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I haven't had one crack that I am aware of YET but it seems like a good way to deal with the problem of inconsistent rates of wood expantion in that area might be the bolt on fingerboard extension. I have done a couple of them but currently I have been glueing the fingerboard down.

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Tickle your guitar daily, and it'll tickle you back.

Todd,

I agree fully with this. That's why I use free floating fully supported necks, with an adjustable neck joint and with a neck block supported by flying butress cf braces. Read and see all about it here: Adjustab;e Neck JointDave White38869.459849537

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

   I don't buy into blaming these types of cracks on humidity and it varying effects on different species of wood and their respective grain/saw orientations. I say this simply because we see so few of these types of cracks. If they were more common across the board and to more builders guitars, I may consider it more of a possible culprit. I've never had one develop on one of my guitars and haven't seen them on any guitars from a arge number of other builders.

   I believe these types of cracks are caused more as a result of structural issues than expansion and contraction from humidity changes.

   I've repaired enough of them to know that string tension and its effects on the neck/body interface and all angles involved in keeping it correct may be a more viable consideration as a cause.

   As the strings pull without mercy on a guitar's assembly, several things happen to cause the distortion of its structure to occur. These things are common to all guitars from every builder whether a small/solo shop or the largest production facilities. It's virtually impossible to build a guitar that is impervious to these effects without jeopardizing tone and volume with too much mass or weight being added.

   First,the neck begins to surrender to the tension both along its length and at it joint to the body. This is the movement that is most commonly blamed for the eventual need for a reset on many guitars.

    Truss rods are used by most builders to easily and efficiently counter the effects of tension along the neck's length, but the neck joint is an assembly integrating the neck and body in most cases so quick practical adjustment isn't possble. There are a few builders offering quickly adjustable neck joints with brilliant innovations in design and assembly, but outside of those few builders, a neck reset is the coomon course of action to correct the effects of tension at the joint.

   Secondly, the arch in the back begins to flatten, although minimally, along with the movement of the neck as it is used as a long lever by the strings to bend the entire guitar into submission to it.

   These are the two effects of string tension that are probably most to blame for the cracks adjecent to or at the end of the fingerboard tongue. As the neck pulls forward, the unsupported fingerboard extension, whether Ebony, Rosewood or another hardwood, can exhibit its superior strength or stiffness to that of the Spruce or Cedar top as it gives in and loses the battle of the two wills and cracks. The cracks can occur along the length of the fingerboard tongue or, commonly, from the corners of that tongue and run right into the soundhole.

   The other areas affected by the string tension that don't affect the area near the fingerboard extension are, of course, the bridge as torque is applied with the saddle being the lever and the the fron edge of the the saddle slot working as fulcrum to twist and distort the top. This is the movement that shows it effects both in front of and behind the bridge.

    Behind the bridge, we commonly see the belly or hump that forms and in front, the top may dip considerably toward the soundhole. In other cases, the bridge or the glue joint between it and the top can fail and it can begin to lift as the strings try to tear it off of the top. I've had guitars here from the builders at the very pinnacle of the industry that are showing these effets clearly and obviously. Some of them cost more than $15,000.00 or are valued at much more, but string tension doesn't pay attention to price or brand name and just has its way with every guitar eventually.

    Someting will always move under the tension of a set of strings. Guitars from different builders will show different effects and to different degrees, but there's a battle going on between strings that are fighting to pull and components that are fighting to stay put and something's got to give in....at least to some extent and I believe that the level of tension offered by a set of strings is a much more formidable foe than humidity and its effects on woods.

    I say all of this assuming that the woods being used are dry to within acceptable parameters and the instruments are build in a stable environment that is with what have become industry standard parameters for temperature and humidity.

Regards,
Kevin Gallagher/Omega Guitars

Good stuff, Kevin, and I agree about the battle between the strings' pull and this lightly built device we call a guitar.

But I'll dissagree on the main culprit for the extension cracks. I see these in winter only up here. When the temperatures drop into the negatives and stay there for 5 months at a time, home heating systems can and will drop the RH to single digits overnight. And without fail, I get phone calls regarding cracks beginning in December...

I've also seen the fretboard shrinkage on plywood topped guitars; where these won't crack, we see a gap between the polyester finish and where the fretboard now resides, and this gap can often be as much as 1/32" on each side. Had that been a solid top, she'd have cracked. The shift is always sideways on these, and never is the fretboard moving towards the soundhole as it would due to the string tension. At least, I haven't seen it yet.

Many of these cracks do result from the neck block tilting back into the top ever so slightly, and some from an obvious fall, yet others from heat problems(the car trunk), as you speak of. But I see way more of them come winter and the dry months...

I haven't had a single one on mine, but 5 or 6 of the lightly glued extensions have popped loose, and most of these also showed the signs of being overly dry. Only one that I know of popped loose before its time, being only a few weeks into its new owner's hands, and while he was most unhappy and quite put off that his new guitar had "come apart", when I explained why I glued it lightly, and after we fixed it, he was okay with the idea of the "fusible link".
Mario38869.5290972222

[QUOTE=ToddStock] In a discussion over on the MIMF, Mario and others have opined that the chief reason for those soundboard cracks adjacent to the fretboard extension is the mismatched humidity-induced expansion rates between spruce soundboard, ebony fretboard, and mahogany or ply neck blocks, rather than poorly supported fretboard extensions.[/QUOTE]

I'm inclined to agree with this. It is my understanding that ebony's expansion/contraction rates due to humidity changes is much greater than that of spruce or WRC.

In my case, the evidence is anecdotal, and specifically refers to classicals. I bought a Jose Ramirez 1a in 1974. It had a WRC top. Several months after purchasing it, I noticed a fine crack in the top, following treble side of the fingerboard. I showed it to the proprietor of the shop where I bought it, and his repair guy. They both told me that it was minor, and nothing to worry about. Well, being a young fellow who'd just spent a big chunk of change on a top-of-the-line guitar, I was still concerned, but I trusted these guys, so I decided I'd just wait and see what happened.

Well, I waited. And I waited. Thirty years later, I sold that guitar, unrepaired crack and all. It had not opened up any further. It was completely stable.

My point here is that the guitar was much too young, IMO, for the crack to have developed due to stresses on the top. In the 30 years I owned it, the guitar had not changed significantly at all in a structural way. E.g., the action was exactly the same as it was the day I bought it. Thus, humidity changes seem to make the best sense to me.

I have owned many classicals since I bought that Ramirez, several even older than it. None of the others have developed a crack along the fb extension. Why? I have no idea. A couple have developed splits in acoustically active areas of the soundboard despite being kept in humidity controlled environments, which I consider much more debilitating than a hairline crack along the fb extension. Why? Wish I knew. But neither of these guitars have cracks along the fb extension. Go figure.

I remind myself I'm trying to understand the specific properties of a specific set of woods, and that each and every one is different in its own specific way.

Best,

Michael

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   I have to throw in my 2 cents. I agree there are some expansion differences between the 2 woods but the reason of the cracking isn't from any one reason. Most modern guitars ( Martins especially ) have the top opened under the fretboard for the truss rod to drop. That separates the expansion issue and they will crack rarely but those that do often will also have alignement at the rosette pushed so that tells me the neck forces where the reason not expanion.
   I have seen some crack where alignment of the rosette is intact and that would tell me the expanion is the culprit so there are more than one issue to content with.
   I do like the A fram structure as it supplies more stability in the area. I can't say one argument is more correct than the other but I can and have seen points for both sides. The few top expanion cracks I did see were from fretboards that were less than quartered.

whilst i can see mario's point, my relatively few years in a similar climate were not enough to convince me that it is the main cause for cracks in this area.

rather, most of the cracks of this area that i have repaired have been clearly attributable to structural problems. while most have needed a neck reset at least, many have also involved loose and even shifted upper transverse braces, loose necks, backs which had come unglued, evidence of impact, etc.

I believe both Kevin and Mario are correct in their assumptions. All environments create different issues to the instrument. With all things being equal Kevin has identified the forces at the root of the failure. ”The relentless pull of the strings”. However Mario has identified an environmental condition that compounds the tendency of failure. The combo of the environmental changes combined with the pull of the strings may very well lead to more of these problems than in the winter.

I look at it this way Kevin has properly identified the major force involved and Mario has identified an environmental condition that the failure is most prevlent.MichaelP38870.4597453704