Questions regarding top and back radius choices

[Eddie McRae]

Maybe this is a mistake on my part, but I've always thought that the norm for radius choices were 30'-40' for tops and 15'-25' for backs. But lately I've noticed several plans that call for 25' top radius and 15'-18' back radius. For some reason, these choices seem a little excessive to me. And I've also been wondering lately about the consequences, if any, of using the same radius for both top and back. So, my questions are:
1) Would using the same radius for both top and back (for example 25') be a problem?
2) Does using a stronger top radius (for example 25') help to compensate for the gap left under the F.B. extension?

[Tim Douglass]

I do a sort of approximate radius, since I don't use a dish, but it is about 25' for both top and back. I think the one caveat with the flatter back is that you may want to make sure that you are building in the lower end of the humidity range.

[Gerry Gruber]

I use 25' for the top and 12' for the back, which I think is close to the norm. I think that as a rule, some radius is better than none, to ensure against humidity swings. I am happy with the 12' back radius. If I were to change anything it might be to go with a slightly flatter soundboard, to help keep my bridge height down. Kevin Ryan builds with a completely flat soundboard, and gets away with it - I think his thinking is that the upward and forward pressure on the bridge typically gives the soundboard a bit of "radius" anyways.

As to the gap left under the fingerboard extension, if the issue is that you are trying to attach a flat fingerboard to a curved soundboard, then less radius (i.e., bigger radius number, such as 30'+) would help in this respect. Some people try to flatten out their upper bouts to address this. I don't. My bridges are typically 3/8" high, which is a bit on the high side, but I compensate for that by removing mass from the bridge away from the saddle slot.

[Eddie McRae]

The gap issue that I'm referring to is created by attaching a back-angled (for ex. 1.5 dgrees) neck to the body. This of course creates and leaves a gap under the fingerboard as it continues to rise due to the back-angled heel. This conditions worstens with a flatter soundboard. I'm thinking a stronger radius for the top will actually create a slight rise in the top at the point of neckmount, thereby helping to eliminate the amount of gap under the FB. The resonators that I build have a flat soundboard with a 1.5 degree neck mount. I have to make a 1/8" FB shim for each of them to fill this gap.

[Gene Zierdt]

I use a 25' radius for the top, and then plane/sand the sides to flatten out the upper bout before I attach to top. The 25' radius creates very close to the right amount of drop-off to give me the 1.5 degrees on the side-to-top angle, and allows the fretboards to lie flat on the upper bout (ie, closing the "gap" Eddie is talking about). The 25' radius also puts the projection of the fretboard surface at the bridge right around 3/8", which is what I want. I use 15' radius for the back.

[Rodger Knox]

I use 15' for half of the top and the back. That is, the top is flat from the neck block to the bottom of the soundhole, and then curves down with a 15' radius. I don't use radius dishes, but I do use sanding sticks with a 15' radius.
I profile the sides prior to bending, and I develop the EXACT profile of the sides using a CAD program, which allows me to start the top radius where I want. This simplifies a couple of things. By using a 15' radius starting at the bottom of the soundhole, there is enough drop at the bridge location so that the neck does not require any angle. Also the fretboard fits flat on the flat part of the top, also simpler.

[Gerry Gruber]

I think to get a good sense of what your options are and the consequences in terms of the fingerboard overhang are to get a nice big piece of drafting board paper, and sharp pencil and nice long ruler, and draw your guitar - side angle - at full scale. Or if you are handy with CAD, so much the better. It might be a challenge to draw a 25 foot radius without CAD, but it's possible. I've done it. Years ago I built a Les Paul knockoff and was concerned about the neck angle and how it related to the arched top, and before I started making sawdust I made a lot of pencil shavings. it turned out fine, but the diagramming in advance was a huge help. It was the only electric I made, but the geometry turned out fine.

[Eddie McRae]

Thanks! I think I will try drawing it out and then experiment with different radii until I find one that closely matches the FB angle. As for the back, I'm still undecided about changing to a stronger radius.

[Barry Daniels]

I use a 25' radius for the largest part of my top but I brace the upper part to a 60' radius which most closely matches my fretboard angle.

[Rodger Knox]

+1 for drawing it up to scale, and CAD is even better for working out the details.
Barry, do you use a spherical or cylindrical top? I remember someone doing that, with a transverse curve but no longitudinal curve. Maybe it was Howard Klepper...

[Michael Lewis]

The transverse curve is much easier to achieve than a true dome, and in a guitar top situation should be plenty strong. Charles Fox has used the 'cylindrical' section top because it works so well and is much easier to make than the dome.

[Clay Schaeffer]

I've used "cylindrical " tops on octave mandolins. This method seems to lend itself to the bouzouki / octave mandolin sound - strong attack and moderate to short sustain. I think it could be easily overdone on a guitar.