In the neck of a guitar there is a metallic rod called the 'truss rod'. This allows the curvature of the guitar neck to be adjusted, which is necessary when changing the string gauge, or when drastic temperature and/or humidity changes occur (for example when moving the guitar from one country to another).
The truss rod in fact looks like this:
The tension in the strings on a guitar causes the neck to be pulled upwards. This tension obviously varies with string gauge hence why you need to adjust the truss rod when change the string gauge.
When tightened the truss rod itself becomes curved in the opposite direction from the neck (i.e. downwards as shown below). It is this fact that allows the truss rod to counteract the upwards pull caused by the strings.Types of Truss Rods
There are many types of truss rods, such as the common single truss rod, the non-adjustable truss rod, the dual action truss rod and the double truss rod.
The single truss rod is the truss rod shown in the picture at the top of the page and can be tightened to give the neck back bow (bend the neck downwards to counteract the upwards pull of the strings).
The non-adjustable truss rod is placed in some guitars (although not so common), and it is there just to provide reinforcement for the neck.
The dual action truss rod can be used to add back bow, but can also be used to increase the curvature of the neck upwards (upwards bow).
The double truss rod refers to having two truss rods in the neck. This is so that more force can be applied by the truss rods and is used when in guitars with higher string tension, like a 5 string bass.The Reason for Requiring a Truss Rod
A string on a guitar basically vibrates in an oval shape. This means that the middle of the string vibrates with greatest amplitude, which you can actually see if you sound an open string. For this reason there needs to be a thing called "relief", which is a bow half way up the neck. This bow prevents the strings from contacting the frets when playing (which causes 'buzzing'). Here are some over-exaggerated diagrams to clarify what I mean:
This is an example of where the truss rod is too loose. The relief in the neck is so great that the "action", which is the distance between the string and the fret, is huge. This will cause difficulty in pressing some of the frets and should be readjusted immediately (seek a specialist).
This is an example of where the truss rod is too tight (back-bow). The relief is non-existent. This means that either you get "buzzing" or you won't be able to sound an open string at all. Again seek a specialist.
This is the ideal. There is a small amount of bow in the centre of the neck which accommodates for the way the string vibrates.Checking Your Neck Relief
There is no such thing as an ideal value for neck relief. It will depend on the set-up of the rest of the guitar and your style of playing etc... The neck relief on a guitar can be roughly estimated by pressing down on the 1st fret, and the fret where the neck meets the body of the guitar.
For example on one of my guitars this would be as shown above by the arrow.
While you are holding these two frets you should check the gap between the string and the top of the 7th fret, where the bow should be greatest. This gives you a general guide to how much relief your guitar has. If this gap is no existent then you either have a completely straight neck or back bow in which case your truss rod should be readjusted.Adjustment of the Guitar Truss Rod
It's not really advisable to adjust the truss rod yourself unless you really know what you're doing - I would suggest you give it to a professional. However I'll complete this section in case you are curious about how the truss rod is adjusted. I take no responsibility in any damage caused by you adjusting the truss rod yourself.
The truss rod can usually be accessed from the headstock, and is normally covered by a plastic cover which first needs to be removed.
When adjusting the truss rod the strings should be loosened. Small adjustments should be made, the guitar tuned, and the relief checked. This is should be carried out until the relief is set to the desired amount.
The truss rod adjustment is made using a hexagonal key. Turning the key in a clockwise direction will tighten the truss rod, which will apply a downwards force on the neck. Turning the key in an anti-clockwise direction will loosen the truss rod, allowing the string tension to pull the neck upwards.
Labels: Guitar, Guitars, Rod, Setup, Truss