Fingerboard - body 'extension'

[charles Tauber]

Originally Posted by jeff crisp

It was no doubt hard to tell from Charles and my conversation, but we both agreed, and many other people, including the person who I have learnt the most from, that the main cause of cracking in the said area is from neck block rotation.

Not to split hairs or be argumentative, but I was very specific in what I stated.

I stated the following:

1. I don't use the popsicle brace and don't know why others do. I conjectured that many do it because it is "traditional", others might do it to attempt to counter stresses and/or humidity-related wood movement.

2. I don't know any better or worse than anyone else what causes the cracks in the top on the sides of the fingerboard. Until sufficient, targeted testing is done that isolates specific test parameters, we have various subjective opinions, rather than objective "facts" regarding the causes.

3. My opinion (read: hypothesis) is that the in some cases the cause of the cracks is humidity related while in other cases stress related. There are examples I've seen of cracked tops with no apparent rotation of the neck block, examples with lots of rotation of the neck block and no cracking of the top and examples with both rotation of the neck block and cracking of the top.

I don't see the point in arguing that one opinion is more correct than another's. Each is based upon the individual's experience and his or her interpretation of that experience. Different people have different experiences and different interpretations of that experience. That isn't a substitute for scientific, objective evidence. In the absence of scientific, objective (repeatable) evidence, each should do what works for them. Sharing of ideas, opinions and experience is good when people do so with an open, inquisitive mind.

That's my opinion. Back to the regularly scheduled discussion....

For the more objectively oriented, one could do some first-order approximations and perform some calculations to determine what the humidity related stresses in a top would be for a given change in humidity. By calculating how much the top would shrink cross-grain versus how much the fingerboard material would shrink cross-grain for the same change in humidity, one could determine if the stresses so induced exceed the cross-grain strength of the top material. That would give some indication of whether or not that could be a plausible mechanism of top cracking. Admittedly, it's much easier to just argue one's opinion.

[Tom West]

Jeff I disagree with your reason for the cracks. Carry on.
Tom

[jeff crisp]

Originally Posted by Tom West

Jeff: My feeling about the popcicle brace is that its main use is to help prevent cave in at the sound hole should the top crack along side the fret board and the UTB also let go.
Tom

If the UTB let go, the guitar would have a large problem.

Originally Posted by Tom West

Jeff I disagree with your reason for the cracks. Carry on.
Tom

While there are always exeptions, especially when dealing with timber and string tension and treatment of a guitar. As far as the primary reason goes you are welcome to disagree with me, but many dont, including Charles, no matter how he trys to weave it.

Carry on yourself Tom.

[Alan Carruth]

I've always felt that the cracks along the sides of the fingerboard were caused in large part by 'shrinkage hysteresis' and the stress riser of the fingerboard glued to the top. We all know that wood shrinks as it dries out, and swells back up when the humidity rises. Recent research suggests that if the moisture cycling goes between the same limits (say, 90% to 30% and back the 90%) the wood never quite regains it's initial width, and shrinks a little further with each cycle. Ebony is not the most stable of woods, and it's strong enough to pull the spruce along with it. This tends to giver a higher stress in the upper bout along the fingerboard edge than you see in the lower bout, say.

From what I can see, the popsicle stick brace is there to provide sufficient glue area to keep the top from shifting inward under string tension when it cracks and you lose the (small, but real) shear strength of that area along the grain. Much will depend on what sort of glue was used: hide glue is pretty good in shear, so long as it's not shocked, but some of the modern glues will cold creep. I've seen Martin tops from the 70's that have shifted inward due to fingerboard edge cracks where the popsicle stick brace was still glued down, but the glue line moved . It didn't help that the inside surface of the top looked as though it had been smoothed with a garden rake...

A 'chin' extension under the fingerboard end might help reduce cracking along the edges of the board if it extends past the fingerboard on either side. I've used this sort of block for some time: so far, so good. I make it so that it extends 1/4" or so past the fingerboard at the upper end, and tapers to the same width as the fingerboard at the sound hole end. The taper helps avoid a stress riser along a single grain line.

A better way to prevent the neck shifting forward is to use an 'A' brace that plugs into the neck block on the upper end, and runs through the upper transverse brace to butt (or, better, inlet) into the upper arms of the X braces. This spreads the stress, and acts in compression. Again, I've been using this for some time. I've had at least two instruments take severe blows to the back of the neck from being dropped or knocked over, with no problems except the big ding on the back of the neck. Experience with guitars that don't have the A brace leads me to believe both of those would have resulted in top cracks and a shifted neck without it.

I will leave the discussion about the acoustic effects of the popsicle stick brace to those who can hear better than I can.

Shaving the fingerboard extension thinner will not effect the shear strength of the glue line between the extension and the top, and thus should not diminish any addition of stability that confers. It might if the board is made so thin that it buckles, but I think you'd have to go pretty thin to do that. Shaving the board thinner should, however, reduce the shrinkage stress and the size of the stress riser, so in might help.

Gotta go...