A truss rod is a steel rod or set of rods that run down the center of the guitar neck. Its purpose is to counteract the pull of the strings against the neck and to allow adjustment of relief or bow in the neck.
Imagine the neck like a bow. The more the strings are tightened, the more the neck will bow. The rod prevents this by applying a counter force within the neck.
Every guitar is different but typically your truss rod may need adjustment when changing tuning, changing string gauge, or when your guitar has been subject to environmental/seasonal/humidity changes. For example, from winter to spring, or summer to fall, you may see a change in your guitar. Any time your guitar's playability changes after it has been setup (for a specific tuning and string gauge) it is due to the neck needing a re-adjustment.
This can change a buzzing neck to near perfect again, or from that uncomfortably high string action back to how it was when it first left the shop. Most players agree, a near straight neck is the most comfortable to play on but this is subjective to personal preference.
WHY SET THE RELIEF?
When a string is plucked, strummed, or picked, it vibrates in an elliptical pattern. This is why we need a slight bow (relief) in the neck to match the natural motion of the string (mainly in the lower register of the fretboard). Otherwise, the strings smack against the frets, causing buzz, poor tone and poor volume.
If you play hard on the strings, you will exaggerate the movement of the string and depending on your preference, may need more bow in the neck, and/or a higher action.
Players with a lighter touch will often have straighter necks with lower action. This is why electric guitars may have less relief than acoustics (although not as a rule). If you play bass, you will want more relief than on a guitar as the strings are that much bigger and vibrate in relativity to their size.
So let’s get started!
WHAT YOU'LL NEED
There are a few golden rules when adjusting your truss rod. First, only use the rod to keep your neck as straight as it needs to be…do not use it to adjust your action! Second, only use the proper tools. If you do not know what the proper tool is, check with the manufacturer. Truss rod nuts are easy to strip, and once they're stripped, they are expensive to fix! Finally, do not force anything; an eighth of a turn can make a drastic change. Remember, if the truss rod feels tight or doesn’t seem to be doing anything, take it to a professional.
Things should look as pictured below. If you have really poor fret quality, then this method may not work for you.
HOW MUCH TO ADJUST
The bigger the gap, the more relief is in the neck. Normally, we want just a small amount of relief in the neck for optimal playing, about the same as the thickness of a business card or less.
If there is a gap, tighten the truss rod clockwise until you have something closer to what we’re after. Slide a business card in between the top of fret and bottom of the string (or ruler). Make sure you are placing the card parallel with the string or ruler (Fig. E). If there is no gap (Fig. F), you may be either dead straight or back bowed, so you will need to loosen the truss rod counter-clockwise to get the results we are after (Fig. G). Hint: If the strings are buzzing in the first few frets and open strings, it is a clear indication of a back bow.
Remember, do not force anything to get the results you are after. If it is hard to adjust or spins freely you may need professional assistance.
For the technically minded, you can start by setting your relief to around .008”-.011” for most electrics, .010”-.012” for acoustics, and .015”-.020” for basses (this is where an automotive feeler gauge comes in handy). This tolerance is measured as described above.
Since many business cards can be .015”- .020” thick, you’ll want to consider giving the truss rod another ¼ turn one way or the other to dial in minimal relief.
Hint: Some necks may take some time to settle after adjustment. Specifically, maple necks can be stiff and may not settle immediately after making an adjustment. It is often best to check periodically for any change in relief and re-adjust as necessary, using the method above.
The final step in setting your relief is to test it out! If it plays better, you’re done! But take note, straightening the neck may actually make your guitar play worse! Consequently, when the truss rod is tightened, the height of the strings are lowered, which can create string buzz if your guitar isn't setup properly. Likewise, it can make a poor fret condition more noticeable. So, if your guitar plays worse after adjusting the neck, it is either too straight for your playing style, or it is a sign that more work is needed. Thankfully, you can easily reverse the changes you’ve just made. Good luck and stay tuned in for more setup articles!