Looking for kevlar outer, poly inner string

Discussion in 'Strings' started by webbeing, May 1, 2013.

  1. webbeing

    webbeing Rookie

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    Anyone happen to know of a string with kevlar as outer layer and poly inner core? Thanks.
     
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  2. corners

    corners Legend

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    Why do you want such a string? The advantage of copoly strings is their slick and hard outer surface.
     
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  3. slowfox

    slowfox Professional

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    Maybe it's a chocolate - peanut butter thing. Who would've thought they'd be so great together..?
     
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  4. webbeing

    webbeing Rookie

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    The string (if available) will be used in the main. The kevlar outer surface resists tearing/notching. The poly inner core enables more power and especially snap-back for spin.
     
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  5. gameboy

    gameboy Hall of Fame

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    Interesting, a string that combines the worst traits of both Kevlar and Poly. You got me intrigued...
     
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  6. corners

    corners Legend

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    Kevlar mains are great in the mains. They snapback wonderfully as long as the cross string is smooth and slippery. Adding a poly core wouldn't help very much. Adding a syngut or natural gut core would help, but nobody's making such strings. There was a research group at MIT some years ago that developed a string like this - the ACES project - but for some reason it never reached the market.

    Check out threads and posts by travlerajm. He's been using kevlar mains with slick copoly crosses for some time and claims that this is the most spin-friendly setup available. The kevlar mains snap back with gusto and unlike with copoly mains, which get notched, the braided kevlar just keeps snapping back for the life of the string. For more comfort and pop he's recently been using kevlar mains with Ashaway Monogut ZX, a 100% Zyex monofilament that is smooth like copoly but much less stiff and more elastic.
     
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  7. J011yroger

    J011yroger G.O.A.T.

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    Kevlar doesn't snap back, it is nearly inelastic.

    A poly core with a Kevlar outer wrap would not work, because the Kevlar would not stretch. If you put a rubberband inside of a hemp rope, the rubber would not allow the combination to stretch any more than the rope would be able to alone.

    Chain...weakest link...blah blah blah.

    J
     
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  8. corners

    corners Legend

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    It's not as elastic as natural gut, but it's much closer to being perfectly elastic than it is to being "nearly inelastic." Kevlar returns close to 90% of the energy in a collision.

    And it certainly does snap back, or can, if the cross string, string pattern and tensions combine to create the right conditions for snapback. If it didn't, I doubt that you would have ever played kevlar mains/copoly crosses. If the kevlar mains didn't snap back with the ball in that setup you would have observed poor spin production combined with a very high rebound angle that was also highly dependent on the speed and spin of the incoming shot you were trying to return. It would have had horrible control, in other words. And it might have even hurt the arm of a strongman such as you.

    This paper includes an experiment using Kevlar mains and RPM Blast crosses in a 16x10 string pattern (figure 10, role of cross strings). I think it shows pretty clearly that Kevlar mains snap back and provide additional spin just as copoly mains or gut mains can do (or syngut mains, for that matter, at least until they are notched). Travlerajm claims that Kevlar mains with copoly crosses is the most spin-friendly setup he's tried.

    I don't think that's necessarily the case. Most Kevlar strings are braids, which naturally have some play or inherent slack along their length. I think it would be possible to make the inner core of a more resilient material (and poly would be a poor choice) that was functionally shorter than the outer braid of Kevlar so that the inner core bore most of the tension while the outer braid was not stretched entirely taught. The problem would be that the braid would bear more and more of the load as the inner core lost tension due to stress relaxation, and consequently the playing qualities would change dramatically from the beginning to end of its playing life.

    A team at MIT tried to develop a nylon wrapped in Kevlar string, which they called ACES:

    http://web.mit.edu/3.082/www/team1_f02/results.html
    http://web.mit.edu/3.082/www/team1_f02/design.html
    http://web.mit.edu/3.082/www/team1_f02/kevlarwear.html
    http://web.mit.edu/3.082/www/team1_f02/ACES.html

    As far as I can tell, the objective of this team was to design a durable string with greater energy return than Kevlar that had a very high-friction surface. Of course, they were operating on the old belief that high ball-string friction is most important to spin generation. Of course, we know now that that is not the case. Low inter-string friction is more important.
     
    Last edited: May 3, 2013
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  9. J011yroger

    J011yroger G.O.A.T.

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    Been trying to reply all morning. Hectic day.

    J
     
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  10. Tamiya

    Tamiya Semi-Pro

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    Did some mucking about with Kevlar few wks ago.

    Being so unelastic mine seems to rely more on how soft the frame is;
    seems to act more like a bow & arrow situation.
     
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  11. J011yroger

    J011yroger G.O.A.T.

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    When you string Kevlar, it doesn't stretch at all when pulling tension as compared to any other string. It just takes the slack out of the braid, and that is it. If you try to deform it more than slightly, it just snaps.

    When the ball impacts the Kevlar strings, it is mostly the frame that gives, instead of the string. So the distortion of the hoop, and subsequent return to shape are what accounts for the string movement and snap back. As the poster above said, more like a bow and arrow where other strings are more akin to a slingshot.

    I am convinced that this is why all 20 of my K90s caved in once I switched to the Kevlar blend; because the frame takes the lion's share of the impact.

    I didn't see anything which suggested that, all the paper said about Kevlar mains and poly crosses was that the cross when strung tighter provided additional spin.

    If you summon up my original Jolly-Rig thread, or the one where I go looking for a new setup and test a bunch of strings, I explain that it must be strung tightly to work or else it is god-awful after a little bit of play.

    Because the Kevlar has no "stretch" to it and can't be deformed much (I know the physics definitions of resilience and elasticity and don't think either of them convey what I am looking to say) it can't be stretched enough during stringing to allow it to stay tight as the material wears away. As you get towards the end of the life of the stringjob the Kevlar mains just flop around loosely, and if not strung tight this will occur about halfway through the stringjob.

    If you take a Kevlar hybrid and play with it until it is about 75% used up, then cut the crosses out, you will see that the Kevlar mains are almost fishnet loose.

    I have read Traverler's posts, and I don't think he swings particularly fast because his claims closely approximate my personal findings at lower swing speeds.

    Kevlar/Poly blend plays incredible, and it's spin generation is linear as swing speed increases. It is solid, and extremely predictable. You swing X fast you get X*Y spin. With full poly (the good ones anyway) the output is more exponential (more akin to you swing X fast you get (X/2)^2*Y spin.).

    The Kevlar blend is like a deal with an honest businessman, "You put this in, I give you this in return."

    The loose full poly stringjob is like a deal with the devil. "You swing this fast or more and I will make everything good, but if you don't..."

    I would certainly love to see it, (For Science!) however I think internal string friction between the nylon core, and loose outer braid would in technical terms "gum up the works".


    This is interesting because they tried a wrap, looping the Kevlar around the nylon like a coil spring, so that when the string stretched space would be allowed to form between the coils of Kevlar, instead of having a braid which wouldn't give.

    J
     
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  12. corners

    corners Legend

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    I think your K90s caved because kevlar is 3 times stiffer than many copolys, not because it lacks elasticity. Elasticity is one of those things that most people have an intuitive sense of, but that sense is usually not correct. When people think of "elastic" they typically think of rubber bands. Rubber is very elastic, but it is also not at all stiff (it is very flexible). Natural gut is kind of like this - it is very elastic and also very flexible (not stiff). Kevlar is also elastic, but unlike rubber and natural gut, it is also very, very stiff. It returns 90% or so of energy in an impact. It wouldn't do that if it weren't elastic. But because it is so stiff, it reaches a much higher peak tension during impact than other strings. It's that peak tension - and extremely high peak force - that caved your racquets, not a lack of elasticity.

    You need to read the whole article, maybe. But it's very clear that the experiment with kevlar/copoly hybrids was done to figure out the effect of cross string tension on the sideways sliding and snapback of the mains, in this case kevlar mains. What the section I referred you to showed was that, in extremely open 16x10 patterns, if the cross strings are too loose the kevlar mains were too free to slide sideways - they slid so far sideways that they did not have enough time (or energy) to snap back as the ball was leaving the strings, and so no additional spin was produced. When the cross string tension was doubled, spin increased by 40% because interstring friction increased enough to prevent the mains from stretching too far sideways, and they then had enough time and energy to impart additional spin on the ball.

    Here's the text:

    ROLE OF CROSS STRINGS. Does the tension in the cross strings affect spin? To investigate that, two 16x10 hybrid (Ashaway Crossfire Kevlar mains and Babolat RPM Blast crosses) setups were prepared. One racquet had 60 lb. mains and 60 lb crosses and the other had 60 lb mains and 30 lb crosses. The idea was two-fold: first to create a very spin friendly setup (i.e., super-stiff mains sliding on stiff, slippery crosses in a very open pattern), and second to see if altering just the cross string tension in such a setup would matter.

    The cross strings at 60 lb. produced almost 40% more spin than the 30 lb. crosses. The greater inter-string friction due to higher cross tension probably constrained the mains from moving too far forward and creating a large normal force offset.​

    What you're describing is simple tension loss. Kevlar loses lots of tension. As it loses tension it tends to "bag out". And this will tend to promote excessive main string movement of the type described above. The mains will slide too far sideways during impact and not have enough time or energy to snap back with the ball. If loose enough, and/or if the crosses have become quite scuffed and dented, the mains may not snap back at all, and you'll find them out of line after points.

    This is the same thing, though, that happens with copoly strings, which also lose lots of tension. The mains lose tension and start sliding sideways too far. At the same time, they are getting notched and the crosses are getting scuffed and dented, increasing interstring friction.

    When you combine excessive sideways sliding with increased interstring friction the snapback mechanism no longer works properly. The mains, if they snap back at all, snap back too late to impart additional spin. Instead, they bunch up "under" the ball, creating a counter-spin torque that results in less spin and a very high rebound angle. (This is what the TW Professor referred to above as the Normal Force Offset.)

    In my opinion, the reason why you found that kevlar had to be strung tight was because it needed to be tight initially otherwise the tension loss that followed would disable the snapback mechanism too quickly, and the whole stringjob would be useless. So maybe it was too tight at first but after losing 20 pounds of tension it was still able to snap back; whereas, if you had strung it looser initially, it may have played fine for an hour but then been too loose to snapback.

    If you want to be certain if kevlar mains snap back or not, simply compare a kevlar/copoly hybrid with a full bed of braided kevlar.

    You might be interested to read the TW Professor's most recent paper on how strings "go dead."

    And also about Trav's recent experiments with pre-stretching Kevlar (down a way in this thread): http://tt.tennis-warehouse.com/showthread.php?t=460719

    Only a wookie could match your swingspeeds, so that makes sense.

    I think I understand what you're saying.

    You might be right, but there would be ways around that, I would think. But it's really not in manufacturer's best interests to make better Kevlar strings. They would prefer that we broke more fragile materials quickly and paid for more as often as we can afford.

    Yeah, I wish that string would have made an appearance. The numbers they cite for energy return were near-gut performance! The stiffness was somewhere around a syngut too, so they wouldn't have broken your K90s. If the wrap held together it may have been the ultimate main string.
     
    Last edited: May 5, 2013
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  13. J011yroger

    J011yroger G.O.A.T.

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    I feel that you are jumping to a conclusion with the Kevlar/poly blend.

    There was very little said in that paragraph, with two test points and a result. With no further investigation into the why or how of it.

    I think you are filling in the blanks with what you want to believe.

    I have a few ideas for an experiment, just need to work out how to set it up in the shop with some old frames and measuring tools.

    J
     
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  14. corners

    corners Legend

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    LOL, that is not the case. I think you should read the whole paper, because I don't think that you've understood the short section I referred you to.

    But let's start with the abstract of the paper we've been discussing:

    Many factors have been offered as influencing spin: string material, tension, gauge, pattern, stiffness, texture, shape, ball-to-string friction, and string-to-string friction. Many of these theories have been addressed elsewhere at TWU (see String Comparison menu above). Here, we will consider the role of string stiffness (as determined by material, tension, and pattern) in spin production. The ITF (International Tennis Federation) conducted tests in 2006 and 2007 concluding that string stiffness was an important factor in spin production.1,2 We will expand on those tests here.

    Until recently, the prevailing theory regarding string stiffness and spin has been that the firmness and lateral rigidity of a stiffer string will create more ball-to-string friction due to more squashing and embedding, resulting in more spin. It is true that friction accounts for most of the spin produced by the stringbed, but it does not account for most of the difference in spin between stringbeds. Recent research suggests that lateral main string movement is the most important factor determining this difference ("Which Strings Generate the Most Spin?" and "String Movement and Spin" and "String Snap-Back and Spin"). Consequently, the explanation of how string stiffness might affect the spin differential has changed. The question now becomes, "Does string stiffness affect tangential string movement and its associated storage and return of energy to the ball, both rotationally and translationally? And if so, how much and under what circumstances?" This article presents evidence that string stiffness does affect the magnitude and efficiency of lateral string movement and that it does so most in very open string patterns. In fact, for any given string, there was a 2-6 times more spin between stringbed setups, and for any given setup, there was a 70-400% difference in spin between strings. How can this be?​

    And then again to the paragraph concerning the experiment with kevlar mains:

    ROLE OF CROSS STRINGS. Does the tension in the cross strings affect spin? To investigate that, two 16x10 hybrid (Ashaway Crossfire Kevlar mains and Babolat RPM Blast crosses) setups were prepared. One racquet had 60 lb. mains and 60 lb crosses and the other had 60 lb mains and 30 lb crosses. The idea was two-fold: first to create a very spin friendly setup (i.e., super-stiff mains sliding on stiff, slippery crosses in a very open pattern), and second to see if altering just the cross string tension in such a setup would matter.

    The cross strings at 60 lb. produced almost 40% more spin than the 30 lb. crosses. The greater inter-string friction due to higher cross tension probably constrained the mains from moving too far forward and creating a large normal force offset.

    Great, look forward to hearing all about it.
     
    Last edited: May 6, 2013
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  15. J011yroger

    J011yroger G.O.A.T.

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    ^^ Sent you an e-mail reply if you didn't get it please let me know, and I will try harder.

    J
     
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  16. Pickle9

    Pickle9 Semi-Pro

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    [​IMG]
     
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  17. Pickle9

    Pickle9 Semi-Pro

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    Kevlar doesn't "snap back". It just doesn't move that much to begin with. Kevlar typically stretches just 3.5% before it breaks (http://en.wikipedia.org/wiki/Aramid).

    [​IMG]
    I think your idea is correct, that if you can make something stretch more (increase x) while its stiffness (in tw terms lb/in) decreases at a slower rate (k), you will increase the force exerted (the snap).
    I don't want to say it's impossible but in reality you can't make something stretch more without decreasing its stiffness, and you can't make something stiffer without reducing the amount it stretches because materials follow hooke's law (unless they are overloaded, like a rubber band).
     
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  18. travlerajm

    travlerajm Hall of Fame

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    I may not be a wookie, but I can use my magical Jedi powers to overcome my slow swing speed to hit a serve with a half-dead ball and a shortened frame that's still rising when it hits the back curtain on the famously slow UW indoor courts:
    http://youtu.be/3SJ7QnTH0ok
     
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  19. J011yroger

    J011yroger G.O.A.T.

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    I did not mean to imply that the force was not with you.


    J
     
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  20. J011yroger

    J011yroger G.O.A.T.

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    Anybody have a sacrificial frame which they feel like stringing with piano wire?

    J
     
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  21. polytheist

    polytheist Rookie

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    kiss the grommets goodbye too
     
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  22. o0lunatik

    o0lunatik Rookie

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    I ran 3/64" (18G) stainless steel cable in one of my racquets on a drop weight at 40lbs and cheap wilson syn gut in the cross. The ball bite and spin generation was insane... topspin action constantly dropped balls like Nadal. It was suprisingly springy, and with a fast swing for top spin, I was ripping felt like plucking feather off of a chicken. I enjoyed it a lot, but the balls did not last long at all. I wouldn't recommend as a daily racquet, instead as a fun racquet.

    Gamma used to have metal strings branded under Gamma Edge (something like that).

    I am also a big fan of Kevlar too! It dropped tension like a bit*h at first, but after it settles, it plays very consistant. So string the main real high like 68lbs, then cross with a twisted edge poly at around 55lbs, and the kevlar will stay in place. You wont get that snap back action from kevlar, but kevlar is spin friendly due to its braided surface. I spent over a decade of playing with Kevlar and experimented at many diff tension and with many different cross strings and material and have come to say that kevlar is awesome!
     
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  23. travlerajm

    travlerajm Hall of Fame

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    I suspect the increased stiffness of the steel cable really helped with the extra spin by flattening the balls more.

    We kevlar exponents are a minority. If you haven't tried the new Ashaway Monogut ZX string as a cross with kevlar mains yet, you might want to give it a try. It is slippery like poly, but doesn't drop tension like poly. It has an initial break-in period where the tension drops a lot, but like kevlar, it plateaus off and plays well for a long time after that.
     
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  24. J011yroger

    J011yroger G.O.A.T.

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    I imagine that it broke balls with the amazing frequency which Kevlar does.

    J
     
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  25. J011yroger

    J011yroger G.O.A.T.

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    Funny how proponent and exponent mean similar things.

    I never thought about it before.

    J
     
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  26. treo

    treo Rookie

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  27. travlerajm

    travlerajm Hall of Fame

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    Something tells me that I won't be able to resist an experiment with metal strings. Maybe a steel mono cross with a braided steel main? Should I pre-stretch?
     
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