Truss Rods

[Brian T. Williams]

Hello,
I am not a luthier or repairman but as someone who likes to be sure of his facts, and being involved in a discussion regarding guitar set up recently, I was hoping someone could clear this up for me.
I have always understood that the truss rod is purely for setting the amount of neck relief, and whilst it can affect the guitars "action" or string height, it should not be used for this purpose, as many an overtightened and broken truss rod will testify.
As I understand it, "action" or string height should be set at the bridge, after the neck relief and nut depth are all correct and after all these, intonation follows.
The discussion in question, concerned a Fender "Acoustisonic" with a neck tilt adjustment. The neck relief was perfect as was the saddle height, but the player wanted a slightly lower action and I suggested that the neck tilt was the way to go, NOT the truss rod.

Thank you for looking, I would really appreciate your thoughts.
Brian.

[Roger Häggström]

Yes, the truss rod gives stiffness to the neck with an added possibility to change the neck's relief. The string height will go down a little when the top of the fretboard moves down if you straighten the neck and decrease the relief.

The relief should be small, about 0.15 mm in the middle of the fretboard before anything else. I put a 0.4 mm feeler gauge between the string and the 1st fret and put on a capo to emulate a ballpark correct nut height (with all strings at tension). Then I adjust the saddle down or up to the desired string height at the 12th fret. I use feeler gauges to measure the current string height at the 12th fret and calculate how much adjustments are needed at the saddle to get the height I want. With the saddle height correct I then adjust the string height at the 1st fret filing the string cuts in the nut.

If the string height is too high with a good setup of the relief and nut, then you either have to lower the saddle or reset the neck.

[Brian T. Williams]

Thank you Roger, I do set up my own instruments following the guidelines and have been very happy with the results over the years.
I just worry about all the wrong information dished out by would be "experts", that result in at best, poorly setup guitars, or at worst disastrously wrecked guitars!
Cheers......Brian

[Russell Vance]

Hi Brian, What Roger said with the addition:

Over the years we have come to view the string path and subscribed string ellipses (different at every fret) as part of the setup equation.  We are electric sort of peeps, but its a general application.   We setup "active" - which is a process of not using defined measurement at various frets as this changes with neck geometry, neck integrity,  string gauge, pick attack and a few other things. 

We take into account three points in space, the nut, the bridge saddle(s) and the fingerboard area from the half scale point to the end of the fingerboard (which is generally not affected by the truss rod relief - but more so by string take-off angle from these frets).  We think of the neck adjustment process as a series of eliptical paths originating from each fret.   So, for instance,  we may flatten a neck and raise the bridge saddle height to change the geometry to clear trailing frets (or the usual "rising tongue" found in a lot of instruments) - doesn't change the action height but gives us clearance in the upper register frets.  We will also often dial in a bit of drop away in the high number frets to adjust the action there (where the truss rod has no effect) which makes the fingerboard a tale of two sets of geometry - one part fixed and one part responsive to the trussrod.  Adjustable neck tilt as found on many Fender electrics and others can also help getting this geometry equation solved a little in our favor.

One final thing is how geometry fits into working out why some seemingly identical instruments feel different than others - as the break angle of the strings over the saddles increases and decreases as a function of adjustments to the general string path up and down, so too does the feel or stiffness of the strings and the tone of the instrument.   Small things but they all add up.

Regards, 

Rusty. 

  

[Roger Häggström]

I might as well describe my method in more detail too.

I don't use a truss rod when restoring old 12 fret to the body parlor guitars, instead I use a stiff carbon rod. I sand a small relief into the fretboard at simulated string tension of the neck using an aluminum sanding bar with a negative 0.15 mm relief before fretting. I then trust the carbon rod to keep the shape of the neck over time. After fretting and at simulated string tension of the neck, I again shape the tops of the frets to the relief in the direction of the strings with the same aluminum sanding bar. The deepest part of the relief is placed bang in the middle of the fretboard, around fret 7,5. I finish off the relief shaping sanding down a small "slope off" behind the 12th fret. After that the usual crowning work is done.

The small "slope off" behind the 12th fret is needed for low string height at the 12th fret. With a really low string height I have to give the string height at the nut a little more height, to avoid back buzz, then you get with a zero fret with the same height as all the other frets.

[Brian T. Williams]

Thank you....great information and an excellent insight into guitar setup!

As I mentioned, whilst I have no ambition to enter into Luthiery or guitar repairs myself, I hold all you guitar builders and repair technicians in very high esteem and watch with great respect, your craft & skills.

One more question please if I may......how much does the headstock break angle contribute to a guitars "playability"? 

PS......I was correct then, in suggesting this chap adjust his string height by the "neck tilt" adjustment on his Fender Acoustisonic, considering the neck relief and bridge saddle height were at optimum measurements?

[I have no intention of performing this adjustment for him; I just didn't want him cranking the truss rod to the extreme, as was his intention, and snapping it in the neck]!

[Mark McLean]

Yes, you were correct in your advice to your friend.  If he is lucky enough to have a guitar with adjustable neck angle this is the ideal mechanism for setting string action - after he has set neck relief using the truss rod and has also checked string height at the nut. 

And all the stuff that Rusty and Roger have added is icing on the cake.