Truss rod

A truss rod is a guitar part used to stabilize and adjust the lengthwise forward curvature (also called relief), of the neck. Usually it is a steel rod that runs inside the neck and has a bolt that can be used to adjust its tension. The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921^[1], although the idea of "truss rod" can be encountered in patents as early as 1908^[2].

Application

Cross-section of an electric guitar neck showing the position of the truss rod

When the truss rod is loosened (i.e. moved towards the guitar's body), it allows the neck to bend slightly in response to the tension of the strings. Similarly, when tightened (i.e. moved towards the guitar's headstock) the truss rod straightens the neck by resisting the tension of the strings.

It is desirable for a guitar neck to have a slight relief in order that reasonably low action is achieved in the high fretboard positions, while at the same time, the strings ring clearly in the low positions. Improved action in the high fret positions also allows for more accurate intonation, to be achieved with less compensation at the bridge.

Relief achieved through the truss rod combines with the height of the bridge to affect the playability of the instrument. The two should be adjusted in tandem. Too little relief contributes to a neck that feels stiff and lifeless, while too much feels floppy, slow and imprecise.

Truss rods are required for instruments with steel (high tension) strings. Without a truss rod, the guitar's wooden neck would gradually warp (i.e. bend) beyond repair due to applied high tension. Such devices are not normally needed on instruments with lower tension strings, such as the classical guitar which uses nylon (previously catgut) strings.

Truss rods also allow the instrument neck to be made from less rigid materials, such as cheaper grade of wood, or man-made composites, which without the truss rod would not be able to properly handle the string tension. The neck can also be made thinner, which improves playability. In fact, the possibility of selecting cheaper materials is specifically touted in the 1923 patent as an advantage of the truss rod. Prior to the introduction of truss rods, the neck would have been made of a very rigid wood, and relief was achieved by planing the fingerboard: more expensive material, and more demanding construction technique.

Construction and action

Truss rods are frequently made out of steel, though graphite and other materials are sometimes used.

The truss rod can be adjusted to compensate for expansion or contraction in the neck wood due to changes in humidity or temperature, or to compensate for changes in the tension of the strings (the thicker the guitar string, the higher its tension when tuned to correct pitch).

Usually, the truss rod of a brand-new instrument is adjusted by the manufacturer before sale. Adjusting the truss rod is not recommended for a novice, as guitar necks can be easily damaged beyond repair in the process. Luthiers or more experienced players can adjust it when necessary (e.g. if strings of a very different tension are to be used). Turning the truss rod clockwise will tighten it, counteracting the tension of the strings and straightening the neck or creating a backward bow. Turning the truss rod counter-clockwise will loosen it, allowing string tension to act on the neck and creating a forward bow (higher string action).

Some guitars (notably Rickenbackers) come with dual truss rods that are more stable and not affected by seasonal climate changes, they are also more difficult to adjust and should be serviced by a professional. Though uncommon there are also left-handed truss rods that need to be rotated in the opposite direction of a normal truss rod.

Location and adjustment

Truss rod adjustment bolt visible from the side of headstock

The truss rod tension is usually controlled using an adjustment bolt (a hex nut or allen-key). Depending on the model of guitar, this bolt can be located:

• On older Fender-style electric guitars with bolt-on necks (and vintage re-issues) — on the heel of the neck. Adjustment of such truss rods can be done by a Phillips screwdriver and requires prior removal of the guitar's pickguard or neck.

• On newer Fender-style electric guitars — behind the nut, uncovered and can usually be adjusted by a 1/8" (3 mm) Allen wrench.

• On set-neck electrics — under a cover-plate behind the nut. Gibson & Epiphone guitars have their truss rod bolt covered with a signature bell-shaped plate. Most Gibson electrics have a 5/16” (8 mm) or a 1/4" (6 mm) hex adjustable truss rod nut that can be adjusted with a hex box spanner wrench.

• On acoustic guitars — inside the guitar body, accessible through the sound hole, or on the headstock. Martins use a 3/16" (5 mm) Allen wrench and Gibson uses the same as for the Gibson electrics above.

• Counterclockwise adjustment will decrease the truss rod tension (correct an underbow) and clockwise adjustment will increase the truss rod tension (correct an overbow).

Installing a truss rod in a newly constructed guitar requires woodworking capabilities. Special tools are required including a router with a variety of bits and ability to work with metals. Completed truss rods can be purchased through suppliers or manufactured according to specifications given in literature. Be sure of your measurements when constructing a guitar as they all play into the guitars full scale and playability. Consult a professional luthier or literature before constructing your own guitar.

Dual-action truss rod

A dual action truss rod can be adjusted for both upbow and backbow. The original design truss rod does not compensate for a lack of relief or backbow because in most cases guitars do not suffer from a lack of relief. It is important to note that dual action truss rods adjust the opposite way to a standard truss rod to compensate for too much neck relief.

See also

• List of guitar-related topics

References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (April 2009)

1. ^ US1,446,758 (PDF version) (1923-02-27) Thaddeus McHugh, Neck for musical instruments.  — first patent known on truss rod. Note that the diagram in McHugh's patent document shows an incorrect design for the truss rod channel. The channel is shallower in the middle, causing the truss rod to be installed with a backbow. Tightening the rod will increase neck relief, acting in concert with string tension, rather than reduce relief in opposition to string tension. This looks like a deliberate ploy to secure a patent without fully disclosing the invention, in hopes of foiling naive imitators who try to follow the patent without consultation and licensing.
2. ^ US964,660 (PDF version) (1910-07-19) George D. Laurian, Stringed musical instrument.  — a patent on instrument that is "designed like harp guitar", but it mentions "truss rod" concept "to prevent a head springing under the strain of strings".

Links

• Bass truss rod adjustment