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The Art of Selecting a Monofilament String: Perception versus Reality

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

Racquet technicians these days need to be more like artists -- though hopefully without the berets and pointy goatees. They can no longer just rely on basic science and engineering to "create" a masterpiece racquet setup for their customer. Rather, they have to weave their technical expertise with a thorough understanding of their client to select a string (or strings) and string-bed setup that is unique to his or her strengths, abilities, and desires. Says USRSA Certified Master Racquet Technician and veteran stringer John Gugel, "starting with a string you trust is essential."

Compounding the problem is the vast array of string choices available. This is especially true of monofilaments, which in the public mind are often lumped into the single category of "polyesters." Of course, not all monofilaments are polyesters and not all monofilaments are created equal. Some have more power, some more durability, and some better tension holding. Basically there are three types: "poly" (100% polyester); "co-poly" (polyester blended with other softer polymers); and "polyketone" (polyetheretherketone, PEEK or Zyex^® for short). In this piece we'll look at perception versus the reality of three key performance characteristics of these different monofilament types: dynamic stiffness; tension holding ability; and spin.

Dynamic Stiffness
Dynamic Stiffness can be likened to elasticity. It's measured in pounds per inch, and in layman's terms, is a measure of how much a string stretches when it impacts the ball. The more stretch, the more power a string returns to the ball and the less impact it has on your arm and elbow. The less it stretches, the more energy is lost in flattening the ball and the less total power is available for rebound.

As in golf, the lower the dynamic stiffness 'score,' the better. The chart below shows comparative data of several monofilament strings, including the best selling Natural Gut. I have to qualify this chart by saying that, except for the Dynamic Stiffness numbers which were provided by USRSA, all the other tests were done in Ashaway's own lab, which is not an independent, certified lab. Nevertheless, the numbers should be close enough to give you a clearer picture of how dynamic stiffness works.

The first column shows the tensile strength of the various strings, which is the force required to stretch the string close to its breaking point. Note that the leading polyester mono has better tensile strength than gut while all the others have less. So it's stronger, and one would think, provides more power. But not so. Look at the next column, "Elongation at 55 lb.," which is the amount a string stretches at racquet tension, expressed as a percentage of length. Polyester and co-polymer strings stretch less than half as much as gut, while the Ashaway Zyex string stretches more. This means that polyesters are much stiffer and return less power to the ball.

By comparison, the dynamic stiffness of Zyex is the closest to gut of any synthetic string material. That's why we call it a "manmade polymer equivalent to natural gut." This means it absorbs and returns impact force more like gut. It controls the ball more like gut, especially in thinner gauge strings, and holds tension as well, or perhaps even a bit better than gut. Zyex is impervious to moisture and offers excellent abrasion resistance to enhance durability. And, of course, it's cheaper than either gut or the high-end polyesters.

Tension Holding
Top pros are notorious for stringing their racquets to suit whatever conditions they will be playing in: if it's hot and humid, they'll string at one tension; cold and dry, they'll string at another. They do this to ensure that their racquets will be at their precisely preferred tension when they get out on the court to play. They want consistency, but even so, stringers often hear complaints.

So, what are the keys to tension holding? First, different string materials react differently to weather conditions. Nylon, for example, is hygroscopic: that is, it absorbs moisture, which adversely affects tension-holding properties. Think cooked spaghetti versus raw. Polyesters are somewhat less hygroscopic, but only PEEK polymers are impervious to moisture.

Second, polymer chemistry also plays a major role in tension-holding properties. To see just how big a role, John Gugel of Racquet Quest recently ran some tests, the results of which he has graciously allowed us to present.

John strung three leading brand strings -- a polyester monofilament, a synthetic gut (nylon), and Ashaway MonoGut^® ZX polyketone monofilament -- at 60 lbs. on identical racquets, stored them under stable temperature and moisture conditions, and measured string tension for 21 days. None of the racquets were played with or disturbed in any way, so the tension loss data shown in the chart below represents purely the tension response of the different materials, isolated from all other effects.


As you can see, all three strings lost tension in the first couple days after stringing as they "settled" into the frames. The polyester and synthetic gut both lost about 13% during this period, while the MonoGut ZX lost 9.5%. After that, the synthetic gut continued to lose tension fairly steadily, with a couple plateaus of stability in between. Interestingly, the polyester actually gained tension again until day 8, after which it lost it fairly rapidly, which might explain the pros' complaints about tension despite their restringing efforts. The MonoGut polyketone lost the least tension of all three strings, and held it steady the longest.

Overall, synthetic gut lost the most tension at 17.3%. Polyester was the second biggest loser at 15.2%, and was by far the most inconsistent. The MonoGut ZX polyketone lost least at 11.9%, and was most consistent throughout the trial. To put that another way, the polyester string lost nearly 30% more tension than the MonoGut ZX, and the nylon synthetic gut lost over 45% more.

So, why is this? If you took organic chemistry, you might remember that a polymer is a compound comprised of repeating structural units, often called a backbone. Polymer backbones consist mainly of carbon atoms, although other elements can also be included. These chains are held together with various types of bond. The simplest, for our purposes, is a covalent bond, which is a chemical link between two carbon atoms. An aromatic bond involves a more complex molecular structure that, in terms of tension holding properties, is much stronger.

So, here's the deal: nylon (a polyamide) is a long chain carbon polymer with simple covalent bonds. When used in string, it loses tension because the bonds break down. Polyester is a more complex polymer chain with one aromatic bond. It holds tension better than nylon, but as we saw, is very inconsistent. Polyketone, or PEEK, is an even more complex chain with three aromatic bonds. Thus, it holds tension better, longer, and more consistently than other strings. And consistency wins.

Spin
In tennis these days -- much like politics -- spin is all the rage. Everybody wants to improve their "up and down" game, and play like the top pros with "heavy balls" that drop like stones just inside the base line. And to generate that spin many are also emulating the pros by using the same "high tech" high-performance polyester strings, aka "gut on steroids." But what they're finding, unfortunately, is that polyester strings don't work as well for them. They feel like they have to work much harder with these strings to get less, and get worked over pretty hard by them in return. One fellow likened playing with them to "hitting a brick wall with a baseball bat."

So is polyester all it's cracked up to be when it comes to spin? The answer is yes, and no. In our discussion of dynamic stiffness, we talked about how stiff polyester strings are and how they actually generate less power than other string materials like gut and the new polyketone materials like Zyex. So then, how is it possible for a string that generates less power to also generate more spin?

Crawford Lindsey and company at Tennis Warehouse University have studied spin extensively. Their comparative study, Which Strings Generate the Most Spin, concluded that polyester strings do indeed generate more spin: on average, about 25% more than nylon and 7% more than natural gut. But they also point out that:

1.   The player has by far the larger role in generating spin. Racquet head speed, angle of racquet, and angle of swing basically trump all other considerations. Other non-equipment factors include incoming ball speed, angle and amount of topspin.
2.   While string material, tension and stringing pattern do play a role, the key factor in this regard is lateral string movement and snap-back. Polyester simply moves laterally more easily than nylon or gut, and thus produces more spin.

In a follow-up report, String 'Snap-Back' and Spin, they ask "What is it about the string/material that makes it move and stretch laterally?" They suggest stiffness, tension loss, and slipperiness as the main candidates. Polyester is stiffer, loses more tension, and is slippery. Stiffness helps overcome inter-string friction; lower tension decreases inter-string friction; and slipperiness (low coefficient of friction -- COF) enables movement. These, they say, "may all act in combination to facilitate string movement and snap-back."

So the key element here is inter-string friction.

In another report, String Movement and Spin, they point out that, "because polyester string is stiffer than other strings, it deforms the ball more, which loses energy, and this results in less power for the same swing. Because the ball had less speed, players swung faster to get more depth. But swinging faster (at an angle to the ball) causes more spin, so they could swing faster again, and so on -- more speed, more spin, more speed, more spin."

In short, high performance polyester strings work for the top pros because they are top pros. They swing harder and faster, hit at better swing and racquet angles, and are more likely to hit with the sweet spot on the racquet. Thus, they are able to do things with these strings that the rest of us can't. But even the pros have their limits with polyesters. Many are stringing at lower tensions or going to hybrid setups to reduce stiffness and impact shock. And you'll notice that when they do occasionally off hit, the ball goes zooming away like a rocket. High performance polyesters are like formula-one racecars: they take a lot of skill to drive.

So, what's a poor recreational player to do (or even a pro who's sick of getting beaten up by his string)? Is there another way to generate spin?

Although Crawford Lindsey and company have yet to study it scientifically, reports we're getting at Ashaway from players of all stripes indicate that polyketone-based strings like our MonoGut ZX do indeed generate spin like polyesters, and do so easier and more comfortably with less impact shock on players' arms and shoulders. This is because of PEEK's low dynamic stiffness (high elasticity). It stretches more and returns more power to the ball. Polyesters stretch less, and lose more energy in flattening the ball. Thus, less total power is available for rebound.
This is true regardless of whether a string stretches perpendicular to the string bed as in a straight hit, or lateral to the string bed in generating spin, so long as friction does not get in the way. The reason nylon generates less spin than polyester is because nylon strings are stickier than polyester. Their snap-back gets stuck. Gut is less sticky than nylon, but is still stickier than polyester. Our evidence suggests that polyketone is as slippery as polyester, especially when configured as a monofilament in MonoGut ZX. So you get more spin with less work. We call it Dynamic Spin.
This gives you the best of both worlds. Rather than a highly specialized 'formula-one' string, polyketone or Zyex strings are like Porsches. You get all the power and handling of a finely built racing machine, but it drives easily and you can take it on the road.

The Trend Towards Hybrids
I believe these characteristics of polyester monofilaments - stiffness, inconsistent tension holding, and 'formula-one' spin - are responsible for two trends I've noticed. One is that recreational players are losing their infatuation with high-end polyester. They don't like the impact, and they're realizing that normal everyday players just don't get enough benefit from these strings to warrant the added cost. Instead, they're turning back to more traditional multi-filament and synthetic gut strings, which offer a good combination of performance, economy and durability.

The second trend is the huge explosion in hybrid stringing. And it's easy to understand. Who would want a 100% polyester string bed that is so stiff and unforgiving? Players of all stripes -- including most of the players I saw at this year's US Open - are using polyesters for stiffness and durability, but mixing it with a softer string for playability. Now everyone's game is unique and string preferences range widely. But from my perspective as a string manufacturer, I don't think today's polyesters are the end of the story.

Artful Considerations
As a racquet technician these days, you need to call up your artistic side to thoroughly know your client and to select the monofilament string and the other variables that best respond to their needs. For example, would you put the stiffest, deadest poly in the racquet of a 12-year old junior player, just because he wants to mimic his favorite pro player? Wouldn't you rather nudge him to a softer choice? By the same token, say your best copoly customer is getting good results, but having to battle shoulder and arm fatigue. How about trying to match a polyketone with their favorite poly in a hybrid setup for more comfortable and satisfying results? Or, say you have a strong senior player raised on natural gut. Might they not like the opportunity to try a full bed polyketone or gut hybrid to improve tension holding and playing performance under all weather conditions, and economize on their string budget?

PEEK or polyketone polymer is an excellent choice for many of these situations. The dynamic stiffness of Zyex PEEK is the closest to natural gut of any synthetic string material. It absorbs and returns impact force more like gut. It controls the ball more like gut, especially in thinner gauges, and holds tension as well, or perhaps even a bit better than gut. Plus, Zyex monofilaments generate spin as good as the high tech polys, and the choice of 1.27 mm MonoGut ZX or the slightly thinner 1.22 mm MonoGut ZX Pro allows you to more closely tailor string choice to your client / player's style of play.

“Ashaway MonoGut ZX represents a unique product that seems to get better with age so to speak," says John Gugel. "I mean, the more I string with it, the better it responds to the process and the player. I believe it is a matter of understanding how to get the maximum benefit from MonoGut ZX. If you have not tried MonoGut ZX lately you should. Like a fine wine it just gets better with age and knowledge!”

ZYEX^® is a registered trademark of Victrex Ltd.

The dynamic stiffness of Zyex® PEEK is the closest to natural gut of any synthetic string material. It absorbs and returns impact force more like gut. It controls the ball more like gut, especially in thinner gauges, and holds tension as well, or perhaps even a bit better than gut. Plus, Zyex monofilaments generate spin as good as the high tech polys, and the choice of 1.27 mm MonoGut® ZX or the slightly thinner 1.22 mm MonoGut ZX Pro allows you to more closely tailor string choice to your client/player's style of play.