In today's steel string acoustic guitars, the truss rod is as common as the soundhole. But is a truss rod really necessary? What would happen if it was removed entirely and/or was replaced with something else, such as a carbon fiber stiffener?
A truss rod is a rod, bar or beam installed into a channel cut into the neck and secured beneath the fretboard during the construction of the guitar. Originally intended to compensate for the pull of the strings on a neck that would otherwise be subjected to unwanted excessive forward bow, the truss rod has graduated from a simple hardwood stick to a clever machine that permits manually-induced backward or forward bow.
A six-string set of D’Addario EXP11 Light Gauge steel strings applies 159 lb (72 kg) of tension on a 25.5″ scale length guitar. A light gauge 12-string set from the same supplier (EXP36) is rated at nearly 242 lb (110 kg) of tension!
Most guitar necks are made of wood and, without supplemental support, prove to be no match against the pull of steel strings. Over time, as the neck bows forward, the strings may continue to elevate farther away from the fretboard, resulting in an unplayable instrument (you can strum the strings, but you can no longer comfortably press them down against the fretboard to change their pitch).
Comparatively, nylon string guitars exert much less tension on the neck, typically ranging from 75 to 100 lb (34 - 45 kg). Historically, truss rods were rarely found in such instruments as the wooden necks were (are) inherently stiff enough to resist the pull of the strings. However, as countering string pull is not necessarily the only function of truss rods, they are also being included with modern nylon stringed guitars.
Early renditions of truss rods were simply fixed stiffeners. Extra-stiff wooden bars or metal square tubes, T-bars, I-beams, U-channels, etc, went a long way toward prolonging the inevitable forward bow with some being more successful than others. They eventually fell out of favor as adjustable versions were introduced.
Wooden trusses are, well, wood. To be rigid enough to be effective they are, by necessity, heavy hardwood. As such, they are subject to the same environmental factors the rest of the neck is subject to. By contrast, to be light enough to be usable, fixed steel trusses are still subject to bending and, once bent, may form a rather permanent forward bow.
I believe the fault of the early fixed stiffeners to be one of material availability, more so than adjustability (or the lack thereof).
The simplest form of an adjustable truss rod may be the Compression Rod. A steel rod is permanently secured at one end of the neck, compressed into a slight concave bow in a channel cut into a dead flat neck, and held in place via a shaped filler strip. The opposite, threaded end of the rod is exposed for adjustment purposes.
Modern versions of the Compression Rod may thread into a barrel bolt on the end that is to be retained, where older versions featured a peened rod against which a thick, rolled washer is affixed, even welded in many cases.
On acoustic guitars, that fixed end is commonly anchored at the heel of the neck and the threaded end passes through a recess routed into the headstock. Most electric guitars reverse the orientation, exposing the threads at the body end of the neck. Tightening the nut on the threaded end of the rod (against the wood through which the rod passes) attempts to shorten, and subsequently straighten, the embedded bowed rod, effectually compressing the back of the neck and lengthening the fretboard to counteract the string pull. Tightening such a rod with no strings attached will result in a slight back bow of the neck, and loosening it should return the neck to dead flat again. Since the strings are attempting to pull the neck into a forward bow, the result is a balance between two opposing forces.
The Compression Rod has proven to be quite effective over the years, but is not without faults. The embedded rod is held in a pre-determined curve based on an educated guess regarding the future movement of the neck. Over time, the compensation effected by the tightening of the compression rod may no longer be sufficient, as the rod can only move from its original, slightly bowed position toward straight (at best). Many a compression rod has had additional threading applied in order to effect greater pressure. Over-tightening often results in a snapped rod, a very undesirable condition. Due to the one-way adjustability of the design, it is often difficult (if not impossible, without special jigs) to return the neck to the straight, dead-flat condition needed for fretboard/fret work, setups, etc. When the compression rod hasn't broken (yet), and the threads haven't stripped (yet), over-tightening has been responsible for producing a visible split down the back of the neck which may or may not be easily repaired.
Like the Compression Rod, the Single-Action (or single acting) Truss Rod employs a uni-directional adjustment to counter the forward bow of the neck resulting from the pull of the strings. Unlike the Compression Rod, it is a self-contained machine, requiring only a rectangular slot in which to reside beneath the fretboard. It is not fixed to either end of the neck, nor does it need to be compressed into a curve.
A Single-Action Truss Rod is comprised of two parallel rods, one slightly longer than the other, that are joined at one end (two rods may be welded together, or one long rod may be heated, folded in two, and hammered tightly together). At the other end, one rod is fixed into a block through which the second, longer rod passes. Typically, the two rods are bound together with tape or a sleeve made of heat-shrink material. The end of the longer rod is threaded and, as a nut is tightened against the stop block (into which the shorter rod is terminated), a bow is developed; concave for the longer rod and convex for the shorter.
Installed into a slot in the neck in such a way that the longer rod (the rod having the adjusting nut) is farthest from the fretboard, tightening the nut of this style of truss rod will introduce a back bow in the neck. If you were to reverse the installation, and rotate the rod 180° along its length in the neck slot where the longer rod (the rod having the adjusting nut) is up against the fretboard, tightening the nut would result in a forward bow in the neck.
If care is taken when designing and assembling this style of truss rod, and if the guitar neck is removable (such as is the case with a bolt-on neck), the truss rod may be reversed at will in order to adjust for various conditions.
Rods can snap, welds can fail, and threads can strip, so it is important to make small adjustments with care.
Can one truss rod introduce both a forward and back bow in a guitar neck without having to be un-installed, rolled over 180° and re-installed? Enter the clever, if not ingenious, Dual-Action Truss Rod (double-action, double acting, twin-action, etc). Its self-contained, two-way adjustability overcomes the uni-directional limitations of both the Single-Action Truss Rod and the Compression Rod and provides a greater degree of neck bow control. Turn clockwise to induce a forward bow, and counter-clockwise to bow the neck backwards. This reversing of direction is accomplished by threading the rod through the stop block, as opposed to simply passing it through a hole in the stop block, as is the case with the Single-Action Truss Rod.
Along with ease of installation, requiring only a simple, straight channel, it is understandable how this design skyrocketed it to truss rod super-stardom. But does the Dual-Action Truss Rod have a downside? More neck material may need to be removed to accommodate it and, unlike the Compression Rod, the channel it occupies must be left open perpetually requiring greater care when attaching the fretboard. It is typically heavier than its single-action cousin, has been known to introduce the dreaded intermittent rattle (Ugh!). As with any steel Truss Rod of adjustable design, the Dual-Action Truss Rod is subject to failure (welds, stripped threads, snapped rods, etc).
Despite its shortcomings, the the Dual-Action Truss Rod is ubiquitous today.
The use of carbon fiber for stringed instrument neck reinforcement is now several decades old, and it is easy to understand why. The fact that carbon fiber often weighs less than the wood it replaces and is certainly many times stiffer and stronger can be reason enough to incorporate it. Fibers comprised of long chains of carbon atoms are embedded in a matrix of epoxy resin resulting in material having properties similar to steel with the weight of a plastic.
Steel and plastic are said to be homogeneous and isotropic, meaning their properties (mechanical, electrical, thermal, etc) are equal at all points, in all directions, throughout the material. Carbon fiber, on the other hand, generally exhibits strength along the axis of the fibers only, similar to wood. Carbon fiber can be specifically constructed to approach isotropic (called quasi-isotropic) properties by layering the laminates in off-axis direction (think: plywood).
With or without the adjustability of a truss rod, a guitar neck needs to be as stiff and lightweight as possible. Many builders have chosen to supplement an adjustable truss rod with parallel bars of carbon fiber running the length of the neck (see the photo, above).
Relief may be one of the most misunderstood topics in the world of acoustic guitars.
Current philosophy:
Relief refers to an ever-so-slight but deliberate forward bow in what would otherwise be a perfectly flat fretboard, thought to be necessary in order to prevent or mitigate the incidence of contact of the oscillating strings with the metal frets.
It is believed that relief may be helpful in reducing string buzz resulting from extremely low action setups, reduced tension (lower-than-concert pitch) tuning(s), heavier gauge strings, aggressive strumming, etc.
In an effort to satisfy as many customers as possible, many acoustic guitar builders seek to construct a guitar that will handle any possible playing or setup scenario. For such builders (and subsequently most all of the guitar-buying public), an adjustable truss rod is considered to be a required component.
Belief in Relief:
Relief is only as necessary as you believe it to be. Consider the following:
The nut is slotted to hold the strings in their properly spaced position. Most importantly, the nut acts as fret #0. To demonstrate this, the nut can be moved back toward the headstock, using it solely to maintain the spacing of the strings. The strings may then be laid directly across a fret wire installed where the nut used to be/would have been, in that zero fret position on the fretboard.
The strings form a flat plane, extending from fret #0 (or the nut) to the crest of the saddle. The measurement of this distance is referred to as the scale length. The fretboard, when installed, during a re-fret, and prior to beginning a proper setup, also forms a flat plane, extending from fret #0 across fret 20 (or 22, 24, etc). The two planes are not parallel, but they do intersect at fret #0. The strings gradually increase in distance away from the surface of the frets as you move toward the saddle.
The 12th fret wire is positioned at a point that measures halfway between the nut and the saddle. This is the center point of any given length of string and this is where action (or string height off the frets) is measured.
An un-fretted (or "open") string has the widest swing or oscillation path, which happens to occur directly above the center point, the 12th fret. As you fret a string, pushing it down to make contact with the fret wire, you (temporarily) shorten the scale length for that string. Subsequently, you alter the string's center point, its position of widest swing, moving that point closer to the saddle. Shorter strings will have narrower oscillation paths.
Truss rods, when used to induce relief (forward bow or curl) in the neck, are literally lifting the headstock and, subsequently, fret #0 up and away from their original position. This raises the strings away from the frets, resulting in a higher action. Using a truss rod to un-curl such a neck, forcing the fretboard flat again, has the opposite effect, lowering the strings back toward the frets. Armed with a hex wrench, it is no wonder why the typical guitar owner continues to believe that action adjustment is the primary purpose of the truss rod.
Pause long enough to consider the actual location of the truss rod in the neck and precisely where its bowing effect occurs (and doesn't occur) in relation to the path of the oscillating strings. Acknowledge the proper role of accurate neck geometry and the benefits of a flat fretboard, and you may begin to see the irony of the widespread "belief in relief."
0.1mm (0.01") is considered to be low relief, while 0.76mm (0.03") is considered to be high relief (though many guitars are set to 0.10" and even higher). If you are inducing relief to prevent strings from buzzing, you likely are in need of a proper setup made by a qualified technician or luthier.
Perhaps. And that is totally accomplishable by adjusting action height. If switching between such tunings (concert pitch and well-below concert pitch) is going to be a frequent event on the same instrument, a guitar having an adjustable truss rod may be a better choice. As an alternative, you could just use a second guitar that is optimized for the intended purpose!
You sound like a perfect candidate for a truss rod replacement.
Most (if not all) luthiers and guitar technicians acknowledge that neck relief, while related, is to be treated independently of string height (action). Otherwise, you end up chasing your tail trying to get the setup perfect. Assuming we are starting with a neck having an adjustable truss rod, and that neck is set to be dead flat (planing and radiusing are perfect, frets are perfectly consistent in height, string height at the nut is perfect, etc.), when we correctly set up the guitar for playability we first establish the desired relief independent of string action, then address the string height at the saddle. Sadly, many tweak and fiddle with the truss rod to address action, which only succeeds in complicating what would otherwise be a very straightforward setup process.
A growing contingent of builders and players have become convinced that these steel fixtures (Square tubes, T-bars, I-beams, U-channels, Compression Rods, Single-Action Truss Rods, Dual-Action Truss Rods, etc) have no place inside their instrument necks and are actually detrimental to both the playability and the sound of their guitars. They contend that the weight of the added steel does not make for an optimally balanced neck, and the metal fixture may have/has a negative impact on the overall tone.
The goal of acoustic guitar construction is to capture as much string energy at the saddle (and bridge) as possible, with the least amount of damping. On guitars that have a dual-action truss rod, an entire section of the structural frame supporting the length of the strings (the neck, directly beneath the fretboard), which could have been tasked with transmission of energy, has been removed to accommodate a potentially damping-inducing mechanical tool.
Regardless of the issue of relief, the truss rod and/or stiffener(s) cannot simply be removed, as the need to counter the pull of the strings remains and most wooden necks have failed to successfully resist the pull of the steel strings without supplementary assistance.
If one believes neck relief to be essential for optimal playability, it can be readily calculated and introduced into a flat, immoveable neck design. Long before the advent of carbon fiber, guitars were constructed using fixed necks, having pre-set neck relief, so this is not a new thing. Today, when I am asking if an acoustic guitar needs to have an adjustable truss rod, I am really asking if it is absolutely necessary to be able to adjust neck relief.
Can you honestly tell me that you don't know what gauge strings you prefer to play with on any given acoustic guitar?
Can you actually demonstrate, if you are truly among the unique few who are constantly swapping out string gauges and/or dramatically altering tunings ON THE SAME GUITAR, that you make truss rod adjustments each and every time?
If you can honestly answer “Yes” to either question, let me be the first to congratulate you, and add that I do not see any benefit in your pursuit of a truss rod alternative.
The Dragonplate D-Tube Neck Beam is a carbon fiber structural component specifically designed for the task of replacing a truss rod altogether. This 16″ hollow beam weighs a mere 1.8 oz (0.05 kg). The well-known Martin-style adjustable truss rod weighs more than three times as much at a whopping 6.06 oz (0.17 kg). Comparatively, weight-wise, the D-Tube is practically non-existent!
There are multiple versions of this product available, but I chose to use a straight tube 3/4″ wide x 1/2″ high x 16″ long, as the groove I would have to cut into the neck, being of equal depth throughout, would be straightforward to rout.
The groove may be cut via CNC, milling machine, a router table, jigs and fixtures with a handheld router, or with hand tools (I use a router table). In addition to the D-Tube, a few supplies are necessary:
• Neck blank
• Neck profile template
• Router and router bit
• Epoxy
The D-Tube compliments all varieties of neck construction. Whether a neck is laminated or solid wood, whether the headstock is one piece with the neck or attached via a scarf joint, whether the neck to body attachment is a glued dovetail, a bolt-on mortise-and-tenon, a butt-joint loose tenon, or even an integrated Spanish heel; all are able to receive this truss rod replacement.
I happen to prefer beginning with as stiff a neck as possible, such as may be achieved by laminating quarter sawn material.
Up to this point, these steps would be required for any conventional acoustic guitar neck built this way, with or without a truss rod.
The only difference between constructing a neck with a D-Tube and constructing a neck with a truss rod and Carbon Fiber stiffener combination is in the number and size(s) of the groove(s). The D-Tube requires that I rout a single channel down the center of the neck and epoxy in the Neck Beam, as opposed to routing three slots, the center one for the loose-fitting truss rod and the remaining two for carbon fiber stiffeners (which would also need to be epoxied into the neck).
Tool recommendation: I use a 3/4″ wide Whiteside 1411 3/8″ radius round nose (Core Box) router bit that perfectly matches the half-round profile of the “D” shaped carbon fiber beam.
A trip to the router table provides me with a couple of advantages over using my handheld router with a centering base. Dust collection is a given at my router table and can be harder to control with a handheld tool. Additionally, the fence allows me to cut grooves quickly, safely and accurately. With a handheld router, I need to be much more vigilant regarding the proximity of the router base to the neck. More importantly, my neck blank does not extend far enough beyond my groove's start and stop points to safely register my router if using a centering base (dedicated router base having two (2) perpendicular pins spaced equidistant from the center). If I didn't have a router table I would employ a jig or fixture to both cradle the neck and to guide the router along. Otherwise, I would have to alter my construction techniques, attempting to rout the groove prior to dimensioning the neck.
For my application it is necessary to rout a 3/4″ wide by 1/2″ deep groove or channel down the center of the face of the neck (the side to which the fretboard will be attached). I choose to keep the D-Tube flush with the face of the neck blank, where it will be in direct contact with the back of the fretboard, as opposed to recessing it slightly and filling the recess with a thin strip of wood. At 1/2″ deep, this material will sit very close to the back of my finished neck at the headstock end, so I want all the clearance that can be afforded me. I can rout a through groove from the headstock to the heel block, or stop shy of the end. I cut the groove in three (3) ever-deepening passes over the router bit.
The groove is cut ever-so-slightly deeper than the height of the D-Tube. This accommodates a thin layer of epoxy beneath it. Once the epoxy has cured and the surface is trued-up again, the D-Tube will (should) be perfectly flush. I want 100% contact of neck material to carbon fiber at all points. I fashion a plug that will fill the void (the portion of the groove not filled by the carbon fiber), butting up squarely against the end of the D-Tube, a process which takes just a few extra minutes at the bandsaw and the disc sander. I have also successfully used 3/4″ hardwood dowel stock for this purpose. Alternatively, I can shape the end of the D-Tube to conform to the compound radius left by the stop dado, or simply have plugged the end of the D-Tube and filled the small void with epoxy.
As an alternative to stopping the 16″ D-Tube short of the dovetail tenon end of the neck, I can use a longer 20″ D-Tube that runs the full length of the neck with material to spare. This can project out onto a dovetail or tenon, or across a raised fretboard's extension. I don't currently do this, as I prefer to leave that area as solid wood. That solid wood then forms the upper portion of the tenon of my neck joint. Replacing that solid wood with a hollow D-Tube has not provided me with an obvious benefit (yet).
Note that on the opposite end of the neck I have run the carbon fiber up into the headstock, rather than terminate the groove short of the point where the fretboard ends at the nut, as I would with a truss rod slot. It has been a long time since I fashioned a neck without using some form of carbon fiber stiffener, and I have always extended those stiffeners into the headstock area. Having witnessed a few cracked headstocks from other builders, I believe this crucial juncture of the neck benefits from the extra attention. As others have rightly pointed out, be aware of the dimensions of the D-Tube, the slot in which it resides, and the target thickness of your finished neck.
The groove gets coated with a thin layer of epoxy. The D-Tube is hollow and I don't want glue inside of it. Because I do not flood the groove with adhesive there is no need to plug the ends of the D-Tube. Here I am using 3M Scotch-Weld 2216 2-part epoxy. It has proven to be very effective. I have also had success with Superbond Epoxy 1:1 from Fiberglass Coatings.
To simplify cleanup, I would suggest applying tape (painter's, low-tack, etc) to the surface of the neck blank prior to adding the epoxy. Two narrow strips, one down each side of the groove, or a single, wide strip whose center is removed with a razor knife will also mitigate the need to aggressively remove any squeeze-out and subsequent hardened epoxy. Why bother with tape, you may ask? I have no desire to re-establish my neck geometry, which can (would likely) be altered when attempting to sand off epoxy squeeze out. Your mileage may vary.
Both the carbon fiber D-Tube and my custom wooden plug are pressed into place. I lay a piece of parchment paper over the neck and lightly apply a few small clamps, merely to prevent any shifting during cure. If you are unfamiliar with epoxy, unlike PolyVinyl Acetate (PVA) or Aliphatic Resin (AR) glues, it does not benefit from high-pressure clamping; in fact, heavy clamping is advised against.
The neck is left to dry overnight.
I have successfully bonded many fretboards to neck blanks containing carbon fiber stiffeners using protein-based glues (Fish or Hide Glue) with no adverse results. My preference is to use a very thin application of epoxy to attach fretboards and, while fretboard removal may be a bit more difficult, there is absolutely no concern regarding obtaining a sufficient bond with the carbon fiber. If you are using an AR glue, such as Titebond or the equivalent, you may want to consider routing your groove a bit deeper in order to add a wood filler strip. Be aware that increasing the depth of the groove, already 1/2″ deep, in order to allow for a thin veneer on top of the carbon fiber could potentially compromise the overall thickness of the neck (The manufacturer does offer both a smaller, straight tube as well as a tapered tube that may be used if neck thickness really becomes an issue).
After releasing the clamps and lifting off the parchment paper, I removed any accumulated, cured epoxy using a scraper. When setting the depth of cut for the groove, I had taken the time to ensure the D-Tube would sit just shy of the surface of the neck blank. I was rewarded as the scraper just kissed the surface of the carbon fiber as all the squeeze out was removed.
What do you do if the tube is slightly proud of the surface of the neck? While I have not had this issue, you would need to try to make the surface flush again without altering your neck geometry and without weakening the D-Tube. I would suggest sanding it on a perfectly flat surface. And then try really hard NOT to do this on the next neck!
Cut-off wheels, such as the type used with rotary tools, are my preference for cutting carbon fiber, and I have had good success with diamond impregnated jig-saw blades. Using a Dremel, I remove most of the section of D-Tube that extends out over the headstock. The balance gets sanded back nearly flush with the headstock at the disc sander. For final dressing of the face surfaces of the neck and headstock, I rely on strips of adhesive-backed sandpaper affixed to a dead-flat marble slab. A few careful strokes across that slab are all that are needed to complete this stage of the neck construction. Alternatively, I could saw the waste off and true it up with a sanding block. At this stage, it is critical to maintain the dimensions of the neck, so sand with caution.
Tip: Protect your airways and eyes when machining carbon fiber! That includes any and all routing, sawing and sanding.
I completed the neck in the same way it would have had I installed any other form of Truss Rod. With this neck I have now completely replaced the truss rod (or combination truss rod / carbon fiber stiffeners) with the Dragonplate D-Tube Neck Beam.
My tests have resulted in absolutely no flex with extra-light or light gauge strings at standard tuning, and (surprising) negligible flex with heavy gauge strings at standard tuning. A medium gauge set of strings (13-56) introduced just under 0.1mm of relief at the 6th fret on a 14 fret (5th fret on a 12 fret) body joined neck.
A qualified “Yes”, depending on your goals. I have found it to be the case on 6-string guitars using medium-gauge strings and below, and 12-string guitars using light-gauge strings. Your experience may differ.
I have done it! Zero deflection on a 12-string neck using RotoSound Jumbo King lights (11-52)!
Not for me. Not anymore!
I am convinced I can build a noticeably lighter guitar having perceptibly increased sustain and improved overall tone.
This neck is impressively lightweight! Equally impressive is it's rigidity when it is installed on the guitar; it does not move under string tension! For my purposes, that is a good thing, as it assures me that I retain complete control of any relief I may choose to build into my necks - or not. Even though the neck has been developed separately from the body, the overall feel of the instrument is more unified.
UPDATE: In my guitars having no center soundhole (where I incorporate my Shoulder Port design), the soundboards are noticeably more responsive using necks having D-Tubes. When the soundboard is vibrated at the resonant frequency of the box, I can measure greater responsiveness in the upper bout, as I move toward the neck block, much more so than on my guitars having center soundholes.
Thank you, Dragonplate!