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| Confused about back bracing http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=11294 |
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| Author: | Philip Perdue [ Thu Mar 15, 2007 6:15 pm ] |
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I have recently seen photos of back bracing that are quite different. I have seen at least one back that had really wide bracing and one that had really tall and th in bracing. I think that I like the thin tall bracing myself but have a question. Although most of us likley overbrace I have a wonder about the thin tall bracing. Is there a point where the bracing can be too thin to give the support needed for the back. I know that if it is thin and tall it is at minimum as strong as a lower but wider brace. I just wonder if there is a minimum surface area needed to support the back. Thanks Philip |
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| Author: | Sam Price [ Thu Mar 15, 2007 10:52 pm ] |
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Great question, although a little late for me now, as I have braced and profiled my back. I would also like to chime in and ask whether a stiffer wood, say maple would make great back bracing, and would it need to be as "high" as a softwood? Yes, I did watch some of Ervin's vids when they were available on this site... |
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| Author: | Brock Poling [ Fri Mar 16, 2007 12:08 am ] |
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On the average thinner and taller is much stronger than flatter and fatter. The strength of the brace increases by the cube of the height, while the strength only doubles for each unit of width you add. So imagine you have a brace that is 1 unit high and one unit wide. If you double the height it is 8 times as strong as the original (2^3) While if you make it the same height as the original but double the width it is only twice as strong. This is why you can gain strength by making braces really thin and tall. You can remove a lot of weight from the brace by thinning it without sacrificing (much) strength. But small changes in height have a profound effect. With respect to the back braces, mine generally run between 1/2" and 7/16" tall on SJ size guitars. I suspect I could go lighter though. |
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| Author: | tippie53 [ Fri Mar 16, 2007 4:06 am ] |
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Brock is right on the explaination. The difference I see is that stiffer ins't allways stronger. Thinner and high bracing take strength from the lateral and adds it to the vertical. Since we are not building bridges you want to match the need you have for what you are building. I use both bracing styles . I like to build vintage spec pieces for my clients and often use the older style martin "broomstick" bracing. This will allow the back to have more influence to the tone. The thinner bracing is more stable and will help to avoid the problems with the old style bracing that is affected by humidity changes. Both have thier place . john hall blues creek guitars |
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| Author: | gozierdt [ Fri Mar 16, 2007 5:54 am ] |
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From an engineering standpoint, there is a limit to how small (thin) you want the beam to be across the width. There is phenomenon known as "buckling", that occurs when a vertically stressed beam bends (buckles)to the side, rather than slowly curving in the vertical plane. Buckling produces a very sudden decrease in strength. You can demonstrate this in your shop by taking a piece of clear wood and cutting it about 3/4 x 3/8. Clamp one end to the bench with the other end overhanging. Pull straight up, and the beam will slowly deflect vertically. Now cut the width down to something like 3/4 x 1/16. As you pull vertically, the beam will start vertical, then suddenly twist and buckle to the side. There aren't any hard and fast rules as to how wide to keep the beam, but I like to keep the width at a minimum of 25-30% of the height. I also taper them to lighten them up, while keeping the better part of the strength in the vertical plane. |
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| Author: | CarltonM [ Fri Mar 16, 2007 8:45 am ] |
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I've always found the tall and thin bracing style to be more appealing visually and intellectually (less mass, right?). However, recently Alan Carruth offered a short statement that the wide and flat back braces give the guitar more bass response. That's certainly got me rethinking old opinions; and asking, "Who WAS that masked man?!" |
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| Author: | Alan Carruth [ Fri Mar 16, 2007 9:35 am ] |
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Lute tops commonly used bracing that was 5 times as tall as it was wide, so that probably sets an upper limit for that. Tall narrow bracing will tend to be _stiffer_ than low, wide bracing, for the given weight, as has been said. It may not be _stronger_. A high stiffness to weight ratio seems to be very important in the top. It's not at all certain that's the case for the back. Backs and tops do different jobs. In fact, there are probably a lot of diferent jobs the back can do, and every maker figures out which ones they're most interested in. Try some different things and see what works best for you. |
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| Author: | nickton [ Fri Mar 16, 2007 7:53 pm ] |
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I could also see that over time if a brace were so tall and thin as to make it more delicate and vulnerable to something maybe falling on it and accidentally cracking it, you'd have another problem. Durability is a factor to consider. I know somewhat heavy little items can over time have a propensity for falling in a guitar hole and tossing around inside- like maybe a steel slide or capo. I hope that doesn't sound too.... 
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