ZX is strangely stiff at 30 lbs.
Both Dunlop Black Widow and Volkl V-Torque are much much softer at that tension.
Perhaps ZX gets softer, relative to polys, as the tension goes up?
Zyex is unusual as a material in that its stiffness only increases a small amount relative to increases in tension. By contrast, polyester and nylon become progressively, and appreciably, stiffer the tighter the string is tensioned. In this regard, Zyex is a bit like natural gut. So a Zyex string at 60 pounds will not be much stiffer than one at 30 pounds, while a poly will be much less stiff at 30 than at 60.
Nevertheless, this does not adequately explain your perception that Black Widow is "softer" at 30 pounds than is Monogut ZX. According to the
TWU String Database, when Black Widow is at an actual tension of 23 pounds and Monogut ZX at an actual tension of 27 pounds, BW's stiffness is 170 while ZX's is 85. So, while BW at this super-low tension is significantly less stiff than a syngut at normal tensions, it remains much stiffer than ZX. However, this is stiffness in the longitudinal direction, and one of the great things about the TWU database is that we also can look at two other measures - deflection and peak force - when the old-fashioned stiffness measurement doesn't match our on-court experience.
Deflection is a measurement of how far the string deflects upon impact in the plane perpendicular to impact, so it is a measure of perpendicular stiffness, which is probably more relevant to how a string plays and feels than is longitudinal stiffness. Here we find that the differences between the two strings is much smaller: At the same tensions (actually, during the same impact) above, BW deflects 33.9mm and ZX deflects 35.2 mm. Not much difference, although BW is still stiffer than ZX.
Finally, the peak force measurement gives us a measure of how much force is transmitted by the string to the racquet frame. Here, BW exerts 29.8 pounds of force and ZX 26.1 pounds. Again, the two strings are quite close, but ZX is still "softer", insofar as it transmits less force than BW.
So how to explain your observation that BW at 30 is softer than ZX? One thing to look at is impact angle. All of the above measurements come from an impact directly perpendicular to the string - head on. But during actual play, very, very few impacts are like this. Even on flat shots, because the ball is usually rising or falling during impact, the angle of impact is almost never square like this. (Flat serves might be a good example of when the impact could be square.) So it could be that BW feels softer for some reason that has to do with the difference between the ball-string interaction during head-on versus oblique impacts.
One explanation might be lateral main string sliding. We know that when the main strings are free to slide sideways that the ball then stays on the strings longer (greater dwell time) than if the strings were stuck together. Increased dwell time results in decreased shock and a softer feel. BW, as a slick, hard-surfaced poly, slides very well upon itself, so it's reasonable to conclude that on all but very square impacts the main strings will slide to some degree, increasing dwell time and decreasing shock. However, ZX, although not as hard and slick as poly, is also known to slide and snapback, at least initially.
These facts suggest that when brand spanking new ZX might feel softer than BW at 30 pounds, but that after some play, when the ZX loses some of its surface luster and stops sliding freely, that BW feels softer. Some players have complained about ZX producing a "clicking" sound, which suggests to me that after it's become notched and scuffed a bit, that it starts to stick together at impact. This could, through the loss of main string sliding, lead to increased shock and a harder feel the older the string gets.
In any case, if you find BW to feel softer at 30 pounds there seems little reason to use ZX at such a low tension. BTW, how would you compare the power level of BW and ZX at 30 pounds?
