Guitar back curve

[phavriluk]

I might have used an incorrect term to described the shape of a guitar back. I know it ain't flat, and its shape is bowed on a roughly twenty-something-foot radius. Is that back intended to be a section of a sphere 2x the radius or is it meant to describe a side to side arc of that twenty-something feet, the center dimension being flat. Looking at the bracing I infer that the back is flat down the center and curved at right angles to the center line. Am I holding a valid assumption?

Thanks very much folks for any correction/enlightenment that is offered.

[tippie53]

If you are speaking of Martin spec it can be 15 or 20 foot radius that is 30 or 40 foot diameter. Don't loose too much sleep if there is a flat spot .
Most likely your RH may be the culprit. This is not a tonal feature but one for strength and stability during RH changes. It is best to glue top or back braces with the RH is low. I like to do this when I set my dehumidifiers at 35% for 24 hr. The higher the RH when you glue the more unstable the brace structure.

[phavriluk]

John, I think I'm thick inna head. Is a guitar back curved from neck to endpin as well as side to side or only side to side? Looking at the braces, which only run at right angles to the center joint in the back, if there is curvature along the center joint, how do we introduce it? Meanwhile, thanks very much for responding to my question.

[johnnparchem]

John, I think I'm thick inna head. Is a guitar back curved from neck to endpin as well as side to side or only side to side? Looking at the braces, which only run at right angles to the center joint in the back, if there is curvature along the center joint, how do we introduce it? Meanwhile, thanks very much for responding to my question.

It is curved head to tail as well as side to side. The curve is introduced when the rims are profiled in typically a 15 or 16 foot radius dish. The rims will hold the shape of the back when you glue it on.

[tippie53]

think SPHERE

[phavriluk]

Aaaah. John, thanks very much indeed. Another bit of the information puzzle filled in. Some of our wood shapes are jigged into place on subassemblies and others are pulled and clamped into place dduring final assembly. I'm trying to parse out as best I can which get into place without the need for locating force (side supports, for instance) and which need to be forced into conformity (present discussion of back assembly).

Much gratitude.

[Darryl Young]

Yeah, the back braces create the side-to-side arch and the rim creates the arch from tail block to neck block.