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Hello OLF,

I'm looking for help in deciding top and back radius. I'm making an OM type instrument with spruce top and mahogany B&S. I was planning on using 28/20 because I've heard that's what Martin uses on their OM's (I figure copying something tried and true for a 1st guitar is not a bad way to go). Here on the forum, there seem to be many using 25/15, though. What are the benefits of these radius. Is it because more arc = greater stiffness = better tone. Does a 25 top set you up for cleaner FB extension to top mating.

I'm sure this one's been discussed mucho, but sometimes it takes a lot of searching in the archives to find exactly what yer lookin' for

Tanks!

I use 28 for the top and 15 for the back. This is what I was taught to do. I can't tell you what the big difference would be between 28 and 25, my guess is it doesn't make a big difference. But since I have never used anything other than 28 and 15 maybe I'm not the best one to comment.

There are a number of other issues that come into play when you start talking about mating the FB extension to the top. I would think you can make it work equally well with either 28 or 25 depending on how you go about it.

Maybe some other people around here will be able to help you more.

Josh

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Josh House

Canadian Luthier Supply
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It works for me and I do make some adjustments for everything to match well and have the proper fret board extension drop off at the sound hole. But you are right, many different radiuses would work and once you make the adjustments, if any are required, for what ever you use you will be good to go.

In fact I use 15/25 for OMs, d*eads, SJs, and L-OOs - I guess this makes me a two dish pony......

Myself, I was taught 28' top with only the two x-braces radiused and all the remaining braces flat bottomed. The back, 22' braces except number two (visible through the sound hole) being slightly tighter in that I place a wedge about 3/16" high under each end. I can't tell you why - I don't know, but I can tell you the small jumbo I made like this sounds better than any guitar I have ever played / heard; but I am a bit biased.

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Jamie M

The purpose of doming is to provide more strength to the plates than a flat plate would provide. The back dome is really not very critical at all and can vary a good bit. IMO the top radii should be one that works well with the back angle of the neck as to eliminate excessive sanding or scrapping of the top in the fretboard extension area for a flush fit. I find a 28' radii works best with a 1.5 degree neck angle. 25' works ok with a 1.5 degree neck angle also but required more sanding or scraping attention to flush out the fretboard extension and there by thins the top under the fretboard extension near the sound hole more than I would prefer.

It can also be used to shape the sound of the instrument and it's projection.

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

There is truth to this as well. but the initial intent was to help alleviate the inward cupping between the FB and the bridge

Michael,

I'm ashamed to say that I don't know my guitar history that well to know when it was first introduced and why (maybe someone like Rick Turner knows) , but the Larson brothers were using big arching/doming back in the 1920-30's to create a sound, and Howe-Orme well before that.

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Im interested in this topic as Im rapidly approaching this consideration on #1 ...
I bought 25' and 15' dishes from Shane
as I saw it mentioned most predominantly and I figured as alot of people were experienced with these radii,... I could get help regarding any issues that might come up with these,,,
(in particular)
Now Im pretty sure Rod True (among many others) builds flat-tops - I have been meaniing to pick his brain regarding this feature of his builds,,, mabye he will chime in?

Also I have to say Im quite curious to hear a side by side comaprison of a domed/non top... Im sure Ive played a domed top and a flat top,, back to back,, at some point in some music store over the decades - however I wasnt directly assessing this aspect...

Mabye we could get some builders to post soundclips of thier guitars in this thread - radiused and not ??
Id really appreciate it - if so..
Anyhow studying the process of building these radii is something I have been focusing on lately and Im happy to see some mention of it here... good stuff and thanks to the knowledgable among us here for sharing thier wisdom,,!
Cheers
Charlie

I use 16 and 28, if that means anything. Again, it is what I was taught to use.

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Reguards,

Ken H