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Thread: Radius: Tops & Backs-Random Numbers, Designed Ratios?

  1. #1
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    Default Radius: Tops & Backs-Random Numbers, Designed Ratios?

    My hope is that the title of this post suggests that I've pored over everything I can find on this forum related to this subject and that I'm trying to better understand the thinking behind building choices.

    I know that Pete has reported using 22/12; Chuck, 25/15; Dom, 20/10 (http://www.ukuleleunderground.com/fo...ur-preference).

    Then there's Ken and Pete's Discussion of a Martin style "O" soprano- 5'2" or boat curve (http://www.ukuleleunderground.com/fo...-Dish-required)

    Discussions in general focus on structural integrity with reference to shrinkage/humidity, forces acting on the bridge and top, and by general reference to stiffness and plate thickness and bracing pattern. (Forgive the incomplete lists, I'm a real beginner).

    Here's what I'm asking the Pros: Why did you pick the radius numbers you use? How is the ratio of top to back radius used in your design thinking? Finally, how does the position of the radius "pole" factor into your design--under the bridge for the top? Different for the back?

    My questions were stimulated by Ken Timms remark sometime ago about radically changing radius "depending" and my reading of David Hurd's works. Apologies to both of you for distortion or simplification.

    If radiusing is "free stiffness" then it seems that it's a significant variable that, like bracing, effects sound as well as structure. What is your experience?

  2. #2
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    25/15 but don't over intellectualize this. Make some, decide what you like for your style and sound. It's too easy to spend too much time thinking and not enough time doing, especially when you're new to the craft.

  3. #3
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    Thanks for your encouragement, Rick. I am building and that process continues. I love this (and I don't have to earn a living doing it). These are the questions that get formulated in a separate process...best for late night when sawdust and tool clatter have given way to playing or reading. Again, thanks

  4. #4
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    I use a dished solara for the top and 15' for the back. I build Spanish style. The 15' back is just cos. Don't over think things.
    Liam Ryan.
    Cairns, Australia.
    Stump Jump Ukulele Co.
    Stump Jump on Facebook

  5. #5
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    I did a Little reading hear on dishes, and I ended up going out to the shop and making a 20' dish for tops and a 15' for backs... or close to that. So far I am happy with the result.

    I find with a 20' radius on the top I can set the neck at 1/2* off of 90* and the fretboard extension will sit flat to the top and I will also end up with a string height at the bridge of 1/2" off the top... at least on a tenor, I think it was still pretty close to that on the last concert I made, but I really don't rememeber, regardless I have not had a problem with the string height one any of the ukes soprano, concery, or tenor that i have built.

  6. #6
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    I build with the Spanish Heel and a dished solera only in the lower bout. Couldn't tell you what the radius is, 'cause I've not a clue. The dish on a tenor is about 2.5mm deep at the saddle position, but it tends to relax just a bit after the go bars come off the braces. 15' radius on the backs 'cause I've got 2 dishes that I made for guitars. 25' wasn't enough, so 15' looked good to me.

    There are just so many variables to add to the mix that I wouldn't worry too much, or at all about the radius you build with. Settle on one or a method and go for it. You'll have much more fun working on how thin to make the top, and the style, size and number of braces along with that bridge patch.

  7. #7
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    Alan, special thanks for your specs and candor. If I'm understanding you correctly, the depth of the dish (2.5mm) at the saddle in the lower bout of the solera, assuming something around a 9" lower bout width (22.86cm) results in a 8.57' radius. Maybe my estimates are off. If not, very instructive.

  8. #8
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    The dish in the solara isn't a radius, it's just dished out with a small plane. The dish starts just below the lower transverse brace and just in from the lower bout linings. This way the linings don't have to be cut to an ever-so-slightly obtuse angle. The upper bout is flat. When the back is glued on, the neck is set back using a small spacer the same thickness as the depth of the dish.
    Liam Ryan.
    Cairns, Australia.
    Stump Jump Ukulele Co.
    Stump Jump on Facebook

  9. #9
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    Liam has got it. Might have got it from me.

    In the Spanish Heel style I build, the upper bout is flat. Makes all kinds of things easier to deal with in a build. Upper and lower transverse braces are flat. It's just the lower bout that for lack of a better analogy ends up like a small pot belly. You know what they look like fellas. It's what happens to all of us when we hit 50.

    I use a 3 - 4mm or so shim right under the nut area to pitch the neck (depends on instrument scale length) when I go to glue on the back. This will force the upper bout to flair just a bit. Not noticeable when instrument is complete. Once back is glued on to holds the neck in this position.It puts a set to the neck so you can get proper sting hight over the soundboard and action at the 12th fret.

    If you do repairs on instruments with a Spanish heel, this is how you do a neck reset. You slip the heel on the back to get better action. (Simplified explanation, but you should get the idea.)
    Last edited by Allen; 02-06-2012 at 10:16 PM.

  10. #10
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    You guys are really being patient with me, thanks. I understand the dishing ending slightly lower than the brace and above the lower bout lining. With the depth of his dishing at 2.5 mm centered at the bridge, I assumed a spherical arc, used a hypothetical 9" lower bout width and ran a formula to calculate the resulting radius of the arc based on a ballpark "chord" length of 9". I used this calculator: http://www.liutaiomottola.com/formulae/sag.htm (tech editor for GAL). Sorry for my confusion. The Spanish heel and flat upper bout part I understand.

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