String Theory is a regular segment that delves into the science and/or technology of strings. Today, we open up Rod Cross and Crawford Lindsey’s book Technical Tennis and take a look at how several relatively unexpected racquet variables—from string patterns to head size and grommet systems—can affect stringbed stiffness and, by extension, performance.
Stringbed stiffness—the combined measure of all factors, from string material to racquet stiffness, which affect the extent to which the strings deflect upon impacts—exerts a significant influence on a racquet's feel and performance. For example, decreasing the stringbed's stiffness (say, by dropping the strings' tension) will increase ball velocity and depth, not only because looser strings return more energy to the ball upon contact, but also because they increase the ball’s vertical launch angle off the stringbed.
Of course, for most players, there’s usually a price to pay for that extra power: A loss of control. Like a trampoline, lower tensions fundamentally increase “dwell time”—i.e., the length of time the ball stays on the strings upon impact. And added dwell time, in addition to increasing rebound velocity, decreases margin for error when timing the ball.
(Why? Because with longer dwell times, as Technical Tennis authors Rod Cross and Crawford Lindsey explain, “the ball will stay on the strings through a longer part of the arc of your swing both vertically and horizontally,” accentuating “the consequences of errors in the timing of your swing.”)
For the most part, then, the old saying still holds water: “String loose for power, tight for control.”
So what makes for a soft or stiff stringbed? (Or, for longer or shorter dwell times?) The reference tension (high or low) set by the machine on which the racquet is strung. The string’s gauge (how thick or thin the string is) is another factor, as is the string's material (polyester, synthetic, or natural gut, for the most part). Tension, gauge, material: these are variables that most string-savvy tennis players are familiar with.
But what about the shape and configuration of the stringbed itself? You may not realize it, but your racquet’s design also affects the stiffness of your stringbed—and, by extension, how it feels and performs. Consider these three additional string-altering variables:
String pattern, which refers to the number of main strings (up and down) and cross strings (side to side) in the stringbed, has a large bearing on how the stringbed plays. Given that the tension, material, and gauge of each string is the same, “an open pattern, 14 mains and 16 crosses,” Cross and Lindsey explain, “will be less stiff than a closed pattern like 18 mains and 20 crosses. Fewer strings resisting the ball impact will naturally stretch farther and thus be softer.” The result? All other factors being the same: The looser the string pattern, the more pliable the strings, greater the dwell time, and greater the power; conversely, the tighter the string pattern, the stiffer the strings, smaller the dwell time, and greater the control.
Head size, or the size of the area where the strings are, can also affect how the stringbed feels and performs. As I noted in a recent Midweek Mailbag, stringing a racquet with a 90 sq. in. head size at 50 lbs. is not the same as stringing, say, a 100 sq. in. head at 50 lbs.; the former’s stringbed will play stiffer—precisely because the strings are shorter and, all other factors being equal, shorter strings play stiffer than longer strings. “A 60-pound string 10 feet long is very easy to push sideways,” Cross and Lindsey write, “but if it is only one inch long, it will be very difficult to push sideways.” In principle, then, larger head sizes play looser, producing greater dwell time and greater power; smaller head sizes play tighter, producing shorter dwell time and greater control.
But it’s not just head size that affects string length; the racquet's grommets—i.e., the components that come into contact with and suspend the strings within the frame—can alter string length and, subsequently, how the stringbed behaves. Again, Cross and Lindsey provide the way: “In a frame with traditional grommets, [typically plastic pieces] which stick out from the inside edge of the frame into the string area, the anchor (where the strings bend) is at the end of the grommet. Racquets without grommets or with very large grommet holes (allowing free string movement and deformation) are anchored at the outside of the frame, giving the strings an extra length over which to bend and stretch.” The inference? With more traditional grommets, the stringbed plays stiffer, with less dwell time and more control; with more experimental, “open” grommets, the opposite occurs—a softer stringbed, with greater dwell time and power.
UPDATE: Indeed, an oustanding question is, as Jack says below, "the relative impact of each factor." I.e., it's important to account for not only how but to what extent each racquet variable affects stringbed stiffness. To be honest, I haven't yet figured this out. Look for an answer in a future edition of String Theory.