The physics of a dropweight tensioner

Discussion in 'Stringing Techniques / Stringing Machines' started by lethalfang, Dec 9, 2007.

  1. Carolina Racquet

    Carolina Racquet Hall of Fame

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    I'm about the same age and used to live there in the 80's. Played a lot of tennis with Jim Savage from Okemos. I think he still plays either out of the MAC or Court One.

    When did you previously play before your extended break?
     
    #51
  2. monticore

    monticore Rookie

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    i just got a drop weight machine with linear gripper from eagnas, i made a post in this forum but figured i would ask here about my problem.

    could the linear gripper act as a counter balance and effect the final tensions.

    example if the gripper finishes at 9 o'clock could it be enough of a counter balance to need less tension on the string to keep the rod level.

    i have been getting varying results with my digital scale from 18lbs to 23lbs depending on the final resting spot of the gripper. (weight set at 20) all other parameters the same and scale not being removed from mount between tries.

    cory
     
    #52
  3. Hominator

    Hominator Hall of Fame

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    Didn't know how accurate a regular drop weight could be. Given the accuracy, is there any advantage in getting an automatic drop weight over a regular drop weight? Thanks.
     
    #53
  4. 0d1n

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    You mean something like those "Stringway" machines ??
     
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  5. Hominator

    Hominator Hall of Fame

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    Yes. I'm now comparing a Stringway ML100 with an Alpha Pioneer DC. The difference between the two is more than $300. If the Alpha can come really close to the accuracy of an auto drop weight, I think I'll save some $ and get the Alpha...

    Thanks for your help!
     
    #55
  6. 0d1n

    0d1n Hall of Fame

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    Quite interested in the answer to that one as well. The difference is even bigger over here...an ML100 is almost 3 times as expensive as an acceptable fixed clamps/6 point mounting system "normal" dropweight.
     
    #56
  7. Technatic

    Technatic Semi-Pro

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    The M90 is also automatic and much cheaper!
     
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  8. weksa

    weksa Rookie

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    I just have one thing to say to the OP: Go Bears!
     
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  9. swfh

    swfh Semi-Pro

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    Quick question for you. I'm taking ap physics b (ninth grade) and I learned torque to be F x d x sin fee. Why did you use cos?
     
    #59
  10. Koz

    Koz Rookie

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    You pick Cos or Sin depending on what angle you know (or what angle you want to use). In the original post we use cos because the angle we measure is the angle Q, which is the angle between the horizontal direction and the arm. Cos gives the correct properties for this setup. It has a max value (1) at zero degrees (when the bar is horizontal), and a minimum value (0) at 90 degrees (when the bar is vertical). In comparison sin has a max value at 90 degrees. So if we used Sin in the equation we'd be saying you get max tension when the weight arm is pointing straight up or straight down! We know this isn't true, because if you're really good at balancing, you could balance the weight arm vertically without ANY tension on the string! You definitely can't hold the arm horizontal without tension on the string though!

    [​IMG]


    However, you could also look at the problem from a different direction. If you defined the angle Z as the angle between the lever arm and the vertical direction (instead of the horizontal direction), your tension would be max when Z=90 degrees. Z=90 degrees is the same position as Q=0 degrees.

    The original tension equation in the 1st post was:
    f = (M g / r) (cos Q) R

    If you use Z instead of Q you'd just get:
    f = (M g / r) (sin Z) R

    Note that the physics didn't change at all, you just looked at it differently the 2nd time. So you pick Sin or Cos depending on how you look at the problem.

    It's good you ask the question here, because if you can make sense of this now it will be really helpful for you in your AP class. A lot of times you'll learn an equation one way (with sin), but you'll be given a problem where the angle you're given means you need to use Cos instead. Try to do a quick "does this make sense" check when looking at these problems. Ask yourself "what happens when the angle is zero?" or "what happens when the angle is 90?" If the value is biggest at zero, you know to use cos. If it's biggest at 90, you know to use sin.

    So in this case, you ask "what happens when Q is zero?" well the bar is horizontal and you get your max tension. Which trig function is maxed at zero? Cos.

    If you asked "what happens when Q is 90?" you'd have to think about it...if Q is 90 then the bar would be standing up vertically. If you had it perfectly balanced so that it was standing up all by itself, how much tension would you have to apply to keep it vertical? Zero lbs! In other words, your tension is a minimum (zero) when the arm is vertical. What trig function is a minimum at zero? Cos. So you use Cos for this problem when given angle Q.

    Note, if you are dead set on using sin in your equation, you still can't use Sin(Q), but you can use Sin(90-Q), since that's the same thing as Cos (Q). It's just whatever way you're most comfortable with, or what makes the most sense to you.
     
    Last edited: Jan 17, 2012
    #60
  11. Irvin

    Irvin G.O.A.T.

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    Just remember SOH-CAH-TOA. Here is a short video that explains.

    http://www.youtube.com/watch?v=_S35Ht4imhs
     
    #61
  12. swfh

    swfh Semi-Pro

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    I got it. Thanks for the response. My teacher wants it to be perpendicular to the force, so i was a little confused. It makes perfect sense.
     
    #62
  13. Speed Kat

    Speed Kat Semi-Pro

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    I only let it "freefall an inch or two sometimes on the crosses to help get rid of some of the excess slack on days where due to particular strings and string pattern combos, things get a bit jammed and tight. Also, instead of pressing down on the lever at the last string to compensate for slack, just increase the tension by a couple of pounds although this is a controversial practice too.
     
    #63
  14. Ray_cn

    Ray_cn New User

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    It's a great post. Recently i'm interesting in the stringing machine too. And read some docs about that.

    For the dropweight machine.

    If considered the string tension lose eg. the string will be elongated. For example sofer strings. The dropweight will not be a accurated system. In my pic, left, the string is pulled at a constant tension, and continuously whether the string is elongated. Right, if the string is elongated eg. tension lost, the lever will not still holding the horizontal position. For example, extreme situation if the string losed too much, when the Q=-90degree, the string tension is 0. Stringer have to adjust the tension for the tension lose. Increase the Q between the arm and the string gripper. One hand hold the 1-2kg lever, and one hand hold the string gripper, and the eye have to watch the indictor. It's complicate operation. I saw some video, if the stringer need to adjust the Q, at least adjust 2-3times.

    To avoid the string elongated, many stringer using Pre-stretch method. It's easy to control.

    I agree with your opinion about the F=ma, the speed should not be fast. and your example catapult, I always using a manual Manual crane for example, it is easy to find in a car repair shop. And easy to use to elevate a car. I think that is why someone adjust the Q many times then the tension is much higher than correct.

    If the string is hard to be elongated, the tension is excatly accuracate, if the string is softer, the tension maybe need more experience to adjust.

    [​IMG]

    The Stringway machine invients a method to make "The tension is the same for every angle of the lever" It's great, but I don't have that machine, wish to learn how they do that.
    http://www.stringway-nl.com/USA/

    Learned much from this topic and reply, it's great.
     
    #64
  15. dyldore

    dyldore Rookie

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    I didn't look through the entire thread so I apologize if this has been answered, but what if the drop weight goes passed horizontal? I'm curious because I ordered the klippermate and am not sure how much of a pain the non-ratcheting tensioner is going to be.
     
    #65
  16. Lakers4Life

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    I think he answered that in the first page. If the bar were to dip slightly below horizonal, 1.2 deg can be as high as 15 lbs.
     
    #66
  17. enderx1x

    enderx1x New User

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    If I understand correctly, extra tension only occurs when external force is added to the bar, such as dropping it or pushing on it. If the drop weight ends up past horizontal, but was lowered slowly then it is still fine. 5-10 degrees past horizontal would result in about 1 pound less tension.
     
    #67
  18. Lakers4Life

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    I just realized that too, but what about torque? I've never really used a DW other than to mess around with. I've even had a ML200TT that I hated with a vengence, but that's another story.
     
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  19. enderx1x

    enderx1x New User

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    The torque for 5 degrees over horizontal and 5 degrees under horizontal should be the same, as for any other number of degrees.
     
    #69
  20. Technatic

    Technatic Semi-Pro

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    Theory of "automatic" dropweight

    To get the total picture it might be good to add the theory of the “automatic” dropweight to this discussion.
    It shows why the tension is independent of the angel of the lever.

    [​IMG]
     
    #70
  21. norcal

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    Bumping this thread because I have a drop weight stringer and never saw this before...pretty great thread.
     
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  22. MixedMaster

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    I'm glad you found it also!!! I, too have never seen this thread and the info is really great. This thread makes me think much more highly of my drop weight.
     
    #72
  23. MathieuR

    MathieuR Professional

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    Agree 200%!
    A crank-lock-out looks more "sophisticated", but the string-tensions you get with a drop-weight are closer to the reference-tension. (and so the dropweight is in my opinion far superior to a LO)
     
    #73
  24. eelhc

    eelhc Hall of Fame

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    There you go again... I have a turntable, turntable brake, mounting system and clamps that looks more "sophisticated" as well :)

    Not unless every dropweight is a Stringway.

    The OPs analysis is accurate... no surprises there. Levers, Pulleys... Physics 101 from Freshman engineering.

    But the OP did not take into account the friction the center mains strings encounter because they have to be pulled over the throat since most drop weight machines are not 360 degree rotation. Also... and I maintain that this is as important as the set tension is how the string is pulled and worked (perhaps the OP didn't take Sophomore year Material Science yet). On a dropweight with a ratchet, a main or a cross that's been worked through multiple ratchet cycles is NOT the same as the one that the tension was nailed with a single drop. Also, it's very important the stringer lowers the bar methodically at a consistent, deliberate rate. Even on your Stringway, the dropweight must be cradled and lowered in this manner to be consistent (just like I have to turn my crank).
     
    #74
  25. MathieuR

    MathieuR Professional

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    Yes, I think a dropweight is underrated and deserves more "respect"

    You are giving me a straw man here ;) . I am not arguing the necessity of a quality stringtable/clamps

    True, a Stringway is "special"; but every dropweight can, when correctly used, give a stringtension close to reference-tension.

    Having to pull "over the throat" has to be done with all machines lacking a "concorde"-system

    I agree there could be minimal differences, but not substantial. The ratchet works like the brake of a lock-out. But unlike a LO, you keep tensioning the string till you removed most of the creep.
     
    #75
  26. eelhc

    eelhc Hall of Fame

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    You might not be arguing the quality of the turntable, mounting mechanism and the clamps, but it is germane to the discussion because I see no dropweight tables with better turntable + clamps (again we're leaving Stringway out and we've already discussed the deficiencies of the Stringway mounting system in another thread). So why is it that we do not see a stringing machine with high end turntable, mounting system, clamps and a dropweight tensioner? Gamma makes a fixed clamp dropweight but they do not have a model with the better version of these features. You would say that Gamma, like me are not enlightened (along with Tennis Warehouse who string all their customers racquets with a NEOS 1500).
     
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  27. Zielmann

    Zielmann Semi-Pro

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    Electric constant-pull makes it easier to string faster. When you're stringing in volume, or when the fastest turnaround possible is required, this is a must. Drop-weights are able to be just as accurate and consistent, but they're a little more fiddly to get there.
     
    #77
  28. eelhc

    eelhc Hall of Fame

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    Given everything else is equal... I've not seen a dropweight with a comparable turntable, mounting mechanism and clamps to electronic in a high volume shop.
     
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  29. Zielmann

    Zielmann Semi-Pro

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    All those features add to making stringing more efficient. IMO, when looking at electric versus drop-weight, the tensioning with the drop-weight itself is the main bottleneck in speed.

    I see drop-weight machines as geared toward the self-stringer, who wants to save cost in the long-term. Perhaps they'll string for friends/family every once in a while, but aren't making a business out of it. They don't mind spending 45-60 minutes on a frame because they're not stringing all that often. High-end machines are geared more toward professional and high-volume stringers, who want/need every advantage in speed they can get.
     
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  30. ricardo

    ricardo Hall of Fame

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    I fully agree and approve this statement.

    High-end machines are designed for professional stringers who string everyday and whose primary concern is volume rather than quality.
    Drop-weights are for those who primarily string for themselves once in a while.
    Drop-weights cannot match the volume of high-end machines but they can match or even surpass the quality of stringing.
     
    #80
  31. MathieuR

    MathieuR Professional

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    Fully agree.
    And a dropweight easily surpasses the quality of a stringjob done on a LO-machine.
     
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  32. Irvin

    Irvin G.O.A.T.

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    That's like saying driving around Talladega Superspeedway in a VW may be a little slower but it is just as much fun (or scary) as driving in a NASCAR car going 200 mph. I could not agree less.
     
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  33. Zielmann

    Zielmann Semi-Pro

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    Driving a VW around an oval might actually be more scary, if it can't maintain high enough speed to hold a line on the banked corners :p

    But I agree with you. I wouldn't agree that high-end machines are for quantity over quality of string job. Of course it's dependent on the stringer, but the high-end machines are also going to produce a high-quality string job. And they're probably capable of a higher quality string job than a dropweight, too. But generally, the people using dropweights aren't quite as concerned about that, or don't notice it. As long as they're consistent in their own stringing, they can make an adjustment to get it strung how they want.
     
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  34. Irvin

    Irvin G.O.A.T.

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    The bottom of the track is pretty much flat, and you don't have to go 100+ mph to stay on the banked section.
     
    #84
  35. eelhc

    eelhc Hall of Fame

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    To me it seems that dropweight owners have some unfounded inferiority complex. This needn't be... they should just be happy with the machine they can afford/have and try to the best job with the tools available. The argument over accuracy because gravity, Newton really gets silly. To me... I settled on a high end lockout crank (NEOS 1500) after owning just about every type of a machine save for a high end electronic. Why? because it gives the best and most consistent results. The whole CP argument too is silly when the same DT/Stringbed is what one is after when the dust settles.

    Professional stringer's primary concern is quantity over quality? Complete nonsense. They need both. Otherwise they won't be professional stringers for long.
     
    #85
  36. Irvin

    Irvin G.O.A.T.

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    As long as a pro has enough rackets to complete the match I doubt they're too concerned about quantity. But the quality better always be there. One would think if the quality of Prince, Tecnifibre, Babolat , and Wilson high end eCPs were in doubt at l sat one major tournament would bring in a DW machine.
     
    #86
  37. MathieuR

    MathieuR Professional

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    If, for some reason, an eCP could not be used, and a stringer needs to chose between a LO or a dropweight.....
     
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  38. Irvin

    Irvin G.O.A.T.

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    #88
  39. eelhc

    eelhc Hall of Fame

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    #89
  40. MathieuR

    MathieuR Professional

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    As we are going in circles I give "my" resume:
    - an experienced stringer can get a good result on any machine, LO, dropweight, eCP (preconditions: clamps do not slip, do not damage the strings, and have limited drawback; stringer adjusts the reference-tension on feedback of tennis-player).
    - a dropweight is a constant-pull-machine (Stringway is an "automatic" cp-dropweight)
    - a lock-out is NOT a constant pull machine
    - a lock-out will give with different strings, with the same reference-tension, bigger differences in resulting SBS as a drop-weight, caused by the difference in the creep of the strings.
    - the "extra-time" needed when using a drop-weight, is the time "skipped" by the lock-out. This "skipped" time (NOT saved time) is causing tension-loss while stringing.
    - When using "a" refence-tension, the result of a drop-weight will be closer to the result of an eCP as the result of a lock-out
    - On a lock-out the stringer needs to adjust the reference-tension differently depending on the type of string.( This would mean that if you use such this type of kitchen-balance for a recipe, you need to add to the scale 10% for sugar, 20% for butter, 30% for flour to get the "correct" result. Wouldn't it be easier if the scale really shows the weight? )
     
    Last edited: Nov 24, 2015
    #90
  41. ricardo

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    I fully agree.

    On a drop-weight, I can physically see when I get to the reference tension because there is an immediate physical feedback.
    When the bar is at horizontal for at least 10-15 seconds, then I know that most of the creep has been taken out.
    If the bar drops then I know that I have to adjust and re-tension until I get a steady horizontal bar, which take time.

    On a lock-out, I will be like stringing blindly.
    The lock-out will lock as soon as it reaches the reference tension.
    There is no physical feedback that the string is still stretching and losing tension.
    But I know that some strings, especially multis like Monogut ZX, are very stretchy.
    So if I string Monogut ZX or some other stretchy strings, I know that as soon as the LO locks, there will be an immediate tension loss
    but I don't see it because there is no physical feedback.

    I think TW uses lockouts because they have a lot of rackets to string.
    I agree that LO are superior for quantity stringing.
     
    #91
  42. eelhc

    eelhc Hall of Fame

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    "I have to adjust and re-tension until I get a steady horizontal bar, which take time."

    This is the problem with a dropweight. Sometimes you have to adjust and re-tension. The string has not been worked the same way. When pulling tension, Spot on horizontal on the 1st drop IS NOT the same as 2 or 3 drops to get the bar horizontal. The section of string being tensioned has been worked differently.

    On a lockout, once the reference tension is reached, the tensioner locks, there is no going back/forth. On a high end electronic and a Stringway, the string is continuously pulled to reference. All of these tensions are superior to a dropweight with/without a clutch. Tennis Warehouse is doing fine job for their clients . Both quality and quantity.

    The tension loss is not germane.... I assure you that as soon as you clamp off Monogut zx on a dropweight it is losing tension and will continue to lose tension. The clamp is "locking out" the tension inside the frame. As soon as you clamp, the section of string between the clamp and the dropweight tensioner might still be "pulling" but the section of string between the clamp and the opposite grommet is "locked out" and losing tension.

    The set tension is just higher on a lockout to achieve the same DT. That's all.
     
    #92
  43. MathieuR

    MathieuR Professional

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    Please explane "how different" this works out. The string is never overstretched in this proces. ANd when you have a ratchet, the tension is maintained, a bit like the corrections of a eCP

    ?? Please elaborate. Just stating is no "proof"

    Yeah..... But you only clamp the string AFTER 99% of the creep is gone. And yes, there will still be tensionloss, same as on a high end eCP

    That's all. LOL! "Just higher", but the problem is, that "just higher" is different for EVERY type of string, for EVERY different reference-tension used. Of course you can get used to this, especially if you only string using one specific string, one specific reference tension.

    And I did not see you comment my "statement", that the stringing-result of a drop-weight ( given "a" specific string, and "a" refence-tension) is much closer to the result of a eCP as the result of a LO. ( and of course, on that LO you can make the reference-tension "just higher"; but how many tries you need to find the "just higher"??)
     
    #93
  44. ricardo

    ricardo Hall of Fame

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    This is very true.
    Using the same tension increment to offset tension loss during stringing is like 'stringing blindly'.
    In effect, you will be over-tensioning pre-stretched strings or strings that do not stretch that much
    and under-tensioning strings that stretches a lot.

    To get as close to reference tension, you have to be very familiar with the string you are using
    to determine the right tension increment.

    There are, I think, almost a 1,000 different strings out there.
    Personally, since I only string my racket, I am only familiar with at the most 20 different strings.

    With a drop-weight there is no guesswork and I don't have to be familiar with 1,000 different strings.
     
    #94
  45. eelhc

    eelhc Hall of Fame

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    Think of it as a physics/material science problem... The string is either a natural or polymer material. Pulling + stretching uses less of that material to fill the space between the grommets. So in the end, if the same amount of material fills that space as long as it was worked (stretched/relaxed) in the same manner.
     
    #95
  46. MathieuR

    MathieuR Professional

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    Could you please elaborate, I do not understand what you are trying to say.
     
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  47. eelhc

    eelhc Hall of Fame

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    What's your technical/scientific background? Any physics/chemistry or material science courses? Think this through scientifically...

    The background: A material's properties can change depending how it is formed and worked. One can stamp a piece of sheetmetal steel to shape, or hammer it repeatedly to get the same shape. The properties of the resulting material is not the same. Many other materials including including plastics and polymers behave in a similar manner. There are many, many other ways to change the properties of materials but for our discussion, the forming of the material is sufficient.

    Applying this to tennis strings/stringing. A tennis string out of the package has a given diameter and length. As one pulls tension on the tennis string, the length of string will increase, and the the diameter of the string will decrease. The stretch is not consistent as there is friction in the grommets. On most dropweights (including Stringway machines without the concorde system) there is additional friction on the center mains since the string has to be pulled over the throat but for the same of simplicity, leave this variable out of this discussion. In the end, if the section of string in the frame, is the same amount of material stretched over the same distance (grommet to grommet) in a consistent manner (smooth single pull as in a crank, single drop on a manual dropweight, automatic dropweight or a high end electronic), it doesn't matter what was used to pull the string , the material (string) between the grommets retain the same properties in each case. But depending on the string, all of this is somewhat moot since even after completing the string job and tying off, the string will continue to lose tension. For example, I play a full bed of poly or hybrid it with multi crosses, taking it off the stringer and playing right away with it is very different than letting it sit for a couple of days before playing (this is true for any type of stringing machine).

    I find your issue with lockouts unsubstantiated because there's no scientific basis for it. Again, same amount of material that is worked (formed or stretched) in the same manner that fills the same space... will have no difference.

    Now let's take a look at a manual dropweight more carefully..In particular, the ratchet. With skill and practice, one can get the bar to level on the first drop consistently. But this takes a lot of experience, feel and knowledge of string types. Most people will take 2~4 drops, sometimes even more to get the bar level. The material in this case is not formed or stretched in the same way as a lockout, automatic dropweight or a high end electronic. On each drop prior to the bar going level, it is pulled to a lower tension beforehand. It's hammering away at that sheetmetal vs just stamping it. The properties of the material will be different.

    You have less than 500 posts here, not sure what your stringing background is but there are people here with years of experience and hundreds, if not thousands of racquets who have crank lockout machines. Saying things like lockouts should be banned demonstrates your ignorance and will get you ignored fast.

    Edit... corrected grammar and clarified some segments...
     
    Last edited: Dec 20, 2015
    #97
  48. jim e

    jim e Hall of Fame

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    .........
     
    Last edited: Dec 20, 2015
    #98
  49. Karma Tennis

    Karma Tennis Semi-Pro

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    "

    What does this statement actually mean?

    "Quality" is a subjective term in this context rendering it somewhat meaningless to me.

    An example ... I've had an experienced stringer string racquets for me on a very expensive high end Wilson stringing machine for several years. Recently, I acquired a very old LO machine and started stringing racquets for myself just for a laugh. In reality, I didn't expect to get much out of this venture because I didn't think I would be able to do it to an acceptable level.

    Well, Thanks to a lot of Irvin's videos, I'm up to my 10th racquet, and to be frank, from the 5th racquet on I couldn't tell the difference in playability between my old expert stringer's jobs and my own. RacquetTune shows them coming off both machines with similar tensions - I know, I know - more importantly the tension loss over time seems to be almost identical - I get the same number of playing hours either way. Applies both to full beds of Poly and Poly / SynGut hybrids.

    My expert stringer can string me a racquet in about 25 mins but he charges me $$$ to do it. It takes me about 70 mins ... my goal is to get to 55 mins

    The LO serves my purpose beautifully. So from my point of view, your assertion that dw machines easily surpass the quality of LO machines is totally false. I can't see how a dw machine would make my string jobs "better" because the LO does them fine. I'm actually glad that my first foray into stringing is with a LO machine.

    Of course, I qualify everything I say here by making the point that I only string racquets for personal use. Perhaps the "Did it Yourself" feeling is clouding my judgement ... but really, I don't think so.

    For me the point of the exercise is to string racquets to a standard I find acceptable. The LO does that for me.
     
    #99
  50. MathieuR

    MathieuR Professional

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    the Netherlands
    I studied physical chemistry, and have a master degree from a Dutch Technical University. So I do think I can see things from a "scientifically" perspective ( which doesn't guaranty I am right, but I can read and understand a technical explanation)


    Agree. Fact is, that if using the same "reference tension", there is difference in "material that fills the same space" when you compare a lock-out and a drop-weight

    Love to see a reaction on this one...


    Of course my "statement" is meant to be provocative. On a lock-out you will be able to get excellent results, as proven by many stringers. I myself am stringing now for four years. Started on a battered Klipper with slipping flying clamps, then a Tyger-dropweight (fixed 2step clamps), then after 6month an almost 30 years old Stringway automatic dropweight. I now have strung in total something like 600 frames, with "all" types of string. I now have 2 Stringways fully functional (one with glidebars, that is the "old one", and one with 2way swivel-clamps. I like the glidebars best. ( and I have 4 Stringway-machines that I need to revamp).
    I am here to learn, and I sometimes I want to be a little provocative to stimulate reactions, but I never intended to insult or ridicule people. If I did unintentionnally I'm sorry!!

    But imagine this situation:
    we are both on an ATP-tournament, you with your lock-out, me with my Stringway (imagine ;) ). And "a" player asks us to string his racquet, and he brings a string that we both do not know. And he asks: string my racquet at 25kg, mains and crosses. Which stringing machine will give the result that matches the expactations from that player?
     

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