wall and to have a large “selection map” or photos of each racquet with their price and physical properties printed underneath. The actual frames could then be kept in a back room, like shoes in a shoe store. Shoes come in so many sizes that it is often impossible to display them all in one room.

An even better way to display racquets might be to organize them as a database on a computer, to which your staff and customers would have access.

The Racquet Selection Map published each year in RSI, which plots each racquet in terms of maneuverability (swingweight) and a “power/control” formula, can be a useful guide. But research has shown that the power of a racquet depends mostly on its swingweight, for two reasons. One is that the swing speed of a racquet depends mainly on its swingweight. It also depends on how fast the player wants to swing the racquet; but if he or she wants to swing it at high speed, then the physical parameter limiting the speed is its swingweight.

The second reason is that, for any given swing speed, the ball speed off the racquet depends mainly on its swingweight. The ball speed also depends on where on the racquet the ball is struck; ball speed is a maximum near the middle of the strings or slightly toward the racquet tip. But for any given impact point on the strings, the ball speed depends mainly on the swingweight.

Racquet stiffness is also a factor if the ball is not struck near the middle of the strings, but swingweight is the most important factor. If the ball is struck near the tip or throat of the racquet, then the racquet will vibrate and the outgoing ball speed is reduced slightly due to the energy lost in creating those vibrations.

Mapping with Swingweight

High swingweight racquets, therefore, are more powerful, but not by much. The point here is that low swing-weight racquets can be swung faster, and that almost makes up for their lower “inbuilt” power.

A good design for a racquet selection map would be to have racquet weight along one axis and swingweight along the other axis. That way, the two most important properties of a racquet are displayed in a way that is easy to understand and that doesn’t involve any complicated mixture of racquet properties that might confuse the reader. (Editor’s note: RSI is considering adding this type of racquet weight/swingweight chart to our annual Racquet Selection Map.)

Racquet stiffness, head size and width are also important factors that affect the feel and performance of a racquet, so they should also be listed for each racquet, together with a few words under the selection map to guide and educate the reader. For example, wide-body racquets are relatively stiff and are preferred by recreational players since they don’t vibrate as much as flexible racquets. In that respect, they have a larger sweetspot where racquet vibrations are relatively weak so the shot feels good.

Narrow-body racquets are preferred by players who use a lot of topspin since the ball doesn’t clip a narrow frame as easily as it clips a wide frame. Large-head racquets are preferred by recreational players since such players tend to hit the ball all over the string plane. Smaller head racquets are preferred by good players since they are more maneuverable.

There are a lot of factors that go into finding the perfect racquet for your customer, but we can’t underestimate the importance of swingweight in this decision. Making sure players are using the right frame for their games is an important part of keeping them in the game — and buying equipment in your store.

Rod Cross is an Honorary Associate Professor of Physics at Sydney University, having retired in 2003 to work on the physics of sport and forensic physics. Tennis is his main interest, and he has written two books on the physics of tennis with Crawford Lindsey and Howard Brody. Cross also has written the books, “The Physics of Baseball and Softball” and “Evidence for Murder,” based on his forensic investigations into the death of a model in Sydney. For details, visit physics.usyd.edu.au/~cross.

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.

From the June 2011 issue
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