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A R T I C L E S
Truss Rods and Neck Relief
by Bruce Morrison
As a string vibrates, the middle of its length is displaced further than its ends are. The greater a strings' diameter, the lower the frequency it will produce, and the further its middle is displaced as it vibrates. Therefore, the greater a string's diameter, the higher it must be set above the fretboard to prevent buzzing. Although a string is set at the correct distance from the fretboard, some additional clearance is necessary or it will buzz. That additional clearance is called neck relief. Too much neck relief makes a guitar's action uncomfortable, while too little allows the strings to buzz.
Depending on their brand and gauge, a set of strings exerts between 120 and 190 lbs. of tension on a guitar. That tension bows the neck producing too much relief. The purpose of a truss rod is to counteract string tension just enough to provide the correct amount of neck relief.
To check the neck relief, tune the guitar to pitch and hold it in playing position. Place a capo at the 1st fret then fret the 6th string at the 15th fret. Ideally, there should be .005" to .007" clearance between the bottom of the 6th string and the top of the 7th fret, measured with a feeler gauge. Repeat this on the 1st string. You should have the same clearance. The truss rod needs adjustment if there is less than .005" or more than .009" clearance.
Generally this adjustment is required twice a year. The first time is just after entering the heating season; the second is just after leaving it. In some years the humidity of July and August will make an additional adjustment necessary. Should you change to a different gauge or brand of string, it's likely that the truss rod will need readjustment.
If the instrument doesn't have a truss rod, a certain amount of neck relief has to be built in during construction. That relief will vary with the seasonal changes and with a change of string tension due to changing brands or gauges of string. If the relief gets too far from specification, the procedure used to correct it is called a "plane and re-fret". That means that the frets are removed, then the correct amount of relief is planed into the fretboard before the guitar is re-fretted.
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