Steel string guitars require the addition of a truss rod to be installed in the neck of the guitar to counter the additional tension steel strings place on the guitar. Classical guitars, (in most cases) don’t require the addition of a truss rod because the tension placed on the guitar from nylon strings is minimal in comparison.
There are a number of subtle differences between how steel string acoustics and classical guitars are designed and constructed. This is largely due to the strings. Both nylon and steel strings place different amounts of tension on the guitar, especially the neck and headstock. This additional tension must be accounted for or the structural integrity of the guitar will become compromised leading to problems with fret buzz, intonation and playability rendering the guitar more or less useless.
Of course, there are always exceptions, and these are becoming more common as we will cover shortly.
What is a truss rod?
In construction, trusses are known as supporting structures, typically used in roofing and bridge building. Much like trusses, truss rods are adjustable steel rods used to support and stabilise the neck when under tension.
They are installed in a channel beneath the fretboard with one end fixed to the guitar and the other end featuring an adjustable nut.
To make an adjustment, an allen wrench (also known as a hex key or allen key) is used. You can typically find the end of the rod beneath the fretboard where the neck joins the body of the guitar, at the external edge of the sound hole.
On electric guitars, the adjustable end is on the headstock and requires the truss rod cover to be removed to gain access.
Turning the hex key clockwise shortens the truss rod which in turn straightens the neck, pulling the strings closer to the fretboard and increasing the tension on the neck of the guitar.
Turning the hex key counterclockwise lengthens the truss rod, which reduces the tension on the neck allowing the strings to bow the neck more inward.
In almost all cases the goal is not to completely straighten the neck but instead to allow a small amount of relief. Ideally somewhere between .003″ and .008″. Allowing the neck to bow slightly inward (relief) reduces the likelihood of fret buzz.
Two way truss rods
Two way or double action truss rod systems are a more recent advancement on the single style model. Two way systems are fixed to each other instead of the neck itself and as a result tightening and adjustment can be made both inward and outward.
Some classical guitars do have truss rods
As mentioned there are exceptions and a number of modern classical guitars do come with truss rods, allowing the neck to be adjusted accordingly. My Córdoba C5 for instance does, whereas my Yamaha doesn’t.
Alternatively, the luthier may also insert a section of graphite beneath the fretboard or the neck itself will feature an insert of hardwood on the back of the neck. While not adjustable, both measures can help improve the stability of a guitar’s neck.
To explain why some utilise truss rods and some do not, it helps to understand a little about the differences in tension between steel and nylon strings.
The first three, or highest 3 strings on a nylon guitar (E,B and G) are single strands of nylon. The top three strings (D, A and E) consist of copper coated strands of wrapped nylon.
Due to the lower tension of nylon strings compared to steel, the sound waves produced have a higher vibrational amplitude. For those unfamiliar with the term, this simply means the range of vibrational movement outward from the centre (the point of equilibrium) at the peak of vibration.
Because of this, classical guitars mostly have wider necks than steel strings to prevent the strings clashing with each other. The wider the neck the more stable the neck is under tension. However, many guitarists believe the wider string spacing results in a guitar more difficult to play compared to a steel string.
High tension nylon strings
In recent times high tension nylon strings have become increasingly popular and some classical guitar manufacturers are beginning to produce hybrid classical models along with standard classical guitars. These can be built with a narrower neck than standard classical guitars, due to the increased tension resulting in a lower vibrational amplitude of the strings, and for many this means an easier guitar to play.
However, because of the additional tension placed on the neck, an adjustment is sometimes required to maintain stability.
If comparing low to high frequency strings. The additional tension applied on the neck is somewhere between 10% and 12%, or the equivalent increase in tension if you were to tune the guitar a semitone higher.
While this mostly sounds like a positive development. It depends which side of the fence you are sitting on as there is a tradeoff in regard to the additional weight (classical guitars are typically very light) which is also believed to slightly reduce the responsiveness of the guitar.
Humidity and the impact on Guitar Necks
Another reason you might see a truss rod installed on a classical guitar is to address concerns over humidity. Wood tends to react to seasonal changes, especially if the fluctuations are on the extreme side. Additionally the guitar may be shipped from a completely different environment or you may travel with your guitar. In any of these cases a truss rod can help maintain the stability of the guitar neck.
Some guitarists actually make seasonal adjustments to counter the effects of humidity.
Another reason truss rods are being included more often may be due to the increasing number of classical guitars produced with cutaways. A cutaway tends to reduce the stability of the guitar neck in comparison to a full body guitar.
Can a truss rod be installed on a classical guitar that doesn’t have one?
Yes, classical guitars suffer from warped or bowed necks often, especially less expensive models. A skilled luthier will be able to install a truss rod to allow the neck to be adjusted, saving the guitar from becoming unplayable. The process involves removing the fretboard and carving a channel that the rod can then be installed into.
Classical guitars that do come with truss rods
In my opinion, while there may be an increase in the weight of a classical guitar, the differences are only slight and most of the objections to installing a truss rod are largely based on tradition which serves very little practical purpose.
As a result I believe more classical guitars will include thems over time, which I see as a positive development. I’ve listed some popular manufacturers below who already do so.
All Córdoba guitars feature a double action truss rod. As already mentioned, I own a Cordoba C5 and have found the ability to adjust the neck relief useful especially as the guitar required some slight adjustments on the first day I brought the guitar home.
Much like Cordoba, La Patrie guitars also feature a double action truss rod.
The Takamine TC132SC features a dual action truss rod.
While the majority of Yamaha classical guitars don’t, the Yamaha CGX-171-CC features a truss rod.
As discussed, while classical guitars do not often feature truss rods due to a lack of tension on the guitar itself due to the nylon strings, things are changing and it is becoming more common.
Modern manufacturers such as Cordoba build all their modern classical guitars this way and this is only set to continue as the demand for neck stability regardless of string tension or environmental changes increases