What kind of neck reinforcement or truss rod should I use for my instrument?

[Liam McGillivray]

I'm building an instrument which doesn't fit into any existing category, but it's closest to a semi-hollow body fretless bass guitar than anything, although it will probably be played upright.

I was going to build it without an adjustable truss rod, and use a square steel tube and maybe some carbon fibre rods. But I should ask if I do in fact need it to be adjustable. The strings will be quite loose, but I want the neck to be very stiff so that it gives more sustain to the upper harmonics (like a piano, which has a cast iron frame). Will either a good steel or carbon fibre rod remove the need for adjustability? Classical guitars manage to work without an adjustable truss rod, with some newer ones using carbon fibre reinforcement. But because my instrument will be fretless, variations in curvature won't be as detrimental (although I do want a slight curve to the fingerboard).

Here are some factors to consider:

• Fretless
  36" scale length
  Want it to be as bright as possible.
  Neck should be as stiff as possible to sustain high frequencies. (I think that's how it works)
  3 strings loosely tuned. The open strings range from B0 to F2.
  Live in Vancouver; a humid place.

What's stiffer? Carbon fibre or steel? Will either alone be good enough to keep a fairly constant curvature? Is it possible to have an adjustable truss rod in addition to a square steel beam?

[Brian Evans]

Carbon fiber is a composite made up of carbon fiber strands in an epoxy matrix, thus can be quite variable. You can make it so it's a springy, flexible structure, or a very stiff unyielding structure. Steel is not variable at all, all steel has roughly the same stiffness (but with different yield points), so a steel tube will be quite uniformly stiff and very predictable. Carbon fiber is usually considered to be about 5 times as stiff as steel in a given structure, and weighs less as well. So you either go all overboard and make the neck stiff as all get out, or you go all engineering and calculate how stiff you need it to be, and design the structure to achieve that stiffness. So the answer to your question "will it be stiff enough" is "maybe, but probably if you design it right". You need to take your design scale length and pitches, choose strings and find out what the tension will be - bass' often have surprisingly high tension forces, way higher than any guitar, and loose floppy strings might sound like a neat idea but any string has a window of pitch and tension that it wants to see in order to play in tune, have good intonation, and since you are doing a fretless, have a good harmonic overtone sequence that is in tune with the fundamental pitch that you are trying to tune to. It's very possible for an under-tensioned string to be out of tune with itself and sound just bad.

In terms of a truss rod, the longer the unsupported neck is, the more it needs a truss rod. The more guitar-like the neck (slim, narrow) the more it needs a truss rod. If your design is like a Fender Jazz bass neck and joins the body at the second octave (24th fret equivalent) it will need a double acting truss rod just to get it straight, let alone keep it straight. If you have a thick neck, big and round like a double bass and joins the body at the first octave (12th fret) then you can almost certainly get away with nothing but a wood neck and no reinforcement at all, just like an upright double bass.

Brian

[Bryan Bear]

I could have this wrong so hopefully someone will correct me. . . I don't think carbon fiber is 5Xs stiffer than steel in a given application. I think it can be much stiffer for a given weight or another way to look at it comparable stiffness would be X times lighter in carbon fiber than steel. Of course the height of the beam has a profound effect on the stiffness so it is hard to really compare stiffness and weight without considering dimensions.

Edit: I changed my references to 5Xs; I don't have any reference data and was just using the 5Xs number provided above. . .

[Barry Daniels]

Mr. Bear is correct. Steel is stiffer when looking at the same cross sectional area, whereas carbon is stiffer based solely on weight. And since the size of the bar is more important in a neck that a slight difference in weight, steel is probably optimum if you are looking for maximum stiffness. All that being said, I still use carbon rods in my necks since I don't want so much stiffness that the truss rod becomes difficult to adjust which may be the case if you put steel rods next to a truss rod.

[Gordon Bellerose]

As has been said, you can design in a desired stiffness through the dimensions of the neck and materials used.
Again, steel will be stiffer but heavier. Steel may also overpower a truss rod. Like Barry, I use carbon fiber.

Regardless which stiffening rod you end up using, the truss rod is still a good idea, IMHO.
As instruments age, and as they move with weather and humidity changes, a truss rod will give you the adjustability you need, to make the instrument more playable.