IMO spin strings are just for marketing. A proper technique is required to create spin rather than the string. RF used to have all natural gut racquet-bed before and was still able to create lots of spin.
Topspin is generated by four distinct factors:
1. Technique. This cannot be overstated enough. Proper mechanics, more than anything else, affect spin production. I won't go into all of it because I don't understand it fully myself. However, I'm fairly certain that this is a fact and can be vetted by many other talented coaches both on these boards and in the real world.
2. String pattern. Generally, more open string patterns generate more spin. A 16x19 frame will generate more spin than an 18x20. This does not mean the 18x20
cannot generate much spin, on the contrary: the 18x20
can generate quite a lot of spin (see note #1 above). Be that as it may, with the same strings and technique, a 16x19 will generate a little more spin than the 18x20. Not by leaps and bounds, but there's enough of a delta factor that should influence your decision.
3. Mains string movement. The mains need to snap-back into place quickly in order to maximize spin. This can only be done if there is a low coefficient of friction between the mains and the crosses,
and enough room (tension-wise) for the mains to move in the first place.
3a. Coefficient of friction. See this
link for more info. Essentially, it is in your best interests to have
the cross strings to be as slick as possible in order to maximize spin. The mains should be slick too, but the crosses are more important. As long as the cross strings are very smooth, then the mains will be able to move freely. If the cross strings are rough, then they will lock the mains in place, preventing string movement, inhibiting spin potential.
3b. Tension considerations. If the cross strings are too tight, then they will exert too much force on the mains, which inhibits them from moving. This is irregardless of coefficient of friction. It doesn't matter how slick your strings are: if you take an 18x20 pattern frame and string it full poly @ 65 lbs all the way around, then those mains aren't moving at all. They're so locked in place that your spin production will be negligible. Instead, for tight 18x20 patterns string the crosses 3 lbs (or 1 kg) looser than crosses. For open string patterns such as 16x19, 2 lbs looser should be sufficient. For "spin" technology frames such as 18x16, 16x15, etc., string mains and crosses @ the same tension.
4. Strings: Poly vs. nylon vs. gut. Just because you've got a smooth poly in the crosses doesn't mean you can put a synthetic gut (nylon) in the mains and expect crazy spin, because you won't. Nylon doesn't "snap back" like poly does. The strings will be moved out of place upon ball impact, but the strings aren't going to force themselves to "snap back" to their original positions
before the ball leaves the string bed. They will after the ball leaves, but that is after the fact and does not help with spin production.
However, if you put a smooth poly in the cross, and then put almost any other poly OR natural gut in the mains, the mains
WILL snap back
before the ball leaves the string bed.
This is what affects spin the most: the fact that the mains are so hell-bent on returning back to normal that they literally
take the ball with them in the process. This forces the ball to rotate even faster.
This is why dead poly isn't as effective as fresh poly is: the main strings lose some of their elasticity and they don't return back to their original position
before the ball leaves the string bed. Any time the mains become sluggish and slow and don't want to return back to their original position, you're losing spin potential on your frame.
