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| Brace shaving -- a cautionary tale http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=27653 |
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| Author: | truckjohn [ Tue Jun 01, 2010 10:09 pm ] |
| Post subject: | Brace shaving -- a cautionary tale |
Hey all, I picked up this guitar on the cheap this past winter for a retop project -- Fender Athena Dread... Why on the cheap? Well.... The bridge was mostly pulled off and the top was split in front of the bridge.... I figured it was just another example of a cheap Asian guitar with a bad bridge glue joint... until I put my hand inside.... Turns out that someone decided to do a little brace shaving inside there.... The lower X legs were *Hugely* scalloped... Directly under the bridge... The center line of the thinnest area of the scallop is *RIGHT* at the saddle centerline.... and the scallop is maybe 3-4" long total... Anyway, I will take some pix when I get around to routing off the top.. I feel like it is good to show the wreckage that people cause when they do a little too much of a good thing... Attachment: Fender Athena 1.JPG Thanks John |
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| Author: | John Platko [ Wed Jun 02, 2010 10:59 am ] |
| Post subject: | Re: Brace shaving -- a cautionary tale |
A little reality is always nice to see. After the FEA posts of top displacement I'm ready to believe that tops don't need braces! John |
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| Author: | Pat Hawley [ Wed Jun 02, 2010 11:34 am ] |
| Post subject: | Re: Brace shaving -- a cautionary tale |
What size is the bridge plate? It looks to me like the crack in front of the bridge would be in the bridge plate area which I don't understand regardless of the over-shaved braces. Pat |
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| Author: | truckjohn [ Wed Jun 02, 2010 1:02 pm ] |
| Post subject: | Re: Brace shaving -- a cautionary tale |
That instrument has a sufficiently large bridge plate. Similar to a modern Martin D28 -- Certainly not the giant plank of lumber like an old Double-X Gibson or a modern cheap Tak (Which looks like it starts as a 6"x8" plate and covers everything from the X-intersection to the 1st tone bar...) I did some "Back of the Envelope" stress/strain and torque calcs on the guitar top... Granted, they fell into the "Quick and dirty" method bucket.... but they did tell me something interesting... Turns out the Top alone should have more than enough strength to resist the string pull alone... It is the "Buckling" effect from the bridge rotation that gives us hang... My own current personal opinion (And it's nothing more than that currently...) is that (Ignoring tone) the structural function of most of the bracing on the top is to prevent the thin top plate from buckling... or in simple terms -- to help the top retain it's flat or domed shape... Think about the party trick -- a grown man standing on an empty beer can.... The empty can will support his weight.. so long as the sides are intact... Tap the side and the whole can buckles and smooshes into a pancake... This is how I currently think of the top/bracing system.... Thanks John |
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| Author: | John Platko [ Wed Jun 02, 2010 1:52 pm ] |
| Post subject: | Re: Brace shaving -- a cautionary tale |
Wouldn't the top being connected to the sides be enough to prevent buckling? |
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| Author: | Alan Carruth [ Wed Jun 02, 2010 2:49 pm ] |
| Post subject: | Re: Brace shaving -- a cautionary tale |
Truckjohn wrote: "Turns out the Top alone should have more than enough strength to resist the string pull alone..." Bingo! If the load on the top were purely tension/compression, and the top could resist buckling, it would be plenty strong enough to take the pull of the strings. You'd only get that kind of load if the strings were in the same plane as the 'center of moment' of the top: the imaginary surface halfway between the upper and lower surfaces of the wood, AND if the top were flat. I can pretty well guarantee that you won't see any actual guitars built that way. John Platko asked: "Wouldn't the top being connected to the sides be enough to prevent buckling?" Eventually as the bridge twists upward there would probably be enough tension on the area of the top behind it to stop the distortion. I've seen a few tops that were made without bracing, or with very light bracing 'to get more sound'. They might sound remarkable at first, but then, a foghorn in a Beethoven symphony would sound 'remarkable'. They don't hold up very well, though. Now, in theory, you _could_ make an unbraced top that was stiff enough to resist bridge torque for as long as you'd like; all you have to do is leave it thick enough. I'm not sure how thick that would be, but I am pretty sure that it would be too thick and heavy to sound like much if it was solid wood. |
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| Author: | John Platko [ Wed Jun 02, 2010 5:26 pm ] |
| Post subject: | Re: Brace shaving -- a cautionary tale |
I don't know if you saw this thread, Al viewtopic.php?f=10101&t=26735 At the end Jim Watts showed the results from his FEA analysis of a braced guitar top under string load. It moved very little. He confirmed his analysis with measurements. I get similiar results with my own crude FEA model. I'm still scratching my head over this. It seems that the guitar tops are built plenty strong for the static forces at the bridge. Any idea what I'm missing here? John |
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