THE TRUSS ROD

 
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THE TRUSS ROD is a steel bar placed in the neck of a guitar to counteract the bending force exerted on a neck by the compressive load of the strings.
 
The earliest adjustable truss rod was designed by Gibson engineer: Thaddeus McHugh in 1921 and featured a curved rod positioned higher in the center and lower at both ends.
 
The idea behind the patent was that the low position of the anchor points would pull the bottom of the peg-head toward the bottom of the neck heel. This worked to a point, but when the rod got tight it tended to straighten and forced the neck into a hollow rather then a bow.
 
In the early 1930's, Gibson inverted the rod so that it was lower in the center and higher at both ends to exert a post-tensioning effect and raise the center of the fretboard when the rod is tightened.
 
Truss rods are typically adjustable and numerous mechanical systems exist. However, T-bars, composite rods, and square tubes are not true "truss rods" and fall into their own non-adjustable categories of structural neck beams. (Siminoff, 135)
 
Many guitar owners think that by adjusting an adjustable truss rod it will raise or lower string action. That's not accurate. A truss rod will not move a neck up or down like an elevator.
 
Simply, the function of the truss rod is to counter-balance the pull of the strings. Think of a two part scale with weights on both sides. If 2 lbs is on one side then 2 lbs needs to be on the other side so that the scale is balanced.
 
A guitar strung to regular tuning (A 440) exerts approximately 130 psi pull against the neck, therefore, the truss rod will counter balance that "psi" for a level fret board commonly called a finger-board. Thus, that's the function of the truss rod.