Truss Rod - Needed or Not Needed?

[julyboy]

With such a short neck on mandolins is there really a need for a truss rod? Does it really serve a useful purpose on such a short neck instrument? Thanks for any information.

[Roger Moss]

I've seen some old ones without so I guess they can do without but I feel better with one. There's an awful lot of tension on those necks.

[Mandobart]

The thing about routing out a channel for a truss rod is - you weaken the neck making it more likely you'll need to adjust the truss rod. There is no truss rod in my Morris F4, or my Cricket fiddle 10 string mandola, 21" OM or 26" 10 string 'cello. All of these have solid steel reinforcement which actually strengthens the neck.

As a mechanical engineer, I prefer a stronger neck that resists bending (aka higher moment of inertia) to a weaker neck with a limited means to compensate for the additional weakness.

As a musician with over 40 years playing lots of stringed instruments, the number of times I've adjusted or felt the need to adjust a truss rod is - zero.

[Bob A]

I've been playing a 1921 F4 for maybe 40 years with no problems with the neck, and no trussrod. I don't use heavy gauge strings. Also have a Lyon & Healy mandolin with no trussrod, running light strings.

There's a fair amount of wood in the pre-trussrod era Gibsons. I don't think they have much of a problem, but then again, it's possible that the ones with problems were weeded out by attrition a lifetime or so ago.

Running heavy strings on a bluegrass mandolin with the 15th-fret neck-to-body joint might be a different story, of course.

[pops1]

I like a truss rod as everyone doesn't play the same or like the same action. A truss rod lets you adjust the neck to your preference, where a solid neck with carbon fiber will possibly stay straight it allows for no adjustment. I have arthritis in my wrists and need a low action, a very small relief, more on the the bass side gives me a lower action and less hand pain. Without a truss rod it would be doable, but not as easily.

[Bertram Henze]

...a stronger neck that resists bending (aka higher moment of inertia)

Moment of inertia is at work only with moving - especially rotating - objects. The static resistance to bending is just bending stress that equals string tension.

Apart from that, I'll say steel string tension adds up to a force that's better countered by some kind of steel reinforcement in the neck (adjustable or not), in order to avoid that longterm inelastic creeping wood can undergo; I have seen a bouzouki once that had no such reinforcement: you could shoot an arrow from it. I have a mandolin that developed an uneven "relief", i.e. the camber is most pronounced between frets #5 and #12, with straight plateaus above and below.

[Dave Hanson]

I play a 1917 Gibson A and a 1914 Vega Whyte Laydie banjo, both without a truss rod and both as good as the day they were built.

Dave H

[darylcrisp]

I prefer and will only buy any instrument with a truss rod, so I can adjust relief to suit my style of play. I have osteoarthritis in my left thumb joint and I have to set up all my instruments to play low and easy with little relief as possible.

I've owned a few instruments with no adjustable TR and sold them rather quickly as they had too much built in relief-and no way to adjust any of that out.

d

[Ivan Kelsall]

Dave - My own Clifford Essex ''Boudoir Grande'' banjo,made in 1914, that i had back in 1965,had a solid Rosewood neck with no truss rod. In fact,i'd bet that the majority of UK & US banjos made back in the early 1900's, were 'truss rod free'.
After i'd bought my Weber mandolin,i contacted Bruce Weber re. 'neck relief'. Bruce told me that the short scale of a mandolin didn't really need 'any' relief. In fact Weber sends (or did at that time), all it's instruments out with 'flat' fingerboards = no relief. I suspect that truss rods are required more to correct any possible warping of the neck / fingerboard than anything else.
It's been fairly common practice amongst some luthiers who build mandolins & guitars,to use Carbon Fibre 'rods' of one shape or another embeded in the neckwood,to reinforce the neck against possible bending & warping. As long as the neckwood is seasoned well enough,that's possibly all that's needed. However,the neck on my once owned Weber 'Beartooth' "A" style,did develop a warp in it. So,maybe taking things for granted re.warping isn't 100% safe.
Regarding the need for fingerboard 'relief',despite being not absolutely required because of the mandolin's short scale length,some folk like it. So,it boils down to 'personal preference',as most things do. So choose what suits you - carefully !,
Ivan

[Tobin]

Moment of inertia is at work only with moving - especially rotating - objects. The static resistance to bending is just bending stress that equals string tension.

Actually, moment of inertia is an engineering term that is very relevant here. You are thinking of rotational moment of inertia, but in statics, when we define the loading of a beam, the internal forces we look at are shear and moment. Moment is the bending resistance of the beam. Moment of inertia is the universally accepted term for the second moment of area, and is a section property of the member.

Typically, the moment strength of a beam is defined by the yield strength (or plastic strength, depending on what you're calculating) of the material times the section modulus, or M = Fy*S. The section modulus is different from the moment of inertia.

Section modulus (S) is relevant for determining the actual bending strength of a member, along with the bending strength of the material (Fy or Fb, etc.). Moment of inertia (I) is relevant for deflection, along with the moment of elasticity (E).

So when we're dealing with a composite member like a reinforced mandolin neck, the additional stiffness of the neck isn't noticeably improved via the section modulus or moment of inertia. Those are dimensional properties that aren't significantly changed. The real improvement in strength comes from the increase in yield strength of the composite member by adding steel or carbon fiber or whatever. And the real improvement in bending resistance comes from the increase in the moment of elasticity.

[Bertram Henze]

Moment of inertia is the universally accepted term for the second moment of area, and is a section property of the member.

Oh, that one. Yes, that one applies. I didn't know about the linguistic shortcut, though.

[Folkmusician.com]

On a steel string mandolin, you do want a truss rod. The question is, does it need to be adjustable? I am of the opinion that you only need an adjustable truss rod "if you need an adjustable truss rod".

If the mandolin is made well, a non adjustable carbon or steel truss rod seems fine.

No reinforcement att all is asking for trouble.

[Tobin]

After thinking about my previous post some more and wanting to minimize confusion, I should add that a mandolin neck isn't really a "beam" when it comes to the forces we are talking about. It would be more accurate to think of it (and analyze it) as a column. It is being axially loaded in compression, with an induced moment at the end. That induced moment is basically "torque" applied to the end, since the tension of the strings is not in-line with the centroid of the neck. The total tension of all 8 strings, multiplied by the distance from the centroid, would be the induced moment.

The induced moment is why necks tend to bow, and why a truss rod helps.

So the analysis of the forces and resistance in the neck is what we would call combined loading. Axial compression plus an easily calculated moment applied at one end (which will be resisted by a moment at the other end, where the neck is connected to the body). Still, though, the sectional properties and material properties are relevant. They just go into the equations differently. A column analysis will use such fancy and exotic terms as the radius of gyration in addition to moment of inertia and section modulus. And in the case of a mandolin neck, the section properties are not consistent along the length of the neck, so it would require some annoying differential equations or other methods to get a real handle on the internal stresses at various points inside the neck. Fun stuff for us engineering nerds, LOL.

[Folkmusician.com]

Though I really would like to re-read Tobin's post a few more times and get a better grasp, I have only had one cup of coffee this morning!

It is interesting!

[Jim Hilburn]

A brief history of the trussrod (at Gibson anyway) is that all the oval mandolins up until '22 had a wide fingerboard, short neck (12th fret join) and a triangular maple re-enforcing insert to stiffen the neck. They were also built with a handful of wood making them nearly impervious to bowing.
When they built the F-5, they (Loar?) wanted a longer, slimmer, narrower neck. It may be a chicken or egg thing, but Ted McHugh had either invented the TR concept specifically for this purpose or they decided they could make the lighter neck because he invented the TR.
In either case, they deemed it to be needed and has become the standard most have duplicated.
Obviously, many have used carbon fiber beams with great success, something that wasn't available in '22, but pretty much everyone believes some form of re-enforcement is needed.

[Bertram Henze]

and in the case of a mandolin neck, the section properties are not consistent along the length of the neck, so it would require some annoying differential equations or other methods to get a real handle on the internal stresses at various points inside the neck. Fun stuff for us engineering nerds, lol.

That would be FEM, then...

[Eddie Sheehy]

I have several mandolins with no truss-rod and never any problems with them - considering most are over 100 years old. I do prefer a modern mandolin to have a truss-rod - but I have never even opened a truss-rod cover, not even to see if there really is a truss-rod.
Some builders reinforce the neck with: laminates, carbon-fibre, steel rods (unadjustable) etc.

[Eddie Sheehy]

QUOTE=Folkmusician.com;1491262]Though I really would like to re-read Tobin's post a few more times and get a better grasp, I have only had one cup of coffee this morning!

It is interesting! [/QUOTE]

I have insomnia so I'll try reading at bedtime tonight...

[DataNick]

For me the decision is based thusly: Set-up/Neck Relief, Set-up/Neck Relief, Set-up/Neck Relief....Just about every time I've wanted to mico-adjust my set-up(those of you who are anal about your set-up can relate) a truss-rod adjustment was needed.

A friend of mine owns a big-name boutique F5 mandolin(the builder is well known) that had no truss rod. The neck began to warp and he had to have one put it. A real big pain in the you know what, cause the mando is a monster! and he didn't want to give up on it!

Even if strength/re-enforcement could be guaranteed, to me wood changes, and a set-up will always require tinkering at one point or another resulting in a truss rod adjustment...YMMV

[Tobin]

A brief history of the trussrod (at Gibson anyway) is that all the oval mandolins up until '22 had a wide fingerboard, short neck (12th fret join) and a triangular maple re-enforcing insert to stiffen the neck. They were also built with a handful of wood making them nearly impervious to bowing.

I have heard a lot of theories out there about how using multiple wood species is somehow better protection against bowing. I'm not sure it's true - at least, not for the reasons that some people think. There's nothing inherent about using multiple species of wood that makes it stronger, unless one of the species is more strong and stiff, in which case it's going to take a larger share of the load (much like a truss rod does) due to its material properties. The maple insert you mentioned would do this, if the maple is stronger and stiffer than the other wood used in the neck.

Many instrument necks are built in a split construction method where they book-match two pieces of wood and put a divider strip down the middle, like Gibson did. This doesn't necessarily make it stronger in bending, but it does help resist side-warp of the neck from drying or shrinking of the wood. Since the pieces are book-matched, and assuming they dry/shrink together, any lateral movement by one piece is resisted equally by the other (they will be fighting each other for directionality of movement, but they are equally matched, since they are cut from the same piece and flipped opposite of each other).

And there is, to some small degree, some additional strength from the glued joint. Often, a laminate member can be stronger than a solid member due to the glued joints. This is why modern LVL (laminated veneer lumber) is popular in construction. It's resistant to bowing and warping, and can be stronger than a solid member. Whether a mandolin neck gets this same benefit would depend on the quality of the glued joint.

The other thing to keep in mind with necks is that the fingerboard plays a role too. When it is glued to the neck, it becomes part of the composite member. Due to the tension from the strings, the fingerboard is on the compression side of the neck. Ebony seems to be very good in compression, and as long as the glue (which is in a shear plane) holds, that fingerboard is doing a lot of the work in resisting bending. The truss rod just below it, or the wood of the neck if no truss rod is present, will usually be in tension. The presence of the truss rod actually changes the neutral axis of the member and relieves the wood in the neck from doing as much work. But the fingerboard, which is always at the compression side of the member, doesn't get much of a break.

That would be FEM, then...

Indeed. Finite Element Analysis would be one way (probably the most popular way) to skin that particular cat.

[julyboy]

Thanks to all of you for the valuable information you have shared. I have gained some good knowledge from this discussion.

[Mandobart]

Moment of inertia is at work only with moving - especially rotating - objects. The static resistance to bending is just bending stress that equals string tension.

You're thinking of the polar moment of inertia. Glad to see there's at least one other enginerd on the board Tobin...

And Tobin is spot on with the description of the neck being more like a column in compression with a moment due to the tension not acting directly through the axis/centroid.

As far as routinely adjusting action with the truss rod - that doesn't sound right. My understanding is the truss rod is intended to adjust the bow of the neck, which will affect the action. However that's not the reason for the truss rod (to adjust action). There are better ways to do that.

[DataNick]

As far as routinely adjusting action with the truss rod - that doesn't sound right. My understanding is the truss rod is intended to adjust the bow of the neck, which will affect the action. However that's not the reason for the truss rod (to adjust action). There are better ways to do that.

If my comments were the basis for this, I should have clarified that when I need to fine-tune(not routine) my set-up (I'm thinking of 4 distinct mandolins) to "set the neck relief" as in more or less, a truss rod adjustment was needed. Set-up infers more than just action: nut slot depth/height, bridge placement,etc.

As far as action, once my set-up is dialed in, I control action mainly by adjusting the wheels on the bridge...YMMV

[pops1]

I am like DataNick maybe even more anal. I like a low action so and my instrument goes between lots of humidity in the summer to very little in the winter. Yes I humidify my home. I even go to the extent that I file in a slight relief on the G string so I can go lower. It helps the arthritis in my hands and playing for dances that may go 18 minutes long is fatiguing for me with anything but low action. I carry tools to raise or lower my bridge and have done so between songs on the stage of an opera house concert. I keep it on the verge of buzzing so any change is buzzing or hard to play. I also have a gauge in my case I made to check the height of the strings at the 12th fret so I can keep it where I want it and not be guessing. For me the adjustable truss rod is important. I have also had a well known makers mandolin, without an adjustable rod, bow and have too much relief. I filed the frets to correct it, but at some point it will have to be dealt with more permanently.

[Ivan Kelsall]

From what i've read & have always been given to understand,is that truss rods are required for 2 reasons. 1) To allow a degree of fingerboard 'relief' - a very shallow 'dip' in the fingerboard,which allows the strings when fretted at any one fret,to adequately clear the fret in front of it,so as not to cause the strings to buzz/rattle against it. 2) To allow the neck to be 'forced' straight' again in the event of it warping. However,a 'twisting' warp such as occurred in my Weber "A" style neck,isn't correctable simply using a truss rod.
Personally - i'd prefer to have a truss rod & NOT use it,than NOT to have one & then need it,as in the case of Data Nick's friend. Unless any piece of 'neckwoood' can be safely said not to have any interior stresses at all,then i'd opt for a truss rod every time,
Ivan