Tuesday, October 19, 2010

How does a truss rod work?

The truss rod makes adjusting the bow in your neck possible.  They can be found in all modern electric, acoustic, and bass guitars.  some vintage guitars didn't come with a truss rod, but that's ok, as long as the action is not too low or too high.

Strings put a lot of tension between the tuners and the bridge, so naturally the neck will bow a little under the force, a little bow in the neck is a good thing.  Unfortunately wood is prone to warping due to temperature and humidity changes and can bow too much causing high action or even back-bow making the action too low and causing fret-buzz. 

A truss rod is a metal rod that runs up the length of the neck under the fingerboard and has a nut at one end for adjustment.  There are many different types of truss rods but one of the most common is the twin-action truss rod shown below.  They make it possible to adjust for either bow or backbow.

The twin-action rod is two threaded rods connected to blocks at both ends.  At one end there is a nut for adjustment, if you turn the nut clockwise it shortens the bottom rod and forces the rod and the neck to bend backwards and correct bow.  If you turn the nut counter clockwise it lengthens the bottom rod and forces the neck and the rod to bend forwards and correct backbow.  Here is a picture of how the truss rod fits in a small channel routed in the neck under the fingerboard.

The other type of rod is a single-action rod that only corrects for bow in a neck.  This type of rod requires a curved truss rod channel.  The rod itself (shown in green) is curved and when you tighten the nut it forces the rod to try to straighten out, thus straightening out the neck and correcting for too much bow.

Generally to adjust a truss rod you need to make very slight adjustments at a time, you may not even see results for 24 hours after the adjustment is made.  Go slowly to avoid breaking your truss rod, it is a very expensive repair.

For more information on how to set up your guitar, see our setup blog entry.