Originally Posted by SystemicAnomaly
This is not what Rod Cross says in his book, Technical Tennis. I suggest you give it a read before passing judgement. There is no other way to explain the behavior of the ball flight and bounce. Rod Cross very explicitly states that regular sidespin has ZERO direct effect on the bounce direction. Whereas spiral spin does not affect flight trajectory but does influence bounce direction.
Gravity has nothing at all to do with imparting spiral spin to the ball. Maximum spiral spin is achieved at contact with the racquet, not sometime later.
In tennis, it is true that we usually cannot impart as much spiralspin to the ball as we can topspin (or sidespin) for a serve. However, it is not at all negligible as evidenced by the bounce direction. There is an underhand shot in tennis that exhibits quite a bit more spiral spin than the twist serve. Players will sometimes take a vicious swipe left-to-right (or right-to-left) to the back/underside of the ball. It is as if you were drawing a smile on the lower part of the ball (but not on the very bottom of the ball). This action will usually produce a bit of underspin with some regular sidespin and a massive amount of spiral spin if executed correctly. With this novelty shot, I've gotten the ball to bounce nearly parallel to the net due to the presence of spiralspin.
I respectfully disagree. I have not read Technical Tennis in many years, but have read all of his short publications I must say that I have found several occasions where I disagree with Cross. And in at least one occasion, I have found his interpretation to be easily disproved. It is possible that this is another.
It is easy to show that a very decent twist serve can be hit with a dual-wheel ball machine that delivers zero initial spiralspin.
And again, the spiralspin in an underhand sidespin serve applied to the underside of the ball is maximized at the bounce, not at the strings. The same is true of a twist serve.