#1
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Top bracing on guitars with a tailpiece
When building a guitar that has a tailpiece, is the top braced differently since it will not have to bear the string tension in the same way as a guitar without a tailpiece?
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#2
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Yes, typically the bracing would be different. Manouche guitars tend to (more or less) be ladder braced with no x-brace. In addition they often have a pliage -- a sharp bend in the top that goes across the lower bout to help support the downward vertical pressure of a floating bridge.
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#3
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Archtops are tailpiece guitars and are obviously braced totally differently to flat top guitars, as are the Gypsy style guitars, as already noted. The bracing on most of a flat top top is designed to support the vertical and horizontal string load, which is actually pulling towards the neck and up on a regular flat top bridge, and to distribute the load and the string vibrations around the top. With a tail piece, the load is vertical down towards the body, but the string vibrations still need to be distributed. I would probably either use ladder bracing, a combo of parallel bracing with flying wings out to the side, or an X-brace shifted down so the X was under the "feet" of the bridge, rather than outside the bridge plate. I might include longitudinal braces not attached to the top, running between the neck and tail blocks, so the tail was tied directly to the neck to take up string tension loads. Except for ladder bracing, I do all of these on my archtops from time to time.
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Brian Evans Around 15 archtops, electrics, resonators, a lap steel, a uke, a mandolin, some I made, some I bought, some kinda showed up and wouldn't leave. Tatamagouche Nova Scotia. |
#4
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The main issue with a tailpiece is getting enough break angle over the saddle to keep the strings in contact as they vibrate. This doesn't require as much angle as some folks seem to think it does, but you do need 12-15 degrees of break. If the tailpiece is attached with some sort of 'gut', the effective pivot point will be at the edge of the top where the gut comes around the lower saddle. If there's some sort of a hinge that can be moved below the edge to some extent. For a 'normal' bridge height, with the strings 1/2" or so off the top, it can be hard to get enough break angle to work.
Some banjos use a tailpiece that extends out over the top a fair distance, and has an adjustment that presses it down toward the top, thus producing more of a break angle. Of course, in doing this you're also pushing down on the top. The same would happen if you put a tall bridge on the guitar, and tilted the neck back to get the action right. The added down pressure can collapse the top unless it's thick and/or heavily braced, and that added stiffness and weight don't do much for the sound (except add sustain...). Arch top guitars use the vault of the arch to fight the tendency of the top to collapse. Another strategy is to 'crank' the top, as has been said. The top is literally creased across it's width at, or slightly behind, the bridge, thus enabling the strings to break over the saddle top at a greater angle. Normally this is done by cutting a kerf on the inside surface of the top so that it can bend. The bend is usually reinforced sometimes with a cloth patch, and there's normally a fairly heavy cross brace just in front of the bend to help resist the down force. This can work well structurally for as long as the brace stays glued in place. Acoustically it breaks the top up into a couple of short sections, which doesn't allow it to vibrate well at low frequencies. A few people have used a bridge that works more or less like a classical guitar 'tieblock' bridge, or like the Ovation pinless bridge, to get the break angle, and tied the strings to a tailpiece. The objective is to take the tension load off the top allowing it to be more lightly made, with the hope that it will sound better. The old Stella guitars used this setup to convert a 6-string to a 12, more or less so far as I can tell, and I've been told that they didn't even beef up the bracing. The point, I guess, is that simply introducing a tailpiece may not solve many problems; there is a whole suite of features that need to be thought about. Bracing is one of them, but not the only one, or, maybe, even the most important. |
#5
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You sir, are a wealth of knowledge
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#6
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Quote:
However, I do think the pinless bridge/separate tailpiece idea is still well worth exploring. It does typically exist on various guitar-like folk instruments from Latin America and Spain, such as the Cuban tres, the Puerto Rican cuatro, and the Spanish bandurria. Not sure how these are braced. I have modified some cheapie guitars with a pinless/tieblock bridge to use a separate tailpiece, and it seems to be pretty satisfactory for mitigating a deforming top/separating bridge problem. There's still significant torsional vibrations imparted on the soundboard, but much of the force normally pulling on the soundboard is relieved by the tailpiece. Being unable to A/B compare before and after, I'd not be able to say there's a very noticeable change in tone. The ones I've done used both X-bracing and classical-style fan bracing. Last edited by Tim E; 09-10-2020 at 10:32 PM. |
#7
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Tim E wrote:
"I can't recall seeing old Stella 12 strings using this scheme of routing the strings through a "tieblock" of sorts. " I seem to recall having seen that in some pictures, and descriptions of same, but I have no direct experience with old Stellas, so I'll take your word for it. In the one picture I have handy of Leadbelly he's got his hand over the bridge.... "There's still significant torsional vibrations imparted on the soundboard, but much of the force normally pulling on the soundboard is relieved by the tailpiece." I've done a fair amount of work on the way strings drive normal 'tie' or pin bridges, and also arch tops with tailpieces. On a tie/pin bridge the torque signal the twice-per-cycle tension change puts in doesn't produce significant output power, but can change the timbre if the height of the strings off the top is changed. Arch tops with tail pieces don't seem to get any signal from the tension change. It might be hard to parse the input from a 'hybrid' where the bridge takes part of the string load, but in any case I suspect it's not a big power source. |
#8
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After I posted, I started thinking I had seen some photos of Stellas with the strings-through-tieblock design, but I've been at a loss finding a picture of one online. One possibility is a photo of Texan songstress Lydia Mendoza, who played 12 string, posing in 1948 with an instrument which clearly has the strings-through-tieblock design. However, I'm unable to tell if it's a Stella, as I simply don't know them very well. And the company's output isn't nearly as well documented as the big players of the time. FWIW, that instrument pictured may not even be hers, may have been a studio prop. There are plenty of photos showing her playing, but few that can pinpoint definitively what they were, or their design. It's possible she used instruments made by local luthiers who catered to the Latino population of the time, as there seemed to be at least a couple in Texas, more often remembered these days for their Bajo Sextos.
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#9
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Oh good: it's nice to know I may not always be wrong....
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#10
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The Forest Products Laboratory lists the mechanical properties of a wide array of woods ( https://www.fpl.fs.fed.us/documnts/f...tr113/ch04.pdf ). Included is a listing of ultimate tensile strength for many woods of interest to luthiers. The weakest soundboard wood in the listing is Western Red Cedar, with a tensile strength of only 6,600 lb/in^2. If we make the outrageous assumption that only the wood directly between the bridge and the tailblock is resisting string pull, and we assume a top that is 0.1" thick and a bridge that is only 4" wide, we find that this "weak" soundboard can support 2640 pounds of string pull. Total string tension of a 12 string guitar will usually not exceed 250 lbs.
Resisting the direct pull of the strings is NOT a significant issue when engineering the guitar bridge or string termination, whether at the bridge or at a tailpiece. Resisting the torque that the strings exert on the saddle/bridge/soundboard/bridgeplate is a significant issue if the strings terminate at the bridge. As MC5C notes, even light braces that have a significant longitudinal component and are positioned under the wings of the bridge are an effective method to manage that twist (aka "bellying"). If the soundboard is freed from neck angle control duties and bracing geometry limits bellying, all soundboard bracing can be light without compromising the instrument's integrity. Whether that light bracing improves the guitar's tone is a matter of implementation and taste. |