^^ Thanks for the report Trav. Let's assume that the tension you chose, or achieved given the short strings, etc., was not the issue. Do you have any hypotheses for why gut/ZX was so much less spin-friendly and rebounded the ball at such a high angle compared to Kevlar/ZX or gut/copoly?
Comparing the stringbed deflection of the ng/zx in my POG with the kev/zx in my blade by pushing on it with my thumbs, the deflections are similar (with maybe 10% more deflection on the ng/zx).
There is snapback with both, but the nat gut is much easier to pull laterally with my fingers - it requires much less lateral force. Pulling laterally on a main in the center of the sweetspot with my central 3 fingers with max finger squeeze, the kevlar main deflects about 5-6mm laterally. The ng main deflects about 9-10mm laterally with same force. A huge difference.
Based on this, I think this is what's going on:
The lateral force of ball impact on the stringbed is probably much greater than the force I can produce with my fingers. So with both types of main, the ball is able to deflect the mains laterally the maximum amount (until the bottom main of the ones contacting the ball butts up against the top main of the ones not touching the ball). So the amount of slide is the the same. But the amount of lateral recoil force generated by the kevlar main is almost double the lateral recoil of the ng main (applying Hooke's law of F=-kx to my observations with the lateral finger pull).
If the above hypothesis is correct, it suggests that a significantly higher tension is needed for nat gut mains to achieve the same spin level as with kevlar mains (because the nat gut is too soft to be optimized for applying tangential force to the ball). But I'm not sure if upping the tension with nat gut would ever make it as stiff as kevlar?
I believe there is another important factor in play too. In stringbeds that lock (instead of allowing interstring sliding and snapback), I've found that maximum spin is achieved by using the stiffest possible main with the most compliant possible cross. That is, the cross/main compliance ratio is an important factor to spin generation. An example is the locking combo of 18g kevlar (very stiff and under high stress @65 lbs) crossed with 15L syn gut (very compliant and lively under low stress at @65 lbs) - generates a lot of spin.
The higher the cross/main compliance ratio, the better the ratio of lateral/normal stretch of the stringbed. ==> better spin.
To clarify, High cross/main compliance ratio adds spin in mulitple ways:
1. Stiffer in normal stretch => more ball compression => more spin.
2. Stiffer in normal direction => less denting => lateral force acts more tangentially rather than to change rebound angle => more lateral force converted to spin.
3. Compliance in lateral direction =. more lateral stretching = more tangential force applied to ball => more spin.
I think the ng/zx has a crummy cross/main compliance ratio, while the kev/zx has an excellent cross/main compliance ratio.
I mentioned this earlier in the thread: The kev/zx combo is the first stringbed I've ever tried that combines the dual spin-friendly benefits of high cross/main compliance ratio with good snapback. Makes for an amazing serving string at the right tension (my current formula plays best in the plateau at last 2/3 of its life when I get through a stiffer break-in period).
Thinking about this more, cross/main compliance ratio with ng/poly might actually be relatively high, because the poly loses so much more tension than the nat gut. It also explains why some players report good results stringing nat gut mains significantly higher tension than the poly crosses.
The spinniest setups I've ever used have all been kevlar/poly setups after the poly had dropped tension much more than the kevlar; stiff kevlar main at high tension with low tension poly cross == spaghetti spin. This gives me an idea for a fun experiment, as I've never tried it with large tension differential from the get-go (kev main with either poly cross or zx cross).