Tension loss happens with any string - the reasons for the extreme differential are independent of tension loss.
There are a few misconceptions about stringing kevlar mains at 90 lbs in a hybrid.
1. It may seem counterintuitive, but a kevlar hybrid at 90/40 is much, much softer than the same hybrid strung at 60/60.
There are 2 reasons for this, and neither is obvious until you think about it.
The first reason applies only to kevlar:
Kevlar is almost 4x stiffer than the ZX (and most other types of string too).
When you string the kevlar in a hybrid at 90/40 lbs, the racquet hoop shape will distort by several mm. For each mm the hoop length squashes, the hoop will widen by a mm. This means that for every mm the mains are allowed to shorten by racquet distortion, the crosses must stretch longer by a mm.
Since kevlar is 4x stiffer than ZX, for every lb in tension increase of the ZX crosses due to hoop squashing, the kevlar's tension will decrease 4 lbs.
When I lean my weight (with about half my body weight or ~80 lbs force) against the hoop to squash it when unstrung, I can easily deflect it about 4mm (the amount that the frame gets squashed by my 50-lb differential with kevlar/zx). And taking the 50-lb differential and multiplying by the number of mains (16, say) to roughly estimate the initial squashing force, you get 800 lbs. This is 10x the amount of force it takes to squash the hoop by 4mm. So by a first order approximation, we can estimate that the remaining differential after the hoop has squashed to relieve the 800lb of pressure is about 1/10 of the initial differential, or about 5 lbs. I.e., in equilibrium after taking the racquet off the stringer, the mains are still ~5lbs tighter than the crosses.
Since kevlar is 4x stiffer than ZX, for every lb in tension increase due to hoop squashing, the kevlar's tension will decrease 4 lbs. From this now we can estimate the actual average tension. For the 50-lb differential with kevlar/ZX, the kevlar would need to drop 40 lbs and the ZX would increase 10 lbs. So the average tension for the mains and crosses is 50 lbs. And assuming the 5lb actual differential, the kevlar would be at about 52.5 lbs, and the crosses would be at 47.5 lbs.
In other words, my 90/40 hybrid is actually a 52.5/47.5, which doesn't sound that bad, does it?
This would not be true if I strung the frame with, say, a full bed of syn gut ro a full bed of poly. 4mm of squash is enough to almost fully relieve the 50-lb differential for kevlar, but the racquet would have to squash 3 times that (~12mm) to relieve the 50-lb differential with syn gut (or poly).
I don't know, but it's quite possible that a 12mm squash would crush a tennis racquet, while 4mm squash is well within it's elastic range. So kevlar is the only type of string that I can confidently use in a hybrid with 50-lb differential without worrying that I would crush my frame.
Also, if mains and crosses were the same type (or two types having similar stiffness), then the calculation of the average tension after racquet distortion is simpler. A 90/40 string tension would simply average out to the mean of 90 and 40, or 65 lbs. Obviously 65 lbs is a lot tighter than 50 lbs. So counterintuitively, stringing with kevlar mains likely gives you a softer overall stringbed than if you had strung the mains with a much softer string.
The second reason why 90/40 is more comfortable than you might expect is more obvious - the increased lateral freedom of the mains within the stringbed plane greatly reduces the shock of the ball impact on the stringbed, giving much more of cushioned feel than if the string movement were more restricted. I think other posters have noticed this effect.
My 90/40 kevlar ZX hybrid is actually more comfortable than 66/46 was, and 66/46 was much more comfortable on my arm than 56/56 was.
Now you might be thinking, if 90/40 is actually 52/47, why not string at 52/47 to begin with? The answer is that the frame squashes in this case too. 52/47 becomes 50/49 as soon as you remove it from the stringer. So my effective differential with 90/40 is still >5x greater than starting with 52/47, which certainly amplifies the ESP effect.
To simplify calculations, I have ignored the effects of tension loss, but even with tension losses, the premises are still the same.
For anyone wanting to try 90/40 - it sounds odd, but I recommend only doing it with kevlar mains ensure both arm health and frame health.
hope you manage a speedy recovery.
