Guide to Polyester Strings
Polyester strings are the choice of many professional players and have surged in popularity since their introduction in the 1990s. There are now a huge number of polyester strings (or ‘polys’) available on the market.
This guide covers some of the pros and cons to polyester strings, provides suggestions on what types of players should use them and explains the key things to consider when choosing a polyester string.
Key Features of Polyester Strings
Advantages:
- Enhance spin due to the ‘snap-back’ effect.
- Durable, owing to the low coefficient of friction (less abrading and notching where the mains and crosses touch – the main reason for string breakage).
Limitations:
- Lose tension faster than other string materials.
- String playing life is very short – the strings can feel ‘dead’ after only 10-20 hours of play.
- Feel stiff, uncomfortable and ‘harsh’.
- Do not absorb shock well and can contribute to arm injuries like tennis elbow.
Who Should Use Polyester Strings?
The main reason that players choose polyester tennis strings, despite their apparently short lifespan, is the greater topspin potential. Fundamentally, however this spin potential relates to spin that the player already knows how to produce. Polyester strings suit players with a large, aggressive and fast swing and are therefore aimed at those who play at a higher competitive level.
Many recreational players will be best suited to a synthetic gut or multifilament string, or a hybrid setup, until they develop good technique and can generate fast racket head speed.
In general, polyester strings are not recommended for the following player types:
- Players with a short, uncomplicated swing style are unlikely to benefit from polyester strings.
- Anyone with tennis elbow or arm injuries should avoid polyester strings.
- Junior players are advised not to use polyester strings for risk of injury.
- Beginners will not benefit greatly from polyester strings and will see much bigger gains in topspin by improving technique.
The main downside of polyester strings is that they lose tension quickly – a problem that is particularly significant for top professional players, who are highly sensitive to even small changes in string tension and require consistent performance from their equipment. To keep their rackets in optimal condition each time they take to the court, whether it be for practice or for a match, professionals typically have their rackets strung just a few hours before play and will discard a racket after only 1-2 hours of use. In long matches, it is common to see players request freshly strung rackets during the match. If a match was postponed, they may even have all rackets restrung the following day without having used any of them!
Recreational players are generally less sensitive to gradual changes to string tension and will unconsciously adjust their swing and playing style to compensate for the effects of tension loss. Nevertheless, players at any level who choose to use polyester strings should be prepared to restring their rackets frequently in order to maintain consistency in their technique.
Choosing a Polyester String
With so many polyester strings to choose from, comparing strings can be a little overwhelming. Manufacturers commonly market their strings with claims of superior performance, so how can we judge the accuracy of these claims and evaluate the performance of one manufacturer’s string against another?
Fortunately, there is independent performance test data available for most of the leading strings on the market. Analysing this data and comparing the performance of one polyester string to another requires an understanding of four key characteristics – stiffness, spin potential, tension loss and energy return.
Stiffness
The stiffness of a string is its ability to resist stretch. The more it stretches, the softer it is, and the less it stretches, the stiffer it is. Stiff strings include those made from polyester and Kevlar, while multifilaments, synthetic gut and natural gut are all considered soft strings. Stiff strings provide more control, less power and less comfort than soft strings.
Some polyester strings are softer than others, often as a result of blending polyester with additives or other softer polymers – creating a co-polymer (sometimes referred to as a ‘co-polyester’ or ‘co-poly’ string). The development of co-polys has led to differentiation between ‘stiff’ polyesters and ‘soft’ polyesters, which have greater elasticity and provide more comfort.
Tennis Warehouse University, a free online source of scientific research on tennis strings and rackets, has published test data on stiffness for a number of strings on the market. The chart below presents the dynamic stiffness for a selection of poly / co-poly strings on the market:
String stiffness measured at medium tension (51 lb) and fast swing speed
By comparison to the polyester strings above, which cover a range of dynamic stiffnesses up to nearly 300 lb/in, most synthetic gut strings have a dynamic stiffness in the region of 170 lb/in, while natural gut strings have the lowest dynamic stiffness – typically around 100 lb/in. Kevlar strings, as the stiffest strings on the market, can have a dynamic stiffness up to 600 lb/in.
It is important to note that the stiffness of a string also depends on its tension. A string at high tension is stiffer than a string at low tension. In the range of normal stringing tensions, however, the relative stiffness of one string to another will remain the same – a string that is stiffer than another at 60 lbs will also be stiffer at 50 lbs.
String stiffness measured at low (40 lb), medium (51 lb) and high (62 lb) tension and fast swing speed
The chart above shows the stiffness of a selection of strings – including polyester, synthetic gut, multifilament and natural gut strings – at three reference tensions. Even at a tension of 40 lb, the two Luxilon polyester strings have a higher stiffness than the softer synthetic gut string at 51 lb and the multifilament and natural gut strings at 62 lb.
Tension Loss
All strings undergo stress relaxation (tension loss) after being pulled to a fixed length. Most tension is lost during stringing and within the first few minutes after stringing (up to 15 pounds in one minute).1 Tension loss stabilises quickly after the few first minutes but rapidly increases with the first few hits (as much as 20 pounds in 20 hits). After that, the rate of tension loss again stabilises, but all strings continue to be subjected to ‘creep’ – a gradual loss of tension due to the equalisation of tension of molecule chains over time. With each subsequent impact a fraction of a pound more tension is also lost.
Strings at high tension tend to lose tension faster than strings at low tension.
Polyester strings are notoriously bad at maintaining tension. However, some polyester strings are more resistant to tension loss than others. Softer polys often struggle more when it comes to tension loss and performance longevity.
In addition to the data on string stiffness, Tennis Warehouse University has also published test data on tension loss for a number of widely used strings. The total tension loss measured for each string includes the tension loss immediately after stringing and the tension loss after 20 hits. The chart below summarises the measured total tension loss (as a percentage of the reference tension) for a selection of strings:
String tension loss measured at medium tension (51 lb) and fast swing speed
As tension goes down, so does stiffness. Nevertheless, even after losing 20-35 pounds before you have played a game, stiff polyester strings can still produce a stiffer stringbed than softer nylon strings that lose only a small amount of tension.
Spin Potential
Tennis Warehouse University defines the spin potential of a string (installed in both the mains and crosses) as the ratio of the ball-to-string coefficient of friction divided by the string-to-string coefficient of friction. The higher the ratio, the greater the spin potential.
The ideal combination to maximise spin potential is therefore:
- Low string-to-string friction, allowing the main strings to slide along the crosses and snap-back.
- High string-to-ball friction, causing the string to ‘grab’ the ball during the snap-back.
This combination means that the strings will slide and snap back while grabbing and spinning the ball.
The test data published by Tennis Warehouse University for a selection of strings is presented in the chart below:
String spin potential measured at medium tension (51 lb) and fast swing speed
Stiffer strings (i.e. polys) tend to be ‘slippery strings’ with low string-to-string friction. On the other hand, rough, textured or shaped polyester strings with a higher coefficient of friction ‘grip’ or ‘grab’ the ball. Textured strings include Luxilon ALU Power Rough while shaped strings can be hexagon-shaped (e.g. MSV Focus Hex), gear-shaped (e.g. Völkl Cyclone), or even square-shaped (e.g. Völkl V-Square). The high coefficient of friction of textured and shaped strings, however, applies to both string-to-string and string-to-ball contact. Optimisation of spin therefore relies a little on what the developer of Tennis Warehouse University, Crawford Lindsey, refers to as the ‘Goldilocks Principle‘ – the string must not be too rough or too smooth, or the spin potential of the string is compromised.2
Tests have also shown that as string tension / stiffness increases, spin increases. Again, however, the Goldilocks Principle applies – stiff materials and high tensions can generate too much inter-string friction and may constrain lateral string movement, therefore limiting the spin potential.
Energy Return
When a ball impacts the strings, the strings deform, absorbing some of the ball’s kinetic energy before returning it to the ball. The energy return of a string is related to resilience and is a measure of the amount of energy returned to the ball from the total energy that went into stretching the string during impact. A higher energy return indicates a higher efficiency.
Energy return is expressed as follows:
Energy return (%) = COR2 x 100
The COR, or coefficient of restitution, is a ratio of the rebounding ball speed (v2) to the impact (or incident) ball speed (v1):
COR = v2 / v1
All strings are about equally powerful and resilient. Strings are designed to return as much energy as possible and there is no significant difference in COR or energy return between different string materials, tensions, or gauges, or whether they are old or new. However, strings are viscoelastic, meaning they do not return all the energy used to stretch them. Most strings typically return 80-95% of the energy absorbed, with some energy inevitably lost.
The small difference in energy return between different strings relates to the amount of internal damping, otherwise known as hysteresis or hysteretic damping – energy dissipation as heat as result of friction created by the folding/unfolding, sliding and rubbing of polymer chains inside the material when it stretches and relaxes. The more viscoelastic the material, the greater the damping, the more friction is induced and the more energy it loses, resulting in a lower energy return.
String energy return measured at medium tension (51 lb) and fast swing speed
The energy return of a string is determined using a steel hammer instead of a tennis ball. This is because in a tennis ball collision, around 50% of the incoming energy goes into ball deformation, with the other 50% absorbed by the strings. The softer the strings are in comparison to the ball, the larger is the fraction of energy stored in the strings and returned to the ball. The stiffer the strings, the more the ball is compressed, and the fraction of energy lost increases. A steel hammer head does not deform and therefore does not absorb any significant amount of energy. All the kinetic energy of the ball goes into the string, whereupon most gets stored as elastic energy, with some energy lost as heat.
A higher energy return therefore does not necessarily mean a faster or more powerful shot. The energy return reflects the percentage of energy returned from what is actually stored, not the total amount of energy returned. In other words, the string may be efficient, but if it is stiff, the energy of impact goes into crushing the ball and not into deflecting the string. Thus, there would be less energy to give back to the ball. Decreasing the stiffness of the stringbed, either by decreasing tension or by changing to a less stiff string material or thinner string gauge, allows more energy to be stored in the strings and therefore returned to the ball.
There is a limit to the effect of loose tension, however. Strings that are too loose will dissipate energy, as the strings begin to move more within the string plane, rubbing against each other and generating more friction, through which energy is lost. Lowering tension too far – i.e. beyond the range of typical playing tensions – will therefore produce less power. You can’t play tennis with a butterfly net!
Another thing that can influence energy return is pre-stretching. Pre-stretching a string (i.e. stretching the string before it is installed) increases its stiffness and energy return while decreasing deflection and dwell time. This is because pre-stretching forces the coiled polymer chains to straighten and realign in the axial direction, removing some of the viscous play normally evident in the unfolding and sliding of chains. String manufacturers often do their own form of pre-stretching, where the string is stretched at high temperature to get the desired alignment of the polymer chains. When cooled, this alignment is set, determining many of the string’s properties.
Note on String Gauge
The gauge (thickness) of the string affects its performance. In general, the following rules apply when selecting the string gauge:
- Thinner strings have greater elasticity and play better: more control, more power and better comfort.
- Thin strings create more ‘bite’ and offer more spin potential.
- Thinner strings might be expected to lose tension faster than thick strings, but this is not always the case – it depends on the string.
- Thinner strings will break sooner as a result of notching.
The string performance data presented in the sections above are for 16-gauge strings (or nearest equivalent).
Summary
The string test data allows strings to be compared based on the crucial parameters of string behaviour. In particular, a string’s stiffness and tension loss rate can be checked in order to weigh up its performance.
Based on the test data, which can be found in the String Performance Database on the Tennis Warehouse University website, the performance of the selected strings can be summarised as follows:
| String | Price | Stiffness | Tension Maintenance | Spin Potential | Energy Return |
|---|---|---|---|---|---|
| Babolat RPM Blast | £££ | High | Good | Low to Medium | Good |
| Luxilon 4G | ££££ | Very High | Excellent | Low | Excellent |
| Luxilon ALU Power | £££ | Medium | Average | Medium | Average |
| MSV Focus Hex | £ | Medium to High | Good | High | Very Good |
| Solinco Confidential | ££ | Medium to High | Excellent | Medium to High | Very Good |
| Solinco Hyper-G | ££ | Medium to High | Excellent | Medium to High | Very Good |
| Solinco Hyper-G Soft | ££ | Very Low | Good | Medium | Very Good |
| Solinco Tour Bite | ££ | High | Excellent | Medium | Very Good |
| Völkl Cyclone | £ | Low | Average | Medium | Average |
| Völkl Cyclone Tour | £ | Very Low | Average | Medium to High | Average |
| Völkl V-Square | £££ | Low | Average | Very High | Average |
| Yonex Poly Tour Pro | ££ | Very Low | Average | Low | Good |
The most important thing to consider is how the strings change after being ‘broken in’ – i.e., the stiffness after the initial tension loss. Consider two rackets: one strung with String A and the other with String B, which is a little bit stiffer than String A but loses tension more quickly. The racket with String B will play stiffer at first, but a day after being strung it may actually be the racket with String A that plays stiffer.
What matters is that when you walk onto court, the stringbed stiffness reached by that time is the stiffness you like to play with.
References:
1. ‘The Physics of Tennis: How Tennis Strings "Go Dead" – Part 1’, Crawford Lindsey, Tennis Warehouse, 21st March 2013.
2. ‘Spin and String Stiffness’, Crawford Lindsey, Tennis Warehouse, 4th March 2011.
