텐션과 스핀의 관계?

혜랑님 글 참고하세요^^


이번 호주오픈 최대관심사는 나달의 4대그랜드슬램 연속우승 그리고 더 나아가서
나달이 올해 모든 그랜드슬램대회를 석권하면서 진정한 그랜드 슬램의 위업을 달성할 수 있는가였지만
모두 알다시피...
sports trivia는 참 피곤한 일 같다.
이것저것 깔아놓은 어플 중 나의 tennis trivia가 있어 테스트를 해보니 참담한 결과가 나왔다.
질릴 정도로 많은 자질구레한 과거의 기록들이 태산만큼 쌓여 있고(나의 테니스 B.C.는 물론이고 A.D.9년 사이도 마찬가지)
해를 거듭할 수록 아니 매게임마다 새로운 기록과 사실과 에피소드들이 쉴새없이 쏟아져 나오고 있다.
스포츠에서 배드민턴 셔틀콕이 가장 빠르게 움직인다고 알고 있었는데
위에 링크한 글을 보니 바스크족의 전통경기인 jai alai가 the fastest sport in the world(300 km/h)로 알려져있다하고
(왼손잡이는 하이 알라이를 못하게 돼있다고 ㅋㅋ)
골프공이 몇 년전 328 km/h 기록을 냈다고 덧붙였는데 그 몇 년 사이에 새기록이 안나왔을 것같지 않다.

토끼해가 아니어도 테니스에서는 오래 전부터 파워와 스핀이라는 두마리 토끼를 쫓았지만
샷이 어디 이 둘만의 결과랴? 각도, 깊이, placement에 패턴이나 작전이 들어가는 게임스타일....

번역은 너무 수고스러운 일이라 간단히 요약 해보면,

"Technical Tennis: Racquets, Strings, Balls, Courts, Spin and Bounce"의 저자 Rod Cross와의 대담

- polyester줄은 나일론 줄에 비해 대략 25% 스핀이 많이 걸린다.

- 공이 줄과 접촉해있는 시간은 5 miliseconds에 불과

- 과거엔 공을 잘 감싸는 줄이(gripping) 보다 스핀을 많이 만들거라 생각했지만
과학적으로는 마찰이 적은 줄이 오히려 스핀이 많이 걸린다.
마찰이 적은 줄일수록 main 줄이 옆으로 많이 움직였다 되돌아오면서 공 측면을 때려줘 더 많은 스핀이 생긴다.

- 테니스에는 잘못된 상식이 많은데
가는 줄이 탄성이 높다거나, 텐션을 낮추면 공 속이 상당한 정도로 증가한다(공속의 증가는 실제 1%정도)거나
dampener가 tennis elbow를 예방 내지 경감해준다거나,
어떤 라켓이 다른 라켓보다 sweet spot이 넓다거나(대부분 라켓의 sweet spot은 직경 2 milimeters에 불과)

- 스핀은 공과 라켓면이 반발할 때의 탄젠트 값에서 나오는데 대개는 0.2이다.
이 값이 1.0 이라면(spaghetti strings이라면) 이론적으로 스핀량을 두배로 늘릴 수 있겠지만
테니스줄은 꼰사이기 때문에 줄에 따라 다르지만 대략 10 % 정도 스핀을 높일 수 있다.

- 하지만 선수가 head speed를 높이고 임팩트 직후 좀더 가파른 라켓면으로 만들어 주면
20% 부가적인 스핀을 만들어 낼 수 있기 때문에 줄이 전부는 아니다.

- 이상적인 테니스 줄이라면 인장강도나 발열점이 높아 잘 끊어지지 않고
시간이 흘러도 또 반복되는 충격에도 텐션이 떨어지지 않고 탄성을 유지하는 것이겠지만
그런 제품을 현실적으로 기대하기란 (string 업체 입장에서 이런 제품을 개발하면 바로 망하는 지름길)

- 마지막으로 테니스공이 표면에 만들어내는 볼 마크과 hawk eye 판정의 문제점
볼 마크는 원(직각으로 떨어지면)이기보다 길쭉한 타원인데다
공의 펠트섬유가 2센티 이상 뻗어나올 수 있어서 여전히 논란거리지만 어쩌겠는가?

올려주신 글 잘 읽었습니다.
좀더 원리를 설명해 주는 자료를 찾다가 참고가 될까해서 첨부합니다.
모두 번역할 수 없지만 결론만 간략하게 정리하면 다음과 같습니다.

높은 스트링 텐션은 더 좋은 컨트롤과 더 많은 스핀을 주는가?
스트링 텐션은 스핀에는 영량을 주지 않지만 스트링의 움직임과 임팩시 공이 라켓면에 머무는 시간과 라켓면의 접촉거리에 영향을 준다. 이러한 인자들에 의해 선수들의 임팩시의 감도 뿐만 아니라 공의 경로에 영향을 끼친다.
핵심은 높은 텐션은 보다 일관된 샷과 탑스핀 볼을 보다 쉽게 칠 수 있게 한다.(제 의견: 높은 텐션으로 짧아진 사거리를 보충하기 위해 더 높은 헤드스피드로 공을 쳐야하고 이 스피드에 의해 탑스핀은 더 잘 형성된다.)
만일 높은 텐션으로 인한 벽같이 딱딱한한 느낌이 싫다면 더 낮은 텐션으로 줄을메되 기억해야 할것은 스트링 모션으로 생기는 원치않는 효과를 최소화하기 위해 정기적으로 줄을 갈야줘야만 한다.

원문: http://www.racquetsportsindustry.com/articles/2005/01/does_higher_string_tension_giv.html

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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.)
ball contacting strings
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.
ball-racquet collision speed
Figure 2a
ball-racquet collision speed
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.

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