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I am building a couple laminated necks with butternut and walnut and I would like to reinforce them with carbon fiber on both sides of the truss rod. It is for a steel string with a 25.4" scale length. What size would be appropriate for this? Thanks

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1/8" x 3/8" x about 13-15". I size mine to end just behind the tenon.

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

Maybe it is the engineer in me, but I have a hard time understanding why someone would want to build the neck flat when we know we want some relief, add an adjustable truss rod so that you can control that relief, and then add Carbon fiber beams so the the truss rod becomes ineffective...

-jd

Bill,

I run a .06 x 3/8 thick piece on its side under the truss rod and I run 3/8 tubes on the outer edges following the taper of the neck. It has been working well for me. I also have been leaving a thick neck extension piece under the FB extension and extending the CF into there. It has made a big difference with limiting any distortion in that area for me

I used to use the 1/8 x 3/8 flanking the truss rod and that also worked pretty well but I feel better about my new method.

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

My truss rod is still effective, my necks don't warp, and they're stiff enough for excellent playability. They're also stiff enough to prevent the neck from absorbing sound from the body.

I use .200" X .250" carbon fiber rods. I stop both rods pretty much at the start of the tenon. The other end is ~1 1/2" from the nut on the treble side, and ~3 1/2" from the nut on the bass side. The allows a little more relief for the bigger bass strings, where they really need it.

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I didn't mean to say it, but I meant what I said.
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Woody, please don't take this personally... this is just an example for discussion. Your geometry is much better than others I have seen.

1) when you mix high modulus materials (carbon, steel) with low modulus materials (mahogony) in structure you create a situation where the high modulud material carrier all of the load. In this case, the overall structure is going to be weaker (but stiffer) than without the carbon as there is a very obvious weak point in the open cavity at the headstock end of the truss rod. The stiff carbon and truss rod will act as a very effective lever arm to transmit force to this area.

2) it definitely helps that the carbon rods stop short and are different lengths as it leaves a longer area of more flexable material to absorb some of this force.

3) Placing the carbon (and truss rod) in the center of the neck is the worst place to put them if you are wanting to add stiffness. A carbon fretboard would be better. Thin carbon strips that extend the full thickness of the neck producing carbon skunk-stripes would also be better. (this assumes that stiffer is better which is another issue completely)

4) The quarter-sawn Mahogony between the truss rod and the carbon bars is loaded in shear -this is the weakest direction and may result in splitting of the neck.

5) having a dielectric spacer between the carbon and the truss rod is likely a good thing as it will help prevent galvanic corrosion of the truss rod. If the glue used to attach the fretboard is conductive, you may still get some corrosion.

6) in normal use the load on the neck is pretty low (they work fine without reinforcement) so none of this may matter.

follow up question : why do people think stiffer is better ? my guess is that surrounding high modulus carbon with low modulus mahogony will give a higher resonant frequency and higher dampening than the mahogony on it's own.

-jd

JD: I my mind very little relief is needed in a well set up guitar.Up to .008 is max for me and generally less.Even two CF rods 1/8x3/8 can be adjusted with the truss rod to provide the needed relief. The CF rods also help to prevent twisting in a neck that might otherwise twist.A stiff neck helps to put more into the top rather then fetter it away in the neck. I've done my last seven this way and would not go back to just the truss rod.
Tom

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A person who has never made a mistake has never made anything!!!

For what it's worth, my CF rods parallel the outside edges of the fingerboard just 1/2" in from the edge. They go into the headstock and all the way to the rails i use to support the fingerboard extension. No problem adjusting the truss rod.

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Randy Muth
RS Muth Guitars Website
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Randy, could you post a picture of that setup? Beth

Here's an example. The neck has 1/8" of waste on each edge at this point for final carving after the fingerboard is glued on.

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Randy Muth
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Randy, I'm curious......is your intention for the CF to help prevent twisting of the neck? I haven't completely thought this through, but when string tension is applied the back of the neck is in tension and the fretboard has a compressive force so it seems (remember I said I haven't thought this out) that the face of the neck (where the fretboard glues) and the area just under that where the Cf is located would be on a mostly neutral axis. Running the CF through the headstock and through the fretboard extensions makes a lot of sense.......and it seems where you have them would also help prevent a twist........and allow the truss rod to work nice.

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Formerly known as Adaboy.......

Darryl, you are absolutely correct. The neutral axis is right around where the fingerboard glues on. However if you draw it out, 3/8" goes pretty far down into the neck, especially if you parallel the outside edge, so I never fully understood the argument that they are ineffective.

Also, that argument pertains to stiffness. I'm more concerned with stability. Since carbon fiber doesn't relax over time, I would think the neck would be more stable. Finally, I think running them the way I do would help prevent twisting, but I'm no engineer.

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Randy Muth
RS Muth Guitars Website
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By the way, I no longer use a router table insert. I use a 3 1/2 HP Milwaukee fixed base router attached directly to the underside of the table. Sweet!

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Randy Muth
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Has anyone ditched the adjustable rod entirely in favor of a 1/2" x 1/4" or so CF bar in the center? Possibly still keeping the 2 outer CF bars as well. Almost a full CF core neck. Super stable, stiff and light. Maybe TOO light on dread/jumbo size guitars, unless you go with a maple or rosewood neck. And then you can just sand in whatever relief you want to the fingerboard before fretting. Isn't most of the curve needed within the 1st to 5th fret range anyway? Might be easier to get optimal action if you're not mixing in the neck bow from string tension and truss rod center point variables into the equation.

Actually, I wonder if a rosewood neck even needs any reinforcement at all... might be stiff enough to take 180lbs as a solid wood neck. I guess the adjustable rod wouldn't make much weight difference to such heavy wood though, so you might as well put it in anyway.

My experience has been that any time an instrument crosses my bench for repair work and does not have a truss rod, I usually wish it did. I don't know if it is poor initial design or what, but over time the instruments I've seen without a TR develop more relief than is desirable. A TR make setups and relief adjustments a breeze, but without one they are a major pain.

Josh

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Josh House

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It seems from reading the responses on this that most are adding Carbon in the pursuit of increased stability, not stiffness.

Spacing the rods further from the truss rod would help with this.

solid pultruded carbon rods are denser than mahogany, so using them is not going to make the neck any lighter.

-jd

So, for those of you doing repair work, how much of an issue is twisting or instability in a neck with just a TR and no CF? If the neck was made with excellent wood, and made well the first time (as we hope on a higher end production or small shop guitar), do you really see twisting or instability? Beth