IntroductionBack to Top
In todays 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 were removed entirely and/or replaced with something else, such as carbon fiber?
Truss rods, available from several sources and in more than one shape and size, all share one thing in common: they have been engineered to stabilize, and compensate for the potential movement of, the neck of the steel string guitar.
A six-string set of D’Addario EXP11 Light Gauge strings applies 159 lb (72 kg) of tension on a 25.5″ scale length guitar. A light gauge twelve-string set from the same supplier (EXP36) is rated at nearly 242 lb (110 kg) of tension! Comparatively, non-steel 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 nylon stringed instruments as the necks are inherently stiff enough to resist the pull of the strings.
Truss rods are set in place during neck construction, embedded into a channel in the neck just below the fingerboard. With nothing but the wood of the neck to resist the pull of the strings, a wooden neck having no truss rod and under high steel string tension may simply continue to bow forward, eventually making the instrument un-playable, as the strings continue to elevate farther away from the fingerboard.
Non-adjustable Truss RodBack to Top
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 but, as they possessed no inherent adjustability, they eventually fell out of favor as innovations were introduced. Wooden trusses are still, well, wood. To be rigid enough to be effective they are, by necessity, heavy. And 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 was one of material availability, more so than adjustability. More on this topic in a moment...
Non-adjustable Truss Rod
The Compression RodBack to Top
A single-action truss rod, or compression rod, is typically secured at one end, compressed into a slight concave bow on a dead flat neck, and held in place via a filler strip. The opposite, threaded end is exposed for access. On acoustic guitars, the fixed end is commonly anchored at the neck joint and the threaded end passes into a recess routed into the headstock. Most electric guitars reverse the orientation, exposing the threaded end at the body of the guitar. Tightening the nut on the threaded end attempts to shorten, and subsequently straighten, the embedded bowed rod, effectually compressing the back of the neck and lengthening the fingerboard to counteract the string pull. To demonstrate it’s functionality: with no strings attached, tightening this rod would result in a slight back bow of the neck, and loosening it would (should) return it 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, 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 not be sufficient, as the rod can only move from it’s original slightly bowed position toward straight (at best). Many a compression rod has had additional threading applied in order to apply more 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.
Standard Compression Rod
Twin Compression Rods ('70s era Guild 12 String)
The Dual-Action Truss RodBack to Top
Enter the clever, if not ingenious, dual (double, twin) -action truss rod. It’s self-contained, two-way adjustability overcomes the limitations of the uni-directional 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. Along with the ease of installation, requiring only a simple, straight channel, it is understandable how the design skyrocketed it to truss rod super-stardom. Does the dual-action truss rod have a downside? More neck material must 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 fingerboard. It is typically heavier than it’s single-action cousin, has been known to introduce the dreaded intermittent rattle (Ugh!) and can also fail (welds, stripped threads, snapped rods, etc). Despite it’s shortcomings, it is ubiquitous today.
Dual-Action Truss Rod
Dual-Action Truss Rod: Clockwise for forward bow
Dual-Action Truss Rods: Counter-clockwise for back bow
Carbon Fiber StiffenersBack to Top
Carbon fiber is a strong, stiff, lightweight composite material that boasts exceptional stiffness-to-weight and strength-to-weight ratios. 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. A carbon fiber part would need to be specifically constructed to approach isotropic (called quasi-isotropic) properties.
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. Many builders have chosen to supplement an adjustable truss rod with parallel bars of carbon fiber running the length of the neck.
Carbon Fiber Neck Reinforcement
ReliefBack to Top
Relief refers to an ever-so-slight but deliberate forward bow in the neck. This can be desirable for reducing string buzz, especially for extremely low action setups, lower-than-concert pitch tuning(s), or in accommodating aggressive strummers. A builder may seek to mitigate against customer dissatisfaction by constructing a guitar that will handle any possible playing or setup scenario. Such an instrument will provide for adjustable relief by incorporating an adjustable truss rod (with or without carbon fiber stiffeners).
At this point, for many, the discussion is over; an adjustable truss rod is a required component in an acoustic guitar. However, a growing contingent of builders and players have become convinced that these steel machines (metal square tubes, T-bars, I-beams, U-channels, single-action truss rods / compression rods, or dual / double / twin-action truss rods) inside their instrument necks are 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 has a negative impact on the overall tone. The truss rod cannot simply be removed, as the need to counter the pull of the strings remains. What is a luthier to do?
If one believes neck relief to be essential for optimal playability, it can be readily introduced into the 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 ask 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.
- Q: In real world scenarios, how much relief is actually introduced by an adjustable truss rod?
A: 0.010" is considered to be low relief, while 0.025" is considered to be high relief.
- Q: What about flex (the expected forward bow of a flat neck when strings are applied)?
A: My tests have resulted in no flex with light gauge strings at standard tuning, and (surprisingly) negligible flex with heavy gauge strings at standard tuning (just enough to yield some relief).
- Q: Is a carbon fiber structural stiffener suited to the task of accommodating differing string tensions?
A: A qualified “Yes”, I believe so.
- Q: If I were to tune the guitar down two or more steps, wouldn't I need a little more clearance to avoid fret buzz?
A: Perhaps and, if switching between tunings is going to be a frequent event, an adjustable truss rod may be better suited.
Dragonplate D-Tube Neck BeamBack to Top
Along with my own subjective experience, I had obtained sufficient anecdotal evidence regarding neck stiffness and carbon fiber to purge a neck of a truss rod altogether. I selected the Dragonplate D-Tube Neck Beam, a $75 carbon fiber structural component specifically designed for the task. 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
Neck Construction and PreparationBack to Top
Neck design is of little consequence when considering the installation of a D-Tube. Whether a neck is laminated or solid wood, the headstock is one piece with the neck or attached via a scarf joint, the neck to body attachment is glued dovetail, bolt-on mortise-and-tenon, butt-joint loose tenon, or even an integrated Spanish heel; all are able to receive this truss rod replacement.
The laminated mahogany block I selected was sized to accommodate two (2) of these necks by reversing the template and flipping it vertically. The necks were separated at the bandsaw.
Neck Blank with Template
My roughed-out necks must to be trued up prior to cutting any grooves, so they go the the bench to be hand-planed. At this stage I am ever mindful of the 15° angle of my headstock as well as the point of intersection of the headstock at the fretboard; a single pass with a hand plane will alter the dimensions.
True up the Neck
Installing the D-TubeBack to Top
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 truss rod is in the number and size(s) of the groove(s). The D-Tube requires that I rout a single groove into the surface of the neck and epoxy in the Neck Beam, as opposed to routing three slots, one for the truss rod and the remaining two for carbon fiber stiffeners.
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.
Core Box Router Bit
A trip to the router table provides me with a couple of advantages over using my handheld router with, say, 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. If I would have to use a router this way (if I didn't have a router table), I would employ a jig to both cradle the neck and to guide the router along.
At the Router Table
It is necessary to rout a 3/4″ wide by 1/2″ deep groove / channel down the center of the face of the neck (the side to which the fingerboard will be attached). I choose to keep the D-Tube flush with the face of the neck blank, as opposed to recessing it. 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 opt to rout a through groove from the headstock to the heel block, as opposed to a stop dado, which I accomplish in three (3) ever-deepening passes over the router bit.
Routing the Through Groove
The groove is cut slightly deeper than the height of the D-Tube to accommodate a thin layer of adhesive. 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. Alternatively, I could have attempted to 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.
Groove / Channel Plug for 16″ D-Tube
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 fingerboard's extension. Note that I have run the carbon fiber up into the headstock, rather than terminate the groove short of the point where the fingerboard 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.
16″ D-Tube and 20″ D-Tube
The groove gets coated with a thin layer epoxy. The D-Tube is hollow but I have no need to plug the ends, as I do not flood the application with adhesive. I use the same 3M Scotch-Weld 2216 2-part epoxy that I use for all my significant carbon fiber-to-wood bondings. 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. Epoxy, unlike PVA or AR glue, does not benefit from high-pressure clamping; in fact, heavy clamping is advised against. The neck is left to dry overnight.
Note that a debate still exists regarding whether or not to add a filler strip of wood above the carbon fiber. The contingent adding the filler suggest that a (micro-thin) wood strip, applied over the carbon fiber and secured with epoxy, will somehow provide a superior glueing surface for the soundboard. Technically, the argument rests on the adhesion capacity of the glues and materials in use, in conjunction with the concerns over delamination. I have successfully bonded fingerboards to neck blanks containing carbon fiber stiffeners using protein-based glues (Fish and Hide Glue) for some time, now, with no adverse results. These fingerboards can be heated and removed. I have also used epoxy to attach fingerboards and, while fingerboard removal can be tougher, there is zero concern regarding obtaining a sufficient bond with the carbon fiber. Perhaps a greater concern when considering adding a filler strip is the fact that the groove cut for the D-Tube is already 1/2″ deep. Be aware that increasing the depth of that groove 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 offers both a smaller, straight version along with a tapered version that may be used if neck thickness becomes an issue).
D-Tube Glue Up
After releasing the clamps and lifting off the parchment paper I removed any accumulated, cured epoxy using a scraper. My go to scraper these days was designed by luthier Al Carruth and is available through StewMac. 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.
The section of D-Tube that extends out over the headstock gets sanded back nearly flush 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. (I could also saw the waste off and true it up with a sanding block - Tip: Wear a mask when machining carbon fiber!)
Sanding on a Marble Slab
I have found that, while carbon fiber machines nicely, it can dull my tools rather rapidly. Having an effective (read: fast, accurate) means of keeping my hand tools honed makes the process much more enjoyable.
The neck blank is surprisingly light and stiff at this point compared to necks containing a truss rod. But the question remains: What, if any, effect will the strings have on moving this neck away from dead flat? I have now replaced the truss rod (or combination truss rod / carbon fiber stiffeners) with the Dragonplate D-Tube Neck Beam.
Truss Rod Replacement
Cutting the Neck AngleBack to Top
To cut the desired angle into the heel of the neck, I use a bench-mounted router jig manufactured by Chris Klumper of luthiertool.com. A neck blank is clamped to a vertical plate, neck heel pointing up. The jig expects a perfectly centered truss rod slot to have been routed down the length of the neck. Two (2) locating pins fixed to the vertical plate accurately position the neck, side-to-side, and the routed slot (for the truss rod) allows the entire neck blank to be accurately positioned vertically. A dial indicator, juxtaposed to an adjustable horizontal plate that will support a hand held router, lets me calculate and dial in the precise angle needed to mate the neck heel to the body. It accomplishes this by letting me compensate for the (desired) offset between the plane of the fretboard and the height of the bridge. As I am constantly altering guitar designs, not having to stop to think in terms of angles is of tremendous benefit to me.
Neck Angle Jig
The D-Tube eliminates the truss rod slot which my neck angle jig relies on for positioning. In order to make use of my neck angle jig’s two (2) centering pins, I have three options:
- Precisely locate and rout two holes (short slots, to allow for some vertical positioning) into the carbon fiber D-Tube
- Rout an initial, narrow slot in the neck blank, as though I were going to install a dual-action truss rod, align the slot on the pins of the neck angle jig, cut the tenon, then rout the groove for the D-Tube, glue in the D-Tube, etc.
- Radically modify my neck angle jig to precision center a precision cut, perfectly symmetrical neck blank, into which the the carbon fiber D-Tube has been precisely centered.
I opted for the first approach, and installed the D-Tube into the neck blank, then precisely located and routed two short slots into the carbon fiber D-Tube for subsequent mounting of the neck blank onto my neck angle jig. This involved creating a router base having a centering capability. This base contains two pins that are spaced equidistant from the center. With the neck held securely in a vise, the proper bit mounted in a hand-held plunge router, the custom base centered on the router, and the router resting firmly on the fretboard plane of the neck with the centering pins straddling the edges, I manually rotate the router to engage the pins against the edges of the neck. This centers the router bit on the neck and allows me to plunge cut two short slots in the areas where the pins on my bench-mounted neck angle jig expect to find holes. It worked flawlessly.
ConclusionBack to Top
Once complete, 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. Even though the neck has been developed separately from the body, the overall feel of the instrument is more unified, closer to that of a guitar constructed having a Spanish heel.
My guitars are already light, as I incorporate carbon fiber in other components. The only metal on my instruments, apart from the strings, is found in the tuning machines. Responsiveness and sustain is much more noticeable than with the average acoustic guitar. Though I have not yet measured it, I believe the addition of the carbon fiber D-tube may have actually increased that sustain!
At the outset of this article, I asked two questions about truss rods that I believe I can now answer:
- Q: Is a truss rod really necessary?
A: Not for me. Not anymore!
- Q: What would happen if it were ...replaced with ...carbon fiber?
A: I can build a noticeably lighter guitar having increased sustain.
Thank you, Dragonplate!