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Racquet power formula

Q: I find it interesting to read and ponder the tension-versus-power debate in your magazine and website. It all seems to make sense regarding energy in versus out, less than one percent velocity change with various string tension, etc. It makes for great conversation at the tennis club trying to explain to people.

However, there is one thing I don’t understand that you could help me with. If string tension doesn’t affect power significantly, then string stiffness doesn’t affect power significantly. Therefore, a small head racquet and a large head racquet should produce the same amount of power. The steel ball would bounce the same height whether the head size is small or large.

In the racquet selection maps you have printed in the past, you have a power formula that takes into account the length, head size, flex, and swing weight to come up with your figure. Why is head size part of the formula, it doesn’t seem to make any more sense than including the string tension. Am I missing something?

A: If you hit in the center of the oversized stringbed, or more precisely, at the node, you will get the small increase in power that you have noticed from our power formula. However, the key to oversize racquets is that the sweet spot feels bigger both because you have less energy loss in the ball due to the softer stringbed, but also because there is less loss in twist around the long axis on off-center hits. The larger head has a greater twist weight so less energy goes into racquet rotation around that axis. All else being equal except the head size (head width, really), the ACOR (ratio of the outgoing speed to the incoming speed on a stationary hand-held racquet) at every point on the stringbed should be higher for the larger racquet. So, as most players using racquets with bigger heads don’t hit in the center of the racquet, they are indeed getting more power (though it is difficult to know how much) and also control.

There is another aspect to this. ACOR is the real indicator of power, as the May 2003 issue of Racquet Tech magazine explained. The power formula that we have used in the past is an admittedly flawed indicator. You can’t take it too literally. It is just a starting point for racquet selection and is not meant to be a scientific calculation by any stretch of the imagination. When we came up with it, the industry just didn’t have anything else to go on. If we could measure ACOR of every racquet (and we might soon be able to do just that), then there would be no question as to which racquets are more powerful. Even knowing ACOR, there is a caveat: Weight and swing weight are a couple of the factors that influence ACOR, so if you increase the mass of your racquet to create more power potential, you have to be able to swing the now-heavier racquet as fast as you did your lighter racquet in order to realize that additional power.

From the May 2004 issue
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