Can the "hump" be prevented?

[HurleyRidge]

I have a mandolin with a hump in the fretboard, where the neck joins the body. What caused this? Can it be prevented? What would it cost on average to get it repaired?

[sunburst]

I have a mandolin with a hump in the fretboard, where the neck joins the body. What caused this?

Different things can cause it.

Can it be prevented?

Sometimes.

What would it cost on average to get it repaired?

The price of a re-fret including planing the fingerboard ($300 give or take).

[Rick Turner]

Build them with carbon fiber reinforcement under the fingerboard from nut (or beyond up into the peghead) to fingerboard's end. That's what I do with guitars and it works. The hump or dip is one of the classic problems with traditional fretted instrument construction. We don't have to put up with that anymore; we have new materials to work with that can eliminate that issue.

[HurleyRidge]

Thanks. Is the hump caused by changes in humidity. For example if it had been left out of the case for a long period of time, hanging in a room?

[mandomedic]

It also possible that the neck angle has changed and the fingerboard extension is pushing against the top, causing the upward bump, so you may want to check that... Sunburst is right on by saying in essence that to correct it if the neck in not needing resetting, to pull the frets, plane the fingerboard, and refret... what brand and model is the mandolin? Kenc
www.kencradio.com

[HurleyRidge]

I'd rather not say what brand and model. Thanks for the posts though.

[MikeEdgerton]

OK, we had this thread locked down yesterday, let's refrain from having it happen again. It would appear that the OP has his question answered.

[Paul Hostetter]

Is the hump caused by changes in humidity.

No, it's a consequence of string tension, and fairly normal, more with guitars than with mandolins. John ("sunburst") had the cure right. Rick knows about the ounce of prevention, though I haven't seen that applied in mandolins.

[HurleyRidge]

Thank you, so from what I gather this is a common occurrence and is not the fault of the builder. It is too bad you can't string it backwards and reverse the effect.

[Rick Turner]

It is a common occurrence, and it's one of those things that should be dealt with with modern building techniques. Let's just say it's a traditional problem...for better or worse, mostly worse.

Those who do not know history are doomed to repeat it, and all that...

[8ch(pl)]

I had an el Degas import from the Orient. It had a hump right where the neck joined the body. nThe instrument was unplayable past fret 7 and even then with a high action.

When I finally got a new mandolin, I ran the el Degas througha bandsaw to look at the neck joint. Two quarter inch dowels held the neck in place.

[MikeEdgerton]

If you take a look at the Roger Siminoff book you'll see the same neck joint suggested for building an F-5.

[Big Joe]

The cure is a plane and refret. This can be often prevented in the building process by installing the neck and fingerboard and letting it settle before fretting. Most manufacturers and builders fret the neck/fingerboard before installation and this can be prevented by fretting after installation. It may still develop a hump in time with various conditions mentioned above, but much less likely.

[barry k]

I believe in the 50's and 60's a famous maker mandolins had the pinned joint, or at least some of them did. And the mandolin in question has a dove tail joint and no adjustable truss rod. Rick, are you saying to run a layer of CF fabric between the neck and fingerboard? Or on your mandolins are you using a CF rod to stiffen the neck. If it's the CF rod, where does the adjustable truss rod go? ,or does the CF material get imbedded on top of the adjustable truss rod where normally you would put a filler piece?

[Rick Turner]

I say run two 1/8" x 5/16" CF bars on either side of the truss rod (which will now be pretty much window dressing) from above the nut down to the end of the fingerboard. Dado them up into the fingerboard about 1/16 or 3/32" and down into the neck. You'll have the most stable foundation for a fingerboard surface you've ever seen.

The fingerboard doesn't even make contact with the top over the neck block in the body with this design, so it's much more like a violin in that way.

Bear in mind that I say this a primarily a guitar maker, not a traditional arched top mandolin maker, but the issues are the same. The foundation for the fingerboard goes through a fundamental and major change there at the body joint. It's like a dirt road going onto a wooden bridge that then ends on a granite cliff.

Paul saw the guitar I built for Henry Kaiser right after it came back from what is one of the worst climates on Earth...Antarctica. There were no playability problems with that instrument, and there haven't been since I built it. I see no reason why the principles used in it's construction wouldn't work just great on a mando. The fingerboard doesn't touch the top; it's fully cantilevered out over the top and it's supported by the carbon fiber bars. Miniaturize the design and it would be great on a mandolin.

All this refret and plane it stuff is a band aide on a built-in problem. It's "fixing" something that doesn't have to be wrong in the first place.

Of course I'm suggesting moving on from the '20s Loar design. I suspect he would, too, if he were still alive. I know that we love to crystalize things in 1923 as the be all and end all. I just think we can look more objectively at some of the problems that we now see and deal with them.

[sunburst]

The (pretty much) inevitable "hump" or "rising extender" is pretty consistent within a given instrument design. It can be anticipated and accounted for during the building process and well controlled without the use of CF.

It doesn't really have anything to do with the type of neck joint, by the way. As long as the neck joint is well designed and well done it will be solid. There is something at work where the neck becomes the neck heel that has a technical name that I can't remember (a structural engineer friend of mine told me what it's called, and I tried to remember so I could sound smarter when I'm trying to explain it, but the truth is I'm not smart enough to remember), but the bending forces of the strings pulling up on the neck become somehow concentrated where the beam that is the neck is cantilevered from the heel and body of the mandolin, so the neck bends a little bit right there. The extender doesn't really rise, it just looks like it when you sight down the neck because the neck has "bent" up just before the heel. In my mandolins, that's only about .015", and by milling a .015" or so drop-off into the neck/extender beyond the 12th fret, it all straightens up after the string tension "soaks in".

[Rick Turner]

John, that's like saying that there's an absolute and inherent design flaw in these instruments; we can't design the problem out; so let's compensate on every one.

I believe the basic problem can be eliminated by making a fingerboard and CF structure that basically overpowers the problems. I also think that attaching the fingerboard to both the neck and the top is a fundamental flaw in the basic design. You sure won't see that on violin family instruments...

Look, there are a ton of things we can repair or compensate for. Why not fix the basic problem instead of living with it?

Oh, I just remembered; it's because nothing past 1924 is allowed...

I'd like to live in Loar's footsteps, not under his gravestone.

[bryce]

Rick, In a 1 1/8" (at the nut) wide mandolin neck would you still think it safe to use 1/8"cf + 3/16"rod +1/8"cf =(7/16") of neck reinfircement? Or would you leave out the 3/16" adjustable rod. I am playing with the thought of using 2-1/8"x3/8" cf bars,but don't know about putting a 3/16" rod between them because of all the wood I'd be removing.
But I'm not sure some customers would like the thought of not having an adjustable trussrod. Thanks

[sunburst]

David, I've done that; 1/8"X1/2" CF | 3/16" steel | 1/8"X1/2" CF in a mandola neck. The "adjustable" rod is just along for the ride in between those pieces of CF, but it all fit. That neck was 1 3/16" or 1 1/4" at the nut IIRC.

[sunburst]

John, that's like saying that there's an absolute and inherent design flaw in these instruments

Yep.
We can compensate for it or overpower it, but there it is either way.

[Rick Turner]

Well, how often does anybody really adjust the rod in a mando neck? Maybe the thing to do is just go with a nice big chunk of CF in there, dial the relief you want into the fingerboard before you fret it, and be done with it all for good. Normally with guitar necks I like the adjustability, but maybe the trade off of lnot having one would be ultimate stability, no hump, and perhaps better sound. Many guitar makers have noted that they prefer the sound of guitars with CF reinforced necks, and on the few vintage guitars into which I've put CF rods I can say that they sounded better with the CF than before they did without it. The best was a Dyer harp guitar that was a nice guitar pre-CF and an incredible one afterwards.

[Rick Turner]

The point of this thread was "can the hump be prevented". I believe it can, and I think that as long as you have a flexible hinge point at the neck joint, you'll have a problem. But overpower that hinge...which is only made worse by fret slots...and you've addressed the issue of preventing the hump. Sure you can plane the 'board, and it will probably be fine for a few years or until the instrument goes through some humidity changes. Then the problem comes back. It sure does with guitars. So throw some CF in there literally as a part of the fingerboard structure, and you've greatly improved the stability of the playing surface of the instrument.

My guitar fingerboards made this way are literally stronger and stiffer than the neck itself. It turns the whole notion of where the strength and stability lies upside down and puts those qualities closer to the strings where it does the most good from a playing point of view. I've not tried this with a mando yet, but I can't see why it wouldn't work just fine and take care of the issue this thread is all about.

This is an outgrowth of a patent I got when I worked for Gibson. That covered making the fingerboard out of CF in a roughly "T" cross section with either a single or double web...the double would allow a truss rod. We made a few prototypes, and they were fantastic. Political issues got in the way of production, shall we say...

[bryce]

Thanks John.

[Rob Grant]

8ch(pl) wrote:
"When I finally got a new mandolin, I ran the el Degas through a bandsaw to look at the neck joint. Two quarter inch dowels held the neck in place."

To which Mike replied:
"If you take a look at the Roger Siminoff book you'll see the same neck joint suggested for building an F-5."

I've use the mortised joint that Roger Siminoff has in his book without any problems. The two dowels are only used to "lock" the mortise. Properly cut and glued the neck literally becomes part of the headblock with this joint. I believe 8ch(pl) is actually referring to two dowels alone holding the entire neck to the headblock like in my last thread: "Junk repair"

http://www.mandolincafe.com/forum/showthread.php?t=47822

You'll definitely end up with a "hump" in the fretboard when this substandard method of attaching a neck fails.

Check around the "heel button" and see if there is any sign of cracking or separation. On a properly built mandolin or mandola that button is suppose to be part of the back plate. I don't know how this "junk" ever made it on the market.

[John Arnold]

There is something at work where the neck becomes the neck heel that has a technical name that I can't remember
It's called a stress riser.

on the few vintage guitars into which I've put CF rods I can say that they sounded better with the CF than before they did without it.
One thing we can agree on is that CF will change the sound. Whether it is better is a matter of opinion. One of my best guitars was one with a nonadjustable CF reinforcement. But like it or not, we guitar makers have the 1937 D-28 as the 'standard', and mandolin builders have the '23 Loar.

[Michael Lewis]

'37 D 28 and '24 Loar. Great stuff "inside the box". I love making the traditional designs but also want to push the envelope a bit from time to time, so I like to stand on that box and reach a bit higher.

[barry k]

Still trying to wrap my brain around all that stuff that is suggested to be put in a tiny mandolin neck. I Agree with a fellow poster , that if there is no adustable truss rod present, it raises some doubtfull questions on the customers part as to why its not there. Thats why I had only built 2 "A" models without an adjustable truss rod and all my "F"s and 2 points have them. Really didnt know that this "hump" was such a inherant problem with mandolins though, I had only seen it on 2 occurances. Dont know how many mandolin builders are out there now, a lot more now then when I first started, but i think we all do things fairly much in the same manner, and just adding in our own little quirks or secrets to the mix. We must be collectively convinced that the original F5 style is our gold standard, because not very many stray off that path. I personally strive to put out a structurally sound product, never short cutting or taking chances or experimenting at a customers expense . I have been fortunate and blessed to be allowed to build 90+ instruments with very few problems, and if my health and my hands hold up, will continue to do so.

[Rick Turner]

As did Martin, Gibson, and Loar, Michael. Our task is to keep on their path, not to be stuck at the roadside rest stop. It becomes a graveyard eventually...

[8ch(pl)]

My el Degas had no mortise at all, the 2 dowels were in the end of the neck and into the endblock. In other words, the neck was pegged onto the body.

[Stephanie Reiser]

John's advice on the treatment of the hump is solid. I've planed and refretted several, and just this weekend did another, to remove that hump.
I'm in the middle of my first CF neck instrument. THis is a first for me. I have a 12-string guitar coming up next that will have two CF bars in it.

[Big Joe]

We have dealt with many many fingerboard humps on guitars as well as mandolins. It is not unique to mandolins but just as common in guitars. First, Rick has some valid points that it can be dealt with in most cases with the way it is built. I don't know that one has to go to exotic means like CF to accompish this. It can be done without sacrificing convention. Still, CF is a great material to work with and is able to accomplish things that might otherwise seem impossible. We have been using CF for various things in repair and building for nearly two decades. We have found it to be sonically invisible for most repairs. We often work with the cloth itself and go from there. IT is a wonderful product for stabilizing poor necks and can be done without losing the truss rod.

The issue is two fold. First, can a mandolin...and by extension...a guitar be built without the neck/body hump. The answer is YES. At the very least it can be postponed for a very long time with the way the instrument is built. This can be done with either CF or without CF. It is unfortunate that so many mandolins...even new ones...have this hump. It is not complicated to repair, but the only way to correct it is to plane and refret the instrument. This can be a good time to change the fret size or to have custom inlay work done or to have the board radiused or flattened. The good news is that it is not a permanent problem.

There is a lot of good information on this topic in this thread. We have a great group of luthiers who each have different experiences in dealing with these issues and the open discussion helps not only the readers to learn, but also that each of us may garner a bit of new information from one another. Great question and great answers.

[Bill Snyder]

If you take a look at the Roger Siminoff book you'll see the same neck joint suggested for building an F-5.

Probably not. The El Degas probably had a doweled neck joint like this which is not anything like the Siminoff joint.
Siminoff's joint just uses dowels to help lock an otherwise good joint. The drawing below relies entirely on dowels to hold the neck on and in place.

[TomTyrrell]

I've become convinced that adjustable truss rods aren't really such a good thing for mandolins. On a banjo there is something around 15 inches of "thin" section of the neck from where the heal thins out to the nut. On a mandolin it is more like 5 or 6 inches. If there is a hump around the 12 - 17 frets the truss rod isn't going to be able to fix it.

I just think a nice stable reinforcement that the owner can't screw around with is a better solution for mandolins.

[MikeEdgerton]

Probably not. The El Degas probably had a doweled neck joint like this which is not anything like the Siminoff joint.
Siminoff's joint just uses dowels to help lock an otherwise good joint. The drawing below relies entirely on dowels to hold the neck on and in place.


I sit corrected.

[Rick Turner]

Both Orville Gibson and Lloyd Loar sacrificed convention...Let's be part of their innovative tradition. No, don't throw out the great stuff they did, but rather think more like they did...keep pushing forward. The results will show in tone and playability. Look objectively at the F mandolin and preserve all that works while using the past 85 years worth of advances in materials technology and engineering knowledge to make those subtle improvements that will result in your instruments representing a new golden era of both respect for the past and acceptance of the future. I use hot hide glue where it seems best...and epoxy and CF where they seem best. I like trying to understand the best of all eras and combining them.

One of my jobs as a builder is to reduce the workload on repair luthiers of the future...sure, most of us reading this thread can plane and refret a fingerboard, but should we have to in order to compensate for an inherent weakness? Refrets should be to replace worn out frets on a great playing instrument.

CF is not terribly exotic these days. Yes, it is a drag to work with, but so are some of the woods we work with, to say nothing of finishes! Note that celluloid binding and purfling were once exotic materials...as were steel strings at one point.

[Rick Turner]

Both Orville Gibson and Lloyd Loar sacrificed convention...Let's be part of their innovative tradition. No, don't throw out the great stuff they did, but rather think more like they did...keep pushing forward. The results will show in tone and playability. Look objectively at the F mandolin and preserve all that works while using the past 85 years worth of advances in materials technology and engineering knowledge to make those subtle improvements that will result in your instruments representing a new golden era of both respect for the past and acceptance of the future. I use hot hide glue where it seems best...and epoxy and CF where they seem best. I like trying to understand the best of all eras and combining them.

One of my jobs as a builder is to reduce the workload on repair luthiers of the future...sure, most of us reading this thread can plane and refret a fingerboard, but should we have to in order to compensate for an inherent weakness? Refrets should be to replace worn out frets on a great playing instrument.

CF is not terribly exotic these days. Yes, it is a drag to work with, but so are some of the woods we work with, to say nothing of finishes! Note that celluloid binding and purfling were once exotic materials...as were steel strings at one point.

[TomTyrrell]

<< Both Orville Gibson and Lloyd Loar sacrificed convention >>

I don't look at it as they sacrificed convention, they murdered it with malace aforethought. Both used a lot of what was then state of the art technology, it seems only logical to continue improving the instruments with state of the art technology.

I often wonder how Gibson and Loar would react today if they saw so many brand new instruments being made in the same way they were made nearly a century ago.

[bryce]

This may not be the same thing, and please excuse the drawing.
Some of the Hump problems I've had brought to my shop were not really humps, but too much relief in the fingerboard. Some were in fact fixed with only tightening the trussrod. I guess the ones your talking about are more severe. These folks heard a buzz, and sighted down the fingerboard and saw what appeared to be a hump. I tightened the rod and raised the action back to the now flatter fingerboard and all was well.
I have worked on just as many that had to be planed down (or at least had to level these frets)but thought I'd add this as a first try for people who don't have the tools, etc to fret and may not be close to a luthier.

[sunburst]

Just a couple of related thoughts;

New technologies are not always an improvement. CF can certainly make necks stiffer and more stable and can improve especially long necks like guitar and banjo necks. I use it sometimes when I feel it will help, but I don't like working with the dirty, nasty stuff, so I seldom use it in mandolin necks. I prefer adjustable necks rather than stiffened necks most of the time, and CF can reduce adjustability. Mostly, though I don't feel a need for CF in my mandolin necks. I choose my neck wood carefully for stiffness and stability, I understand about the stress riser (thanks, John :) ) and I have no trouble anticipating and controlling it. It's common engineering practice to understand ones materials and build to accommodate the materials response to the stresses it will undergo in use. My mandolin necks are consistently straight and true and nearly never need adjustment, so I don't see that adding CF to them would be an improvement. I don't suggest that my way is the only right way, each builder is free to use what works for him/her, I'm not stuck emulating the Loar mandolins, but sometimes I like to use the "if it ain't broke, don't fix it" approach.

And one more thing. About a year ago I replaced about 10 frets in a 1924 loar (with replica fret wire, thanks Dan). The frets from 11 to 29 were original, that part of the fingerboard hadn't been planed in 84 years, no fingerboard hump, no CF in there and none needed.

[swampy]

I don't want to hijack this thread, however, I have the same problem right now, and I'm ready to replane the neck.

How exactly do I go about doing this without chipping the fret-slots. Is this actually a sanding job, or do you use a plane?

[Paul Hostetter]

Planes seldom work because most boards have pearl dots. Not to mention the fret slots and chipping and so on. I have always done this with sanding blocks and a straight-edge. But if you're asking questions like this, I hope it's not a very valuable instrument you're poised to practice on. Frank Ford probably has a good page on refretting at www.frets.com.

[swampy]

Thanks.

I have a lot of woodworking experience and a little luthiery experience so I feel confident I can do the job, or at the very least fix the many errors I will probably make. And no, it's not an expensive mando, just my stew-mac project.

[John Arnold]

I like to plane fingerboards rather than sand, but it is not always possible, due to the grain. IMHO, the frets slots do not complicate the planing. If anything, they make it easier. You need to remove the dots because a hand plane will chip those. Removing and replacing the dots is frequently done anyway, since the surfacing may make them too thin. A hand scraper is also something I use quite a bit on fingerboards.
Sanding is perfectly acceptable, but I try to avoid it as much as possible.

[D.E.Williams]

To augment what Rick has been saying...

I build two 1/8"x3/8" CF bars/rods into my guitar necks. What I've noticed since switching to that is that the stiffer neck also seems to provide better sustain in the instrument, and also seems to make the tone of the neck consistently good up the entire length of the neck. No dead spots. I plan to do the same with any mandolins I build, if I build any beyond what I plan on building for myself.

The next comment is partly just my opinion...you can weigh its value if you wish, and throw it out if you find it to not apply.
I can't speak to mandolins, but at least with guitars if you don't get the neck/body joint to be perfectly supplementary (180 degrees), you've built that hump into it right from the start. Proper neck relief should be barely visible by sighting down the length of the neck, but measurable in conventional ways with straightedge and ruler etc. I agree wholeheartedly with Rick (nothing new there) about using newer techniques and technology to improve on older designs, as it's probably in the same spirit as what the original designers were all about too. If you run the CF up into the fretboard, and right through into the fretboard extension, you eliminate the eventual possibility of that hump. At least, probably within your lifetime. As to the planing and refretting, I would think a neck reset would be more in order, as I believe the planing is only a band-aid approach to a symptom of a problem, not that it is a bad option necessarily. Same thing with guitars, planing fretboards and bridges etc, will only put off temporarily what will be necessary in the future. And then when you reset the neck, the bridge and the fretboard will often need to be replaced. Don't fear a reset if your mando needs one.

[sunburst]

Don, mandolins aren't guitars. Neck re-sets are almost inevitable in conventionally built flat top guitars because of the cumulative effect of string tension on the body. That doesn't happen with carved mandolins, normally, for several reasons. Mostly, it's the arch and thickness of the top and the adjustable, movable bridge. A properly carved arch is very stable under string load and seldom distorts from constant string tension. A typical mandolin can work well with the bridge adjusted within...probably a 1/4" range of height, and well made mandolins normally don't ever need neck re-sets if they aren't damaged.
The OP's mandolin most likely does not need a neck re-set, and adjusting the bridge is not like shaving away a flat top guitar bridge to lower the action.

[D.E.Williams]

Points well made, and accepted. Although I wasn't drawing a comparison between a guitar bridge and a mandolin bridge. I was merely saying how it wasn't an ideal way of dealing with a guitar that has moved toward that inevitable reset. I'm certainly no expert with mandolins (read - don't know much at all)...but I have to think that this hump could be dealt with differently than planing the fretboard...and unless I'm not getting this at all, I suspect this hump is at the intersection of the fretboard and the extension, so perhaps if the fretboard extension and neck were built to be integral, this could all be avoided entirely, at least for a mando such as an F5 style that has the extension.

[Rob Grant]

D.E. Williams hit the nail on the head when it comes to my experience with "humps" in mandolin fretboards...
"I suspect this hump is at the intersection of the fretboard and the extension, so perhaps if the fretboard extension and neck were built to be integral, this could all be avoided entirely, at least for a mando such as an F5 style that has the extension."

The base of the extension sits directly over the headblock (with the top in between. If the head block/neck joint shifts in anyway, you'll get a "hump." This really has nothing to do with trussrods or neck stiffness, it has to do with lack of integrity between the neck and the headblock. All the "humps" I've seen on mandolin fretboards have been caused by faulty neck joints.

You can actually produce a hump on a "junk" join by performing a bit of "instrumental torture" on a cheap instrument (see photo).

[Rick Turner]

Note that when we say "plane" we really mean "make plane" or at least "make true" by any method that works. Most people do the job by sanding.

[Nick Triesch]

Several years ago I took a very nice late model Flatiron F5 Artist to a very well know guitar/instrument repair shop here in San Diego. I wanted them to look at the "hump" that you folks are talking about. The head guitar maker/repair person told me that most of the mandolins that have come into the shop over the years had the hump. He told me it was the way mandolins were constructed plain and simple and they just showed up! . But he also said that in most cases it was just a hump and did not interfere with playabilty. He was right! My mandolin was set up with very low action and did not buzz. He said not to worry about it unless it gets really bad someday. So if you think about it the hump is really normal for most F type mandolins and it usually happens with in a few years...not over many, many years. Even if they cost several thousand bucks! I'm ok with the hump!! Nick

[sunburst]

...I suspect this hump is at the intersection of the fretboard and the extension...

The "hump" I normally see in both guitars and mandolins starts at about the 12th fret, or whatever fret is about 2 frets clear of the body, and is a result of the stress riser where the heel of the neck starts. It is not at the neck joint.

[Rob Grant]

For an example, here is a photo of my last "hump" (Down lads, I'm referring to a fretboard!<G>)

It does start at the 12th fret, but in the case of this cheap mandolin which uses two dowels alone to fasten the neck to the body the 12th fret is actually directly above the point where the neck joins the body.

This is the same instrument covered in "Junk repair" http://www.mandolincafe.com/forum/showthread.php?t=47822

[D.E.Williams]

The "hump" I normally see in both guitars and mandolins starts at about the 12th fret, or whatever fret is about 2 frets clear of the body, and is a result of the stress riser where the heel of the neck starts. It is not at the neck joint.

Not disputing that John, but I think we may be talking about two different kinds of humps. if you're seeing it at the 12th fret of a 14 fret instrument, then it's more likely a result a different hump than what I was referring to.
There can be a hump-ish thing happening closer to the 12th fret if it's a result of an overly tensioned truss rod. If it's going to cause an issue in the neck, it will move at the point where the curve of the heel is supporting the neck, or more precisely, just before it, since the neck is weaker right before going into the heel. The stress is going to take the path of least resistance, which means that the heel, which offers far more resistance than the neck shaft will stop the backward movement in that location. Hence a hump.
It could also be a result of what's actually happening at the 14th fret neck / body joint. I'm sure it can easily telegraph out to the 12th, considering it's pretty tough to bend something as rigid as a guitar neck at a single point without it affecting the surrounding materials. The 12th fret has a ton of "meat" of the neck and heel under it to support it in that location. The biggest stress riser is at that neck/body joint. You have the tension of the strings trying to rotate the neck around the edge of the body under the 14th fret, and you also have the truss rod pulling the neck back. (A good bit of the truss rods tension is concentrated at the 7th fret, where most warping & bowing take place, but that's not crucial to the conversation per se). Part of the hump is usually a neck that is set back too far, or if it's been on awhile, the natural caving inward of the top in the fretboard extension area from string tension. That location, the neck/body joint is essentially a fulcrum point. Now if we want to get into Moments of Inertia and all that, I would have to go back and dig out my old college texts...but we really don't need to go there! Anyway...... most humps that I've personally seen are more from an overly-aggressive neck back-angle, or from the top caving in toward the sound hole over time.

On another note...since I'm redesigning the F5 on my own in my CAD system, as so many before me have done, since thread is giving me some thoughts about doing what Rick has suggested. I'm thinking of designing the neck and fretboard extension support to be one unit, or at least combined in such a way as to eliminate the possibility of that hump. Not sure exactly what it's going to entail just yet, but I have some ideas.

[Paul Hostetter]

It is not at the neck joint.
But it's really close. C'mon!

[Stephanie Reiser]

The "hump" I normally see in both guitars and mandolins starts at about the 12th fret, or whatever fret is about 2 frets clear of the body, and is a result of the stress riser where the heel of the neck starts. It is not at the neck joint.
John, are you saying that the heel wood expanding across the grain, i.e. up, (just before the joint) is what causes the hump? If so, this makes alot of sense to me.

[bryce]

Rob, I'm still confused. What I see in your last picture is what I call the dropoff. I start a slight taper downward in my fingerboards from the 14th fret toward the bridge. If the neck has any relief, this will appear as a hump when sighting (or straight edge)down the fingerboard. Since my fingerboards are usually very flat, this causes no problem. I guess this is a different "hump" than what you are referring to.

[sunburst]

John, are you saying that the heel wood expanding across the grain, i.e. up, (just before the joint) is what causes the hump? If so, this makes alot of sense to me.

Not really.
It's not easy to explain, apparently, and I believe we're talking about a few different things in this thread and calling different things "humps". What I see in most necks is a bending forward of the neck shaft concentrated at the place where the neck shaft curves into the neck heel. If you sight down the neck, it will appear as a rising of the fingerboard extender, starting at about the 12th fret on an F5 (or similar). It is the reason for the fingerboard "drop-off" commonly milled into guitar fingerboards, and the reason I mill a similar drop-off into my mandolin neck/extenders. In guitars, like Martins, where the neck angle is such that the fingerboard extender has to be deflected down to glue to the top, it will sometimes look like a hump at the neck joint (14th fret), but milling the 'board straight from the 12th fret to the end will make it look like a simple drop off, and the stress riser will straighten it out, at least to some extent.

[Rick Turner]

It often is a real hump where you have relief in the neck from nut to about where the heel starts it's descent away from the fingerboard and then you have the drop-off from the general area of the heel/neck block to the end of the fingerboard.

And sure, you can "plane" it level, but then it's likely to reappear some time later as humidity changes and string tension win out over what you've done.

I still maintain that if you do a decent job of supporting stability in the fingerboard surface...however you do that...and make that support more or less independent of the changes happening below the fingerboard, you can solve the whole issue from the get-go.

There's always a repair solution, but good engineering along with respect for tradition (not blind adherence to it) can solve some of the problems that show up in instruments years after they've been made. A lot of what I do is to study what goes wrong in 10 or 25 or 50 or 100 years with the instruments I love, and then try to figure out how to retain the good qualities while designing out the problem-causing features. Much of this involves trying to mentally separate what parts of the instruments contribute primarily to tone and which are more about structural integrity. Hence my use (for instance) of both hot hide glue where I think it's right and epoxy and carbon fiber where I think they are best.

I respect historically accurate copies and all; I just think we can do better, even though it may take another 75 years to prove that.

[Rob Grant]

bryce wrote:
"Rob, I'm still confused. What I see in your last picture is what I call the dropoff."

Don't worry, I'm confused too!<g>

This in the "hump" that I've seen in several mandolins with faulty neck joints, but I don't think it is necessarily the "hump" that John and Rick are referring to. Probably "dropoff" would be a better term for the situation I've shown in the two photos. You can still easily play the instrument in the photos, it's just that the action is a bit higher then it should be from about the 16th to
20th fret.

Basically I created this "Dropoff/hump" when I stressed the neck into a proper rake angle without removing the fretboard. (see "Junk repair" http://www.mandolincafe.com/forum/showthread.php?t=47822 ). The owner didn't want to pay for the extra labour and material costs needed to level the board.