Do short-scale steel-string instruments need a truss rod?

[Liam McGillivray]

I am making two electric stringed instruments of my own design. One of them is a 2-string instrument with a 22"/56cm scale length, with strings tuned to A3 & D4. The second is a 4-string instrument with a 18"/46cm scale length. I haven't decided the tuning for the second one, but the lowest note will be in the range of D3 to G3.

Will these have enough tension * tension to take advantage of an adjustable truss rod? I am already planning to use carbon fibre neck reinforcement, but will a truss rod still give much benefit? For the longer 2-string instrument, I can use a guitar truss rod. But for the shorter 4-string, it will be harder (although I might be able to get a mandola truss rod).

Mandolins are made either with or without truss rods, but browsing through forums they say that they're not really needed. An 8-string acoustic instrument would have more tension than a 4-string electric, but then mandolins are only 13".

[Brian Evans]

I would say you can do those instruments with non-adjustable truss rods. Steel strings kind of like to have around 25 lbs of tension, so if you choose your string gauges with that in mind, you'll end up with pretty low overall tension on the neck.

[Barry Daniels]

Steel strings kind of like to have around 25 lbs of tension

What? Please explain this.

[Brian Evans]

Just a rule of thumb, nothing exact. A starting point, if you will. A typical set of light gauge (012 - 052) on a typical 25" scale guitar will be in around 150 lbs of total tension, so average 25 lbs. Obviously there is a wide range of gauges and scale lengths, but I find a lot less than that to start to feel floppy, a lot more than that starts to be overly stiff for my taste. When I had a 7 string and I wanted a gauge to buy for the low B string I looked at gauges based on tension for that pitch.

Brian

[Liam McGillivray]

.. but I find a lot less than that to start to feel floppy, a lot more than that starts to be overly stiff for my taste.
Brian

Do you mean "a lot less than that thickness/tension of strings" being floppy, or are you referring to the neck being floppy?
I think I have a pretty good idea of what strings I need to achieve a certain note. Unfortunately, gauge isn't an exact measurement of how much tension is required for a given note, as there are other factors (density, stiffness, etc). However, I don't think I need any help with choosing my strings.

For the longer 2-string instrument, I'm going to go for lighter-gauge roundwounds for more sustain; typical for an electric guitar. For the shorter 4-string instrument, I want a more percussive sound, so I will want higher tension. Most likely 3 of them will be plain steel, with a flatwound for the lowest string.

[Brian Evans]

I was referring to the feel of the strings as floppy. My personal experience is too much tension and I don't enjoy the sound, too little tension and the string has intonation issues. I've used those tension numbers as guides for many things, including my lap steel, but they are just starting points. I actually tend to often tune down from E-E to D-D on guitar, which in turn drops the overall tension of a set from the 150 lb range to the 135 lb range. I seem to like the feel of that range of tension but prefer the sound of the thicker strings (more mass). So its an experiment. I agree that gauge isn't the ultimate guide, but D'Addario has a site where you can get tension numbers for their strings at many different tuning points (pitches) and scale lengths. You can get some insight from looking at mandolin family instruments for gauges and scale lengths, mando-cellos and what-not. Good luck with your project!

[Mark Wybierala]

Just browsing and came across this post again. IMO and my intent when I use a trussrod, the trussrod is employed to extend the longevity of the instrument and insure that instruments coming off an assembly line can all be adjusted to a reasonable level of playability. Wood strength and predicting the way it will react under string tension is far from being exact. How it will react to changes in temp, humidity, and age is an unpredictable element and especially now when we are forced by necessity to use wood that 100 years ago would have been inferior to high quality instrument makers. Consider that you'd never see a trussrod in a violin, viola, cello or a contrabass. You might say that fretted instruments require a higher degree of accuracy or they are more difficult to work to obtain the perfect amount of relief under string tension and in some ways this is true. I'd like to think that a few of my instruments might survive to be 100 years-old and beyond so on the ones I put my heart into, I use a trussrod -- they're cheap insurance. But then, sometimes you're just clowning around and making something silly like this:
This was a squire strat. I removed the length of the first five frets in a long scarf joint, glued it back together without the trussrod and added 3 strings to create the Strat-O-Pixie. Ten years later, the relief is still just fine without the aid of a trussrod. Its tuned to A and with the 3 additional strings it has about 50% more tension on the neck. The odd bridge is used so I could employ an under-saddle piezo pickup. No trussrod and its doing fine but then, I was just clowning around.

[Beate Ritzert]

The archtop i restored recently has not truss rod. Its neck is really thick yet comfortable, and it is perfectly stable with .012 strings - the neck itself should be capable of .013 strings. The neck is made of birch which has the largest Young's modulus among the "common" northern European woods. The guitar is roughly 60 years old.

One of my basses has a fixed "truss rod" made of Machiche wood, 16 mm, and a very thick fingerboard made of Katalox. Stable for 14 years.
The Machiche rod should have roughly the same Young's modulus as Rick Toone's hollow aluminum bar.

So it can be done, it requires some estimate, thorough choice of wood and some planning. And, of course, the flat necks some players love, cannot be made that way.
Regarding the large variability of the wood properties trussrod-less instruments are nothing for industrial production. But in a small shop and with measuring and each piece of wood for stiffness - why not?

BTW: how would thermal treatment of wood affect its mechanical properties?

@Mark: bowed instruments have short and thick necks which are inherently stiff, even if the fingerboard is made of pear (the wood with the smallest stiffness used as fingerboard for the violin family). Stiffness increases with the 3rd power of the thickness, and with the 3rd power of the reciprocal of the length