NEW TWU (3-21-13): How Strings "Go Dead"

Discussion in 'Strings' started by TW Professor, Mar 21, 2013.

  1. TW Professor

    TW Professor Administrator

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    There is a lot of talk about how polys "go dead" and there are a lot of contrary opinions of what going dead means. I asked playtesters at TW and some say the ball flies and control is lost and others say the string hits like a board -- two seemingly contrary opinions.

    So what really happens? This experiment set out to find out and the results are reported here:

    How Strings Go Dead
     
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  2. McLovin

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    #2
  3. ChicagoJack

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    Mr. Lindsey -

    Wow. Thank you. Thank you. Thank you. Thank you. You've correctly identified an issue that has needed serious consideration for quite a while. While the effect of strings, racquets, and inherent rebound power / ACOR is given careful study in your book, the going "dead" issue is given only a brief mention. This in depth study is truly appreciated!

    -Jack
     
    Last edited: Mar 21, 2013
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  4. newyorkstadium

    newyorkstadium Semi-Pro

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    Thanks TW Professor.

    I'm confused by this article. So all strings lose spin potential, but nylons lose less then poly's? Although, in your words the "decrease in tension should also decrease the friction between strings"

    I'm guessing the increase in stiffness is the dominant factor. The main thing I read is that dead strings "hurt your arm".
     
    Last edited: Mar 21, 2013
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  5. kaiser

    kaiser Semi-Pro

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    Thanks Prof, great experiment with very intreguing results, much appreciated!
    I'm trying to get my head around how these findings can be reconciled with the increased spin with no concomitant loss of control reported for sub-40 lbs tensions in the "Low, low tensions" thread.

    You conclude in this paper:
    "The lower tensions and perpendicular stiffness of many polyesters leads to longer dwell times and greater deflection. This keeps the ball on the racquet for a longer arc of the stroke, potentially creating "power" problems with the ball going deeper, wider and higher than desired. The decrease in perpendicular stiffness also contributes to the sensation that the strings get "mushy" or behave like a trampoline. A loss of control is the end result."

    Following this line of reasoning, shouldn't we expect to see the same phenomena and resulting loss of control when stringing at very low tensions in the first place? I don't observe this in my own rackets.
     
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  6. Sander001

    Sander001 Hall of Fame

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    "Should" is a key word in science which is used as a preface for the next phase of the experiment.
     
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  7. TennezSport

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    Nice work, thanks.....

    This is a very nicely done string baseline analysis and explanation. It makes it clear to see how complicated it gets when you add in racquet size, string pattern, weight and playing styles to the string life, but at least you have a place to start understanding poly string. Thanks for the data.

    Cheers, TennezSport :cool:
     
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  8. arche3

    arche3 Banned

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    Puts all the ttw wanabe scientists to shame. Good job!
     
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  9. Overdrive

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    So a low-tension co-poly lasts less than a high-tension co-poly?

    Just for verification.
     
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  10. fgs

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    kaiser,

    i think you don't see the difference between stringing low from the start and getting to the same tension by repeated impact on the string. after several hundreds of impacts, the strinbed will behave differently than a fresh stringbed installed at a lower tension.
     
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  11. TW Professor

    TW Professor Administrator

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    fgs is correct above. Strings behave differently depending on how they arrive at a given tension. I have done experiments where I get to and test a string, say at 40 lbs, in several ways. I might test the string freshly strung at 40, or strung to 60 and manually lowered to 40, or strung at 60 and raised to 80 (as if impacted) and then lowered to 40, or by stringing at 60 and repeatedly impacting until I get to 40. All the methods will arrive at different test data for the same 40 lbs. The tension is one thing, but how it gets to that tension is another.
     
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  12. TW Professor

    TW Professor Administrator

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    All strings that I tested increase in friction COF which will tend to decrease spin. However, the rate of that decrease and at what level you will notice it depends on the string. Most nylons don't have much string movement to begin with, so it doesn't matter much that the COF increases.

    Yes, decreasing tension should decrease the friction between strings, but at the same time it is increasing due to wear. One or the other will win out. If you feel it getting stiffer, then the increasing friction due to wear is the winner.
     
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  13. newyorkstadium

    newyorkstadium Semi-Pro

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    Is it possible the lower tensions and perpendicular stiffness could balance out with the increased COF? So the playability stays the same.

    If you find that a string ends up too stiff, can't you just string lower?

    What happens to spin potential if the COF is going up, but the decreased tension also decreases the friction between strings?
     
    Last edited: Mar 22, 2013
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  14. DonBot

    DonBot Semi-Pro

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    I notice a drop in playability when my full poly bed starts to etch really bad, I always assumed people complaining about a dead full poly string bed being boardy were victims of etching. I can't imagine the strings slide much at all when you have 16g poly etched half way to breaking.
     
    Last edited: Mar 21, 2013
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  15. JT_2eighty

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    Great thread and responses!

    Love it when the TWU prof gives us a new experiment to read.

    I think the phenomena of 'string going dead' has been one of the most needed experiment out there; great stuff!!
     
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  16. pvaudio

    pvaudio Legend

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    I will just say this: that was absolutely fantastic. Thank you SO much for spending the time researching this topic. :)
     
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  17. lawrencejin

    lawrencejin Rookie

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    This is why I keep coming back to TW forums. Thank you for a great article!
     
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  18. tlm

    tlm Legend

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    I sure do thats why I like higher tension because it gives better control than low does.
     
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  19. mikeler

    mikeler G.O.A.T.

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    I've only had time to read the Conclusion but I agree so far!

    Great job on this TWP.
     
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  20. ricki

    ricki Professional

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    So do I understand, that from polys you tesed here are best RPM Team and Luxilon Original?
     
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  21. kaiser

    kaiser Semi-Pro

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    Thanks for that clarification, very interesting to hear that you actually tested this and got different results for the same tension achieved from different starting points and via different pathways. From your paper this was not clear.
     
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  22. pvaudio

    pvaudio Legend

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    My favorite part was the notion of contradiction in the conclusion which EVERYONE has experienced (well, of those who believe in death :) ). Dead strings should have greater spin via less friction, but they have less stiffness to them, so you don't realize it. I'm so glad you looked into this objectively. I can attest to the fact that dead Scorpion is an exercise in patience although it's playable in a hybrid. However, the moment I switched to a fresh stick (popped the cross), everything I had been hitting simply became 10x easier. So, note to everyone here: don't hone your technique with dead poly!
     
    Last edited: Mar 22, 2013
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  23. bad_call

    bad_call Legend

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    becomes more dramatic with those "lively" polys...for those who relish drama. ;)
     
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  24. TW Professor

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    I suppose. The fact that some strings go dead in 2 hours and others 10, etc., is testimony to the fact each string is losing tension and stiffness and gaining in COF in its own distinctive way.

    Yes, of course. But that might change the rate at which stiffness and COF change relative to each other. So, you might prolong playability or hasten its death. Remember, the history of how a string gets to its pre-impact tension seems to make a difference. The stringing reference tension is one of these historical variables.

    It is just a matter of which tendency changes the COF the most. The increase in COF due to wear may be more, less, or the same as the decrease in COF due to the tension going down. But you probably won't find something that always stays in balance because the rates of COF and tension change also change as the number of impacts increases.
     
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  25. TW Professor

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    It was actually part of another experiment, so I did not include it. Maybe it will appear on TWU some day.
     
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  26. TW Professor

    TW Professor Administrator

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    Thanks. I'm glad everyone finds this experiment interesting. It began last July and has been an ongoing off-and-on project. As you can imagine, hitting and sliding and videoing each string thousands of times is not for the faint of heart. In fact, there were many instances of the Professor going dead also.
     
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  27. fgs

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    thank god or whoever it is that obviously kept the professor alive and kicking, i.e. hitting.

    there is one issue about this going dead which i still can't really figure out: same stick, same tension, same player, two different strings (names don't make any difference) and two different behaviours or ways to die.

    1. strings obviously lose tension and therefore deflection of the stringbed is bigger. this leads to some spraying and obviously a higher launch angle which is mistakenly interpreted as an increase in power, balls all of a sudden go long or wide by a unusually wide margin. this is obviously described as going "rocketlauncher-mode".

    2. in spite of the strings having lost tension and obviously the deflection is also bigger, the balls are hardly making it over the net. this is most commonly described by fellow-members as the string is playing like a board.

    intuitively i would say this has to do with the way the material is "ageing" (material fatigue) and with the way this aged stringbed is returning energy. while in the rocketlauncher-mode there obviously still seems to be a reasonable energy return (i'm not talking deflection angles here which translate into control in the perception of the striker), in the "board-mode" the energy return must have quite dramatically dropped compared to the fresh stringbed.
     
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  28. maxpotapov

    maxpotapov Hall of Fame

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    Seems to me, that PolyStar Turbo is the most unique string in how it ages.
    This is the only string that feels better as it wears out.
    Is PolyStar Energy the same string, smooth version?

    EDIT: I'm referring to hybrid setup. If used in full bed it locks in place due to notching and denting just like any other poly.
     
    Last edited: Mar 24, 2013
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  29. JT_2eighty

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    Having used both, I think they are the same material.

    That would be a great string to test for the TWU, it definitely defies the feel of all other polys... it stays soft for a very long duration.
     
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  30. corners

    corners Legend

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    Interesting that only one poster, Kaiser, asked the Professor a question relating to elasticity loss or resilience loss. Attributing "going dead" to the loss of these things has been the favored theory of many on these boards. But this new paper by the Professor doesn't really look into "resilience" or "elasticity" loss as a potential cause of "going dead."

    Professor, thanks a ton for a great paper! However, now we'll be needing a String Wear and Tear Database with categories for cross string scuffing, main string notching, cross-string denting, etc.
     
    Last edited: Mar 22, 2013
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  31. kaiser

    kaiser Semi-Pro

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    As a matter of fact, I commented on the general conclusions of TWProf on lower tensions and perpendicular stiffness and concomitant loss of control. But it is indeed remarkable that in this analysis loss of 'resilience' or elasticity does not appear to play a role.
     
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  32. pvaudio

    pvaudio Legend

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    It actually plays a fundamental role. It's just bundled in there mathematically with the notion of the strings having stiffness, and them being able to move and return to their original position. For some very in depth and boring theory, wiki "linear elasticity". :)
     
    #32
  33. Xonemains

    Xonemains Semi-Pro

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    Thank you TW Professor,

    It was a long read but I learned alot from this thread, cheers to you!
     
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  34. corners

    corners Legend

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    I understand what you're saying about it being "bundled in their mathematically", insofar as resilience and elasticity can be calculated using stress/strain curves, but I'm not seeing either characteristic playing a fundamental role in this new paper. But perhaps I'm being dense.

    The Wikipedia article you referred to on linear elasticity has this to say in the introductory paragraph:

    "In addition, linear elasticity is valid only for stress states that do not produce yielding."​

    But when we are talking about strings losing tension we are talking about stress states that produce yielding. Strings permanently elongate (and lose tension thereby) by plastically deforming, and to plastically deform they must have been stressed beyond their yield points. So I'm not sure all the calculus in that article is applicable to tennis strings losing tension and "going dead." Or am I missing something?

    Could you explain for us where elasticity loss is documented in this new TWU paper, where it is bundled in mathematically? It would really help out all the non-engineers among us.
     
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  35. Sander001

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    I will grab some WD-40.

    [​IMG]
     
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  36. julian

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    Last edited: Mar 23, 2013
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  37. Sander001

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    WD-40 helps lubricate the strings so they slide against one another more. In theory this should help the dead feeling.
     
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  38. monomer

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    Another factor to consider is that both nylon and polyester are hygroscopic materials - they absorb moisture. Testing these materials at different temp's and relative humidities can give considerably different results provided that they have reached equilibrium.
     
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  39. corners

    corners Legend

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    It's my understanding that polyester is hydrophobic, which is why it is a favorite of sport/outdoor/mountaineering clothing manufacturers.
     
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  40. pvaudio

    pvaudio Legend

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    I will do my best. :)

    The concept is that a material only has a stiffness when it is in the form of a body or system. What that means is that saying that the stiffness of steel or nylon is x is not correct. The reason is because steel could be used as a girder, a piano wire, or in steel wool. The material is the same, but the stiffness be it flexural (bending), torsional (twisting), axial or otherwise is completely different. So, when people talk about stringbed stiffness being x, that is perfectly acceptable because it's a system. A stringbed of Tour Bite 1.10 and 1.30 strung at the same tension will not have the same stringbed stiffness as each of the individual string's properties will be different due to the force applied to them. Much the same: aluminum foil under any force that doesn't break it is not nearly as stiff as a 1cm sheet of it under the same force.

    So, to get around this issue, people use what is known as the modulus of elasticity (Young's modulus) to describe materials. The lower this number, the greater the flexibility. The higher the number, the less flexible the material is. Stiffness of a system is directly proportional to the modulus of elasticity of a material and the tension applied to it (in the case of axial stiffness which is what we're interested in). The higher the mod of elasticity of the material, the lower the stiffness.

    Now, something is only considered elastic to any degree as long as it's below its elastic limit. This is the point where the force applied to it permanently deforms it and it can no longer retain its original shape. It is also called the yield strength which is a bit more intuitive title. This creates what you can consider to be a "new" material with different properties than before. A system made up of this "new" material will have its stiffness be lower (aside from those materials designed to stiffen under load until failure). Since stiffness is defined as the ability for something to take a force and not deform, if the system is less stiff because the strings are permanently deformed, then the strings no longer behave elastically.

    Perhaps this is clearer:
    http://en.wikipedia.org/wiki/Stiffness#Relationship_to_elasticity

    Hopefully that makes sense. :)
     
    Last edited: Mar 23, 2013
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  41. pvaudio

    pvaudio Legend

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    Just an important note: I am not refuting or disputing the TWU findings in the slightest. :)
     
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  42. julian

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    Please elaborate more

    So I have dead poly strings ( I am a coach of a tennis team).
    My player brings his racket.
    I tell him strings are dead.
    He says -I will bring WD-40 and they will be alive.
    Is it what you are saying?
     
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  43. julian

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    No,it does NOT

    I will post TOMORROW why the question of corners is very good
    but your explanation does NOT address the issue stated by corners
     
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  44. Sander001

    Sander001 Hall of Fame

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    It's what I'm hoping.
    I think it would help to get the lubricant in between the strings.
     
    #44
  45. sundaypunch

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    This is a great article.
     
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  46. pvaudio

    pvaudio Legend

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    I am interested to hear this actually. You still can't have a system that stores and returns energy (ball loads up the stringbed and then rebounds) and has stiffness that is not elastic. It's simply not possible for that to happen.
     
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  47. corners

    corners Legend

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    Adding lubricant to "dead" strings is a great, real-world experiment.
     
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  48. monomer

    monomer Rookie

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    The vast majority of thermoplastic polyesters are hygroscopic. Any string manufacturer will be required to dry the material to a very low moisture level prior to processing. This is not required with most plastics. They also would see a considerable difference in mechanical properties if they test the string "dry as molded" or at 50% RH.
     
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  49. TennezSport

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    Excellent post.......

    PVs explanation earlier was very good and right to the point. Think of it like a spring. In great condition the spring will stretch and then return to its original shape with x amount of energy expelled. However, if the spring is overstretch passed its elastic limit (relating to racquet string), it will not return to its original shape and energy is lost. No matter how much lubication you use it will no longer respond.

    What makes the issue even more complicated is that the materials used, ingredients added, method used to create the string and coating will all have an effect on how the string will perform and how long it lives. Luxilon guards the secret to their final coating process like the Coka Cola formula for a good reason. Hope this helps.

    Cheers, TennezSport :cool:
     
    #49
  50. maxpotapov

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    If a player hits perfectly flat, then yes, lubrication will not make any difference. But if ball is hit with a spin, then lubrication will definitely "revive" a stringbed.
     
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