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I'm about to brace the back of my new project, and I got to (danger ahead) wondering about deviating from the norm of four braces at right angles to the centerline to something else.

I originally thought I'd use five thinner braces instead of four. Not much of a deviation, really.

I then did a quick search through Google Images of back bracing and ran into Trevor Gore's design for his 'active' back, which uses three transverse braces instead of four and then uses, for want of a better term, four finger braces 90 degrees apart starting at 45 degrees from the center line radiating from the farthest-south lateral brace which is located in the center of the lower bout.

Another design which teased me was to use an x brace on the back with one transverse brace (from P. H. Carey, calls it his 'spider' bracing). Not much material there and it preserves the dome nicely.

Comments, suggestions, advice, anybody?

Thanks!

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Peter Havriluk

Are you after a live or reflective back?

The overall flexibility is more important than the bracing pattern, but if you’re tuning your back braces, the Gore ‘asterisk’ pattern tends to yield a clearer monopole pattern. I tune my back plates but just use 4 1/4” braces...

I use Trevor's design.



It is relatively easy to work through the sound hold to tune the lower brace. So it is easier to tune the back relative to a 4 brace. It looks cool through the sound hole like I know what I am doing.

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http://www.Harvestmoonguitars.com

Ed, damfino what I want. I have no way, or the judgement, to measure much of anything. For my past projects I just used the four braces we're accustomed to seeing. I was hoping to build a back that would hold more of the domed shape, hence my interest in something that looked like it would be an improvement over the 4-brace pattern. Gore states his customers prefer his bracing acoustically over the 4-brace model, but I have no idea if mimicking that without any testing would work and at least do no harm.

And John, how on earth high are those braces?

Thanks, folks.

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Peter Havriluk

Keep in mind that Trevor’s design uses a 10’ radius and very stiff braces (10mm wide x 20mm tall), which results in a high frequency response that can then be intentionally tuned down by carving the middle of the lowest cross brace.

If you’re using a typical 15’ back radius, he recommends even stiffer braces to help effectuate the higher frequency back.

If you’re not following his system to tune the back, it might be best to use the typical ladder pattern this time.


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It is worth thinking about Ed's question of whether you want a live back (light, responsive) or reflective back (heavier, a foundation for the top responses). It is not a question of good or bad as they each have their attributes. You have probably already read the brief discussion by Trevor Gore here:
https://goreguitars.com.au/main/page_ab ... acing.html

I am guesing that you probably want a reflective back, which is the more common thing for a steel string. There are lots of ways to achieve this. It is certainly less critical that the bracing of the top, as it is not load bearing. you can do all sorts of things that will work well - and some can look different from your average factory made guitar.
Here is one of mine with an x-brace.


And another. OK, I know it is a bouzouki rather than a guitar, and a three piece back, and nothing like what you are building. But my point is you can be a bit inventive here and it will work out OK.

Now if you really want an arch in your back check out the old Vega or Howe-Orme cylinder guitars - or the modern homage to this style made by Shelley Park or Nigel Forster.

The number of back braces can vary a lot and still work. I believe Torres in some cases used as few as two and some small bodied Martins have as many as five. Some have no bracing but rely on the arch of the back to hold it's shape instead.
One thing I try to keep in mind is the movement of the back with humidity changes. Across the widest part of the guitar back I use wide "flat" braces that don't resist the "flattening" (shrinking) of the back as much as a tall stiff brace will. For a guitar that lives in a perfectly humidity controlled environment it may not matter, but my guitars live in whatever the ambient conditions happen to be. I would rather the back lose some arch rather than crack. Conversely in humid conditions it can swell and increase the arch. As with most of the people here I try to do my critical glueups between 40 to 50% R.H.

I can muse on this forever...talk about overthinking!

It's time to make a choice and commit the bracing to wood. And Mark McLean's picture appeals to me enough to copy the idea. The 'x' brace takes me in a direction that preserves the dome, and the combination of 'x' and lateral bracing in the lower bout seems to offer me the support I wanted to help avoid 'ashcanning' during finish. Gonna find out.

Thanks, everybody for all the comments, analysis, and advice. I guarantee that I'll appreciate anything anybody wants to say.

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Peter Havriluk

One advantage to X bracing is it runs at an oblique angle to the grain of the back, so less stress should be created with changes of humidity compared to conventional lateral bracing.

Peter, I built that following the plans available here in the OLF marketplace (the OLF SJ plan) - good value if you want a plan to follow. I made this one long before reading the Gore/ Gilet books about live back design, and certainly made no attempt to “tune” the back. It would fall into the category of a non-live (reflective) back, like a Martin-style ladder braced back. The two things that you need to achieve are:
1. It should be dimensionally stable
2. The main note on tapping the back should be noticably higher than the main resonant note of the top.
Exactly how you get to those goals can be your own adventure.
Oh yeah - and what Clay said about the RH when bracing and closing the box is critical. You probably know this already - but we should repeat it again and again for any novices reading this forum. It is very disheartening to have your panels turn inside out like a potato chip, but that is exactly what will happen if you glue up your braces in conditions of high humidity and then the panel dries out later.

I'm afraid I haven't been following anybody's plans but my own, which I derived by TLAR 'analysis' of StewMac's Martin Herringbone plan (I shrunk mine to 93% of the original and took a SWAG at the bracing. Seems to have worked well enough). Four guitars, four different thicknesses, three scale lengths, 12-fret (1), 13-fret (1) and 14-fret (two different scale lengths). All built in the same mold. The soundholes have moved around as has the bracing.

I have the luxury of all my building materials stored in my basement whose dehumidifier has been holding to 30-and-a-bit percent for the last two years. And the four projects that came out of that shop have behaved themselves nicely.

But your advice can't be overemphasized. Stable environments with wood that's lived in that environment for an extended time makes for few surprises.

It's been a fun ride.

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Peter Havriluk

My approach is just like yours Peter. I apply the TLAR principle all the time, and never build the same thing twice. It would be hopeless if I was trying to make a living from lutherie - but it makes it fun as a hobby.

Same hymnbook. A lovely sound...

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Peter Havriluk

I have tried this bracing system a few times and it does work well for maintaining the back radius. Jeff Traugott was my inspiration.

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Craig. I have never seen this before. Looks very interesting - can you explain what is going on here?

Looks to me as if the longitudinal brace over the center strip is intended to lock in the dome. I think when left to themselves the four lateral braces describe a cylinder.


All a guess on my part. I'd love to hear from CraigSZ.

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Peter Havriluk

In my internet searching I came across this design on Jeff Traugott guitars. It made sense to me in that the center of the back was supported in the longitudinal direction by the floating brace. I think it helps to stabilize the back radius and also the neck/body joint to some degree. It takes a little more time to execute but worth the effort. The only drawback for me has been the difficulty in tuning the back frequency of the completed instrument.

Craig.

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Just to further confuse the issue, here's another one, CF strips hold the back braces in a curve. aim is to maximise stiffness and reduce "vibrating" weight. Back is cylindrical rather than domed.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I've used a lot of different back brace schemes over the years. At the moment, I'm using a ladder brace setup much like Martin's: two tall and narrow upper braces and two low and wide lower ones. I'm finding this to be relatively easy to get 'tuned' so that it works the way I want. Every scheme sounds a little different, but you have to keep in mind that most of the sound is in the top. The back has some effect, but not all that much unless you really go wild. Most of the time that just results in a poor guitar.

The closest thing I know to a purely reflective back is the Ovation. That back simply doesn't move at any useful frequency, and so doesn't color the sound one way or the other. A back that is 'active', with a 'main back' resonance that can couple with the 'main top' mode, can increase the sound output in the lowest range. Other than that any resonance of the back tends to steal energy from the top and reduce the output of the guitar at some frequency. These 'dips' in the spectrum do contribute to 'color', so you don't really want to eliminate them totally: the lack of those is what limits the 'color' in the tone of Ovations.

In short, don't over think this. Whatever you do with the back is unlikely to either make or break the sound of the guitar by itself. Concentrate on getting the top to work well, and then see if you can get the back to compliment that in whatever way seems sensible to you.