Rambozo96
Senior Member
Hi Arfy. Actually the truss rod adjusts the flatness of the fretboard, it has nothing to do with neck angle.
The pivot point for neck angle is the joint itself. The tension of the strings pulls on the neck, bringing the headstock closer to the bridge, and the neck joint is the "weak spot" where compression and deformation occur.
The truss rod is contained entirely in the neck, so as the neck is pulled forward, the trussrod is pulled with it. It can't exert any counter-acting force on the joint itself.
The neck itself also flexes slightly under tension, but with no string tension it's actually is slightly "back-bowed". That's so the neck (or more precisely the fretboard )will be brought flat under string tension at pitch.
This chart may help explain why adjusting the rod does have a slight effect on action, but it's secondary and very limited:
So if the neck's diving into the top, then adding some "back bow" (tightening the rod) will tend to bring the action height down, but again, only slightly. It's not actually changing the neck angle.
The truss rod also controls "relief", where a small amount of up-bow is desirable to allow the fretted strings to clear higher frets to prevent buzzing.
Hope I explained that well enough.
If I had known that the truss rod played a very small role in the overall action it would have saved me and a friend from wrecking a truss rod on an old Ventura 12 string. Of course with the neck angle problem we didn’t lose much but we didn’t help matters either