The truss rod. It’s the long metal thing that runs down the length of your guitar neck’s interior, routed deep into it and hence unseen, but nonetheless the unsung hero of steel-string instrument design. Why is it there and how does it work?
The truss rod is there to do one thing—to keep the neck of your guitar straight and stable, keeping your instrument in tune all the way up its well-aligned neck. It does this job well with ingenious simplicity and efficiency, constantly counteracting the tremendous physical forces that continually conspire to bend, warp and bow the neck, preventing proper intonation.
Chief among these powerful physical forces is string tension. You just wouldn’t believe how much tension steel strings put on the neck of a guitar or bass. It’s a lot (even a set of light-gauge strings can put well more than 200 pounds of tension on a neck). Enough, in fact, to actually bend a wooden neck into a concave bow, or maybe pull one side of a neck out of whack while the other side remains OK.
These are problems that didn’t exist with old-world guitars, as their gut and later nylon strings didn’t exert enough tension to pull a neck out of shape. But with the advent of steel strings in the early 1900s, the effects of greatly increased string tension on wooden guitar necks became a real problem.
Another continuous physical force acting on your guitar’s neck is climate. Guitars are made of once-living plant material—wood, namely—that can be plenty strong, but it’s not like it’s steel-reinforced concrete. Wood is not 100-percent rock-solid stable; it reacts to climate variations in temperature and humidity by expanding, contracting, warping and bending. All of which are natural and normal.
Truss rods were introduced in the early 1900s to counteract these forces by reinforcing wooden guitar necks with adjustable steel rods (graphite is also commonly used now) that served as a sort of “backbone.” Indeed, the truss rod strengthens, straightens and supports the neck of your guitar in much the same way that your spinal column strengthens, straightens and supports you. They also allow instruments to be made from less rigid and perhaps less expensive woods and other materials that otherwise wouldn’t be able to withstand the forces of steel string tension. Further, truss rods also allow thinner necks, which improves playability.
What makes the truss rod so indispensable is that it’s adjustable. A nut at one end of it (often accessible at the headstock end of a guitar with an Allen wrench, although it’s sometimes at the butt end of the neck), allows it to be tightened or loosened as necessary. A concave (dipping) neck bow can be corrected by tightening the truss rod, thus resisting string tension. A convex (arching) neck bow can be corrected by loosening the truss rod, thus allowing the neck to bend forward slightly in response to string tension.
The amount of curvature along the length of the neck is called “relief.” A very small amount of concave relief is often actually desirable rather than having a ramrod-straight neck. The truss rod is adjusted to provide just the right amount of relief. In a combination of precise measurement and personal preference, the amount of neck relief, combined with string height and length settings of the bridge, determines the proper intonation and ease of playability of an instrument.
Truss rod adjustment is one of the most common guitar maintenance items. Novices might prefer to leave it to authorized service personnel, but it should be neither time-consuming nor terribly expensive, unless the neck is really seriously messed up. It’s quite commonplace for more experienced players to make minor truss rod adjustments themselves, with an eighth or quarter turn or two often sufficient to correct a visible neck bow. It is possible to “max out” out or even break a truss rod—the former signified by an inability to crank the nut any further because it’s already gone as far as it can go; the latter signified by a sickening snapping sound—but these two scenarios occur relatively infrequently.