MIMF

If you could build your perfect neck with perfect stiffness - like Ken Parker's carbon fiber cored neck - would you do that, or do you value the ability to adjust relief more than the lack of need to ever adjust relief? I am toying with the idea of building a neck stiff and stable enough to not need a truss rod, but I can't decide if the ability to adjust is important enough to build necks flexible enough to bend under tension of a truss rod. The neck would be machined aluminium with spruce or balsa wood core and wrapped with maple for looks and feel.

Adjustability is VERY important. Just because a neck is very stiff does not mean it won't move some and need adjustment.

Aluminum necks have been done (I forget the name) and they are reported to be very cold and uncomfortable in the hand.

Martin learned to put in trussrods..... at least after a dozen decades.

Martin has admitted that they only went to adjustable rods because of consumer demand. They had fewer warranty issues with solid, non-adjustable necks.

I offer mine either way. All of my personal guitars and mandolins are solid(steel tube), non-adjustable. Heck, my telecaster knock-off has a one-piece spruce neck, with NO reinforcement whatsoever... All are straight, and don't move with the years or seasons...

How do you adjust relief without a trussrod? I'm not challenging, just asking.

Traditionally Martin repairmen gor very good at 'compression fretting'. They had several different sizes of fret wire, where the crown was the same but the tang variied in width. A heavier tang would force the slot open, bending the neck back slightly, while a narrow tang would allow it to bend up a little. By judicious mixing and matching they could control the relief.

In a sense adjustable truss rods are of more use to the maker than the player. They can also be a big pain for makers. They're useful because they allow us to make a one-size-fits-all neck in a sense. Most players tend to stick with a certain gauge of string, and if you know what that is you can set the neck up for it. If you don't you would need to guess. With a rod we don't need to worry about what gauge of strings the customer will use so long as the rod can handle the load.

On the other hand, that presupposes the customer knows how to use the rod properly. Many don't. You can do a lot of damage that's hard to undo quickly by using a rod wrong; as a means of adusting the action height rather than the relief.

How do you adjust relief without a trussrod

You don't have to if it's built correctly and is stiff enough. In the rare event that it must be adjusted, see Alan's post....

13 years ago i have built a bass without an adjustable truss rod - it is perfectly straight, but indeed, the string tension has to be chosen with a bit of care. It may not become too high.

I might have mentioned it somewhere - i have been using a central stripe of Machiche, 16 mm wide, and a fingerboard of Katalox, 100 mm thick. Fretless. 80 cm scale (32"), 5 string. The neck thickens to full height after the 14th fret, so the thin section is kept as short as possible. If it was fretted, the neck would probably be too straight...

That demonstrates that necks without adjustable rods can be built. The ingredients are:
wood with a large Young's modulus - like Ebony, Katalox, Machiche. Among the Europane ones mostly Birch, Olive - here i am still searching. It is hard to find estimates because most people mix "hardness" with stiffness - which is a wholly different thing: hard woods like Maple or Birch have a surprisingly SMALL Young's modulus and need to be handled with care in this application.

(my old archtops have necks made of Birch and no trussrod).

The neck needs to be relatively thick - stiffness goes with the 3rd power of thickness, so 0.5 mm more or less matter. The thin area should be as short as possible. So give it a volute an thicken it as early as possible, maybe on the bass side in order to be safe.

Another option is being used by Rick Toone who uses a square rod of aluminum. Its Young's modulus is larger than the one of the strongest woods, but not so much that my approach of chosen hard wood would not be an alternative; You have to use relatively thick walls to obtain the necessary stiffness. Again the height is important - any stiff bar should be as high as possible in the 1st experiments. Maybe builders with more experience can systematically reduce the stiffness in order to control playability and sound. But the 1st attempts should be "make it as stiff as possible".

I am going to evaluate this a bit further when it comes to building the neck of my six string.

Stiffness is only part of the issue; the other is cold creep.

Lignin is a thermoplastic. When we bend sides we heat it up to the point where it will flow more or less freely, and this allows the cellulose fibers of the wood to move into new positions, where the cooling lignin holds them (more or less). Like many such materials, such as tar or hard candy, lignin will still flow even at low temperatures, albeit slowly. Any continued bending stress on wood will eventually cause it to move, and take a set in the direction of the force. Since strings are above the neutral axis of the neck they will eventually cause it to creep.

Stiffer woods, of course, bend less under a given load, which slows things down. Some woods also seem to resist creep better than others. However, there's no way to make a purely wooden structure that won't do this eventually.

A truss rod can apply a countervailing force against the upward pull of the strings. In effect, this cancels out the bending by converting the load to simple tension. It doubles, more or less, the compression on the neck, but that's so far below the limit load the wood can take it's not an issue.

Reinforcements, such as steel or aluminum tube or CF, don't stop the wood from creeping. However, as it does creep it transfers the load over to the rod. Since the rod itself doesn't creep this limits the amount that the wood will move; once most of the load is being taken by the rod the creep slows to imperceptibility. A really stiff rod, such as the CF D-tubes, essentially makes a neck that won't move.

Thanks. BTW: even stone will creep - that's the mechanism enabling continental drift (actually thermal convection in the earth's mantle, but also that requires creeping).

Two of my guitars are about 50-60 years old (the two archtops i recently restored). One required a correction of the neck under heat 30 years ago, the other one shows some deformation from the tension even a very slight permanent one. But it is still not a problem.

So it appears possible to make a neck from wood alone, but You have take great care with the wood selection, and the neck will come out pretty thick. Something many guitar players do not like. Moreover, You have also to take care it will not come out too heavy, which is often not desired - these stiff woods all have a pretty large density. So making a good neck entirely from wood appears to me pretty tricky - although i do advocate for it.

(Somewhere in the depth of my hard disk i must have a spreadsheet for estimates of such headstocks. Will of course also work with carbon fiber or aluminum reinforcements.)

It appears to clear to me, that such experiments are ok for an amateur luthier - but for someone who gets his living from lutherie and who has to meet the expectations of his customers it'll probably be a no go.

BTW: after some first estimates on the stiffness of Rick Toone's aluminum trussrod i am a bit surprised that it works...