Most everyone posting in this thread is missing the main point:
Sprialspin exists in tennis, but it is not applied directly by a tennis racquet.
For easier visualization, consider the extreme case of an underspin spiralspin serve.
At the moment of contact with the stringbed, the racquetface is aimed about 45-degrees upward. Thus the strings contact the surface of the ball about midway between the bottom of the ball and the back of the ball, and maybe a little bit to the side of the ball.
And at the moment of contact with the stringbed, the swingpath of the racquet is very fast in a right-to-left direction.
I think we all agree that the effect of this type of serve is a sharp bounce to the right.
At the moment of contact, 0% of the spin is sprialspin, because the spin axis is perpendicular to the flight path. But as gravity acts on the ball through its arc, the flight direction changes. At the moment of the bounce, the ball is moving downward in roughly a 45-degree angle. The flight path has veered by 90 degrees over the arc of travel, but the spin axis has stayed the same. Nearly 100% of the spin is now spiralspin at the moment of the bounce. Thus, gravity has converted all of the spin to sprialspin.
This also happens in a twist serve, but the conversion to spiralspin is not as extreme.
When you serve in pingpong, you can "corkscrew" the paddle into the ball by rotating the handle around the impact point. So a pingpong paddle can apply a torque around the z axis. You can't do that with a tennis racquet. A tennis racquet only applies normal force and tangential force to the ball. These two vectors together are not enough to cause spiralspin - you need a third vector. That third vector is supplied by gravity.
A twist serve is really not that complicated.
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Last edited by travlerajm; 02-08-2013 at 07:18 AM.