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Not a repair person here so not much experience with neck resets. I was always under the impression that the body distortion that made a reset necessary was a straightening out of the longitude arc in the back of the guitar. The pivot point of rotation being at the top edge of the body where the fingerboard hits the body. That would result in a straight board going into a sky slope shape. But a lot of posts on resets mention the fact there is a gap between the board and top after reset which requires a tapered wedge to correct. That leads one to think that body distortion is only happening by a gradual raising of the top or the top north of the sound hole in moving down. Also I know there are folks out there who use CF to prevent that type of movement. So all repair people what is the real scoop on what distortions are taking place and how can we prevent them from happening????

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Some people have described it as the guitar being swallowed through the soundhole. On old parlor guitars that have been strung in steel this is especially evident. As the soundhole area collapses the neck pulls up, and as the soundboard 'bellys" in the lower bout the bridge rises.
The "X" brace partially solved the problem by stiffening the area which eventually collapses. Stronger soundhole reinforcement as some are doing now with the "A" brace should also help.

It isn't the back that flattens , Wood will not get longer but the top will "crush" as the cells collapse. This is not uncommon. The neck block will rotate from the stress and the top humps up from the compressive force. Over time this will happen to most guitars , unless you over build them.
Finding a way to control these forces have been going on for years and I have not seen anything short of an over built guitar that will hold this from happening. I use a 3 faceted top system on my guitars and I have not had to reset a neck on anything yet with the oldest one at 14 yrs.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

What Clay said.

edit: and John. We were writing at the same time.

Some builders use struts running between the back at the waist and the top end of the neck block, forming essentially a triangle with the struts being the hypotenuses, taking the stresses in compression. Seems an effective strategy, though that still leaves the lower bout which can also distort under string tension over time. It might be too early to tell how effective the struts are. There might be some tone issues with the struts, as that would stiffen the guitar in an area of a major antinode. Anybody have any info on that? Alan?

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

Makes me wonder about the effectiveness of a "C" type neck block. Having a little more surface to distribute the load should only make for less long term movement. One would think that having the extra material up that far wouldn't be a sacrifice of tone. But as mentioned, the lower bout will still belly, not that there's anything wrong with that.

Danny

Is it always the soundhole area that collapses? At least it seems that is the area that tends to sink. Cutting the soundhole removes a significant area that weakens the top in that location. Add all this up.......and doesn't it make the case for stronger bracing either side of the soundhole? Using taller soundhole braces adds little weight and I would guess it's impact on sound would be minimal (possibly no impact) considering where the additional stiffness would be located.

I put a 4" soundhole on the 00 I'm currently building. Considering the larger soundhole and the points mentioned previously, I went with taller soundhole braces than what Martin normally uses. Hopefully it will help.

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Formerly known as Adaboy.......

Pressure from the strings torques the bridge, which tends to depress the top in the soundhole area. It doesn't always happen, but the odds are that it will.

The back can and does flatten on lightly-built guitars. I mean guitars that are lighter built than the typical Martin. If the back does flatten, the distance between the neck block and tail block increases slightly. The wood is not stretching, but it is bending.
However.....
Most of the body distortion that causes a reset to become necessary is in the top. This torque on the bridge causes the top to take an "s" curve, shortening the distance from the neck block to the tail block.

Martin uses taller soundhole bracing in their A-frame design, and those braces pass under the UTB and are mortised into the neck block extension. The result is much stiffer in a critical area.
Another way to guard against sinkage is to arch the bracing, particularly the UTB and the upper legs of the X-bracing.

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John

John Hall: What do you mean by a three faceted system on your tops ?
Tom

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John Arnold: In re: to the movement of the back. Are you saying brace not tall enough or thinner back or most likely a combination ?
Tom

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A person who has never made a mistake has never made anything!!!

While the back looses the arch of the braces the back doesn't lengthen. The braces often flatten. Wood cannot stretch that is a fact of the wood properties. Wood moves across the grain and cells may collapse so wood may shorten. The neck block will still rotate. I have seen buttress systems that tied in the body. There are a number of forces that apply. You have 2 rotational forces one on the neck block and the 2nd on the bridge along with a compressive force on the top. John Arnold is correct on the A frame using a higher brace that actually ties into the upper X brace. Here they are more a truss brace than sound support. Also in the 15 series the rosette was a decal so there was no need for soundhole reinforcement.
The original design of the sound hole braces were to help avoid cracking in the rosette area and not so much to strengthen . For those that want to see how I do my tops here is the link. http://www.youtube.com/results?search_q ... M1uRfoD0BE

You can influence the stiffness of a structure by the shape. This doesn't change the overall yield strength but will work opposite of the crumple zones in a car.
In this system we have a neck angle a flat area between the top of the sound hole and a domed area behind the bridge.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I believe that one of the easiest things to do to put off neck re-sets as long as possible is to beef up the sound hole braces. Reinforcing the hole will help keep the top from collapsing under string tension. This can be done without over bracing the rest of the top and doesn't seem to affect tone or sustain very much. I also use a much larger neck block than most others and a fairly heavy transverse brace.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Like I said, wood doesn't stretch, but flattening the back will increase the distance between the ends. Conversely, arching a back reduces the distance between the ends. It is simple geometry.
For example: Draw half a circle. The distance between the ends is the diameter. The length of that curve is 1/2 the circumference, or pi times the diameter divided by two (1.57). If you flatten out that semicircle, it becomes 57% longer.

Braces provide most of the stiffness.
I have seen backs flatten out on older Gibsons, primarily those with the small triangular braces.

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John

Thanks for the answers. Guess I did not make myself clear enough in the original post. Did not indicate that the wood was streaching but that the longitudinal arch was straighting out thus being longer.If the back is longer the neck block rotates.I then assume the point of rotation is as stated in original post.That would give a ski slope shape to the board which would be removed by the reset neck....?? I wonder about folks who use CF in braces,if that has any effect on the tendacy of overall distortion.
Tom

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John Hall: Still not sure what you mean by a three faceted bracing system. May you could elaborate.
Tom

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Yes to what John Arnold is saying that the curve from the neck to the tail can add a very small about of length from flattening . Agree with that.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Because the string tension is just above the soundboard, the compressive forces in the soundboard are much greater than the tensile forces in the back(due to leverage)

Certainly no engineer here Jeff but the neck on a guitar looks like a crow bar to me with the fulcrum being at the top edge of the guitar. Looks like about a 3.5 times advantage against the back to me.
Tom

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Jeff : After a bit of consideration my last post is not very valid because the force is being applied parallel as you stated. If it was applied at 90 degrees to the neck what I said would be valid...?? Guess that would not make much of a guitar....!!! Like I said ,no engineer here...!!
Tom

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A person who has never made a mistake has never made anything!!!

To what John Arnold is saying and to this I can agree , if the back flattens fore to aft that will allow the back to lenghten but as they flatten side to side and the back still has the arch from neck to tail the back will not lengthen.
Resets can happen for a few reasons but mostly it is the top that is caving in from the neck block rotating and the top caving in . On a Martin the back arch is not just carried by the bracing the sides are also shaped to the radius and on martins the back will not loose the fore to aft arch as some other makers will.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Yes, the neck block rotation causes a rise at the end of the fretboard over the body that goes away after the neck is reset.

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