Acoustic Guitar Construction

Folks- first post here I think - or at least in MANY MANY years.

I am planning a small bodied guitar - perhaps even 3/4 size. And I am thinking (perhaps too hard!) about neck length (not scale length).

I suspect there is going to be a 'duhhhhh' at the end but, not guts no glory, looking good being overrated. And 'high math' is involved

So - take two guitars - perhaps two Martin knockoffs - a 12 fret to body slot head - 0 size and a 12 fret to the body slot head - 00 size

Both have 24.9" scale lengths.

Both have a total length of 37 3/4".

The body length on the 0 size is 19 1/8", the 00 1/2 inch longer at 19 5/8"

So - is the only difference in these guitars really in the body length - or do you need to add that 1/2 to the neck length? I am trying to get my head around what this does to the neck length, the resultant location of the fret board and and then the bridge location. (and for that matter - all the bracing). I can certainly just look at various plans but I am trying to understand the math and the implications of that math to the design build.

I am waiting for the duh.... I feel like i must me missing something really obvious here.

I really think about scale length and it appropriateness to the size of the guitar. I generally want a scale length that places the bridge close to the center of the lower bout with a 12 fret join. A 14 fret join without changing the scale length moves the bridge closer to the sound hole. That is one of the original reasons for long scale necks. Martin found it easier to make more frets available by going to a 14 fret join, this pulled the bridge up. In response design wise they lengthened the scale to push it part of the way back down.

The neck length (the area between the body and the head stock is set by the scale length and which fret number intersects with the body. The 12th, 13th, and 14th are common. Ignoring the body size the bridge location is set by the scale length The leading edge of the saddle is placed on the scale length plus compensation.

In the case of the knock offs you called out the headstock is probably longer/shorter if the body is longer and the length of the guitar is the same. Generally
the extra length is just in the body of the guitar. The area between the bridge and the tail of the guitar is a little longer. The bracing is built around the the body size, sound hole location and the bridge location. There are preferences for locating the X and there are preference for how open and closed one wants the X. In all cases the the legs of the X brace should cross the corner of the bridge wings.

Thanks John. That was helpful. I will absorb that a bit more and perhaps come back with some follow on questions / thoughts.

A quicker answer would be: the length between the nut and the saddle slot in the bridge is fixed by the scale length. With the scale length set, the length of the neck between the nut and the body join depends on where on the scale length the body join happens. (generally the 12th or 14th fret)

Yes - that I understand.. But to extrapolate from that - it therefore seems that (for instance) a 12 fret to the body 000 size slot head neck should be identical to a 12 fret to the body 0 size slot head neck - assuming same scale length and what changes is the resulting bridge location (and subsequently locations of sound hole, bracing, etc). Am I right or am I still missing something? Thanks in advance!

stevegr wrote: ↑Wed Apr 20, 2022 12:33 pm Yes - that I understand.. But to extrapolate from that - it therefore seems that (for instance) a 12 fret to the body 000 size slot head neck should be identical to a 12 fret to the body 0 size slot head neck - assuming same scale length and what changes is the resulting bridge location (and subsequently locations of sound hole, bracing, etc). Am I right or am I still missing something? Thanks in advance!

Yes, The scale length determines the distance between the nut and any fret. So for any two guitars with the body join at the 12th fret, the part of the neck between the nut and the body of the two guitars with the same scale length should be the same.

I am careful stating the nut to a fret distance, as there can be small difference how the 12 fret aligns with the body and the length of the headstock.

Very good - thanks for helping me work through this mental map.