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The other day I broke a string and was forced to use one of my playing partner’s racquets. It has a much higher stiffness rating than my racquet, which is pretty flexible, but I actually struggled to generate my usual pace with it. What’s the deal: Aren’t stiff racquets supposed to be much more powerful?—David L.
Defining a racquet as “powerful” based solely on its stiffness rating can be misleading. There are other specs involved—weight, distribution of mass, string pattern and set-up, length of frame—that will certainly affect a racquet’s power potential. Perhaps your friend’s racquet is much lighter than yours, or its geometry made it more challenging to maneuver. There’s no getting around force = mass x acceleration. That frame may also have been strung at a higher tension or with a stiffer string than you’re accustomed, which can require more effort to hit with pace and depth. But the general rule of thumb is that firmer racquets tend to lead to bigger hitting.
Just to clarify, the flexibility of a racquet is generally revealed by its RA rating. This is typically measured with an expensive piece of equipment such as a Babolat RDC (racquet diagnostic center) machine. The range for most commercial racquets is from around 60 (lots of flex) to the low 70s (rigid). Manufacturers may put this rating on the frame, but it’s easily accessible at knowledgeable pro shops or online retailers.
Keep in mind that a frame’s unstrung stiffness measurement will be higher than when it is strung. Another consideration is that the stiffness rating is taken at one particular location on the racquet; it’s possible that due to variances in thickness and construction it could be firmer or more flexible at different points. So it’s conceivable that a racquet with a low RA rating can actually feel stiff at certain impact locations and vice versa.
The basic reason why frame stiffness impacts power—don’t expect a sophisticated physics lesson—is energy transfer. At contact all frames bend somewhat; the more flexible, the more energy it absorbs. If the ball stayed on the string bed for the entirety of the frame’s return to stasis, all that potential energy would be transferred into the shot. (Think Wile E. Coyote landing in a bendy palm tree and then getting flung into oblivion). However, a tennis ball remains on the strings for a fraction of a second and leaves before the frame fully rebounds. So, all other things being equal, a stiffer frame bends less and transfers more energy into the ball than one with more flex.
This translates into a firmer racquet providing a more uniform hitting surface with a higher margin for error. The user doesn’t have to center the ball as precisely to produce a workable shot. For me, this is one of the more noticeable contrasts—mishits aren’t punished as severely and typically land deeper in the court. When hit in the sweet spot, there may not be a discernible difference in power. However, when a flexible frame is contacted off-center, there’s a greater disparity in performance with a better probability the ball flutters with less pace and depth.