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Your welcome, I did this several years ago and it seemed like it might help clarify things.

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Jim Watts
http://jameswattsguitars.com

I must be missing something as I can not see why there is no issue having the thinnest part of a brace in the bright green area on the static load map.

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http://www.Harvestmoonguitars.com

Do you think that a guitar that has the ability to be overdriven by a heavy handed player might sound OK(or better)in the hands of someone with a lighter touch John? (he said fishing for more of that good ol' free advice)

John, I'm not sure I understand your question.
I'm not saying there is a problem, if you look at guitars under tension the area behind the bridge does rises up and the area in front of the bridge dives down.
So I suppose the answer to you question is that the stresses aren't usually great enough to cause a problem, just some bulging. Now like I said the picture is greatly exaggerated and the bulging is not a lot.
If you really curious, take a dial indicator and map out a guitar top without string tension on it and then with the string tension, you'll find it matches the deflection distribution map pretty well. This is the way how I verified my model some years back.
BTW: I'm enjoying watching your build over with the Australian guys, it's looking great!

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Jim Watts
http://jameswattsguitars.com

Jim,

I think your model is really cool and it does seem to map to what I see when I string up my guitars ignoring the exaggeration. I thought the modeled showed the importance of the x-brace to keep the bridge rotation under load to within 2 degrees or so.

Thanks for the kind comments on my falcate braced guitar project. I will have to post some pictures here at some point. I sure am learning a bunch.

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http://www.Harvestmoonguitars.com

My experience with seeing tops that failed , that bracing on the pictured top has issues. But beside the structure , it is the tonal issues. The physics on the top are acting in rotation on the bridge , this is a fact , the rotation will cause the top to sink in front and rise behind the bridge. The less the brace supports this part of the structure the bridge will separate as the top will pull off the bridge starting on the back of the bridge.
I have seen many tops fail and all were over scalloped at the bridge area.

As for making a guitar that will be good for a finger picker and headroom , the more the braces flex the lower the head room. The point is that the bracing should work with the top to control the balance. Once you go beyond that , you will lose tone and volume. I did try something thinking that if I made the top braces more flexible I could fool the top . I did heavy scalloping to about a 1/4 in and used a 9/16 brace .3 wide. What I ended up with was a top that had no attack , light on volume and was to bass sided. After 2 years the guitar was unplayable as the top behind the bridge rose and the top in front sank.
The point is that when you study the top and the way it moves , along with the way the top is stressed , the lower part of the top is in tension. Martins use a scalloping that is peaked well behind the bridge structure. This helps to keep the bridge from over rotation. I have guitars out now for over 15 years so I can see longer term effect.
head room is just that. And there is no guitar that is good for all players. That is why we have scallop , 5/16 and 1/4 in braces. All are ingredients for the recipe we call a guitar.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Careful, guys. What Jim posted was a deflection plot.

The trick in designing bracing is to have enough in there to limit the rotational deflections, whilst allowing as much "panting" movement as desired, because that is what produces most of the sound. To an extent, it is possible to separate these functions.

Simon Marty, a well respected builder of classical guitars, has no continuous braces under the bridge. Indeed, the brace ends under the bridge do not even meet, never mind overlap. Diagram below:


Whilst the loading regime is the same on a classical guitar as on a steel string guitar, the actual torque loading is approximately double on the steel string guitar, so components need to be sized accordingly.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Don Teeter concluded over the years that the "X" scallop low point is 21/64" its in his book with a drawing. I image loads of guitars have been retro'd or built to that spec. Also -- what about ladder braced guitars?

My eyes have trouble seeing the 64ths any more so I would call 21/64 a real 5/16th, the 1/4" the brace in question or 4/16" is 20% lower in height than 5/16. So that is a loss of a lot stiffness from what Don Teeter concluded.

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http://www.Harvestmoonguitars.com

Do you guys think this 1972 Bennett Tolliver classical that just came into by shop might be over braced?

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I have no idea, but that's a really cool x-ray!
Hows the patient sound?

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Jim Watts
http://jameswattsguitars.com

Wow... I am overwhelmed with the response and the time and effort people put into answering this question. I guess this is one of those questions that goes to the heart of the matter - balancing tonal response with longevity and durability in an instrument.

A special thanks to John Hall for his lengthy responses. John, I printed them out, and they will go in my binder of reference articles!

And, long story short, I went with my gut instinct (and John's recommendation) early last night and re-braced the guitar. One factor was that a previous student did the same thing three years ago... put his heart and soul into building a guitar, only he scalloped the x-brace way too low (What is it with teenagers and chisels?). We went ahead and finished it, and it played and sounded great for about 6 months. Then the bellying and rotation started. Last I heard, he wasn't playing it much. I didn't want a repeat of that... too sad.

So, thanks again to everyone who responded. This forum is such a great resource!
Ken

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Ken Mitchell
Durham, NC

@ Ken,

I am glad you made the decision to error on the side of being safe. I probably would have done the same if the guitar was someone else's and not mine.

@ Trevor,



I am aware of what it is that Jim posted, I have an Industrial Design degree so the concept isn't new to me. He posted what I was trying to explain and was drawing up in CAD to try to explain the point I was trying to get across. His plot did that better than what I was doing. No one actually did answer my question about the bridge/top/bridge plate (sandwich) having a stiffness greater than the torsion put on it by the strings, in effect making the low scallop of the braces less of a factor than people may think.

Again, I am not an expert and don't claim to know but was asking the question so I could understand better. John answered a question but didn't explain what I was asking. So I am still unsure about the whole concept that the "sandwich" doesn't add stiffness to that area.

@ John,



Thanks for the explanation you gave. I do have the concept down of what each brace does and the stresses put on them (and I do understand that a guitar isn't a floor...) However you did not address the question that I posed. Which is fine because I am guessing it is more complex than I may think. You did state exactly what I was pointing out that when the bridge rotates it is in front of and behind the bridge that the top moves. I contend that the area of the bridge rotates because that whole area is so much stiffer than in front or behind and the "sandwitch" is far more stiff than the braces beside it (the low points of the scallop) which contribute to the system as a whole..


I find this thread to be one of the more interesting ones on the forum in a while, although I don't claim to know the answer I am suspecting most people don't either. Maybe there isn't an answer and only variables...

That is one of the things I am learning about guitar building is there are a lot more variables than there are people who have opinions...



Cheers,
Bob

My 2 cents worth .. in fact, the top and a floor are not really all that different .. its just that a floor in comparison is vastly overbraced, and the ends are tucked at full height !!! .. jump on a floor in an house that's being built without the interior walls drywalled yet .. does it make a booming sound ??? of course it does ... that's the Helmholtz freq of the house you hear ...

Some one above mentioned that taking the brace from 5/16 to 1/4 is a change of 20% in height ..that translates to about a 60% change/loss of stiffness. Thats pretty significant under the bridge ... try that with a floor joist ... hope you wanted that fridge in the basement ... ( the inspector would fail that house )

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Explain to me how that area can possibly flex under normal string tension especially relating to a guitar that has a large belly. I agree that it will rotate but it will not flex.

As I said, I am not an expert but I do know how a guitar works and I know how physics work but maybe I am missing something here.

Bob

I think you are misunderstand what happens. I agree that you just won't flex the bridge structure of the plate and bridge but they act on the top. The braces in this area help distribute the stresses applied by the bridge structure. With the brace scallop at this point being the lowest on the brace coupled with this being the lowest point of stiffness of the brace and the highest torque load on the top , over time the rotation of the bridge becomes the dominant force and the braces can't hold it. The wood then creeps allowing the the rotation to deform the top . The more the bridge rotates the more the glue line on the bridge is stressed. Wood will cold creep so this is a time duration issue.
On the old Martins the scallop will start usually just in front of the bridge and not peak to the lowest point well behind the bridge so the braces will still have the top supported at this point. Now when we get into the scalloping , this isn't just about stiffness and strength , it is about the manipulation of the top and how and where it moves. We are manipulating the why the string load is changed from stress to work.

In making a guitar , as I stated earlier , it is just 1 ingredient in the construction "recipe". The steel string is not like a classical so I am speaking of X braces , Steel string guitars. Learning to mate the top and braces to get the sound that you as a builder want isn't going to happen overnight. It will take some time.
Just to share my experience of what I learned to make my ear happy , when I started I did deflection testing on the top and brace stock. Over time this chart showed me pretty much what the charts at ASE show ( http://www.esm.psu.edu/courses/emch213d ... s/wood.php ). As I started to learn more I have to admit that I started to make more patterns on the braces , and this got me in the ball park of what I wanted out of the top. The art of building a guitar is how well you execute this mating.
I learned more from my failures than I did from my successes as now I could look back to see what caused it and work to avoid that. In most all cases it was over manipulation. I lost strength of the top become too floppy. So you can have a top that has structural integrity but doesn't allow the proper movement to make good guitar noise.
Structure is most important in that you want the guitar to withstand the forces applied but not much more than that. The trick is to learn where and what on the braces help the top to move to make the work exchange efficient without loss of structure. Alan Carruth has pointed out how to influence the top to get the movement patterns.
Finding what works for you is the hard part but the journey is enjoyable. The longer I do this the more I know I have so much more to learn.


http://www.esomogyi.com/principles.html

Also as a side note . look as the cross section of a brace. It is fascinating at how the same amount of wood , in cross section can be so much more stiff just by shape. A 1 inch sq piece is wood is not even close to a 1/2 in by 2 in piece so you can see you can manipulate the shape of the brace to make it stronger and lighter.

then there is joint integrity , getting the mating surfaces to be a clean and precise as possible.
then glue , which is the right one ?
Tite bond is prone to cold creep
Hot Hide Glue and Fish do not
Tite bond is better at filling gaps
HHG and Fish glue are harder glues
tite bond is a curing glue and must be removed to reglue
fish and HHG are drying glues and can be turned back to glue by adding water.

ahhh way too much to comprehend for now

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Over think much?
Charts, graphs, vectors, vibratory oscillations, dipoles, monopods, iron filings, sparkle sprinkles, you folks sure make it complicated!

Still one of the best bracing patterns ever designed...OMG!



Just couldn't resist...carry on!

the Sella is a great guitar , a great blues guitar using a different bracing structure from X braces , so you see ,there is no one way to make any one thing.
We all have Grandma's chocolate cake recipe but no one makes it like Grnadma.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Lighten up John, just having some fun...
Seems like no one builds by feel anymore, it's all reductionist, charts and graphs.
Somehow I doubt Grandma's chocolate cake was spec-traumatized. She probably build by feel/taste. Maybe sprinkles though...
Stellas were more than "blues guitars". Seems even Nick Lucas played them. May have a ladder surprise later this year...

Well my question is pretty simple. Why are you scalloping? A properly sized tapered brace will produce a very fine sounding guitar.

Tim

Just a question after looking at the dwg. I am a beginner and this is just a curiosity question.

There is always talk of rotation at the bridge and I gather this drawing assumes an even force across the bridge.

So I just wondered if the bridge experiences torsional stresses due to different string tensions and / or bridge pin position?

Thx

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Wake up and smell the rosewood!

Haans
Love the old Stellas and the X ray is a great learning tool.

They are versatile and and here in the states they have a strong following. Here in my area they are used more for blues than anything else but I agree , the old ones are great . Then newer ones not as much.

I was poking fun about the cake
LOL

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

The rotation is still there. Under a string load the applied force is spread out over the whole bridge structure. The math would show that the size of the bridge plate and position in relation to the string and pins would be more of the end equation . The larger the bridge plate the more of the stress is distributed to the top. The smaller , the more it would be focused.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Scalloped floor joists for better home acoustics. Should be able to sell that to somebody Tony.

20% in height ..that translates to about a 60% change/loss of stiffness. That's a pretty good summery.

a) Martin did it

b) It looks cool

c) It makes one feel more luthier like shaping the braces.

I have to replace the bridge. The owner says it's loud.

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It's not what you don't know that hurts you, it's what you do know that's wrong.