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Carbon Fiber in the neck: where to put it?

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Carbon Fiber in the neck: where to put it?

Postby Jason Rodgers » Tue Sep 08, 2015 12:26 am

It has become standard practice by many to use Carbon Fiber (CF) reinforcement in the neck. The most common orientation is to install two 1/8" wide by ~3/8" tall rods on either side of the truss rod. Some use tubes. Others sandwich CF sheet/tape/veneer in their neck layup (vertically). I've seen some cool D-tubes used, as well.

What other types of CF products (aside from full CF fingerboards and neck shafts) and orientations are possible and/or useful, and to what degree? (Too much, and no truss rod adjustability; too little, and what's the point.)

For example, I have used 1/4" diameter CF tubes alongside the truss rod, and while they add stiffness, they're not as stiff as the rectangular rods.

A neck lamination of maple/CF/maple/CF/maple would yield a fairly stiff neck, but what about a .005" CF sheet (unidirectional fibers) under the fingerboard?
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Re: Carbon Fiber in the neck: where to put it?

Postby Barry Daniels » Tue Sep 08, 2015 9:39 am

A sheet under the fretboard would add little stiffness due to the wrong plane orientation. I use small bars (0.92" thick by 1/4" high) on either side of the truss rod. Works fine for me.
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Re: Carbon Fiber in the neck: where to put it?

Postby Todd Stock » Tue Sep 08, 2015 11:11 am

Optimum is as far from the neutral axis as possible and as little on the neutral axis as possible, but that is tough to achieve. Compromise is as suggested...vertically oriented bars on either side of a single trussrod. I continue the CF through the nut area to get some bang for the additional mass, but only us CF for necks that I know will likely need it for stiffness...so usually just ABG.
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Re: Carbon Fiber in the neck: where to put it?

Postby David King » Tue Sep 08, 2015 12:29 pm

Your best bet is to wrap the neck in a CF sock and vacuum bag it. That's basically what all the CF necks out there are, a light wood core and a couple of laters of CF cloth with most of the epoxy matrix sucked out. Rick Turner, Ken Parker and Geoff Gould were probably all doing that starting more than 40 years ago. You end up with a torsion box which is very stiff in all directions but especially resistant to twisting which is something wood necks are quite prone to.
The exceptions are Steinberger and Moses.
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Re: Carbon Fiber in the neck: where to put it?

Postby Randolph Rhett » Tue Sep 08, 2015 12:33 pm

Careful with two CF bars on either side of the truss rod. The temptation is to space them well out, but remember that the back of the neck is curved. If you space them to far out you will carve through the wood and expose the rods. Ask me how I know :-( ...
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Re: Carbon Fiber in the neck: where to put it?

Postby Jason Rodgers » Tue Sep 08, 2015 9:58 pm

This is all good info, as I have no education in the material sciences and engineering. Thanks!
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Re: Carbon Fiber in the neck: where to put it?

Postby David King » Tue Sep 08, 2015 11:34 pm

Just keep in mind that steel is a lot stronger than CF and a whole lot cheaper.
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Re: Carbon Fiber in the neck: where to put it?

Postby Jason Rodgers » Wed Sep 09, 2015 12:42 am

David King wrote:Just keep in mind that steel is a lot stronger than CF and a whole lot cheaper.

Ain't that the truth!
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Re: Carbon Fiber in the neck: where to put it?

Postby Todd Stock » Wed Sep 09, 2015 10:02 am

Lots of confusion between strength and stiffness on most forums...hope this helps:

Guitars are stiffness-driven structures - if stiff enough to do the job, they are almost always going to be over-strength, so strength is not something to worry about for structural components, because we are working nowhere near those limits. What we want, generally, is stiffness at minimum weight, or a good 'stiffness-to-weight' ratio.

Unidirectional CF (where all the fibers run in the same direction) is about four times stiffer per unit weight in the fiber direction than steel, aluminum or titanium, so it's much more efficient than those materials in providing an increase in beam stiffness. With the cost of a pound of CF down to about $5 versus $150 in 1970, it's cost is not much more than standard mild steel hardware store shapes we're stuck using if we don't want to pop for a custom extrusion. There are a lot of good reasons not to use CF in complex structures where stamped or forged steel or cast aluminum will do (many automotive applications), over where titanium is needed to handle higher temps (turbine blades, etc.), or where we want specific fatigue and damage tolerance characteristics associated with metallic materials, but for stiffening the neck of a guitar, CF will provide the same stiffness increase as steel, aluminum, or titanium at just 1/4 the weight increase where we are talking similar formats (rectangular bar, hollow rectangular or square tube, etc.). Further, accommodating the CF reinforcement is not going to result in compromises with neck cross section as can be the case with aluminum (aluminum is about 1/3 the weight of steel per unit volume, but only about half as stiff, so a larger section is needed for the same stiffness increase...it's the Ford versus Chevy commercial).

Where CF and steel/aluminum/titanium will differ is in internal damping properties. Keep in mind that energy loss due to damping is a function of the degree of internal energy loss (loss coefficient), frequency of the vibration, and mass...CF has a higher degree of damping (different loss coefficient) than many metallic materials such as steel, but because mass of an equally stiff CF neck reinforcement bar is 1/4 that of steel, overall energy loss at frequencies we are concerned with is likely to be no worse steel (depends on other factors such as whether reinforcement is bonded/unbonded/bedded in silicone in neck, adhesive used, fit, and actual composition of the reinforcements.

Not saying steel is bad and CF is good, just noting that steel will be much heavier for desired change in stiffness.
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Re: Carbon Fiber in the neck: where to put it?

Postby Todd Stock » Wed Sep 09, 2015 11:24 am

And for what it's worth, uni CF is about 10 times stronger than standard structural steel (ultimate tensile strength of 4100 to 6300 MPa versus 400-550 MPa) and 5-6 times stronger (tensile strength) than 1090 mild steel, which is likely what you are buying from most hardware stores. If you'd like to spend a bit more, you can buy higher strength steels (up to 5200 MPa for Micromelt steels), but you'll have to get into designer boutique tech alloys ($$$$) to get tensile strengths that exceed plain vanilla CF.

Bottom line: best materials bet to increase stiffness: CF. Best materials bet for durable, inexpensive adjustable truss rod to control relief: steel with a bit of bronze for bushings. Want stiff neck with positive two-way adjustment: 1/8" x 3/8" CF times two and a good two-way rod.

That aside, I don't use CF in mahogany guitar necks...not really necessary in my experience unless the customer insists. I do use CF in ABG necks and for some alternate neck woods like spanish cedar.
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Re: Carbon Fiber in the neck: where to put it?

Postby Jason Rodgers » Sat Sep 12, 2015 6:44 pm

Todd Stock wrote:Bottom line: best materials bet to increase stiffness: CF. Best materials bet for durable, inexpensive adjustable truss rod to control relief: steel with a bit of bronze for bushings. Want stiff neck with positive two-way adjustment: 1/8" x 3/8" CF times two and a good two-way rod.

This may be an elementary topic for folks who have been building for a while, and I have been at least reading about it for a decade and a half, but I'm trying to understand it better. My wonderings also came from a manufacturing perspective. The neck requires the most precision and steps in its construction (at least with electrics, relative to the body), and I'm thinking about how I can simplify. That's where the CF sheet under the fretboard question came from. Ok, so short of wrapping the whole unit in CF, "standard practice" with CF reinforcement bars is what it is for a reason: it works the best!

When it comes to putting those two CF bars in the neck, is it necessary to give them their own channels, separated from the truss rod by 1/8" or so web of wood? Can they be epoxied on the walls of a single, wider channel with the truss rod? That would be a 1/2" wide channel 3/8" deep (assuming that your double-acting truss rod is 3/8" tall). Or would this centralized support not be able to resist twist as well as wider-spaced CF rods?
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Re: Carbon Fiber in the neck: where to put it?

Postby Todd Stock » Sun Sep 13, 2015 7:18 am

We want to prevent the CF rods from twisting and reducing stiffness in the desired plane, so def needs to be glued into neck, and separate channels increases glue line area. Is that additional area needed? Only where you anticipate partial glue line failure in shear and the truss rod not acting as a support to prevent twist. I suspect the real reason why most builders use CF in separate channels is that it's easier to do the glues (CF reinforcement and closing the neck out) and slightly less risk of buckling/crippling failure due to adhesive failure.

FWIW, I worry far less about carving close to the CF than a truss rod channel - the CF is bonded into the neck and is essentially just another laminate...it could be replaced with wood and the only impact to shaping and load-carrying would be reduced stiffness...no blow-out and cross-grain failures like we see with non-bonded adjustable truss rods.
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Re: Carbon Fiber in the neck: where to put it?

Postby Greg Martin » Tue Nov 03, 2015 10:45 pm

For what its worth, Ive used cf rods on every guitar Ive made (20) except for 1. That 1 was problematic and I ended up removing the finger board and routing in cf rods, after a lot of grief it finally was dead straight and stiff and had plenty of truss rod travel. using cf allows the builder to choose figured woods with out sacrificing stiffness/strength. Ill always build necks with cf weather I use single action or double action truss rods. Ive got ready to use routing jigs for it too.
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Re: Carbon Fiber in the neck: where to put it?

Postby Glenn Howland » Wed Nov 04, 2015 8:26 am

I'm new to this.

I have a number of one piece mahogany neck blanks in rough bandsawed form that I'd like to use up before moving to a less wasteful laminated neck design. Most of them are pretty straight grain, but all share short grain/severe runout in the headstock, making them vulnerable to a "Gibson fracture".

I'm thinking of routing two short, 1/8 " channels in the headstock / nut area, perhaps from halfway down the first fret area to the first set of tuning machines, and use short cf bars to reinforce the neck area just north of the nut. I anticipate using a truss rod adjustable from the inside of the guitar, so there would be no truss rod access issues. I also plan to use a somewhat beefy headstock veneer plate.

Practical idea or am I making a mountain out of a molehill?
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Re: Carbon Fiber in the neck: where to put it?

Postby Todd Stock » Wed Nov 04, 2015 10:59 am

The problem with running the CF through just the transition area is avoiding a stress concentration at the ends of the rods. For peg heads, I taper the rods on the Y axis to nothing to eliminate the hard spot (I taper to the peghead back angle) , and continue all the way to the tenon or dovetail to eliminate the potential for hard spot (stress riser) just below the nut where you proposed to end your reinforcement. You could taper both ends of the rod, but milling the slots will be a bit of a pain...not just a case of milling the slot with a plunge router, as that creates as much of an issue as a hard termination of the CD.
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