By Dan Erlewine
When a quality guitar leaves the factory, the truss rod is adjusted according to the maker’s taste in setup specs. But a change in string gauges, climate (especially a change in humidity), or simply the player’s taste may require an adjustment, even on a new guitar. If you have a guitar that played great when you got it, but has developed a higher, stiffer action over time, it may be time to learn how to adjust your truss rod so you can keep the action just the way you like it.
An adjustable truss rod is a slim steel rod embedded in the neck. One end is threaded for an adjusting nut and is accessible at either the peghead or through the soundhole. The other end is anchored to give the adjusting end something to tighten against.
There are two styles of adjustable truss rods: single-action (“one way”) truss rods, and double-action (“two way”) rods. One-way rods straighten the neck against string tension and upbow; two-way rods not only straighten the neck against upbow, but can also force a backbowed neck into either a straight or upbowed configuration.
Your truss rod needs adjustment when the neck of your guitar has too much or too little upbow or too much backbow.
Tightening or loosening the adjustment nut adds or lessens pressure on the rod and neck. As a general rule,tightening the nut moves the neck away from the string pull and removes upbow; loosening the nut allows the neck to relax into an upbow again (especially when helped by the strings’ pull).
Controlled upbow is known as relief.
However, with a one-way truss rod, if the neck warps away from the string pull, no amount of loosening the truss rod will pull the neck straight, because the truss rod only works against the pull of the strings. This is why, in the 1980s, double-action truss rods began to be used widely.
Two primary signs tell you that your truss rod needs adjusting:
- There’s a noticeable change in the action; the height of the strings over the frets has become either too high or too low. The most common scenario is that the strings get higher as the neck upbows from the string pull.
- Some strings buzz on the frets between the nut and the fifth fret. This indicates that the neck is either too straight or it is backbowed from the truss rod’s slow, constant pressure over time.
See more Basic Guitar Maintenance articles.