Official Luthiers Forum!

I typically put two 1/8 by 3/8 carbon fiber rods on either side of a truss rod channel in the necks that I build. So, recently, I had a repair job to do on a neck that I had made. The truss rod's socket had crumbled, making it impossible to adjust. I removed the fingerboard, removed the trussrod, and for some reason just decided to cram the cavity full of carbon fiber and epoxy it in place, epoxy the fingerboard back on, clamp the entire neck, including fingerboard onto a machined straight piece of steel and go from there.

The fingerboard was dead flat with no string tension. There was a slight amount of relief under tension, but the most interesting thing was the really noticable increase in volume. Rather dramatic really. If the relief remains constant, I am wondering about the real value of a trussrod. Certainly there has always been a temptation for repair folks to do some string height adjustment using a truss rod. I consider that a drawback.

Any of you builders of steel string guitars not use trussrods? I know old man Martin disdained their use.

you are mistaken on Martin and the truss rod. They used them from the get go. It was the adjustable truss rod they fought . Martin used a number of different truss rods , square tube , Tee and ebony during the war years , then back to I think Tee then in the 80;s they did the 1 way adjustable and now the 2 way.
I have used CF on some of the older ebony necks . I like it .

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I'm of the belief that you can't necessarily make universal statements about the neck's effect on tone. Like everything else, it seems to depend on the rest of the system.

There are modern builders building without truss rods. I know because someone brought a 10 year old guitar in for me to repair that had way too much relief. It sure becomes a pain for the owner when they are looking at a re-fret in order to fix the action when a turn of the truss rod would have fixed it.

Certainly one can build a stable neck without a rod but it takes some serious engineering to make something stiff enough to resist 180 pounds of force for decades.

_________________
http://www.chassonguitars.com

You are right John
I meant to say adjustable truss rod. Am I not correct that adjustable rods were only used when the senior Martin turned over control of the company to his son.

Kent
If you consider that the 180 pounds of tension is a vector that is mostly parallel to the neck rather than acting at right angles, it should not really be that difficult to engineer a neck that is strong enough to resist the pull. I think a blanket statement that goes something like this:"the stiffer the neck, the more energy is transferred to the top" is probably a pretty safe statement.
Or how about another example. "the shorter the blanket, the colder the feet"
Just thinking out loud

your statement about Sr Martin is more to the point and correct. There is a dampening effect from the neck to a degree. May i ad that It isn't safe to make a blanket statement that the neck is that much of effect to the guitar's tone as it is just a small part ot it.. It is more because of the relation of the structure and the synergy of the box. You need to look at the whole and not just one part , the bracing and don't forget the bridge and that structure.
So much to learn and so little time

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

John K. ...Interesting effect you got there, and one that begs to explore the cause. I know that Mario uses a solid rod (non adjustable) in his guitars and I know from 1st hand experience that they are volume, tone and sustain monsters and are quite light. Of course, he has a lot more going on as well but he's a firm believer in the neck contributing significantly to the overall sound. Don't expect him to weigh in here (unless I mis-represented his neck building strategy <BG>) but you know where to contact him should you need more details.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

Subject: Carbon fiber vs trussrod re...volume



Another way to look at it is that the vector changes as the neck pulls up so that as it deforms, even more force is applied....

And the number of guitars out there with necks that move around tells me that it's a real issue. I'm sure it can be done but it either takes a bombproof system (look at McPherson) or something more conventional where every element is well conceived and can't creep.

I've also played guitars with fairly floppy necks that were plenty loud and great sounding. I just think you decide on your priorities and keep tweaking things till you get what you want. For me, having some adjustment in the neck is a priority.

_________________
http://www.chassonguitars.com

_________________
There is no difference between the man that thinks he can....and the man that thinks he cannot.

Guys I realize that there is a lot of variables going on. That is the reason that this particular "experiment" was so interesting. The variables were pretty well controlled. Same bracing, same wood, same finish, in fact, same guitar. Only difference was the substitution of carbon 1/4 x 1/2 carbon fiber rod for the two way adjustable truss rod. Now that is an experiment worth noting.

The only variable is the subjuctive evaluation of the tone before and after. I admit to being falable, but the difference was unexpected and quite noticable.

Just thought it might be a point of interest.

JJ, thanks for posting that interesting fact about Mario's build strategy.
In fact I recently had a commission wherein the customer specified no truss rod, lots of carbon fiber in the neck (he certainly got that) and a light a build as I could get. He said that top belly was not a problem for him. I am interested to see how this one sounds ( and how long it lasts )

The problem the truss rod is there to address is 'cold creep': the fact that wood moves plastically under even a small sustained force, and takes a set over time. The plastic nature of wood is a good thing when it allows us to bend sides, but it's more of an issue when you want something to stay put. As an example of how small forces can cause big problems over time: 'Old Ironsides', the oldest commissioned ship in the US Navy, has to be taken out to sea and turned around once a year. If it sat at the dock in the same position the sun on the masts would cause them to bend to the south: an annual turnaround helps keep things plumb.

Adjustable truss rods allow you to put a countervailing force on the neck: the strings pull up and the rod pulls down. You end up with more compression in the wood, but since even spruce can handle up to about 5000psi in direct compression with no problems, 400# or so on something as big as a neck is a piece of cake. BTW, the aircraft folks say that you're safe if the initial deformation under the max load on a wooden structure is no more than 1/3 of the amount you can live with in the long term.

Neck reinforcement, whether it's a hollow steel tube or CF, limits the amount the neck will pull upward. At the beginning some of the stress is taken up by the wood, and some by the reinforcement. As the wood relaxes over time, more and more of the load is taken up by the reinforcement (assuming that doesn't deform plastically as well). Eventually the load the wood is taking is so low that the creep slows down to almost nothing, and the neck is 'stable'.

CF is strongest in tension or compression, and less useful in adding stiffness when it's loaded in shear. Dave Hurd showed that a CF rod under the fingerboard is just about on the 'center of moment' of the neck, and is thus pretty much shear loaded. Setting it deeper should make it more effective.

As far as the stiffness of the neck and the sound of the guitar goes; that's a can of worms. The neck bends a lot in the lowest vibration mode of the entire guitar body, so its stiffness is a major factor in the pitch of that mode. If that 'C-1' corpus mode (AKA the 'neck mode') is high enough in pitch to couple with the 'A-0' or 'Helmholtz' air resonance it can really effect the tone of the guitar, particularly in the low range. Usually the C-1 mode is lower than the A-0, but a lot depends on the neck stiffness and headstock mass.

While it's true that some string energy does go into vibrating the neck, I'm pretty sure the proportion going into the bridge is so much greater on an acoustic that you can more or less ignore the neck from that standpoint. It might effect the tone, but there's probably not so much energy going that way on any normal acoustic that altering the neck stiffness, even fairly substantially, will make a noticable change in the volume unless there's some sort of frequency coupling going on.

On this point, in what proved for me to be a most enlightening conversation at the ANZLF, one of our members jeffhigh (retired engineer), revealed to us that the tension of the strings above the fretboard induces a 'compressive' load in the fingerboard and the surface below it and a 'tensile' force in the rear of the neck.

http://www.anzlf.com/viewtopic.php?t=1509&start=25

I don't really know why the concept was such a light bulb moment for me now because the principal is so obvious when you stop and think about it. But when Jeff suggested that the resistance to 'tensile' force offered by 'unidirectional CF sheet stock' laminated to the 'back' of the neck would be far more effective at reducing deflection of the neck from compressive force than the common practice of rebating 2 x CF rods into the fretboard surface, it really made me start thinking about trying this out.

My thoughts are to sandwich a strip of .020 or .030 CF sheet stock between the back side of the neck shaft and a .250" decorative back slat most of which will eventually be carved away when forming the shaft. I imagine the CF sheet stock would then look much like a black purfling line, if this is done well it could look quite good, although the transition point from shaft to heel could look a little funky.

Cheers

Kim

I've always thought the compressive load is where most of the creep occurs. The fret slots often leave lots of room that can close up under tension. That's one reason for glueing frets. The glue also can help solidify the end grain and make it less likely to crush under compression.

For this reason, I've always thought the best place for cf would be as fretboard binding. It gets it as far as possible form the neutral axis and putting it out at the edges would help prevent twist as well.

_________________
http://www.chassonguitars.com

The point is that CF works most efficiently under tensile force, it won't stretch. What Jeff Highland has revealed to me is by having the CF as close as you can to the underside of the neck to resist the tensile force, you reduce the compressive force which comes into play at the fretboard. I see this as a method of more effectively reducing the 'cause' rather than trying fighting the negative result.

Cheers

Kim

Hey Kent, CF fretboard binding sounds like an interesting concept.
Sure compressive stresses in the front of the board do play a part and the fret slots are a weak area, which is why the old martins with tight fitting bar frets were renowned for stability. A thin layer of CF under the fingerboard would also work
Why I tend to focus on strengthening the back of the neck is that the shape of the neck gives a much narrower width of wood there at maximum distance from the neutral axis.

One of my relatives works at an aircraft manufacturing facility here in Florida. From him I have a nice pile of CF coming. Sheet stock somewhere in the 60 - 80 thou thickness range. One of the ideas I had in my head was to just simply laminate a sheet of this right down the middle of the neck. That oughta stiffen things up a bit. It hadn't occured to me that I might be able to eliminate a truss rod though.

Chris

_________________
There is no difference between the man that thinks he can....and the man that thinks he cannot.

I am pretty sure that most of the forces on the carbon fiber "beam" are not shear, a force that would move a neck segment like an earthquake displaces a road bed, vertically or at right angles to the long axis or horizontally sliding one segment of the beam over the other. I am pretty sure most of the forces are a combination of tension and compression. There will be some shear forces at the interface between those two components (shear and compression) but I think that can be disregarded since that those forces are spread out the length of the beam. (I don't think the top of the beam will slide past the bottom)

Regardless of the reasons pro and con, the fact of increased volume remains. No nead to explain it away. Just reason why and how it can be put to good use.

So, what does Mario do anyway to compensate for wood creep?

Anyway, the 180 pounds of force compressing the neck is not the problem. It is the 5 or so pounds of force acting to pull the neck up. I seems to me that a neck can be engineered to support that load indefinitely with minimal deflection requiring no additional adjustment rods. If a carbon fiber, or steel or aircraft aluminum rod were imbedded in the neck the forces on the wood would be unloade pretty quickly. No creep.

I can't comment on the building aspect of the adjustable truss rod, but I can say that as a player a neck with a non-adjustable rod just "feels" more "solid" to me. When they're set up right, it's also one less thing for me to go all O.C.D. over (the neck relief)-- you don't worry about it because you can't easily change it.

On my last couple I put the CF as low in the neck as I could (a 1/2" x .06 strip under the truss rod) and as high in the fretboard as possible (just under the fret slots, same size). On one with no truss rod I also put strips as high and low as possible in the outer portions of the neck and fretboard. I have not strung them up yet so no word on how they react but I am very interested in the guitar with no truss. It is very light currently. I do hear something in all of the guitars I have made without truss rods that I prefer. There is an extra "airiness" or tranparency to the sound, especially on the higher strings that I really love. It could be all a coincidence, I want to see if I hear it on this new one.

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.

John,

Thanks for posting your observations. It fired a few neurons for me.

If the increased sustain and volume are secondary to the typical primary goal of CF stiffening necks, I was another one whose light bulb finally got lit as to what CF really is doing to handle the force vectors in that ANZLF conversation that Kim mentioned. (I know there are others here at the OLF that have explained it, including Rick Turner, but I didn't really grasp it.) I'm one who would have thought that a piece of CF sheet running vertically through the entire neck (as a laminate layer, perpendicular to the face of the fingerboard) would provide the maximum stiffening effect to the neck, but now realize that a thin ribbon of CF epoxied at the bottom of a deep channel in the neck (co-planar with the fingerboard), would actually offer more resistance to the neck bending.

Same thing for braces: a vertically laminated brace with CF sheet between Spruce could be distorted out of the intended arch more easily than a brace that is capped with a ribbon of CF.

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Okay...someone needs to pause the class for a moment to catch up the stupid guy! (me)

I thought ANY material would resist the pull of strings better if 'stood up' on it's side. In this case, something that runs the length of the neck but is perpendicular to the plane of the fingerboard.

The below illustration is one of the ideas I had. Everything is fairly simple to see but I'll add one thing. The normal truss rod channel in this illustration would be much larger to afford the use of taller and wider sheet CF.

_________________
There is no difference between the man that thinks he can....and the man that thinks he cannot.

Hi Chris,

Until one of the engineering guys pops in and states the engineering principles in laymen's terms...

Think of an I-beam. An I-beam running the length of your basement, with a whole family of big butt couch potatoes, munching cartons of ice cream, up above the basement on the first floor. The top of the I beam (the horizontal member) is under compression. The bottom of the I beam is under tension (our lard butt family is attempting to make the bottom of the I-beam longer.) The vertical member of the I-beam is adding strength, but its main job is to connect the top and bottom plates, and the bottom plate is the major reason that the ice cream and sprinkles and remote control wielding sumo wrestler lookalikes have not made it to the basement.

Back to guitars. A sheet of CF running through the neck perpendicular to the fingerboard will only make the neck stiffer if the final shape of the CF is stiffer than the wood it replaced, and yes, it is. But, if your goal is not just to be slightly stiffer than wood, but stiff enough not to deflect under tension, then you get back to the key member of the I-beam, the bottom plate. You want that bottom plate to have high tensile strength, that is, not to stretch. That is one huge advantage of unidirectional CF material compared to wood. If you tried to suspend a 200 pound weight on a ribbon of wood veneer (say, 1/32" x 1/4"), the wood fibers would stretch and then break. Do the same with a CF ribbon of the same size; the CF ribbon would not stretch (or break, but it is the lack of stretching that is key.) A thin guitar neck made from wood with no truss rod and no CF would bow forward when strings were brought up to tension. That means the wood at the back of the neck is stretching in length. Now, take that same neck, rout a channel down the middle, and epoxy a ribbon of CF at the bottom of the channel, with the CF ribbon co-planer with the fingerboard. For the sake of the example, glue a piece of wood in to fill-in the rest of the channel. Now, when the strings are brought to tension, the CF's superior tensile strength comes into play, and the neck will not bow.

So, it's not that CF is not strong used perpendicular to the fingerboard, it is that the greatest tensile strength of the neck is achieved by using the CF as the bottom of an I-beam. CF is not a miraculous material that cannot be bent - you can deflect an 1/8" x 3/8" x 18" rectangle with pressure from your hand. The "miraculous" attribute of unidirectional CF is its tensile strength (resistance to stretching), or in the case of something like buttress rods, the key attribute is compressive strength (resistance to changing length when compressed.)

Hope that helps until someone can say it better.

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

what would have helped me...is to have posted the drawing....

_________________
There is no difference between the man that thinks he can....and the man that thinks he cannot.

Or like this...an attempt on my part to illustrate a potential proof of concept?

_________________
There is no difference between the man that thinks he can....and the man that thinks he cannot.

Mcphersons necks are almost completely carbon fiber. They route out a channel and epoxy in a huge carbon bar...leaves roughly .090 of mahogany around the rod. It's stiff I'll give em that. It can't be adjusted though without a trip to the plek, or a lot of handwork. Not the most ideal for the general public. I've seen them warp anyway with the rod.

As for increase in volume....well I've never heard a mcpherson without the rod so I can't comment, but it seems that adding stiffness would lead to more volume, as well as sustain. Really though it's the whole salami not just part. You gotta get a design that works in conjunction with all the parts. A overly stiff neck could be part of that equation, and work well, and other times it wouldn't.

_________________
John Mayes
http://www.mayesluthier.com

In a beam subjected to bending load, the compressive and tensile fibers resist the bending equally. That's what defines which fibers are in tension and which are in compression. If tensile fibers resist but compressive fibers yield, the beam bends until the compressive fibers stop yielding. The neutral axis migrates to put more fibers into compression until there are enough fibers in compression to offer resistance equal to that of the fibers that remain in tension.

(I'm using the word 'fiber' here in the engineering sense of the theoretical elements of the cross-section of a beam, and CF to mean carbon-fiber reinforced polymer.)

So putting CF only at the extreme tensile fiber (the back of the neck) will make the neck stiffer initially, but it won't help much with the long-term problem if the problem is cold creep at the compressive fibers. I don't know whether that's the problem, but others in this thread have said it is and it sounds plausible. The only thing that CF in that position would do is increase the tensile resistance so that more of the wood is put into compression. But if it's failing due to cold creep of the wood under compression, that will still happen, maybe slightly reduced or retarded.

If CF is highly resistant to cold creep, then a solid bar of it on edge anywhere in the neck should be very helpful against long-term bending. A 1/4" x 1/2" bar on edge would have about the same EI (modulus of elasticity times moment of inertia, the measure of a beam's resistance to deflection under bending loads) as an unreinforced wood neck. So the CF will start out sharing both components of the bending load with the wood. If the compressive fibers of the wood yield over time due to creep but those of the CF don't, the CF will eventually take all of the compressive component, leaving the wood of the fingerboard suspended like a decorative element. The only question then would be whether the combination of the CF and any part of the wood cross-section not subject to cold creep is stiff enough to resist the string tension adequately.

I don't know whether CF is highly resistant to cold creep. A supplier told me it is, but John Mayes' comment about warping of McPherson's "almost completely carbon fiber" necks suggests otherwise.