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Why is it that the lower back braces are typically so large? Is it really necessary?

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

No.
There used to be a thinking that wide and flat was better opposite the bridge plate, but that was largely disproven.
I use tall and thin. Works fine and I don't have cave in problems that I had on my first few way back when...

The massive braces make the back less 'active', and the shallower profile makes the back plate tuning lower. There are extensive discussions of back tuning in the archives.

I've tried both styles in the same guitar twice now, and preferred the sound of the traditional big and low back braces. The bass sounds more solid somehow. However, getting all the plates and tunings to interact in a pleasing way is non trivial, and what works better for one person and guitar might not work for another.

I suspect Greg is right when he says getting all of the guitar's construction attributes to work together is the key.

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Joe Beaver
Maker of Sawdust

Critical to understanding and determining if one wants wide, low back braces or tall narrower back braces is a fundamental understanding of active backs vs. reflective backs.

As such the decision as to what's right for you (or me) has everything to do with what your end game is with the choices being active or reflective.

For me since I'm enamored with very light weight instruments I'm keen to work with active backs where the back contributes not unlike a bellows that pumps air as in fireplace paraphernalia.

My back braces, the lower two.... are typically 5/8" wide and not very high either with final height determined by my own method for voicing.

One of the things that I had hoped to gain greater insight into when entering the repair business some years back now and working with David Collins in a VERY busy repair business these days was and is the "serviceability" considerations for guitar building. Since I spend my days these days regluing loose braces, fixing cracks, etc this exposure to how guitars fail and why as well has helped me a great deal with my own building.

Most of the loose back braces that we see that are not RH related in why they failed are because the instrument suffered an impact on or near the back. Be this as it may my take these days is that if serviceability is important to you, and it always should be in my view, AND you are into active backs that need to have some flex to them wider, lower back braces for the lower bout provides the value of much greater gluing area. Greater gluing area should survive better in terms of serviceability if the instrument is abused or an accident occurs.

At the end of the day though there is no correct answer here and either style has already stood the test of time. For me though with serviceability high on my list of value adds and a fondness for very light weight, stiff, guitars with active backs wider back braces serves me very well.

I use 5/16 x 5/8" (to start) lower back braces, and consider my backs very much active.

The understanding most of us have begins with Martin who traditionally made their lower braces 3/4 wide by 5/16 high, apx of course. Modern Martins are kind of the reverse. There are many possible variations between the two poles but what a builder does is either more in one camp or the other, more tall skinny or short wide. Whether what is done functions as a reflective or active back is another conversation. Most would agree that the tall skinny is stiffer than the short wide which would allow the back to move more.
The critical issue though is whether what you want/do will fail or not. Is more likely that tall braces will be less likely to fail. I dont have enough history of Martin to know if a substantial number of the low wide braces failed. Does anyone here?

Actually you can design tall narrow braces and short wide ones to give about the resistance to flex. It all comes down to how narrow and tall verses how wide and short

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Joe Beaver
Maker of Sawdust

I do. I don't see any correlation between failure rate and the style of bracing. I do see failure from differential expansion, or from overheating the guitar.....no matter which style of brace is used. This is because of cross-grain construction.
I believe Martin's longtime use of the low wide back braces was for three reasons:
1) Better bass response from an active back. Martin's early guitars were relatively small, and benefited from the extra bass response of a more flexible back.
2) More flexibility means that the back will sink in low humidity conditions, rather than cracking.
3) More gluing surface means that the braces should stay glued better when subjected to varying humidity causing the back to rise and fall.

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John

Well, my back braces are tall, thin. and laminated. My backs are very active.
As far as varying humidity conditions, my instruments are built in 45% humidity and it is the owner's responsibility to keep them at 45%.

Yep. If you hold the resonant frequency constant, then the short and wide brace style ends up being both heavier and stiffer, so less overall mobility of the back. But an overly mobile back will eat up too much of the top's vibration, so that's not always a bad thing.

Mass can also be varied a great deal by choice of back wood and thickness. So it's just a matter of whether the chosen brace style works with the plate in question, to get the desired mass and stiffness when all said and done.

And you also have to think in context of the longitudinal profile of the brace... do you make them relatively constant height, only scooped at the ends? Tall at the ends and dipped low in the center? A shallow bell curve type shape? How tall should the ends be where they notch into the linings? The end height makes a big difference to resonant frequency and overall mobility.

I usually use 5/16" wide, somewhere between triangular and parabolic cross section, and bell curve shape notched into linings between 1/8" and 3/16" high.

But short, wide braces might be better for mahogany and other lower density back woods, to get the mass up a bit. Especially on small guitars.

Just to add a bit of a twist. Somogyi advocates leaving the ends of his tall narrow back braces 1/4" tall where they enter the linings. He also has active backs that are thin. So do active backs produced by weaker braces and normal plates produce the same results as active backs with strong braces and thinner plates?
Tom

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A person who has never made a mistake has never made anything!!!

Couldn't say. I only do what works for me.
What do you consider a thin plate? What kind of wood? Flat or quarter sawn?

Haans: Never have and never will use flat saw wood. Normal backs .095 or so and thin backs .080 downward for conversations sake. Regular candidates that will support these figures. Let's talk in generalities.
Tom

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A person who has never made a mistake has never made anything!!!

I think it's because the back tends to take more abuse. If larger braces shape the tone in a manner that is pleasing to your ear, "Bonus!"

Only if you want to put Hesh out of business.

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George :-)

Same here, most always. I do have some BRW that goes from quartered to flat out at the edges like old instruments, but most always use quartered on guitars. F5 mandolin copies almost require flatsawn.
Bout the same here on back thickness, up to .1 in the center down to maybe .08 edges. Braces are generously scalloped on the back two. Edges are about 1/8" where they tuck. Bracing sounds thin till you consider that thy are laminated.
Not too worried about abuse. Someone wants to abuse my instruments, fine with me. No one warrants mishandling these days...

bump

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A person who has never made a mistake has never made anything!!!

If the backs have the same resonant frequency (main back monopole) the stiff braces, thin panel back will be the lower mass, so will have the higher monopole mobility, which means if it is on a live back guitar the back will be more active. If you have designed it as a non-live back, (i.e. stiff and heavy) it doesn't make much difference how you got there.

With very dense woods, e.g. Brosimum Rubescens (aka bloodwood), it's difficult to make a live back without taking the panel thinner than you'd really want to.

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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

Probably why I shy away from dense wood for backs and sides. My BRW is so old that it has little resin in it and is very light. Have a set of Granadillo that I just don't know what to to with as it weighs a ton and has huge resin content. Should probably boil it for a week...

Art Overholtzer wrote a book a number of years back in which he discussed soaking wood containing resin in a bath of acetone for a week or so, this was to remove the resin and lighten the wood. Art claimed it made the wood more responsive. I've certainly never tried it but it sounds logical to me.
Tom

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A person who has never made a mistake has never made anything!!!

Thanks Tom, I had forgotten about that!
AND! I just found my old boiling trough!

I've tried a bunch of different back brace schemes, in concert with the 'free' plate tuning method. Several years ago I decided I wasn't getting what I wanted from the various 'X' bracing systems I'd tried, and went over to ladder bracing. I pretty quickly found that I was trimming the lower two braces very low indeed, and ending up with pretty small sticks. Low, wide braces will tend to be heavier for a given stiffness, and still have enough cross section to provuide some strength, so I ended up following Martin's practice. Sometimes the old ways are the best...

I may be confused but it is important to ask because I reject the reflective back scheme. Mr Gore seems to say stiff, I think that means tall/thin, works best for an active back scheme and Mr Carruth seems to suggest low/wide. Have I misunderstood?

No, you haven't misunderstood. You have found 2 differing opinions...
Don't expect solid, scientific answers around any guitar forum. Expect opinions.
As is the usual, take what you can use and leave the rest.