Here’s how to adjust your truss rod for a low-action setup.
Last month we looked at how truss rods work (“The Magic Truss Rod”), so now the big question is, what can we do with this knowledge? As we discussed previously, a wooden neck is mechanically the weakest part of the instrument. Changes in humidity and the relentless pull of the strings affect the neck’s curvature, so to control this shifting we can anticipate having to periodically tweak the truss rod.
For a perfect setup, do we want to have our bass neck straight or curved? If the answer is the latter, how much bowing—also known as relief—is required? Even experienced luthiers and guitar techs debate this, so let’s see if we can shed some light on the topic.
Let’s assume that like most players, you’re looking for a comfortable setup that plays well with very low action. We’ll also assume we’re talking about an instrument with evenly leveled frets and a correctly cut nut, and the intonation has been adjusted. This means that the number of remaining parameters for a good setup is rather small. Two, to be exact: One is bridge height, the other is neck bow. Sounds like an easy task, right?
We need to consider the elasticity of the different regions between bridge and nut. This elasticity defines how each part of an instrument reacts to string tension. Luckily, these regions behave very similarly on most instruments, which makes it easy to predict their role in a quick and easy setup.
The body itself is pretty much immune to bending, and the only adjustable part on it is the bridge. Next comes the upper range of the fretboard with either a neck pocket for a bolt-on, or the thicker part of a neck-through or glued-in build. This transitional area is rather substantial, so here string tension won’t have much of an effect, if any, which explains why truss rods offering headstock access typically don’t reach into that area.
The neck itself is inarguably the weakest part of the whole ensemble, and it gets even thinner and more flexible as it extends toward the nut, so that’s where we need to focus. Many players are afraid of adjusting the truss rod and often don’t know where to start this part of the setup process. But no worries—there are two simple steps that almost guarantee a perfect setup.
First, adjust the bridge so the strings don’t buzz against the highest frets—the ones right above the neck-body joint—when you play in this region. (Or the strings buzz only as much as you are willing to accept when fretting up there.)
Next, tighten the truss rod until the open first string starts to buzz. Now back the truss rod off minimally—just enough to eliminate that buzz. Okay, you’re as good as done. At this point, any instrument with level frets, a properly slotted nut, and a smoothly working truss rod will be perfectly set up for a low action. If you experience areas with too much buzz, then the condition of your nut or frets isn’t ideal. But there are ways to compensate for these issues, if they’re not too extreme: Any buzzing in the lower range of the fretboard calls for loosening the truss rod, while buzzing in the upper regions requires raising the bridge.
And we can now return to that initial question of straight versus curved: After following this two-step procedure, it’s very, very likely the neck will not be straight, but rather slightly bowed forward. Why? Next month we’ll try to answer this question without using too much math.