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Old 01-09-2013, 05:56 AM   #1048
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Join Date: Dec 2005
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If the snap-back of the strings contributes to spin, then why
does Fed use string savers to prevent string movement?

He also changes racquets (to ones with fresh string jobs) fairly
frequently in a match.

Originally Posted by ChicagoJack View Post

What Deformation and Impact Angle Means To The Ball "Bite" Issue: You cannot have any more, or any less ball bite, when the ball is already squashed into the string bed like a bug on a wind shield. One hundred percent ball bite occurs, every time you hit the ball, no matter what strings you use. What can be said is that there is biting a little bit sooner during the 3-5 milliseconds impact, and there is biting a bit later as the ball exits the string bed. However, this biting a little bit better as the ball leaves the string bed is important to spin production only if the mains have traveled over the crosses, leaving them positioned for the snap back. See photos [a] [b] provided below. Link here.

Quote [1] "String manufacturers have always marketed tennis strings according to their ability to grip the ball via sticky coatings, rough/grabby surfaces, or "biting" shapes. The underlying theory is that greater friction between the string and the ball will cause more spin. This was universally accepted as true until it was demonstrated that for all impacts less than 50 degrees away from perpendicular, the strings will bite the ball to the maximum extent possible, no matter the texture, shape, or material of the string. This is important because when biting occurs, friction ceases. This result assumes that the stringbed is laterally rigid i.e., the strings do not move sideways, or, if they do, they do not snap back into position. This was the situation in the pre-polyester string days. As we will see below, polyester changed everything" [1] - Crawford Lindsey, How Ball to Spin Friction Affects Spin, Feb 2012. Link here.

Quote [2] "Friction is important to spin, but not in the sense that most people think it is. In the past, two assumptions were the foundation for the conventional wisdom that (1) a rough string surface creates more spin by increasing the string's bite, grab and push on the ball, and (2) that inter-string motion would lessen that grabbing and thus should be minimized. So, to gain maximum spin, the goal was to use a string with both a high ball-string friction and a high string-string friction to create a rigid surface parallel to the string bed. As so formulated, both of these assumptions have proven incorrect. First, experiments show that for a tangentially non compliant surface, the spin will be nearly the same for all magnitudes of string-ball friction for impact angles typical for tennis shots. For reasons explained below, both rough and smooth strings will produce the same spin on a laterally rigid surface. (2nd paragraph deleted for brevity) The role of ball-string friction in this process is that it influences both the amount of lateral string movement and the torque the snap-back exerts on the ball (though the magnitude of its contribution is yet to be ascertained by experiment). It is here that ball-string friction contributes to the difference in spin performance between string models, but only if the strings move. Otherwise, ball-string friction produces about the same spin for every string. And whether the strings move or not depends on the static and sliding friction between strings. [2] -- Crawford Lindsey, August 2010, Revised February 11. Link here.

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