Guitar and Bass Repair: Improving Tone, Action and Sustain Without Spending a Nickel Part One

The design and mechanics of adjustable truss rods are straightforward and simple. Despite this simplicity, truss rods and their effect on instruments are often misunderstood. For example, there is a common misconception that tightening a truss rod will lower the action of an instrument so the lightest touch will sound a note. In fact a truss rod’s effect is more subtle, but it can still significantly improve (or worsen, if adjusted incorrectly) the playability of an instrument. A properly adjusted truss rod can improve an instrument’s action, tone and sustain, eliminate fret buzz and prevent the neck from warping. The following article will reveal the design and mechanics of single action (traditional) truss rods and explain how to tell if yours needs adjustment by reading neck relief.

Components and Mechanics of Single Action Truss Rods

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The construction of single action truss rods is very simple (Fig. 1). Most consist of a 3/16-1/4 inch diameter steel rod with threads on one or both ends, an adjustment nut (Fig.2), a washer and, in most cases, anchor nut (Fig. 3).

Rods with an anchor nut screwed onto one end are peened (the metal is hammered so that it "mushrooms") below the anchor, preventing it from moving (Fig. 3). A single action truss rod that does not have an anchor nut may instead have a 90 degree bend at one end to secure it within the neck.

Single action truss rods are installed deep below the fretboard in a channel in the neck. Generally speaking, the deeper a rod is set in the neck, the more effective the rod will be. Luthiers and manufacturers must be careful not to place a rod too deep however, as adjustment could cause it to break through the back of the neck if the wood below the rod is too thin. Truss rods may be installed with the adjustment nut either at the headstock-end of the neck or where the neck heel meets the body. In either case, the result is a rod that has two points of contact at both ends inside of the neck.

Tightening the adjustment nut of a single action truss rod pulls the anchor and adjustment nuts closer together, causing the washer and anchor nut to compress the neck. Since the rod is deep in the neck there is more wood above it than below, so the back offers less resistance and is therefore compressed. The compression causes the neck to bend backwards or backbow. Herein lies the function of the rod. As the strings pull the neck into relief or an upbow (Fig. 5), the backbow caused by the truss rod counteracts the pull of the strings, thus keeping the neck straight. With an understanding of truss rod mechanics, you will find it much easier to adjust a rod to improve or correct an instrument.

How to Read Neck Relief

Before adjusting a truss rod you must understand the condition of the instrument’s neck. To determine the condition of a neck, you must read its relief. Here we will discuss two different methods.

1) Look at the neck from the side as in figures 4 and 5. Fret an outside string at the first fret with your left hand and at the twelfth fret with your right. Next, observe how the frets between the first and twelfth touch the string. If all the frets touch the string, the neck is either dead flat (Fig.4) or has some backbow. If there is some space between the frets and the string then the neck has some relief (Fig. 5). To determine if the neck is twisted, use the above method to read each side. If one side has more or less bow than the other, or the sides bow in opposite directions, the neck may be twisted.

2) Using this method, it is easier to distinguish a flat neck from a backbowed neck. Sight down the side of the neck from the headstock to the body. Now compare the curve of the neck to the outside strings on both sides to see whether the neck is flat, backbowed or upbowed and check for twisting.

Using both methods one after the other will give the best indication of the neck’s condition. After determining the bow of the neck, you can prepare to adjust the truss rod and optimize the instrument’s relief.

Note: A common misconception is that a dead flat neck is perfectly adjusted. In fact, the majority of necks operate best with a slight amount of relief (upbow). Strings vibrate in an elliptical pattern, so a perfectly flat neck may interfere with their vibrations causing fret buzz.

Understanding truss rod mechanics and neck relief is key to optimizing the playability of any truss rod-equipped guitar or bass guitar. The next Xylem article will discuss how to adjust truss rods to correct for specific problems and optimize relief to improve an instrument’s overall playability.

If you have questions or comments about this article please email Xylem: anthony@xylembassguitar.com

Copyright © Anthony Olinger, Xylem Handmade Basses and Guitars 2008.