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We are going to start bracing the top and have a few questions if you please.

We're using some scrap cedar (?) and I thinned it a bit too much as I was having too much fun making a pile of shavings before it occurred to me that I might need to check the thickness... went to about .95 or so. So I am thinking rather than get too paranoid and beefing up the braces that we would try a lattice aka Lars's most famous television thread. Just appeals to me and I would be less prone to make them overly heavy (Whatever that means...).

So I started looking at lattice bracing and bracing in general and I have a couple of questions.

1.) I have some hesitation for putting this link up as I hate to hold someone's work up for inspection, but it IS on the web for all to see. Please feel free to delete it if it is deemed unfair to the builder.
http://www.lodgeguitars.com/lattice with carbon fibre on top.JPG
This can't be right. Not that I would want to use carbon fiber yet, but does it look like the CF used there is almost like shoelaces??? How can that possibly stiffen the bracing? I have seen other examples (Smallman's) and it looks like he used very stiff cf strips.

2.) My assumption is that a lattice brace will resist the top bulging more efficiently than A bracing. T/F?

3.) I am curious why, with all the effort to reduce weight via the mysterious "scalloping", wouldn't it make sense to shape the braces in a proven, most efficient I beam shape for the type of stresses we're concerned with? It would seem to be very simple to take a gouge out of each side of a brace to reduce weight rather than all that scalloping, though scallopin' do look purdy. I didn't find any good laymen descriptions of how the flanges on an I beam increase it's strength. If any of you engineering types know of a handy-dandy calculator that I can plug in flange and web dimensions and get a "strength" answer, that would be most excellent. I really want to compare an I beam with flanges of x width against one whose flange on one side is zero (~triangular brace).

I know. The KISS principle. But we are playing and not too concerned with "Can we do it?". If the answer to #2 is True, then it seems the way to go.

Thanks!
Rob & Robbie

Rob and Robbie here are some of my thoughts for you.

First .095 is way to thin for cedar. I don't know if you are just getting some bracing practice or if this top is destined for an actual guitar. If you plan on using it on a guitar I would start with a thicker top. To give you some idea how thin this top is compared to a "number" since we don't have any deflection testing data a number that is typically thrown around here on the forum for cedar thickness is .130. You can see that your top is much thinner.

Now as for "I" beams it depends on your very own personal definition for the purpose of the braces on a guitar top. What you may say is this whack job Hesh talking about? What I am offering here for discussion is this: Is the purpose of bracing on a guitar top to structurally reinforce the top against the forces that a guitar encounters with string pull etc or is the purpose of bracing to act as meridians for the transfer of vibration throughout the top - or both?

I am in the both camp. To me braces structurally reinforce the top but they also need to be efficient at transferring vibration.

Now your idea of sticking to the tried and true is what I would recommend too but not when it comes to "I" beams on a guitar top. I can't see the link - it does not work but why not work with a proven design for bracing a top?

IMHO some of the mass that braces need to hold the top together is also mass that produces sound and necessary to produce sound. One of the ways that we can tell if a guitar is under braced is that it is not very loud lacking the critical mass of structure to "sound off" well.

Anyway I always get in trouble with the folks who want to go off and try to reinvent the wheel. To me just understanding how a conventionally braced guitar really works will probably be a life long pursuit but a very worthy one.

Thanks Hesh.

Yeah, i know the top is wayyyy too thin. It was thin to start with (this is the old barn board thing and they are cut so the end profile is a wedge where the smaller side is .110) and i just was enjoying the process too much without thinking. Trying to get all the punky wood off maybe. My goal is to build one just to figure out the process with junk wood before i build a real one. Yes, this is going onto a "Guitar" or what i am now calling a UGLO - unidentified Gutarlike object - just because i want to follow the entire process through. If it works in the end, Hooray for me... if not, i heat with wood.

My Bad on the link.. sorry. I should have previewed.

The link is to an example fo a carbon fiber lattice top that i didn't get. Not to anything with I beams. ...and you really have to open it full size to see what i am talking about.

http://www.lodgeguitars.com/lattice%20with%20carbon%20fibre%20on%20top.JPG


I agree with you with being in the "Both" camp, BUT... i don't think (sheer guessing here) that bracing need to transmit vibration like wood that you'd use for a xylophone, for example, else the smallman design would be crap. Balsa just ain't that kind of wood. So the braces have to resist the pull of the strings while allowing the various vibrations to be transmitted to the top without deadening it by adding (too much) mass. I am just leery that because i made the top so thin, that i would be tempted to greatly over brace with a traditional design, hence the lattice (if that is in fact more resistant to the string pull?)

I'm not under any pressure to make a great sounding guitar (that comes later when i use "THE Wood"), so i am free to play.

Rob

The Smallman is a classical, which means reduced string pull, however, his tops are less than 1mm thick, and they are cedar. You could probably get away with a lattice of some type, but I would think, if you are building a steel string, that an X brace would be important structurally, with a lattice in the center, and some kind of wing braces on the edges. You are, however, now making a hybrid of some kind with little to go on. Building to a plan is better on early guitars.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Actually that is a perfectly fine way to use the carbon fiber. It’s most effective when it’s on the outer most surface, furthest from the load, in this case the top of the braces.

You are correct that the lattice structure spreads the load better than the traditional martin style bracing. Hence the tops are usually thinner than standard.

If this is you first instrument however I wouldn’t build in this style. The height of the braces with the carbon is super critical and you should really have an understanding for top flexure before you attempt this.
The lattice style shown in the link also complicates the bridge/bridge plate design.

If your going to keep building get use to the idea of scrapping something if it’s not just right.

_________________
Jim Watts
http://jameswattsguitars.com

Boy... I'm glad i don't run a country else my inability to say what i mean would put us in hot water.

No one is understanding me. And here i though it was alway just my wife...

Waddy:
Yes. That's what i liked about the lattice design lars had posted in his television thread. It used the A bracing but on the lower bout it used a lattice configuration.
Plan? what plan? This is a complete "winging it" experience.

Jim:
It sounds as if you are not seeing what i am seeing. On the link, the CF look like it is draped over the lattice, like wet spaghetti... it doesn't look stiff to me at all like you'd see on a smallman... make sure you look at it at it's full resolution. Take a look at where the CF ends on the braces, it's all frayed and hangs loosely.

I have no intention of using CF in this build (or any most likely). Too much complexity would damage my brain.

Yeah, i do realize that i'll have to take things apart and redo them. But that's exactly my intent. I'd rather make mistakes when i am trying to understand the build rather than when i am building a real guitar. I figure i'll understand better what questions to ask if i go through the most painful process i can, rather than making a cake from a box.

Thanks all!

Rob

I dont get that carbon usage either. Now if the lattice pieces were made of a wood/carbon/wood laminate and vertically installed that would be strong.
but sound???

Rob & Robbie asked:
"Not that I would want to use carbon fiber yet, but does it look like the CF used there is almost like shoelaces??? How can that possibly stiffen the bracing? I have seen other examples (Smallman's) and it looks like he used very stiff cf strips."

CF has an extremely high Young's modulus: it takes a lot of force to stretch it. By putting it on the upper and lower surfaces of the brace you're getting the most stretch on the CF for a given amount of bending of the brace, so it makes the greatest possible contribution to the stiffness there. It is, in fact, just like an I-beam.

Without looking at the pic in question, I suspect they're using loose CF fibers, which are then stuck down and rendered stiff with an epoxy or CA matrix. THis is what Smallman uses, as I understand it. There are pros and cons to doing it this way, but there are pros and cons to everything.

"2.) My assumption is that a lattice brace will resist the top bulging more efficiently than A bracing. T/F?"

Probably true if you do it right.

"3.) I am curious why, with all the effort to reduce weight via the mysterious "scalloping", wouldn't it make sense to shape the braces in a proven, most efficient I beam shape for the type of stresses we're concerned with?"

Scalloping is not done to reduce weight, even if that's what the people doing it think they're doing. It's a method of locally reducing the stiffness of the brace, so that the top will bend more in that area. This gets us to the main reason for not using I-beams; tunability, or the lack thereof.

There's a reason why the 'tried and true' designs are the standards; they work well. In fact, so much of the sound of the guitar is more or less taken care of in the design that the differences between 'good' and 'great' guitars are hard to measure. But, as is always true in such cases, those small differences are extermely important.

Now, wood is a variable material. Two pieces of the same species might differ in Young's modulus along the grain by 30% or so, and across the grain the differences can be much greater owing to the extreme sensitivity to grain angle. Thus two guitar tops made and braced to the same specs from the same species of wood can vibrate in very different ways.

Thus there is a need for some method of 'fine tuning'. There are lots of ways of doing it, and everybody knows that their favorite is the best way and the rest are worthless. However, all of these methods of fine tuning involve shaping the braces in some way or other to alter the stiffness distribution.

You can't do that very well with an I-beam. The stiffness there depends mostly on the width of the caps and the height of the web: you're pretty well stuck with the height of the web, and can only take so much off the caps. That is, in fact, one of the big problems with the Smallman design: once you've built the top it either works or it doesn't. When it doesn't there are only a few things you can do to get it to work, and if they fail you have to take it off and start over.

At any rate, most of the weight of a normal guitar top is in the top, with only about 1/4-1/3 of the weight in the bracing (if you don't count the bridge). Going to a 'more efficient' brace section, such as an I-beam, might save you 10% of the brace weight, but that's not much of the weight of the whole top, and you've lost the option of fine tuning. Besides, making proper I-beams that don't themselves introduce problems, such as stress risers, is not trivial, and is time consuming. This, again, is the idea behind the lattice and sandwich tops: they reduce the need for thickness in the top membrane itself by reducing the unsupported span between braces, and accept a doubling (say) of the brace weight to cut the top weight in half, or better. Overall the weight is reduced by going to a sort of 'distributed bracing' model.

You can certainly learn a lot by jumping right in and experimenting. The problem, IMO, is that without some baseline experience you might not know what the heck you learned. These things are a lot more complicated than they might seem, which is why we spend so much time on lists like this arguing about them.

Rob, it looks to me that he took tows (bundles) of fibers and wet them with epoxy an, then placed them on top of the braces. A pretty messy process and less than optimal as far as the structure of the carbon fiber goes, but still effective. The composite then cures in place, but you end up with stray and splaying fibers as you point out.
Sorry for misunderstanding your post. Happens to me too.

PS if you just having fun go for it. I guarantee you it will sound like some type of guitar. You might even be surprised. So, with a thinner top like that, yes I'd use a lattice.

_________________
Jim Watts
http://jameswattsguitars.com

Al, thanks for the detailed thoughts. I like your reasoning about being unable to change an IBeam... makes sense to me. I also like you comment about where the weight lies... that also makes sense! Additionally you are right about perhaps not being able to recognize what lesson i learn... i won't know until i know.

The only reason i am jumping to the lattice is that the top is already too thin, not because i went that direction on purpose. Just got mezmerized by the "zing" of the plane across the surface. Never really used a sharp one before and i had just tried out the scary sharp method and when i woke up...

Jim: I think you and Al seem to agree on the epoxy thing. Not sure that it still makes any sense to me as from a newbie view, i whould think that most of the strength is just the epoxy itself unless your fighting the pull of the top towards the nect and not the rotation of the bridge. Maybe that's what i have wrong? My thinking is the the main purpose of the braces is to resists the rotation of the bridge. Is that wrong?

I do think i'm going with the "A" lattice and see what happens. Maybe i'll change my mind when i rethink all those cuts and intersections.

Thanks again guys!

I was in almost the exact same position 6 months back I'd over thinned my spruce top, but I carried on with it against most advice here, and the few proper luthiers who've looked at it say it's gonna sound good and is a responsive top so hey. If I were in your shoes and it were me I'd do a double x-brace, make it a 13 fret to the body to keep the tension down, make the bridge patch thicker with a stiffening brace behind it to keep the centre stiff. I was advised on mine to make the braces narrower but increase the height to between 5/8" and 3/4". I'm glad I did what I've done I made so many mistakes on number 1 and I wanted to do it on the cheap so as not to muck up expensive woods I'm coming in at £50 if you include finishing materials.

I've read hear how your to "build on the brink of disaster", "make it so once it's strung it looks 4 months pregnant not 9" and lots of other it's you first one and most newbies over brace apparently I'd urge you to continue with it and fly in the face of tradition and you'll find how close you can build to the limit, and then hopefully not need to tweak it years down the line, if it does belly it's your guitar so you'll know how to fix it!

Take into account I'm on number 1 but those are my feelings.

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Formerly JJH

I learn more from my mistakes than my successes

Rob, the fibers are on the order of several hundred times stiffer than the epoxy. They don't feel that stiff when your handling them as cloth or tows, but that's due to the very small diameter of each fiber.
You are correct in that you're battling the rotation of the bridge. The body and neck also want to fold together however.

_________________
Jim Watts
http://jameswattsguitars.com

John,

13 frets to the body won't make a difference in string tension. Perhaps you mean a shorter scale?

Rob,

Maybe at this juncture, it would be beneficial to give up on this top and build what you originally set out to build. It would serve you better in the long term, educationally speakin'.

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formerly known around here as burbank
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http://www.patfosterguitars.com

Cheers pat yeah I meant 13 body to fret join and a 24.9" scale length

Sorry

_________________
Formerly JJH

I learn more from my mistakes than my successes

It sounds as if the CF thing is one of those things that i'd need to experience before it makes sense to me.

John: Thanks for the encouragement. You didn't say how yours turned out in the end though. I'll have to go back and search if you have posted something in the past...

Pat: The only time i think i lose out from a learning experience perspective is if i give up on it. It could be that it will sound like a dog. The wood is kind of punky in the tap tone, so it probably wasn't anything good to build with in the first place - there's a reason it was on the side of a barn! But in the least, i'll learn what not to start with for the next one...

My thinking at this point is that i want to see if there's anything that can be done to make it workabkle and i just don't know if a traditional bracing pattern would be enough.

If cedar is reccomended to be in the .130 range and mine is somewhere around .90 wouldn't a lattice make the best sense? Al brought up the point of "If it's done right", but what does that mean? The right sized squares? The right height and width of the braces? What seems to make sense to me is something like Lars had done.. the link to the image he had posted is http://www.luthiersforum.com/forum/download/file.php?id=7843&mode=view.

So my plan at this point is to still move forward. The top doesn't look bad. It would look decent if i hadn't managed to lift a couple of splinters when chiseling out the rosette, but i can fix that later. Anyway, there's enough effort into it at this point just to want to see what the final result will be. With all her imperfections, she's still my baby!

Something you may want to consider - building any guitar is going to take a significant amount of time. Student grade sounboards are very inexpensive, if you look hard, you can probably find one for around $20. If you look really hard, you might even be able to find a 2x8 or something like that the home despot that could be re-sawn into several tops putting the price below $20/top (if you don't include your time).

When faced with similar decisions, I ask myself whether the savings in cost is offset by potential losses in time. Do you really want to sink that kind of time into something that even optimistically speaking will be o.k. at best, or spend the extra 20 bucks and get something that's pretty much guaranteed to work?

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

The simplest analogy I've seen for how a carbon fiber cap (well, an I-beam, actually) works is this one:

Imagine you've got a piece of rope behind your back with handles on both ends so that when you grab the handles, the rope is taut across your back and the backs of your arms. If you try to move your arms forward (a bending movement), you can't because the rope would need to stretch. So, you're using something that's good at resisting stretching (the rope) to resist bending.

In an I beam, when you put a load on the top the bottom layer of the I would need to stretch for the beam to bow, and so the load is actually being carried by the resistance of the bottom layer to stretching rather than the resistance of the top layer to bending! It's really genius design, and (like so many other things) 'nobody' knows the name of the guy that came up with it. Everyone knows who Henry Ford is, but nobody knows who invented the internal combustion engine

So far as use of carbon fiber in braces, though: if you look at the load that's on the brace, the carbon fiber caps are on the wrong side to resist it effectively. I think the most effective looking CF-inclusion in bracing I've seen has been vertical strips of laminate in braces (which should allow the CF to be put into tension and compression effectively). That would also allow reshaping the bracing, though it'll be Hell on your tooling!

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Unless you're building an arch top....or a bandura

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Just came across this comment on another site in reference to a guitar that Rick Turner was building years ago.



Who knows what the final effect was on the overall tone of the guitar he built, or whether he continues to play with braces like that. In the end he pretty much raved about the guitar, but he had a number of improvements listed for the same build.

Just another one of those things that "maybe" is worth trying at some point. Nice to know i shouldn't throw the thought out completely.

Hi Guys, I am new to this forum but have lurked for a while and learned a lot. I have especially liked how Allen C. and others have taken the time to respond to the questions many aspiring luthiers have.Having come from an engineering background I too have thought about the use of I-beams.From my understanding on the flexural loading of I-beams if you slowly plane off the flange away from the glue joint you will slowly move the neutral bending axis toward the soundboard, changing stress loading of the fibers as you go, as well as the overall stiffness.How this affects the damping factor of the beam and the resultant sound is anybody's guess,but that is what is so great about throwing around ideas,who knows where it will lead?

efialtis: interesting comment. I would have only thought about messing with the flange away from the glue side. Surely the glue-side must be a dangerous place to play for obvious reasons, but it does open up options that i would not have considered otherwise... ala: "Gee, I wonder what happens if i stick this key in a wall socket?".

Thanks!

Hi Rob, I like you was thinking of the flange away from the glue joint. I wouldn't want to mess around anywhere near the soundboard.That being said, in the extreme case where you totally planed off the upper flange but kept the same thickness i.e., plane the sides of the strut till you only have the center web, you now have a T-beam with a neutral bending axis closer to the top and less stiffness. Regards, Chris.

Hi Rob, just to clarify I meant the bending axis would move closer to the soundboard. When you take the soundboard into consideration anytime you glue a rectangular strut to it you are making a T-beam with the soundboard being the flange.Regards, Chris.

lol - ok i see where my first reading of your post shocked me...



I read that as the soundboard side, but i see you meant the other side. Darn! You're not as radical as i thought you were!

When you are trained as an engineer you can't afford to be too radical or bridges fall!! That dosen't preclude you from thinking outside the box when mulling over new ideas.Regards,keep the ideas flowing.