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 Post subject: Radius for Top and Back
PostPosted: Fri Jul 28, 2017 9:15 am 
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Joined: Tue Jul 25, 2017 7:14 pm
Posts: 28
Good Morning Members,

Studying the why's and how's of the top and back radius. Still not understanding the reason the radius (or beveled, curved) top and back exist to begin with. Also trying to understand how to carve the sides to accept the curved top radius and back radius. Looking for further areas of study and would like any suggestions please. I understand some acoustics are designed without a top and back radius (they are flat). Appreciate the help! Be well!


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PostPosted: Fri Jul 28, 2017 9:40 am 
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Joined: Sun Jun 25, 2017 3:58 pm
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Location: St. Louis area
Goto StewMac site and find their guitar kit. Download the PDF instructions for their dreadnaught kit. There you will find the sanding board technique for radiusing the sides for different radius. You tube has lots of sanding dish videos showing a different technique. Radius top and back adds structural integrity to overcome forces of string tension.

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PostPosted: Fri Jul 28, 2017 9:50 am 
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Thank you Kabore.
Can you please recommend a basic youtube video that explains this technique in a simple and understandable approach? Tried searching youtube this morning and found nothing that explains the how's and why's of top and back radius in an understandable format. Perhaps I'm using the wrong search words. Thank you.


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PostPosted: Fri Jul 28, 2017 10:01 am 
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Okay, I searched under "acoustic guitar sanding dish radius top and back" and found some videos but to clarify, i'm still focusing my search on the "WHY" instead of the "How To".
Really studying the physics of the acoustic build and would like to learn and understand "WHY" the radius is needed for the top and back. Once I understand, I can move to the "HowTo". Thank you.


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PostPosted: Fri Jul 28, 2017 10:40 am 
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Joined: Mon Jul 28, 2014 7:50 am
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Location: Chadds Ford, PA
Why radius -- probably tied up in tradition, strength, stability, or aesthetics.

For me, I just follow convention (I do what Martin does -- good enough for me).

Regarding how-to, I follow John Hall's videos/technique, simple and effective.


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PostPosted: Fri Jul 28, 2017 10:43 am 
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You can build a guitar that's completely flat. However, the curve increases the strength. The strings cause a lot of tension; the curve on the top helps to mitigate some of the stress. As far as the back radius, which is more than the top, I'm going to go out on a limb here and suggest that not only does the back radius lend strength, but projection. I used a back radius of 15' on one guitar, and a radius of 10' on another. The guitar with the tighter radius, the 10', projects a lot more and the sound resonates with great sustain. Part of it could be wood choice, but I credit the radius.


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PostPosted: Fri Jul 28, 2017 10:43 am 
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Joined: Sat Apr 10, 2010 10:50 pm
Posts: 2281
Location: Seattle
Sunset wrote:
Okay, I searched under "acoustic guitar sanding dish radius top and back" and found some videos but to clarify, i'm still focusing my search on the "WHY" instead of the "How To".
Really studying the physics of the acoustic build and would like to learn and understand "WHY" the radius is needed for the top and back. Once I understand, I can move to the "HowTo". Thank you.


You can build a true flat top and back guitar; some guitars and a lot of ukuleles are. A radiused top and back provides some resilience against humidity changes; without the radius on a dry day you may end up with a concave or even a cracked top or back. Also with care (follow John Hall's youtube videos) you can match the top radius with the neck angle on a steel string guitar. When I want a very active back I use a smaller (10 foot) radius, to prestress the back allowing me to thin the back to remove mass but still have the stiffness I am looking for.

If you really want an in depth understanding of the physics of a guitar I would suggest you purchase the Gore\Gilet Contemporary Acoustic Guitar Design and Build books http://www.goreguitars.com.au/main/page_the_book_overview.html. You can successfully build without this information if you are following plans from existing successful guitar. The Gore\Gilet book show that all of the components of a guitar work as a system. The design book purpose is show how the various components interact allowing one to create their own design.

If you do build a true "flat top" you still need to sand the proper neck angle into the upper bout so that you fretboard extension can lie flat on the top (about 1.5 degrees).

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