-What is neck relief
-Why guitars need neck relief
-How to measure neck relief
-How to adjust the truss rod
Neck relief refers to the amount of forward bow that a neck has under string tension, it is one of the most important aspects of a well set up instrument. On guitars without a truss rod, like classical guitars, the relief is factored into the building process by the Luthier. However, in most acoustic and electric guitars there is an adjustable metal rod (truss rod) which, when tightened will bow, and pull the neck back to counteract the tension of the strings. Most guitars now feature dual action truss rods which, as the name suggests, can tighten in both directions, the use case for this would be if the guitar already naturally has a back bow, you would need the truss rod to be able to pull it up into a forward bow, which is not do able with a traditional single action rod.
One of the main reasons why we want to have relief in the neck is because of the vibrational path of the string. The string (represented by the dotted orange line) will vibrate when played, the path of this vibration (represented by the solid orange line) will be at its furthest distance from the strings resting position, halfway along the strings length. fig.a) represents a neck with an adequate (albeit over emphasised) amount of relief, see how the forward bow of the neck closely matches the vibrational path of the string, this will allow the guitar to play cleanly and without buzzing in any position on the fingerboard, too much or too little will inhibit how well the guitar plays. fig.b) is representing a dead straight neck with no bow, see how the the halfway point of the string collides with the surface of the neck, the neck is not compromising to the strings vibrational path and therefore obstructs it, this will likely not lead to an unplayable instrument, but an instrument which experiences more frequent and instrusive string buzzing than desired. In fig.c) we have a back bow, think of the string as a long HGV lorry, and the neck as a small canal bridge, the lorry will get halfway across and get its underside grounded on the asphalt. This will, in most cases lead to dead notes and intolerable issues regarding string buzz, as when you fret in the first position the string will be obstructed by the curvature of the neck, causing it to 'ground out,' like the lorry over the bridge.
Note that whilst adding this forward bow (relief) to a neck will raise its action, you should not rely on the bow of the neck to correct an action issue, you should set the proper amount of relief before considering action height, and then raise/lower your saddle(s) accordingly.
Before you adjust the truss rod you need to be able to measure the relief. All you will need is your guitar with strings at pitch, a capo, and a set of feeler gauges. You want to start by placing the capo on the first fret position, making sure the strings ring clearly off the first fret. Now, with the guitar in playing position, fret the low E string on the fret which meets the body, this will usually be 12/14 on acoustics, and around 17 on most electrics. What you have done at this point is you have made the guitar string a straight edge, and we can use this straight edge to comparitively measure how much the neck deviates from this straight line.
Now take a feeler gauge, size 0.008" is a good starting point, and feed it under the string and above the fret on the 5th, 6th, 7th fret area, we are looking for the feeler gauge to slide in between the gap of the top of the fret and bottom of the string. The reason I did not make clear exactly which fret is because the point at which the gap is biggest can vary and you want to test multiple frets to see which has the largest gap, this is where we will measure the relief. Now, if your feeler gauge is passing through easily without disturbing the string, that suggests you have >0.008" of relief, and if the feeler gauge is passing through but lifting the string you have <0.008" of relief, if you notice that the string is completely touching all the frets between the capo and your finger, this suggests that either, the neck is completely flat (no relief) or the neck is in a back bow, neither of which are conducive to a well playing instrument. Usually measuring just the low E is acceptable, however it is good practice to measure both E strings, if the relief is different between those two outer strings, that suggests the neck has a warp.
When it comes to adjusting the neck to get the correct relief you need to be sure you are using the correct truss rod tool, research your manufacturer and model to find what
type/size is correct, the most common are 5/16"(Gibson,) 3/16" OR 1/8"(fender,) 5mm(Martin,) 4mm is very common on eastern made guitars. Truss rod adjustments can be found under a cover on the headstock, on the heel of the neck, or in the case of many acoustics, underneath the fingerboard extension in the body. When turning the key it is important to make sure the tool is secured as you do not want to strip the allen key adjust or nut, one ill thought out turn could lead to a challenging and expensive repair. That being said, if you are careful you should not worry, just remember to stick to making very small adjustments, fractions of a turn at a time, and make sure before turning the key, it is well secured in the nut.
So now you know how to adjust it, how and when should you? If your relief is too much, you want to tighten the truss rod clockwise, and if the relief is too little, or your neck is bowed, you want to loosen the truss rod by turning anti clockwise. Simply measure, then adjust, measure, adjust etc, until you have your desired relief. Generally anything >0.015" is considered too much, and anything lower than 0.002" would be too little. For most acoustic guitars I will aim for 0.006", for most electrics 0.008", and basses often are comfortable at 0.010". There is reasoning for these differing numbers, and there is also logic behind using differing amounts of relief based on string gauges that are out of the scope for this page.
It is good practice to always turn the truss rod anti clockwise a little first, you see a truss rod only has a limited range of movement, as you tighten it, it will get increasingly stiff, take this too far and you risk damaging or breaking the truss rod, so before you tighten it, you want to establish how tight it already is. There's no fixed, 'too tight' when it comes to truss rods, it's just a feel thing that comes with time and experience, however, if you feel like you are having to strain to tighten the rod further, it's a good idea to stop there! One thing to note with dual action rods is you may have a situation where you turn it clockwise (to tighten the rod) and you feel it loosen, don't be alarmed, this is because the truss rod tightens in both directions so you are just undoing the tightening of it in the anti clockwise direction.