Replace X- braces without removing top?

[Tom Frei]

My first build was probably too thin on the X braces. Big Question- ugh. Any practical way to :
thicken ( add??) More volume to the X braces or..,
Replace X braces without removing the top? They go under the kerfling.

[Alan Carruth]

I've replaced the UTB on guitars a few times, and it should be the same sort of thing, but harder. Usually these have been old Gibsons, and UTB is just gone, so I don't have to pull them out. I remove a section of liner where the brace is to go, and glue the new one in. Then i make up a 'bracket' to support the brace ends on the sides, and generally epoxy that in. With an X brace you probably would want to fit the lap joint off the guitar, and glue them in one at a time. When they're in you put a patch of some sort over the joint.

The trick is getting the geometry right. I'd try getting a bright light that will shine through the top, and mark all the positions on the outside with masking tape before removing the old bracing. You can get the angles and distances right on the outside, and then transfer everything inside. Maybe it will even fit!

[Tom Frei]

Ok I don't quite get the " bracket" thing.??
I use hide glue, is this easy to scrape off? Probably heat gun it?

[Barry Daniels]

I just recently replaced the X-braces in a Martin after taking the back off. It was difficult enough that I would never try it through the soundhole.

One option would be to laminate some spruce strips on top of the existing braces.

[Alan Carruth]

One of the nice things about hide glue is that it sticks to itself perfectly, unlike Titebond and the ilk. You don't really need to clean it up.

I wonder if a modern variation on the trick that Torres is said to have used would help with the problem of the glue gelling on you. He used to glue the back braces into their notches in the liners and dress them down to the right curve to produce the arc of the back that he wanted. He then glued the back itself on. This presents no real problem in the joint between the back and the sides: you do a little at a time. The problem is getting a good joint between the back and the braces. I've read that he used to spread glue on the braces and allow it to gel before placing the back. Once the plate had been glued around the edges he'd paint a line with a small amount of alcohol on the outside of the back where the braces were, and light it on fire. Apparently he sometimes got a little too much alcohol, and the heat discolored the back along those lines.

I'd paint a glue line on both the brace and the top, if possible. Get the brace in place and wedge it up with posts or jacks so it's snug against the top. Use a heat gun (carefully!) or a hair dryer to warm the top from the outside, and watch with a mirror inside to see when the glue starts to well out of the joint along the edges of the brace. When you see that you clamp over the top using a caul, such as a piece of 1/4" plexi to help spread the clamp force. I think you'd need to do the two braces separately, allowing a day or so in between so that the first one has a chance to dry pretty well. This will not be easy, but it seems plausible.

[Peter Wilcox]

Sounds like whatever you do it's a PITA, so why do you think they're (probably) too thin? What does the top profile look like when strung to pitch?

[Tom Frei]

It's an old build, closer to a jumbo than a Dred. I'm pretty sure theyre too thin. It's just Top and Sides. No back. But I think the top is dished also

[Tom Frei]

I was thinking about laminating more bracewood but I think it would kill the tone. Im a tone nut.

[Barry Daniels]

As you can see above, I replaced the x-brace with the back removed. It can be done but takes a bit of finagling. You have to carve the braces down and scrape till you get a clean surface. Excavate the brace end that extends into the kerfing. Make the new braces with enough length to extend under the kerfing. When installing the braces I found that you can twist it into place so that the ends pivot into place. After gluing into place there will probably be a few gaps in the kerfing that can be filled with small wedges of mahogany. I will try to get out into my shop in the next few days and post some photos of the final results.

[Alan Carruth]

There's always Trevor Gore's two degree test. He says that if the top and bracing are right you should get two degrees of forward rotation under full string tension as compared with no tension. Note that changing the string height off the top has a big effect on this; it's not all about the structure.

[Mark McLean]

Yes - like Al Says you can do the bridge rotation test, or the top deflection test (both in the Gore/Gilet books, or we could describe it to you if you don't have the books). Both are tests of the structural performance of the braced top. It would be worth doing these to provide some objective evidence before you conclude that your top is inadequately braced. Maybe it is actually great? Dimensions alone don't tell you how it will go.

[Tom Frei]

Ok. I would post a pic and dimensions when I dig it out of storage.
It's a Carpathian top with adi braces. I can get thickness of braces etc.

[Alan Carruth]

The species is not nearly as important as the density, but I don't suppose you measured that.

[Tom Frei]

No never heard of density measurements. I'll be lucky to get done with what I've got. It's adi and Carpathian although now I'm going strictly Adi tops.

[Alan Carruth]

It turns out that the Young's modulus along the grain (E-long) tracks the density quite closely in softwoods. All softwoods follow the same rule, with about 60% of the samples I've run being within 10% of the Young's modulus you'd predict based on density alone; not bad for a natural material. All softwoods have a very similar cell structure, which probably accounts for this. There are lots of outliers, of course, and it's always best to measure if you can.

E-long, in turn, predicts the bending stiffness of the top at a given thickness, so you can use that to determine the 'right' thickness for the top once you've worked out the values for your system. It's not unusual for samples of spruce of the same species to differ in density by +/- 20%, even when they look very similar. Again, the only way to be sure is to measure.

[Barry Daniels]

Here are a few photos of my X-brace replacement on a '70's Martin D-18. Both ends of the braces are tucked so it took a little maneuvering to get the new ones installed. I excavated the brace ends and cut a piece of kerfing out to allow the new brace to be pivoted into place. Once glued in I plugged the kerfing with a small wedge of mahogany. The lower X-brace was glued in and allowed to cure before the top one. I still have a bit of scalloping to do before closing up the box.

[Bob Gramann]

Nice work. Is that a Musser brace on the bridgeplate?

[Barry Daniels]

No, it's a PMTE (Proulx Magic Tone Enhancer) brace. It has a vertical lamination of carbon fiber. It is the main deviation from the Martin design that I am using.

[Bob Gramann]

I’ve been attempting to do the same thing by controlling the stiffness of the bridgeplate—treating it as a brace.

[Barry Daniels]

Another thing I am doing is angling the grain direction of the bridge plate. Should keep it from splitting with the top and resist bulging from the string tension. I am going to remove the popsicle brace too and replace it with a small maple plate.