A Little Relief
Do we need an adjustable truss rod? Unlike wood, carbon fiber is not affected by temperature or humidity. Some guitar builders using carbon fiber proclaim that they don’t need to install a truss rod because necks made from carbon fiber are already stable and will not shift, warp or bow over time. However, while that may be true, these builders miss a crucial point: neck relief. The neck should not be perfectly straight.
Update (Feb 2, 2014): It’s my thinking now that neck relief is yet another myth. I now set up necks without any relief and it’s a charm to play! I guess it’s time for another post exposing yet another myth. See comments below. I agree with Dave: “There is absolutely no reason a neck should not be straight.”
Strings oscillate side to side and also up and down. This oscillation is most pronounced at the center (the 12th fret). A little bit of neck relief —a slight bow— is required to allow the strings to vibrate freely. You can check neck relief by placing a ruler across the frets or capo at the first fret while pressing down at the last fret. The clearance at the half-way point (typically the 8th fret) is the neck relief. There is no ideal value for neck relief. It will depend on a few factors, most importantly your playing style. However, for an electric guitar, 0.025 mm (0.001″) string relief is typically recommended. This value is sub-millimeter, so be sure to use a feeler gauge if you want to be precise.
So, do we need an adjustable truss rod? Yes. It is the only means to make the neck relief adjustable. Different string gauges will pull from 40 kg (88 lbs) to 50 kg (110 lbs) of force. The string’s tension naturally tends to bend the neck giving it a a slight concave curvature. The stiffness of the neck and the truss rod counteracts that force. An adjustable truss rod will allow us to control just the right amount of relief. Too much and the action will be too high making the guitar a pain to play. Too little and the strings will not have enough freedom to vibrate freely which will result in string buzz.
Fit and lite
Allow me to present the carbon-glass fiber truss rod. I’m quite pleased with the results. A truss rod of comparable strength made from steel is at least 3 times heavier. A stainless steel rod 6.3mm dia. 510mm in length weighs 127 grams whereas our hybrid carbon-glass fiber rod (the one I am using) 4.8mm by 10mm, 510mm in length is just 41 grams.
(Click to zoom)
The rod is made of 8 layers of carbon fiber (right now, I am using carbon fiber twill weave cloth but in the future I intend to use unidirectional carbon fiber cloth). 4 layers + 4 layers of carbon fiber sandwiches 6 layers of fiber glass weave for a total width of 4.8mm.
The head is machined aluminum alloy with a stainless steel allen adjustment screw. The adjustment screw has a finer thread than typical truss rods. This gives us finer control over adjustment of relief. Recall that we merely want 0.025mm of relief. The rigidity of the combined bamboo and carbon fiber neck is more than adequate to counteract the string’s pull. All we need is a little nudge.
The tail is anchored to the other end of the neck near the body using a short stainless steel rod anchor. To avoid galvanic corrosion —when one conductor (aluminum) corrodes when in electrical contact with a different type of conductor (carbon fiber), the whole thing is sealed with a few coats of clear polyurethane.
This is a single action truss rod. Modern guitars are equipped with double action truss rods. A double action truss rod has the ability to pull or push thereby compensating for back or forward bow. We don’t need that. Our hybrid composite neck is guaranteed not to back-bow.