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Double pulling

Q: We string both tennis and racquetball frames in my shop. For tennis frames, we always pull each string individually, and we never have a complaint. For racquetball frames, we sometimes have to double-pull — pull two strings at once — from the head. If we pull at the customer’s desired tension, he often feels that the racquet is not strung properly, is “weak,” the strings move and need to be adjusted after each shot. I’ve found, however, that I get a much better string job if I set the reference tension 2 pounds higher than the customer requests. My machine is properly calibrated, so why is there such a difference?

A: Any time you attempt to pull around an angle, you lose tension due to friction. You can demonstrate this with some string, a dowel, and a weight. Tie one end of the string to the weight, and then wrap the loose end of string all the way around a dowel. While you hold the free end of the string, observe how little effort it takes to keep the string from slipping as you lift the weight. Now, slowly unwind the loose end, and observe how you must increase the effort needed to prevent the string from slipping around the dowel.

When tensioning two strings at a time, the friction is enough to produce dramatic reductions in the tension on the string that is farther away from the tension head. The only tension that is “transferred” from the string closer to the tension head to the string farther from the tension head occurs when the friction of the string going around the outside of the frame is less than the internal friction (a.k.a stretch) of the string. That is, if it’s easier for the string to stretch than it is to overcome the friction where it contacts the frame, the string will stretch and no tension will be transferred to the other string.

Increasing the reference tension doesn’t compensate for this tension loss, although it may mask it enough that your customers don’t complain.

From the November 2005 issue
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