Tweaking your trussrod.  Or, what's really going on inside there?

Back in early Septenber of 2010 a member of GuitarsByLeo sought some detailed information on how the current (CNC-era) trussrod works.  Dave McLaren of BBE/G&L responded with a very descriptive post of how the thing works.  That thread can be found here.

I requested a trussrod for the purpose of photographing it to help with a visual look at how it works.  Dave and Steve Grom were kind enough to supply me with one and the photos below are a start on my project of getting a pictorial presentation available.

This is a work in progress, so photos and text will probably change as the project goes forward.

Each thumbnail below is clickable to display its associated fullsized image.

Further discussion can take place on the forums at BassesByLeo and GuitarsByLeo.


As Dave explained in his post linked above, the G&L trussrod is unlike a traditional trussrod that is curved and acts on the neck by applying direct logitudinal (crush) pressure on the wood.  It is also not a dual action trussrod that can apply force in either of two directions.  Instead, it is a sort of passive/active device that is installed as a straight assembly and applies primary longitudinal force to itself.  This primary force is then made to apply a lateral force to the top of the neck, under the fingerboard, which counteracts neck bow resulting from string tension.

Covered - Whole rodThis is an image of the entire trussrod as it would be installed in a G&L bass.  The far end of the rod, the butt end, is rectagular in shape and fits tightly in an interior slot near the butt end of the neck.  The entire assembly sit just under the fingerboard.  You can see from the image that the trussrod is round on one face and flat on the other.  These are the faces of the active and passive parts of the assembly.

Note the black covering  This is a slippery shrink wrapping that covers the active and passive parts of the trussrod.  This covering protects the trussrod from glue when the fingerboard is attached.
Covered Adjuster 6This is the adjuster end of the trussrod with the nut spun on to make contact with the active sleeve.  You can see the shiny adjuster and how it's in contact with a gray cylinder, or sleeve.
Covered adjuster 3This is the same view as directly above, except that I've backed off the adjuster a few turns to show the end of the active components.  The passive part is the threaded rod.  The active components are the adjuster nut, sleeve, and the rectagular-section bar you can see the end of in this image.
Covered adjuster without nutOne more shot of the adjuster end of the bar, this time with the nut removed.  Here you can clearly see the end of the sleeve and bar.
Covered butt end 1This is the butt end of the trussrod assembly.  As you can see, it is basically rectagular in section and is installed into a slot in the butt end of the neck.  Once installed, the overall assembly cannot turn or twist.
Covered butt end 2An end-on view of the butt end of the trussrod.  The round piece is the passive rod, which is threaded at the other end for the nut.  The rectagular piece is the end of the active part of the assembly.  The two parts are welded together at this end.
Uncovered butt end 3Here's the butt end of the trussrod showing the welding.  Note the grinding marks.  You'll see these at both ends and it's done to clean up after the welding.  However, this end must be ground or milled to close tolerance so that, once covered, it fits tightly into its mating slot in the neck.
Uncovered adjuster end 1Now things should start falling together.  I've taken the cover off so you can see the inner workings.  Here you can see that the active bar is welded to the sleeve, which is free to slide up and down the passive rod as the nut is adjusted.  

Uncovered adjuster end 2Same shot as above with the nut backed out a few turns.
Uncovered midrodThis is a closeup of the uncovered mid section of the assembly.  No tension, so the rod and bar are in contact with each other.
Uncovered whole zero turnThis shows the entire trussrod assembly with no tension being imparted on the active bar.  This is roughly how it would sit in the neck.  If the fingerboard were removed, you'd see the flat of the active bar at the same level as the underside of the fingerboard.
Uncovered half turn 1This is the assembly with a half turn of tension.  The nut pushes the sleeve toward the butt end of the neck.  This in turn causes the bar to flex laterally and apply upward pressure on the underside of the fingerboard.
Uncovered whole turn 1Same photo as above, but with a full turn of tension.  You can see that the bar has flexed even more.

Bear in mind that because this trussrod isn't installed in a neck and has no fingerboard to offer resistance to the movement of the bar, the movement you see here is exaggerated and not totally representative of how it actually bears up against the fingerboard.  In operation on a guitar or bass, the bar will make contact and flatten somewhat against the fingerboard, applying pressure across a broader area than these photos depict.  And because of the design of this trussrod, surprisingly little pressure is actually applied for a given neck deflection.

I'll add more to this in the future to help provide an even better illustration of how this trussrod works.

Copyright 2010 Ken Baker and bassesbyleo.com