SPAGHETTI STRUNG RACQUETS
In 1971 a German horticulturalist, Werner Fischer, invented a new way to string tennis racquets. It generated so much spin on the ball that it was eventually banned by the International Tennis Federation in 1978. One of the problems was that players of relatively low standing were able to beat top players by generating much more spin than the top players could at the time. Ironically, the modern game of tennis has evolved to the point where players like Nadal can now generate almost as much spin as Fisher could in the 1970Ős with his spaghetti strings. Modern players run back and forth across the baseline in long boring rallies, rarely daring to come to the net since they can easily be passed down the sideline or overhead with a topspin lob. That was one of the reasons that the spaghetti stringing system was banned in the first place.
The spaghetti stringing system is shown in the photo, as it originally appeared in a Fisher aluminum frame. The same system could be used in a modern graphite frame, but it is now illegal. In the interests of science, I recently filmed Todd Reid hitting with the racquet to get his opinion. Todd won the junior boys event at Wimbledon in 2002. He said it was much easier to generate topspin and would use it if it was legal. The video clip (at 300 fps) shows a forehand where significant spin is generated even when the racquet head is not rising steeply to meet the ball. The video needs to be advanced one frame at a time to see the marks on the ball properly.
At first sight, the stringing looks something like a relatively neat Afro hairdo. The main features are:
There are only five cross strings and they are not woven with the main strings, as they are now required to be.
The main strings are all tied together at the five locations near the cross strings, using five thin strings looped around each main string, so that all the main strings move together. If one of the main strings is pulled sideways, all the main strings move sideways.
There are two sets of main strings, one on each side of the racquet, the cross strings being located between the two sets of mains. So, there are three separate layers of strings.
At points where the main strings intersect the cross strings, short sections of plastic spaghetti tubing are threaded onto the main strings. They serve the same function as modern Ňstring-a-lingsÓ that can be inserted between the mains and crosses to reduce friction and wear between the mains and crosses.
Small, sharp blobs of glue are attached to the strings to get a good grip on the ball but it is doubtful that they do anything useful. The strings will grip the ball regardless of whether they are rough or smooth, and the resulting ball spin is the same regardless of the surface texture of the strings.
How it works
It is clear that Fisher didnŐt come up with his stringing system entirely by trial and error. There was method in his madness. The system was deliberately designed to impart as much spin as possible to the ball. The object of the exercise was to allow all the main strings to move sideways when the racquet struck the ball, with the least amount of friction possible. If the ball is struck at right angles to the string plane, then there is no sideways movement of the strings. If the racquet head is rising upward (as well as forward) when the racquet strikes the ball, then all the main strings will move down when they contact the ball. As the ball starts to come off the strings, all the main strings snap back to their original position, giving the ball an extra vertical kick, thereby imparting additional spin to the ball.
If there is too much friction between strings they cannot move freely in both directions. If strings get stuck then players need to straighten the strings after every point, as they did when using woven strings, at least until polyester strings become popular after about 2005. However, most players assumed that polyester was better because the strings donŐt move, so there was no need to straighten them. What they didnŐt know was that polyester strings do move freely and they snap back into place immediately as the ball comes off the strings. Polyester is more slippery than nylon or natural gut, and can slide freely even when woven. Modern polyester strings generate slightly more spin than nylon or natural gut strings, but not as much as spaghetti strung strings. Modern players learnt to generate extra spin by swinging up steeply at the ball using an extreme Western grip for a forehand and a double-handed grip for a backhand, and by tilting the racquet head forward slightly.
If the main strings arenŐt tied together, then only those strings in contact with the ball are pushed sideways. The ball digs into the string plane so some of the main strings are pushed sideways to the left and some are pushed to the right. When the strings snap back to their original position, some generate extra spin but others act to reduce the spin. Fisher overcame the problem by tying all the main strings together. The effect can be seen in this video where some of the main strings have been removed in order to show what happens to the cross strings. Strings at the front edge of the ball are pushed forward, but strings at the back edge are pushed backward (advance one frame at time).
The physics of it
The following papers describe the physics of the extra spin effect. The effect is due to the increased elasticity of the strings in a direction parallel to the string plane. Some of the energy given to the strings as a result of tangential displacement of the strings is given back to the ball when the strings return to their original position. Spaghetti strings are more elastic than woven strings in that respect. The ball itself also stores some energy in the tangential direction, but the energy is mostly lost in the rubber and the cloth cover. On the other hand, a superball spins rapidly when it bounces since the ball is highly elastic in directions both perpendicular and parallel to the surface on which it bounces.
R. Cross, Impact of a ball on a surface with tangential compliance, Am. J. Phys. 78, 716-720 (2010).
R. Cross, Enhancing the bounce of a ball, The Physics Teacher, 48, 450-452 (2010).
A modern spaghetti strung racquet
The following photo and movie shows a modern racquet strung with a polyester string that is not woven. As such, it is illegal but the physics is very interesting. There is much more string movement than with a woven string and considerably more spin is imparted to the ball. In the movie, the ball approaches from the right with backspin at 30 m/s, and it bounces off the racquet (which is hand held) with lots of topspin. The strings are pushed forward but then they snap back and push the ball back almost along the line of incidence. With woven strings, the ball would bounce off to the left and at lower spin.
Spaghetti strings in close up. Not the original design but a modern version where the strings are not woven. This string pattern is illegal but the physics is more interesting than the legal, woven pattern
2Mb QuickTime movie filmed at 600 fps in Sept 2012 to measure the spin. You need to advance the movie one frame at a time to see the string movement during the bounce.
Most top players these days prefer to use polyester strings since they are relatively slippery. The main strings are pushed sideways (or down in the diagram) by the ball and then snap back into place as the ball leaves the strings. The snap back action exerts an upward force on the ball, as shown in the following diagram, giving the ball extra spin.
The upward force has an additional effect, in that the ball is projected higher over the net. As a result, the ball can land out if the snap back effect is too large. Players often say that the strings then have too much power. In fact, it is not the extra ball speed that causes the ball to land out, but the higher exit angle off the strings. The effect can be seen in the above movie.