Originally Posted by TW Professor
I don't know if this qualifies as "Executive," or even "summary," but here you go ...
When the ball bounces from a racquet, the ejection force is not evenly distributed across the ball. Instead, the sum of all the local forces from the strings ends up to be in front of the center of the ball as it slides across the stringbed. This off-center force pushing up in front of the ball tries to create backspin. The distance offset in front of center is called "D". So, the force that creates your "power" is also a topspin inhibiting force. That's not good . . . that's bad.
When main strings move sideways with the ball, they bunch up in front of the ball and push up from there. This adds to the counter-topspin effect. That's not good either. Bad.
But the sideways movement also stores sideways elastic energy, which, upon snap-back, pushes the ball backwards (slows it down, creating high launch angle) and adds topspin. That's good. -- (The amount of energy given back is summarized as a sort of "sideways power". I suppose we could also call this "Spin Power". But we don't. We call it eT.)
Both D and eT can be measured or calculated. They are quantifiable. That means we can compare. That is good too.
Good against bad. Which wins? In this case we have a ratio of the pro-spin to anti-spin agent: eT/D. That ratio ends up with funny dimensions - 1/mm - so we multiply by the constant R (33 mm, radius of ball) to get a nice, neat dimensionless ratio of spin-influencing agents: eT * R / D.
We call this ratio "Spin Number." As spin number goes up, spin and launch angle increase.
D and eT tend to move in the same direction but not by the same percentage amount. If you have a working knowledge of these relationships, then, as a player, stringer, coach, retailer, or manufacturer you can design, string, recommend, choose, etc., racquet setups to customize factors that influence eT and D in ways optimizing the end result. This is a stringer's dream come true -- it's and extra arrow in the quiver, tool in the tool-belt, when promoting your customizing/optimizing skills. You can then make recommendations based on patterns, tensions, materials, gauges, stiffnesses, friction numbers, etc. that will change eT compared to D in ways beneficial to the customer. You don't actually have to know these numbers, you just have to know how altering different variables might change their relationship.
The image database and the experiment write-up offer both visual and cognitive approach to these concepts.
But concepts aside, of the patterns/strings tested, the basic ranking of spin/launch angle from high to low is: spaghetti, Steams (16x15), diagonal, polyester, nylon, zyex.