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What Radius do you use and why?

I've been watching Peter M of Cornerstone guitars build for a couple of years and now I'm venturing into a build for myself.
Peter builds with a flat top and they are wonderful guitars with a great sound.

Well I was looking into getting a Radius form for the back of my guitar and as I was looking
I found all of these different forms available from an ebay seller.

Here is his list of available options 12' 15' 20' 25' 28' 35' 40' 52' (True Martin) 65' (R Taylor signature)

Well I thought it would be great to hear from others what they use for both the front and back of their guitars but mostly ..... WHY ?

This should help beginners like myself to gain an understanding of the concepts out there.
Most likely I will go with a flat top since the advantages I see are it is easier and sounds nice if done right.

Thanks for any input given that will stretch this brain of mine.

Dennis

None.

Because...

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Thanks for the response Alain.... I assume you are talking about the top...

What about the back of your guitars are they flat also?

I kinda figured it would be a challenge to everyone to have a reason other than that is just how I started or I'm not really sure but that is what Martin uses or Taylor uses so that is the dish I made or ordered.

Unless I get educated that will be my answer.

Great question Dennis! I'm very interested to hear what others have to say about it.

I have only used 28' because I've read it results in a perfect (or near perfect) neck angle, and also because it's the only radius for which I have a dish.

I use 28' for my tops as it seems to give me a nice set-up angle. My backs are 15'. I only build steel string guitars.

25 top 15 back,Like charlie said,that`s the dishes I have.Also these are the radiuses that Robbie O`Brien uses in his instructional videos.I`m not sure there would be much difference in 28 or 25 .They work well for me.Although I true up the upper bout ala Hesh`s tutorial.
James

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James W Bolan
Nashville Tennessee

I am 15 on the backs and 28 on tops. The back radius is for movement and strength and isn't a sound influencer

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks guys for sharing what you do ..... I haven't seen Hesh's tutorial but I would assume he is flattening out the upper bout to work with what ever angle he builds his Neck block to and making that area a nicer fit for the finger board to attach. Basically the same Peter does with a flat top guitar in establishing the neck angle.

Ok so far the best bit of knowledge gained is that 28' seems to works best with the neck angle....

I'm now making my back radius dish since I had a jig loaned to me from Steve Saville.... sure is a messy job but better than spending $60-90 plus shipping and then wondering if I picked the best size......

15' on backs, 25', 40' 65' or flat on tops. I don't use radiused rims. I shape my rims to make the neck angle work out with whatever radius I'm using.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

What do the different sizes 12' - 15' - 20' do differently as far as strengthening in a back?

Hey Woody.... Since you use so many different top configurations maybe your one who can best answer what you find in the differences...

My reasons are kinda........................... strange I guess. With my building style, a dread with a flat top sounds kinda muddy. Using a 25' radius on a dread helps me keep them balanced. I'll go flatter if a client prefers more warmth. Using a 40' radius on my "B" body (between a MJ, and SJ) helps me keep good bass response, but maintain balance on this size guitar. On OMs and smaller I use a flat top with most tops woods, but use a 65' radius for Cedar, Redwood, or Englemann. Humidity control during the build is very important, but a flatter radius makes it even more important.

For me the top radius is a good way to fine tune a build, without changing your personal bracing, and building style. I've outlined what I usually use. If someone want's their guitar to sound....crispy, with alot of sparkle, and long sustain I'll use one of my......steeper radii. (smaller number) If they want alot or warmth, with quicker decay I'll use a flatter radius. I've also done crazy stuff like a radius on the "X" , but flat tone bars, but can't say what effect it had.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

I'm not sure what radius I've always used but I know its on the round side.
I'd be skeptical to believe that the radius would have any noteworthy affect on the tone, but rather serves to prevent the top from collapsing overtime. The radius functions like an arc, which will resist the downward pressure from the bridge.

Thanks Woody .... That makes sense to me
I appreciate your sharing. The good thing is here in So Cal humidity is not as hard to control.
It really makes sense that flat tops would be more affected by the changes.

If you take a handsaw and play it like an instrument the more you bend it the higher the pitch. I don't know if it is like that with a guitar top but that's what it makes me think about...

28' top radius with a 1.5 degree neck angle.... seems to lay out very nicely with a low action.

Just curious.... has any one tried any type of compound radius for their tops? like 25 for the top to bottom axis and say either 20 or 30 for the side to side axis? I am going to be cutting some radius dishes soon and was thinking about trying one like this.
Brian

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Hi guys
interesting reading ....

I use 28' for my tops as it seems to give me a nice set-up angle. My backs are 15'. I only build steel string guitars.

28' top radius with a 1.5 degree neck angle.... seems to lay out very nicely with a low action.

Im assuming that you guys glue your utb flat with no radius ?

thanks .tomas

Hey, I'm here for you to tell me. Ya gotta remember I know nothing. So .... Will it be higher???

My personal opinion is that the tighter the radius (the lower the number), the stiffer the top becomes. If that's true, then it only makes sense that too tight a radius will sound "tight" like an overbraced guitar.

Personally, I built with a 28' radius top then sprang it onto flat sides on my current 000 build. So it's probably some compound radius that becomes a 28' radius near the center. I used a 16' radius for the back and sanded the sphere shape onto the back of the sides.

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Formerly known as Adaboy.......

I do. My tops have a 25 foot radius above the bridge, and are flat below the bridge. That is to compensate for the torque on the bridge from the string tension. I have always used level sides where the top joins, but I do radius the sides for the back.
I also use a compound radius on the back. It is about 15 feet across the grain, and 25 feet lengthwise.
Why? because the main reason for the back radius is to allow some room for the back to shrink if the guitar is exposed to low humidity. Nearly all that shrinkage is across the grain. Also, using the flatter radius lengthwise results in a much flatter joining edge, which will allow the back to vibrate more (plus IMHO it looks better).
With a perfectly flat top, there is no room for shrinkage. Expose the guitar to low humidity, and it is much more likely to crack than an arched top.
I haven't noticed a raising of pitch due to arching the top, at least with the system I use. My assymetrical top arch is probably equivalent to about a 35 foot radius, and I believe the flat sides make the top a bit looser.
But if you were to use more top radius (less than 20 feet, say) AND radius the sides to match, I am sure it would stiffen the top enough to raise the pitch.

If you mean the #1 cross brace above the soundhole, I put a slight arch in it (about 0.05").

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John

Hey Filippo I agree things are always more complicated than any simple analogy can describe.
I guess that is why I'm asking for input in the first place.
My thought is maybe only a few here really know why they use the Radius they use.

So far the only response with an analysis other than works good for neck angle was from Woody. Whether he is right or not I don't really know but my thought was that most things that I found to be under some form of tension usually brings out a higher pitch and I'm thinking the tighter the radius the greater the tension. So it kinda made sense to me. I'm sure that is probably not always true but in general somehow it feels right. Now I've done a few things in life the wrong way because at the time it felt right.

Now when Nils said I thought ....I bet it does have and responded with simplicity.

Anyway, I always do appreciate your extra layering of thought as guitar building is very complex and
the more input I receive the better chance of gaining understanding....

So, back to the original question ... What Radius do you use and do you have a reason for what you use?

Thanks John...



That is the first comment that has me thinking that a radius top might be worth the extra effort.
Yet many of the best builders use flat tops as their standard method.

You know my answer already....

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

and a flat top will definitely be my first but I want to understand what is the value of the different methods

I also figure people ought to know why they do the things they do even if it is just that's the way I was taught.


By the way I only made a back Radius dish not one for a top.