How similar is the mechanism of slippery strings and spaghetti strings? A4: A spaghetti strung racket has three planes of non-intersecting strings compared to a.
Interview with Yoshihiko Kawazoe by Joshua Speckman for January/February 2011 ATLANTIC MAGAZINE “The
New Physics of Tennis:
Unlocking the mysteries of Rafael Nadal’s killer topspin” By Joshua M. Speckman, The Atlantic Magazine: Technology http://www.theatlantic.com/magazine/archive/2011/01/the-new-physics-of-tennis/308339/
Kawazoe's answers to Speckman's questions about the mechanism of topspin improvement: Q1. When did you first become interested in studying tennis strings? A.1 I became interested in studying strings when I started studying dynamic characteristics of tennis racket and developing the prediction and estimation system for tennis racket performance in 1987. Because, I was crazy about tennis and enjoyed participating local tennis tournaments almost every weekends, so I knew very well about wooden tennis racket suitable for me by experience, but it was difficult to pick up a suitable composite racket for me among variety of many types of new composite rackets, I wanted to know the properties of tennis racket myself by experiments with personal computer we could use in the laboratory. Until then I was studying mechanical engineering with automobile engine or something. Q2. Why did you become interested? A2. I became interested because I found that the so-called string tension is just the pretension when strung and different from the actual tension during impact. The actual tension is much larger than the pre-tension of strings. Furthermore, stiffness of string bed is important for impact force, impact time or dwell time, frame shock vibrations etc. and is different from so-called string tension. I found the key factors that the damping coefficient and the stiffness of a ball and the stiffness of string bed or string plane radically increase with their deformations, and this is closely related to the coefficient of restitution between a ball and string plane. Accordingly, I could explain why string tension does not affect the coefficient of restitution and the contact time between a ball and string plane. It was described in my papers.
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Q3. When did the hypothesis that spin generation is improved by slippery strings first occur to you? Did you see this hypothesis in someone else’s work, or was this your own idea? A3: We happened to find the fact that spin generation is improved by slippery strings when we did the experiment with ultra-high speed camera in order to investigate what is happening during impact with notched strings and the lubricated notched strings, because my partners said that the impact during tennis play feels mild and the main (longitudinal) strings does not stay side ways with lubricated strings at the cross intersections. My partners who are wife and her husband are tennis fans and enjoy tennis frequently and someday he said how about lubricating strings with engine oil or something answering to his wife saying that returning the slided strings side ways back to original position is troublesome. We found that the main (longitudinal) strings stretch and slide side ways more and they spring back by lubricating the notched strings, as a result the ball is given more spin when the ball is released from the strings. The notches at string intersections reduce the spin rate of the ball. Furthermore, we found that the spin rate of a newly strung racket without notch is larger than that of a racket with lubricated used strings. It was found that stringing new is necessary for spin performance, not for ball power. Q4. How similar is the mechanism of slippery strings and spaghetti strings? A4: A spaghetti strung racket has three planes of non-intersecting strings compared to a conventional single interwoven string plane according to the paper written by Goodwill and Haake (Goodwill, S & Haake, S., The engineering of Sport 4 , pp.231-237. Blackwell Science, (2002) Oxford.) The two sets of mains are threaded through hollow plastic rollers. The mains are connected together using thinner strings called spaghetti strings. The strings on the spaghetti racket are not interwoven and therefore can easily deform laterally (in the plane of the string bed) during impact. They can then return this stored energy to the ball during the restitution phase of impact. Laboratory tests showed that the spaghetti racket imparts almost twice as much spin on the ball compared to a conventionally strung racket, for a typical oblique impact. So, the mechanism of slippery strings and spaghetti strings is similar. Q5. How important to your research has been the 10,000 fps camera that you use? A5: It is important to analyze the accurate contact time, spin rate, ball velocity and the timehistorical behavior of ball and strings.
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Q6. Would an even faster camera improve your research? Would you be able to answer questions that are now mysterious? A6: The 10,000 fps camera is enough for tennis impact. Q7. Was your 2005 paper, “Super high-speed video analysis of tennis top spin and its performance improvement by string lubrication”, the first academic research testing the ‘slippery string’ hypothesis? A7: Yes, I think so as English papers. . Q8. Polyester strings are naturally slippery, and some manufacturers have added other materials to make them even slipperier. How does the spin generation of new polyester strings compare to new nylon strings? A8: The most important thing is that the notches at string intersections reduce the spin rate of the ball. So, strings being difficult to produce notches are better. Q9. How does the spin generation of new polyester strings compare with nylon strings that have been lubricated? A9: There is no big difference between the strings without notches, because it is needed the parallel force to make mains slide sideways. There seems to be no big difference between the forces with the different type of strings. Q10. Have you done tests with natural gut strings that have been lubricated? How does lubricated natural gut compare to lubricated nylon? A10: Lubrication is effective to notched strings and not so effective new strings. We tested with nylon strings without notches, with notches and with lubricated notches under help of a professional player. We also tested with natural gut strings without notches and with notches under help of a professional player. Q11. We now know that low string/string friction improves spin generation. Does high string/ball friction also improve spin generation?
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A11: There is already high string/ball friction, because the strings are woven at intersections on the string plane. Strings biting and temporarily holding the ball is important. Ball sinking into the strings is important. This process has a significant effect on the rate of spin of the ball. So, additional friction does not seem to improve spin generation. Q12. Many string manufacturers are designing strings that have low-friction coatings but are “shaped” or textured (hexagonal, etc.). It seems that the goal is to have low string/string friction but high string/ball friction. Do you think this is the closest thing to a legal “spaghetti strings” on the market? A12: There is big difference quantitatively between slippery strings and the spaghetti strings being not woven and can easily deform laterally, so rule violation. Q13. Do you think thinner strings produce more spin by reducing string bed deformation and therefore reducing string/string friction? A13: Nonlinear dynamics of strings and the notches production mechanism tells us, No. Q14-1. Gosen has developed strings with square cross sections. They claim these strings generate more spin. Were these strings developed with your help? Or is Gosen only using what they learned from reading your papers? A14-1: I did not help them. Q14-2. It seems that Gosen’s square strings would resist notching. Do you think so? A14-2: Although it seems to be interesting, I don't know if it is effective or not. It needs experiment. Q15. Is energy return (or resilience) important for spin generation? If we had a polyester (stiff, low-resiliency), a nylon (medium stiffness, medium-resiliency) and a natural gut (low stiffness, high resiliency), and each of these strings had identical friction characteristics and were at the same tension, which would produce more spin? And why?
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A15: Energy return (or resilience) is important for spin generation. The most important is that the notches at string intersections reduce the spin rate of the ball. The spin rate of a newly strung racket without notch is larger than that of a racket with lubricated used strings. Stiffer strings less deforms. Lower stiffness strings more deforms, but my paper says that actual stiffness of strings radically increase with deformation during actual tennis impact. Nonlinear dynamics with string deformation - force properties tells us that there is no big difference between them at actual impact, although accurate prediction and estimation of string-ball behavior and spin behavior are not so simple. Q16. How slippery is Mira-fit? Is it more slippery than a very slippery polyester string like Babolat RPM Blast? A16: Mira-fit is a lubricant, not strings. It is effective to the used strings with notches. Even newly strung racket with no use, notches are easily formed. Applying lubricant makes the strings slippery, allowing more lateral sliding before springing back to their original positions. When no lubricant is applied, not only do the strings show less lateral sliding, but the strings are less likely to return to their original positions. Q17. When was Mira-fit developed and what was your role in that development? A17: Mira-fit was developed around in 2002 and 2003 by Mr. Okimoto at San-ai company. I was asked to analyze the effectiveness and properties of Mira-fit using high-speed cameras．He expected it increases elasticity and make string power revive. At first, it was not predicted it would affect the spin performance so much. Q18. Australian physicist Rod Cross has proposed that natural gut generates more spin than nylon because it has natural oils inside its structure that reduce friction at the string intersections. Do you think this is correct? Have you studied this? A18: We did experiment with nylon and with natural gut. According to the results, new natural gut generated more spin (3000 rpm) than new nylon strings (2800 rpm), but the players (hitting testers) are not the same, because they are separate experiments. We are concerned with the comparison between the newly strung racket and used racket, because we wanted to know the merits of newly stringing of a racket.
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Q19. Do you think the ITF will ban or regulate slippery strings like they did with Spaghetti strings? Why or why not? A19: ITF cannot find the reason to ban, because new strings without notches generate more spin than lubricated used strings, and there is no big difference between newly strung different kinds of strings without notches. Strings that are difficult to form notches become slippery, generate more spin, bring interesting games with spin and prevent tennis elbows because there is less shock and vibration. Q20. Players often complain that polyester strings “go dead”, although it’s not clear what this means. A20: I don't think that polyester strings “go dead”, because there is no reason. I do not think there is disadvantage of polyester strings for players. I am not sure for string makers, it may be a bit difficult to process and produce. Q21. Your 2005 paper “Super high speed video analysis of tennis top spin and it’s performance improvement by string lubrication” and the introduction of the MiraFit product seems to alert the ITF to the “slippery string” hypothesis. In 2005 and 2007 the ITF did experiments very similar to those you had done. Do you think your paper solved the mystery of strings and spin for them? A21: I do not understand the accurate meaning of this question, but I think my study solved the key mystery of strings and spin for them. Q22. It seems that string companies are now aware of the true mechanism of top spin improvement. Have any companies come to you asking for help in developing or studying strings? A22: Some companies contacted us, but they did not ask for help in developing or studying strings. Q23. To my knowledge there are several strings now on the market that are designed explicitly to take advantage of the slippery string mechanism for top spin improvement. These include:
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Babolat RPM Blast – silicone low-friction coating for spin Gosen Polymaster – square, 1.02mm, thickness for spin Luxilon Alu – PTFE resins for spin Babolat Addiction (nylon) – PTFE resins for spin Prince Recoil – PTFE (Gore-tex) wrap for spin Yonex 850 Pro (Spin) – slippery coating for spin Are you aware of any other strings that have been designed to be slippery? A23: Thank you very much for your precious information. I am not aware of any other strings that have been designed to be slippery. I am waiting for your information in the near future.
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