For the next few weeks I’ll be talking set ups.
Today I’ll be talking trussrods! Most people I encounter have a very vague idea of it’s operation and what it actually does.
While a truss rod can affect string height slightly, that is not its purpose. It’s purpose is to adjust relief in the neck. To explain, if a neck is perfectly flat there will be buzzing. A string that is attached at both ends will vibrate in an arc. A neck that is too flat will interrupt this vibration, so a certain amount of relief or concave curve in the fret board is necessary to allow the vibration.
If the string is stopped at the first and last fret you can use a feeler gauge (available at any auto supply) under about the ninth fret on an electric and seventh on an acoustic, in between the fret and the string to give you your current relief or lack thereof.
Before we get into the actual adjustment, I should explain the difference in rods. There are basically two types: Single and double action. The single is nothing more than a 3/16th diameter rod threaded on one end and anchored to the neck on the other. It can be adjusted at either end, headstock or base (think Gibson and vintage Fender) and when it is tight it takes the concave bow out of the neck. It will not, however correct a convex bow
This is where the second type comes in. The double action rod consists of two rods or a rod and flat piece of steel The bottom rod is always a rod threaded on both ends and threaded in opposing directions. The top can be a rod or steel bar that is anchored to the bottom rod at both ends. The system is independent of the neck and does not require attachment. In fact on some guitars it can be removed with out taking the fret board off. This setup can also correct convex bows as well as concave.
As you have probably guessed, I prefer the latter and use it on all my guitars unless the customer specifies otherwise.
There has been talk that the single rod is superior in tone because it is anchored to the neck but I haven’t found any evidence to support this.
Now we get into adjustment: No rod will be able to take a severe warp out of the neck, so don’t even try. A broken truss rod can be very expensive to repair. The most important part is to only adjust the rod an 1/.8 th turn at a time and should be “helped” by using light pressure with your hands. A small amount of torque can produce a good amount of relief, so go easy. Relief is usually set between .007″ and .010″. Some players, bluegrass flatpickers come to mind, like even more because of their aggressive technique.
Hope this helps. Next time we’ll tackle nut height and spacing.