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.”
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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)
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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.
Further Reading
I know this is old, but relief is BS. There is absolutely no reason a neck should not be straight. The action across the whole neck should be linear to assure precise intonation. Relief essentially shortens the neck slightly and non linearly. Action should only be adjusted by bridge height or neck pitch ,a linear adjustment, which will maintain intonation
Dave, how the times have changed. I actually agree 100%!!! I learned about this sometime after this post was published, while doing more research. Now I setup necks without any relief and it’s a charm to play! I guess it’s time for another post exposing yet another myth.
I beg to differ about relief, it is necessary depending on the circumstances. But this doesn’t need to be a matter of opinion. The way to know if relief is necessary is the same way to know how to set relief, which is based on the symptoms. If the tendency to buzz is different depending on which frets are being played, then the relief is incorrect. The symptom of too little relief is more buzzing when playing down near the nut than up the neck, the symptom of too much relief is the opposite. Of course, you may need to adjust the action to start to get a little buzz to check it out.
Now, having said all this, it may be that with the shorter neck of a guitar compared to bass guitar, and if your playing style is a lighter touch, that the proper neck relief is so small as to be immeasurable, leading to your observation. I’ve done modeling of this, and this combined with my personal experience has led to the conclusion that necessary relief corresponds to playing style. If you play harder you’ll need more relief. It is absolutely true that on my basses and with my playing style, that if I don’t dial in some relief, I have to set the action unnecessarily high to avoid buzzing on frets near the nut. I dial in some relief, and I get uniform response up the neck.
I prefer science to religion in these matters.
Agreed. It’s 2023, and I’ve arrived at the same conclusion as you. Basically, it depends.