The following videos were taken at 600 frames per second. The racquet was hand-held. The incoming ball was incident at 56 mph and at 40 degrees to the stringbed. The incident spin was zero. These movies were taken solely to see string movement and were not the movies used to obtain the spin data presented in the table.
Other movies were analyzed to obtain the data displayed in the table below. In these movies, the racquet was clamped into a secured holding device. The incident spin was -3820 rpm. The incident speed and angle were the same — 56 mph and 40 degrees.
To see string movement in the impact videos, it is best to use the control knob and/or the right and left arrow keys to progress the movie a frame at a time.
Experimental Spin Results
|Babolat||Tonic Gut 16||52||Gut||56||40||-3820||1642|
|Babolat||Tonic Gut 16||62||Gut||56||40||-3820||1524|
|Luxilon||ALU Power 16||52||Polyester||56||40||-3820||1777|
|Luxilon||ALU Power 16||62||Polyester||56||40||-3820||1626|
|Luxilon||ALU Rough 16||52||Polyester||56||40||-3820||1803|
|Luxilon||ALU Rough 16||62||Polyester||56||40||-3820||1780|
- All the polyesters had more spin than any of the nylons (a gut snuck in above some polys however).
- The average polyester spin compared to the average nylon is +20.2%.
- The difference in spin between the highest and lowest poly is +15.9%.
- The difference in spin between the highest and lowest nylon is +26.8%.
- The difference in spin between the highest string and the lowest string is +49.3%.
- Something about classes of strings and strings within a class seems to make a difference in spin — but what?
- NOTE: Topspin from the court point of view is backspin to the strings (hence -3820 rpm)
Does the string type and tension make a difference to spin? Spin depends on many factors. It primarily depends on the incoming ball parameters, such as spin, speed, and angle of trajectory, as well as the swing parameters, such as the racquet speed, angle, and tilt. But does it depend significantly on the string itself? And if so, what property of the string is most important to contributing to spin?
In the last several years, certain types of polyester strings have gained a reputation as producers of extra spin. For years this remained simply as anecdotal evidence without any scientific corroboration. Also, until recently, evidence from laboratory experiments supported the following conclusions:
- String texture or shape makes no difference to spin
- String pattern makes no difference to spin
- String tension makes no difference to spin
- String gauge makes no difference to spin
- String material makes no difference to spin
Research showed that no matter how you changed these variables, the ball and string gripped each other equally. In all cases of typical tennis swings, ball "bite" was the same. In all cases it was found that the bottom of the ball bites the string, which means that the contact point comes to a stop and friction drops virtually to zero since there is no longer any relative movement of the ball bottom to the strings (remember, the ball is traveling across the strings in one direction and the spin is usually reversed in the other direction, so the two cancel at some point during contact). But even in the labs, "overspin" started showing up — that is, more spin than theory predicted. A couple candidate explanations emerged — first, the force pushing up from the strings can be in front of, through the middle, or behind the center of gravity of the ball, depending on how it compresses on the strings. If the force is off-center it will create a spin-causing torque. To add top spin, the force would have to act behind the center of the ball. Unfortunately, experiments showed the strings most often push up in front of the center of mass, tending to reduce top spin. The other explanation was that the sideways movement of the strings caused the extra spin. This is obvious now, but just a few years ago players and researchers thought of string movement as either strings moving and getting stuck (and thus not causing spin), or as not moving at all.
About the same time that researchers were noticing these theoretical anomalies, more and more players started to report that they got much more spin from polyester strings compared to other materials, in particular, at the time, Luxilon strings. One thing was evident — 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.
So polyester's stiffness caused less power that in turn allowed players to swing in ways that produced more spin and speed, a combination known as a "heavy" ball. In this case, it wasn't that the string was more "spinny" but that it behaved in ways to allow the player to create more spin-friendly strokes.
But then another thing was noticed — these polyester strings didn't seem to move in the stringbed. When players hit the ball with spin, their strings did not move to the side and get stuck, like in the "old" days when you would see players straightening their strings between every point.
But the fact that the strings were in the same position after the shot as before could also mean that they moved and snapped back into position by the time you looked at them. What if the strings moved and snapped back into position while the ball was still on them? That would produce a tangential torque on the ball that would cause more spin. Theoretically, strings that stored the most energy in tangential string movement and returned to position without getting stuck by friction would spin the ball more. The key was that the moved string had to have enough energy to overcome the friction on its return to equilibrium and that the ball still had to be in contact with the strings long enough and with enough force to create sufficient friction to increase spin by the observed amounts.
The videos above, for the most part (and given the restraint of only seeing 600 fps), seem to support this view that tangential string movement is a contributing factor in spin. Fourteen identical racquets strung with 7 different strings at two tensions (52 and 62 pounds) were tested. You can see from the videos that polyester strings tend to move farther, more strings move, and strings move in more places (front, center, back of ball) than with either gut or nylon. The measurements presented in the table above also confirm that many polyester strings do indeed cause more spin. Unlike the movies above where the racquet was hand held and the ball had no incoming spin, the data above was acquired by firing the ball at a secured racquet with a speed of 56 mph, -3820 rpm spin, and an angle of incidence of 40 degrees.
But questions remain. Is it string movement alone that makes the difference in spin between certain polyesters and other strings? And what is the main cause of the kind of string movement that increases spin (not all movement will increase spin — movement without "snapback" will reduce spin)? Is it due to shape? Texture? Gauge? Tension? Stiffness? Impact duration? Or is it a combination of these that conspire to optimize frictional and other torque producing forces to produce the most spin? Can all string types be made in a way to maximize spin? The two friction demonstration movies above show that there is a big difference between the way nylon and polyester strings move against each other (and note that the polyester in the film was actually the polyester that produced the least spin in the experiments).
One last note is necessary to put these differences in spin in perspective. We are not talking about huge differences in forces here. Even though we see that there is a 49.3% difference in top spin between the highest and lowest strings in the experiment, there is only a 11.8% difference in the amount the spin changed on the racquet. The change in spin while in contact with the racquet was 5623 rpm for the highest and 5027 rpm for the lowest. The difference in total change in spin compared to the highest spin changer was about 1 to 5% for the polyesters, 5-12% for the nylons, and 3-5% for the gut. The difference in strings thus depends on how you look at the data and what your purpose is. As tennis players, we are only interested in the spin that is added after the work of reversing the ball's incoming spin is done, which is by far the biggest part of the work done. What is important to us is thus a percentage of a percentage of the impact event. So there is not a big difference between strings in the total friction event, but there is in the part of it that matters on court.