The Patent diagram for Gibson’s first truss rod often leaves today’s luthiers scratching their heads, as it seems to have been installed incorrectly! Roger Siminoff – historian and author of The life and work of Lloyd Allayre Loar – recounts the development of Gibson’s patented truss rod neck reinforcement.
“Gibson’s first truss rod system was designed by Gibson engineer Theodore “Ted” McHugh (U.S. Pat #1,446,758 – filed: Apr 5, 1921 – granted Feb. 27, 1923) and featured a truss rod with a high center and with both ends of the truss rod embedded low in the neck,” says Siminoff.
McHugh’s idea was that the low ends of the truss rod would pull the bottom of the neck heel toward the peghead and create a ‘bow’ (high area) in the center of the fretboard to correct any ‘hollow’ (low area) caused by the tension of strings. However, McHugh’s design was flawed; as the truss rod was tightened, the truss rod wanted to straighten, and this created a force that would put a ‘hollow’ in the fretboard. Important to note that McHugh’s design did work during initial tightening; as the truss rod was tightened, the neck wood could compress more easily than the ebony fretboard, and this caused a quasi bi-metal effect in which the neck would be forced into a mild bow. But, with further tightening, the tension of the truss rod would become dominant, and it would reverse the action resulting in the fretboard being drawn into a hollow. These two forces working against each other did create a stiffer neck, but made adjustments difficult.
“The first few hundred or so Gibson instruments (of various models) to be fitted with the new truss rod design featured nickel-plated truss rod covers – later changed to the black truss rod cover.
“Gibson continued to use this faulted design for almost 15 years when it began to invert the truss rod and position it with a low center. (the image in the photo below shows a cross-section of a mid 1970s Gibson electric guitar neck).
“Gibson, and many other instrument makers have used the low-center rod with great success for the past 80 years! The neck in bottom of the photo is cross-section of a 1925 Gibson tenor banjo neck showing McHugh’s high-center truss rod.”
