I admit I didn't read the article that carefully. I will try to read it again tonight. In the meantime, are you saying that if I normally string the poly at 40 lbs and if I pre-stretch and then string it at 35 lbs, the resulting spin performance would be equivalent?
Harry
The five pounds was just a ballpark, but yes. The Professor pre-stretches most strings he tests in the lab. The data on pre-stretched copolys shows that, in general:
If a particular copoly is strung at 60 pounds it might have 200 stiffness and return 90% of the energy in an impact. It also might lose 15 pounds of tension, settling in at around 45 pounds of tension.
If you pre-stretch that same string and then string it to 60 pounds it might show 230 stiffness and return 93% of the energy in an impact. It will also lose less tension, maybe 12 pounds.
So if you want the pre-stretched string to play like the normally strung one you could string it at 55, at which tension it might have 200 stiffness and 90% energy return and lose only 12 pounds, settling in at 43 pounds of tension. Since the stiffness is now equivalent to the regular string strung at 60 it will play the same when fresh but will lose less tension and so maintain its playing characteristics for a longer period of time.
Again, though, these are all ballpark figures. Previous research that the TW Professor has done has shown that string tension is important to spin, but probably more important is maintaining a smooth, slippery string surface. When that smooth slippery surface is lost to denting, notching or scuffing, the main strings stop snapping back and so stop generating the extra spin that copolys are famous for. And when that happens they play much stiffer too. So now you've got loose strings that don't give much spin but that feel stiffer than when they were new. It turns out that this is what we call "dead" strings.
And keep in mind that pre-stretching a string might make it slightly more fragile (according to some reports by people who have been experimenting with extreme pre-stretching over the past two weeks), so do it at your own risk.
It's important to note too that this paper is not really about pre-stretching. The professor "pre-stretched" a piece with hundreds of impacts, then re-tensioned that piece of string to its original tension, and then "pre-stretched" the string with several hundred more impacts, and then repeated this cycle several times. He did this not to test the effect of pre-stretching, but in order to test the hypothesis that copoly strings die because their internal structure becomes fatigued by repeated impacts and that this fatigue results in a loss of elasticity or resilience. But his experiments showed that this does not happen. If anything, the string "improved" slightly after these cycles of repeated impacts, tension loss, and re-tensionings.
The two "going dead" papers will lead to a complete paradigm shift in the way we view copoly strings and how we might get them to play well for longer periods of time.
It's very cool, too, in my opinion, that these two papers were written kind of in response to questions raised by posters on this forum. TW is really doing us players a great service by providing us, for free, TW University.