Tennis Industry magazine

 

String Selector Map 2004

Cure your customer’s arms, heads and games with the right string.

By Crawford Lindsey

Choosing a string can be an unbelievably bewildering affair with absolutely no guide to choosing except pure chance and anecdotal advice — until now! The String Selector Map is the only quantifiable way to compare and select from the over 350 strings tested in the RSI/USRSA lab and listed here. Post it in your shop. Your customers will thank you — with more business.

Every choice concerning string is about its effect on stringbed stiffness. Material, construction, gauge, and tension decisions all influence stringbed stiffness, and it is the stiffness that influences the ball response and what you feel. It’s all about stiffness — dynamic stiffness, that is. Dynamic stiffness refers to how much the string at given tension will deflect when it is impacted with a ball or experimental hammer of a given energy.

Stiffness and Feel

For any given amount of energy in the impacting ball or hammer, a soft stringbed will display greater deflection. It will also have a lower peak impact force and tension, as well as a longer dwell time. Consequently, when comparing two strings on the String Selector Map, the softer string (to the left) will exhibit the above characteristics compared to a string with a greater stiffness (to the right). This difference in stiffness and the labels “soft” and “stiff” are not meant to indicate “good” or “bad,” however. Some players will call the softer string “comfortable” while others will call it “mushy.” And while some will call the stiffer string “crisp,” others will call it “harsh.” But whatever you call it, and whatever type of “feel” you prefer, all strings with the same stiffness values (within some plus or minus range) should in theory feel very similar, at least for a certain amount of time until differing rates of tension loss change the relative stiffness of the two strings.

Any string located on the same vertical axis has the exact same stiffness, regardless of tension. Likewise, any string on the same horizontal axis is at the same tension, regardless of stiffness. Therefore, no matter how much tension has been lost, all strings on the same vertical axis should feel about the same. All strings along the same horizontal axis can feel widely different from each other, even though they are at the same tension. The key determinant is not how much tension a string of a given material, construction, or gauge loses, but how stiff it is after it loses the tension. String plane stiffness depends primarily on string stiffness (which is related to material, construction, and gauge), tension, headsize, and the string pattern.

Tension Loss and Feel

A string’s tension and stiffness never stay the same. String performance is never the same from one hit to the next. Strings lose tension and become softer with every hit and every second that passes. Strings on the Selector Map that have lost more tension in the testing will probably continue to lose more over time than others. However, the effect of this tension loss on what you feel depends on several factors: the stiffness to begin with, the stiffness that feels best to you, your sensitivity to change, and how you interpret what you feel at a given stringbed stiffness compared to another.

Tension Loss and Going ‘Dead’

When a string loses tension, it becomes softer. That is all that changes in a string. It does not lose power, resiliency, elasticity, or go “dead” in any quantitative way. It simply becomes less stiff, and thus feels “different” to the player.

In fact, in the testing, a hammer will be ejected at essentially the same speed whether you raise or lower the tension. The string doesn’t change its “power” or elasticity. When you change tension and hit a ball, the change in stiffness will affect how much energy will go into ball deformation. This, in turn, will determine how much energy is lost, because the ball loses almost half of the energy that goes into it.

So when a string loses tension and, according to some players, goes “dead,” it doesn’t lose power. The power potential of the string is the same (if not more). The difference is how much energy goes into the string vs. the ball and the result this has on performance and perception. In all cases, the string gives back about 95 percent of the energy that goes into stretching it. So as tension declines, the string actually takes in, and thus gives back, more energy (which translates to ball velocity). So tension loss does not equal “dead” in terms of “power” (ball velocity), but in terms of diminished “force of impact,” shock, and feedback.

Tension Loss, Shock, and Feedback

You don’t get the same “punch” when you hit the ball with diminished tension. The ball may go faster and farther, but it feels like less oomph. And because you have actually lost “control” as witnessed by the ball going farther (i.e., not where you are aiming), you may back off your stroke speed, which lessens the feel of impact oomph even more. So you haven’t lost power, but you have lost the feel of providing the power, being in control, and getting feedback confirmation from the racquet punch. In essence, you have lost shock, not power. The impact doesn’t shock you as much. Not as much shock is not as much “feedback.” So, in one sense, when you choose a string, you are choosing the level of shock that feels good or proper to you. (“Feeling good” doesn’t mean that the level of shock that you like is good for you, however.)

Tension Loss and Consistency of Feel

But, however you interpret the change in feel as tension goes down, the fact is that the feel does change. So, consistency of feel is important.

A string at the top of the Map (more tension loss) had to change tension more over the test range to get to its current spot on the stiffness scale. It is likely that those strings will continue to lose tension (thus, change in feel) at a more rapid rate than others farther down on the Map. But strings do stabilize with time, and the rate of loss continually slows down. Consequently, a relatively “fast changing” string may still be slow enough to stay within your “feel range” for an acceptable amount of playing time. That will depend on your sensitivity and response to change. The Map does not tell us the range of stiffness values that will “feel the same” to any given player.

Nonetheless, a rule of thumb does apply here. Any strings at the same stiffness value on the Map should feel close to the same for a certain amount of time, no matter what amount of tension loss was required to arrive at that stiffness. The amount of tension loss is irrelevant as long at the resulting stiffness feels good to you. So, tension loss is not in itself a necessarily good or bad thing. But, if you are unhappy with how long your string maintains its feel characteristics, then try a string of the same stiffness, lower down on the Map.

Tension Loss and Control

Losing tension affects control (or the feel of control) in three ways. First, it may be that the lower tension results in more energy return to the ball from the strings, and the ball will travel farther (tests show this is very small — in the range of 1 to 2 percent more velocity and only with very large tension changes) with the same swing. Secondly, a lower tension will result in a longer dwell time. If you swing at the same speed, then the ball will stay on the strings through a longer part of the arc of your swing. The angle of your shot will therefore be affected. And third, if you hit off center, the ball will twist the racquet for a longer time and affect the launch angle.

Sometimes players describe this as a “trampoline effect,” or by saying that the “ball flies all over the place,” or “I’m spraying my shots.” The answer? Move to a string to the right of your current string at the same tension loss, or move down to a lower tension loss string at the same stiffness.

Stiffness, Sound, and Feel

Losing tension changes the sound of the impact. The pitch is lower. You can go from a “twing” to a “twang.” “Twings” sound more responsive, elastic, powerful than “twangs” — if that is what you are used to, anyway. This twanging will affect the psychology of your play. The secret is to tune your racquet. Stiffness is the key factor in twings or twangs. Twings will be to the right and twangs to the left of your current string on the Map.

Conclusion

String selection is definitely part science, part psychology, and part art. If you integrate all these aspects, it can also become a very good business. If you can demonstrate and communicate your proficiency to your customers and help them understand their own relationships with their strings, you will surely become a true Zen master of stringing, retailing, or coaching.


The Geography of ‘Feel’


Test Procedure

We tensioned each string to 62 pounds and allowed it to sit for 200 seconds. Then we struck the tensioned string five times with a force equivalent to hitting a 120 mph serve. The tension loss represents the total amount of the relaxation over both time and impact. The stiffness value is a calculation derived from the amount of force created at impact to stretch the string. Lower values represent softer strings and lower impact forces. Higher values represent stiffer strings and higher impact forces.

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About the Author

Crawford Lindsey  is co-author of The Physics and Technology of Tennis and Technical Tennis

 

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