Radiusing the back
 

Let's start out by stating that I am no luthier. I am an average woodworker that decided to build an acoustic guitar. With that said, does the back have to be radiused or can it be a plain flat back?

I would appreciate any feedback anyone can provide.

Thanks,
Chris

I am no lutheir either but guitar backs are often somewhat bowl-shaped or arched. Certainly all the ones I've had have been. I believe this helps with projecting the sound out of the sound-hole as well as adding stiffness (3D shapes are inherently stiffer than flat ones), but I don't think it's a requirement.

I would be surprised if someone popped up and said every guitar ever has always had a rounded back. For your first guitar, it might be sensible to avoid unnecessary complications; there is not often 'better' when it comes to guitars, just 'different'

Enjoy your project, I built an electric once, purely for the enjoyment of doing it and it was very satisfying!

This allows you to increase the total air volume of the instrument. It can also mean you do not require back bracing as the bowl is structurally stronger. How it changes the acoustics is beyond my knowledge and better answered by a real builder.

I built a Martin kit guitar (12 fret 000) about 25 years ago. Before the days where the internet provided us with a wealth of information. I basically had the minimal instructions from Martin and one reference book.

In the kit, the braces came pre-shaped. The flat back was glued to the arched braces, providing the arch/radius to the back. The bigger trick was mating the arched back to the sides.

Guitar tops and backs are radiused to increase section strength and reduce moisture content-variation shrinkage stresses. If the wood dries lateral shrinkage will cause a change in the arch radius before cracks occur.

There are many books that tell you how to radius the top and back. This free build manual will help you understand building.

https://www.stewmac.com/freeinfo/i-W...V.doc.1219.pdf

Don't start until you understand what is required. Most critical is neck angle which starts with the neck block shape.

Jon pretty adequately answered the question.

One can build whatever one wants. However, the way guitars are usually built today is the result of a lot of trial and error over many years by many makers. If one stays generally within those parameters one is nearly guaranteed to produce something that everyone will recognize as sounding, playing and looking like a guitar. The further one deviates from those standard designs the less likely one is to produce a standard guitar. It depends on what you want to achieve. There is nothing wrong with diverging from the beaten path, just be aware that there were reasons for it being the beaten path.

Thank you
 

Thank you for all the replies. This is my first build and I am sure I will have many more questions. Thanks again.

One word: cracks.
Wood shrinks, and the bracing restricts it from getting smaller. It literally pulls itself apart. If you insist on making the back flat, I would caution you to get the back as dry as possible before gluing on the braces. Otherwise, refer to my first sentence. If it doesn't crack, it most certainly will go concave when the shrinkage happens. Same applies to the top. A concave top can adversely affect the neck angle.
Arching does stiffen the plates. It is always a balancing act between structure and sound.

Thank you
 

Thank you John. I appreciate your insight.

Chris