Finally starting to brace my first build

[kencierp]

Here's an extreme case of too much humidity -- this guitar has a only a 40' radius X and tone bars -- it was stored with a wet sponge in the case. If I recall correctly it took several weeks of normal RH for it to shrink back to size.

[kencierp]

Oh I would certainly agree that over time the front edge of the bridge rolls down -- I would think that is were most of the torque of the strings is applying pressure. On the other hand I have seen no evidence that the neck block is rotating forward and tipping -- that would require crushing the sound board, compressing the sides, stretching the back? or pulling loose from the back, failed glue joint, folding /bending the shoulder brace and bending the fingerboard extension. The short legs of the X failing to carry the load makes more sense to me.

[Kevin Sjostrand]

Ken, in short you just described Armagedon coming to the guitar!

Kevin

[tippie53]

It is the rotation of the neck block that causes the need for a neck reset . Often the visible result other than what Rick Davis pointed out is the dipping of the upper area of the sound board . In older Martins before 1939 the neck block as larger than the modern size ( 1 9/16 compared to 1 3/8 ) The popsicle brace was used to control the top caving in.
Often people get a modern guitar reworked to old spec and forget or don't know about this feature . In short the top crushed the rosette and splits the top as Rick described. Rick I am sure has seen enough of these that often it takes a wedge under the fretboard to get the frets back on plane and flat and true.

[deadedith]

Why are the reinforcement strips around the soundhole so miniscule in many of the guitars I've seen? Are they meant to prevent cracks or to prevent cave-ins, etc? If the latter, would not heftier be better?

[tippie53]

The small sound hole supports are there for support under the rosette . Finger bracing that go out to the sides from the X braces are just for helping to control cracking in that area . If you tried to make a guitar to withstand all the forces you would have made it too heavy . There is a balance involved . If you made a heavy brace under the rosette area the top will still deform and you would pop the heavy brace off , or they will distort from the load and time .
It is all about load , and time . Over time any load will cause a certain amount of deformation. The A frame bracing martin uses is better for load than the traditional bracing scheme but it still will distort and resets will be required to reset the geometry.
I have seen buttress system that try and control that load stress but often they are cumbersum and add more mass to the instrument. Wood being wood, it will do what it want to , as the load is applied . The forces involved can couple as does the neck block rotation and the bridge. I have switched to tucking my joint and using Hot Hide and Fish glue as the glues are less prone to creep that tite bond. That is not to say that Tite Bond is not a good glue , as it is , it is just I am leaning more to traditional building techniques.

[kencierp]

I suppose my view is a bit/ (really) narrow -- I see the all other movement being casued by the failure of the X brace legs -- I thought Martin moved the intersection closer to the point of rotation (saddle) and enlarged the bridge plate to help prevent cave-in?

[nkwak]

No pics but I chiseled the finger braces down to 1/4" in height and am in the process of reducing the height on the two side sound hole grafts down to the same.

As for the neck block rotation, the plans I'm following employ a neck block extension instead of a popsicle brace. I made mine out of birch plywood and thicker than the plans called for, mostly so that I could rout a channel for the truss rod access:

[tippie53]

Martin has been doing that since 1985 and it won't stop the rotaion , all that it will do is make the cave in more pronounced at the extension. I tried that very thing in the early 90's and stopped as that pulled the top down and cracked the top.
It may be best to allow for some deflection in the top . The traverse brace will carry most of that load and in a less concentrated area .

[deadedith]

Does anyone have an illustration or a picture the shows the forces and their directions that are at work on a guitar top? That would be worth a thousand words or so..
Thanks