Thursday, September 21, 2006

Truss rods demystified

When I used to repair guitars for a living, I noticed that soooooo many people feared the mysterious device known as the truss rod. "Don't turn it more than a quarter turn per day, or you'll break the neck!". I heard variations of that, many, many times. That's definitely not the case. There's alot of mojo that goes into a guitar neck in general, as far as alot of guitar players (and guitar salesmen) are concerned. This probably comes from the fact that, even in high production environments, there's alot of hand work involved in the neck-making process, which leads to each neck having it's own personality. The truth is though, there's no secret juju we luthiers use when making necks. We're using techniques you'd use when making a nice piece of furniture. Of course, we need to make sure that the nut and all the frets are in exactly the right place and on a level plane, we need to make sure a hand can traverse it comfortably, and we need to make sure it can stay straight when string tension is added. That's it, really. That last part is where the truss rod comes in.

An adjustable truss rod is found on just about every electric guitar out there, and nowadays, on most acoustic guitars. This is simply one or two rods with a nut at one end, which counteracts the tension of the strings when tightened. Unless the neck is extremely poorly made, turning the truss rod nut isn't going to break or damage the neck. You'll need to adjust the rod every time you change to a different string gauge. This is because a thicker string requires more tension than a thinner one, at the same tuning. That tension bends the neck upward causing the strings to rise away from the neck - what we call high "action". Turning the truss rod nut clockwise counters that added string tension, thereby straightening the neck, and lowering the action. Of course, there are many other factors that can lead to high action (or the need for high action), but that's beyond our scope for today's post.

To check how straight your neck is, start by holding it in playing position. As a side note, any adjustments done to a neck, whether they are as simple as a truss rod adjustment, or as involved as a fret level & dress, are best done in the playing position. I actually use a special rotating table and jig to put my guitars in the playing position for fretwork. It's all about gravity. Anyway, back on topic. Be sure your guitar is in tune. This is extremely important, since what we are doing here is all about dealing with the forces string tension places on the neck. With your left hand, press the sixth string at the first fret, just like you are playing an F. While holding that down, use your right hand to press the sixth string fret which is in line with where the body meets the neck - usually the fifteenth or sixteenth fret. This turns the sixth string into a straightedge, covering the "flexible" length of the playing area of the neck, and allows you to see the degree of "dip" you will need to adjust out with a tweak of the truss rod nut. With your wrench (or screwdriver, depending on the type of truss rod), turn the nut clockwise to counter the dip. Try a quarter turn at first, and see where you are. Since we're working with a wood neck and fingerboard, no two necks will have the same stiffness, and different types of truss rods will adjust at different rates - so there's no secret formula there. Adjust, and recheck. Always be sure to leave a small amount of dip (we're talking tiny here).
If you are adjusting a neck with the trussrod nut up at the headstock, it's as simple as it sounds. However, this can be a time consuming process if you have a Fender or other guitar with the truss rod nut at the heel-end of the neck, which requires you to check the straightness while in tune, loosen the strings, loosen the screws that attach the neck to the body, tilt out the neck, adjust the rod, put it all back and tune it up, and re-check the neck's straightness. If it's not where you need it (and it probably won't be the first time), you need to do the whole thing again. Still simple, but time-consuming.
Of course, anytime the truss rod is adjusted, the distance from the nut to the bridge saddle has been changed, so intonation needs to be adjusted. We'll talk about that one in the future, as it's another adjustment which does not require incantations or mutilated chickens (unlike pickup winding). At least now you'll have a straight neck with reasonable string action :)


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