Double top bracing

[Eldon Howe]

Randolph, I'm using 1mm top and bottom skin both are spruce and 2mm nomex.
I was thinking of thinner than normal x bracing and maybe shorter than normal, not to impede the vibration of the top. I'll probably use spruce less than a 1/4” in width and 3/8” in height.
Any thoughts on just using carbon a fiber strip bonded to the inside of the spruce top?

Robert, don't leave the post this is your post. It's all good.

[Randolph Rhett]

I don't think one strip of CF will do much. It's very strong in tension, not in compression. You need an I-beam of CF or tube for strength.

That said, I doubt you need any bracing. Have you tried deflection testing? You may be surprised at how stiff your top already is.

[Robert Freemond]

Randolph, Can you provide some numbers for deflection testing. Do you use weight or string tension.

[Eldon Howe]

Just strung up the guitar with all six strings to pitch.
The guitar in the mold with long spreader in place.
The top has initially settled 2mm.

I thought maybe the carbon fiber strip ( as a brace) might act as a thicker skin and the nomex would act as a compression member and then the outer skin as normal, just a little thicker?? I do see your point though.

[Eldon Howe]

It's now been 24 hours under full tension all six strings. The top has still only settled 2mm without any bracing.
Is that a lot?
Will it slowly drop more over time?
Another thought, without bracing would the lower bout get wider over time? Maybe one flying brace to stop gradual drift?

[Robert Freemond]

Eldon, What did you use to laminate all the pieces together. A vacuum bag, or a press system.

[Eldon Howe]

a vacuum bag kit from Joe wood worker.

[Randolph Rhett]

Again, I don't know that anyone can tell you for sure. My CF tops deflect about .02" with 20lbs. That is quite a bit stiffer than your top. Of course I don't know exactly what tension strings you have or the break angle. However, I consider it excellent news that twenty four hours and you've seen no further movement. You've cut your f holes, right? The test is somewhat meaningless if you haven't finished cutting into the top.

I have two suggestions:

Leave it a couple of more days. If you still have no further movement, my gut is you should be fine.

Second, how does it tap? Is it musical or dry? Does it sound like a bass or Tom drum, or like a snare?

If it doesn't move, and it has a good voice don't mess with it further.

As far a "spreading", that is not really what traditional archtop bracing is for. The issue is more that the f holes weaken the top along its perimeter. The braces are to keep the dome from collapsing by buckling at the neck or tail where the load is being carried.

If traditionally braced guitars don't "spread" I don't see why yours would.

Braces are not neutral to the tone. My philosophy has always been to use as little bracing as possible. Adding extra bracing just to be sure is not like spraying one more coat of finish before level sanding just to be sure you don't sand through. Bracing will radically change the voice of the guitar.

In any case, good luck. Start a build thread. I'm sure many of us would like to see your progress.

[Eldon Howe]

My sound hole is along the upper bout and cut in. Some what like a Ken Parker archtop.
The bridge is way tall a good 1/8” taller than needed, so the break angle is quite steep.
I'm using .010-.047 D'Addario strings.

However now my top is settling between the bridge and the neck block.
Quite a bit, so I reduced all the string tension. Still only 2mm of settle at the back side of the bridge.
Now it looks like I need an X type brace with the x crossing mid way between the neck block and bridge. Or a flying brace that kicks to the waist.

[Barry Daniels]

An x-brace would probably do a lot more to produce sound from the top than flying buttress braces.

[Randolph Rhett]

Sounds like you do need bracing. Parallel or X. I don't have enough experience to comment on how each color the tone, but I do know parallel is much easier to do.

[Eldon Howe]

I would assume that the flying braces would add no tone at all.
And would only stop the neck block from pitching downward toward the bridge.
The head stock moved up as the cantilevered fingerboard went down with the top.

One thought is to install 2 flying braces from the neck block to the waist on each side, just below the spruce top .
A partial filler if needed could be glued in between the flying brace and the top.

[Randolph Rhett]

I'm sorry, I guess I didn't know what you meant by "flying braces". I assumed you meant attached to the top. Here is one of mine. For me "axial bracing" proved essential. However, CF has little compression strength. I'm surprised that your head and tail are rotating together. If they are, word of caution. Wooden axial braces tend to expand and contract with humidity. Err on the side of very dry well seasoned wood.

[Jason Rodgers]

Throw an X brace in there, make it a little taller than you think you need, then carve it down as you experiment with your top loading. If you didn't like the effects of the X, you could even carve it off (very carefully, not to damage your skins).

[Eldon Howe]

Randolph, Yea that’s what I'm thinking.
I don't have any CF tubes like yours, Well I do have a 1”x1” square CF tube and I'm sure that would be over kill.
I do have some well cured bass wood that I could put a layer of CF on the top and bottom.

Do your cf tubes vibrate? They have a clean look from the top as well I bet.
I'll try to get some photos of what my project soon.

[Eldon Howe]

photo I guess my photos are to big I keep getting this notice,

Sorry, the board attachment quota has been reached

[Barry Daniels]

Spruce would be a whole let better than basswood, which is soft, weak and acoustically dead.

[Eldon Howe]

I would think that the flying braces or axial bracing not touching the top should be neutral in tone, hence the CF basswood composite.
I will use spruce for X bracing glued to the bottom of the top.

Typically the X part of the bracing is just in front of the bridge for structural reasons.
My top only settled 2mm at the bridge, but a quite a lot between the bridge and the neck block.
So I'm thinking of placing the X closer to the neck block ??

[Barry Daniels]

Normally, the x-brace is placed so that the legs of the bridge are supported.

[Eldon Howe]

Barry, supporting the bridge legs with the x brace is what I've done in the past. I'm just thinking that being my top has only settled 2mm at the bridge maybe no support is nessasary there. my bridge is over 7" long and that should spread the load and in a sence act as a brace.