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Getting the lead in

Q: According your Racquet Mass Mover on-line calculator, adding weight less than 9 centimeters from the butt moves the center of percussion (COP) toward the tip, which is exactly what I want. Is this really true or is there a problem with the on-line calculator? When I add 20 grams to the butt of my racquet, the COP doesn’t move up (I tested it by hitting my hand against the string).

A: The COP on any racquet is a calculated distance from the butt cap, although it can be verified if you have the ability to measure the apparent coefficient of restitution. For example, given a hypothetical racquet weighing 300 grams, with a balance point at 30 cm, and a swingweight of 300 kg·cm², the COP will be 60 cm.

Adding 20 grams (which is a lot of weight to add in a small space) at the butt cap changes the COP by 2.07 cm. At the same time, the balance changes by -1.88 cm, and the swingweight changes by 2.00 kg·cm². Two centimeters of change in the COP should be noticeable, but keep in mind that adding this much weight in this example also changes the recoil weight by 16.88 kg·cm² and the hitting weight by -8.61 kg·cm². Thus, the racquet will feel more comfortable (due to the higher recoil weight, but it won’t transfer as much of its mass into the shot (because of the lower hitting weight).

The changes in these other measurements could account for your subjective impression that the COP did not move after you added mass to the butt cap.

For more information about ACOR, see “Raw Racquet Power” in the February 2006 issue of RSI.

From the September 2006 issue
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