Trussed up

The purpose of the truss rod is to keep the neck from bending forwards excessively once the strings are pulling on it. The neck profile and width are not constant and being able to predict how much it bends along its length is not something easily worked out (well not by me, anyway).

Perhaps I'll just have to fit something and hope. It shall therefore be known as a Trust Rod system. Rickenbacker use two (Rod and Jane?) and I am sure somebody out there has tried three (Rod, Jane and Freddy). Maybe not...

I've had guitars before that have a mysterious rattle from inside the neck on a particular note. It doesn't seem to get to the pickups directly, but it makes the note quieter or lose sustain quicker. To avoid this, I am shying away from a double action Trust Rod and instead going for the traditional bar in a curved channel version. This gives me the option of fine tuning its response to some extent and no other metallic parts for Rod to clang against.

The worst case scenario I can think of is a neck that bows forward with one curve, and a truss rod system that compensates for a different curve. The difference between these could result in a neck that cannot be adjusted flat, it will always have some sort of buckle in the middle.




To get around this I managed to get Chris at work to assist me with a simple finite element stress analysis model of a typical neck profile. String tension was added to see what happens in use. I don't know how to use mechanical CAD systems so I just directed from afar.



Measurements of deformation were taken at 20mm intervals and entered into a spreadsheet which could then be analysed to see if any sense could be made from it all.

With my approximate Jazz bass neck profile, the deformation was remarkably similar to a square law. This meant to me that for every 20mm step along its length it had curved by the same amount as the previous 20mm. A quick surf brought up the Engineering Fundamentals site which shows that for a constant cross section (ie a plank) the deformation is a cube law.

Hmm - been dazed and confused for so long its not true, wanted a straight neck, never bargained for you.

I'm still thinking there's a lot more to truss roddage than meets the allen key, but for now I think a single radius channel is the place to start. Over a 600mm ish length I am going for a peak curve of 6mm, which is a radius of around 7.5m.

I don't know why, but I feel that if I can put a mild downwards slope with this curved channel as it heads towards the body, it can only help negate the effect of the strings.

Trust Rod! Don't trust Jane or Freddy.