Types of string
There are four main types of string used in tennis:
Natural Gut - made from the intestines of cows. It takes three cows to supply enough intestine to make one racquet of string, and the process of turning the intestine into string is timely and therefore more expensive than other types of string. It is the softest string on the market.
Nylon - cheaper alternative to natural gut. Recommended for recreational players. It is more durable and stiff than natural gut but less durable and stiff than polyester.
Polyester - most popular for professionals. It is a stiffer string and loses tension faster than natural gut or nylon. It is generally more durable and therefore if you break strings quickly this might be the string for you.
Kevlar - very stiff string. It may be of use to you if your polyester string breaks too quickly for your liking or you are looking for a stiffer stringbed.
String can also be classified as multifilament (multiple filaments/strands wound together) or monofilament (one filament/strand). Usually multifilaments are softer but less durable than monofilaments.
Stringbed stiffness
The type of string will alter stringbed stiffness, but the type of string is only one of several features that can. Stringbed stiffness is the only problem that needs to be solved when it comes to string modifications. If you want more depth, have a softer stringbed. If you want more control, have a stiffer stringbed.
Features that alter the stringbed stiffness include the:
type of string (natural gut, nylon, polyester, kevlar; multifilament or monofilament)
tension (how tight or loose the string is)
gauge (how thick the string is)
racquet head size
string pattern (# of up/down and cross strings)
and grommet hole size (the holes that the string is threaded through).
Altering any of these features alters the stringbed stiffness. If one feature is altered which reduces stringbed stiffness but another feature is altered which increases stiffness by the same amount, the stringbed stiffness will be the exact same and the strings will play in the same way.
Modifying stringbed stiffness and its effect on performance
String features used to soften the stringbed:
Larger racquet head
Open string patterns (where strings are more spread out e.g. 16 × 19)
Loose string tension
Soft string material (e.g. nylon)
Big grommets/string holes
Effect on performance of a soft stringbed:
More power
Less control
Less shock/vibration
Same spin
Higher rebound angle
String features used to stiffen the stringbed:
Smaller racquet head
Closed string pattern (e.g. 18 x 20)
Tight string tension
Stiff string material (e.g. polyester)
Small grommets/string holes
Effect on performance of a stiff stringbed:
Less power
More control
More shock/vibration
Same spin
Lower rebound angle
While softening the stringbed does increase power, it does not increase the power by nearly as much as people think. For example, dropping string tension by 10 lbs will lead to less than a 2% gain in ball velocity of a ball travelling at 96.5kms. However, a lower string tension will produce a deeper shot giving players the impression that they have a lot more power than they actually do.
Lower string tension creates more depth by:
Producing slightly more power
Keeping the ball on the racquet slightly longer. With the racquet movement containing a vertical swing, the ball leaves the racquet at a slightly higher trajectory
The ball hitting the strings and rebounding at a higher angle. This is due to the ball plowing into the strings, making a hole in the strings that slows the ball more than it would in a stiffer stringbed, and results in a higher launch off the strings.
On creating depth, the authors write: “Hitting the ball one degree higher will make the ball land four or five feet further” and “If the ball is hit one mile per hour faster, it will land about one foot farther”. While the authors don’t provide specific data on how many degrees softening the stringbed adds to the trajectory of the ball, the authors do mention it is significant. For example, if you’ve ever hit a ball with a broken string, the ball can fly several metres further than intended.
How often should I restring my racquet?
A common question players have is how often should I restring my racquet? Well, it really depends on whether you can adjust to a drop in tension. Losing tension means you'll hit the ball deeper which could be good or bad. Another problem with a loss of tension is you'll some control. Losing tension can reduce control by:
Increasing power
Increasing dwell time of the ball on the strings, changing the release point in the swing and modifying the trajectory of the ball. In other words, the ball doesn’t come off the strings as crisply.
Increasing twisting of the racquet which changes the launch angle of the ball off the racquet. Due to the additional dwell time of the ball on the strings, when an off centre contact with the ball is made, the racquet will twist for longer, displacing the intended string direction of the racquet.
Increasing the steepness of the ball's rebound off the strings, increasing depth
Professionals change their strings every day to ensure that they are always playing with a stringbed of a very similar stiffness (or softness). Recreational players do not need to be so pedantic. The authors suggest recreational players could change their strings every time the tension drops by 20%, though this seems like an arbitrary number and is difficult for a recreational player to assess. Recreational players could use the same strings for two years or more if they wanted to, and the loss of tension could even add some benefit to their game, namely giving players more depth, more power, and reducing arm issues (as lower tension = less shock/vibration).
String tension loss
Players can be finicky about their string tension, but surprisingly, once a racquet is restrung string tension is lost before the player even makes their way to the court. Even during the restringing process the string goes through tension change. First, restringers string the ups and downs (otherwise known as the mains), which slightly alters the head shape of the racquet, pulling the racquet tip towards the handle and making the racquet more circular in shape. When the crosses are added, the crosses pull the sides of the racquet closer together and push the racquet tip back away from the handle, stretching the mains. This means that the crosses will usually have less tension than the mains do.
It is impossible for a restringer to string every string at the same tension. By the time they’ve tensioned just a few mains, the first main will drop in tension. After 30 minutes, most strings lose 5-10lbs and after 5 impacts with a ball and within 4 minutes, the tension of a racquet can drop between 6-19lbs. Therefore, to find a good restringer the most important thing isn’t how evenly tensioned each row of string is but how consistent the stringbed stiffness is with each restring.
Prestretching string can reduce the rate of tension loss and increase string stiffness. This is why professionals will often have their string prestretched. Similarly, the authors point out that 50% of thinner gauge string is stiffer than their thicker, same brand counterpart because during restringing the thinner string stretches further, reducing the amount of stretchiness the string has (making it stiffer), and reducing the rate of tension loss.
Misperceptions
Psychology plays a role in how players interact with their tennis equipment. Based on the authors’ research it is clear that players are not great at predicting how much power, spin, or tension is present in the racquet when they swing. This is because our perception is largely based on how the handle of the racquet feels against the hand during contact with the ball and the sound of contact with the ball, which doesn’t give the player a full picture in which to make accurate judgements about their string.
Players will often misattribute a tighter string tension as creating more power or spin because:
The impact sound is more pingy, sounding more powerful than the dead sound of a low tensioned racquet, even though the low tensioned racquet is more powerful
The ball leaves the strings sooner and the impact force on the strings is greater over a shorter period of time, providing a heavier thwack and giving players the impression that they are hitting faster. While the ball stays on the strings for less time, the power of the shot is not actually greater.
A tighter tension decreases power so players can compensate by unconsciously swinging faster, producing more power and spin.
Players hit with slightly less power when their strings are tight, so the spin to speed ratio is higher. While the revolutions of the ball stays unchanged (e.g. 1000rpm) the speed of shot is slightly lower (e.g. from 50kmph down to 49kmph), so the spin is slightly higher in comparison to the speed. The loss in speed makes it seem like the ball is spinning faster.
Conclusion
As a player, it can be fun to alter the features of the stringbed. Just know that strings do little to affect power and spin but will have more of an influence on depth and control. Use a stiffer stringbed for control and a softer stringbed for depth. If you are a recreational player and want a more significant increase in power or spin the best way is to learn to swing faster or to modify your technique. Professionals and semi-professionals can benefit more from altering their stringbed stiffness as they look to make small, incremental improvements in their game.
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