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This is off-topic for this thread, but I'm a big fan of non-adjustable necks. Every guitar and mandolin I own is solid, non-adjustable. All are perfect and have remained so. Martin did not use an adjustable truss rod until 1985, and have said that they experienced less warranty work and complaints with the non-adjustable necks.

But I don't believe CF on its own is stiff enough; we need steel.

EDIT: the issue with the guitar in this thread, and others like it stems from extremes in humidity, made worse by overly thick ebony fretboards that may not have been properly seasoned, and very likely not built in an ideal shop RH environment. My own(personal) instruments will go down to, and remain at, 30-35% RH in winter, and up to around 60-65% RH in summer, with hours of exposure to 90%+ at late-night jam sessions at festivals. The necks have remained stable. I have a 1951 Martin 000-18 that is strung with mediums, and sees the same environment as my other ones, and it still has a dead-flat neck. 61 years, no movement!

My FB came dry and stayed here for a couple years. Glued it at 45%. I have other guitars "out there" and they are fine. The only thing different was that it felt less stiff than usual, i would guess because the side grain was curly.

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Build log

It that case it is most likely that you unfortunately had a piece of wood that was truly unruly. Perhaps also paired with a neck that was also slightly weaker and/or more prone to movement than typical. Especially if it was truly well seasoned and built well, and the others you have built are doing fine. In this case, the best suggestion is to not over-react and change what you're doing, just because of this one odd failure. Adding some CF or steel to your necks is still a good idea, and will add insurance, but don't get too caught-up in trying to make them, and everything else, so strong that your instruments will suffer a weight and/or tone penalty.

We can do everything "right", but when using natural materials with as many unknowns as wood has, we can still get bit. That's why we provide warranties and must be prepared to replace/repair our instruments when the inevitable happens.

Grumpy, do you perceive any "down sides" associated with using an adjustable truss rod? Just curious, thanks again for your thoughts.

Best,
Trev

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http://www.PeakeGuitars.com

Customers using the adjustable truss rod to adjust the action is a major pet peeve.....

I also firmly believe that a stiffer, solid, non-adjustable neck leads to a better sounding, more powerful instrument. I've seen it first hand when changing necks on guitars, and I'm not the only one who noticed.

When I use an adjustable rod(the one way, aluminum U channel "Martin style" rod), my goal is to build the neck such that the neck itself will need minimum tension on the rod, so much so that most of them are only finger tight when they leave here. As such, they behave much like a solid, non-adjustable neck, yet still have the ability to be adjusted if necessary as the years pass.

Bear in mind that we are talking about two different animals here.

The steel string guitar with twice the string tension and a narrower neck
The classical guitar with low string tension, wide and thick fretboard.

The steel string guitar, with the extra string tension, unless you really screw up the construction or put it in the shower is not likely to backbow at all, and any reinforcement or adjustable rod is aimed at limiting excess relief.

Classical guitars have been built for centuries without neck reinforcement and generally hold up well against string tension.
Unfortunately player demand for ebony boards does expose them to movement with humidity and without higher string tension this can lead to backbow.

My preference now is to use more stable woods for the fretboard on classicals and to use a double action rod.
My last few builds have used african blackwood and ringed gidgee.

I have bought one of Mark Blanchard's adjustable 2-way trussrods for my first build, and a fine piece of engineering it is too ...

I just have a gut feeling, however, that a simple 3/16" compression rod, installed in the neck blank and tensioned up to give a back bow, which could then be planed out, prior to fretboard installation , might be an optimum solution ... the string tension would equalise the truss rod compression (which could of course be adjusted) , so the neck would be in an ideal state of equal tension from both sides ...

I may be totally wrong here, however, and if I am, I welcome any elucidation from more experienced builders ..

Mario, I used the default figures for Hog and Ebony in David Hurd's spreadsheet when I last did the calc. as I wanted a third party source for the theory/code. I converted Hurd's spreadsheet to SI units (so it made more sense to me!!) and iirc the default values for hog and ebony are pretty close to what I measure as typical for the species. Anyone who's interested can download Hurd's spreadsheet from here and put in whatever numbers you like. It's easy to configure his model for different woods and sizes of CF bar. Hurd's is a uke site, so remember to change all the relevant dimensions to guitar sizes. The calc. I did was for a typical hog/ebony neck for a classical guitar, 6.5mm fretboard, 21mm total neck thickness at fret 1, iirc. Adding two CF bars gave the same result as adding ~1mm to neck thickness (i.e. going from 21mm to 22mm)

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

regarding ebony as the enemy: i have a bit of (mostly flawed with mis-drills, chips, off-mark fret indicators, etc )well seasoned 7mm ebony fretboard blanks i've had since the mid 1980s when my older brother worked at a higher end guitar factory in the USA. it is nice, undyed, naturally-black ebony, so i think its the real deal, whatever that is. anyway, it has always been dead straight- it doesn't seem to fluctuate much from week to week, year to year, or decade to decade. i believe it is possible that the original classical builders used the thicker ebony fretboards as a stiffener, to oppose the seasonal flexing of the inferior and porous mahogany...ebony can be strong, stable stuff!

my little 'stash' is pictured, along with a 710mm baritone neck i made using the same ebony
(BTW my parents used to get this scrap from the shop and burn it in their woodstove- a LOT of this and rosewood, probably honduras, was used as kindling. FWIW it burns almost like a torch or candle, as it contains so much natural oil; it emits a lot of soot)

Unfortunately, having a loose blank which stays straight tells you little about it's lengthwise expansion and contraction unless you measure it's length accurately under different humidity regimes.
And mahogany is much more stable in that sense.
Glue a large wide unsealed ebony board on top of a mahogany board which is at least semi sealed and that is a combination which will move with changes in RH

Well, it seems stable on its own, you can't really see when it is moving from 500mm length to 501. But when the same length of mahogany only moves to 500.2, and the two pieces are bonded, the lamination will definitely bend noticeably.

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yes, i failed to think of this. yet i should have, because i just made 2 of those wooden humidity indicators.....as you see i am not a pro

but maybe if the mahogany/spanish cedar is treated and finished in just the right way, it will react to humidity in the same manner as the ebony.......all i can say is this: i own 4-5 vintage, cheap classical style guitars, 35 to maybe 60 years old...only one is "steel reinforced"....none of them exhibit neck bowing problems. thay all appear to be mahogany+rosewood laminations. and i DO NOT keep them in a climate controlled vault...i live in central USA and at times the humidity is unbearable in summer(fog forming outdoors, at night, at 70F) and furnace-induced droughts of maybe 15-20% humidity in winter.....

I forgot to mention the owner of the guitar 2 other of mine, plus 3 other luthier guitars. All 6 stay on the same rack and the others don't seem to have any major problem.

So it must have been something rotten with that specific ebony FB.

Since today I am thinning several FBs for my future builds, i looked in the pile and found a curly ebony board, sister cut to the problem one.

On my bench there is another ebony, a bois de rose and an Amazon. After they were all thinned to about 6.5mm I started flexing them vigorously with both hands. All 3 current FBs bend about the same amount, while the curly one is noticeably less stiff. Probably 50% more deflection.

So the conclusion could be: a good quality, stiff FB can fight bending by itself. A weak one will allow all the expansion force pour down in the neck and bend it.

If this is correct, then any extra stiffness we can add to a neck (hardwood spine or CF/metal bar(s)) will help prevent or reduce humidity movements.

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I used the default figures for Hog and Ebony

There we have it. In the weaker examples, which are the ones that need it most, the advantages of adding CF will increase well past your 10% finding. And for Spanish Cedar and other lighter, weaker woods, still more so.

Adding two CF bars gave the same result as adding ~1mm to neck thickness (i.e. going from 21mm to 22mm)

First, most players will not want a 1mm thicker neck, especially today, where most players want a thinner, "faster" neck, more so in steel string guitars, but possibly in the classical world, also.

Second, did you factor-in the neck's profile? Most of the neck's material is closer to its neutral axis, and very little is left at the back, where most of it was carved away.

What size were these CF bars, and where was their placement in the neck?

On the subject of those ebony boards not moving for years, as was said, they won't show movement until glued to a more stable, and/or stiffer backing. Much like the homebuilt hygrometers recently discussed. Whenever we glue 2 materials with different expansion/contraction rates, we see movement, even if by themselves, each piece would have appeared perfectly stable.

Grumpy, serious question for you.
With a stiff non adjustable neck, how are you controlling for relief.
I prefer to have about 0.005", if you get too stiff will pulling in this amount of relief still be possible, or do you prefer none?

Actually, the curly grain alone may be enough explanation of the extreme longitudinal expansion. Grains like this: \/\/\/\/\/\ and when they expand, a lot of the sideways movement is actually aligned with the long axis. I hadn't ever really considered that before, but it's a very good point toward minimal runout in fingerboard and neck woods.

And thanks to Mario for many great posts in this thread I thought I was the only one who doesn't like adjustable rods. More downsides for me are weight and potential for rattle. I rather like the look of the truss rod cover on the headstock, so I'd actually be sad to lose that by going solid

Classicals definitely do benefit from their naturally higher action, which is necessary due to the lower string tension and resulting larger vibrational arc of the strings. Steels can be extremely low, but the tolerances are equally extreme, and having the wood move around on you even just a little throws it off from ideal. So that's why I still use double action truss rods on steel strings for the moment.

But another advantage to ditching the adjustable rod is that you can then use a 1/2" tall CF bar in the center. A little stiffer than two 3/8" bars on its own, and probably more than that getting it further off the neutral axis. May even be able to squeeze two side-by-side in there for double the effect.

I wonder how well it would work if you just cut a channel along the back of the neck 1/16~1/8" deep, epoxy some carbon fiber tow in there, and stick a strip of wood over it, similar to how Fender electrics have their truss rod installed. Then inlay a CF sheet into the underside of the fingerboard to take the compressive load.

But really, the D tube seems like the best design, if I can just get over the price and installation difficulty... plus the hollow core would be perfect for sympitar type instruments where you have strings running inside the neck

Whilst you would be getting the CF tow in a good position relative to the neutral axis, you would actually have a very small crossectional area of the CF.

With a stiff non adjustable neck, how are you controlling for relief.
I prefer to have about 0.005", if you get too stiff will pulling in this amount of relief still be possible, or do you prefer none?

I really do prefer none at all, but they always pull-in a few thou. In any case, if it didn't pull-in enough relief, we can easily build-in a few thou of relief while dressing the frets.

I agree with the tow just at the outermost edge; not enough of it. While it's true that we get the most benefit by placing any reinforcement as far away from the neutral axis as possible, everything else leading to that outermost edge also adds stiffness and strength, such that the overall cross-sectional area is also important.

Were the CF bars in the "10% finding" not 1/2" tall? My apologies if I missed it, but I automatically think of any reasonable neck reinforcement as being at least 1/2" tall. 3/8" just isn't much of anything.

So Mario, have you actually done the calculation? Download Hurd's spreadsheet and you will see where the sensitivities really are.

That was included for comparative purposes, to demonstrate the influence of neck geometry.


Read back through the thread and check out Hurd's site. It's all explained.

The point is that you can get around all of these problems using a well known, tried and trusted solution called an adjustable truss rod, which you still need if you want to adjust relief whether or not you use CF, which the vast majority of guitars don't bother with because it's not required and as normally used does very little for you.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I have been aware of David hurds spreadsheet for a while but only just tried it out now.
Very easy to use and you can see the results for combined stiffness EI under a number of scenarios
I got results of about 10% improvement with 2 X1/8 by 3/8" bars (which is what stewmac sells)
It did not really matter if I used the default figures for mahogany or a lower value, I still got a 10% improvement

It did not really matter if I used the default figures for mahogany or a lower value, I still got a 10% improvement

That points to a problem, right there....

I don't have the time or inclination to re-read this whole thread, nor go through someone else's site to look for omissions, oversights or errors. I simply ask questions when and where I see a reason to ask a question. This is all a part of a process called "peer review"; it doesn't mean the one asking the questions thinks he/she is smarter or more knowledgeable than you, it is simply a review process that allows everyone to be certain that all Ts were crossed and all Is were dotted, and that no details(for example, was the neck's shape considered in the calcs, and if so, was it a soft, gently rounded shape or a "V" shape?) was overlooked, as is far too easy to do when dealing with armchair engineering.

You've still left a lot of my questions unanswered....

Actually it does not, what it shows is that the 3/8" CF which a lot of people use does not produce remarkable stiffness gains, because it is still relying on the stiffness of the wood at the back of the neck
But just going up to 1/2" deep bars as you do gave a 25% gain

You're asking me (or Jeff) to re-write responses because you can't be bothered to go back and read them?

Well, I've pointed you to all the answers. I can't help you any more if you aren't prepared to put in even that minimal amount of effort.




See? That's the sort of thing you find out, if you're prepared to make some effort. It still leaves you with 75% of the problem, though. I'll bet that the majority of people are using Stewmac sized CF bars because they think Stewmac ought to know that what they're selling will work.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor, I don't need to visit any other sites, or do any math. I've got an engineering degree on my wall here in the office, and long ago figured out what works for my instruments, and just as importantly(if not more so), what has and hasn't worked for millions of others.
I simply want to know the parameters you used for the figures you throw about, for the sake of everyone involved, including yourself. So, I asked some simple questions that could have been answered in shorter sentences than your retributions.

Still waiting for the answers, too....

. They're all already in this thread, Mario. All of them. Jeff found enough information to completely replicate my results.

You can take a horse to water...

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au