How much downward force is actually exerted on the UTB ?
 

Being in the (very) early stages of my first build, I have been pondering this question for some time. I even googled it as best I could and the only relevant hit I came up with was this thread on the MIMF

The contributors there (one in particular) seem to be under the impression that the UTB needs to be fairly massive in order to withstand a substantial downward force from the glued-on fretboard extension. They obviously view the neck block as a fulcrum, the fretboard as a lever, and the strings as the force exerting the leverage on the fretboard.

In the past, this would have been my view as well, but now I am not so sure. It seems to me that the neck block, properly glued in to the back, top, and sides, with a well fitted dovetail joint, is more than capable of withstand the compressive force exerted by the string tension all on its own, and that the end of the fretboard above the UTB is exerting very little (if indeed any) downward force on the UTB.

I am aware that fretting methods, and also the goodness of fit of the extension to the plane of the upper part of the soundboard in the first place could play a part in determining how much stress the UTB is under . I am also aware of the plastic deformation which happens over time, necessitating an eventual neck reset, but I am skeptical that the downward pressure on the UTB has anything to do with that. Basically, the UTB is just helping to strengthen and stabilize the area round the soundhole, nothing more , IMO.

I doubt that the experiment has ever been carried out, but it would be fascinating to measure accurately (on say a standard Martin dreadnought as a benchmark) the distance between the underside of the UTB and the back directly underneath with the strings on and up to tension, and then … remove the UTB apart from an inch long piece in the center, and do the same measurement with the strings on and up to tension. If the two measurements differed, it would be a simple matter to determine the force exerted on the UTB simply by removing the strings, and placing weights on the end of the fretboard until the second measurement was replicated.

An invasive experiment admittedly, but by no means destructive. I am quite sure John Arnold could execute the experiment and have the guitar back on the road before his morning coffee break.

It would of course be easy to calculate the vectors and come up with an exact figure mathematically if the neck block were in fact a fulcrum, but since it isn't, such an approach is inapplicable.

Murray, I couldn't tell you. Maybe a quick trip to the science lab at the local university could yield a force plate and other equipment that can measure this.

Being of the Somogyi mindset, I've adopted the use of an L-shaped neck block that runs right into the UTB. I do believe that the unitization of the L-shaped neck block to the UTB, anchored at the linings and kept in position by the top, does lend to increased resistance to neck block "rotation." Because, I believe, the top area under the fretboard extension is actually in compression as well as torsion. Since the fretboard wood is harder and stiffer, that is what helps the neck from rotating about the neck block area, and not the force of the fretboard into the top.

I had mentioned in a previous thread that I actually used this theory to repair a couple of cheap guitars, that would have normally required a neck reset that would have cost far more than the guitar was worth. I simply unglued the fretboard from the top, pulled the neck back slightly, and reglued the fretboard in essence pre-loading that area. Cheap (only had to remove and replace 5 frets) and fast.

A statics diagram with forces and moments will tell you. We know the amount of torque on the bridge. If the string tension is 180 pounds and the strings are 1/2" off the top, then the torque is 90 inch-pounds. Subtend the distance from the bridge to the UTB (about 8 inches), then you have about 11 pounds of downward force at that locaton.
The amount of force is not excessive for the standard braced top, but over time the wood will tend to distort.
The other way to analyze this is to look at old guitars, where top sinkage in the soundhole area is common. Martins made before mid-1939 have a UTB that is no wider than 5/16", and 5/8" tall. Those are more susceptible to sinkage than later Martins, which have a 1/2" wide UTB and a flat popsicle brace.
My approach is to use subtle arching of the UTB (50 foot radius), which helps counteract the force that causes the top to sink. This arching also helps produce a straight fingerboard over the body, since the neck is pitched back slightly. Also note that the compressive force between the bridge and the neck block aggravates the situation. The compression of the top will cause it to move either up or down, depending on the shape of the top and the bracing. Having the slight arching of the upper bout helps push the top upwards instead of downwards when it is compressed.
This is why violins resist top sinking so well....the arching is 'helped' by the compression of the strings, which push from the tail block where the tailpiece is attached.

Force (string tension) times sine of break angle (in degrees)

I agree with Louie. The compression force into the side of the UTB (in line with the strings) is greater than the downward force. The soundboard wood itself is in-line with the compression force, so that goes a long way toward resisting it. But being thin, it needs the brace to keep from buckling up or down, and from shear creeping into the soundhole.

My UTB is wedge shaped, 1/2" wide at the base and about 3/4" tall, so it's stiff in both directions. Definitely overkill, but my belief is that a little extra mass and stiffness in that area is more likely to improve tone than to hurt it.

The neck joint is the worst place to have any long-term deformation. One degree of bend there makes a much larger change in action than one degree of bend half way between the joint and the bridge. So by having extreme stiffness in the upper bout, high stiffness in the soundhole area, medium stiffness at the bridge, and low stiffness around the lower bout, I can get maximum bridge mobility for a given long term action rise rate.

My acting theory is that the cave in across the sound hole is because many old guitars were not reinforced around the sound hole.

One thing you see is the sides tilting in slightly and the lower bout caving into the soundhole - not just the end of the fretboard.

It's true that there is 10 or 12 pounds of down force - but there's 180 pounds of force pulling the neck towards the tail. Think of the old game where you stand up on an empty coke can... It will support your weight until you tap a dent into the side... Then it collapses.

And that load is mostly spread across the top.... But there's a big hole right there at the waist. Most of the bracing north of the bridge is there to help the top retain it's dome... Because that helps it resist caving in. But most old guitars had almost nothing at the soundhole... Especially old ladder braced guitars... And you are more likely to find an old potato chip ladder braced guitar than it is to find one that's not deformed.

I really like an upper bout A frame or at least some substantial soundhole bracing that connects the lower bout bracing to the upper bout bracing. So far so good...

Thanks

The UTB and popsicle braces are extremely widespread, so presumably they work better than alternatives. Does anyone know if / when an upper X-brace was tried instead, and what caused it not to be adopted? I'm thinking of an X between the soundhole and the neck block. Or maybe 2 X-braces, one on each side (bass and treble), both "north" of the sound hole.

I know Somogyi and others have used multiple X-braces below the sound hole (lattice bracing, etc), so why not above?

Something like this:

http://i.imgur.com/7N43Rwdl.png

Or this (sort-of double X):

http://i.imgur.com/VMy3erb.png

Or this, which I guess is the A-frame idea Truckjohn is referring to?

http://i.imgur.com/HXLE20f.png

It seems to me intuitively that the downward force will vary considerably according to how far the neck block extends into the body ... eg the beveled neck block on a D16 will IMO result in much greater downward pressure on the UTB than for example the Charles Fox neck block which extends well into the body .... by 3 inches or more IIRC. However, I have learned over the years that intuition is fallible more often than not, and may well be in this case ...

I have no idea but as John A mentioned I arch that brace too and make it good and strong. Once I learned to arch it I now have perfectly straight fretboard extensions and that makes me very happy :)

What ever amount of force there is there over time as we have all seen it causes damage and since it's a relatively dead acoustic zone on the top anyway then why not strengthen it.

I also almost always use an A-braced system there now as well. I've always hated seeing older guitars with deformation around the sound hole.

It could indeed make for an interesting experiment. Problem is, to find out just how much structure is needed there, you might need to wait years.

This is a case where repairing old guitars can tell you what doesn't work. Some were notoriously weak in the upper bout area, and they often sounded great -- for a while.

If x-braces in the upper bout worked, I have a feeling that Somogyi would have jumped right on it.

Rotation of the neck block is countered all sorts of ways: struts running to the waist, block extending to the UTB, the long foot found on classical guitars,to name a few.

I also use a-braces, taken from some mid-low range Martins, where two braces are tucked into the upper legs of the x-braces and run close to the soundhole, through a substantial UTB and are tucked into the neck block. After eleven years, I've had no signs of sinking. Knock on wood (pun not intended).

Pat

Have any of you held a neck (off the body, of course) by the end of the fingerboard?

It's so weak and bendy out there it simply can't put much downward force on the top of the guitar!

Think more in terms of the neck block rotation, and the transverse brace resisting the downward sinking at the soundhole.

Tops of nstruments with cantilevered fingerboards sink there, too, without having the fingerboard touching the top at all. . .

Yes yes yes. John (as usual) gives sage advice.

In a Martin-X style bracing pattern, the upper bout really does very little vibration. There is no good reason in my mind to avoid building with a thick and strong transverse brace. This will help avoid the downward dive of the fingerboard extension over time.

As John stated, a subtle arcing of the transverse brace can be a very good idea.

In addition, you may wish to think of "fold points" on the soundboard. These are points where the lack of braces, or concentration of force from braces to a certain more weakly braced area can allow the constant tension to "fold" or deform the soundboard. A common location for this folding/deformation with x-braced steel string guitars is near the soundhole and waist and at the soundhole end of the x-braces.

Since the x-braced guitar has a significant portion of top removed within the functional portion of the soundboard, this weakens the top's ability to resist folding & deformation. Hence, the x-braces should not be overly scalloped at the sound-hole ends. And the sound-hole bracing (whichever style is used) should ideally transfer the tension so that one "fold point" is reinforced by overlapping of a different brace structure.

Hope this makes sense and adds some insight for you.

Yes, it (the fingerboard extension) won't resist folding, but along its length, it can help transfer the "pull" of the strings into the transverse brace.

So that would be arcing on both long sides? The top side (where the glue goes) to hug the dome of the top, and the bottom side as well? Like this?

http://i.imgur.com/LNUjIYA.png

This is great, thank you.

I haven't, but that makes sense. Especially with the fret slots cut.

(apologies to OP for the threadjacking)