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Thread: carbon fiber instead of truss rod

  1. #101

    Default Re: carbon fiber instead of truss rod

    I've done a bit of reading and my conclusion is that the opponents of my theory are quite incorrect in their assumptions.

    Dave and Rick are correct on one point: the neutral axis does not move, only because it is theoretical and based on everything Dave said it was -- profile, MOE, etc. However they are both wrong in stating that this neutral axis does not undergo net compression from the state of no string tension to a state of string tension. It does in fact compress under this load. I was wrong in calling this a shift in the neutral axis, it is nothing more than a net change in the length along a plane due to external forces, and my drawing showing little or no change in the compression at the back of the neck still holds true, only it does not represent the neutral axis, it represents stability between state changes, non-movement. The area of the cross section nearest the fretboard is subject to the most compression in a neck's profile, purely as a result of the component force vector parallel to the fretboard face, and therefore the most net shrinkage. A CF rod installed under no string tension (obviously) and then later compressed under string tension, is in fact subject to net compressive forces, and purely so in almost any arrangement within the neck, but even moreso nearest the fretboard. Whatever happens after equilization of forces during stringing, happens around the centroid, the neutral axis. But from a state of no strings to strings, that rod's very first job is to fight compression.
    That's all I meant to say from the getgo............ and you can quote me on it.

  2. #102
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    Default Re: carbon fiber instead of truss rod

    And what if you put the carbon fiber rod at the back of the neck? Do you still think it's under compression?

    You just don't get it...

  3. #103
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    Default Re: carbon fiber instead of truss rod


  4. #104
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    Default Re: carbon fiber instead of truss rod

    OK, here's another way to understand it...

    Truss rods work in tension. Tightening the nut applies more tension to the rod just like tuning up strings apply more tension to the strings. Yes, the tension on the rod applies compression to the neck, but it's doing so through the force of tension. You are balancing the tension of strings with the tension of the truss rod, and your neutral axis is in compression. But there's still a neutral axis...the balance point between the forces in opposition. Think of the neutral axis as being like a fulcrum point.

  5. #105

    Default Re: carbon fiber instead of truss rod

    Rick, you're not trying here. My illustration is quite clear: the back of the neck is in tension due to the vector of pure bending moment. I'm admitting, you guys are right on this part man. But what you're not taking into consideration is the state change from no strings to strings. You're acting as if that neutral axis were made out of kryptonite, that it were physically incapable of being compressed from one state to another of equilibrium. This is just simply not the case. What you have is that once that rod under the fretboard is under compression, it is then subject to any tensions around the neutral axis, the bending moment. But only when you factor the first part in.
    Yes it is under tension when placed in the back of the neck! It is infuriating that you neither seem to read what I've written, or ask questions on points I may be being unclear on! My argument is that there are two components and as far as I can tell you're only seeing one. The neck first undergoes net compression, and this is why that first G string drops down two steps when you put the rest of the strings on, the fretboard remaining flat the while. If the neck were only subject to the forces you are saying it is, if you strapped 150 lbs pulling away from the nut, you'd have massive bending around the neutral axis, and massive relief as a result -- the back of the neck under intense tension and the front of the neck in intense compression. But the 150lbs is split into two things. One of the above, the other which is not being considered because you are forgetting "to put the strings on"..

    You would do well not to cast off other's thoughts without any numbers or justification in the future. You can't reason your way out of this with reputation. If I'm wrong about the strings adding net compression, then please show me why. A piece of wood in pure, net compression in a vise retains its neutral axis through the compression where, let's say, the block shrinks 1/64". The neutral axis has shrunk 1/64" in this case! Any residual bending moments placed on it after that, revolve purely around the neutral axis.

  6. #106

    Default Re: carbon fiber instead of truss rod

    we co-posted, but Rick, I already admitted the neutral axis. ?

  7. #107
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    Default Re: carbon fiber instead of truss rod

    I think this is a great discussion, and I get a little lost in the science of trussrods, but to me the issue of string tension is about the back of the neck stretching as much as the fingerboard area compressing. This is where the entire neck is weakest and wants to give way to the upward pull of the strings. With a traditional adjustable rod the point is to counteract the string force within the neck and relieve the stress on the back of the neck.
    So if your going to use CF exclusively it makes sense to me to do it like A. Mowry and install a big stiff bar in the center of the neck which will allow you to get it deeper in the neck than with a pair of bars on either side. That seems to be how CF is used in conjunction with an adjustable.

    To Nick. When mandolins started to be popular through bluegrass and it became apparent that there were very few options for buying a good mandolin, the pioneers of the small builder market almost exclusively modeled their instruments on the Loar which included the adjustable.
    But in the meantime in the guitar building world, while some stuck with tradition ( actually, tradition in the steel string would lead to a steel t-bar) there was and explosion of innovation, and Rick was right there in the thick of it.
    Alternate materials and methods have slowly been working their way into mandolin construction, but there are plenty of adjustables to be had. You've named many of them. But there are plenty of examples of CF necks that have stood the test of time.

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    Default Re: carbon fiber instead of truss rod

    At this point, we just may be arguing on the same side of the engineering, but looking at it from two different points of view.

    If you take the strings off of a mandolin with a truss rod, is the fingerboard now in tension...being stretched? You betcha... And probably the neutral axis has moved because its location is dependent on the forces acting on the beam.

    So maybe we don't have a problem, Houston...or Cape Breton...or Dave.

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    Default Re: carbon fiber instead of truss rod

    Brain, I swear I'm gonna charge you a consultancy fee for teaching you physics, since I ain't gettin' paid for the time I have to put into these posts.

    You are overlooking something huge, namely the anisotropy of both wood and CF composites. A crystalline metal is isotropic, meaning that its' properties are the same in all directions. Composite materials like wood and CF are enormously anisotropic. Along the direction of the fibers, the MOEs are extremely high - multiple megapascals. At the atomic level, closest packed metallic crystals are pretty uniform in all directions. To be compressed, the atoms have to be pushed closer together; to be stretched, the atoms have to be pulled farther apart. What resists the atoms being pushed closer together is repulsion between the nuclei; what resists the atoms being pulled farther apart are the various types of attractive forces between atoms - electron-nuclei attractions, Van der Waal's attractions, etc. Now, even with an isotropic material like a metal, if the shape is long and thin, like a beam or an instrument neck, the sum of the forces in the long dimension is much greater than in the other dimensions. Add in the anisotropy of wood and CF, and the forces are overwhelmingly greater in the long dimension.

    You were getting warmer with your first diagram. That the moduli are so high in the long dimensions means that the resistance to pushing the atoms closer together or pulling them farther apart in that direction is very great. Wood and CF composites very effectively push back against the long blue vector in your first diagram. But the little red component vector in your first diagram is the one that does the bending, since the moduli are much, much lower in that direction. And once you have bending, you have a neutral axis, and the atoms above the neutral axis in the beam or neck are pushed closer together, and the atoms below the neutral axis are pulled farther apart. Hence, you have shear from compressing above the n.a., and shear from stretching below the n.a. Ergo, the material above the n.a. has to be in compression, and the material below the n.a. has to be in tension. When we make necks from the traditional materials at hand, we are attempting to resist the shear almost entirely with the high longitudinal M.O.E.s of those materials, since the M.O.E.s in the other directions are so much lower.

    Btw, no "assumptions" here. The formalism of the statics and dynamics of beams goes back over a hundred years that I know of. The technology for good laboratory measurements is not as old, but still goes back, what, seven decades or so?

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  10. #110

    Default Re: carbon fiber instead of truss rod

    Dave, that's not a problem. I will gladly offer you a couple pints of German suds!

    But you owe me some back, now I need a beer!

    I think we are in agreement, as Rick said. You reiterated and restated a number of things I just put forth, either because I did not say them in the scientifically proper way, or what I don't know. But I just agreed with what you wrote, Rick says we're in agreement. Could we get a:

    <handshake>

    You may not agree that there is as much value placing a truss rod directly under the fretboard, but I think I have presented a fair argument that at the very least could inspire the more mathematical, computer-inclined person to actually simulate some of these things. Not only forces once the instrument is happily strung, but the before and after as well. Which materials are taking the most stress, and where. It seems just crazy that we're battling this out over the cloud of uncertainty we all agree is there -- and it is a cloud without testing and numbers. Would not an article of this nature create some stir in a future Guild journal? The world needs these numbers....

    ps, I did not overlook all you said, the diagram is purely a description of forces and how I estimated they would have effect. Estimates have their role in many theory applications.

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    Default Re: carbon fiber instead of truss rod

    Suds are good. Btw, you didn't give your early impression(s) of Cape Breton. Find a house yet?

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  12. #112

    Default Re: carbon fiber instead of truss rod

    Will be arriving shortly! My better half is there, so by default, as they say: "home is where the heart is."

  13. #113
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    Default Re: carbon fiber instead of truss rod

    I love Cape Breton! Good music.

  14. #114
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    Default Re: carbon fiber instead of truss rod

    But there are plenty of examples of CF necks that have stood the test of time.
    27 years on mine and still straight and true.

    "You can pick your friends, and you can pick your nose, but you can't wipe your friends off on your saddle."

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    Default Re: carbon fiber instead of truss rod

    Informative thread guys. I'll add that in my personal experience and it also depends on the neck wood density and dimensions. I have a Morris A4 (longneck hybrid) that has a very slim neck with a non-adjustable steel rod in it. It is dead flat unstrung, but under string tension the neck bows considerably. Enough that it is impossible to dial in an action as low as I would like without buzzing at the upper frets. With j74s it's a serious problem. j73s are more manageable, but far from perfect. By comparison, and my teens A1 stays dead flat with j74s with no reinforcement at all (beefy, short neck).

    My take-away is that if you like slender necks and heavier strings, you may want the ability to tweak the neck relief. Especially if you intend to experiment with different string gauges. With beefier or shorter necks, it may not be an issue.

    I ordered my new mando with an adjustable rod. It should be here soon!

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    Default Re: carbon fiber instead of truss rod

    Jim, when Porsche switched from air cooled motors to water cooled I was not upset like many other sports car fans. I knew the new motors would have more power and run cleaner for many years to come. It was a better idea. But as for the great mandolins from the masters I feel the adjustable truss rod is a very good idea that has stood the test of time. Like my old Gibson A type that is 86 years old and still very straight and many the truss rod mandolins after it, the truss rod is time tested. Really, I think CF is fine, just not for me. I want the option to adjust. We all know that the truss works. We all know that CF in the neck probably works very well. I always get comments on my instruments on how easy they play and how straight the necks are. I bet if you were to do a study, most mandolin players would want the ability to adjust. Because most players I know set up mandolins to play just like they want. I understand the truss rod is not for adjusting action but it does so anyway, Nick
    ntriesch

  17. #117

    Default Re: carbon fiber instead of truss rod

    You know what else is time tested. War. Yeah, you know those diplomatic French, trying to talk stuff out and all, look where it got them in WWII.
    Slavery. Hey, work for free, why not? They built one rich nation, why not others?
    Hey, maybe CF will be time tested one day. Then we can count it among the great time tested things of history.
    Your arguments are priceless Nick.

  18. #118
    kestrel
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    Default Re: carbon fiber instead of truss rod

    Price-less.

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    Default Re: carbon fiber instead of truss rod

    This thread has obviously generated a bunch of interest. Folks will read all this stuff and will use it to buy their mandolins now and in the future. It's all really about to tweek or not not tweek? That is the Question. I want to tweek! I just got home from work and I will grab my Weber and a hot cup of coffee and work on the Butch song, "Black Mountain Aire". Almost got it wired! Nick
    ntriesch

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    Default Re: carbon fiber instead of truss rod

    I wouldn't buy an instrument weighing the decision heavily one way or another based on this thread. I think that if an instrument is built by a good maker, either system will work; there are just trade-offs. I would buy an instrument based on liking the tone as the top criterion, then go to playability and how I feel it's likely to stay stable...or not. I'm after results in a new instrument, not stories. If I want stories, I'll get a vintage instrument with lots of ghosts in it. Back stories about the why and wherefore are great, and I certainly sell instruments with that in mind, but tone is king and trumps all else.

  21. #121
    Registered User amowry's Avatar
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    Default Re: carbon fiber instead of truss rod

    I think it's been illustrated here that there's more to it than the question of adjustability. If that were the only issue I'd choose adjustability in a second. However, for me, the advantages of CF outweigh adjustability. Different strokes...

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    Default Re: carbon fiber instead of truss rod

    Well stated, Andrew.

    Whether a neck is reinforced with CF or an adjustable rod does not necessarily mean it will be a good neck. As always, it depends on HOW it is arranged. I have done set ups on some Pac rim imports and from a domestic maker that the truss rods were absolutely useless. Rods either glued and locked in their slots or set too high in the neck beam. I have also seen CF reinforced necks that flexed too much due to the fact that the CF was either too small or not placed strategically. The more flexible the neck the more critical the reinforcement. The design parameters set forth by tradition usually work well enough, but there exceptions. Some necks warp, twist, or bow regardless of the adjustable truss rod.

    Flat top guitars are still being made in the traditional design that will guarantee the neck will need to be reset at some point in the future. Rick Turner makes acoustic guitars that probably will not need that procedure because he has designed the instruments to resist the typical forces and deformations. Tradition has it's place, but remember, what now is Tradition was at one time the new kid on the block.

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    Default Re: carbon fiber instead of truss rod

    Thanks for the mention, Michael. Actually, you can "reset" the neck on my guitars in about ten seconds, and that's if you fumble with the Allen wrench. The neck angle is totally adjustable, and that is the means for adjusting action, which in turn means you don't have to mess with the bridge saddle height.

    And I've got carbon fiber assisted back braces, and carbon fiber flying buttresses, and two 1/8" x 1/2" CF rods in the neck, ............and an adjustable two way action truss rod.

  24. #124

    Default Re: carbon fiber instead of truss rod

    That's some mighty reinforcement, Rick.

    To add to what Michael said, there is also a question of how you treat the rod at installation. Was the surface properly readied, is the adhesive the right one, was it simply laid in a bed of glue leaving a thick glue line, or was it clamped to maximize squeezeout.

  25. #125
    Registered User grandcanyonminstrel's Avatar
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    Default Re: carbon fiber instead of truss rod

    Michael:

    'Careful about giving away trade secrets....Martin neck resets are a nice reliable source of regular income for anybody that works on guitars; almost as reliable as old Kay basses, which get a lot of carbon fiber retrofits under the fingerboards!

    j.
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