In a different thread, a question came up about measuring and adjusting relief. We opted to move it to a new thread specific to the topic and share it in case anyone wanted to join in the discussion. Be forewarned, I am not a luthier. I do maintain my own guitars and do set up work for others. I am happy to share what I know, but be advised that I don’t know what I am talking about.
As we get into measuring and setup, we might end up venturing into other aspects, but we’ll start with the original topic of relief.
First a little background on relief. If you sight down the neck of a well setup guitar, the neck will generally have a slight curvature towards the strings from around the 9th or 10th fret down to the nut. Relief is this “normal” curvature of the neck. Most guitars have an adjustable truss rod that allows one to increase or decrease the amount of relief. Changing relief is not the primary method to adjust action. Changing relief does change the action, but if the action is excessively high or low, relief alone will not correct the problem. What relief allows you to do is get similar action all the way up and down the neck. That is to say if your action is a bit too low and you get fret buzzing, if your relief is set correctly (and your frets are level), you would expect to get get buzzing up and down the neck. With too little relief you get more buzzing in the lower frets.
How do we measure relief? There are several methods. An easy one is to use the string as a straight edge. To start, put a capo on the first fret. Then use your finger to hold a note on the low (or high) E string at the 13th fret. The string acts as a straight edge. Relief is measured at the point where you have the maximum height of the string over the frets. This is generally done at the 6th or 7th fret. A typical range you might see is .004" to .012" Martin specifies a max of .010" from the 7th to 9th frets. Of course that’s a pretty small gap, so how do you measure it? A business card is typically about .012 and paper about .004. So if there is a relief gap that is more than a business card or less than a piece of paper then it might be suspect right off the bat. The easiest setup fix I did was on a friend’s new and freshly setup D-18. The owner complained about some fret buzz on the lower frets. Just using the string as a straightedge revealed the neck was actually back bowed slightly (there was anti-relief) and the fix was simply loosening the truss rod a small amount (2 turns of 1/8 of a turn or less). This type of measurement (with the string) is generally sufficient to have a good idea of your relief, but you can get more exact if desired. If you have feeler gauges, then you can get a better measurement, but even then the flex of the string makes getting an exact measurement impossible. If you really want to know the exact number, you will need to use a straightedge about 18" long (a rafter square works nicely). It won’t give the way a string does, so you should be able to get an exact measurement with a straight edge and some feeler gauges.
So what is the right number? It depends on other setup factors, your playing style, the gauge of strings, and probably a few other things I haven’t thought of. The point being, there is not a universal “right” number. With that said, the aforementioned range is a pretty good general guideline. When I set up a guitar I start with it pretty close to flat. It will generally have some buzz like that and I will add enough relief to eliminate that buzzing. I like guitars with little relief, typically in the .005" range (many bluegrassers like a ton of relief). If the neck is dead flat and doesn’t buzz, then I can probably lower the action a bit if desired. If I get to .010" or so and I am still buzzing and my frets are in good order, then probably something else is going on that will need to change to raise the action (nut, saddle or neck set angle).
Well, I have kind of rambled. The point being, the first thing to do is evaluate the relief you currently have. You can do that by sighting the neck and measuring with one of the two previously mentioned techniques.
I could have saved a bunch of rambling… here is an article by Frank Ford. He knows a great deal more than I about this stuff:
frets.com/FretsPages/Musicia … tradj.html