In order for the rod to bend the neck, it only needs to be to one side of this neutral plane. Essentially, by turning the adjusting nut and thus shortening the rod, you are crushing the backside and stretching the front side into a convex arc. This is fairly rudimentary stuff, which is why I like it—no zizzing or dripping, and no software to crash. It’s possible to over-tighten a single rod, so if you’ve cranked it for more than two complete turns, I’d stop there. Then call a pro because if he breaks your rod, he’ll have to fix it for you!
Two-way truss rods. Despite the success of the simple, single truss rod, there is always room for experimentation—if not improvement—and over the years many have tried to create the perfect prop to support our favorite playing surface. One of the best-known derivations is the two-way, or so-called double-acting truss rod.
This contraption usually employs two parallel rods that are stacked, and then welded or screwed to each other at one end. The free ends are threaded and can be adjusted back and forth at a joining block, causing the entire arrangement to bow in two directions. The neck is still being compressed and stretched the same way as with a single rod, but the idea is that this device can be inserted into a neck at any depth and it will still bend the neck.
The plus side is that on the rare occasion a neck bows too far backwards on its own, some double rods can be used to coax the neck forward—either by reversing the direction of the adjuster or physically removing the rod and reinstalling it upside down. The downside is that this type of rod weighs twice as much as a single rod and takes up twice the space. It can also be prone to weld failure when over-tightened.
Variations on the theme. The original Gibson rod was inserted into a curved slot that was closer to the fretboard in the middle of the neck, the exact opposite of the Fender design that slopes upwards at each end. In both cases, the curve may have been adapted to provide access to the adjustment as opposed to any kind of mechanical advantage.
Other builders follow Parker’s theory with a straight rod because there is no added power to a curved rod as long as the neutral-plane rule is observed. In fact, I equate the curved rod to pulling on a clothesline full of wet laundry in an effort to make the line straight, when what you’re really trying to do is move the pole it’s tied to.
Tonal and physical considerations. This leads us to the other side of the truss-rod equation. How does it alter the sound and sustain of the guitar? Unfortunately, I don’t have enough space here to go into it, so that will have to wait until next time. Until then, go forth and adjust without fear. Just don’t over-tighten.