Official Luthiers Forum!

From what I understand, the primary purpose of the popsicle brace is to keep the upper bout from cracking in line with the fretboard. If that's the case, why does it extend across almost the whole width of the bout? Tradition?

Thanks in advance

Steve

engineering If you make it only an inch longer past the fretboard you will be ok. I like to make the neck blocks as they were in the pre war time , the neck block was 1 9/16" not the 1 3/8 they use today

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Steve: My feeling is that top cracking at the fingerboard edge is more likely due to differential shrinkage between the board and top. The popcicle to me serves the purpose of helping to prevent the fingerboard from moving back into the soundhole when cracks do happen. My approach is to use a shorter brace but one that goes beyond the diameter of the sound hole. Thus for a 4" hole I use a brace at least 5" long. Some folks don't use any brace here,I've only done this once with disasterous results.No problem if no cracks develop in the soundboard. Just my opinion of course.
Tom

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A person who has never made a mistake has never made anything!!!

The design came about as the upper area of the Martin guitar did change from the 37 era . The neck block was originally 1 9/16 , then that was changed to 1 3/8 . the old traverse brace was 3/8 by 3/4 then Martin had to deal with many top cracks from the fretboard shearing into the sound hole. The traverse brace went to 1/2 in wide by 5/8 and the Popsicle brace went in.
I do many conversions and replications of the pre war design and with Hot Hide glue , tucked bracing I have had good results. The only difference I do is that I do keep the wider traverse brace . Wayne Henderson does the same thing. The shear may start from a differential if the fretboard and t once that happens the top dives into the hole. It really doesn't matter but once it happens there is no doubt the upper part of the top fails and the neck block rotates.
I don't think you would loose any real amount of tonal quality if you put 2 small cleats in , as long as the block is the same size of the pre war block. The glue surface area is important to help control that stress. I have seen some "converted " guitars that have the Popsicle removed. Without any support they can fail in this area pretty quick.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Is there anything wrong with putting a second utb instead of the Popsicle brace?

You need more glueing surface than just a second UTB to sufficiently compensate for the forces that are pulling the neck/fretboard into the soundhole.

I do both. Put a second UTB and a popscicle brace (sort of...) in between the two, although with the grain parallel to the top grain.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

While it sort of works, the popsicle brace and UTB doesn't seem like a very rational bracing system for the upper bout. I don't buy the often heard explanation that expansion/contraction of the fretboard causes the soundboard to crack. It may contribute something, but I think the main issue is that the top of the heel block is being pulled into the soundhole by the string tension, and simultaneously that rocking motion is forcing the upper bout apart. This is pretty easy to see when the top is off while retopping a guitar.

The soundboard is in tension across the grain in the area of the fingerboard extension, and there is very little in the way of bracing to counteract it. With time, of course it will split. IMO there should be some triangulation here, and I've always added it to the beaters I've retopped and fixed up. The new Martin 'A' frame bracing solves this I presume, and so does an upper bout X.

Expansion? No. Contraction? Definitely. Ebony or rosewood are much more unstable than quartered spruce, and the fingerboard is usually thicker than the top. Couple that fact with the usual practice of gluing the two together with a water-based glue, and shrinkage is pretty much a given.
There are two supposed functions of the popsicle, and IMHO, it is not particularly effective in facilitating either one of them:
1) Prevent the top from sinking. In that regard, the UTB, being much taller, is the main deterrent to a sinking top. If you are worried about that aspect, it is much better to make that brace a little taller, or to make it from a stiffer material. You can add another brace if you feel it is necessary, but considering the 'cube rule', it is much more efficient to just use one taller brace.
2) Prevent the cracked top from shifting into the soundhole. This is the main reason given for the width of the popsicle, but shifting can only occur when the glue fails. The fact that the popsicle is cross-grain to the top, and is normally about 1" wide, only encourages glue failure, due to differential expansion.
I have repaired a significant number of 'shifted' guitars, both with and without a popsicle. Some of the 14-fret Martins before mid-1939 (the ones that had no popsicle originally) had been repaired with the addition of a popsicle, only to fail once again. In some cases, the 'popsicle' was a block of wood many times larger than Martin would have used.
What those failures all had in common was cross-grain construction.
My method for repairing these failed guitars is to install a trapezoid patch. It is normally a piece of vertical-grained spruce that is 0.100" to 0.160" thick. It is installed so that the grain is parallel with the grain in the top, and it fits tightly between the neck block and the UTB. This tight fit serves to prevent any movement of the neck block. The trapezoid shape is dictated by matching it to the width of the neck block where it butts up against it, and it widens to about 4 1/2" at the UTB. That serves two purposes. It 'spans' the width of the 4" soundhole, distributing the compression load from the string tension. It also prevents the stress riser that occurs when the edge of the applied piece is parallel with the grain in the top.

The ones I have measured were 5/16 by 5/8....the same as the X-braces.

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John