Truss rod adjustment and neck angle

[hng]

I have a question that I can't answer myself. Please give your comments.

I know that truss rod adjustment and neck angle are not related. To measure a neck angle with a 24' straight edge ruler, a neck must be straight. Otherwise, the following will happen in my observation:

* Decrease neck relief, counter clockwise truss rod adjustment, will make the straight edge measurement lower than a bridge top.

* Increasing neck relief, clock wise truss rod adjustment, will make the straight edge measurement higher than a bridge top.

My question is the following:

If the straight edge measurement is depending on truss rod adjustment, why can't I use a 10" straight ruler to measure a guitar neck angle from 14th fret to a bridge top?

As far as I know, a straight line from 14th to the last fret will not be affected by truss rod adjustment.

[fazool]

It sounds lihe you aren't doing it right.

With string tension in place (tuned to pitch), adjust neck with truss rod until it's dead flat.

Now place long straight edge to measure angle by check where it targets the bridge.

If/once neck angle is correct, then adjust truss rod to create a little relief so strings don't buzz.

Truss rod adjustment doesn't affect neck angle at all and you can't properly measure neck angle with a bowed (relief) neck.

[Alan Carruth]

Truss rods work by pulling the head end of the neck down, to counteract the upward pull of the strings. If you put a straightedge on the first fret and the body fret, and measure the angle between that and the side, you will see a change when you adjust the truss rod. What doesn't change is the angle between the shoulder of the tenon at the neck heel and the neck surface. That, of course, stays the same.

None of this means that the purpose of a truss rod adjustment is to change the neck angle: it's there to control the relief. The fact that it does alter both the neck angle (depending on how you measure it) and the action height, is a side effect. Still, it does mean that the proper order for making changes is to:
1) make sure the frets are level,
2) adjust the truss rod to get the correct relief (whatever that is for your playing style), and
3) look at the neck angle.

Unless the neck joint is coming loose, so that there's a gap someplace around the heel, a change in neck angle involves some sort of distortion, either in the neck itself or in the box it's attached to. Usually it's the latter if there's a truss rod that can be adjusted to get the correct relief.

I know, I'm being pedantic.

[Rodger Knox]

Originally Posted by hng

I have a question that I can't answer myself. Please give your comments.

I know that truss rod adjustment and neck angle are not related. To measure a neck angle with a 24' straight edge ruler, a neck must be straight. Otherwise, the following will happen in my observation:

* Decrease neck relief, counter clockwise truss rod adjustment, will make the straight edge measurement lower than a bridge top.

* Increasing neck relief, clock wise truss rod adjustment, will make the straight edge measurement higher than a bridge top.

My question is the following:

If the straight edge measurement is depending on truss rod adjustment, why can't I use a 10" straight ruler to measure a guitar neck angle from 14th fret to a bridge top?

As far as I know, a straight line from 14th to the last fret will not be affected by truss rod adjustment.

Why would you measure the neck angle? Unless you are setting the neck, there's no reason to measure the neck angle. That's not the easiest/best way to check if a neck needs to be reset.

If you can put strings on the guitar, and tune them to pitch, and get the relief in the ballpark (0.003"-0.008") then measuring the action at the 12th fret and the height of the saddle above the bridge will tell you if the neck needs to be reset. If the difference between the measured action and desired action is more than half of what can be removed from the saddle(there needs to be some saddle above the bridge, how much depends on several factors), it needs a neck reset. Otherwise, it's fine.

[hng]

Thank you all for your comments.

My intension was just to check the neck angle to see how good my guitar. With strings on and tuned, I measured the neck angle at neck relief 0.005, and the straight edge is right on top of the bridge. Now, I went back to my style, neck relief 0.010, the straight edge touched a little bit lower than the top of the bridge as a side effect of the neck being pulled forward.

My question was why don't we measure the neck angle with a straight edge from 14th fret to the bridge? With this thinking, I don't need to touch truss rod adjustment since the straight light from 14th to the last fret will not be changed by truss rod adjustment.

[Rodger Knox]

Originally Posted by hng

My question was why don't we measure the neck angle with a straight edge from 14th fret to the bridge? With this thinking, I don't need to touch truss rod adjustment since the straight light from 14th to the last fret will not be changed by truss rod adjustment.

That is correct, the truss rod will not affect those frets. I still don't see the point of the measurement. The straightedge to the bridge is a measurement that builders make while a guitar is under construction, and cannot yet be strung and tuned to pitch. There's easier and more reliable ways to evaluate a guitar once it is completed enough to put strings on it.

[Howard Klepper]

Originally Posted by hng

Thank you all for your comments.

My intension was just to check the neck angle to see how good my guitar. With strings on and tuned, I measured the neck angle at neck relief 0.005, and the straight edge is right on top of the bridge. Now, I went back to my style, neck relief 0.010, the straight edge touched a little bit lower than the top of the bridge as a side effect of the neck being pulled forward.

My question was why don't we measure the neck angle with a straight edge from 14th fret to the bridge? With this thinking, I don't need to touch truss rod adjustment since the straight light from 14th to the last fret will not be changed by truss rod adjustment.

We don't do that because it would not be measuring the neck angle at all. The neck ends at the 14th fret. The fretboard extension over the body is not part of the neck, and it does not change where it points to on the bridge when the neck angle changes.

You could eliminate most of the effect of the truss rod on the straightedge test by using an 18 inch straightedge and placing it from about the 5th fret to the bridge. Using the straightedge can be misleading for the very reason that the frets over the body are irrelevant to neck angle. If the straightedge is touching at the last fret but not at the 14th, it will point misleadingly high on the bridge.

[murrmac123]

OK ... permit me to put forward what I consider to be the definitive definition of correct acoustic flat-top guitar neck geometry. ( This presupposes that the guitar should be capable of being adjusted to the lowest possible buzz-free action, even if that is not the requirement of the current owner).

Firstly, the frets should have been leveled professionally...or maybe I should say to professional standards (no reason why an amateur cannot achieve professional results with the correct tools)

Secondly, the nut slots should be shaped correctly ... the base of every nut slot should be in exactly the same plane as the first two frets.

Thirdly ...with the first two conditions achieved, the relief on the neck should be set to whatever the player requires... this can vary from zero to like .020" ... or maybe even more ... there is no god-given "correct" relief.

Fourthly, with the first three conditions achieved, when the string is depressed at the first fret and the last fret, the straight line formed by the string should make contact at the first fret, and the last fret and also with every fret between the body joint fret and the last fret.

If there is any kind of gap between the body joint fret and the string when the string is thus depressed, then the upper frets require a "fallaway" sanded until such time as the string does make contact with the body joint fret.

These four conditions take care of the "elementary" geometry.

Now we come to the most important parameters.

The gap between the underside of the bass E string and the surface of the soundboard should ideally be exactly 1/2" . Some people say that on smaller guitars the measurement should be fractionally less ...like 7/16" ... I do not believe there is any valid evidence that deviation from 1/2" is either necessary or beneficial.

So finally, with all these conditions met, the action at the 12th fret should be to your requirements ... only the player can say what the action should be ... but irrespective of what the individual requirements might be, I can state categorically that all the previous requirement should be met in order to ensure a perfect flat-top acoustic guitar geometry.

EDIT : once again , Howard cross-posts while I am composing my own contribution ...

[hng]

Thank you all. Your comments are very helpful.
This forum is wonderful.

Thanks again.

[hng]

@murrmac123

How can the desired action be achieved if the distance between the underside of the low bass E and the surface of the sound board is ideally maintained at 1/2"?

From your comments, I understand that the saddle can't be lowered. Thus, to get to the desired action, one has to adjust neck angle or the truss rod. Is my understanding correct?

[murrmac123]

Originally Posted by hng

@murrmac123

How can the desired action be achieved if the distance between the underside of the low bass E and the surface of the sound board is ideally maintained at 1/2"?

From your comments, I understand that the saddle can't be lowered. Thus, to get to the desired action, one has to adjust neck angle or the truss rod. Is my understanding correct?

I was describing the ideal geometry .... in real life the ideal is rarely achieved, and most instruments do end up with the string/soundboard gap at less than 1/2" after the action has been set correctly.

The saddle normally does need to be lowered to achieve the desired action. If it brings the gap down to 7/16" it's not the end of the world.

[Rodger Knox]

Originally Posted by murrmac123

I was describing the ideal geometry .... in real life the ideal is rarely achieved, and most instruments do end up with the string/soundboard gap at less than 1/2" after the action has been set correctly.

The saddle normally does need to be lowered to achieve the desired action. If it brings the gap down to 7/16" it's not the end of the world.

That depends on the design height off the soundboard, and the constructed "stiffness" of the top. On most factory guitars, the design height is 1/2" and the top is "stiff" enough that dropping to 7/16" does not make a significant difference in tone or output. That is not necessarily always true.
The "stiffness" of the top is actually the stiffness distribution, which is affected by the bracing design and stiffness. It's not something that can be easily measured and cannot be represented by a single number.

[Bruce Sexauer]

Many fret technicians work a fall off into the their fingerboards above the body join to insure that the fingerboard extension never gets in the way of the string path. In such a case projecting the extension to calculate the bridge height would be futile. Others, myself among them, include the extension as a fully functional part of the fingerboard equation. Assuming the fingerboard and the truss-rod are completely healthy, you could actually get the results you seek with a short straightedge on the extension. But the assumption is pretty big, and how the maker of the guitar you are looking at approached the issues is largely an unknown; every time!

Once the guitar is properly set up and playing as it should, a competent tech merely has to glance at the bridge and saddle to instantly know whether the geometry is right, and in which direction it is off, if it is. It is when building the guitar, before the strings go on, that calculations need to be made to insure that the geometry is going to be in the ball park.

IMO, at the very least!

[Rodger Knox]

Originally Posted by Bruce Sexauer

Many fret technicians work a fall off into the their fingerboards above the body join to insure that the fingerboard extension never gets in the way of the string path. In such a case projecting the extension to calculate the bridge height would be futile. Others, myself among them, include the extension as a fully functional part of the fingerboard equation. Assuming the fingerboard and the truss-rod are completely healthy, you could actually get the results you seek with a short straightedge on the extension. But the assumption is pretty big, and how the maker of the guitar you are looking at approached the issues is largely an unknown; every time!

Once the guitar is properly set up and playing as it should, a competent tech merely has to glance at the bridge and saddle to instantly know whether the geometry is right, and in which direction it is off, if it is. It is when building the guitar, before the strings go on, that calculations need to be made to insure that the geometry is going to be in the ball park.
IMO, at the very least!

Thanks, Mr. Sexauer. That's a point I've tried to make whenever I see "the straightedge hits just... on my new guitar" threads, including this one.

There's a difference between setting the neck angle on a new guitar and checking the neck angle on a completed guitar. The new guitar usually does not have the bridge attached when the neck angle is set, so it cannot be strung up. The luthier uses the straightedge on the frets to estimate where the neck needs to go. He has to take into account how much the top will pull up and how much the bridge will rotate under string tension, so it's not a direct measurement, but a direct measurement cannot be made without string tension. There's absolutely no reason to use a straightedge to check the neck angle on a guitar that can be strung and tuned to pitch.