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What is the purpose of the popsicle brace
http://www.luthiersforum.com/forum/viewtopic.php?f=10102&t=15147		 Page 1 of 1
Author:  burbank [ Thu Dec 27, 2007 6:21 am ]
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The forward -X thread had some points that I thought warranted discussion, particularly grumpy's reference to the ineffectiveness of the popsicle brace in preventing top cracks (which I agree with). I'm open, shoot me down if you've a mind to!

We've long heard from many sources that the function of the popsicle brace (PB) is to prevent the top from cracking alongside the fretboard extension from shrinkage and how ineffective it is in that role. But for awhile now I've wondered if perhaps the popsicle brace was meant to counteract the sheer force of the neck block being pushed (or pulled, depending on your perspective) toward the bridge rather than to prevent top cracks at the fretboard. Seems unlikely to me that Martin would have thought the PB could possibly prevent top cracks. If the top is going to shrink, it's just going to shrink and crack whether there's a PB or not; the number top cracks we see there would seem to support this. Maybe the thinking was that with the huge difference between spruce and ebony in how they react to moisture, if the top did crack from shrinkage, the PB would provide sheer strength lost from the top cracks.

Martin's use of A-frame bracing that replaced the PB on some models would not have any effect on top cracks from shrinkage, as far as I can tell. It covers almost no area beside the FB extension where tops often crack. Seems like Martin would have the A-frame brace along with the PB if they thought the PB kept top cracks at bay.

I'm using Martin here only as an example, BTW.

So, what do you think? Is the PB just meant to prevent cracks along the FB extension or to provide sheer strength? Or both?



Author:  Michael Dale Payne [ Thu Dec 27, 2007 6:32 am ]
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Humm???? My opinion is the upper transverse brace is the main buttress against the FB extension causing inverse belling. I understand the upper Transverse graft as a anti-splitting mechanism (in my opinion popsicle stick is a misleading and bad name for this brace because it should be much thicker than a popsicle stick)
Author:  Alan Carruth [ Thu Dec 27, 2007 6:59 am ]
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I've always thought that those cracks were more from fingerboard shrinkage than top shrinkage. Ebony is so dense that it seems to never stop shrinking; popped fret ends and concave fingerboards on classicals are other symptoms of this. If the 'popsicle stick' is supposed to prevent those cracks, it's not very successful. For that I use a 'chin' that extends out beyond the fretboard on either side, and runs almost up to the shoulder brace. So far....

I've always thought of the popsicle stick as being there to prevent the neck shifting inward once the top does crack. It has plenty of gluing area which is loaded in shear for that sort of deformation, which should help. Sadly, if you're using a glue that 'cold creeps' it's not going to do the job. I've had more than one repair come in with the neck shifted inward by as much as 1/16" and the glue line still 'intact'. The glue joint has not broken, it has simply allowed the parts to slide past each other. One particularly bad one was a '70s Martin. The inside of the top looked like it had been smoothed off with a garden rake, and that one had shifted by at least 1/8".

The 'popsicle stick' might work if it is glued down well with HHG. Rather than that I've gone to using a small 'A' brace, with the legs of the A starting at the upper arms of the X, running up the sides of the soundhole, and through the shoulder brace, to inlet into the neck block. I've had two guitars with that construction take serious knocks to the back of the neck, which I'm sure would have shifted the neck otherwise, and they showed no damage except for the big dings in the necks.   
Author:  j.Brown [ Thu Dec 27, 2007 7:02 am ]
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Alan, do you have any photos of your configuration?
Author:  burbank [ Thu Dec 27, 2007 8:10 am ]
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Michael,

I agree that the upper transverse brace would prevent the top from sinking under the fretboard. Owing to its bulk in both height and width, it would resist both "reverse bellying" and sheer forces that want to move the neck block closer to the bridge.


Alan,

The mishaps those guitars experienced indicate that the A brace must be quite effective. Is there a reason why the "chin" doesn't extend to and come into contact with the shoulder brace?
Author:  David Collins [ Thu Dec 27, 2007 9:56 am ]
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I agree "popsicle stick" is a poor and misleading description, so I'll
assume authority to make a unilateral decision and re-title it the "flat
shoulder brace". Or that's what I like to call it anyway.

I've thought of it's intended function pretty much as Alan said, to support
the top from shifting inward by increasing the sheer strength along the
top grain. I also agree that it often fails, probably more so with PVA glues.
Removing it entirely without replacing it with any other reinforcement in
the shoulders is rather foolish in my opinion though.

What I've found interesting is in Gibson's shoulder bracing in recent years.
Their flat shoulder braces appear around 3/16" directly under the
fingerboard, then are scalloped down to a very thin 1/16"ish immediately
at the edges of the board. Then they only extend from the board edges to
about half way to the lining. It makes no sense to me whatsoever, and
they seem terribly prone to top bulging at the shoulders. It seems like
ever other Gibson I see made in the last 10 years needs a neck reset or at
least the fret hump dressed out at the extension. I had one customer that
returned 3 of the exact same model and had them replaced under
warranty for this same problem over the course of a single year. The
fourth he immediately sold when he received it.

Now I certainly wouldn't blame only the ultra thin shoulder brace for this,
as I doubt the longer and only slightly thicker Martin version would save
these tops entirely. Still, it's obvious that their shoulders need some
better reinforcement.

I guess all I can say is that I have no intention of building any guitars
with the conventional flat shoulder brace. That area needs more support
than it offers, and there are plenty of variations on blocks or A-bracing
that are better in my opinion.
Author:  Bruce Dickey [ Thu Dec 27, 2007 1:40 pm ]
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This is one reason I adjust the truss rod at the headstock. It allows me to put in two transverse braces when Martin used tubular or Tee style rigid non-adjustable truss rods.

So it's a Gibsonesque feature, but with a Martin double Transverse Brace. Even use it on shortened cutaway tops, it's there but short.
Author:  bob J [ Fri Dec 28, 2007 12:27 am ]
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It's to insure that absolutely NO SOUND emanates from the upper bout
Author:  Alan Carruth [ Fri Dec 28, 2007 9:12 am ]
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There's a pic on another thread that shows the A-brace pretty much as I do it.

I used to try to butt the 'chin' into the shoulder brace, but the problem is that it's hard to get it into really firm contact. Even a little slack will allow it to move enough to start the neck shifting, and once it starts it's gone. That happened on one of my gfavorite guitars, a 12-string that got knocked off a table. The neck only shifted inward about 1/32" or less, and the action hardly changed, but the writing was on the wall. I can inlet the small A braces and get them to lock.
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