Do You Place the Truss Rod Flush to the Bottom of the Fret Board or...

[LHawes]

...do you route the truss rod deeper into the neck with a shim over the top so the truss rod interacts more with the interior of the neck?

Thanks

[Alan Carruth]

I rout it lower down in the neck, if only to provide some wood over the rod to dress off level before gluing the fingerboard.

Otherwise it somewhat depends on the type of rod you're using. A plain rod that is put under tension with nut on one end should be installed below the 'center of moment' of the neck. When the neck bends upward the wood on the upper surface is in compression, and the wood on the bottom is in tension. The tension and compression forces drop off as you go in from the upper and lower surfaces, and there is a plane in the middle where they cancel out, the 'bending center' or 'center of moment'. If you put a simple tension rod above the CoM it will actually bend the neck upward when you tighten it. The further from the CoM the rod is the more effective it will be. The exact location of the CoM depends on the wood and the construction. of the neck. One person reports that a computer model places it just at or a slightly below the top surface of the neck wood when the neck is mahogany and the fingerboard ebony. A harder neck wood or softer fingerboard would move thaty upward. On a neck that tapers, so that it is thicker at the body end, the CoM would be lower down at the base of the neck.

A self-acting truss rod, that has something like an aluminum channel or fixed rod that the adjustable one works against, can be at any depth, but probably benefits some from being lower down too.

A fixed rod, such as a CF rod, or the old Martin plain steel reinforcement rod, is usually more effective lower down too. CF and steel have a much higher Young's modulus than the wood of the neck, so they're most effective on the 'tension' side of the CoM, where most of the forces are shearing. CF, in particular, is not nearly as stiff in shear if the fibers mostly run along the rod. The hollow D-tubes made of woven CF are another matter, and will work any place in the neck.

[redir]

I use the modern Martin designed rods and put them *almost flush against the fret board. I design it so that the truss rod can be removed in the future so a bit, and I mean a very little bit like 1/64th inch, of airspace makes it easier to remove.

[LHawes]

Originally Posted by Alan Carruth

One person reports that a computer model places it just at or a slightly below the top surface of the neck wood when the neck is mahogany and the fingerboard ebony. A harder neck wood or softer fingerboard would move that upward.

Hi Alan,

Thank you and trying to understand your informative post. Would it be accurate to say that placing an adjustable truss rod (I'm using an older LMI design) higher up in the compression side of the neck might, or does, have the most effect at counteracting that compressive force?

[Bruce Sexauer]

I settled on LMIi’s TRST single action rod many years ago. I install it immediately under the fingerboard after wrapping it in a layer of electrical tape to prevent rattle. My building system makes it unlikely that I will ever have a back bend to combat, and the consistency in adjustment is the best I’ve encountered. The fit should be snug, but so much so that the wood is distorted. More wood between the rod and the back of the neck is always a good thing. IMO, of course.

[charles Tauber]

I stand corrected. See below.

[nikpearson]

Originally Posted by Alan Carruth

I rout it lower down in the neck, if only to provide some wood over the rod to dress off level before gluing the fingerboard.

Otherwise it somewhat depends on the type of rod you're using. A plain rod that is put under tension with nut on one end should be installed below the 'center of moment' of the neck. When the neck bends upward the wood on the upper surface is in compression, and the wood on the bottom is in tension. The tension and compression forces drop off as you go in from the upper and lower surfaces, and there is a plane in the middle where they cancel out, the 'bending center' or 'center of moment'. If you put a simple tension rod above the CoM it will actually bend the neck upward when you tighten it. The further from the CoM the rod is the more effective it will be. The exact location of the CoM depends on the wood and the construction. of the neck. One person reports that a computer model places it just at or a slightly below the top surface of the neck wood when the neck is mahogany and the fingerboard ebony. A harder neck wood or softer fingerboard would move thaty upward. On a neck that tapers, so that it is thicker at the body end, the CoM would be lower down at the base of the neck.

A self-acting truss rod, that has something like an aluminum channel or fixed rod that the adjustable one works against, can be at any depth, but probably benefits some from being lower down too.

A fixed rod, such as a CF rod, or the old Martin plain steel reinforcement rod, is usually more effective lower down too. CF and steel have a much higher Young's modulus than the wood of the neck, so they're most effective on the 'tension' side of the CoM, where most of the forces are shearing. CF, in particular, is not nearly as stiff in shear if the fibers mostly run along the rod. The hollow D-tubes made of woven CF are another matter, and will work any place in the neck.

This is one of the best explanations of truss rod function and positioning I’ve seen. You have a way of explaining things that uses the proper technical terms without being opaque. I see you as the David Attenborough of lutherie?

[redir]

If you hold the truss rod in your hand and tighten it then you will see that the very center of the rod 'raises' up making an arch from the nut end to the opposite end. Once installed in a neck that arch is pushing up on the center of the neck against the fretboard as the two ends push down or at least remain stable. So it seems to me that even if the adjustable rod is placed on the plane of the neutral axis it would still do it's job. A carbon fiber rod along the neutral axis is useless though. In fact once I was educated on this concept I just stopped using them.

[Rudy4]

Originally Posted by LHawes

...do you route the truss rod deeper into the neck with a shim over the top so the truss rod interacts more with the interior of the neck?

Thanks

I prefer a channel that allows the rod to be adjusted from either chosen end and easily removed if necessary. Choosing the right type of rod will allow it to be inserted in a straight through channel, no weakening of the neck / headstock transition area by requiring a larger access pocket.

I create a channel just slightly deeper than the rod height, glue the board directly over the channel, and use a brass rectangular rod to "ramrod" through slightly dampened pieces of paper towel to remove any glue before it dries.



If you use heel access to the truss rod adjustment the channel can be filled and the headstock overlay added after the fretboard is glued on. That produces a clean channel, removable rod, and no truss rod access cover at the headstock.

Here's a filled channel before the headstock overlay is added, truss adjustment at heel end of the neck.

[Alan Carruth]

Charles Tauber wrote:
"The position of the centroid, and hence position of the neutral axis, are properties of the cross sectional shape (geometry) of the beam and are independent of the material used."

Are you sure? Ebony has a much higher Young's modulus than mahogany, and I'd think that would shift the centroid upward. How about the extreme example of an ebony fingerboard on a balsa neck?

[charles Tauber]

I stand corrected, see below.

[JonWint]

"The centroid, neutral axis and moments of area are properties of the geometry, independent of the material."

That's not true. Materials with different "E" values change all three.

[IMG][/IMG]

Fixed reinforcement like a carbon fiber composite or "T" bar will result in a 3-material neck composite section; mahogany/CFC/ebony.

[charles Tauber]

Originally Posted by JonWint

"The centroid, neutral axis and moments of area are properties of the geometry, independent of the material."

That's not true. Materials with different "E" values change all three.

[IMG][/IMG]

Thanks for the correction.

[Henning]

Originally Posted by JonWint

"The centroid, neutral axis and moments of area are properties of the geometry, independent of the material."

That's not true. Materials with different "E" values change all three.

[IMG][/IMG]

Fixed reinforcement like a carbon fiber composite or "T" bar will result in a 3-material neck composite section; mahogany/CFC/ebony.

The figure and symbols here are referring to Hookes law:
I havn't been doing solid mechanics for years. But it might be a good idea to straighten out the concept.
The model is a one dimensional problem. Sigma , σ, is the tensile stress linearly, proportional to its fractional extension or strain ε by the modulus of elasticity E. from wikipedia