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| Question for the Engineers Re UTB... http://www-.luthiersforum.com/forum/viewtopic.php?f=10101&t=41759 |
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| Author: | meddlingfool [ Thu Oct 24, 2013 10:50 pm ] |
| Post subject: | Question for the Engineers Re UTB... |
Greetings.... There have been a few threads lately that has got me thinking. (Ouch). Is it a mathematical fact that the UTB can be taken all the way down to 1/8" at the ends where it meets the linings without losing it's strength? Does the UTB need to be resting either on the linings, or resting on the sides themselves to be effective, or is there a little wiggle room cutting its pocket? Thanks |
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| Author: | Tim McKnight [ Fri Oct 25, 2013 11:10 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
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| Author: | Barry Daniels [ Fri Oct 25, 2013 12:50 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
I think it is a bit more complicated than that. Reducing the cross section of the UTB at the ends does not weaken the brace as much as reducing it in the middle. I have not done an statics analysis of this, but it just make intuitive sense to me. One reason is that you would not expect as much load being applied to the ends of the UTB as might occur in the middle. Also, loads at the end of the beam are partially carried by the nearby sides. |
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| Author: | meddlingfool [ Fri Oct 25, 2013 1:38 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
So Todd, You notch into the linings but not through your sides? |
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| Author: | Tom West [ Fri Oct 25, 2013 3:17 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Over the years it has been stated that the fingerboard extension exerts a fair amount of downward pressure on the area that it covers on the top. Most extensions I have seen are relatively flexible and I doubt they are capable of much transfer of force in that direction. There are shear forces between the board and top from the rotation of the neck on fulcrum point at the top /side junction. I have a couple of guitars that are a number of years old and they do not have the extension glued to the top. They to this time seem to be acting normally. It just makes me wonder more about the effect the extension has on the top. I do use a decent size UTB and leave the ends at least 1/4" in height. Any other thoughts on the effect of the finger board extension. Tom |
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| Author: | murrmac [ Fri Oct 25, 2013 4:03 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Tom West wrote: Over the years it has been stated that the fingerboard extension exerts a fair amount of downward pressure on the area that it covers on the top. Most extensions I have seen are relatively flexible and I doubt they are capable of much transfer of force in that direction. There are shear forces between the board and top from the rotation of the neck on fulcrum point at the top /side junction. I have a couple of guitars that are a number of years old and they do not have the extension glued to the top. They to this time seem to be acting normally. It just makes me wonder more about the effect the extension has on the top. I do use a decent size UTB and leave the ends at least 1/4" in height. Any other thoughts on the effect of the finger board. Tom Excellent observation, Tom. Purely from a personal point of view, I am building my second (flat-top) instrument with a fretboard extension totally independent of the soundboard ...cantilevered with two CF reinforcing rods inset into the underside of the fretboard, with two routered slots in the soundboard to accommodate same. Credit to Randy Muth for this idea btw ...not a concept I came up with by myself ... I have never been totally happy with the whole "fretboard extension glued to soundboard " syndrome ... just so many things to go wrong ... |
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| Author: | meddlingfool [ Fri Oct 25, 2013 5:28 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Murmac... I was just in Edinburgh for fringe... Would have hit you up for pints had I known.... And now, back to you regularly scheduled program... |
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| Author: | John Arnold [ Sat Oct 26, 2013 1:31 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Instead of looking at this area as accepting downward pressure from the fingerboard extension, I see it in terms of the compressive force from the string tension, coupled with some downward force from bridge rotation, acting through the X-braces. The compressive force can do two things....depress the top, or cause the top to bulge outwards. The outward bulge (which is stabilizing due to the counteracting force from the bridge) can happen if the top has some radius in this area. A small amount of radius on the UTB is acceptable, and actually helps prevent the extension from dropping off when the neck is pitched back. This neck pitch on a typical Martin design is between 1 and 1 1/2 degrees. Tapering the ends of the UTB does not reduce stiffness as much as it reduces strength. This strength reduction is due to the grain runout that happens when the brace is cut across the grain in the tapered section. I like to leave the ends of my UTB's about 0,220" tall at the kerfing. They are fit snugly in a notch in the kerfing, but inside the sides. I have repaired many guitars that had sunken or cracked tops in this area, simply because the UTB and/or the X-braces were not tightly fitted in the kerfing, allowing the braces to peel loose from the top on the ends. Some of my early guitars featured a UTB that was black locust, which is one of the stiffest domestic woods. They could be made less than 1/4" wide, and still would be very stiff. |
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| Author: | nyazzip [ Sat Oct 26, 2013 2:39 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
if this UTB/upper transverse brace is made too heavily, what are the consequences? is it an overall diminished volume, or does it affect certain frequencies more? from what i read here, it sounds like the most important brace on a flattop guitar, almost akin to a "bass bar" on a violin/cello/et al... |
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| Author: | Clay S. [ Sat Oct 26, 2013 8:04 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
One point to consider is that the UTB is glued to the top, so assuming the peel strength is not exceeded the top should help distribute the load where the UTB meets the sides. I would not equate the UTB's function to that of a violins bass bar. It is supporting a relatively inactive part of the top (whether or not we argue that it is what helps make it inactive  ) I see it as mostly adding strength, and hard to "overbuild".
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| Author: | Alain Moisan [ Sat Oct 26, 2013 11:27 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Clay S. wrote: One point to consider is that the UTB is glued to the top, so assuming the peel strength is not exceeded the top should help distribute the load where the UTB meets the sides. +1 |
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| Author: | Tai Fu [ Sat Oct 26, 2013 11:36 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
I always felt the UTB in addition to the piece of spruce glued behind it is to prevent that classic fingerboard crack, ie. give it a bit more reinforcement to stop the top from cracking where it meets the fingerboard. Often guitars that had this crack usually have loosened glue joint due to heat, and in many cases these guitars do not have that spruce patch behind the UTB. It is my opinion that the patch does add quite a bit of strength, and together with the UTB should make the guitar fairly resistant to the fingerboard crack unless it was left inside a hot car under string tension. This time I decided not to overbuild the UTB as I had traditionally done, and will see how it turns out. However behind the UTB there is a spruce patch, in addition to a wooden block glued on top of that to sort of support the exposed truss rod that extends past the neck. This should provide a very strong backing for the fingerboard extension and hopefully makes installing frets easier. |
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| Author: | meddlingfool [ Sat Oct 26, 2013 12:01 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
The UTB is the one part of the guitar that for me has no upper limit to the amount of stiffness I'll accept. |
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| Author: | Tai Fu [ Sat Oct 26, 2013 12:09 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Well if you want to go overkill I guess you could always glue a 1" thick piece of maple as UTB... |
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| Author: | murrmac [ Sat Oct 26, 2013 5:59 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
meddlingfool wrote: Murmac... I was just in Edinburgh for fringe... Would have hit you up for pints had I known.... And now, back to you regularly scheduled program... Next time you are in Edinburgh, give me advance warning, and we can debate the stiffness of UTB's ( among other issues) over a pint or two in the Hanging Bat, which just might be the best real ale pub in the world. |
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| Author: | murrmac [ Sat Oct 26, 2013 6:25 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
John Arnold wrote: Tapering the ends of the UTB does not reduce stiffness as much as it reduces strength. This strength reduction is due to the grain runout that happens when the brace is cut across the grain in the tapered section. You have gotten me confused now, John. As an old site carpenter, I kind of visualize guitar top bracing in the same way as I perceive flooring joists (strength ways at least) and back in the day there were no qualms whatsoever about reducing the depth of a joist at the ends ...didn't reduce stiffness or strength one whit ....but maybe the analogy is not in fact appropriate ... |
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| Author: | ChuckB [ Sun Oct 27, 2013 10:21 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Todd, have you seen many UTB's shear where it was tapered to 1/8" at the linings? Assuming that the UTB is not undersized and the UTB has a tight fit in the lining pocket up to the side but not through. Thanks Chuck |
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| Author: | Alex Kleon [ Sun Oct 27, 2013 10:34 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Would a laminated UTB be practical, if it were the same dimensions and mass or a solid one? Say 3 or 5 laminations? If it is practical, what would be the best configuration of the plies? Sorry to get off track. Alex |
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| Author: | Alain Moisan [ Sun Oct 27, 2013 10:54 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Todd Stock wrote: - I taper UTBs to 1/4" at the ends, and ensure they are well supported at the rim. I do this because of the types of failures I have seen on UTBs and related structures have causes in either excessively limber UTBs or failure to support the ends of the UTB and successfully transfer the loads applied to that brace to the rest of the structure. For myself, I have never seen a UTB failure, regardless of the way the ends were taper of fit to the linings, that wasn't the result of an accident (a blow or the guitar being dropped). Have you seen such a failure only due to the normal loads the neck joint applies on the area? |
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| Author: | Tai Fu [ Sun Oct 27, 2013 11:57 am ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Only UTB failure I have ever seen is from damage due to heat or impact. |
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| Author: | Alan Carruth [ Sun Oct 27, 2013 2:28 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
I've replaced UTBs in old Gibsons a couple of times. They used to taper the brace ends down to almost nothing, and then rely on the liners crushing at the brace ends when the plates were glued on, rather than actually bothering to cut inlets. Sometimes it was the end of the brace that crushed instead. I'm sure there is a down force on the UTB for the simple reason that when those old Gibby's came in, the neck was pointing up and the area above the soundhole was depressed, and there was enough peeling stress to cause the brace to come unglued. I'll note that the only times I ever saw that degree of distortion were those cases where the brace was broken off short of the liner and peeled up: apparently it does not take much wood to withstand the down load at the ends so long as it's properly supported by the liner. The upper bout on the guitar is much more about air vibration than wood I think: that area of the top doesn't do much until you get to the high frequency stuff, and the brace is close enough to 'rigid' that fairly large changes in it's stiffness probably won't matter. I have to say that this is an opinion (albeit, I hope, a reasonably informed one); I don't have data from controlled comparative studies on this. Since I use an 'A' brace to resist the compression load of the strings on the end of the top I don't feel the need to use a really heavy UTB. OTOH, I use one, don't usually pare the ends down lower than, say, 1/4", and see to it that they're properly supported in their inlets. OTOH, I don't see the use of a large UTB as a liability. The difference between the lightest and the heaviest one you might see would be ten or twelve grams. That would be a lot down in the middle of the lower bout, but where the UTB is it's hardly an issue, IMO. I can't see it 'soaking up a lot of tone' because it's not moving to speak of. It could even be that having it heavy would help the tone. Again, without data to go on it's just speculation. |
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| Author: | Parser [ Sun Oct 27, 2013 3:05 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
There's no doubt that the structure is stronger if the UTB is pocketed into the sides and is firmly supported in all degrees of freedom. As for the brace itself, the thicker the brace is, the stronger it is. I think the real question - as some have alluded to - is how strong does it really need to be? |
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| Author: | Jeff Highland [ Sun Oct 27, 2013 3:40 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Don't forget the other functions of the UTB Besides resisting movement perpendicular to the soundboard, it is also a major player in distributing force around the soundhole. One detail that I see on some makeys UTB's that concerns me is a large hole for truss rod adjustment, coupled with aggressive rounding of the profile which can leave only a small sliver of wood at the inner face. |
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| Author: | murrmac [ Sun Oct 27, 2013 4:21 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
Thanks to Todd for his elegant and lucid explanation of the various terms... stresses/strains/stiffness/strength etc... Like I said earlier , I am just an old carpenter with antediluvian intuitive assessments of where the stresses and strains occur in any structure ....floors...roofs...guitars ...it's all one to me. Which brings me to another question (which is in fact related to the UTB issue) Some builders ( Rick Turner being perhaps the most notable example) use CF rods or tubes at the neck block to create a triangulation to counteract the deforming stress caused by the string tension. Now, as far as I am aware, (and I may be wrong here, and am open to correction) these Cf reinforcements always run from the top of the neck block to the brace on the back, (the one underneath the soundhole) My question here is ...(and is stimulated by my experience of having made dozens if not hundreds of ledged and braced wooden gates)..would it not be a more efficient construction if the CF tubes/rods were to be anchored on the base of the neck block, and angled upwards to rest in the ends of the UTB ? EDIT: I should have done a Google search earlier, but I see that Kent Chasson has implemented exactly what I am talking about ..no doubt he will stop by and comment ... |
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| Author: | Jeff Highland [ Sun Oct 27, 2013 10:45 pm ] |
| Post subject: | Re: Question for the Engineers Re UTB... |
murrmac wrote: Thanks to Todd for his elegant and lucid explanation of the various terms... stresses/strains/stiffness/strength etc... ) My question here is ...(and is stimulated by my experience of having made dozens if not hundreds of ledged and braced wooden gates)..would it not be a more efficient construction if the CF tubes/rods were to be anchored on the base of the neck block, and angled upwards to rest in the ends of the UTB ? ... Or even better if you used some bracing from the top of the neck block, fixed to the soundboard and running around the soundhole towards the bridge................like a martin A brace or functional equivalent |
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