The Ultimate Poly Pre-Stretch Experiment!

Discussion in 'Strings' started by travlerajm, Apr 14, 2013.

  1. travlerajm

    travlerajm Hall of Fame

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    Hypothesis:
    This post was inspired by an interesting recent thread questioning whether the tension stability of poly strings can be improved by pre-stretching.

    To truly test this idea, I decided to try an experiment with a TRUE pre-stretch of poly. That is, a true pre-stretch requires holding the tensile stress for a long time to allow the string to creep under the the tension. A true pre-stretch cannot be accomplished in a few seconds.

    In short, I pre-stretched a poly at 70 lbs and held it under tension for more than an hour.
    Here are the details of Part I of this experiment:

    String:
    Prince Tournament Poly 16g.
    I chose this string because it’s my favorite string to use as a slippery poly cross when it’s under appropriate tension. It has a glassy smooth dent-resistant surface (harder and smoother than any other string I’ve encountered). It’s downside is that it has the poor tension stability of a typical poly string.

    The setup:
    I cut off a little bit more than 20 ft of string. To anchor one end of the string, I created a lasso at one end to loop around the end of my kitchen bar countertop.

    I set up my Gamma X2 dropweight stringer on the carpet 20 feet away, and placed the other end of the string in the jaws of the tensioner.

    Before pulling tension, I pulled the string enough to straighten it (but still with plenty of slack), and with a sharpie pen marked off a 10-inch gauge length for the string in the relaxed condition.

    To adequately anchor the stringer to the floor, I first attempted combinations of luggage and furniture, but nothing in my condo seemed to be heavy enough and conveniently shaped to suit the job. I decided that for this first experiment, I’d need to stand on the base of the stringer in order to anchor the stringer securely enough – so that’s what I did.

    The process:
    I set the weight at 70 lbs. Pulling tension with the dropweight was extremely difficult, because raising the dropweight 90 degrees did not release much of the tension (normally, 90 degrees translates to a significant percentage of the total length under tension, but that is not the case with the full 20 feet under tension.)

    I found that I could muster enough torque with my hand to advance the ratchet through one click (10 degrees of rotation, or about 4mm length extension) with each raise of the dropweight (my hand is quite sore though).

    After each added click ( raising the dropweight 10 degrees above horizontal), I would wait a few minutes for the dropweight to creep back to horizontal.

    As I continued to repeat this cycle, the creep rate got slower and slower, so that it was taking more than 10 minutes for the string to creep another 4mm.

    Something else interesting started to happen too. After a while, I started to notice that the initial equilibrium position of the drop weight (after adding a click) was getting higher (because the creep rate was no longer fast enough to balance the tension increase).

    Eventually, the equilibrium position got high enough that there was no longer an equilibrium position (without forcing the dropweight down to horizontal, which I was afraid to do, for fear of exceeding the string’s ultimate tensile strength). If I tried to take my hand off the dropweight while it was 20 degrees from horizontal, the tension in the string would slowly pull the dropweight back up. A few times in a row, I patiently held the drop weight steady until the creep was enough to reach a quasi-equilibrium again. And the final time, holding it in place for 10 minutes was not long enough, so I risked gently forcing it almost to horizontal, which worked.

    I performed these cycles for about 90 minutes total time under 70 lbs.

    Initial observations:
    After the stretching process, I asked my wife to take a measurement of the distance between the gauge markers while it was still under tension (I couldn’t do it myself, since I needed to keep my body weight on top of the stringer). It measured 10.5”.

    Then I released the tension and measured the new relaxed gauge length: 10.14”.

    This means that the string creeped by about 1.4% during the process. For a typical racquet, that’s about 2.5” in total string length.

    This also means that the string can still stretch about 3.5% elastically under 70 lbs of tension. This can be used to calculate a tensile elastic modulus – I’ll do that later.

    I found it interesting that after the stretching process, there was no hint of coil memory left in the string. Rather, it had a new memory: it wanted to be a 20-ft-long straight rod.

    More to follow…
     
  2. travlerajm

    travlerajm Hall of Fame

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    Part II:

    Strung up my siliconed & shortened diablo mid.

    I re-used the 18g kevlar mains that were already in the frame that had been mostly broken in (about 3 hours) but been in there for a year, and then restrung the crosses using the pre-stretched tournament poly. I used 45 lbs in both mains and crosses (I tightened up the mains a bit without re-weaving to get back up to 45 lbs reference, since kevlar is easy to cut and tie on an extension).

    Bouncing the ball on the strings in the living room, feels crisp with short dwell time like a fresh bed of kevlar/poly normally does at that tension. But seems a little more lively than normal, as if it has better energy return.

    Will test later against the wall...
     
    Last edited: Apr 14, 2013
  3. corners

    corners Legend

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    Great experiment Trav. Look forward to hearing how it plays and holds tension.
     
  4. travlerajm

    travlerajm Hall of Fame

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    I worry that I will really like the pre-stretched stringbed a lot, and then there's no going back.

    I'll need to go to the hardware store to get some bricks and sandbags to anchor my stringer to permit an hour+ pre-stretch every time I want to re-string my racquet. Or maybe I'll just stick with the ZX crosses.

    I just did a quick calulation based on the creep and elastic deformation:

    My 70-lb, hour+ pre-stretch (to 1.4% creep) is equivalent to letting a stringbed strung at 45 lb reference tension relax all the way down to 18 lbs. Should make a big difference in tension stability.
     
    Last edited: Apr 14, 2013
  5. corners

    corners Legend

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    LOL

    Have you considered something like bench clamps or a bench vise?

    That's about how much tension Tournament Poly 16g lost during TWU's pre-stretch protocol. After being so stretched and then re-tensioned to 62 pounds the string lost 17 pounds, compared to 25 during the pre-strech. It looks like Tournament Poly is pretty prone to creep and stress relaxation, but the difference between 25 and 17 pounds of total loss is pretty significant. I think you can expect it to hold tension much better after the application of your (very arduous) pre-stretch protocol.
     
  6. travlerajm

    travlerajm Hall of Fame

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    Actually, I suspect that my pre-stretch procedure will have much larger effects than the TWU pre-stretch procedue.

    The behavior of the string while stringing was much different than when I string it without pre-stretching. Without a pre-stretch, the dropweight will creep about 10 degrees/minute with every drop during the stringing process (roughly from my memory). With the pre-stretch, there was zero noticeable creep. It was as if it was a completely different string.

    Given this observation, I'm guessing that I've probably captured at least 90% of the creep/relaxation curve. In contrast, the TWU protocol only captured about 30% [(25-17)/25] of the creep/relaxation curve.

    Anyway, the on court results are what matter. We shall see...
     
  7. travlerajm

    travlerajm Hall of Fame

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    Tried the pre-stretched bed against the wall tonight.

    It played amazing. Terrific bite and rpm. Short dwell time, but soft muted yet full-bodied feel. Nice control.

    The 45 lbs was slightly softer than my usual preference, but still great all-around and rebound angle not too high. It seems to have more energy return and pop than kevlar/poly usually does, without sacrificing accuracy.

    Unfortunately, the poly snapped after 40 minutes of wall hitting (equivalent to about 1.5h singles). It snapped on the outside of frame between 3rd and 4th string from the tip when I caught a serve off-center near the frame. So tentative conclusion is that the pre-stretching process can make the string more fragile and sensitive to stress concentrations.

    Also: I couldn't resist making a few purchases today in preparation for building a home pre-stretching device (a wire-frame shelf unit that can take 500 lbs per shelf, a 100-lb set of disk weights, and a set of pulleys).
     
    Last edited: Apr 16, 2013
  8. pvaudio

    pvaudio Legend

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    Goodness, that sounds like some sort of Inquisition equipment you're building :lol:
     
  9. Power Player

    Power Player G.O.A.T.

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    It's much better to string on a constant pull machine with poly. simulates prestretching, but does not weaken the string as much as actual prestretching does.
     
  10. corners

    corners Legend

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    Deleted post
     
  11. travlerajm

    travlerajm Hall of Fame

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    Ok. I officially set up my home pre-stretching device in my lliving room this morning:

    Components:
    1. A "3-Step Project Stool" from Home Depot as a frame (which is basically a 5-foot-tall step ladder with steel tube frame).

    2. Four 3-inch diameter clothes line pulleys.

    3. 3/16" diameter Neon twine.

    4. A 20-lb nylon sand-filled disk weight.

    -----

    I'm doing a pre-stetch on 16g Prince Tournament Poly again.

    Step 1: I attached 2 pulleys to the top crossbar of the frame, and 2 pulleys to the crossbar at the base. I used 2 small loops of twine (each triple-knotted to avoid knot slip) to attach each pulley to the bar (I needed two loops to orient the pulleys correctly).

    Step 2: I looped a "safety net" of twine through the disk weight and around the top crossbar to catch the weight before it crashes to the floor in case the string were to break.

    Step 3: I anchored one end of the 20-foot segment of string to a loop of twine wrapped around the top cross bar.

    Step 4: I threaded the string down, up, down, up through the 4 pulleys.

    Step 5: I placed the disk weight on top of a 3-foot tall bar stool (to give me some slack while I attached it). I looped some twine through the hole in the disk weight, and then tied the other end of the string to the twine.

    Step 6: I marked off a 10" gauge length with a marker while the string was still slack.

    Step 6: I made sure that the string was properly seated in all 4 pulleys, and carefully removed the bar stool and gently lowered the disk weight to tension the string.

    Observations so far:

    I have the weight hanging about 2 feet off the ground. I proabably have about 19 feet under tension. It looks like the 20 lbs of tension is too low -- I can't notice any creep happening.

    Next step: I'm going down to my car to grab more disk weights. Will add another 20 lbs and see how it works.
     
    Last edited: Apr 20, 2013
  12. corners

    corners Legend

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    Good ol fashion industry and ingenuity. Tournament Poly is a good one for this as it loses a lot of tension (according to the lab tests) but has that surface hardness and lubricity that we look for. If you can get it to hold tension better with the pre-stretching you'll have a winner.
     
  13. travlerajm

    travlerajm Hall of Fame

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    Update:

    I attached another 20-lb disk weight to the first one. So now 40 lbs of constant tension.

    This seems to be working, the weights dropped about an inch due to to slow viscous creep in the first 15 minutes after the initial stretch. I noted the height of the center of the outer weight was about 18.75" off the floor when it started. Now it's at 17.75".

    I plan to let it hang for at least 24h, maybe even a few days - however long it takes for me to stop noticing the movement. I have about a foot of clearance between the ground and the weights.
     
  14. travlerajm

    travlerajm Hall of Fame

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    Update: 17.5".
     
  15. travlerajm

    travlerajm Hall of Fame

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    17-1/8". (I might as will use this thread as my lab book).
     
  16. travlerajm

    travlerajm Hall of Fame

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    17.0. slowing down.
     
  17. corners

    corners Legend

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    Open lab book, good idea.
     
  18. travlerajm

    travlerajm Hall of Fame

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    16.75".................
     
  19. corners

    corners Legend

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    40# seems to be working nicely.
     
  20. travlerajm

    travlerajm Hall of Fame

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    Yes - still at 16.75".

    I expect it might extend another inch by tomorrow.
     
  21. travlerajm

    travlerajm Hall of Fame

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    16-3/8" - weights were slightly caught by friction against vertical bar.
     
  22. travlerajm

    travlerajm Hall of Fame

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    The Creep Curve so far:

    [​IMG]
     
  23. esgee48

    esgee48 Hall of Fame

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    Looks like it is going to top out around 1.6%. Strange that this is more than the 1st pull with your dropweight.

    Have you checked all the sections of string just to insure none of the pulleys are binding? Just curious.:???:
     
  24. travlerajm

    travlerajm Hall of Fame

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    It seems that the pulleys are doing their job of distributing the tension equally throughout the full length. When I take each measurement, I check to make sure the weight is fully borne by the string and not caught by any friction against the vertical bar by separating the weight from the bar (the duplicate 16.75" height measurements posted above were because of this friction effect). Here's another update:
    [​IMG]
     
    Last edited: Apr 21, 2013
  25. travlerajm

    travlerajm Hall of Fame

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    Update:
    I removed the string from the pre-stretch device this morning. Here is the final 24-h result (based on an unstretched length under tension of 17.3 ft).

    [​IMG]

    Next, I loaded some 18g Ashaway kevlar onto the device. I used flying clamps from my stringer to make it easier to tension each pulley one at a time (which made it easier to load the weight at the proper height).

    I'm curious to see if the shape of the creep curve will look different than the one for poly, given the difference perceived tension stability when in a racquet.
     
  26. travlerajm

    travlerajm Hall of Fame

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    Update:
    Looks like the 18g kevlar is creeping, but at ~half the rate of the 16g poly.
     
  27. Jacklondon00

    Jacklondon00 New User

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    Update us on your playing experience with these pre-streched strings?
     
  28. travlerajm

    travlerajm Hall of Fame

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    Update on playing experience:

    Strung up my shortened (26-3/4") diablo mid (siliconed and leaded up to same specs as my signature frame).

    I used thoroughly pre-stretched ashaway kevlar 18 in the mains (stretched ~4" longer for an 18-ft length), and thoroughly pre-stretched Prince Tournament Poly in the crosses (also stretched ~4" longer for an 18-ft length).

    Initial reference tension: 47 bs.

    Power level: medium-high (nice for serves)
    Spin level: exceptionally high, off the charts (great for heavy forehands)
    Stringbed softness: muted soft feel, softer than my preference, as I prefer firm stringbeds
    Flat shots & volleys: ok, but would prefer stiffer stringbed for better precision.

    After an hour or 2 of play, I decided to snip the ends of the string and re-string with same strings at higher tension. I tied on extensions at the outer mains to keep the same number of mains, but I omitted the bottom cross on the re-string (since poly is too slippery to add an extension without a hulking triple knot).

    Second attempt: re-strung at 51 lbs with same (re-used) string:

    Power level: medium low (but higher than unstretched at same reference tension)
    Spin level: excellent
    Feel: firm but muted (like I like it)
    Volleys & Flat Shots: very precise
    Control: exceptional
    Overall: Extremely satisfied

    Unfortunately, one of my extension knots on the outer kevlar mains slipped after 2h of play. I should have known better than to rely an a square knot. I was really enjoying the hitting experience, and the diablo had overtaken my Blade as the racquet I reached for to play my best quality tennis (but in fairness to my Blade, the Blade needs a re-string).

    So the test of how well the tension will hold up over time is still incomplete. But I can say that I don't think I'll ever use an unstretched poly again. The pre-stretched poly plays better from the get-go. I really like the fully pre-stretched kevlar/poly combo, and will continue to use it.

    Next: will try pre-stretched ZX monogut crosses in my Blade
     
    Last edited: Apr 28, 2013
  29. Jacklondon00

    Jacklondon00 New User

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    Thanks for taking the time and posting the update. I have recently tried several Kevlar/Poly hybrids at 50/56 and 54/60 in a Volkl C10 Pro and there was definitely a window of incredible performance.

    With 50/56 it was good to go from the start, but with 54/60 it required a short break-in period. Unfortunately the drop in tension and control after 5 to 7 hours of play was disappointing, while I could still control most of the hard hit groundstrokes pretty well due to spin, on many flat returns of serve and softer shorts the ball seems to fly.

    I am using Firestorm as a cross, it's the most powerful poly I have come across and holds tension well, despite this I measured a 22% drop in tension with this kevlar/poly combination, while on a full poly setup I have never measured more than a 15% drop in tension, with firestorm or any other poly for that matter. I think kevlar might be the culprit here.
     
  30. diredesire

    diredesire Super Moderator

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    Sup Yev,

    How are you measuring your tension loss? String meter? ERT300? Racquet Tune? Just curious.

    (Not sure if you're aware, but travelerajm was [is?] at UW for a while... Maybe you actually know him in real life..)

    JD
     
  31. travlerajm

    travlerajm Hall of Fame

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    Yev? We used to practice against each other quite a bit in a past life, a decade ago.
     
  32. diredesire

    diredesire Super Moderator

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    Small world, then! (Not sure if you recall -- we hit at a TW get together back in the day, too). I recall there was someone who was looking to get in touch with you for some reason, but I believe the e-mail I had on file for you seemed to be expired...
     
  33. Jacklondon00

    Jacklondon00 New User

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    Hey guys,

    That's interesting, I knew that Trav was from Seattle and possibly UW from his posts, and juding by the timeframe of his posts I asumed you guys knew each other as you have been active on here for so long. I am trying to think now, someone who I used to hit with at UW, who was into tennis strings and majored in material science, and used kavlar, hmmmm, I am really curious now. Any hints?

    I just joined TT last year after starting to play tennis again last January since leaving UW. It took me a year to get back into it but I am definitely playing better than I was 10 years ago, and in better shape now too! But still not up to UW team standards, maybe in ten more years:)

    To answer your question, I use racquet tune, all of my poly's top out at 12-15% after a few weeks of playing 3 to 5 times a week, I am pretty meticulous at straightning out the crosses while also letting the weight sit for 10 to 20 seconds, I also release the previous clamp when tensioning the next cross, this is why I though the numbers were so low before, so when I saw 18- 22% loss on 2 different sticks with kevlar i was really surprised.
     
  34. Jacklondon00

    Jacklondon00 New User

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    Alright, after giving it some thought and thinking back to one of your posts that you used to use a Wilson Pro Staff 4.7 at one point, I am pretty sure it's you A.M?
     
  35. diredesire

    diredesire Super Moderator

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    yep .
     
  36. travlerajm

    travlerajm Hall of Fame

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    Where you at these days? Still in DC? We'll have to get back on court someday like old times.

    To respond to your observation about kevlar tension maintenance, kevlar is misleading because it initially loses a lot of tension in the break-in period (more than other strings). But after it settles (after the first 2-3 h), it hits a plateau and holds tension better than any other type of string. I believe the break-in loss is due to realignment of the braided fibers. Poly is very different -- poly continues to creep and lose tension without ever leveling off.

    Consider the following comparison (data collected on my home-built pre-creeper):
    [​IMG]
    Note how the kevlar has steeper slope than the poly in the first 10 minutes of the test (this I believe corresponds to the break-in period, as 10 minutes on my pre-creeper is equivalent to several hundred ball impacts). But after the first 10 minutes of the test, the kevlar levels off and resists further creeping much better than than the poly (the curves cross each other).

    The key with kevlar is not to string it too loose, and then be patient enough to break it in. The key is to find a tension that you like after the break-in period. For a longer-lasting playability alternative to poly cross (if you don't want to go through the trouble of pre-creeping like I am trying), I recommend the new Ashaway Monogut ZX strings. They have smooth surface, so they give good snap-back with kevlar mains. But unlike poly, the ZX has very good tension maintenance (but also needs a break-in period to settle into it's tension plateau ). The ZX is a unique string -- it has a smooth poly-like surface, but also has elastic stretchiness like natural gut.
     
    Last edited: Apr 29, 2013
  37. Jacklondon00

    Jacklondon00 New User

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    I am in London these days, been here for a while now. Might be going back to U.S. soon but not going to be anywhere near Seattle.

    "10 minutes on my pre-creeper"

    I had a good laugh after reading this. I don't have space for a "pre-creeper" in my place, but I always let the weight sit for 10-20 seconds before clamping which helps a little. I have been following the thread on Ashway ZX, I have two packs at home that I have been meaning to try. I was planing to try some hybrids (Tourbite/ZX vs Tourbite/Firestorm) and (Ashway Kevlar Plus 17/ZX vs Ashway 18g/Firestrom) but I am still waiting on the the other strings to arrive.

    I was just concerned that stringing 56m/60c with ZX might be too high as I hear it's very fragile, but it also might not be high enough since 54/60 with Firestorm lost too much tension after a week of hitting. Although the Kevlar I tried was not Ashway it was Bow Brand Kevlar, so could be just crappy Kevlar.
     
  38. NLBwell

    NLBwell Legend

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    Kevlar does have a big initial tension drop normally. However, because it isn't very elastic, it doesn't change the performance of the string much.
     

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