Neck angle/string tension question

[tdq]

I was hoping someone could answer this for me: How much, if any, should the neck angle move, from no string tension to full tension? e.g. when I lay a straight edge along the neck to the bridge with no strings on, should I expect it to go up a mm or two when the strings are brought up to tension, or none at all? I've tried googling but cant find a clear answer. I'm not a builder at all but I'm fiddling with a home project and wanted a baseline of what to expect.

[dekutree64]

Yeah, there should be some amount of pull-up, mostly from the soundboard flexing, but the whole box bends to some extent. 1/32" or 1mm of bridge rise is a good average allowance for it. Of course the neck itself will bend as well, but that can be cancelled out however much you want by tightening the truss rod.

Think of a guitar as a fancy longbow

[John Arnold]

1/32" is a good ballpark figure. More flexible tops and taller saddles can be a bit more than that.

[Mr Fingers]

Yup. This is one of the "fun" (= maddening!) aspects of acoustic setup. It's the old Catch-22. You want to work on the action, which can sometimes mean working on the saddle, or perhaps a high fret, but whatever you may do with the strings off is a guess based on what you find when tension literally changes key dimensions. The numbers mentioned here are interesting and believable. I've always gone by trial and error, so it's great to have some baseline numbers. IME, truss rod adjustment changes the curve of the neck in a way that brings the nut back "down" vs the string tension that seeks to yank it upwards... and sometimes does.

[Alan Carruth]

You can, of course, measure the 12th fret action height with the strings tight, and then again when they're more or less slack, with only enough tension to keep them from drooping. That will be close. You can also measure the neck relief in both cases to see how much that's contributing.

[tdq]

Thanks for the feedback, all. I'm getting about 2mm from no tension to full tension - I'm working on a cigar box guitar build with 6 strings so I'm not surprised, but I was wondering what I should be comparing it to. This helps - I've got some support in there but I think I can live with 2mm, now that I have a baseline.

[Alan Carruth]

The problem is not the initial deflection, it's the 'cold creep' afterward. Wood under bending stress keeps moving, and takes a set over time. The FAA says that, in wooden aircraft structures, the initial deflection under full load should be no more than 1/3 the deflection you can tolerate over the long term. If your cigar boxes are pulling up 2mm with the initial string load, then they might pull up 6mm over time. Can you live with the action at 1/4".