Different kinds of necks and
truss-rods in electric and acoustic guitars.
In this article you will find some historical notes and a lot of information about your guitar neck, its relationship with the strings and the truss-rod function:
Why are guitar necks equipped
with a truss-rod?
As we know, the necks of electric guitars, and today, sometimes, even nylon string classical guitars are fitted with a rod inside the neck. Adjusting this truss-rod allows us to fix a neck tendency to assume an excessively curved pattern, concave with respect to the straight line of strings between the nut and the bridge, distancing from or approaching to them in the central portion.
These variations can be due to changes in climatic conditions of the room the guitar is in. Dry cold or excessive humidity do not favour neck stability. For this reason it would be advisable not to leave the guitar near a source of heat or steam, nor hanging on a room wall that is in contact with the outside.
This can seem to be an excessive care, but you have to think that our guitar neck is not in a rest position: it is always working. A common light set of strings for classic guitar exerts a tension on the instrument of about 6-7 kg per string, increasing from 6th to 1st string. In a steel string .010-.046 set (light) for electric guitar, tensions vary from about 6 to over 10 kg per string. A set for acoustic folk guitar .013-.056 (medium) can exceed 80 kg of total tension!
Applied pull varies according to string gauge, material and mass, as well as instrument scale and tuning. The very same plain string has two different draws if used, by example, as a G or a B. The same string put on a 24” ¾ scale guitar has a lower draw compared to a 25” ½ scale one.
Different kinds of neck
It is therefore clear that the neck of our in-tune guitar is a potentially stressed part right from the start. Fortunately Mother Nature provided us wood, a strong, durable and elastic material if it is a high grade choice, well seasoned and treated. Theoretically, a neck made of suitably lengthwise arranged laminated wood should be steadier than a single piece neck. In the same way, a maple neck with a glued on maple fretboard should be steadier than a real maple-neck, entirely obtained from a single maple board. Actually this is not always true; there is a remarkable variability due to species, quality, natural or artificial ageing and manufacture.
Artificial ageing in industrial ovens once, and high temperature roasting method nowadays has enabled residual humidity removal from wood, improving neck stability without having to wait years of natural seasoning. (For instance, renowned luthier James D'Aquisto used to work with 9 to 12 year seasoned wood).
Over the years several builders have also been trying to offer rock-solid necks made of synthetic materials, and some of them are very interesting.
In the beginning, guitar necks had not an adjustable truss-rod, because strings were made of gut and silk, and there was little draw on guitars (nylon arrived in 1948). In the early '900s steel strings made up and things changed, though not that much after all. Consider that Martin Guitars did not care about this matter for its acoustic guitars – long since equipped with solid X-bracing tops – until the '20s, when a simple ebony bar was put inside their necks.
Around the mid '60s a solid steel T-bar and later a square-bar were adopted. They were not adjustable, so the builder was certain of its neck quality! In 1985 Martin finally gave in to the novelty represented by the adjustable truss-rod Gibson patented and permanently introduced in 1923!
(Martin trusses till 1985)
Different kinds of truss-rod
The simplest and most popular adjustable truss-rod is the single action one, that fixes neck forward bending (concavity). A metal bar is put inside a curved channel cut along the neck. The bar can be adjusted tightening a bolt at one end of it. Bar resulting movement counteracts string tension if the neck has been stressed until it bows and becomes concave.
Another kind of adjustable truss-rod is the double action or two way one. It consists of two bars inserted in a straight channel and allows to fix an excessive concavity as well as an eventual convexity (backwards bow). This latter situation is a difficult one to solve and somewhat unnatural for a good neck, unless it has faced a particular stress. The double action truss-rod is less appreciated and popular than the single action because many musicians complain a bad influence on sound due to its major metal mass, though there are some single rod models such as the Fender Bi-Flex.
Other solutions seen over the decades are the double single-action truss-rods by Rickenbacker and Guild (12 string), designed for extra strenght and separate adjustment of wound string and plain string sides of the neck.
Adjusting points of a truss-rod
The access to a truss-rod adjusting point can be at the neck basis (by example, in vintage Fenders; in this case the neck must be removed to show the access) or on the headstock, behind the nut (Stratocaster® replicas, Les Paul® inspired guitars, and so on). Sometimes alternative solutions were offered, such as at the neck heel low side (Yamaha RGX TT) or under peculiar shape headstocks (Parker Fly).
Generally the adjusting point is the rod threaded end with a Phillips, hex, barrel or Allen head. In any case, having the right wrench is always advisable, whether it has a metric (European and Asian productions) or Anglo-Saxon pitch (USA productions), in order to avoid damages to this essential part of our guitar.
Well, we know enough now to make our first neck adjustment. A new article will deal with the evaluation of string-fret gap, recommended specs and truss-rod adjustment!