Some bracing advice, please

[Patrick Hanna]

Attachment 81818Attachment 81819Attachment 81820Hello, Friends,
A couple of years ago, I helped a friend build a very successful redwood topped flat top.
We used a simple x-brace with some reinforcement around the sound hole. I am building a second, but I have a couple of problems. I've made this a 14-fret neck, so the x-brace has been shifted toward the neck to keep it properly located under the bridge. But there's a bigger potential problem. I realized belatedly that I've sanded this top down to about .10-inch thick. The previous top was closer to 1/8-th.

Brace stock is quite stiff, tightly grained, perfectly quartered and free of run out. Brace dimensions are currently about 5/16" wide and 1/2" tall. I don't have enough experience to know whether this is tall enough. I realize it's hard for you to advise because you can't flex my top. However, please give me your gut reactions to these options.

Option 1: Plane braces off and fashion higher brace blank.
Option 2: Add a transverse brace between the lower legs of the x
Option 3: Add a tone bar
Option 4: Cap the brace intersection with a longer, taller cap, in hopes of stiffening
one brace member
Option 5: Some combination of the above.

All options shown in blue in these schematic drawings. Thanks for any help you can offer.

Patrick

[bennyb]

Howdy Patrick,
Links don't work for me, but even if they did I couldn't advise. I haven't built a flatty myself. Floating bridge? F holes? I think you should go for it and report back. Or just wait a little bit for some better advice.

Good luck, benny

[Patrick Hanna]

I apologize for my ineptitude in handling attachments. I"ll try to get some viewable plans up shortly.
Patrick

[Dobe]

Try these guys:
http://www.kitguitarsforum.com/board/index.php
Been a while since I did a flat top but there pretty informative.
You'll need to register to see pics but you should check out how prevalent the side sound ports have become on the' Show It Off' section ! Some great ameteur builds out there !!
(links didn't work for me either)

[Patrick Hanna]

Let me try again, one at a time. I should also tell you that the top has 15' induced radius arch. Here's my thought for a transverse brace between the legs of the x-brace. I could probably tie it into the brace tops with caps (like a brace intersection cap). This would add some mass and stiffness around the bridge area, but wouldn't reinforce much else. Bar is shown in blue. Next option will follow.
Patrick

[Patrick Hanna]

Here's another option. A tone bar like this would support the overall curvature of the top behind the bridge, it would provide a little anti-crack protection, and if I graduate it correctly, it should help distribute vibrations. However, it still doesn't stiffen the basic x-brace.

[Patrick Hanna]

I haven't capped the intersection of the x-brace yet. It occurred to me that I could cap it with a taller and much longer than normal cap. This would add height and stiffness to one of the main brace members, and could be faired into the scallops or tapers near the two ends of that member. Proposed cap is shown in blue on this drawing.

Again, the question is: Start over? Add the transverse bar? Add the tone bar? Cap the brace intersection with a long, stiff cap? Or some combination of these ideas?

Many thanks for your patience.
Patrick

[labraid]

I don't think you need to worry about brace height, the top doesn't do a great deal of supporting the bridge by itself, and so those would be most likely fine no matter the top thickness. Your main problem might be that the tone will approach that of a banjo with an overly thin plate. Top thickness contributes to tonal richness.

[Audio Artillery]

So I'm kind of missing the question. Are you saying the top is too flexible currently?

You could have just left the X brace where it was. Some people place the X crossing under the bridge (as I did for a mandocello build), some line it up so the feet are over the braces like you did.

[Patrick Hanna]

Thanks for the replies. To restate the problem, I'm guessing my top is thinner than a redwood top should be. I obviously don't want it to implode, I want it stiff enough to give the instrument a nice voice, I don't want to over-build it, and I don't have enough experience to know exactly what to do. I realize it's tough for you guys to give specific answers because you can't handle the top and judge its stiffness for yourselves.

[Jim Baker]

I always cap the brace intersection. If it were me I would add a brace between the bridge and the tail block. The angled brace may be better as it offers some fore aft stiffness to offset string tension. Just my humble opinions. I also think 5/16" x 1/2" should be stiff enough if you don't thin the sides too much and don't taper too quickly toward the outer ends.