RACKET STRING 101:
AN INTRODUCTION TO STRING TECHNOLOGY

By Steve Crandall
Vice President, Sales & Marketing
Ashaway Racket Strings

Racket string may well be the most important piece of equipment in the game of tennis. After all, you hit the ball with the string, not with the racket. A $60 racket with a great string job will probably play better than a $300 racket with junk string.

Given its importance, it's surprising how few players pay attention to string. Perhaps the problem is that differences in string are mainly invisible, while different racket features can be easily pointed to and explained.

In a common scenario, a player breaks a string, or realizes that his racket has lost its "zing," so he brings it to a stringer and says, in effect, "here: restring this." Too few stringers use this as an opportunity to educate their customers: they respond by simply replacing the player's string with more of the same: in most cases, average quality string that's known generically as "synthetic gut." Even players who do their own stringing are likely to take the easy path and use the same type of string, time after time.

But while synthetic gut is a viable choice for some players, it's certainly not the best choice for everyone. Other types of string provide different levels of power, control, durability, "feel," playability, and a whole host of other qualities. But unless you try them out and do some experimenting, you may never realize the full potential of your racket, or your personal abilities.

Every serious player therefore needs to know the basics of racket string. Once you know how different string characteristics affect their performance, you can assess your needs and choose string accordingly. So here are the essentials of tennis string technology.

The Materials

The vast majority of tennis strings on the market today are made from synthetic fibers. The only exception is gut, which is a natural product derived from animal intestines.

The most common synthetic fibers in strings are nylons, aramids, and something called PEEK - manufactured by large chemical companies like DuPont and ICI. String manufacturers test and compare different fibers, and when they find one with desirable characteristics, they engineer it into a structure keyed toward a particular type of performance. More than one fiber type may be built into a string to combine various properties. For example, the core, or central strength member, might be made of a resilient fiber, for power, while the jacket, or wear layer, might be a tough material, for resistance to abrasion.

Nylon is a do-anything material: it can be very resilient, or relatively stiff, depending upon the specific chemical compound. Nylon is often used as a core material, where its stretchiness can provide a great deal of power. Nylon-cored strings (e.g., Ashaway's Liberty 16) tend to be the least expensive on initial purchase, and are often easier to string than those made from stiffer fibers. Because they combine economy and power, nylon-cored strings are often recommended for the majority of players.

Most strings, regardless of their core material, have nylon jackets. The nylon compounds used in jackets are typically harder and less resilient than the nylon used for string cores, to better resist notching and abrasion.

Aramids, such as DuPont's well-known Kevlar fiber, are used in bulletproof vests, automobile tire cords, and top-end canoes, in addition to racket string. As you would expect, aramids are stiff and durable, and have extraordinary tensile strength, which permits very high stringing tensions. At typical stringing tensions, Kevlar-cored strings (e.g., Ashaway's Kevlar 17) stretch only 5 percent, compared to nylon which stretches 20-25 percent. These qualities provide topspin players with superior ball control, because the string bed remains quite flat and stable.

On the other hand, because they don't stretch much, aramid-cored strings do not provide a lot of power. Rather, they are for topspin power players: strong hitters who don't need more power from their string, but want to enhance their control game.

Aramids can also be used in string jackets, where they provide extraordinary notch resistance, and stiffen the string structure even further.

Zyex is a polyetheretherketone (PEEK) material, introduced by British chemical giant ICI and later spun off as Zyex Ltd. More flexible and resilient than aramids, Zyex is still pretty stiff compared to nylon: at typical stringing tensions it stretches about 14 percent.

Zyex's remarkable characteristic is its "dynamic stiffness,"; that is, it recovers from stretch very quickly. A racket strung with a Zyex-cored string (example: Ashaway Dynamite WB) maintains a fairly flat string bed, which helps in ball control, but the rapid recovery from stretch provides good power. Some players feel that Zyex approximates natural gut in performance. But Zyex is more durable than gut, making it an excellent choice for players looking for a balance of power, control, and practicality.

Gut is a different animal altogether. Manufacturers start with animal intestine and slice it into ribbons. These are processed in a series of chemical baths, then twisted together, stretched, and dried. The string then goes through a polishing machine to make the surface smooth and round, then it is coated with polyurethane to protect it from abrasion, moisture, and ultraviolet light.

Gut is relatively stiff, compared to nylon, but it has very good dynamic resilience, combining the benefits of a stiff string and a stretchy string, and providing good control and power. Gut strings cost more than synthetics, and in some cases they may not be quite as durable. They are still favored by many top pros, however, for whom compromise is not an option. But most players may be better served with lower-priced synthetics that offer similar performance.

String Design

As noted above racket strings have two basic components: a core and a jacket. The core provides strength, and determines the majority of the string's "response" characteristics: how much it stretches, and how quickly it rebounds. The jacket holds the core fibers together, protects them from abrasion, and provides surface texture.

Three basic core types exist. Monofilament cores, found in string commonly called either "tournament nylon" or "synthetic gut", are the most common. They consist of a single thick nylon fiber, or a few medium-gauge nylon fibers that have been chemically bonded together so they are, in effect, a single thick filament.

The second type, multifilament cores, are made from thousands of very thin fibers twisted and/or braided together, but not chemically bonded: they're free to stretch somewhat independently from one another. As a result, multifilament cores are more resilient than monofilament cores of the same material. This generates more power, but at the expense of ball control. Multifilament cores may be made from nylon, aramid, or other materials. Nylon multifilament cores (e.g., Ashaway Vantage Supreme) have a very soft feel, and are often preferred by players with "tennis elbow."

The third type is the composite, or "semi-monofilament" core, in which a small number of relatively thick fibers (usually from three to ten) are twisted or braided together, but not chemically bonded to each other. For example, the core of Ashaway's Dynamite WB string consists of five Zyex monofilaments twisted around a sixth central one.

Jackets may be either twisted or braided over the core. Braiding produces a more highly textured surface, which again provides more "bite." Braided jackets are also more durable. Because of the over-under-over design, the fibers are effectively locked together, so the breakage of a single fiber doesn't mean the imminent death of the string. In contrast, when a fiber breaks in a twisted jacket, it may unravel all the way up to the grommet. On the other hand, strings with twisted jackets often string up more easily.

A final design option is the hybrid string set, in which the vertical or main strings, and the horizontal or cross strings, are of different types. Usually, stiffer strings are installed as the mains, and more resilient strings as the crosses. Popular hybrid combinations include Kevlar mains/synthetic gut crosses (e.g., Ashaway Crossfire 17), and Kevlar mains/natural gut crosses (e.g., Ashaway Gutfire). In fact, all Kevlar strings are almost always used as the mains in hybrids: they are simply too stiff to use by themselves.

String choice can make a big difference in your tennis game. Refer to the accompanying table for a quick summary of these considerations, and look at every restringing as an opportunity to improve your performance.