Why do we brace 'into' the sides of an acoustic guitar?

[Alan Carruth]

My feeling is that any brace that you feel needs to be left taller than about 1/2 the thickness of the top at the end (and preferably lower), and that can't be thinned out gradually enough to flex along with the top, needs to be inlet into the liners, or bracketed, as a precaution against impact damage. This is especially necessary above the bridge, but not negligible below it. I may be being really cautious here, but I really hate to have to repair one of my guitars. I don't think it makes a huge impact on the tone to do so, and the best sounding instrument in the world isn't much good if it folds up.

[ikerstges]

Thanks for the extended and lively answers, I gained some more knowledge again..

If I may invite for some experimental thought: when we would leave the impact-resistance out of the equasion and build a guitar where any structural components got only mounted to the top- and bottom (not even into the kerfing), feathered to 0. I assume that the guitar will still have enough strength to deal with the forces of the strings and during normal play (After all, Torres even managed to build a papier mache guitar!).

Would that improve the sound? Sustain? Projection? Other?

The guitar as airpump, I wonder how the sound would change if the top and bottom would be more freely to move if more loosely coupled to the sides..

[phavriluk]

A difficulty in figuring out whether setting the braces into the kerfing influences the acoustics is measuring it. What is the baseline? Can't do it on the same instrument, and each instrument is unique. Opinions aren't facts, and facts ain't to be had.

[Alan Carruth]

ikerstges wrote:
"The guitar as airpump, I wonder how the sound would change if the top and bottom would be more freely to move if more loosely coupled to the sides.."

Evan Davis wrote his PhD thesis on guitar acoustics, and had a carreer with Boeing in noise control. A few years ago he published a paper showing that the way to make a guitar top (or back) pump more air is not to reduce the heights of the brace ends, but rather to scallop them in the center. Martin, of course, figured that one out a long time ago: Evan just did the math. Thinning the edges of the top and reducing the brace stiffness at the ends does affect the tone, but may not pump more air. At any rate, the air pump function seems mostly to be important in the low frequency 'bass reflex' range. In the high range there's no net volume flow, and the guitar act as a 'multipole source'. The transition seems to happen in the range between about 300-500 Hz, depending on the guitar and the weather and the phase of the moon and the wholesale price of coffee: all the usual variables....

[ikerstges]

thanks! I will look into that!

[LouieAtienza]

Originally Posted by murrmac123

I would assume that it's down to the 180 lbs tension, and the constant vibration of the top which somehow degrades the composition of the glue in the long term, rendering it susceptible to any impact causing the joint to fail along it's length.

Still can't help feeling that it shouldn't happen, since a well glued joint is, or should be, stronger than the wood itself, but there you go ...

Well, there are many reasons why a glue joint fails... improper gluing technique is one, improper glue is another, improper storage conditions yet another. There are also many examples of guitars surviving a century-and-a-half with nary a repair done, which would prove the contrary. I feel the flexibility of top woods and bracing serve to actually prevent these joints from failing.

No one yet has mentioned that gluing braces onto plates violates a cardinal rule of woodworking. Don't try to glue cross grained pieces together, but design a joint that allows for movement. Think breadboard ends on a table where you might glue the center of the joint, but rely on pins in oval holes at the outside edges for movement.

I've always thought about how guitar building seemingly violates many general woodworking practices, but so does humidor making. I don't think it's as much a tragedy when gluing softwoods to softwoods, or softwoods to hardwoods, because the softwoods have more "give" so to speak. think of the many ladder-braced guitars (and lutes for that matter) that exist unscathed. The back is basically mashed in to a radiused dish, and sometimes the top - if it's not mashed to the contours of a solara. There is a certain amount of risk/reward in building a delicate and responsive guitar versus an overly built and unresponsive one. The manufacturer tends to err on the later, while the individual maker tends to go the other way.

[Howard Klepper]

Originally Posted by ikerstges

Hiall!


I understand that these braces have not only just a structural function, but should also support sound transmission in the back and top plates of the guitar. It seems to me that much of the vibration of these braces gets absorbed by their connection to the sides (and dissipates in heat).

Vibration does not flow like a liquid through the braces. That is a common misconception. Letting braces into the linings adds strength, and a brace that is not cut down to the top before it reaches the sides is a stiffer brace and will tend to tune the top to a higher pitch. But loss of energy by transmission from a brace to the sides is a non-issue.

[Alan Carruth]

Thanks for catching that one Howard.

[Slight Return]

Originally Posted by Alan Carruth

The brace ends have to be supported for structural reasons. Most glues, and certainly the traditional hide glue, have poor peeling resistance. Suppose the top is glued down to the rim at the edge, and the brace is simply cut off at the end with no support. Any down load on the top will cause it to flex at the edge, but the brace, due to it's grain orientation and height, is stiffer. It won't move with the top, and will tend to peel loose. Once it starts it keeps going.

Back when I did repairs I ran into couple of Gibsons that suffered from this. Gibson used to use the nose of a belt sander to feather the brace ends down to next to nothing. When they put the back and top on they didn't bother to cut inlets for the brace ends, but relied on them to mash the liners down a bit when the plates were clamped on. The problem is that they used to use spruce for both the liners and braces, and sometimes the liners was the harder. The brace end would be what got crushed. This amounted to breaking it off at the edge of the liner.

At least twice that I can recall somebody came in with high action because the neck was tipping up. When I looked inside there was no upper transverse brace. I asked the customer if there had been a loose stick inside the guitar, and they confirmed that there had been, and they threw it away, not knowing where it came from. The repair involves notching out the liner to fit a UTB of the proper length, with some height at the ends. The brace is glued in, and brackets glued to the sides to support the ends.

In normal use the only braces that feel a downward force at the top braces in front of the bridge. Some makers will reduce the brace ends behind the bridge to zero, and not inlet them. Even the ones behind the bridge can get a down load from time to time, most notably from impacts, and this can peel the brace up unless it's very flexible for some distance in from the edge. I tend to inlet any brace that can't be made that flexible.

Yes, this does affect the sound. I figure a guitar that's falling apart won't usually sound too good, so I try to get the structure right and then get as much sound as I can.

Mega bump here.

Unfortunately, I just re-glued an upper transverse brace on an old Epiphone with this exact issue. Even with just the low E string partially tuned up, it was pulling the neck so far forward, the action was 1/4" at the 12th fret. With just that one string on.

The brace had fallen out, so I re-glued it. It's very solid...however, I just noticed the ends of the brace do not contact the kerfing, let alone go into any inlets (there are no inlets on the kerfing on this guitar).

Anyone have any idea what my options are, having the brace already glued solidly in place? It'll be a struggle, but I imagine I should be able to glue something in there as a 'filler' piece to connect the end of the brace to the kerfing, or somehow inlet the kerfing and do the same.

I've never heard of 'brackets' in an acoustic guitar. Curious how those would work or how I could make them.

I don't want to string it up until I'm sure this is secure, and with the upper transverse brace not contacting even the kerfing, I don't feel safe putting strings on it.

It's a bolt-on neck by the way, four bolt, like Strat style. The neck pocket was severely damaged with huge cracks, which I repaired. But I think this upper transverse brace is the culprit. Doing a quick Google search and finding this thread, I was like, yep..

Put a straightedge on the fingerboard, and you could see the neck angle dramatically shifting with increased string tension. It was so much that it actually crushed the entire area around the soundhole; it pressed the fingerboard extension into that area like an arbor press and it just totally crushed and splintered it.

Anyway. Just got this glued up today, and am hoping to figure out a way to reinforce those brace ends before continuing. Any advice would be much appreciated!

[Alan Carruth]

Chisel out pieces of the kerfed liners at the ends of the brace. Clean everything up nice and smooth. Make triangular blocks that will glue into the spaces and will be wide enough on the top surface to extend out to where the brace has some reasonable height. They need to go down on the side far enough to stay put under the down pressure of the brace. These are the 'brackets'. Glue them in. I usually use epoxy for this, since it doesn't shrink as it hardens and will thus fill small gaps. This is a 'fun' repair....

[Slight Return]

Originally Posted by Alan Carruth

Chisel out pieces of the kerfed liners at the ends of the brace. Clean everything up nice and smooth. Make triangular blocks that will glue into the spaces and will be wide enough on the top surface to extend out to where the brace has some reasonable height. They need to go down on the side far enough to stay put under the down pressure of the brace. These are the 'brackets'. Glue them in. I usually use epoxy for this, since it doesn't shrink as it hardens and will thus fill small gaps. This is a 'fun' repair....

Thanks so much. So the "pointy" part of the triangle goes into the cut kerfing, correct?

Epoxy sounds good for this. Do you simply put them in by hand and feel? I'm assuming you feel for the cut out area of the kerfing, and then when you butt the wider end of the bracket against the brace, you should feel some resistance as you force it into position.

I've only done a couple bridge plate removals and replacements, and I thought those were bad. But around the upper bout you have even less room to move your hands or fit anything in there for gluing. Fun times indeed...thanks again for the tip, I'll see what I can do and hopefully this guitar doesn't implode again under string tension. I'm almost tempted to recommend silk and steels. But I'll do my best and see if plain old 80/20's will be the death of this thing or not.

[Alan Carruth]

There is no canonical 'right' way to do this; it's essentially a jury rig in any case. Normally kerfed liners are about 1/4" wide on the top/back and about 1/2" tall along the side. You probably will need to make your bracket a bit wider than that on the top to catch some wood in the brace, and I usually try to keep the proportions similar to standard kerfed liners just so it looks like I meant it. Often the grain on the bracket runs across the wood of the side, so that the brace is resting on the end grain surface of the bracket. You might find rare earth magnets helpful in putting these in.

[yellowesty]

And why do we need strength in the soundboard, assisted by let-in braces? Because traditional guitar design has depended on the soundboard for structure as well as sound. This demand on the soundboard evolved from gut-strung guitars where forces were moderate enough that structural requirements were limited. The evolution to steel-strung instruments was enabled by stronger soundboard structures (at least after the necks started folding into the soundholes) rather than by a fuller redesign.

Several redesigns have been made, including "neck-through" designs and approaches with internal bracing between the headblock and the tailblock, but have not proved popular. (Possible reasons include added instrument weight and visibility of braces through the soundhole.) When coupled with an elevated fretboard, such designs relieve the top of most structural requirements, though bracing which limits rotation of the bridge is still required. That doesn't necessitate tucking the braces into the sides.