The single truss-rod used on all Fender guitars was originally adjusted at the body end of the neck.

Do you really need to know much more about a truss rod than how it operates? In my previous column [“Bending to Your Will—The Simple Little Rod,” December 2014], I explored the basic mechanics and history of the truss rod. The way it works is incredibly simple: The more you tighten it, the shorter it gets, and in turn, the more it bends the neck backwards. This curve brings the strings closer to the frets as needed.

As we discussed previously, guitar builders originally relied on the strength of the wood alone to resist the pull of gut strings. When steel strings became popular for their superior volume and brilliant tone, builders had to devise a new way of coping with the increased tension. And the adjustable truss rod was the answer.

That’s not to say that the truss rod was the only attempted solution. Builders took to adding heft or multiple laminations, or even experimenting with stiffer varieties of wood to brace necks. Martin used a square, non-adjustable steel rod on some of their models to resist bowing. It was also common for a guitar neck to be fashioned with a bit of backwards bow in the hopes that it could meet the pull of the strings halfway to create a straight and playable fretboard. The problem is that under pressure, wood will still move over time. It’s a phenomenon known as “creep.”

This condition required one of two solutions. The frets could be pulled and the fretboard’s curve reshaped—a difficult task. Or the frets could be replaced with new frets with thicker tangs. Wider tangs force the slots open and the cumulative effect is that the neck will bow backwards. In fact, frets of varying tang thickness were developed to control the amount of bow repair people needed in the areas they specified. Still, this wasn’t something players wanted to do on a regular basis, so the adjustable rod was certainly a welcome invention.

Some builders feel these factors can impose a sonic penalty, or at least alter the resonant frequency of the neck.

For the most part, the adjustable single-action rod provided enough control to keep guitars playing over time and through changes in climate. If you were lucky, an adjustment once or twice a year was plenty. But occasionally, a neck would bow in a small area or even bend backwards on its own—two conditions that a simple rod is not equipped to combat. It was these types of maladies that drove luthiers and engineers to seek more innovative rod systems.

The double-acting rod. As I mentioned before, one of the first improvements was the double rod, which is exactly what it sounds like. It’s a system that employs a pair of rods connected at each end, with the length of one rod being adjustable at one of the ends. A curve is introduced into the pair by lengthening or shortening the adjustable side relative to the other.

When double rods are inserted into a guitar neck and stacked vertically, adjusting them forces the entire neck forward or backward in concert with the bend of the rod. This system does not rely upon the neck as part of the mechanism as found with the single rod, but it does compress or stretch the neck material during bending, so it’s still susceptible to creep. This system doubles the weight of the hardware, and the increased size of the slot needed to hold the two rods necessitates removing more wood in the center of the neck. Some builders feel these factors can impose a sonic penalty, or at least alter the resonant frequency of the neck.

“Neckst” steps. Although the two types of truss rods I’ve outlined work perfectly well, it hasn’t stopped creative souls from pushing the envelope. Variations on the single rod include the use of titanium, ostensibly to improve performance and to reduce weight. Titanium is used in other industries for its high strength-to-weight ratio, weldability, and resistance to corrosion, so the idea of a lighter and stronger truss rod is attractive to guitar builders for obvious reasons. Unfortunately, premium-strength titanium is 10 times the cost of the carbon steel commonly used in guitar rods, and this tends to limit titanium to high-dollar builds.

Rickenbacker is a good example of a company that has traditionally used a pair of truss rods. Photo courtesy John Hall.

Double-action rods have undergone evolution as well. Martin moved to a rod-in-steel-channel design that was somewhat more compact than two rods. A slight mutation of this concept uses a single round rod that’s sheathed in a square channel that can be slotted anywhere along its length to aid in a localized correction.

Perhaps the most famous hybrid is the Rickenbacker twin-rod system. Rather than tying two rods together, this arrangement consists of two side-by-side single rods, spaced about a half inch apart. Because the heavier-gauge strings on the bass side of a neck exert more pressure and require more opposing truss-rod force than the treble side, the theory here seems that the ability to independently bend each side of the fretboard will result in a better playing neck.

Ramifications and side effects. Let’s go back to the argument that rages among some builders about the sonic difference between the various systems. If you like the sound of wood, then it seems the object should be to keep the amount of steel in the neck to a minimum, but there are those who claim that a double-rod’s mass of metal enhances sustain and brightness, and a neck’s resonant frequency can certainly be affected by mass. Either way, I think stiffness is likely the biggest contributing factor.

Another consideration is the physical balance of your instrument. A heavier rod not only adds to the total weight of a guitar, but can also make for a neck-heavy attitude when using a strap.

So these are some of the things that keep me up at night. (Yes, I really need to get out more.) Chances are you’re not too worried about the type of truss rod that’s in your axe as long as it allows you to get the neck playing right ... and at the end of the day, that’s probably all you need to know.