A guitar’s truss rod is an integral part of its operational design and offers players and technicians the ability to closely control the effect of the varying string tension it will be subject to over the course of its life.
Even though this simple feature is so important, it still leaves some people bamboozled by its mystical ways!!
To better explain the truss rod let’s back up through history a little bit….
Way back in the 17th century….told you we were going back!…. when lutes were being replaced by flamenco guitars from northern spain, string tension wasn’t really much of an issue. Not only because the gut and silk strings provided such insignificant amounts of “pull” on the neck but the necks themselves were very, very butch. If flamenco guitars had trouble with forward bow through string tension luthiers would often build pre-relief or back bow into a neck so that it pulled dead straight under tension.
Whizz forward a few years when guitar manufacturers started making steel strung flat top acoustics…now string tension was getting higher and necks certainly couldn’t get any bigger! Both Martin and Gibson realised the need for neck reinforcement….and so the truss rod was born. In its fledgling form the truss rod was either a small but solid length of box-section or T-section steel built into the necks under the fingerboard to counteract the pulling effect of the strings…both were non adjustable.
Now, it’s been long argued who “invented” the adjustable truss rod but i think just about everyone concedes that it was Gibson who first started using them and, to be fair, the design has changed very little since those early days.
The humble adjustable truss rod is a simple length of steel rod with a threaded section at one end and an anchor at the other. This anchor was usually either a flat or a welded “T”-piece. The rod was fitted into a vurved recess in the centre of the neck with the anchor point fixed to one end so that it would not move, the other end had a nut attached to the threaded section which, by way of a square washer would sit on the shoulder of the channel in the neck. The fingerboard was attached so the rod was completely encased other than the protruding nut. When string tension was applied to the neck and it started to forward bow, the nut could be tightened pulling the neck against this tension and towards back bow…simple but effective!
Since this time a few different types of truss rod have been developed including single self-acting truss rods that can be fitted into a straight channel, two-way truss rods that can actually pull back bow out of a neck and dual truss rods side by side that claim to be able to pull twisted necks straight….i, however, have found this not to be the case!
So there you go a brief history of the truss rod…but what does that mean to you?
Since the inception of the truss rod, manufacturers have been able to make guitar necks comfortable and slim feeling since they rely on the rod itself to provide the muscle to deal with string tension. This is all good but it does mean that we rely on the truss rod being in tip-top shape to keep our guitars playing well.
Before we look at the subject of neck relief, how we measure it and how we adjust it i just want to make one thing clear that for some reason people get wrong:
STRING HEIGHT (ACTION) IS NOT ADJUSTED USING THE TRUSS ROD. IT IS THERE TO COUNTERACT THE PULLING EFFECT STRING TENSION HAS ON THE NECK. NECK RELIEF HAS AN EFFECT ON OVERALL STRING HEIGHT BUT IT IS NOT A MECHANISM FOR SETTING ACTION!
Right now that’s out-of-the-way let’s get on with measurement and adjustment! We have established that if we have no counteraction on the strings tension along the neck it will be pulled into forward bow which will make the guitar uncomfortable to play.
The photo above is obviously an exaggerated impression of a forward bow but you get the picture. The strings are too far away from the board in the centre of the neck making it hard to fret notes. So what do we do? In the case of a forward bow we TIGHTEN the truss rod nut to bring the neck back towards back bow. In an ideal world we would bring the neck back to a point where it was arrow straight with no bow or hump, however, this isn’t an ideal world and because of the inconsistencies of fret work and disparity between areas of the fret board the strings are prone to rattle and buzz against frets. Because of this we usually leave a very, very slight amount of forward bow in the neck….this is called relief.
To measure relief we need to take the string height at the nut out of the equation so we put a capo on the first fret…just enough pressure to keep the strings on the first fret. We then remove string height at the bridge from the equation by holding the bottom E string down at the fret at the immediate end of the truss rod’s active length, that is the body joint fret on Gibson style guitars and the last fret on Fender style guitars.
Now we need to look at the gap that exists dead centre ofthese points between the underside of the loe E string and the crown of the corresponding fret…usually the 9th fret for Gibson style and 7th fret for Fender style. This gap is your relief measurement. If there is no gap then your neck is either slightly too straight or you have a back bow. If can get a stack of coins in the gap you have a forward bow. Now, setting your relief is down to personal taste and to a certain extent your playing style. I tend to set the necks on my guitars almost straight with as little relief as possible. I can do this as I don’t set my action super low and my frets are dressed well and often. A realistic bandwidth of adjustment for relief is somewhere between 0 (being dead straight) and around 0.020″ (forward bow). If you are unsure how you would like your relief set then factory specs are a good rule of thumb.
Fender set their relief from factory to between 0.008″ and 0.014″ at the 7th fret.
Gibson set their relief from factory to 0.012″ at the 7th fret.
So if you need to adjust your relief you need to find the adjustment end of your neck. This will either be a recess covered by a plate at one end of the neck or just a recess with a truss rod nut-face just visible. Some truss rod access, as on vintage style Fender guitars, is at the very heel end of the neck requiring you to remove the neck in order to make adjustments. I will say now that if you are not 100% confident in making these adjustments then don’t do it, leave it to a professional. At the same time though it’s nice to know these things. Adjustment for most rods is carried out using either a hex key or a box wrench. Be aware that many manufacturers use slightly different sized fittings….these may be imperial or metric and you MUST use the correct size. If you damage the truss rod nut it could be very difficult to reverse said damage! It is also worth noting that some of the very first adjustable truss rods, usually pre-war, have a reverse action…these are rare though so the rules below should, more often than not, apply to most of us:
If you want to REDUCE your relief and make the gap smaller then tighten the truss rod nut turning it clockwise as you look at it.
If you want to INCREASE relief and make the gap larger then you loosen the truss rod nut turning it counter-clockwise as you look at it.
Remember lefty-loosey, righty-tighty!
BE WARNED!!! Most truss rods are pretty responsive and don’t need much adjustment to cure a slight over/under bow. ALWAYS make adjustments in 1/16 to 1/8 of a turn….no more! Make an 1/8th adjustment…let the neck settle, check relief and adjust again if necessary.
BE MEGA WARNED!!! If the truss rod is tight and you are having to fight it AND/OR the truss rod is squealing and cracking as you turn it then STOP!!!!!!! If you jam or sheer the rod then it’s game over, unless you want to pay out a fortune in repairs your guitar is drift wood!
Just to give you an idea, when i adjust my necks to counteract seasonal changes and shifting in the wood then i would consider a quarter of a turn a drastic amount!!!
YOU HAVE BEEN WARNED!