How Truss Rods Work

All About Truss Rods

The adjustable truss rod is a way to adjust a guitar's neck and it is generally agreed that Gibson invented it around 1921.  Before this (and after by some manufacturers) many builders would use a non-adjustable steel rod or T-bar to keep the neck stable.  In fact, Martin continued this practice until 1980!  This is one of the more important inventions to the modern guitar ever as it allowed for the neck of a guitar to be adjusted for different string gauges and environments and the use of a more playable, slimmer neck.  It has a singular function of countering the tension of the strings on a guitar to keep the neck in optimal playing shape.

A word on neck relief

Relief is the amount of concave curve that is present in a neck as viewed from the side of the guitar.  All guitars have to have some amount of relief.  A perfectly straight neck is not a good thing in regards to tone and sustain not to mention the likelihood of buzzing.  The opposite of relief is backbow and is convex.  There is no amount of acceptable backbow in a stringed instrument.  Relief has nothing to do with action and a truss rod is not a method to lower or raise the action on a guitar.  It is only intended to set the relief.

Example of quick diagnosing relief issues:

• Too much relief  - your guitar will buzz and fret out around the 9-12th fret and up.
• No relief - entire fingerboard is buzzy and sounds thin
• Backbow - below the 5th fret will buzz and or fret out
• Just right - even tone and minimal buzzing over entire fretboard
• Open string buzz (but not fretted buzz) is likely a nut issue and can not be rectified by the truss rod

How to use a truss rod

All truss rods will have either a nut of some sort or a hex/allen key for adjustment.   If you tighten the rod, the neck will have less relief.  If you loosen it, the neck can pull forward thanks to the string's tension and add more relief.  If you tighten it, the neck will become more straight eventually even backbowing.  Your goal is to set an appropriate amount of relief to provide optimal playing condition of the neck by adjusting the truss rod.  Relief is not really subjective either.  To measure relief, you will need to fret the guitar on the first fret (or use a capo) and press another fret at a different point on the fingerboard.  Some people use the 1st and 12th fret, others may use 1st and 17th, or even 1st and last fret.  This means the string will make a straight line between the 2 frets you press and you can see the neck's relief by tapping on the string and seeing the distance it moves.  Take the following 2 examples:

1. You press 1st and 12th fret and tap in between on the string.  It does not move at all.  This means there is no relief in the neck.  You would loosen the truss rod and try again.
2. You do the same but the string moves quite a bit indicating that the neck has too much relief and needs to be tightened.

Your goal is to have the smallest amount of movement when doing the tap test.  In the example above, there should be somewhere between .004" and .007" distance between the 6th fret and the bottom of the string.  Basically between a dollar bill and a common playing card (or just buy a set of feeler gauges).  It isn't much.  If your frets are properly leveled that is...

Will I break it?

It is actually pretty hard to break a truss rod.  What is not hard though, is stripping one.  Always make sure to use the correct size wrench.  A stripped truss rod is no fun at all.  It can take some force to move a truss rod, but it should be easy to do with just your hand and the wrench.  More torque than that is not normal and could indicate a frozen rod or other issues.

How to experiment with your truss rod

Hopefully you are more comfortable with the idea of trying to adjust your truss rod on your own.  A word of advice though:  Always keep track of what you did so you can get back to a starting point.  For example, turn the wrench in easy to count increments such as 1/4 turns.  That way if you get it way out of whack, you can get back to where you started.  Also, flex the neck a bit after each adjustment to settle it.

What to do once relief is good?

You are ready to complete the setup on your guitar by setting action and intonation!  Assuming your frets are level, the rest should be pretty easy.  Remember that the truss rod is not there to adjust action, only the curvature of the neck.