When I developed the current version of racquetTune I did a lot of calculations to determine the correlation between tension and stringbed stiffness. I built a 400 node model in a nonlinear cable software. I applied a load in the middle and calculated the deformation of the string bed. From that I got a rule of thumb that says that if you go from 16x18 to 16x20 the stiffness increases about 5%, and going to 18x20 gives another 5% if you keep the tension constant.
So if you want the same stiffness in a 18x20 racquet as in a 16x18 you should string it with 10% less tension.
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