Wood changes dimension longitudinally between .1 and .2 % with varying moisture. Take the low end - that's .017" dimensional change on a 17" (guitar) neck. It doesn't seem like a lot, but it has a substantial effect.
A truss rod generally has 32 threads per inch. And a quarter turn on the truss rod (which is considered to be quite a bit in a setup) changes the working length of the truss rod by less than .008". So even though the longitudinal dimensional change in the neck as a result of humidity changes seems minuscule, it's still very significant when it comes to keeping a neck true. And since the humidity does change in most of our environments, there is a need for an adjustable rod that can compensate.
I don't doubt that your adjustments make a difference. And I don't doubt that you're very good at it, and your customers get their money's worth. My point, which you seem to be missing, is that the adjustments are required because a wood neck with an adjustable rod is an inherently flawed design. The solution is actually the cause of the problem.
Imagine a neck where most of the force is carried by a steel bar, and the wood is mainly just there because it looks pretty. Properly designed, such a neck would never bend, never twist, never change at all, ever, and never need adjusting. If it elongates all of 17 thousandths of an inch, that would hardly even require re-tuning. The way you arrived at 17 thousandths, I would take a factor of 10 off for real life, so 2 thousandths. A human hair is 2-4 thousandths, so that is the kind of numbers you are fretting over - inconsequential.
Unless you have a neck that is designed so badly that it magnifies even minor atmospheric changes into a problem that has to be corrected, which is exactly what happens with a standard truss rod. You have a curved piece of metal inside a straight piece of wood, pushing it sideways in different directions at different places, and even the smallest change in the wood or the tension of the strings can force the entire assembly into a distinctly different equilibrium. It's a Rube Goldberg cartoon. A properly sized and installed steel bar is not going to do any of that, and it will simply overpower the far weaker wood and restrain it from moving at all. I've got a wall full of them, I've built them, it works, and the wood doesn't crack.
Yes, a guitar neck can benefit from a tiny little bit of relief, but it can also work just fine with none. I have them set up both ways, and they both play equally well. Which says to me, might as well skip all the complications and extra cost, and go with the simple solution that never needs to be fussed with. Unfortunately, you can't do this on a bass, and no manufacturer is willing to do it on a guitar, so don't fret, your job is secure.
The major problem stems from the fact that even under minimal stress, wood will creep over time, and requires some sort of reinforcement, which is actually the primary purpose of the truss rod. Adjustment is secondary, and it introduces a whole host of problems. Again, I do not dispute that these problems exist, I am merely pointing out that they are caused by their own solution, and the whole mess could be easily avoided.
Even on a brand new guitar, you can bet that the truss rod needs to be adjusted, right out of the box. That just shouldn't be necessary, and with a fixed rod, it wouldn't be. When you get a new car, do you immediately have to get an alignment?
A guitar is basically just a beam fixed at one end, under slight compression from some tiny wires strung down the length of one side at a small offset, which induces a small bending moment. This is not something magic, it is quite simple to analyze, it follows all the same rules as everything else in the universe. But when you add a bendy, twisty, not really attached complication like an adjustable truss rod, all bets are off, all you can do is fiddle with it and hope for the best.
The simplest and commonest truss rod design must be a single rod in a curved channel, anchored in the wood at both ends. The Fender "skunk stripe" design, and many others. This design derives its bending force by tightening the rod, which places the neck in compression, and the rod in tension. Since the rod is effectively curved due to the shape of the channel, and the heel is fixed, the effect will be to pull the headstock backwards. The effect of the tension in the rod is magnified by the length and shape of the neck.
So you have the steel acting against the wood longitudinally to produce a lateral force that results in a displacement at the headstock, which is going to be highly sensitive to the balance between the steel and wood components. And, as you pointed out, wood is not perfectly stable with respect to humidity. Even an infinitesimal change in the length of the wood is going to result in a significant change in the tension of the rod, which will in turn result in an exaggerated deflection of the headstock.
A system like this is totally unstable, you could end up adjusting it every day. The only reason you don't is that the wood is pretty well sealed away from the atmosphere by the finish, especially nowadays with poly finishes that are completely impervious. Maple fingerboards are also finished, while rosewood boards are very oily and relatively immune to moisture. The worst thing you could do to such a neck is sand off the finish and leave bare wood exposed to the air.
The fact that it takes just tiny adjustments to the rod - 1/8 turn on a 10-32 thread - shows just how overly sensitive such a setup is. Longitudinal changes are translated into lateral displacements ten times the size or more. Now remember, the rod was put there for long-term reinforcement of the wood against creep. But in the process, what this design does is introduce massive short-term instability. This design should have been abandoned on the drawing board. Instead, it has become ensconced in luthiery tradition.
Now take a properly designed adjustable truss rod, as found on pretty much any Asian or Mexican import guitar. It will have 2 steel rods that act against each-other, regardless of whether it is single or double acting. This internalizes all the longitudinal stresses to the steel, which is unaffected by humidity or even temperature. The rod is relatively free-floating in the channel, and imparts only a lateral force to the wooden neck. Infinitesimal elongations in the wood have no effect on this system. A neck like this will require practically no adjusting once it is set up, as the only variable left is the strings. This system actually works pretty well, although the wood is still prone to twisting and warping as it ages, and the truss rod is going to be no help with that.
Now take a solid steel bar. No adjustment possible, no adjustment necessary. If you did it right the neck is never going to move in any way, the wood can be regarded as little more than padding, although in some designs it actually contributes to the stability of the bars along their weaker axis.
I'll bet that 90% of your customers are "skunk stripes" or the equivalent. That rod design is just so bad, it is a good reason to buy an Epiphone instead of a Gibson, or a Squier instead of a Fender, or a Mexican over an American. I think those rods are also found in most Rics. Ric even took a step backward and went from the (relatively) well designed rod in my 4001 to a 'skunk stripe' in the 4003, although they claim it is better. My 4001 is actually quite stable. I'll bet you see a lot of 4003s.
Now that I look over my little collection, I see that I own quite a few skunk stripes, and they are not hysterically unstable. I credit that to the fact that they are well-sealed against humidity, and they are kept under relatively constant comfortable indoor conditions. In fact, under that totally impervious glass-like layer of poly, I don't think the wood has seen a change in moisture content since it left the factory. And while I do very fine setups, I am simply not a hair-splitter when it comes to playing. There is such a thing as 'good enough', in engineering, and in life, but apparently not in guitars. I wonder how many concerts Geddy Lee has canceled because of the humidity?
Finally, I must repeat for careless readers: I am talking about guitars, not basses. You simply cannot get away without an adjustable rod in a bass, but for a completely different reason. A bass needs a curved neck. A guitar strictly does not, although it is an option.
You seem like a capable guy. Go to the store, get some steel bars and a piece of maple and some other parts, you know what, and build a neck and see if it doesn't work just fine. I have. Or find a nice old Sears Silvertone that wasn't abused, and do your setup magic on that. See how it plays.
I think this reconciles everything you've said with everything I've said. I started this thread to talk about woodworking, not the metaphysics of Gibson's marketing department. Can we get back to that now?
On the other hand, I've never gotten my hands on a guitar, new or used, that couldn't benefit from a proper set up. I'm certainly not disputing the necessity of that.
