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We often talk about CF in necks and many naysayers (rightly) point out that the usual method of inlaying the CF right beneath the FB partly defeats the purpose of increased stiffness as it places the CF close to the neutral axis of the neck. But, being right beneath the FB, isn't that the best way to prevent the FB from distorting the neck?

Recently I got back one of my guitars (classical, no truss rod, Spanish cedar neck and ebony FB) which was held in a too humid environment. The soundboard doming was obliviously puffed up by a couple mm while the neck was backbowed by about 0.5mm resulting in buzzes in countless places. Same as the gluing (water based) of the FB backbows the neck a little, high humidity can do it too since ebony expands a good deal more than cedar/mahogany. The neck had a rosewood spine but it seems it didn't help at all. I've seen other of my guitars puffed up at the soundboard but the necks seemed fine so i imagine there was something especially evil in this particular ebony piece. When I made it I did notice it is slightly less stiff than usual, probably because it had flamed grain.

Anyway this event made me think some more on CF, and wonder if 2 rods placed under the FB would have prevented the backbowing.

I am also wondering if long term issues with necks bowing up are actually created by repeated low humidity hits rather than cold creep from the string tension.

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Build log

I/m no shmexpert , but have used cf in h. mahog cl necks. Personally I doubt they do any harm, but IMHO they add nothing but some reassurance to a client. Gore/gilet aknowledge vy little benefit from adding CF. I/m guessing the water based glue plus the high humidity did it , especially if the ebony was bone dry when glued .Had a coco bolllo fb go wonky on me(upbow) in high summer heat using titebong, the coco was abt 22 yrs old an bone dry. It was a big PITA to remove I replaced it with a 1yrold persimmon FB, looks fine now.

When I last did the calculation, two 10mm by 3mm rods under the fretboard increased the total neck stiffness by ~9% iirc, which means you will still see ~90% of the deflection.

Cold creep increases with the frequency and amplitude of both humidity and temperature swings (and with the static stress level, of course).

It's hard to go past the adjustable truss rod!

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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

It takes more than a little carbon fiber to prevent movement from moisture. I once saw a large wood carving that was bolted to 3" angle iron. It warped and bent the angle iron.

I had a guitar returned from New York once. It was Fall and the heat had just come on so I knew what the problem was but the owner swore the humidity was fine. I use 4 CF bars in my necks and it was still badly bowed. 2 weeks in my shop and it was right back to normal.

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[quote="Kent Chasson"]It takes more than a little carbon fiber to prevent movement from moisture. I once saw a large wood carving that was bolted to 3" angle iron. It warped and bent the angle iron.
quote]


Now that is something I would love to see!!

I've always been puzzled why people seem to prefer ebony over African blackwood for fingerboards. I suppose it's a bit lighter weight, but especially on classicals with no adjustable truss rod, ebony just moves way too much.

I'm surprised you had trouble with cocobolo, given its low expansion rate. Do you think it was just at a different moisture content from the neck, at the time of gluing and/or leveling the fingerboard? What kind of neck wood was it paired with? Mahogany and Spanish cedar should be pretty well matched with its expansion rate, but some of the domestics like walnut and sycamore move more than the rosewoods, so that might have the opposite behavior of ebony on mahogany (backbow when dry, forward bow when wet). Also surprised persimmon was better... that's the movingest wood of all, according to wood-database.com. Don't know of any neck wood that would match with it. Has that guitar been exposed to any significant humidity changes since then?

Too bad about the CF bars being near-useless. Does anyone know if the adjustable truss rod adds a significant amount of stiffness/creep resistance either, or is it there purely for the purpose of countering the forward/back bow of mismatched neck/fingerboard expansion rates when the humidity changes? That is, if you have well matched fingerboard and neck woods, can you skip the truss rod? There is a certain elegance to a solid wood neck.

Another option is those hollow D shaped CF tubes http://www.dragonplate.com/ecart/categories.asp?cID=130. Still might not be enough to significantly counter the humidity expansion force, but probably would prevent any cold creep. Don't like that it takes a monster router bit to install it, though. Pretty pricey, too.

I was the Guinea Pig on the D Tubes...first one to install and give feedback. I have now used on several guitars and have not encountered problems. The first guitar I installed it into was a retro-fit in an electric guitar neck. It was myrtle and would not stay properly adjusted reliably for any length of time (with a double acting rod). The D tube completely eliminated the problem and has been stable since (about 2 years). Another thing is the neck is lightened considerably and another aside is the resonant frequency of the neck becomes higher and adds a liveliness and clarity to the instrument. They also will not rattle

On classicals the ebony problems might be exacerbated by the wider and usually thicker fingerboards. I would guess that a thinner FB has less force to bend the neck. For example 5.5mm instead of 6.5-7mm thick.

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How would a cf rod compare with a steel non adjustable truss rod?

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Probably much heavier and not much stiffer.

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Build log

I would guess that a thinner FB has less force to bend the neck. For example 5.5mm instead of 6.5-7mm thick.

Definitely! Wow, 7mm thick? that's nuts.... I like to keep them closer to .200"(approx. 5mm).

As has been said, if enough moisture is introduced, there's nothing that will stop wood from moving. In ancient times, dry wood was driven into crevices/cracks in large stones/mountains, and then water was added. The expanding wood would split the stone!

There's a reason we all should aim to maintain our shops' RH between 42 and 47%. If we build too dry, problems like bowed necks, domed tops, and popped braces will happen if the instrument is sent to a humid environment. If we build too wet, the reverse can happen, as well as the sad, but all too common, plate cracks and splits.

As for CF or steel in necks, it will help a lot with regards to the normal RH swings most guitars will see in their lifetimes(assuming it was originally built of seasoned woods and in a shop with proper RH control).

As a designer engineer a 10% improvement on an important metric was just at the point that we considered it worthwhile if the cost was not too steep. I have not done any research but one may find that a 10% improvement in stiffness may help in a much greater percentage of the situations the instrument is in. Of course the opposite may be true as well ...

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I agree; 10% stiffer is a valid improvement and nothing to turn your back to. Consider that the vast majority of necks hold-up perfectly well without any CF at all, and that extra 10% adds a great deal of extra "insurance".

Besides, that 10% figure has to be accepted as a rough figure, at most, because of the natural variability of wood. So while an an average piece of neck wood(what wood(s) did you use for your figures, Trevor?)may only see a 10% gain in stiffness, and stiffer examples will see less improvement, the weaker examples, which are the ones that naturally need the most help, will see a lot more than 10%. And of course, we can always use steel, for even more stiffness, and to eliminate adjustable rods altogether.

Do you have any experience, and/or opinions about the D shaped CF rod referenced above?

James

When I started running my rods across into the peg head my necks definitely got stiffer. I had to be sure to level with some tension on the truss rod to get enough relief.

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Do you have any experience, and/or opinions about the D shaped CF rod referenced above?

No sir, though I've looked at 'em on their website. Pricey...

See my post above

I saw it. That's the reference I was referring to. Just looking for other opinions. I'm wonder how the D rod compares to the rods that gave the "10%" increase in the other post.

James

I am not very high tech, so can't tell you about the percentages and such, but I install these and the neck is flat when I string up there is just a slight amount of relief maybe .005". I did have access to a PLEK machine and intended to adjust as needed with fret leveling. But I have moved to different area and now will be back to hand leveling. I am in the process of using on 3 guitars now and will try to get some more useful data.

Dennis, the neck was made of vy dry aged mo. cherry .I glued the coco on last winter when the rh was abt 30%.Then this fall when RH was abt 40-45% I glued on the persimmon to the same cherry neck I left the perssimon at 7mm a tad over 1/4in on a nylon stringed 60/s type folk guitar with a nut neck width at 45mm and the 12 th fret at 54mm on a 650 cl guitar scale. Hope that answers your question. I have used cf on h mahog and sycamore but do not know how to evaluate it/s strengths and weaknesses.

Should we be telling clients to get over it and order rosewood fingerboards? I don't know - I've never used one yet, even on a steel string.

JK

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Jim Kirby
kirby@udel.edu

That sounds like a sensible idea to me, Jim. But I don't see that happening any time soon in the classical guitar world.

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Assuming two identical necks, one cedar/mahogany with ebony FB, the other same wood but with an ebony spine 1/4-1/3" width, full depth. Since the spine will lenghten or contract at the same rate with the FB, is there any chance this neck will bend less than the simple neck?

Ebony spines are common on classical guitars and I've used them on 2 recent guitars (almost 1/3" in width).

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Build log

Since the spine will lenghten or contract at the same rate with the FB,

Incorrect.

The fretboard has many, many times more exposed surface area, and will still expand/contract at a much greater rate than the spline will. The spine will add a good deal of stiffness, but that's all.

Interesting comment, Grumpy (as always!). PRS tried to eliminate adjustable truss rods in their steel string acoustics by incorporating a carbon fiber reinforcement. While I was working there, those of us who were working on the acoustics had a lot of good discussions about the pros and cons...but I could never get past the one point that it's not typical to build an acoustic and not have to adjust the truss rod. The argument for the carbon fiber reinforcement was that if the neck was and always remained straight, then you don't need a truss rod. In my opinion it depends on how the guitar is setup; i.e. if the action is set low, that is just asking for buzzing, etc. down the road as the CF won't eliminate movement, it will only mitigate it. This experience was in regard to steel string acoustics; maybe the higher action on a nylon stringed instrument provides enough of a safety margin?

Trev

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