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PostPosted: Mon Dec 26, 2016 3:36 pm 
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I'm looking for some ideas on why the bridge on the guitar I just completed is rotating enough to cause the the top to flatten in front of the bridge and belly up in back of the bridge. It's enough that when looking at the top with low angle reflected light, it's immediately obvious and it causes me some concern about the long term integrity of the top. This is with a set of 12-53 strings. Switching to a set of extra light 10-47 strings (~30 lbs less total tension) almost, but not entirely, eliminates the deformation under tension.

The two factors I suspect are top thickness/stiffness and the amount of bridge end overlap of the x-braces. Or maybe a combination of those. The guitar is parlor size and it's the first time I've built this design so no prior experience with it. The top is redwood and 0.106" thick. I just started doing deflection testing on the previous guitar. The top on that one was red spruce top which deflected 0.135" over an 18" span under a 5 lb weight. This redwood top deflected 0.204" under the same set up. That larger deflection number still falls in the range of numbers that people have reported in other threads here as being "good" numbers with the same deflection test setup. However, I have no way of knowing how well numbers from different people's set ups compare.

Is the relationship between the amount of deflection and stiffness linear? Is a piece of wood with half the deflection of another piece of wood twice as stiff, or is it more complicated than that?

The other factor I'm wondering about is the bridge location relative to the x-braces. I made the bracing pattern based on existing parlor guitar plans in which the x-braces are located so as to capture just the corners of the bridge ends. I assume the reason is to allow the bridge more freedom to rotate to compensate for the smaller area of the top on a parlor guitar. The first picture below is from the plan for this parlor guitar and the second is from the plan for the larger guitar I build which has had no issues at all with top deformation under string tension.

I want to avoid this in the future so I'm wondering whether I should use thicker/stiffer tops if I keep the x-brace overlap the same, or if I should tighten up the angle of the x-braces to capture more of the bridge ends. The top photo below is from the parlor guitar plan and the bottom photo shows the overlap in the larger guitar I build.

Thanks for any input you have.

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Parlor guitar bridge x-brace locations.jpg

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Bridge x-brace overlap.jpg


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PostPosted: Mon Dec 26, 2016 4:16 pm 
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Measure the bridge rotation in degrees. Attach a ruler to the bridge with sticky tack putty, pointed in-line with the strings. Lay the guitar on its back and set up a reference above it (a desk lamp works), and measure the distance down to each end of the ruler. Then loosen all the strings, and measure again. You can calculate the angle from the length of the ruler and the combined change of both ends. angle = inverse sine(combined change / ruler length)

2 degrees is what the Gore/Gilet book recommends, and it's actually quite a lot. With a 12 inch ruler, that's .4" combined change, so one end goes up .2" and the other end goes down .2" when the strings are loosened. Minimal stiffness is good for parlors, because they naturally tend toward high frequency.

Is there sharp curvature just in front/behind the bridge, or is it fairly gradual across the whole soundboard? The latter case is what you want. Localized deformation means you need better connectivity between braces (note: the bridge is a brace), or else a thicker soundboard. Too much evenly distributed deformation means you need more total stiffness, which can be had either by thickness or brace height. Either way, there's not much you can do about it now without major surgery.

But if there's a "bubble" behind the bass side of the bridge, you could add an extra brace there, working through the soundhole.

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Is the relationship between the amount of deflection and stiffness linear? Is a piece of wood with half the deflection of another piece of wood twice as stiff, or is it more complicated than that?

It's for sure linear for a simple beam supported at each end with a weight in the middle. And I think it is for guitar tops and bridge torque as well.


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PostPosted: Mon Dec 26, 2016 5:26 pm 
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I like the bridge to overlap the X - not just the corners. You can take the top a little thinner this way.

If you make the top thicker - they tend to be brighter and not quite as loud. That's fine if it's what you are after.


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PostPosted: Mon Dec 26, 2016 5:46 pm 
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As I was trained, the X-braces should be full height from the saddle to the X, and an equal distance forward of the X. Many builders scallop in this critical area, leading to the deformation you describe as the bridge rotates forward under tension.

Excessive bridge rotation and belly can be somewhat corrected with a JDL Bridge Doctor. I've installed a couple in guitars coming through my shop, but I consider it a fix for faulty construction.

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These users thanked the author Tim Mullin for the post: Rbello (Tue Dec 27, 2016 8:10 am)
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PostPosted: Mon Dec 26, 2016 6:24 pm 
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Dennis - I measured the bridge rotation using a laser pointer resting on the flat top of the bridge pointing at a vertical ruler positioned 24' inches away in line with the strings. I noted where the laser hit the ruler without strings and then with the 12-53 strings and then solved for the angle of the right triangle. The rotation was 2.28 degrees so a bit more than ideal I suppose. With the 10-47 strings, it was only 0.84 degrees [correction it was 1.95 degrees].

"Is there sharp curvature just in front/behind the bridge, or is it fairly gradual across the whole soundboard? The latter case is what you want. Localized deformation means you need better connectivity between braces (note: the bridge is a brace), or else a thicker soundboard. Too much evenly distributed deformation means you need more total stiffness, which can be had either by thickness or brace height. Either way, there's not much you can do about it now without major surgery."

I would say the deformation is in between the two cases you describe. There is no sharp curvature in front/behind the bridge. It's more gradual. On the sound hole side of the bridge, the flattening of the top doesn't extend past the x-braces. Knowing where the x-braces are, I can just make out the shape of the triangular area of the top between the x-braces and the front of the bridge when looking at it with low angle reflected light with the 12-53 string set on and up to tension. So, I take that to mean that the top isn't deforming as a whole, it's the area between the x-braces that is deforming because of the bridge rotation.

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Last edited by J De Rocher on Mon Dec 26, 2016 7:11 pm, edited 2 times in total.

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PostPosted: Mon Dec 26, 2016 6:31 pm 
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Tim Mullin wrote:
As I was trained, the X-braces should be full height from the saddle to the X, and an equal distance forward of the X. Many builders scallop in this critical area, leading to the deformation you describe as the bridge rotates forward under tension.

Excessive bridge rotation and belly can be somewhat corrected with a JDL Bridge Doctor. I've installed a couple in guitars coming through my shop, but I consider it a fix for faulty construction.

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Thanks for bringing this point up as I hadn't thought of this and you may have hit on a contributor to the problem. I did extend the scalloping of the x-braces forward of the saddle line. It makes sense that that could allow more rotation of the bridge/bridge plate area than wanted.

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PostPosted: Mon Dec 26, 2016 9:42 pm 
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Long discussion in the shop today as I helped get out a 1940 000-18 neck reset. We've had a number of pre-war OM/000 and 00 Martins in for work over the last few months and all were scalloped well above the saddle line - most show a smooth scallop from the X intersection back. It may be the nature of the scallop - a smooth catenary curve, versus what the boss terms (on tone bars) 'two small, conical hills, widely separated by a vast plane' - rather than the shaping itself that is the issue.

The more likely culprit in the opinion of both the boss and our visitor, Mr. Morelli, is a lack of torsional support for the bridge, possibly exacerbated by a lower stiffness redwood top. While the practice here is to use lower stiffness tops and reduce the support for bridges on smaller guitars (as the distance to the tail block and width at lower bout grows smaller), the shift is not that much - a 0000 or D sees the forward face of the X lower leg pass directly through the rearward corner of the bridge in plan view, while a Size 2 sees the rearward (tail block side) face of the X lower leg under the rear corner of the bridge.

If you get a chance to post a note over on Musical Instrument Maker's Forum, you might ask Mr. Mario Proulx about his use of what Mr. Bryan Kimsey has christened the 'Proulx Magic Tone Enhancer', which - in addition to enhancing bass response (so claimed by Mr. Proulx), also increases cross-grain top stiffness at the rear edge of the bridge plate and may reduce excessive belly. The PMTE appears to be an added lateral brace at the rear edge of the plate that is fabricated, but not installed unless it is needed to address the sort of situation that appears similar to your own.

On the Antes Parlor plans and several other sets of plans for guitars drawn by Scott Antes - all needed extensive modification to the bracing layout, dimensions, body outline, and nearly every other aspect of the plans. An example of this is the Antes Parlor, apparently based on a blend of C. Bruno and Martin body shapes and a total lack of actual measurements. If built as drawn with the 24.5" scale called for, the bridge is supported only at the forward corners of the bridge winds. A little adjustment to the scale for a 25.34" instrument, and the bridge moves entirely off the lower arms of the X.

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Last edited by Woodie G on Tue Dec 27, 2016 8:02 am, edited 1 time in total.

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PostPosted: Mon Dec 26, 2016 10:09 pm 
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Rather than mess with the finished instrument you might try a set of silk and steel strings. They will have a little less tension than extra lights and often work out O.K. for the old New Yorker Martins that were originally intended for gut stringing. The sound will be a little "folky" but that is often what is wanted with this type of guitar.


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PostPosted: Mon Dec 26, 2016 10:15 pm 
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Thanks, Woody. I found a couple photos of the PMTE here: http://www.proulxguitars.com/buildup/build6.htm

I can see how that cross brace at the back of the bridge plate could reduce bellying a bit, but if the rotation of the bridge I'm seeing is due to insufficient stiffness in the top I don't see that it it would prevent the front edge of the bridge from still being rotated downward excessively. As an aside, it's interesting to note that in the guitar shown in the photos on that web page, the scalloping of the x-braces starts very close to the x-brace intersection.

Separately, I took a close look at my 1998 HD-28VR and low and behold it has the same triangular flattened area ahead of the bridge between the x-braces such that you tell where the x-braces are located. Not quite as noticeable as on my new guitar, but definitely there. Of course I have no idea it it was there when I bought it new or if it developed during the almost 20 years since then.

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PostPosted: Mon Dec 26, 2016 10:20 pm 
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Clay S. wrote:
Rather than mess with the finished instrument you might try a set of silk and steel strings. They will have a little less tension than extra lights and often work out O.K. for the old New Yorker Martins that were originally intended for gut stringing. The sound will be a little "folky" but that is often what is wanted with this type of guitar.


I agree. I don't want to mess with trying to modify the guitar. I'm going to go with reducing the string tension by using lighter strings so your suggestion is appreciated. I do want to have some ideas going forward though about what to do differently structurally to avoid a repeat. My gut feeling is that a lack of stiffness in the top (minus the braces) may be making the biggest contribution.

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PostPosted: Tue Dec 27, 2016 12:07 pm 
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Bridge deflection under string load is a product of top stiffness and torque. The torque, in turn, depends on how high the strings ar off the top. Lowering the saddle a bit should reduce the torque and deflection in proportion. You don't really need much reduction, so you might even get away without haviing to reset the neck.

Some of the redwood I've seen sold as top wood recently is not up to snuff in my opinion. The last batch I tested had very low long grain stiffness in proportion to the density, and much higher damping than the redwood I've been used to using in the past. Some of what they're selling now is salvaged from stumps, and, at least in some cases I suspect that the wood in the center of the bole was actually damaged by the weight of the tree, leading to the characteristics I saw in that batch of wood. Some stump wood is very good; it's hard to beat 'LS' redwood, which was salvaged, but there are exceptions to every rule. The top thickness you used would be fine for any redwood I've had that I would consider usable. You just have to be careful with redwood.



These users thanked the author Alan Carruth for the post: J De Rocher (Tue Dec 27, 2016 12:48 pm)
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PostPosted: Tue Dec 27, 2016 2:06 pm 
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Apart from the possible lack of stiffness of the top, it's my own opinion that you've not supported the bridge well with the x braces.
Moving forward I would suggest to locate the brace so that the centreline of the brace crosses the rear corner of your bridge. This is always my own pivot point when laying out my braces. I can open or close the X dependent on the body size or tonal quality in looking for.


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These users thanked the author Rod True for the post: J De Rocher (Tue Dec 27, 2016 4:42 pm)
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PostPosted: Tue Dec 27, 2016 4:42 pm 
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Alan Carruth wrote:
Bridge deflection under string load is a product of top stiffness and torque. The torque, in turn, depends on how high the strings ar off the top. Lowering the saddle a bit should reduce the torque and deflection in proportion. You don't really need much reduction, so you might even get away without haviing to reset the neck.


That's a good suggestion but I don't have much room to work with. The action is currently a bit over 4/64ths for the high E and a bit over 5/64ths for the low E. It plays cleanly with no string rattle or buzz. I might get away with bringing those down to right on 4/64ths and 5/64ths without introducing string rattle against the frets on the low strings. That would be a 1/64th reduction in saddle height. Would that be enough to make a significant difference in the torque? The height of the center of the saddle above the top is right on 1/2".

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PostPosted: Tue Dec 27, 2016 10:13 pm 
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If I'm figuring this correctly, you need about a 15% reduction in torque, which should call for a 15% reduction of the string height off the top. It's .5 inches now, and needs to go to about .43, a bit more than a 1/16" drop. Keep in mind that if you're going to drop the action at the 12th fret by 1/64" you will actually need to lower the string height by 1/32"; since the 12th fret is half the string length it gets half of the saddle drop. You need to get about twice as much. All of the suggestions for remedies so far have involved a fair amount of work, and I was hoping it would be easier to drop the string height, but that doesn't look to be the case.


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PostPosted: Wed Dec 28, 2016 12:48 am 
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Alan Carruth wrote:
If I'm figuring this correctly, you need about a 15% reduction in torque, which should call for a 15% reduction of the string height off the top. It's .5 inches now, and needs to go to about .43, a bit more than a 1/16" drop. Keep in mind that if you're going to drop the action at the 12th fret by 1/64" you will actually need to lower the string height by 1/32"; since the 12th fret is half the string length it gets half of the saddle drop. You need to get about twice as much. All of the suggestions for remedies so far have involved a fair amount of work, and I was hoping it would be easier to drop the string height, but that doesn't look to be the case.


Thank you for the suggestion since it would certainly be the easiest option if there was enough room to play with. Right now, it has a set of extra light strings (10-47 instead of the 12-53 set I started with) which pretty much eliminates the deformation of the top. I liked the tone better with the heavier strings though. What I'm thinking I'll try is to make a test saddle to lower the height as much as I can get away with and combine that with an intermediate 11-52 string set and see how that works out.

Thanks to everybody for their input. It's much appreciated. After considering everyone's comments and mulling this over, on the next one of these parlor guitars I will thickness the top to a lower deflection value, move the x-braces to capture more of the bridge ends, and be a bit less aggressive on scalloping the x-braces in the bridge area or maybe use tapered x-braces.

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PostPosted: Wed Dec 28, 2016 9:23 am 
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Do try the lower string height. 3/64" high E and 4/64" low E action should be able to play clean if you have the fret plane shaped just right. Much more comfortable on the fingers, which is especially good for a child. If it buzzes even with low amplitude playing, then it could be an opportunity to improve your fretwork. Read Hesh's posts on fretwork. If you don't have any fall-away past the 12th fret, then mill a bit into the frets.

And my recommendation for the next one is to play this one for a couple weeks before you decide. If this one sounds really good, then just tweak the neck angle and use lower saddle height (I need to try building a parlor with 1/2" string height, but my currently belief is that 3/8" is better for small guitars, to reduce torque and allow lower stiffness). But if this one's tone is not quite what you're after, then try and guess what would make it sound better, and plan your structural modifications based on that.

Whatever you do, it doesn't need to be very much, and may not need to be anything at all. 2.28 degrees not that far over the target 2, and the book does say a little more or less is fine. And as Norman Blake says, never trust a guitar if it doesn't have a belly :) It only becomes a problem if the long-term deformation goes so far that the intonation goes out or the bridge peels off.


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PostPosted: Wed Dec 28, 2016 2:32 pm 
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DennisK wrote:
Do try the lower string height. 3/64" high E and 4/64" low E action should be able to play clean if you have the fret plane shaped just right. Much more comfortable on the fingers, which is especially good for a child. If it buzzes even with low amplitude playing, then it could be an opportunity to improve your fretwork. Read Hesh's posts on fretwork. If you don't have any fall-away past the 12th fret, then mill a bit into the frets.

And my recommendation for the next one is to play this one for a couple weeks before you decide. If this one sounds really good, then just tweak the neck angle and use lower saddle height (I need to try building a parlor with 1/2" string height, but my currently belief is that 3/8" is better for small guitars, to reduce torque and allow lower stiffness). But if this one's tone is not quite what you're after, then try and guess what would make it sound better, and plan your structural modifications based on that.

Whatever you do, it doesn't need to be very much, and may not need to be anything at all. 2.28 degrees not that far over the target 2, and the book does say a little more or less is fine. And as Norman Blake says, never trust a guitar if it doesn't have a belly :) It only becomes a problem if the long-term deformation goes so far that the intonation goes out or the bridge peels off.


I'll definitely try lowering the saddle height as much as I can. I do incorporate fall away in my fretboards so that's not a problem. For myself, I don't like action that low because I find it interferes with string bending (for me at least). My daughter, however, is not a bender of strings so that's not an issue and she she is strictly a finger style player who doesn't hammer the guitar so low action like you suggest could be a good fit.

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PostPosted: Thu Dec 29, 2016 7:12 pm 
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I followed up on the suggestion to try lowering the saddle height. After playing the guitar quite a bit yesterday, I also decided that the extra-light string set had to go, so I put an 11-52 set on and worked on lowering the saddle. In the end, I was able to drop the height by almost 1/32 and not get string buzz playing fingerstyle which will work just fine for my daughter. The bridge rotation angle went from 2.28 degrees with the taller saddle and 12-53 strings, to 1.78 degrees now with the lowered saddle and the 11-52 string set. And it sounds better than it did with the extra light strings. [:Y:]

Thanks a lot for the suggestion and for all the input that will go into the next guitar.

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