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| Buttress, or heel to tail arch? http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10102&t=14541 |
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| Author: | Jim Kirby [ Thu Nov 15, 2007 12:58 pm ] |
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I didn't want to hijack the very interesting buttress thread, so I thought I'd ask my question separately. I've been building classical guitars lately, and any deep investigation into that genre automatically brings up the question of the Smallman design, where you are building the body as a reflex speaker, with a totally rigid structure aside from the top (which is basically an acoustic suspension speaker) and using the soundhole as a port for the speaker system. One of the main objects of this is to be able to build a top with a thickness on the order of a mm. My question is - what if you didn't want to build such a rigid structure, but still want a top a top that can't do anything more than resist the tension and torque exerted at the bridge location by the string pull? Would a simple internal arch spanning from the heel block to the waist and then to the tail block suffice? I'm inclined to think it would, but I haven't built a test instrument yet. Every time I see a picture of a heel block buttress tied back to the waist, I think right away that it should just be an arch all the way back to the tail, to carry the tension through the entire body. Am I deluding myself somehow? Jim |
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| Author: | Rick Turner [ Thu Nov 15, 2007 1:15 pm ] |
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Look at a "Bridge Doctor"...Google it... And why an arch? It's going to flex more than columns under compression. Look at the arched buttress in a Baby Taylor...they flex...arches are wonderful if they're engineered correctly, but just because you throw in an arch does not mean it's stiff. Think triangulated space frame... |
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| Author: | Rick Turner [ Thu Nov 15, 2007 1:20 pm ] |
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Oh...the Smallman design is not an "acoustic suspension speaker"; if there's a speaker analogy, it's a bass reflex. Acoustic suspension speakers are closed boxes with no ports; they have very compliant cones and rely on the spring of air compression for excursion control. They tend to be made with relatively small diameter long throw cones with either underwound voice coils and large magnet gap length or overwound coils. They suck up a lot of amp wattage compared to bass reflex cabinets, too. They're a variant of infinite baffles. |
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| Author: | Jim Kirby [ Thu Nov 15, 2007 1:27 pm ] |
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Well, but the segment of the arch from heel to waist wouldn't be that much more relatively curved than a straight column, so I don't really see any difference there. But maybe arch is the wrong word - I'm thinking a continuous support from heel to tail, whether it be arch or two columns. (I imagine it as a plywood frame, as in the internal structure of a Smallman, but less all-encompassing, and without the assumption of a totally rigid side and back structure.) A set of "columns" from heel to waist and then from waist to tail wouldn't be carrying the compressive load over a straight vector either, so I don't see the problem with an arch. The bridge doctor isn't relevant to the discussion either, as far as I can tell. I'm not talking about supporting the torque on the bridge by any other than the usual means. I was thinking more in terms of supporting the tail block when the top has a minimal level of it's own structural stiffness. |
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| Author: | Rick Turner [ Thu Nov 15, 2007 1:29 pm ] |
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What you're talking about has a long history in the guitar world. Go for it. |
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| Author: | Jim Kirby [ Thu Nov 15, 2007 1:29 pm ] |
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[QUOTE=Rick Turner] Oh...the Smallman design is not an "acoustic suspension speaker"; if there's a speaker analogy, it's a bass reflex. Acoustic suspension speakers are closed boxes with no ports; they have very compliant cones and rely on the spring of air compression for excursion control. They tend to be made with relatively small diameter long throw cones with either underwound voice coils and large magnet gap length or overwound coils. They suck up a lot of amp wattage compared to bass reflex cabinets, too. They're a variant of infinite baffles.[/QUOTE] Rick, yes, my bad. I meant bass reflex. |
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| Author: | Jim Kirby [ Thu Nov 15, 2007 1:34 pm ] |
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Or, maybe the bridge doctor can add to the overall structural support, again reducing the load on the top? At some point, it starts to sound like trying to build a banjo (sorry for the expletive). Then again, I've heard Smallman guitars called similar things! |
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| Author: | SteveCourtright [ Fri Nov 16, 2007 2:43 am ] |
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I recall, and maybe this is not what you were thinking, (not to mention old news) that the Larsson Bros. did something like this idea by directly coupling the neck and heel block together in their steel strings. In one instrument I saw, there was a steel rod which connected the neck directly to the heel block and ran straight down the center of the body (visible through the sound hole). Not elegant by any means, but one example that does support the concept that comes to mind. |
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| Author: | Dennis Leahy [ Fri Nov 16, 2007 7:13 am ] |
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My next 5 guitars, including 3 prototypes (in progress - just started) will have a pair of wood dowels from neckblock to tailblock. I did a pair of triangles on my last one, but kept the force vectors away from the guitar sides. I think that my suspended bracing system with triangles was overly complex, and the straight dowel rods should do the same thing in 1/10th the fitting/asembly time. However, for what you are doing, the shear force of the strings still stops at the bridge, so the top and/or top bracing must deal with that force or the top will (could/can/might?) distort. I did see one classical builder that uses a tailpiece, and that, (along with suspended bracing neck to tail), would indeed resolve the strings' shear force away from the bridge (and allow you to get more experimental with top thickness and top bracing.) Caveat: This ain't Al Carruth, Roger Siminoff, or Ervin Somogyi talking here - it's me, a rank beginner luthier trainee with a lot more ideas than experience. So, take it all FWIW. But, I applaud your experimental query and hope you pursue it, Jim. Dennis |
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| Author: | John Cavanaugh [ Fri Nov 16, 2007 7:48 am ] |
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What if you ran a truss made of carbon fiber rods from the neck block to the heel block? It would be strong and stiff and you could glue it into holes in the blocks with epoxy. There's a company in Minnesota building an electric guitar with a body made from carbon fiber trusses. They say they do it that way for the stiffness. |
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| Author: | Rick Turner [ Fri Nov 16, 2007 7:58 am ] |
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Anyone ever see a banjo? |
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| Author: | Dennis Leahy [ Fri Nov 16, 2007 8:40 am ] |
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[QUOTE=Rick Turner] Anyone ever see a banjo?[/QUOTE] What do you mean? I don't get why this relates to a banjo. Dennis |
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| Author: | Rick Turner [ Fri Nov 16, 2007 8:50 am ] |
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Dowel stick... Coordinator rods... The way I see the long rod vs. triangulated buttresses is that the triangulated system is more stable, and it's not all that difficult to do. I also like having the sides and the back do some of the work. |
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| Author: | Jim Kirby [ Fri Nov 16, 2007 9:35 am ] |
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[QUOTE=Rick Turner] Anyone ever see a banjo?[/QUOTE] Copycat! |
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| Author: | Jim Kirby [ Fri Nov 16, 2007 9:41 am ] |
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For those mentioning trusses or rods straight from neck to tail - I do want to keep this invisible to the casual observer. I need to develop some CAD skills so I can draw something like this up in reasonable time. Seems like, if the computer task doesn't involve a bunch of really ugly partial differential equations mapped onto memory and storage across dozens of processors, I've exhausted my skill set 
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| Author: | Dennis Leahy [ Fri Nov 16, 2007 10:49 am ] |
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[QUOTE=jtkirby] For those mentioning trusses or rods straight from neck to tail - I do want to keep this invisible to the casual observer. [/QUOTE] Jim, I don't want to post the photo again, but I showed an example of triangulated neckblock to tailblock braces (that would be invisible to the casual observer, if you have a *normally* placed sound hole): Anyone Played One of These (thread) Dennis |
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| Author: | Jim Kirby [ Fri Nov 16, 2007 11:09 am ] |
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[QUOTE=DennisLeahy] [QUOTE=jtkirby] For those mentioning trusses or rods straight from neck to tail - I do want to keep this invisible to the casual observer. [/QUOTE] Jim, I don't want to post the photo again, but I showed an example of triangulated neckblock to tailblock braces (that would be invisible to the casual observer, if you have a *normally* placed sound hole): Anyone Played One of These (thread) Dennis[/QUOTE] Dennis - yes I caught that just now. Very similar to what I was thinking of. The tail of that guitar is a little too much of an anthropomorphic trick for me to be a regular feature. But the support system was very similar. I'd like to try a classical version of it. |
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| Author: | Jim Kirby [ Fri Nov 16, 2007 11:10 am ] |
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p.s. Dennis, do you know how much your internal structural support weighs? |
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| Author: | Dennis Leahy [ Fri Nov 16, 2007 11:17 am ] |
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Yeah, that tail/butt was a royal pain in the butt to make, as well.  I won't do another that way. It was half whimsical and feminine, and half because I needed the guitar to indent to be able to get a string from the G string tuning machine to the butt (with a built-in tailpiece.)
Dennis |
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| Author: | Dennis Leahy [ Fri Nov 16, 2007 11:46 am ] |
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[QUOTE=jtkirby] p.s. Dennis, do you know how much your internal structural support weighs?[/QUOTE] Too much! It was my first guitar, so keeping with newbie tradition, I overbraced it. However, only the suspended braces were overbraced, so the guitar is responsive. I was attempting to handle all 180 pounds of shear force within the suspended bracing. How much of the force are you trying to handle?
I have CAD drawings for all of it, and so I could calculate the volume times a typical mass for White Spruce to come up with an answer, but I did not weigh it. I used 1/4" x 3/4" stock. I probably could have gone down to 3/16" x 5/8", and maybe I could have made it parabolic on top and bottom edges to reduce the weight even more. Another possibility As it turns out, all the guitars I'm designing will have offset soundholes, so for me, the centrally located fore-to-aft dowels will be relatively difficult to see for the casual observer. Dennis |
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| Author: | Kent Chasson [ Fri Nov 16, 2007 12:37 pm ] |
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Jim, this weighs in at less than 50 grams and makes an incredibly rigid box in the direction of string pull. I should probably give someone credit for this idea but I don't know who. I've seen lots of different approaches to a rigid box from lots of different folks. I settled on this because it's simple, light, very strong, and as you say, it doesn't look too odd to the casual observer. It also keeps the back involved which I want.
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| Author: | Kent Chasson [ Fri Nov 16, 2007 12:38 pm ] |
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Oh, when I say 50 grams, I mean the carbon tubes and waist blocks. |
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| Author: | Dennis Leahy [ Fri Nov 16, 2007 2:49 pm ] |
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Kent, That's really elegant and minimalist execution. Excellent! Dennis |
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| Author: | Jim Kirby [ Sat Nov 17, 2007 1:01 am ] |
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Kent - That may be just the thing. I have pondered on this addition to the neck to waist buttress. The only concern I had was that there is actually no rigid link in the system that is under pure compression, so the resistance to moving the tail closer to the neck here is carried by the joints in the waist block, rather than the rods themselves (i.e., it the rod ends were pinned rather than fixed, the structure as shown could still collapse.) In a guitar with a top of usual thickness and bracing, this movement is countered (particularly in a classical) by direct compressive or tensile loading of the transverse braces or the top itself. But, if these braces are not there or minimized, and the top is really thin, ?? I envisioned a plywood insert with a continuous arch replacing your two straight runs, and a cross-piece below where the transverse brace on the bridge side of the soundhole would be, that would be intended to fight the tendency of flex in the arch to narrow or widen the waist. Thanks for all the comments, everyone. Jim |
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| Author: | grumpy [ Sat Nov 17, 2007 3:41 am ] |
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My I ask, and I ask this in total honesty, so don't anyone get all worked up, what the end goal is? By that, I mean, what is the desired end result of all this engineering? And why? |
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