Offset hole - brace question

[Dave Bagwill]

A couple more for your comments, which I welcome!!

[ken cierp]

As I understand it, the idea of an "X" brace works well with the normal sound hole location is because the short upper bout legs act as a bridge along the edges of the "hole" preventing the "nut ward" roll of the bridge from caving in the sound-board. There is a limit to the effectiveness of the system -- tens of thousands of "X forward" guitars ("X" intersection close to the edge of the sound hole) end up with the top concave in the center because the string tension/leverage/resistance equation is not perfect. Most makers today have either moved the intersection back, beefed up the braces or both -- this is done knowing full well warranty issues will be reduced and that likely "human ears" are not capable of hearing any difference.

So back to the DB designs which obviously do not have this issue --- the way I see it you do not need an "X" brace. The function of the "A" frame is to prevent the forward roll cave-in, but the "A" is much better able to resist the roll of the bridge, its like a double joist in the flooring system. My suggestion of a longer bridge (surface brace) is to allow the "A" frame to be spread wider just laying under the ends of the bridge wings.

Here's the analogy, my guess is that you have been on an outdoor patio deck where you can feel the vibrations when people are walking around. Also mostly likely you've experienced decks that are rock solid. The guitar top should be built like the former -- hefty enough to hold the load but sensitive enough to sense and react to the input. Spreading the "A" braces will allow this arrangement to be more responsive -- I would not doubt that someone with time on their hands could develop a formula to calculate string gauge tension and the relative "A" frame placement.

[Dave Bagwill]

Thanks Ken! I've seen some classicals with really long bridges so I know they are viable.

Well I'll use the 4 A-frame, keep it light and responsive, use a longer pinless bridge and the other procedures I've made note of over the years, shake everything together in a bag and put it in the oven. Then see what comes out.

Actually, I'm expecting a good sounding instrument. But "I won't know until I've built it"!

[John Parchem]

I agree with Ken, basically moving the sound hole allows an A frame which actually is better than the X at resisting the string tension. Having each A brace tied into the bridge completely spans the length of the guitar body under load. So I would not go back to an X brace as it is a compromise from a better bracing system to deal sound hole.

To support Ken, If I remember my physics correctly, You can spread the A out wider only losing a small precentage of the frames ability to resist the string load. Each of the braces in the A lose effectiveness as they angle off from the center line, but the rate that the braces loses effectivness follows a sine wave. The rate of change in a sine wave starts out small increases toward 45 degrees and decreases after 45 degrees. Your outside A looks to be about 22 degrees or so off of center so cosine (22) = .93 or 93% as effective as brace in line with the load.

With regard to the weight of the braces, a wider bridge and the more angled A frame with 2 braces can give you much better structural support and nearly as good crack prevention as 4 lighter (less height) weight braces. I think it would be interesting following Ken's recomendation of a wider bridge but to throw one of your "A"s away because doubling the braces is not a very efficient way to provide extra structure nor does it add much in the way of crack protection. As Ken suggests thin finger braces in logical place to prevent splits would be better.

I liked Ken's SWAG:

"The one on the left, with only two "A" frame braces positioned at/under the outer wings of the bridge which I would make 6.5" or longer.
The ladder under the bridge is good but I'd add something similar in location to the standard shoulder brace. I'd also place some very thin finger braces in logical places to prevent splits along the grain. I predict it will be on the bassy side."

[Dave Bagwill]

Cool. It's coming down to Mottola having done a good job with his design after all.
Good, I feel better about it now, and learned some things in the process. It's going to be fun.

[Dave Bagwill]

Okay, I pulled out the 1:1 copies of Mottola that I had printed out some time back.

His bridge is exactly 6" wide, and the middle of each brace is 1" from the end of the bridge.
Plus, he has no utb, and the two long braces are not inlet to the kerfing.

Is it a consensus here that the A could be spread further apart;
that the braces need not be a full inch from the end of the bridge;
that an utb should be added m/l in the 'normal' placement;
that there be some minimal provision of finger braces to address crack potential;

Anything else?
How about: the bridge does overlap the lower ladder brace - make sense?
There is a bridge patch between the two longitudinal braces - minimal?

[ken cierp]

Where you place that ladder brace may be dependent on whether or not you use a pin-less bridge. I would not compromise the "A" braces with any notches half laps etc. and would make the ladder with a scallop to bump over the "A's" or even a butress design. I believe the reason your sample has the "A" so narrow is that it is modelled off a jazz guitar -- because of the downward pressure of the string tension the feet of the movable bridge generally are placed on top the "A" frame.