Ever wonder where people get the idea they can adjust their action by turning the truss rod?
We just had an example of a tech from a major acoustic guitar manufacturer.. Let me back up here tell the whole story –
Background
YouTube and web forums are full of “advice” telling you to raise or lower your action by adjusting the truss rod.
Does it work? Will tightening the truss rod lower your action? Does loosening the truss rod raise your action?
Yes. Yes. And yes. But DON’T DO IT!
I’ll tell you why you don’t want to do that in just a bit, but meanwhile, back to our story –
Professional Advice (?)
Our hero had just purchased a brand-new Taylor acoustic guitar, online. When he received it, the action was uncomfortably high, so he gave Taylor a call.
According to our customer, the tech on the phone advised him to just turn the truss rod – it didn’t matter which way at first. If one direction didn’t lower the action, then turn it the other direction.
Having a healthy fear of truss rods, the customer brought the guitar into Hank’s Guitar Shop, where Liam checked it over.
Liam explained that the guitar’s neck relief (the stuff that the truss rod adjusts) was fine. His action problem was due to a high saddle.
Liam also found some nut slot issues and a few high frets – very common in new guitars, regardless of price range.
The guitar was in our shop for about a week and back in our hero’s hands where he is undoubtedly, happily playing it as I type these words.
Purpose of a Truss Rod
Ever play Tug-Of-War, as a kid? Well, while you’re playing your guitar, it’s playing Tug-Of-War!
A steel-string, six-string guitar has a total string tension of over 150 pounds. That’s like a grown man – like me before I married a great cook – pulling up on the headstock with all his weight.
Most guitar necks are fairly narrow and made of wood. What do you think would happen to guitar necks with 150 pounds of pull on the headstock if there were nothing to counter that tension?
Go to any archery range and you’ll get an idea!
The truss rod’s job is to counter the pull of the strings and help to keep the neck straight.
Not perfectly straight, because strings vibrate in a wider arc at their center, so we allow the neck to have a very slight bow (typically 0.010″) in the center so we can have our action a bit lower.
Unintended Consequences
Bending the neck in a bow (headstock upward) or backbow (headstock downward) will raise or lower the action.
But there are side effects that go along with the change in action. Those side effects, depending on which way you bow the neck and how much, include fret buzz in one area of the neck or another.
Another side effect is that raising the headstock also increases the leverage those strings have, so that 150 pounds can have the effect of two or even three times that much tension. That can’t be good for your neck. (Not for your guitar’s neck either!)
Back in the Saddle
Lowering the saddle is the correct way to lower action in this case.
Some acoustic models have several shims under the saddle. Would you know how many to remove?
Let’s say you have an accurate action gauge (unlikely!), and that you’ve been trained in how to read the gauge properly.
You read 110 thousandths of an inch between the low-E string and twelfth fret. You want it to be 90 thousandths. That’s a difference of twenty thousandths, right?
Let’s say there are three shims and each has a thickness of twenty thousandths of an inch. Tell me, how many shims are you going to remove?
Any change in height at one end or the other, will affect the string’s height at its center by exactly half.
So, you would remove two shims.
In our case, this model didn’t have shims, so we used a special nut/saddle jig and adjusted the jig so we could remove the precise amounts on both the bass and treble side of the saddle.