The Acoustic Guitar Forum

Go Back   The Acoustic Guitar Forum > General Acoustic Guitar and Amplification Discussion > Build and Repair

Reply
 
Thread Tools
  #76  
Old 12-17-2017, 11:58 AM
Rodger Knox Rodger Knox is offline
Registered User
 
Join Date: Jul 2013
Location: Baltimore, Md.
Posts: 2,431
Default

Quote:
Originally Posted by LouieAtienza View Post
And yes you are correct about the plate frequencies; I stand corrected. But even so, both plates are not vibrating sympathetically to each other, meaning in a "mirrored" pattern.
That's exactly correct. That "mirrored" pattern is the main air resonance, which disappears when a soundpost is present, it essentially makes the volume of the box a constant, and air is not pumped in and out of the soundhole.

Quote:
Originally Posted by Bruce Sexauer View Post
While the rigid bridge may not "peel" away from the top, the flexible top may be thought of as peeling away from the bridge . . . Perhaps.

Properly made Violin family bridges come very close to bisecting the string's break angle, and thus are driven nearly straight into the top plate.

There is a great deal of what I think of as imperfectly understood conceptual information in this thread.
Very elegntly stated, and if I might quote Frank, the Crux of the bisckit.
__________________
Rodger Knox, PE
1917 Martin 0-28
1956 Gibson J-50
et al
Reply With Quote
  #77  
Old 12-17-2017, 12:49 PM
LouieAtienza LouieAtienza is offline
Registered User
 
Join Date: Feb 2013
Posts: 4,617
Default

Quote:
Originally Posted by charles Tauber View Post
Then “peeling” incorrectly describes the phenomenon that causes separation of the bridge from the top at the glue joint.

The reason for tapering brace ends, as I understand it, is to reduce/eliminate stress conentrations caused by sudden changes in cross section.

Two rigid bodies adhered together can be separated without distortion if the separation forces, in this case a moment, exceed the strength of the adhesive or materials being adhered. Distortion is not a prerequisite for the failure. The question is then how significant a factor in guitar bridge failure is due to the distortion and “peeling” vs. Simply exceeding the material or adhesive strength, or creep of the adhesive. Since hire glue is reported to not creep, it would appear that creep could be removed as a cause in such joints.
I never said that distortion was a prerequisite for failure. Apparently to me it does contribute to it. I did mention catastrophic failure as well. It could very well be that the top deformation causes the bridge to "tip" enough that the "pulling" force overcomes the strength of the glue joint (or wood fibers) in that localized area, which then creates a "chain reaction" of sorts of bridge tipping further, pulling away from the top further, causing it to tip further, etc. Whether it's actually "peeling" I'm not sure, but that is my observation and I was careful to put it in quotes.

I would wager that if you affixed a hardwood board to your wall, and glued a bridge to it, whether with TiteBond or hide glue, you (and possibly I) could hang off that bridge without incident. I would contend if you affixed that same board to the ceiling, you could hang off the bridge.

If stress concentrations were of such concern in bridge design that the wings were tapered, then how come not the back of the bridge? Is it because the bridge plate extends past the bridge back, moving the localized bend point away from the back of the bridge? I'm pretty sure bridge "tipping" has been known ever since the advent of the fixed bridge.

For an extreme example, take two 1/2" plates of Lexan. Fix one on a table, spritz some water on the surface, then press the second sheet down. It would be all but impossible to lift off. You may be able to push or pull it front to back or side to side. Now, do the same experiment, but replace the top sheet with 1/16" Lexan. You still couldn't lift that sheet straight up, but if you got a hold of a corner and bend it up, it would then take little effort to "peel" that thin sheet away. Now take that same 1/16" sheet, and glue bracing over the sheet resembling a box frame and joists as on a ceiling. It would likely behave closer to the 1/2" sheet as far as trying to pull it off the bottom sheet, but it would be slightly more flexible, that if you pushed it far enough over the edge, you could bend the sheet enough that it starts to "peel away" from the bottom sheet. This "top sheet" represents the top of the guitar, and the bottom sheet represents the bridge. Since the bridge is thicker than the soundboard by a factor of about 3 or 4, and made of a denser material, it's unbendable in relation to the soundboard.

My point is, if you mitigate flexing of the top at the point of the back of the bridge, then the bridge and soundboard will tip "together" as if it were on a hinge at that point Dave Malicky describes, leaving any failure in the joint as a result of either glue failure or wood fiber failure. From my ear with my limited experience, the difference would be that the guitar would sound less "vintage" and "dry" for lack of better word, but maybe tighter in the bass. I feel the strings feel a little more "stiff" as well, regardless of gauge.

Quote:
Originally Posted by Bruce Sexauer
While the rigid bridge may not "peel" away from the top, the flexible top may be thought of as peeling away from the bridge . . . Perhaps.
I think this is more eloquently stated than I tried... Thanks Bruce.
Reply With Quote
  #78  
Old 12-18-2017, 05:46 AM
hat hat is offline
Registered User
 
Join Date: Jan 2013
Posts: 1,371
Default

Quote:
Originally Posted by Rodger Knox View Post

There are no good ideas in this thread, but feel free to try the string through and sound posts. You might like a guitar with no bass response. I'm done here.
Comments quite unnecessary sir!
This thread was started as nothing more than an exercise, a discussion of what might be possible, and why or why not. And I feel it was a very good discussion, with a lot of very fine fellow's inputting their expertise and experience into it, including yourself. My 'Hat's ' off to all for a fine discourse!
__________________
______________
---Tom H ---
Reply With Quote
  #79  
Old 12-18-2017, 09:26 AM
charles Tauber charles Tauber is offline
Registered User
 
Join Date: Feb 2011
Posts: 8,381
Default

Quote:
Originally Posted by LouieAtienza View Post
I never said that distortion was a prerequisite for failure. Apparently to me it does contribute to it. I did mention catastrophic failure as well. It could very well be that the top deformation causes the bridge to "tip" enough that the "pulling" force overcomes the strength of the glue joint (or wood fibers) in that localized area, which then creates a "chain reaction" of sorts of bridge tipping further, pulling away from the top further, causing it to tip further, etc. Whether it's actually "peeling" I'm not sure, but that is my observation and I was careful to put it in quotes.
Fair enough.

Quote:
My point is, if you mitigate flexing of the top at the point of the back of the bridge, then the bridge and soundboard will tip "together" as if it were on a hinge at that point Dave Malicky describes, leaving any failure in the joint as a result of either glue failure or wood fiber failure.
Sounds plausible.
Reply With Quote
  #80  
Old 12-18-2017, 12:30 PM
John Arnold John Arnold is offline
Registered User
 
Join Date: Jan 2009
Posts: 4,082
Default

Quote:
Is it because the bridge plate extends past the bridge back, moving the localized bend point away from the back of the bridge?
That is not the case with most Martins made before the large rosewood bridgeplate arrived in 1969.
IMHO, guitars sound better when the bridgeplate does not extend past the bottom edge of the bridge, so that is the way I build them. in any case, a conventional bridgeplate with the grain perpendicular to the grain in the top is not the best for reinforcing the top against peeling. That is because most woods are more flexible across the grain than with the grain. Spruce is much stiffer along the grain than across, making it very suitable for resisting the peeling in the direction of string pull.
The key is to choose wisely....both the stiffness of that particular piece of spruce, and the final thickness. Couple that with string gauge, bridge width (1" wide rectangular or a wider belly) and string height above the top to arrive at the ultimate design.
With the right choices, the guitar can sound great AND last more than a lifetime. The best prewar examples prove that.
Reply With Quote
  #81  
Old 12-18-2017, 01:12 PM
LouieAtienza LouieAtienza is offline
Registered User
 
Join Date: Feb 2013
Posts: 4,617
Default

Quote:
Originally Posted by John Arnold View Post
That is not the case with most Martins made before the large rosewood bridgeplate arrived in 1969.
IMHO, guitars sound better when the bridgeplate does not extend past the bottom edge of the bridge, so that is the way I build them. in any case, a conventional bridgeplate with the grain perpendicular to the grain in the top is not the best for reinforcing the top against peeling. That is because most woods are more flexible across the grain than with the grain. Spruce is much stiffer along the grain than across, making it very suitable for resisting the peeling in the direction of string pull.
The key is to choose wisely....both the stiffness of that particular piece of spruce, and the final thickness. Couple that with string gauge, bridge width (1" wide rectangular or a wider belly) and string height above the top to arrive at the ultimate design.
With the right choices, the guitar can sound great AND last more than a lifetime. The best prewar examples prove that.
Yes John thanks for that clarification. I did point out that the smaller bridgeplate contributes to a more "vintage" sound. While I do like the sound of both, I suppose being exposed to the more "modern" bridgeplate type makes me favor the voice that produces. Of course, I've not been as exposed to true vintage instruments as you have; but I find for me, because of this lack of exposure, there are others who can achieve that "vintage" tone far better than I ever could, so I look for something else. But I agree that proper design and material selection is key.
Reply With Quote
  #82  
Old 12-18-2017, 02:16 PM
Rodger Knox Rodger Knox is offline
Registered User
 
Join Date: Jul 2013
Location: Baltimore, Md.
Posts: 2,431
Default

Quote:
Originally Posted by Rodger Knox View Post
You are right, of course. I was a little harsh, I apologize. Check out the thread Mr. Tauber linked. I'll leave it at that.
Quote:
Originally Posted by hat View Post
Comments quite unnecessary sir!
This thread was started as nothing more than an exercise, a discussion of what might be possible, and why or why not. And I feel it was a very good discussion, with a lot of very fine fellow's inputting their expertise and experience into it, including yourself. My 'Hat's ' off to all for a fine discourse!
Once again, I apologize.
__________________
Rodger Knox, PE
1917 Martin 0-28
1956 Gibson J-50
et al
Reply With Quote
Reply

  The Acoustic Guitar Forum > General Acoustic Guitar and Amplification Discussion > Build and Repair

Thread Tools





All times are GMT -6. The time now is 01:11 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright ©2000 - 2022, The Acoustic Guitar Forum
vB Ad Management by =RedTyger=