Carbon Graphite neck bracing, too much?

[WillB]

Hello all, this is my first post here, and I would like to ask a question:
I see Carbon Graphite being used as a longitudinal neck brace on instruments that also have a truss-rod - electric basses and long 35'' scale 6 string basses / fan fretted for example.
Now, I wouldn't think twice about running some Carbon Graphite down a double bass neck - where there is a deal of tension, but given its rigidity, I am wondering how the interaction with a truss-rod is.

Has anyone here had experience with this?
How do we know when it's too much for the truss-rod to counteract?
I have seen (youtube) C.Graphite rods laid in basses that I can't believe the truss could possibly affect them. What is your experience with this?

I am at an early stage in my design and building, but intend tol be building several instruments - both acoustic and electric.
Thanks for all the great work I see here. And hope to be here a lot.
Will. xx

[johnparchem]

I have used carbon fiber rods in necks with truss rods, and have had no issues providing relief using the truss rod. Part of the reason for this may be that when the fretboard is glued on the rod is close to the neutral axis so it may not really be stiffening the neck all that much. After I was shown an analysis I stopped using them.

But to repeat no problem with the truss rod.

[WillB]

I have used carbon fiber rods in necks with truss rods, and have had no issues providing relief using the truss rod. Part of the reason for this may be that when the fretboard is glued on the rod is close to the neutral axis so it may not really be stiffening the neck all that much. After I was shown an analysis I stopped using them.

But to repeat no problem with the truss rod.

Thanks for the reply johnparchem.
What sort of dimensions were the rods?
Also I am interested in the analysis you speak of that turned you back to truss_rods only?
In light of this, If I were building a 6-er Bass or a 5string even, I would probably consider double truss-rods, although my experience with those still comes down to one rod adjusted and the second just taking the slack out of - but not really using it for relief.

Perhaps I should save the carbon rod for the Double bass Necks, where there is no such adjustment.
may your necks be the perfectness curve.
WXXX

[seeaxe]

You asked about the analysis on the neck with composite materials

When computing strength and stiffness of mixed materials, it is usual to use the modular ratio to convert all materials to 1 equivalent material.

For a maple neck with an E of around 10Gpa with some CF rods that have an E of around 200 GPa or more, then the pieces of CF are represented as maple in the calculation by the rod dimension times the modular ratio, 20 in this example. Depending on the direction of bending you are looking at and how the rod is mounted.

For a typical neck that is around 20mm (not including fretboard) the CF rods might be 10mm deep or less to avoid daylighting them when shaping the neck, by 6mm wide. They are let into a 6mm slot either side of the truss rod. As we are interested in bending along the length of the neck, each CF rod is represented by an equivalent piece of maple would have dimensions whatever depth the CF rod is, but 20 times the width.

You then have to work out the neutral axis of the neck assembly, including the fretboard which you should also convert by its modular ratio (unless its a maple fretboard of course). Thats fairly standard stuff and all in the books. All a bit tedious and best done with a spreadsheet. Once you have done all that you will be able to compute the 2nd moment of inertia (I) and the section modulus (Z) of the composite. If you go the spreadsheet route its then very easy to change a few numbers and "convert" the CF back to maple and do a with/without comparison

Then you will find that John is right. While multiplying the CF rod dimensions seems like a lot, because they end up near the neutral axis, they don't actually have much effect on bending strength or initial deflection. Its kind of the opposite of the I beam analogy. If you imagine an I beam but with flanges in the middle instead of at the top and bottom of the web, that's what you have.

However the CF will have a big effect on creep deflection. Timber is well known to bend in response to constant stress over time and creep deflection can be as much as 3 times initial deflection. With dry wood its more like 1 than 3 but thats what the CF is there for.

In "the books", Trevor states that the CF tow on the falcate braces is there to counter creep deflections not add strength, although it clearly does.

I put CF in my last couple of classical necks and have not actually put any relief in them other than the natural deflection the string tension causes, which I cannot see with the naked eye. Its OK. There was one slight buzz at first that after a few years has more or less gone. I am putting a truss rod in the latest one just to see what happens but I'm not a big fan as it adds considerable weight in the wrong place, and helps unbalance the guitar. Very different for a bass.

So back to original question...eventually Using CF wont be a problem for neck adjustability using a truss rod. In my view the CF will help long term stability of the neck.

[johnparchem]

Richard thanks for your reply, I had not thought about the creep factor over time. On many of my guitars I use a 2 way truss rod so I can dial it out. But for many of my classical guitars without a truss rod it is worth a thought.

[WillB]

Richard thanks for your reply, I had not thought about the creep factor over time. On many of my guitars I use a 2 way truss rod so I can dial it out. But for many of my classical guitars without a truss rod it is worth a thought.

Yes, on my sixer Bass I have dual (single way) rods, and I think for most of my electric instrument needs, 2 way trusses are the way to go.
Would you say dual 2 ways is O.T.T on a 6 string ‘bass’ ?
I have never had to use the force of the truss on the lighter side of the strings, just tension the lower side with the other loosened, then bring the other side till the nut ‘grips’. Thats been my experience, which may be luck.

[WillB]

You asked about the analysis on the neck with composite

Yes! Thank you also Seeaxe! For you informative reply . It certainly looks like I should have the ‘books’ you refer to.
I simply realised, I had not thanked you for such a great reply.
Will.