Neck straightness and Eulers Columns

This first part is old news, but sets the scene for what follows.

We all talk about a guitar neck being “straight”, but most of us know that in practice, the word “straight” is rather misleading - a tiny curve in the neck is essential for most playing styles. This is mostly because strings don’t vibrate in straight lines, and to keep the frets as close as possible to the strings we allow the neck to bend a little, following the “envelope” of the string vibration as we move up the fingerboard from fret to fret.

A slight curve also allows the string to vibrate cleanly even if the fingerboard has distorted slightly due to age or climate. Generally speaking, low actions and light playing styles require straighter necks and vice versa. 

To adjust this “straightness” against the pull of the strings we use a “truss rod”- one of several different devices that apply a reverse bending force to the neck to counteract the forward bending caused by string tension. This isn’t the same as non-adjustable neck reinforcement, although some people still use the same name. There are also “two way” designs of truss rod  that will correct a neck that is bent backward rather than forward, usually caused by frets that are too tight in their slots.

Here is the bit that most people don’t understand. The forward pull of strings, taken together with the backward pull of the truss rod can NEVER add up to a straight line, or even to a single smooth curve.

Basically, a long slender guitar neck, when compressed and pulled forward by the strings but restrained in some way so that it cannot actually move forwards, will not simply shrink lengthways in a straight line. It will take up a vague S shape. It’s a bit like bending your knees and back when carrying a heavy load on your shoulders. 
The mathematics of this was worked out by Leonhard Euler in the 18th century, he examined  the four ways of putting stress onto a long thin column, and worked out the formulae for the maximum possible load in each case by imagining what would happen if the load was slightly  off centre. You can easily google this and see the different shapes that he anticipated.  I have heard some argument about this, so for the engineers amongst you, Euler did his calculations in the “elastic” region part of the stress / strain curve, not in the ”plastic” or failure region. 

I still have a simple mock-up of this that I made years ago to illustrate the point in an exaggerated manner, varying the string tension and the truss rod adjustment can produce a variety of shapes, but never a straight line.

In an acoustic guitar neck with a basic adjustable rod, the closest you can get to a straight line when the strings and truss rod are under tension is actually a slight hollow from nut to about the sixth fret, then a tiny reverse curve from sixth to eleventh, the fingerboard upwards from there remaining mostly unaffected by the truss rod. On an electric guitar, the same thing happens but a little bit further up the neck because of the extra length. This problem is usually addressed by filing off the tops of the high frets where any string buzz occurs, without much thought as to why it is happening.

To get the optimum “straightness”, a guitar maker or repairer will need to allow for string tension, either simply by knowing from experience where to remove a little extra, or by setting the guitar in a device that simulates string tension before levelling the frets. There is now a computer operated machine (Plek) that does all this for you, measuring and plotting out the distortion that comes from string tension and truss rods then grinding down all the high frets automatically to a pre calculated curve. The print outs from these machines show very clearly the distortion involved - the slight S shape of a neck under several different forces.

There is no way round this- all we can do is find ways of dealing with it.

One way is to design the truss rod to bend the neck in a slightly uneven manner, a little more in one part of the neck than another. With the single rod design we can change the initial curve in the rod, or with box section truss rods we can change the internal spacers, but it isn’t so easy with other designs.

Another way round the problem is to sand the top of  the fretboard a little in the appropriate areas before the frets are installed, and high tech makers can use a CNC machine to do this, shaping the fingerboard along its length in a slight reverse S shape before the frets are inserted, removing a little extra wood from the surface in places where it will be forced upwards when the strings and truss rod are tightened. The end result is that common white lie - a “straight” neck. The tops of the frets will look strange when first made, but a nice even curve - “straight” when strung up and adjusted.

At Fylde we do a little of all those things, and finally level the frets while the strings are in place, and under tension.

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