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What's Wrong With a Truss Rod?
Nothing. It is a viable solution to a problem.
I have completely eliminated adjustable truss rods from both my 6-string and 12-string, steel string acoustic guitar builds. I use the DragonPlate® Carbon Fiber D-Tube Neck Beam™ ("D-Tube") from Allred and Associates, Inc., a carbon fiber structural component specifically designed for the purpose of replacing a truss rod altogether. I can attest that the D-Tube is up to the task. It has performed flawlessly for me.
A growing contingent of builders and players have become convinced that these steel fixtures (T-bars, square tubes, I-beams, U-channels, compression rods, single-action truss rods, dual-action truss rods, etc.) have no place inside their instrument necks and may be detrimental to both the playability and the sound of their guitars. They contend that the added weight does not make for an optimally balanced neck, and the (mostly metal) fixture may have a negative impact on the overall tone. YMMV.
A goal of acoustic guitar construction is to transmit as much string energy to the soundboard as possible, with the least amount of damping. To accommodate a truss rod, an entire section of 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 device.
The first patented truss rod was intended to counter the pull of the strings by closing the distance between the fretboard and the now elevated string path, elevated as a direct result of the inability to construct a wooden neck having sufficient stiffness to resist the increased tension of the steel strings, introduced in 1900.
In the application for that patent, filed in 1921, a benefit was identified, a newfound ability:
... to use wood which has not been heretofore considered satisfactory for the manufacture of necks, on account of its not having sufficient strength and rigidity, and further, great care in selecting stock is not necessary.
Wooden necks became slimmer and lighter, as the factor of intrinsic neck stiffness, once a hallmark of lutherie as evidenced by centuries of the intentional symbiosis between wooden necks and catgut strings, was summarily nullified and replaced with the newfound mechanical ability to force a neck, any neck, into shape with the twist of a socket wrench or hex key.
In other words, who cares about neck stiffness, anymore! I can now rely entirely on my truss rod.
Significant strides have been made since that point in guitar design history when the adjustable truss was introduced and the requirement to build sufficiently strong and rigid necks was summarily eliminated. Builders at that time didn't share in the wealth of knowledge we now possess regarding the design and construction of the guitar. Had they had access to the technology of carbon fiber, I think the truss rod story might have turned out differently.
Regardless, a truss rod is not necessary in my guitars.
How do You Adjust for Relief?
I don't.
Utilizing the D-Tube to increase the neck stiffness by several orders of magnitude, and building once again toward a symbiotic relationship between the stiffened neck and the strings under tension (just like luthiers used to, back in the days before steel strings were introduced), my guitar necks now express the slightest amount of forward bow under string tension, just the right amount of relief. My necks are not affected by environmental conditions, not anymore. They stay where they were installed!
The D-Tube necks I have built over the last 10+ years are installed dead flat. A set of light gauge strings will flex the necks on my guitars just enough to measure from 0.08mm to 0.13mm (0.003 in to 0.005 in) of relief while under string tension, depending on how stiff my wooden necks are to begin with. To get closer to 0.20mm (0.008 in) of relief, I need to either sand a profile into the fretboard, or use medium gauge strings.
None of my D-Tube necks have deformed. When the strings are loosened on any of my guitars built with D-Tube necks, the necks return to dead flat, the condition in which they were first constructed. And it's not like they have to travel very far to get there.
If greater relief is requested and demonstrated to actually be necessary on the part of the player (as it was in only one of my guitars built during that period), it is easily formed into surface of the fretboard prior to adding the fretwire.
I have found the D-Tube to be very well-engineered for my purposes. I am able to consistently construct a lighter, stiffer, and more resonant neck for my acoustic guitars.
The D-Tube
For the acoustic guitar, the 3/4" wide D-Tube is available in both a straight or tapered profile. The straight profile is available in two lengths, 16" and 20", and measures 1/2" deep throughout its length. The tapered profile is 20" long.
TIP : I prefer the 3/4" wide straight profile for my guitar necks. I rely on the 16" D-Tube for all tenon-based (dovetail or bolt-on) detachable necks. I use the 20" D-Tube for elevated necks and for Spanish Heel-style construction (non-detachable necks).
The 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!
I position the D-Tube such that it extends fully onto the headstock, significantly increasing the strength of the weakest section of a guitar neck.
Neck Construction and Preparation
The D-Tube compliments all varieties of neck construction:
Solid wood or laminated
One-piece neck or scarf joint
Dovetail, bolt-on, or butt joint with loose tenon
Elevated fretboard
Integrated Spanish heel
I am fairly certain (though I have not tested every possible configuration) that, just like the admission in the 1921 truss rod patent states, the D-Tube would perform admirably on any neck, regardless of the quality of the neck material. However, I prefer beginning with as stiff a wooden neck as possible, such as may be achieved by laminating quarter sawn material into a multi-piece neck.
I primarily rely on Honduran Mahogany or Spanish Cedar for the bulk of the neck, with lamination stripes comprised of most anything else, including Walnut, Maple, Rosewood, Koa, etc. Even stained poplar has made an appearance, or two.
Installing the D-Tube
The D-Tube is designed to be the sole resident in your guitar neck. If you have been supplementing truss rods with twin, parallel carbon fiber stiffeners, you will be pleased to learn that the D-Tube replaces all three items.
To install a D-Tube, instead of routing or sawing three slots (one for the truss rod and the remaining two for carbon fiber stiffeners), one simply routs a single, half-round groove (NOT a square-bottom saw cut) down the center of the surface of the neck. The D-Tube is glued into place and the neck is ready for the fretboard.
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, so I selected a 3/4″ 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 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.
D-Tube at the Router Table
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.
D-Tube Groove
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.
D-Tube Groove Plug
On the opposite end of the neck I choose to 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 - especially critical for one-piece necks. Having witnessed a few cracked headstocks from other builders, I believe this crucial juncture of the neck benefits from the extra attention.
D-Tube Length Options
D-Tube with Scarf Joint
The D-Tube MUST be glued to the groove in the neck. I am increasing the stiffness of the neck. I am not adding an adjustable countermeasure to a pliable neck.
At the recommendation of the manufacturer, 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. Both of these epoxies, the Scotch-Weld and the Superbond, are a paste format, versus a runny liquid format, which I prefer to work with.
You can use your adhesive of choice. Just make sure it will bond to both the carbon fiber and the wood. Hide glue will work, though you do need to clamp the parts tightly.
NOTE : Hide glue may be as well-regarded for what it lacks as for what it yields. It provides a non-creeping bond. It lacks damping, which makes for excellent energy transference. In contrast, some epoxies add a rubber (-izing) compound to increase their peel strength. This is a damping factor. A teeny, tiny factor, but it is a factor.
I believe that damping works to my advantage when installing a D-Tube. Why? It is quite possible to add enough carbon fiber to an acoustic guitar, in just the right places, where the guitar takes on a subtle "plastic-ey" sound. It is just as possible to add the right amount of carbon fiber to an acoustic guitar, in just the right places, where the guitar does NOT take on that "plastic-ey" sound, but benefits from the added strength and decreased weight.
Think: sibilance . If I am approaching "too brittle" of a sound, the last thing I need in a new component is "less damping". I want a "de-esser", a slight damping quality, to avoid that vitreous, glass-like, brightness. On the flip side, when I am building an instrument that would benefit from that extra nudge of brightness, I want to avoid any and all unnecessary damping.
I believe that adhesives possess a sonic quality. Hide glue is very vitreous, very glass-like. The same may be said for the SuperBond epoxy, though perhaps to a bit lesser degree. The Scotch-Weld is much less "brittle" sounding than either of the aforementioned products. That is what I am using, here.
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. I coat the groove with a thin layer of epoxy.
To simplify cleanup, I suggest applying tape (painter's, low-tack, etc) to the surface of the neck blank prior to adding the epoxy and gluing in the D-Tube. Two narrow strips, one down each side of the groove, or a single, wide strip whose center is removed with a razor knife will mitigate the need to aggressively remove any squeeze-out or hardened epoxy. Simply remove the tape once the epoxy has cured and be left with a clean surface.
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. For any who may be unfamiliar, unlike PolyVinyl Acetate (PVA) or Aliphatic Resin (AR) glues, epoxy does not benefit from high-pressure clamping; in fact, heavy clamping is advised against.
The neck is left overnight, giving the epoxy time to fully cure.
D-Tube Glue-up
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 either need to try to make the surface flush again without altering your neck geometry and without weakening the D-Tube, or rout a shallow recess into the back of your fretboard.
SUGGESTION : try really hard NOT to do this on the next neck!
Cut-off wheels, such as the type used with rotary tools, are effective for cutting carbon fiber. Using a handheld rotary tool (Dremel), I am able to quickly remove most of the section of D-Tube that extends out over the headstock.
EDIT : I have since switched switched to using diamond impregnated jig-saw blades for the task.
Remaining material is sanded back nearly flush with the headstock (carbon fiber sands very readily). A disc (or belt) sander works well for this step. 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 I 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 Carbon Fiber D-Tube Neck Beam.
D-Tube Truss Rod Replacement
The fretboard may be attached using your choice of adhesive, including Fish or Hide Glue, Titebond, or even epoxy. I have yet to experience adverse results with any of the above choices. I have had success using a very thin application of epoxy to attach fretboards and, while fretboard removal (should it ever be necessary) may be a bit more difficult (when compared with Hide Glue, for example), there is absolutely no concern regarding obtaining a sufficient bond with the carbon fiber.
Does it Work?
I measured 15 necks fitted with a D-Tube installed dead flat, all being 14-fret, 25.5" scale. I maintained a dead flat fretboard in the majority of those guitars (no relief was sanded into the board). None of the 12-string guitars having light-gauge strings exceeded 0.15mm (0.006 in) of natural forward bow under string tension. To get 0.2mm (0.008 in) of "relief", I needed to either sand a profile into the fretboard, or use medium gauge strings. 6-string guitars with light gauge strings ranged from 0.08mm to 0.13mm (0.0043 in to 0.005 in). I attribute the variations to the inherent stiffness of the wood, seeing as how my laminated necks stay a bit flatter.UPDATE : After more than 10 years there has been ZERO deflection in the plane of the neck built for this article.
A qualified “Yes”, depending on your goals. I have consistent results with 6-string guitars using medium-gauge strings and below, and 12-string guitars using light-gauge strings. I have not had long-term experience with heavy gauge strings, as I don't use them on my guitars.
Yes, it is! Using the D-Tube, I am convinced I can build a noticeably lighter, stiffer, more unified guitar neck which contributes to increased sustain and improved overall tone of my guitar.
Summary
A D-Tube 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.
IMPORTANT : I have not conducted sufficient scientific tests to unequivocally prove that the D-Tube increases sustain and/or improves the overall tone of my guitars; HOWEVER, I cannot otherwise account for the perceptible increase in sustain and improvement in the overall tone of my guitars - guitars in which the only build factor that was altered was the replacement of the truss rod with a D-Tube! I believe I am building a more resonant neck using the carbon fiber D-Tube.
Do you tap your soundboards? I also tap my D-Tube necks. They sound pretty good.
The D-Tube plays an intrinsic and crucial role in my construction of durable, lightweight, responsive guitars having rich and vibrant tone.
Thank you, DragonPlate!
