OK. I’m going to rant a little this week.
A truss rod is not for adjusting action.
A truss rod is NOT for adjusting action.
Despite the fact there is information around the web telling readers to adjust their truss rod to raise or lower action, a truss rod is not for adjusting action. Despite the fact that, until recently, a major instrument manufacturing company included information to the contrary in their support materials, a truss rod is not for adjusting action.
It’s just not.
The side-effect is the problem. The side-effect is why this poor practice has perpetrated.
Straightening a bowed neck with a truss rod can have the side-effect of reducing action.
The action doesn’t reduce consistently along the neck. There will be a larger change in the middle of the truss rod’s length (not necessarily the exact middle of the neck) and the action towards the higher end of the neck may not change significantly, if at all.
Let’s try illustrate things:
Imagine a neck with some bow in it like our exaggerated Figure 1 below.
OK, now we tighten the truss rod and straighten that neck.
Well, yes, the action (Figure 2) gets reduced in part of the neck.
I’ve broken out the ‘shape of the action reduction along the neck to illustrate the inconsistent height change along the neck. I don’t like inconsistency. Inconsistency is hard to work with for consistent results — the clue’s in the name.
What you’ve done in this example is correct the neck relief, not lower the action. Understanding this ‘side-effect’ is an important part of great setups.
Even worse than the inconsistency I mentioned, is that using relief to adjust action can leave you with an instrument with the ‘wrong’ relief for your playing style. Relief is a setup parameter that deserves consideration on its own and not just as a means of altering string height. That's to say nothing of the fact that many reading the truss-rod-action advice may already have an instrument with optimum relief. Rarely do I see advice about measuring relief before making any changes
Doing it properly
It’s far more controllable to first set the neck relief using the truss rod and only then set the action.
For the purposes of our illustration, picture a neck that we’ve already set relief on (straight in this example). Now that relief is set, we adjust action at the bridge/saddles and there is a ‘consistent’ change along the neck.
Note: I didn’t say “the same” change as obviously the strings ‘pivot’ at the nut. However, the action changes consistently and I only need to check or measure action at one position. I know that a change at the bridge will have a corresponding change along the neck.
Now, it’s important for me to mention that things are slightly more complicated in the real world. While I think my ranting above is a good way to consider approaching a setup, there is a certain amount of ‘interaction’ between the action and relief.
This is something that you figure out after doing a few setups and it’s a little difficult to quantify in words. I’ll save an attempt at it for another day as it might entail my rambling a bit.
For now, suffice it to say, that a good setup usually involves a balance of adjustments to relief and action (at the nut and bridge). Often a small change in one will impact how the others have to change.
The Bottom Line
In the real world, relief and action (and nut slot height) are interactive. A change in one affects the feel of the others and may require an adjustment in the others, too.
This interaction is more subtle than the advice to ‘change action with the truss rod’ allows for and I maintain that it’s bad advice. Don’t use the truss rod to change action.
- Set neck relief first.
- Then set action.
- Then play the instrument and adjust one, or both, to taste/feel.
There tends to be a good reason we address setup stages in distinct steps. For example, I order the steps in the Sketchy Setups guides in a very deliberate way and recommend you go through them in sequence. During the ‘fine-tuning’ of a setup, you may (probably will) go back and revisit one or more steps but the initial order is that way for a reason.