MIMF

My sentiments exactly. The upcoming issue of GAL quarterly should have an article regarding the reliability, dimensions, effectiveness and other metrics of 8 truss rods in bass guitar necks that I tested in 2014 for that year's convention presentation. There's an add-on piece about installing a single curved rod with some photos.

Thanks for the explanation on neck access. I can live with through the sound hole. I think I will try the Martin rods mentioned for my next build.

Just got 4 Martin truss rods. They are an inch or so shorter than the LMI rods I've used. With a 2" deep neck block I must move the end point away from the nut or set the other end an inch down inside the neck block. What is the preferred way?

Alan Carruth wrote:...The issue is that the box deforms under the constant string load. This is less of a problem with Classical guitars, with the lower string tension, but even they have issues over the long run...

Thank you for this info, I was thinking about spanish heel on steel strings...

To keep the topic, I have talked with some old school luthiers who adviced me against the two-way truss rod, saying that it would add mass to the neck and therefore it would "suck" energy from the top, what would result in a worse tone...
And that carbon fiber reinforcements don't work well on long therm, because of how it behaves under continuous tension.

Can you guys say your opnion about that?
I never build a steel string, only a classical guitar, but my desire is to learn the steel string...
I always thought that the idea of have a truss rod that WILL fail and make you need to do a neck reset a really lousy thing to do and never understood why people still used the single rod (I came from electric guitar, obviously). That perspective that a two-way could compromise the tone of the acoustic would be a reasonable reason to avoid the two-way, but I still want to think in something that is more reliable than old truss rods, they just don't do it good enough...

So, can you guys give me some insights on this matter?
Thanks a lot

The truss rod has absolutely nothing to do with a guitar needing a neck reset. Also carbon fiber does stand up to long term tension. That is one of its greatest properties. I have no comments on the other myths.

"I have talked with some old school luthiers who adviced me against the two-way truss rod, saying that it would add mass to the neck and therefore it would "suck" energy from the top, what would result in a worse tone..."

I would think just the opposite - a heavier, stiffer neck would be less likely to take energy away from the top. Think of solid body guitars - the reason they sustain so long is the solid body doesn't absorb the energy from the strings as readily.

Double action rods are more needed for lightly strung electrics where "back bow" is a definite possibility. With an acoustic guitar's higher string tension back bow is less likely and some provision for "double action" (for slight back bow) can be built in to a single action rod.

The Blanchard double-action rod I've been using is no heavier than most single action ones, and certainly lighter than the channel rods that Grizzly sells.

A truss rod won't prevent the need for a neck reset, but a Spanish heel will make it impossible to do should in become necessary.

One of my oldies showed up recently in need of some TLC. It's a steel string that has a CF rod, but after 31 years the neck is bowed enough to need to attention. I suspect it has pretty well stopped moving by now, but it moved enough to require some work. Fortunately for me, a friend of mine who also builds bought it, and will do the work. Now I know how that works out in the long run....

I'm with Alan. I've been using Blanchard rods for a few years now and I like the fact that I can order the exact length I need. For me it is well worth the extra expense.