Author Topic: Catastrophic failure of Double Overhand Stopper  (Read 12769 times)

Lexando

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Catastrophic failure of Double Overhand Stopper
« on: December 30, 2013, 04:25:32 PM »
Inspired by http://emedia.leeward.hawaii.edu/frary/changing_strings.htm I decided to restring an old classical guitar using the knot-with-bead method. I didn't like the look of the knots in the article (and couldn't really figure out how they were done) so I decided to use a proper knot which I thought was suited to the purpose.

If you take a look at the attached photo, you'll see I had satisfactory results with the lower five strings.

Unfortunately, the double overhand stopper fails with a bang just as the upper (1st) E-string reaches it's required tension. I leave a good 3/4" tail and can't see any deformation in the knot before it fails.

Working with soft rope I think I can see a mechanism for how the double overhand stopper might pull through which suggests the same would happen to a Fig 8.

The strings are too stiff for a more complicated knot such as the Ashley.

So, can you help me find a knot that will work and still look nice?

Dan_Lehman

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #1 on: December 31, 2013, 08:32:28 PM »
...  I didn't like the look of the knots in the article
(and couldn't really figure out how they were done)
so I decided to use a proper knot which I thought was suited to the purpose.
It appears to me that you used the same knot, topologically,
but you tightened the 2nd tucking of the overhand (which
is what makes it a "double ...".  To those not seeing this
clearly, it is the form of the anchor bend / fisherman's bend
loaded on the tail; I've seen such stoppers in some
commercial-fishing gear (and also loaded the opposite way).
It is NOT the strangle orientation, which knot might be
more commonly presented in knots books qua stopper, btw.

Actually, looking again at your image, it seems that the
A & D (2nd & 3rd from left, bass side) strings are in the
form I say; but the orientation of the monofilament G (!),
B, & presumably broken high E strings is maybe different?

AND, reflecting on what I see in HIS image,
I'm now finding it hard to believe that those strings
are much tight at all --though they sure are straight--,
as the SPart of the knot shows no draw upon the
tucks, and in fact in the case of the A string (2nd...),
it looks as though the tuck has pulled BACK the SPart!?
(that grayish G string also doesn't look all so tight?!).

Is it possible for you to reeve the E string through
the bead a 2nd time (thinking that we could gain
strength by putting the initial bend of the SPart
around the bead, before force reaches a knot) ?

Quote
Unfortunately, the double overhand stopper fails with a bang
just as the upper (1st) E-string reaches it's required tension.
I leave a good 3/4" tail and can't see any deformation in the knot before it fails.
//
Working with soft rope I think I can see a mechanism
for how the double overhand stopper might pull through
which suggests the same would happen to a Fig 8.
What do you see AFTER the knot breaks?
--i.p., do you see a small, squiggly bit of broken-off
guitar string?  --> which would indicate breakage,
not slippage!
That you observed no deformation suggests to me that
the knotted string simply broke.  Does the bead have a
sharp edge (abnormally?), perhaps?

Hmmm, how did you set it?  E.g., anglers
are usually advised to carefully tighten some of
their knots (which also tie monofilament) by pulling
on the tail, with a moistened knot, along with other
parts --and given the relative weakness of the line,
manual pulling can impart significant tightening.

Quote
The strings are too stiff for a more complicated knot such as the Ashley.
So, can you help me find a knot that will work and still look nice?

Ashley's ("oysterman's") stopper isn't so complicated, really,
and in fact shows a simple method for making a stopper
--make a noose, and have it nip the tail.  But his is a knot
that requires firm setting of the overhand component as that
otherwise won't be drawn tighter by loading, and can
thus allow the tightly bending SPart to pull the tail
through and spill.  (One can tie a variation in which the
SPart turns around the knot's rim and avoids this ... .)

But I remain skeptical that your knot "pulled through"
and suspect that the string broke --as asked above.


--dl*
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Lexando

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #2 on: January 05, 2014, 06:21:50 PM »
Hello Dan, thanks for taking the time & trouble to reply.

I couldn't get any more E-strings until the day before yesterday, hence the delay in responding.

Quote
the orientation... maybe different?

All the knots are formed the same - see attached photo "A"

Quote
Is it possible for you to reeve the E string through the bead a 2nd time

Good idea but no, the hole is not large enough.

Quote
suggests to me that the knotted string simply broke.

Yes, I'm sure you are correct (see below).

Quote
Does the bead have a sharp edge (abnormally?), perhaps?

Not that I can see. Also, the string doesn't really bear on the edges.

Quote
how did you set it?

I pulled it hand-tight to begin with and progressively pulled the tail using pliers. For my third attempt, I added a loop -  see attached photo "B" - and it survived for two days. I had just sat down to write this post as "solved" when "bang!"

This time, at least, I was able to find the fragment and identify approximately where the string broke. But I don't understand why.

Any ideas?

I should add that I started with a large loop so I was able to pull through any damage done by the pliers when I made the loop smaller prior to final tightening.

SS369

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #3 on: January 05, 2014, 07:09:37 PM »
Hello Lexando.

Try dressing the stopper as Dan suggested, as I show in the attached photo.
The force into the nub is a bit straighter and the tension on the nipped area (suspected break area) is considerably more circular.
It is worth a try.

SS

Lexando

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #4 on: January 05, 2014, 07:56:33 PM »
Thanks for the quick reply and thanks for the illustration.

It does seem to make a difference, but the string still broke after about 5 mins.

I've done it again, this time dressed as you & Dan suggest but with the loop added back in. It seems to be holding for now so I'll report back in due course (previous version with the loop lasted 2 days).

SS369

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #5 on: January 05, 2014, 08:26:54 PM »
I think your best recourse since the smaller strings probably will give up again and again is to do the  "tried and true" method as shown here > http://www.guitarbitz.com/how-to-restring-a-classical-guitar-i74

The graphics are excellent, clear and neat, and clickable to enlarge.

SS

Lexando

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #6 on: January 05, 2014, 11:01:42 PM »
Yes, of course, that's the way I & everyone else have always done it. But you can see from my photos the damage this does to the instrument over time.

The bead method has (in principle) three advantages:

1) Does not damage the bridge assembly.

2) Sounds better (more acute angle and hence more stability at bridge).

3) Looks really nice.

I don't want to give up just yet.

Edit: BTW I do already know about pre-made bridge beads e.g. http://www.stringsbymail.com/store/rosette-diamond-guitar-bridge-beads-set-of-7-matte-black-13153.html
« Last Edit: January 05, 2014, 11:07:18 PM by Lexando »

SS369

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #7 on: January 06, 2014, 01:10:39 AM »
I didn't like the look of the knots in the article (and couldn't really figure out how they were done) so I decided to use a proper knot which I thought was suited to the purpose.

Sorry, I thought you did not understand the knot they were using, hence the better link that I posted.

Are the beads chamfered in their hole ends? If not there are beading tools expressly for this purpose at craft supply outlets.

Wetting the knot with saliva improves the tightness prior to final tightening and may inhibit detrimental movement that may lead to breakage.

Ashley's stopper just may be a viable candidate if you can dress and snug it sufficiently. Worth a try if your dexterity is up to it.

SS

Dan_Lehman

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #8 on: January 06, 2014, 07:02:19 AM »
Wetting the knot with saliva improves the tightness prior to final tightening
and may inhibit detrimental movement that may lead to breakage.
DITTO --just what I was thinking.

Quote
Ashley's stopper just may be a viable candidate
f you can dress and snug it sufficiently.
Worth a try if your dexterity is up to it.
I see that I didn't come out strongly enough
against this knot, previously --only remarking
that its formation wasn't so hard but did take
some deliberate setting for best results.

Well, I don't think that it should work so well,
as the S.Part makes a hard / 1-dia u-turn
(around the nipped tail), and is really pulling
hard into the overhand component's center
without beneficial tightening of that against it.
The 1-diameter nipped tail could be a slip-bight
and thus 2 diameters, but ... --I'm skeptical.

Btw, rather than E strings, it would be nice to
have simple monofilament fishline of a similar
size, for practice --should be well cheaper!
(OTOH, maybe there is now one *volunteer*
E string for demolition.)
((And I need beads for my fiddling, or something
with like hole size for my fishline.))

>>> My latest musing : Ashley's #525 (with some
alteration, which I'll need to fetch images for),
or
sort of the reverse to what you first tried,
making a full somersault so to speak, and
then tucking the tail through this coil three
times (maybe just twice?),
with the hope that the *pure* turning
("round turn", though we're not sure what
we mean by that term) around 2-3 diameters
results in ... => strength.


--dl*
====

[Inkanyezi] gone

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #9 on: January 10, 2014, 08:42:03 PM »
The classical method of tying strings comes from older instruments, before bone was used as a ridge for the bridge. In old vihuelas, it was only the string itself that bore the end, as nowadays can be seen on rural instruments as the charango and Mexican guitars and the guitarron. The Vihuela often is stringed in the old way too.
http://ifokus-assets.se/uploads/b0d/b0d04c4e91e3a770ef5f9792b63b7ed4/charanguito-02.jpg
http://uploads.ifokus.se/uploads/14f/14f2bf0ae21073d943a0064b1078f126/guitarron-02.jpg

There are some, very few, charangos with a more "modern" bridge with a bone to support the strings, and those really don't sound as a charango; too clean, and the whole bridge assembly is heavier, which renders a softer sound. The high pitch and particular pattern of overtones is different when the string is stopped differently. Same goes for guitars. Of course it is more a matter of personal taste than science, but I find the bead method horrendous. I'm firmly with SS369 in #5.

I don't share your opinion on sound quality. Beads unnecessarily increase the weight of the bridge and also introduce a risk of buzzing, as well as hampering movement/vibration somewhat. When tying strings the usual way, no extra weight is added, and the angle over the bone is usually sufficient. Some players prefer a notched bone, but it is unusual.

I would recommend not to stick to the idea of beads, although if you like to have them, tie the E-string around the bead in much the same way as the classical knot over the bridge. First pass it through the hole, then back over the rounding of the bead and around itself and timber-hitch it there. Make sure that the bead hole is rounded at its edge. 

Normal string wear over the bridge is usually not ugly, but if you wish to decrease that wear and make it look better, you can change the two strips of wood and glue bamboo there instead, rounding it off to the shape that it would take anyway from the strings. When the edge of that strip is rounded somewhat, it doesn't wear like that. Bamboo also is harder than the wood mostly used. Guitar makers have used ivory for the edge of the part where strings are attached, but it is difficult to find legal ivory (mainly seal ivory). Bone also works, but is harder to work, and my preference is bamboo. It is hard enough. The crucial feature actually is the rounding of the edge. The string holes also preferably should be drilled obliquely so the string will not change direction at the side toward the bridge bone. Also on this side, the edge should be rounded, so that the loop where the string is backed onto itself snuggles up to the bridge, without an air gap. This minimises wear, and it is a feature of the best vihuelas made without the bone, but with a classical bridge with directly attached strings.

In the attached image of the bridge of a rustic rural guitar, you can see how the string snugs up toward the bridge by the rounding of the front edge. This bridge is softwood, cedar, and even though it is very soft, the wear is not excessive.
« Last Edit: January 10, 2014, 11:39:46 PM by Inkanyezi »
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Lexando

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #10 on: January 11, 2014, 06:16:58 PM »
Hi Inkanyezi, thanks for your very informed contribution to this issue. I really appreciate the information you have provided and, essentially, I agree with you.

However, the reason I posted in the IGKT forums and not in a guitar forum - and why didn't mention "guitar" in the subject line - is because I'm genuinely interested in what's going on with the knot.

Rather than getting sidetracked by the specific situation (guitar), can we improve the strength of this stopper (or find a better one)? I have a little more info to contribute...

I think the breakage is due to the homogeneous nature of the material and is caused by bending stress, i.e. when the string is pulled into a tight radius most of the tension is applied to the outer edge causing it to tear (I imagine that a multifilament cord would not fail in the same way). Edit: Re-reading Dan's last post, I think I'm just slow to realize this.

The version of the knot in strangle config & with a loop - as per http://igkt.net/sm/index.php?topic=4726.msg30559#msg30559 above - reduces this effect and has survived so far: No breakage after one week :) but I'm concerned that it's still close to it's threshold.

Furthermore, the string in question, is actually one that already broke - I simply didn't trim the other end so I could move it down - and I have the feeling that the pre-stressed nature of this string is also conrtibuting slightly to the current level of success.

So, before I make another attempt to do a neat job with a fresh new string, I was wondering if anyone had any further suggestions?
« Last Edit: January 11, 2014, 06:33:20 PM by Lexando »

[Inkanyezi] gone

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #11 on: January 11, 2014, 06:51:43 PM »
Everyone might not be aware that polyamide (Nylon) is hydrophilic in nature, and that strings that are not stretched contract longitudinally and swell transversely when soaked. They also become more resilient upon absorbing water. When you play, your fingers add some moisture.

When making the knot, this property can be used to one's advantage, and when attaching nylon line to a fishing hook, it has long been a habit to soak the knot with saliva.

I guess this is a clue regarding the knot for a guitar string too. When you wet it, it becomes more resilient as well as lubricated somewhat, so it will easier take its final form in the knot with less stress, and might become less prone to breakage.

I think that it might not only be the curvature into which you force the string, but also its dry condition when you bend it hard like that. If it has absorbed some water, it might take its final form easier.
« Last Edit: January 11, 2014, 06:58:00 PM by Inkanyezi »
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Lexando

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #12 on: January 11, 2014, 08:23:38 PM »
I'm very impressed by your knowledge. I knew about polyamide degradation by hydrolysis but I was not aware that it could be plasticized by exposure to water at room temperature (I don't fish). I'm guessing it's due to hydrogen bonding which could also account for the deformation behaviour you describe.

This is what I'm going to try next:
  • Soak the end of the string in a bowl of water for 10 mins.
  • Form the knot with loop and work it into the strangle config using only enough tension to achieve this in case I overdid the soaking and made the string too stretchy.
  • Give it an hour or so to lose the excess moisture and then try bringing it up to full tension.

[Inkanyezi] gone

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #13 on: January 11, 2014, 09:08:47 PM »
It will not become too resilient by brief soaking. It takes a long time before water is absorbed, and it takes even longer for it to dry out. But at the very surface, it might be the difference between minute cracks and retaining a smooth surface.
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Lexando

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Re: Catastrophic failure of Double Overhand Stopper
« Reply #14 on: January 14, 2014, 08:58:14 PM »
Edit: Forget what I previously wrote here. The string is definitely breaking at the top of the knot, indicated in my original photo "B" with the white arrow & question mark.
« Last Edit: January 21, 2014, 03:59:19 PM by Lexando »