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Old 09-18-2012, 04:17 AM   #20
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Join Date: Feb 2006
Location: Stuck in the Matrix somewhere in Santa Clara CA
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Originally Posted by Bowtiesarecool View Post
It is my understanding that the kick serve bounces away as a result of how the ball deforms when hitting the ground at such a steep angle with heavy topspin. I have seen computer simulations of this effect, but can't remember where to find them.
Are you talking about kick serves, in general, or specifically twist kicks? The term, kick serve, is somewhat generic/ambiguous. Some use the terms, kick serve & twist serve, as synonyms. Whereas others classify the twist serve as a specific type of kick serve. In this taxonomy, a kick serve is any serve with heavy topspin that kicks upward on the bounce. If the kick serve curves one way, in pre-bounce flight, and then kicks off in the opposite direction on the bounce then it qualifies as a twist serve. If the kick does not deviate appreciably left or right on the bounce then it would be a topspin kick (or a topspin-slice kick) serve.

Now the ball, on a twist serve, may very well deform a bit differently than it does on other types of serve. However, this is not really the underlying cause of the way bounces. The ball bounces the way it dues because of the way the ball interacts (rolls/skid/etc) with the ground. This interaction is influenced by the speed, the spins and trajectory on the ball prior to the bounce.

Note that (vertical-axis) sidespin does not directly affect the bounce (height or direction) according to physicist, Rod Cross. OTOH, horizontal-axis spins do have an effect on bounce height and direction. The most common of these is topspin and underspin. A ball that bounces off to the left or right, relative to the pre-bounce direction, indicates the presence of spiralspin as toly has indicated. This type of spin accounts for the left/right deviation seen on the bounce of a twist serve.

I've been talking about spiral spin for 4-5 years in these forums -- there are quite a few threads that discuss this type of spin. In his book, Technical Tennis, physicist Rod Cross refers to it as spiralspin, However, in his TWU article on kick serves, Rod refers it as gyrospin. It is sometimes aka Z-axis spin or longitudinal spin. Table tennis players refer to this spin type as corkscrew spin (or corkspin).
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