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Does Higher String Tension Give More Control and Spin?

By Dr. Simon Goodwill
University of Sheffield, UK

Most players are familiar with the general principle that low tension gives more power and high tension gives more control. The lower tension strings stretch more during impact and thus store more energy. When the ball rebounds from the racquet, more energy is returned, so it leaves with a higher speed. (Though the increased speed is typically less than 1 percent, but that can translate to balls traveling 1 to 2 feet further on a baseline to baseline shot, depending on the speed.)

The claim that higher string tension gives more control is less easy to explain. There is certainly plenty of anecdotal evidence that players “feel” more control when using a high string tension. Furthermore, in the professional game, players like Andy Roddick and Serena Williams are reported to be using string tensions of over 70 lbs. Do they string this way to gain control? And if so, what is the link between high string tension and control?

We will assume that “control” means the ability to consistently make the ball land at an intended location. But there is more to it than that. Many players report that there is an associated “feel of control” when they are hitting their targets. So the question is what is happening during impact at different string tensions to affect both the bounce location and the player’s feel of that shot? We will examine four variables: spin, string movement, impact dwell time, and ball travel distance across the string face.

Spin

It is often assumed that control is linked with the ability to apply spin to the ball. If that is so, then does spin depend on string tension? Players often say “high tension strings bite into the ball giving more spin.” (“Biting” is used in common tennis parlance to mean creating more friction by increasing the space between strings for the ball to sink into; using rougher, stickier, textured, or shaped strings to “grab” the ball; using thinner strings to dig into the ball; or using higher tension to increase surface contact forces.)

Figure 1: View from behind the tennis racquet — (left) ball just in contact with strings, and (right) ball midway through impact.

Fortunately, the spin generated for a typical ball-racquet impact can easily be measured. This has been done at the University of Sheffield in England, and the results showed that the spin on the ball is not dependent on string tension or string type. In that testing it was concluded that all stringbeds are sufficiently “rough” to achieve maximum spin for the given shot. Therefore, even if thin, sticky, and tight strings were used in an attempt to increase stringbed “roughness,” there would be no actual increase in rebound spin.

However, the fact remains that players feel that they can achieve more spin with high-tension strings. Three possibilities arise: (1) the players are simply incorrect; (2) players feel a difference in some other impact related event like more or less dwell time, string movement, or ball travel across the racquet and incorrectly interpret that as more spin; (3) the player, not the racquet, does something differently when playing with higher tension strings that, indeed, produces more spin. So, we did more tests to find the answer.

The study compared two identical tennis racquets, one strung at 40 pounds and one strung at 70 pounds (the same tensions as in our previously mentioned spin study). The impact apparatus can be set up to simulate a player hitting a topspin groundstroke, and we can measure the ball rebound spin using a high speed video camera operating at 240 frames/sec. As in previous testing, it was found that the measured rebound spin for both racquets was identical. So have the players’ perceptions been proven wrong?

Not necessarily. Because tighter strings produce less velocity, the ball will land shorter in the court. To make up for this, the player might swing harder generating more spin. In this case, it is not tighter strings that produce more spin, but the player’s response to tighter strings. In any case, the player is likely to notice the greater spin without realizing that he is swinging faster.

Similarly, even if the player does not swing harder, he may think there is more spin with higher tensions. That is because, although the spin is not greater at higher tensions, the ball speed will be lower, so the ratio of spin to speed will be greater. The ball will then appear to land shorter in the court at slightly steeper angles and to bounce higher — in reality just consequences of less velocity.

If string tension doesn’t influence spin, it can’t influence control through spin. So we are forced to look elsewhere for our connection between string tension and control. Fortunately, we find three variables that do vary with string tension — string movement, dwell time, and ball travel across the stringbed — that might influence control.

Lateral string movement

The impacts were also recorded from behind the racquet, using an ultra high-speed video operating at 3,700 frames/sec to see and measure what happens to the ball and strings during an impact. Figure 1 shows typical views from this camera; Figure 1(a) shows the ball just in contact with the strings, and Figure 1(b) shows the ball midway through the impact. We are simulating a topspin groundstroke, where the racquet is whipped upwards and, therefore, in these images, the ball travels downwards on the racquet during impact. The racquet shown in Figure 1 was strung at 40 pounds, and you can see that the mains strings deform downwards during the impact. However, this downward string motion was generally not seen in our tests for the racquet strung at 70 pounds.

Does this string movement affect control? In theory, if the strings deformed downwards but then recovered to their original position before the end of impact, then they would increase the amount of spin applied to the ball. However, we found that the strings did not recover during impact and were permanently deformed. (Hence, remaining consistent with our findings that spin is independent of tension.) You can tell if your strings do the same by having a look at them after you’ve hit a topspin groundstroke.

However, the amount of movement of the strings will affect the impact because it influences the location at which the ball leaves the racquet. Therefore this string movement will affect the speed and angle at which the ball leaves the racquet and thus where the ball will land on the court. Furthermore, we found that the amount that the strings deform is very inconsistent. It depends on how hard the ball is hit, the position of the strings before impact, and also exactly where on the racquet you hit the ball — i.e., did the ball initially land on one string or on two strings. The lesson is that a low-tension string will give you less consistency in your strokes.

In sum, lower tensions result in more lateral string movement, which, in turn, contributes to more unpredictable ball trajectories. The player may also be able to feel this string movement since it will result in a softer impact.

Dwell time and ball travel on racquet

Different groundstroke speeds were simulated by varying the velocity at which the ball and racquet collided. At these different ball-racquet collision speeds, we measured the distance that the ball travelled along the stringbed during impact (using images such as those shown in Figure 1). We also measured the length of time (dwell time) that the ball was in contact with the strings during impact.

Figure 2(a) shows that the contact time for the balls impacting on the racquet strung at 70 pounds is 20 percent shorter than for the racquet strung at 40 pounds. This is simply due to the 70-pound stringbed being stiffer than the 40-pound stringbed. Figure 2(b) shows that the ball travels consistently further across the stringbed for impacts on the racquet strung at 40 pounds. This is because the ball remains in contact with the strings for a longer time on this racquet, and it therefore travels further across the stringbed.

Figure 2a

Figure 2b

The importance of the contact distance can be illustrated by considering the action of a tennis player hitting a “heavy” topspin shot. The racquet is moved forwards and whipped upwards. The probability that the shot is executed correctly will be increased if the distance that the ball travels across the stringbed is minimized. This highlights that the contact distance will have a direct link to the players ability to play a topspin shot. So, tighter strings will increase the probability of a successful topspin shot.

How does this correspond to what the player feels? Well, players may be correctly identifying that the ball travels a shorter distance across the stringbed when they use a high string tension because the shot feels “clean” or “solid.” A shorter travel distance at higher tensions may be interpreted as “biting” or “grabbing” the ball (i.e., less ball movement). In truth, this shorter contact distance has nothing to due with “biting” but is simply due to the shorter length of time that the ball is in contact with the strings and thus can’t travel as far.

The contact time will also influence your perception of control in another way. For any shot in which the ball does not land perfectly on the long axis of the racquet, the head will rotate during impact. The longer the ball remains in contact with the racquet, the greater this undesirable rotation will be, leading to large errors in your shot precision.

So, the longer the ball is on the strings, the farther it will travel on the stringbed, increasing both the racquet twisting in your hand and the chance for hitting the frame or less responsive parts of the stringbed. Higher tensions reduce all of these unwanted effects.

Conclusion

Changing racquet tension does not affect spin, but it does affect string movement, dwell time, and ball contact distance. These latter parameters all can affect the ball trajectory as well as the player’s feel of the impact.

The main advice is that high string tensions make your shot more consistent and make it easier to hit topspin shots. If you do not like the “boardy” feel of high tension strings, then use a lower tension but remember to restring regularly to minimize the affects of undesirable string motion.

From the January 2005 issue
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