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Good point about the adhesive.

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Formerly known as Adaboy.......

Why don't you use hollow carbon fibre rods instead of solid ones? That would give you the same stiffness with far less weight like an I beam.

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The hollow ones are tubular. The strength we're looking for is in the vertical dimension so, for example, I use a 1/8" x 3/8" rectangular rod. A tubular rod large enough to get the strength in the dimension I want would be too wide.

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Steve Smith
"Music is what feelings sound like"

Hollow rods are not as stiff as a solid rod of the same diameter, but they are stiffer per unit of weight which is not a significant factor in this application.

Hi Don,
I've used Dragon Plate http://www.dragonplate.com/ecart/categories.asp?cID=20
and
Aerospace Composites http://www.acpsales.com/.125-x-.325-Rec ... r-Rod.html
I've had good service from both.

Regards

Ron

Thanks for that info Ron...

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

The issue with 'cold creep' does not seem to be material _failure_: you don't see fiber breakage/crushing on either surface. It seems to me, from what I've seen, that it's a movement in _shear_, and I suspect it has do do with flow in the lignin 'glue' that holds the cellulose structure of wood together. Lignin is a thermoplastic, like taffy or hot met glue, which is why we can bend sides. Many such plastics suffer from cold creep; they act like thick liquids rather than solids, and flow under any sustained load. Us old guys remember 'nylon thump' in tires.

If there were no cold creep any guitar neck would simply pull up until the fiber strain in the neck took up the load, and then it would stop moving. This isn't what happens: the neck keeps on creeping on, although the movement slows down over time. Adjustable rods actually work by putting a countervailing stress on the neck, to balance the force and give a resultant that is simply compression. Non-adjustable rods replace part of the neck material with something that either has a much higher Young's modulus (ebony in a cedro neck on a classical guitar, for example) or that has both a higher E value and no cold creep. With a fixed steel of CF rod, creeping of the neck throws progressively more load on the rod, and the movement stops when the rod is carrying pretty nearly the whole load.

I've always tried to put CF rods as far down in the neck as possible, under the impression that this takes the maximum advantage of the high E value. I Use a wood fill strip between the fingerboard and the CF rod, and epoxy things in. I could be wrong...

and I suspect it has do do with flow in the lignin 'glue' that holds the cellulose structure of wood together. Lignin is a thermoplastic, like taffy or hot met glue, which is why we can bend sides.

///snip///

If there were no cold creep any guitar neck would simply pull up until the fiber strain in the neck took up the load, and then it would stop moving. This isn't what happens: the neck keeps on creeping on, although the movement slows down over time.

Nope, that is incorrect.

The creep in woods does slow over time, but it will NOT continue forever. If it did, nothing made of wood could survive under any load, not even simple gravity. While lignin does appear to act like a glue that holds the wood's fibers together, its true reason for being is to allow trees to bend instead of breaking.

Wood is mostly made up of gazillions of microscopic fibers, all of which have tiny "fingers"(they appear like little hairs), which will hold onto one another just fine, without lignin, but if that were the case, any movement, such as induced by a wind load, on the tree would cause a breaking of some of these 'fingers', with repeated loads eventually break enough 'fingers' to weaken the tree to where it would fail and come crashing to the ground. So in this light, the lignin is more of a lubricant than a glue.

When wood is subjected to a constant load, it will yield(bend, cold creep) the most at first, because the fibers' fingers don't have a tight grip with one another yet, but as the yield/creep continues, more and more 'fingers' grab onto one another, and eventually we have a near-solid material and the yield/cold creep ceases, unless the load is such that the member will fail completely.

Don't think that wood fibers alone can be very strong? Take a sheet of newspaper, just one sheet, twist it into a long thin string, and pull. Pull it straight. If there are no tears in the sheet, it will take a very strong person, and then some, to break it. Yet it's only about .004" thick... All wood fiber, no binders(glues). Or think about the cotton you're likely wearing, held together with with thin cotton thread. All teeny tiny fibers, holding onto one another via their hairy little fingers...

Great explanations Alan and Mario !

What I have learned so far is to place the CF as low as possible in the neck to avoid the upward strain from the constant pressure of the strings.

If you make a bolt on neck - I guess the CF stops right at the body joint ? If making a dovetail joint - is there any value to continuing the CF onto the body - routed through the surface of the sound board into the neck/head block ?

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Thanks Todd. I'd never heard of the 'velcro model' before, and went hunting around yesterday evening through some of the stuff on my shelf to see if _anybody_ said anything like that. Nope. All the references I have talk about wood as you do: cellulose fibers glued together with lignin.

Grumpy wrote:
"The creep in woods does slow over time, but it will NOT continue forever."

I suppose this becomes a semantic issue in the end. In one article I uncovered (Gadd: "Stress Relaxation of Wood" Newsletter of the Catgut Acoustical Society, #41, May '84,pp17-18) the author uses the idea of a 'half life' to talk about the ongoing relaxation of his samples over time. In his case, he made identical sized pieces of spruce, which were then loaded into a fixture that held them in a deformed shape. The fixture was put in an oven and held at a given temperature for some time. The samples were then allowed to cool, and removed from the fixture, and the deformation was measured. If an hour in the rig gave a deformation of 2 units, then two hours would result in 3 units (2+1), and another hour would yield 3.5 units (2+1+1/2), and so on. It's easy to see that in such a sequence you'll never get to zero: every added hour gives a little more deformation. On the other hand, after ten or twelve hours the added movement of another hour won't amount to a lot, and after a few weeks it will be impossible to measure.

Rogers, in another article in CAS Newslatter #36, Nov. '81, p4, "A note on Rib Bending and Lignin", states that lignin is a mix "high molecular weight chemicals that acts very much in the same fashion as the usual thermoplastic" with a softening temperature of around 190F. "Above this temperature it creeps slowly when deformed".

When I did my science project on plastics, back when dinosaurs ruled the earth, one of the distinctions I learned about was that between 'thermoplastic' and 'thermosetting' resins. Thermoplastics have no regular structure, and act like a thick liquid, can be melted, and deform under _any_ stress if it's continued for long enough. Thermosetting resins do have a regular structure, don't melt, and spring back when a load is removed, so long as they have not been stressed beyond a yield point. Cellulose is a 'thermosetting' resin, lignin is thermoplastic.

Wood does not creep noticeably under pure tension or compression. Bending loads impose tension and compression stress in the surfaces, and shearing stress toward the center of the member. Shearing in the thin lignin layer between cells within the wood allows it to creep, and relieves some of the tension and compression stresses in the surfaces. When you bend a side, you heat it above the lignin softening temperature, hold it until the fibers have moved enough to give the shape you want, and and allow it to cool off in the new shape. There will still be some tension and compression forces within the wood, which cause it to spring back, but over time, and with repeated moisture cycling these are reduced and the shape becomes more stable.

Certainly cellulose fibers can be very strong. In the process of making paper these are macerated, felted together, and rolled out into sheets which are strong in all directions, but a little stronger along the grain. I've found (guitar content) that gluing a sheet of paper to the surface of a figured side can reduce the problems of splitting out quite a lot. The paper stays strong, while the heat and moisture of bending weakens the cellulose/lignin mix that glues the cells together, which is one thing that allows splits to start. You have to scrape the paper off later, of course, but it beats tossing an expensive piece of figured wood out.