Official Luthiers Forum!

Bob, I take exception to this. It's kind of like saying guys who machine parts for guitar makers can't hold real tolerances, which we both know is not true.

For those of you who are reading this and have purchased carbon fiber rods in the past from Los Alamos Composites I would like you to know;

1) I produce all of the rods we sell in house. I do not buy from a pultruder and resell.

2) I use only prepreg material from Toray, same as the aircraft industry. Prepreg materials have a precisely controlled resin content, 35% in my case which has a huge impact on the stiffness of the final composite. Composite pieces produced from prepreg are the highest quality of composite parts due to the control of the resin. Prepreg parts are made by laying down layers .005 at a time and curing the part under heat and pressure. This is not slopping resin onto dry fibers and letting it cure.

3) Pultruded product which is the most readily available form of rods is an inferior product to pieces made from prepreg due to pultrusion’s high resin content, assuming the same fibers. It does look better however if that’s important to you.

4) As far as stiffness goes, the rods from Los Alamos Composites are almost 2x as stiff as aluminum while being lighter.

I saw this post earlier and I was really annoyed by it, so I felt like I needed to get this off my mind.
I’m not producing any more rods in the near future, partly because a prepreg type product can’t compete price wise with a pultruded product, and the customer base doesn’t understand the difference. Pultruded product is obviously good enough, in fact nothing is obviously good enough, they made lot’s of guitars without carbon fiber, it does stiffen the neck though which I think is a good thing.

Now to try and contribute something to laurents question. I think aluminum could work great. The only potential problem I see would be it’s coefficient of thermal expansion. I’m sure that it’s a non issue for 99% of the time. High temperatures could move the neck around changing the relief. It’s kind of hard to imagine that happening however. Just a thought.

_________________
Jim Watts
http://jameswattsguitars.com

Your analogy was excellent, apt, and appreciated. I should have specified that there's a pretty small difference between what they can pull off and what a good non-unlimited-budget operation can. I can't hold a tenth, and those guys can, because I didn't have $450,000 to spend on my mill and they do. That little distinction, though, is both invisible and irrelevant to 99.5% of my clients getting precision work.

My last phrase didn't come out right, and I sincerely apologize for the oversight. I'll clear that one up: If I didn't this particular friend, I would absolutely be using rods from Los Alamos, without a second thought. I think that it wouldn't be difficult to make a completely unmovable neck using them. The reason I believe the aluminum will work fine is that CF exceeds the requirements by such a margin that significantly less stiff materials can still do the job, even though they might not be in it's class.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Interesting and educational discussion My totally non-supported intuitive feel on aluminum is that I worry about the deformation mentioned. i.e., it could "take a set" whereas CF will always want to return to straight (or whatever it was shaped to). But I suppose this is just a matter of using the correct alloy. Is there spring tempered aluminum? I figure for the money that Collings' method is really good: thin spring steel slats. The spring temper means it shouldn't take a set, and steel is cheaper than aluminum, and dirt cheap compared to CF.

Peace,
Sanaka

_________________
...imagine there were no hypothetical situations...

Laurent,
I did not know that Collings uses the spring steel slats. But I know that Taylor uses the spring steel slats and that Bod Taylor went to Collings shop to show Bill Collings how to set UV curing finishes. So they are likely using a similar/same approach with the spring steel.

I assume you realize that the shape and orientation of a structural member are both very important parts of its strength in application. A a rod shape is often not ideal. The steel slats you mention above are wide in relation to their thickness. I have never measured one but I have seen Taylor's in their videos. I would guess that they are 1/16" x 4/16" x length of neck. They are likely 4 times deeper than their width. They are oriented with 4/16" dimension perpendicular with the fretboard (into depth of neck) which gives the maximum amount of strength to weight given the direction the neck wants to bend. This would be a VERY rigid orientation and yet take up little space and weight. Obviously, oriented the other way they would add almost no rigidity to the neck. If they were square rods with the same cross sectional area (.015625 sq in.) with same weight per inch they would be 2/16" x 2/16" and would not be near as rigid and yet weigh the same.

I suggest that you look for a better shape for your aluminum or consider spring steel slats like Taylor and Collings.

Ed

FYI,
I believe that Robert Ruck is presently using Titanium tubing in his flamenco guitars for a neck reinforcement, I played two with same and do not believe that they changed the sound in any negative way. The aluminium would have to be heat treated to be of any use. Plain aluminum is too soft and pliable to work without heat treatment.

_________________
John Nowicki

I'll add a couple theoretical points (not from experience).

Where the reinforcement is located in the neck is about as important as the material strength. Note, the strengths mentioned above assume the CF is in tension. My understanding is the strength of CF is significantly less under compression loads. I mention this as some had pointed out the possibility of bridging the neck/fingerboard joint with reinforcement and it's ikely this area of the neck is under compression.

_________________
Formerly known as Adaboy.......

I meant to add that I would prefer aluminum or steel to CF in the areas where there is a compressive load on the neck.

_________________
Formerly known as Adaboy.......

Education seemed to be the main goal when I was making composite aircraft propellers. I understand this lament like no one else. I competed against those who took a far less controlled approach to making their product and spent far too much time explaining why it was better. Ah well....Raytheon gets it. Retail customers.....not so much.

I expect you know this, but if you haven't heard, Aldila, Inc. http://www.aldila.com/ makes their own unidirectional carbon fiber prepreg tape for their golf shaft business....but they also sell prepreg to composite manufacturers. They have a 300 gram per square meter material that made a pretty big difference in our bottom line.

Pultrusion is the least controlled of the manufacturing processes but would likely still yield a part that was much stronger than 6061 aluminum. Laying down unidirectional prepreg tape and curing under heat and pressure, as you described, is the only conceivable way to approach the theoretical properties of carbon fiber without massive investment in processing equipment. You're probably around 85-90% (making a few assumptions) of published mechanical specs for carbon fiber. That puts you well past twice the strength of a comparable aluminum rod.

I hope you didn't take any umbrage with my statements regarding your products. I meant to convey that there isn't much one can do as a retail customer to ascertain what they have in their hands without some depth to their understanding of molding processes. Too often the phrase "carbon fiber" is used as a buzzword that means "indestructable" or "super strong" but the true range of strength of any molded product using carbon fiber is determined by the effort made to control how it is processed. I expect your truss rods are the strongest rods available to the public....based on the molding process you describe.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Imagine bending a 2x4 downwards over your knee.

The material on the outside of the bend (the upper side) is in tension. The material on the inside of the bend (the lower side) is in compression and wants to buckle. The material in the middle of the 2x4 simply keeps the outside material and the inside material from coming together.

The tensile strength of carbon fiber is its most profound mechanical property. At about 650,000 lbs. a square inch, it is many time stronger than steel. Sandwich core construction forces carbon fiber into bearing a tensile load along the outside of a panel that is being bent. There's more to this (modulus of elasticity = bending strength if you prefer to boil it down) but the fundamental truth to "sandwich core construction" is: The further away the inside and outside layers get from each other, the stronger the bending strength of the composite panel because the more directly the fibers along the outside of the bend are forced into tension....where carbon fiber excels.

As a guitar neck bends, the fibers along the back will bear most of the bending load under tension and the other fibers bear progressively less as they get closer to the fret board where the load becomes compressive. Six properly tuned guitar strings pull 200+ lbs. This is a very mild compressive load applied to a carbon fiber rod glued into a groove and is no serious engineering consideration...otherwise we'd see more fretboards buckling. It's the ability of the material right at the back of the neck to carry tension without deforming (a fair definition of modulus of elasticity) that will best determine how far the neck bends under the string load. For this reason, when using a carbon fiber truss rod by itself or in conjunction with an adjustable rod, it is optimal to mount it as close to the back of the neck as possible.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Jim, I'm sorry to hear you are not making the 1/8-3/8 rods any more. I was just about to order some more. This brings up another point. Since I started using Jim's rods a couple of years ago and running them across the neck joint they were almost too strong. The first time I strung one up I had to put a fair amount of reverse force on the truss rod to get some relief. I still use them that way but level the fretboard with the truss rod pretty snug. I don't know why you'd want anything stronger unless there could be some problem with fatigue over the years. This has been a good thread, I've learned a lot. I was one of the folks that just kind of lumped all carbon rods into the same pile.

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.

Another point to all this is:

Regardless of whether you are using a carbon fiber truss rod, the more you plan to have straight wood grain, and also plan to have that grain flow parallel to the BACK of the neck, the more you play to the tensile strength of any particular piece of wood to make that neck stiffer. Wavy grain or grain runout along the neck will produce a relatively weaker neck from a bending standpoint for all the above reasons.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

The plus side to CF being beyond the task of stiffening the neck is that you can also make up for deficiency in any piece of wood that won't twist by balancing the stiffness with CF. I think we're still in the infancy of what composites can do and how we use them in guitar construction, in necks as well as the other parts. They let us separate 'this sounds or looks best' from 'this is strongest'-- we can add strength at will!

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

The CF rods really stiffen the neck, at least the ones sold by Jim. I've had the reverse issue: having to tighten the rod to minimize relief -with mediums-, and I'm always afraid to strip the truss-rod nut.
Do you use heavy gauge strings, or a longer than usual scale? The usual numbers for a 25 1/2" scale are 163.2lbs for lights, 188.8lbs for mediums and 214.2lbs for heavies. So far I've met no one using heavy strings.

_________________
Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

I use light-medium strings but the point was that all strings are around 200 lbs relative to the compressive capacity of the carbon rod.

http://www.mcdonaldstrings.com/stringxxiii.html ......good tension calculator.

The context of my thoughts on this were that since carbon fiber laminates can have a compressive strength of 150,000+ lbs. per sq. inch (processed perfectly), the 200 pound load of the strings to that rod is like a single pixie trying to stomp a volkswagon flat.

....volkswagons do not regard aggressive pixies.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Stuart,

If you ever decide to write a coffee-table book of analogies, I'll pre-order two copies.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Dave Hurd calculated the 'center of moment' for a typical guitar neck, and found that it was approximately at the surface of the neck itself under the fretboard. A CF rod there is in shear, and doesn't do as much as you might think it's doing.

I've been using a 1/2" by 1/10" CF bar, laid up from pre-preg, laid flat, in a 10-11mm deep channel in my classical necks. The space is filled with wood, usually something stiff, and everything epoxied in. I'm assuming I'm getting an 'I-beam' effect, with the CF in tension, and the necks sure don't pull up to speak of.

Laurent,

My first few guitars 30 years ago followed the David Russell Young book. I found aluminum 2024 aircraft grade just as he recommended. I still have two of the guitars. Both are dreadnaughts..the necks have not moved one bit in 30 years. I didn't make them adjustable..simply one 1/4 x 1/2 bar down the center of the neck.
Did it with two dreadnaughts, 2 size 5's, one parlor, one OM. All are OK.
Jay Gordon
Gordonguitarworks.com

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

2024 aluminum will work fine for this - but, it's expensive and hard to find. 6061-T6 is the most common aluminum alloy ( cheap, and available on Ebay, for example) and would work just as well.

Here's why :

All aluminum alloys have the same stiffness ( modulus). It's the strength that varies with the alloy. So, there is no stiffness benefit to using the 2024 alloy.

And, although 2024-T3 is a bit stronger than 6061-T6 ( 50,000 psi yield strength vs 40,000 psi), the 6061 alloy is still more than strong enough. To yield ( permanently deform ) a 1/4 x 1/2 bar of 6061, you would have to bend it to a radius of 62 inches ( or tighter). The neck would have to bend until the relief for a 25 inch scale 14 fret-to-the-body neck was more than 3/8 inch before the aluminum bar in the neck would yield.

That situation is hard for me to imagine -so I'm sure you'd be fine with the 6061.

Phil

hey laurent, i say go for it. might have a brighter sound?
i think the grain of the neckwood is more important than the reinforcement anyway.
you aren't using it as a supporting beam where the load is forcing down in the center, it's more like a bow.
anyway, just my thoughts.
i built an electric strat type (first git build) back in da late 70's (?) with a 1/2" x 1/2" (whoa!) hollow STEEL bar.
heavy as heck, but the sustain! luckily the body was really heavy to balance the weight out.