Guitars prone to Neck resets and bellied up bridges

[charles Tauber]

Originally Posted by Rodger Knox

I agree that bridge rotation probably isn't a good indicator, but can you think of anything that can be measured easily that would be better?

Unfortunately, no. But, in that absence, I don't think bridge rotation is particularly relevant.

I've been interested in this for a while, I have one of those 60+year old guitar that's never had a reset and still doesn't need one. It does have a significant belly behind the bridge, but the action has not risen significantly in 40 years(I've had it over 50 years). I do have it strung with Extra Lights (0.10s), which may make some difference.

Supporting my suggestion that bellying/bridge rotation and neck resets have different causes.

[Rodger Knox]

For those reading this, Mr. Tauber is far more experienced with older guitars than I am, but I'm pretty good at the physics.

Charles, I know you didn't mean that, string tension causes both. I do agree they are different types of failure. Could the point about which the bridge rotates make a difference? Further forward, and it develops a belly. Further back, the top caves into the soundhole. Does this seem reasonable from your observations?

[HNS]

Very very interesting discussion.

IME bellying behind the bridge seldom changes geometry enough to require a neck reset. If the belly is that substantial, then there are likely loose braces or other issues. The top dipping between the bridge and the neck block, and corresponding folding inward of the sides in front of the waist account for most neck resets.

Just in my small collection I have several guitars from the 1930's to the 1950's that have very little belly, but have had the necks reset. All have the original braces and bridge plates intact. Some have had no internal work such as cracks or loose bracing repaired.

[charles Tauber]

Originally Posted by Rodger Knox

For those reading this, Mr. Tauber is far more experienced with older guitars than I am, but I'm pretty good at the physics.

Like you, John and Todd, I'm a mechanical engineer. I have some understanding of basic physics.

Charles, I know you didn't mean that, string tension causes both.

Of course.

I do agree they are different types of failure.

Neither are "failure", but both are plastic deformation - a permanent change in shape that makes the instrument difficult to play.

Could the point about which the bridge rotates make a difference? Further forward, and it develops a belly. Further back, the top caves into the soundhole. Does this seem reasonable from your observations?

I don't think so, at least not within the range of placement typically found on guitars.

I did some stress analysis a while back and was surprised by the result. I had always thought that the "S" shape of the guitar's waist added to the longitudinal strength of the guitar. My analysis suggests it weakens it: a rectangular box resists end-to-end bending better. That suggests that the shape of a particular guitar can have some effect on the ability of the guitar to fold end-to-end (i.e. the upper bout into the soundhole). A deep waist, for example, versus a shallow one. Relative longitudinal position of the waist might also be a factor. But, other variables are also at work.

In recent guitars, I have used solid linings and found that they greatly increase the stiffness of the sides in end-to-end bending - exactly what is needed to resist the deformation that leads to neck resets. The last guitar I made had 1" deep, 1/4" thick linings, making for a very stiff 3-1/2" high side assembly. In 30 years, we'll see if its effective. Double-thickness sides also increase the side assembly stiffness, but I haven't tried that, though others have.

The region behind the bridge is being pulled upward by the bridge: it is in tension. The region in front of the bridge is being pushed downward: it is in compression. This produces the "S" shaped curve of the top.

If you moved the bridge closer to the neck joint, you increase the portion of the top - between bridge and butt - that is in tension and decrease the portion in tension. If, as a limiting case, you moved the bridge to the neck joint, you'd eliminate any compression in the body, eliminating ever needing a neck reset. (Mind you, you wouldn't need the body anymore.) If, as a limiting case, you moved the bride to the butt end of the guitar, the entire length of the top would be in compression, somewhat similar to a tailpiece. It would appear that the placement of the bridge would have some effect on the deformation of the body, if all else was equal, which it never is. The other question would be what effect on sound would there be moving the bridge to a radically different position. Does a guitar with a tailpiece sound "the same" as a guitar with a centrally-located glued-on bridge?

[Bruce Sexauer]

There are some assumptions being made in this thread that are in my opinion inaccurate. Primarily, and already mentioned a couple of times, is that bridge rotation is the culprit. I have not seen this, but rather bridge rotation can crack the top, cause the bridge to come off, break the top across the bridge plate, and ruin intonation, but rarely if ever alters the height of the saddle relative to the neck to a degree requiring a neck set.

Virtually all neck sets that are not due to poor initial geometry ar instead caused by the neck caving into the body. The guitar has a natural weakness at the waist exacerbated by putting the soundhole at that location and further mitigated by many makers leaving no braces more structural that soundhole reinforcement running fore and aft along side the soundhole. Not all guitar are made this way, but since Martin's are and many makers and manufacturers emulate these blindly, the problem is widespread. My engineering
eye sees a virtual dotted line where the fold will happen!

No amount of increased structure around neck body connection is going to solve this problem, and it astounds me that this simple engineering issue still controls the industry to the degree it does. The solutions are several, and the one that I have been using almost since the beginning of my career 51 years ago has meant that there have been just 4 resets (all early work) that I am aware of on my work in all that time NOT COUNTING several more due to my own failure to get it right at the outset and hoping (hah) I was wrong. Take a look, if you like, at any of the braced tops in my several years of posting my work in progress in the Custom Shop here, and think about the sound hole's weakness. Then look at the bracing of a Martin or direct copy of a Martin and it will be obvious, I think.

All opinion, but I do my work from this POV.

[Rodger Knox]

Originally Posted by charles Tauber

Like you, John and Todd, I'm a mechanical engineer. I have some understanding of basic physics.

In recent guitars, I have used solid linings and found that they greatly increase the stiffness of the sides in end-to-end bending - exactly what is needed to resist the deformation that leads to neck resets. The last guitar I made had 1" deep, 1/4" thick linings, making for a very stiff 3-1/2" high side assembly. In 30 years, we'll see if its effective. Double-thickness sides also increase the side assembly stiffness, but I haven't tried that, though others have.

Originally Posted by Bruce Sexauer

There are some assumptions being made in this thread that are in my opinion inaccurate. Primarily, and already mentioned a couple of times, is that bridge rotation is the culprit.

Virtually all neck sets that are not due to poor initial geometry ar instead caused by the neck caving into the body. The guitar has a natural weakness at the waist exacerbated by putting the soundhole at that location and further mitigated by many makers leaving no braces more structural that soundhole reinforcement running fore and aft along side the soundhole. Not all guitar are made this way, but since Martin's are and many makers and manufacturers emulate these blindly, the problem is widespread. My engineering
eye sees a virtual dotted line where the fold will happen!

All opinion, but I do my work from this POV.

It helps to know you "speak the language" in these discussion. Sorry if you thought I meant you weren't good at the physics, I just wanted to let everyone know that was my only reason for being in this discussion, I don't have the experience.

Let's just forget about bridge rotation, it was not a good idea to use that in conjunction with neck resets, the relationship is indirect at best, and probably doesn't exist.

Charles, do you laminate those 1/4" linings? I also use solid linings, and decided that 3 or 4 laminations of 0.10"x 3/4" oak is what I will be using for linings. The last one I went to 0.125" thick oak, and they did not bend as easily as I would like. Ease of bending was what got me started on solid laminated linings, and discovering how much they stiffened up the sides convinced me they were better. I realize that this stiffness will also come from attaching the top, but I believe it's better for the linings to carry some load.

Mr. Sexauer, do you think that solid linings can stiffen the upper bout sufficiently to avoid future resets, or is some modification to Martin bracing necessary? I've looked at one of your bracing patterns, and what I do is pretty close to that. I see that Martin uses very small braces around the soundhole. I agree they need to be heavier, that looks like a weak spot to me as well.

[charles Tauber]

Originally Posted by Rodger Knox

Charles, do you laminate those 1/4" linings? I also use solid linings, and decided that 3 or 4 laminations of 0.10"x 3/4" oak is what I will be using for linings. The last one I went to 0.125" thick oak, and they did not bend as easily as I would like. Ease of bending was what got me started on solid laminated linings...

I can't bend 1/4" thick material. (I'm sure it can be done in a steam box, but I don't have one and won't likely make one just for bending linings.) The laminations I use are about 2 mm thick, typical guitar side thickness.

Serge de Jonge stated that a specific type of willow can be bend solid 1/4" thick over a hot pipe, but that it is difficult to find.

In my opinion, oak is too heavy. I used Spanish cedar, partly 'cause I like the smell and I have lots of it since I use it for classical guitar necks.

[Bruce Sexauer]

Originally Posted by Rodger Knox

Mr. Sexauer, do you think that solid linings can stiffen the upper bout sufficiently to avoid future resets, or is some modification to Martin bracing necessary? I've looked at one of your bracing patterns, and what I do is pretty close to that. I see that Martin uses very small braces around the soundhole. I agree they need to be heavier, that looks like a weak spot to me as well.

The problem, as I see it, has little to do with the sides OR the neck block. Martin's fix appears to have been the massive neck block the extends under the fingerboard, but I see nothing there that keeps the whole unit from tilting into the soundhole. Classical guitars address the issue better IMO by using what we call a "foot" which used the back to add stability to the area, but of course they are dealing with much less force. Increasing soundhole stiffness must help, and my rosette backings are perhaps 6x Martin's, but the real key to what I hope will remain my success is having the X braces connect to the UTB exactly where they hit the rim. This forms a triangle which is supported by the rim itself without adding any rigidity to the key sound producing structure of the instrument, and makes the "fold on the dotted line" aspect disappear.

It is also very important to address the bridge rotation issue, and I believe I have, but that is, as they say, another story.

Guitars design is rarely the realm of structural engineers, and if it were I suspect their priorities would not be in line with my own. But some more influence from thinking along those lines would go a long way in the world of lutherie. ALL IMO, of course.

I do expect my work to out survive the work of many others of my acquaintance, and if our civilization survives my star may rise on this basis . . . not that I expect to know.

[charles Tauber]

Originally Posted by Bruce Sexauer

Classical guitars address the issue better IMO by using what we call a "foot" which used the back to add stability to the area, but of course they are dealing with much less force.

I was taught to make steel string guitars with a classical foot design. The classical foot has done little to prevent the need for neck resets and makes neck resets much more difficult. I wouldn't recommend it for steel string guitars, at least not without some other stuff going on to prevent the upper bout from rotating.

[Bruce Sexauer]

If the construction were classic Spanish, indeed the neck set would be nigh on impossible. If the foot were used with a dove tail join or the often inelegant bolt on method , the reset should not be any harder than usual. Bringing the neck block close to the upper bout back brace should do quite a bit to resist neck rotation.

Originally Posted by Bruce Sexauer

...Martin's fix appears to have been the massive neck block the extends under the fingerboard, but I see nothing there that keeps the whole unit from tilting into the soundhole...

Bruce, I'll poke around and see if I can get an answer, but I don't believe that L-shaped neck block was meant to prevent neck resets. I think they were just creating an enclosure for the truss rod.

[Tone Gopher]

FWIW, I have three Gurians - two that I bought new back in the '70s. My Jumbo retains its original geometry - never had any work and never needed any. My Size 3 had a neck reset at about 5 years age, and has been stable ever since (35 more years).

[mirwa]

Originally Posted by Bruce Sexauer

If the construction were classic Spanish, indeed the neck set would be nigh on impossible. .

I still do ""neck resets"" on these but not the traditional way of steaming the neck off, but machining the fretboard off and putting a new fretboard on, I still classify it as a neck reset, as we are resetting the angle to the body. I have an esteve in my shop right now that I need to do this too.

Steve

[Bruce Sexauer]

Originally Posted by mirwa

I still do ""neck resets"" on these but not the traditional way of steaming the neck off, but machining the fretboard off and putting a new fretboard on, I still classify it as a neck reset, as we are resetting the angle to the body. I have an esteve in my shop right now that I need to do this too.

Steve

How does removing the fingerboard make it possible to change the angle of the neck to the body? I can see a fingerboard thick enough to change its surface enough to do what is necessary (I've done this, but it is not elegant IMO), but without ungluing a plate from the neck block it seems like the thing would stay pretty rigid.

Slipping the back off the neck block is what I think is done, but it is far more "major surgery" than a martin style neck set as the back must be rebound which requires refinish work and some has to be done about the former binding iserts into the button, if the guitar is traditional Spanish. At least on guy I know simply cuts off the neck with the bandsaw and makes a new one and attaches it with a dovetail!