The String Expert 
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The String Expert |
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| Date: | 4/28/2007 |
| Question: | Jeff - thanks for the response to 248. I have one more question: I searched through all the previous questions regarding "the 10% rule" but I never got your specific rationale for the rule. i.e. let's just say for sake of example a player strings his mains at 60 lbs. You'd recommend that player string his ALU Power mains at 54 lbs. My question is this: are the properties of ALU Power such that at 54 llbs. it should "feel like" other strings would at 60? Ultimately, I couldn't agree more with your overall assessment that the proof is in personal experience. My question stems from the fact that when I first strung my racket with ALU Power (was previously using a poly string), I decided to keep the tensions identical. I experienced much more power with the ALU Power-strung racket even though I didn't drop the tension. If I had dropped the tension 10% (which would have actually been a 67 to 60 drop) the change would have been so drastic that I probably wouldn't have liked the string, but since I kept the tension constant and have now been toying around with it, I have grown to love the ALU Power. Sorry for the rambling, but do you see what I'm getting at? We are already talking about string which is more powerful than most of the stuff people are already using, so dropping the tension 10% is a "double-boost," if you will. Was this recommendation created because the string's composite properties are higher-performing under lower tensions? Or because in your test environments you observed that to generate similar string-bed "feel" vs. "string X," you needed to drop tension 10%? |
| Answer: | OK, I'm going to have to get a bit controversial to answer your question honestly... The interaction of a ball with a stringbed is not well understood and there is little physical evidence that demonstrates exactly what happens at the point and moment of impact. The problem is in taking the very precise measurements that would be needed where the impact occurs at an unknown time and position. The best that can be achieved is simulating an impact in a laboratory where the racquet is generally clamped and a ball is fired at it or the ball is stationary and the racquet is swung at it while clamped in a rigid mechanism. Neither of these methods at all simulates the action of a racuet when swung and held by flexible fingers at the end of a flesh and bone arm...! The subjects of the interaction are four flexible, and deformable objects (body, racquet, strings and ball). Ignoring racquet and body deformation, the 'simple' collision of the ball with the stringbed can occur at variable angles and at at various positions on the stringbed and where the racquet is moving in various planes relative to the direction of the ball and is also angled in relation to the ball's trajectory. It is almost impossible to know what the tension of the stringbed is at a particular point of impact let alone factor in the other considerations of direction and speed of racquet motion and angle of incidence at impact. And this is before we start considering the relatve deformations and resulting kinetic energy values of the ball, racquet and strings.... People have enough problems simply measuring the static value of the tension of the center of a stringbed at rest and clamped in a vice...!!! As a result, no-one really knows what happens when a tennis ball strikes a racquet and rebounds in the real world. Some advances in high-speed photography have allowed us to see something of what occurs but we are still unable to take the precise measurements that would give us a greater understanding of the dynamics of the interaction. And, if it is in a real-world situation, then it is impossible to repeat the interaction to make any kind of comparison of what occurs when changes are made to any of the variables involved. Some outstanding efforts have been made to measure, 'mathematize' and predict the outcome of such interactions - notably by Howard Brody, Crawford Lindsey and Rod Cross (their book 'The Physics and Technology of Tennis' is essential reading if you really want to get into the nitty gritty of the subject). But they will be the first to tell you that this is a very imprecise science.... So after all that, I'll return to your question and the thorny subject of what difference does tension make to the 'power' of a stringbed - or, put more simply, to the speed of a returned ball. Actually, very little. It has been calculated that reducing the tension from 60 lb to 50 lb will result in an increase in ball speed of only 1.5% - an amount that would be unnoticeable to any player. Taking into account the fact that hitting the ball 1 inch further furhter from the throat, center, edge or end of a racquet will have a much greater impact on returned ball speed, it will be obvious that the effect of a change in tension is going to be insignificant when compared with the variation in ball speed that is inevitable as a result of connecting with the ball at unpredictale, and largely uncontrollable, points on the stringbed (no-one can hit the ball consistently in the center of the racquet....) That is not to say that changing tension does not have any significant effect - and our experience has shown that a reduction in tension using Luxilon strings results in a better 'feel' and a consequential improvement in performance. That may be something that is difficult to quantify in scientific and mathematical terms but it is something that has been consistently experienced by players - and that's the best answer I can give you in response to your simple - but extremely complex - question..... Jeff |