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I understand the basic concept of why it’s important to be able to adjust a next for relief and such, but pardon a noob question:
What are the pros and cons of an adjustable rod vs simple cf reinforcement?

B


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Not trying to be glib, but you already answered your own question.

Generally wood is cut for "yield" rather than "use". Unless you can pick through the pile you often have to accept a piece with less than ideal qualities. As demand increases and the resource dwindles this becomes even more true.
I think a reason double acting rods have gained popularity is because they can correct some defects (back bow) from poorly selected and improperly conditioned pieces of wood. With single action rods, tightening the rod to induce some "back bow" and then planing the neck flat can allow the rod to act a little bit like a double acting rod without the weight penalty.
Static reinforcement is probably O.K. in short necks (mandolins) or lightly stressed necks (classical guitars) but for normal scale steel string guitars I don't see any advantages.

Why a truss rod? Because a good setup requires measurements within a few thousandths of an inch but unfortunately, over time, most wood will move one way or the other so the ability to adjust the truss rod can save a lot of extra work. Why a double-acting truss rod? Because sometimes the wood doesn't move the way you think it should.

_________________
Steve Smith
"Music is what feelings sound like"

I think it helps to consider non-adjustable neck support and adjustable truss rods as doing two different things.

Non-adjustable neck support adds stiffness to the neck, period. That added stiffness might counteract string tension in the amount you want, or it might not be enough, or it might be too much (i.e., it doesn't allow enough relief).

Adjustable truss rods are there to bend the neck after all other forces (string tension, natural tendencies of that piece of neck wood) have done whatever they are going to do. I guess an added side effect of being made out of metal and being snugly fitted inside the neck is that an adjustable truss rod might also provide stiffness to the neck, but that is not an adjustable truss rod's primary function. It's primary function is to bend the neck to the precise amount needed to hit just the right amount of relief.

Adjustable truss rods are heavier than most forms of non-adjustable neck support. Older instruments didn't have adjustable truss rods. Those considerations might matter to some folks in some situations.

I'm not sure if that is helpful to you, but conceptually, it helps me understand what I want and how to get it.

For me, I like an adjustable rod because not every player needs the same amount of relief, the amount of relief can need to change when you change the height of the strings, and the relief can change when you change string guage.

I like a two way rod. I mill my board as flat as possible. I mill the board with the rod snugged forward from the neutral zone, just snug. Then I fret. This will add some backbow, which I take out by moving the rod forward just a touch more. Then level the frets. This allows about as much future adjustment as possible without the rod maxing out.

Not sure if you can do all that with a non adjustable set up.

I'm with Ed, things change because of the items he mentioned, and also can change with humidity changes.

Whenever sever dry weather moves in around here (10-15% RF) my usually low action fingerstyle guitar takes to bussing. A minor truss rod adjustment and I'm back in business.

_________________
Joe Beaver
Maker of Sawdust

An adjustable rod addresses a problem that can't be dealt with in any other way; 'cold creep'. When subjected to a bending stress wood 'creeps' in the direction of force. This is a result of the thermoplastic nature of lignin; the glue that holds all of the structure of the wood together. Thermoplastics soften under heat, but they also normally will creep under any stress, no matter how small. We take advantage of the ability to heat bend wood parts, but pay for it by having to live with the creep.

FAA rules say that a wooden aircraft structure must be designed so that the maximum displacement under design loads is less than 1/3 of the displacement that can be tolerated. That tells you how much a structure can creep over time. We see this, and the sorts of fixes it demands all the time. A wooden telephone pole that is not guyed back will lean in the direction of a wire lead out to a house. There's no way the weight of that wire can bend the pole noticeably in the short term, but over time it will take quite a set. Wooden sailing ships, such as the U.S.S Constitution, that are kept as memorials, have to be turned around annually. Otherwise the shrinkage of the sun on one side of the masts will cause them to bend to the south.

Trees use a certain amount of pre-stressing and other adaptations to enable them to hold branches up and keep the trunk from bending in unwanted directions. In extreme cases they use a specialized structure; 'reaction wood', to deal with unbalanced loads. Such wood is unstable, with unusual shrinkage in response to humidity changes, and that, along with high built-in stress, and often a 'fuzzy' grain structure that is hard to cut cleanly and finish, makes it unusable for most woodworking.

The wood we do use thus has no real ability to withstand the sort of long-term stress that string tension puts on a neck. Various expedients have been used over the years to mitigate the issue, including deep 'V' section necks to add stiffness, and lamination of wood, such as ebony, that has more creep resistance. Eventually Gibson patented the adjustable truss rod, which solved the problem.

Ideally, a truss rod would allow the maker to produce a downward force on the neck at the nut that exactly balances the upward force of the strings. This would, in theory, double the compression load on the neck, but that's not an issue, since wood is very well designed to handle straight compression. In practice, there are things that make this less plausible.

One is that the frets themselves produce a backward bending force on the neck, because they fit tightly into the slots. This 'compression fretting' system was used for along time as a way to adjust neck bow; a repairman would have frets with the same size bead, but different tang widths, that would allow for fine-tuning of the neck profile on a non-adjustable neck. This requires some skill and practice to get right, and, of course, the right supplies.

Fingerboards typically have a different shrinkage rate with changes in humidity than the necks they're glued to. As the humidity changes with the seasons the neck can bow upward or downward.

The grain of the wood in the neck may not be straight, and that can give it different stiffness from one place to another, and different creep.

A steel or CF reinforcement in the neck will limit creep, but not eliminate it. As the neck moves the reinforcement takes more and more of the load, until a balance is reached where the further creep is masked by seasonal changes and other 'noise'.

Note, too that the wood in the box creeps as well. Over time the guitar body tries to swallow itself through it's own sound hole, and there's really not much you can do about that in a standard design. Some makers are using 'flying braces' from the base of the neck to the waist to fight this; well find out in a few decades how well it works. This is the most usual cause of high action on an older guitar. No truss rod or other neck reinforcement will stop this. Most neck re-sets are needed because of deformation of the box, not because of bending of the neck.

Although tightening an adjustable truss rod will lower the action a little bit that's not what it's for. Over tightening the truss rod to get the action down because the box has deformed will eventually cause the neck to 'creep' into a back bow, and cause fret buzz in the lower positions that is hard to fix.

Rant over....

Not a rant.
Good info


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