What is a Truss Rod? And what does it do?

A Truss Rod is an internal component of the guitar. Totally invisible from the outside, it is a metal rod that runs through the middle of the guitar's neck. You may have owned a guitar for years, and never even known it was there.

So what does it do? Well, first it helps to have an understanding of the tensions exerted on your guitar by the strings fitted to it. An .042", (1.07mm) A string, creates 29.9Lbs (13.56 kg) of tension (this is a string for a steel string acoustic guitar. Electric guitar strings create less tension). Guitars are generally constructed from timber, a material with the ability to bend, and warp. The tension your guitar strings create are enough to bend the neck of your guitar, creating a bow in it. So your strings will be perfectly straight, but the neck under them won't be. There will be a much larger gap between strings and neck half way up the neck, where the gap will be largest.

The truss rod counteracts the tension created by the guitar's strings - the strings pull the neck forward, while the truss rod, when tightened, pulls the neck backwards. There are a few truss rod variants, however most are based on the design Tim McHugh, who in 1921 designed and patented the first truss rod while working at Gibson. This truss rod design is basically a metal rod, anchored at one end of the neck, and running through the centre of the neck - often in a slight curve. The non-anchored end will have an adjustment nut - tightening the nut will straighten the rod, thus pulling the neck closer to the strings. Loosening the nut allows the rod, and thus the neck, to bend more, and the strings will be further away from the neck.

The truss rod may require adjustment for a number of reasons. The most common, is that you have changed to a different string gauge. If you do this, the tension of the strings will have changed, and the amount of bow they cause in the neck will also change. Another reason is changing environmental conditions. Changing humidity has an effect on the timber your guitar is constructed from. Wood shrinks and expands under different conditions, and this can also affect neck bow.

How do you adjust it?

Before you jump in with a spanner and start tinkering, there are a few things you should be aware of, and careful about:
  • Although your truss rod is fairly rugged, if you abuse it, or try to force it, you run the risk of breaking it. This is a bad thing. Truss rod repair is not simple, or cheap. Remember, the thing's burried in the neck of your guitar.
  • A small adjustment can have a large impact. As little as 1/8th of a turn of the nut can make a significant difference
  • Only ever use a tool designed for adjusting your truss rod. Most truss rods are incompatiable with normal spanners and the like. You will probably need a specialist tool.
  • Truss rod adjustment is not used to correct the action on your guitar. You adjust the truss rod to get the relief in the neck correct - after it is correct, you can adjust the action normally.

Before you can properly adjust the truss rod, you need to know what degree of relief is already present in your guitar's neck. It is very difficult to simply look at the neck and judge this, so you use a simple techinque to find this out:

  • Before you start to do anything, tune your guitar properly. If it is out of tune, the string tension will also be out, throwing all adjustments out.
  • Working with your low E string, fret it at the 1st fret, and also at the 14th fret. If you own a capo, it's easier to use it to hold the string at the 1st fret, and use a finger at the 14th.
  • You are interested in the gap between the string, and the 6th fret. Rather than the string resting on the fret, you want a slight gap between them - only about 0.10mm, or the width of a business card.
  • If the gap is larger than this, you need to tighten the truss rod. If it is smaller - or doesn't exist at all - you need to loosen the truss rod.

You've probably realised that this means the neck is not perfectly straight. Most of the time, you want the neck to bend away from the strings slightly. This is mainly because when you strike a string, it vibrates in an elliptical fashion. If the neck was perfectly straight, the string would vibrate against the frets in the middle of the neck, and cause a buzzing sound. A small amount of relief in the neck can help prevent this. If you have a more aggressive playing method, you may need slightly greater relief, to compensate for the increased string movement.

Now that you've determined the current relief on your neck, you can actually make any adjustments necessary. The nut that you will be using to do this will be located in one of a couple of places - normally, either located under a small plate screwed onto the headstock, or if an acoustic, it may be accessed through the soundhole in the body of the guitar. To tighten, you turn the nut clockwise. To loosen, turn it anti-clockwise. No matter whether you need to tighten or loosen it, you should always begin by loosening the truss rod. You should also never need to apply great force when adjusting it. If it's proving difficult to turn the nut, never force it. It's better to take the guitar to a repairer, and have them look at it. This will work out to be far cheaper, and less hassle, than breaking the rod.

The final thing to note, is that adjusting the truss rod can be a slow process. It may take some time for the changes you have made to become fully evident, as the guitar settles into it's altered state. So be patient - if it doesn't appear as though anything's happening immediately, leave it overnight, and reassess it the next day.

So that's it. While it may seem complicated, it's really a very simple process, as long as you approach it carefully, and make adjustments slowly. Being able to do it yourself, will mean you can adjust your guitar so that it plays better, reduces fret buzz, and feels nicer to play. It'll also save you money, not having to get a guitar tech to do it for you.

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