Official Luthiers Forum!

So i was thinking...should i choose the two way truss rod or should i just place two bars of carbon fiber in the neck?what would you choose? Pros and cons?

Adjustable two way rod 1000%...even if you add CF as well.

Truss rod. If you want light weight, use a single compression rod, or one of the new Allied ultra-light rods.

The main drawback of a single compression rod is that if you put the guitar in storage without strings and don't loosen the truss rod, it can permanently backbow the neck. Easily avoidable, but the owner has to be aware of the issue and have a tool to adjust it. Search "gibson truss rod wrench" on ebay for a compact and inexpensive tool to include with the guitar.

A non-adjustable steel string neck requires compression fretting and excellent leveling skills to get the relief just right. Nylon string necks typically aren't adjustable, but they're more forgiving because the vibrating envelope of the string is larger, so the action is necessarily higher, which makes any errors proportionally smaller.

Google "Truss rod neutral axis." CF rods are just about worthless. Unless they are installed very deep into the neck but even then...

Better to have some adjustment for set up purposes.

Given those choices I would certainly choose the two way rod for a steel string guitar. For a nylon string guitar where I wanted to use a light weight cedro neck I might choose the carbon fiber - a way to add stiffness without adding weight.
I generally use a light weight Gibson style single action rod. They can be installed to provide "some" correction of back bow should it ever occur. There are some advantages to a two way rod and for a larger bodied instrument the weight penalty is negligible.

I agree completely CF rods need to be close to the surface of the neck to be effective but could be let into the back of the fingerboard and provide the same stiffness as being set deep into the neck.

I have usually gone with just carbon fiber reinforcement for banjo necks, but they're under far less tension than a guitar neck and tend to hold their "factory setup" a lot better than a guitar does. The CF does do something to the neck that makes it play a lot more evenly up and down the whole fingerboard, on banjo anyway.

I don't think I'd even consider building a steel string without an adjustable rod of some sort. I'm sort of partial to the Martin style two way rods right now.

Dave

The underlying issue is 'cold creep'. Wood takes a 'set' under any sort of bending load, even if the actual stress is well below the nominal yield point. It goes faster at first, but can continue over a long time.

An adjustable truss rod works by canceling out the bending, putting on an equal load in the opposite direction. This adds to the compression load on the neck, but that's normally so far below the limit the neck can take that it's not an issue.

A fixed reinforcement transfers the load to another material, ideally one that doesn't cold creep. Putting a bar along the neutral axis isn't quite useless, so long as the reinforcement is stiffer than a similar sized piece of the wood, and doesn't show cold creep. The neck pulls up until enough of the load has been transferred to the reinforcement, and then stops (in theory). CF works as well as anything for this,and has the advantages of being less dense than steel, and having thermal expansion that is similar to wood.

Which one is 'better' in a given case is a matter of some judgement, depending on the limitations of the case and what you're trying to accomplish.

As a repair guy I can say the following with absolute confidence.....

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id you go with neck reinforcement over adjustable truss rod you will still have to manipulate the neck by compression fretting to control the neck relief. Don't assume it will just stay straight.
As Alan points one you are trying to control the load by transferring the stress to something else. adjustable rod is my first choice

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Oh i see!there are alot of good information here!!! Thanks for the info i will use truss rod then!