Official Luthiers Forum!

In a couple of recent threads I've read reference to "sprung braces". I interprete this to mean that braces are glued/clamped on without being 1st radiused to match the doming of a dish or solera. I'd appreciate some discussion regarding the rational for this approach to building. I take it to mean that flat braces are glued on, and pressed into a radiused dish to form a curved/domed shape.
Elsewhere, I've read some discussion about 'building in tension', mainily about old classical luthiers.
Anyone built 'sprung' and 'unsprung', and seen/heard a difference? Will braces eventually relax, or will they separate from the top from the tension?

CrowDuck

_________________
Chris Nielsen
Soquel, CA.

Yup... flat braces clamped/glued into the dome.

What happens that makes this strong is that you're utilizing the shear strength of the glue - and of course using a glue that doesn't creep.

They do not seem to "relax" any unless there is heat or a glue that creeps.

All things being equal, you could make the top slightly lighter with this method and have the same strength.

_________________
Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

[QUOTE=jfrench] Yup... flat braces clamped/glued into the dome.

What happens that makes this strong is that you're utilizing the shear strength of the glue - and of course using a glue that doesn't creep.

They do not seem to "relax" any unless there is heat or a glue that creeps.

All things being equal, you could make the top slightly lighter with this method and have the same strength.[/QUOTE]

Can you give us your reasoning behind this - that pre-stressing the assembly makes it stronger?

I would think that because of the pre-existing stress in the components that results by doing it this way - higher initial stress in the plate as well as the brace - that it would take less additional stress to exceed the stress limit (strength) of these parts and cause a failure.

The fact that you start out with stress in the parts (including the glue joint) actually makes things weaker, not stronger.


It is true that building assemblies with initial stress adds strength under certain special circumstances. For instance, concrete has almost no tensile strength, but lots of compressive strength. So, if you prestress concrete with pre-stretched steel bars, so that the concrete starts out in compression, it can take more additional load before failing. This is because any load that you add first has to overcome the existing compressive stresses before causing tension in the concrete.

But this wouldn't help for wood. Wood has similar strength in tension and compression.

And, in any case, using un-radiused braces would mean that part of the brace would start out in tension, and part in compression. (and similar for the top or back)   So, it wouldn't help even if wood (like concrete) was much more resistant to one type of stress.

This pre-stressed condition also would not add any stiffness to the assembly, since the stiffness of the wood (like almost all materials) is independent of the stress, until you get to the failure point.


Phil

Phil,

Because the sheer strength of the glue (Hide Glue, Fish Glue... glues that don't creep) becomes a factor in creating and holding the doming.

The stress in forcing a fan brace into a dome is neglible (I would never try this with an X-brace). Little enough to probably barely negate the gain in strength by not introducing runout into a perfect brace. It we nitpicked it further, a brace with no runout in it glued to a top with no runout is probably slightly stronger - but this isn't my point.

There is added strength and stability by taking advantage of the sheer strength of a glue that will not creep.

The dome is held because where each micrscopic part of the brace is glued to a microscopic different part of the soundboard when pushed into the dome, compared to the same components being glued together flat. This is why it holds the dome instead of springing back to flat. Thats my theory anyway.jfrench38950.924375

_________________
Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

Hesh, You're right. I would tend to believe that but i have never built with unsprung braces.

In his videos though, if I recall correctly - he seems to believe it limits the longevity of a good sound.

Torres sprung in his braces and also used very thin soundboards. Many examples exist that have retained their doming quite well.

_________________
Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

[QUOTE=jfrench] Phil,

Because the sheer strength of the glue (Hide Glue, Fish Glue... glues that don't creep) becomes a factor in creating and holding the doming.

The stress in forcing a fan brace into a dome is neglible (I would never try this with an X-brace). Little enough to probably barely negate the gain in strength by not introducing runout into a perfect brace. It we nitpicked it further, a brace with no runout in it glued to a top with no runout is probably slightly stronger - but this isn't my point.

There is added strength and stability by taking advantage of the sheer strength of a glue that will not creep.

The dome is held because where each micrscopic part of the brace is glued to a microscopic different part of the soundboard when pushed into the dome, compared to the same components being glued together flat. This is why it holds the dome instead of springing back to flat. Thats my theory anyway.[/QUOTE]

jfrench-

I agree with you about the importance of a strong and stable glueline.

And that's a great description of how the glue serves to maintain the domed shape in a stressed top.

I just think that the glue joint will serve the same function - but even better - if it doesn't have to work quite as hard to keep the braces bent to a stressed shape.

Even with radiused braces, the glueline keeps the plate stressed (because no one radiuses plates, right?) into the right shape. So there is always some shear stress in the glueline.

I just think that the less work the glue has to do, the better, in terms of both the strength and stability of the finished top. The less stress there is in the parts to start with, the more additional stress they safely take.



Phil

Joshua, I have been springing in my fan braces on my recent guitars and have noticed a real improvement in tone. Are you springing in the harmonic bars as well? My concern is that a hunk of spruce 8x16mm or whatever will fight a bit too hard to come back straight.

Roger,

I'm with you. If its difficult to press a brace into the dome then its probably not a good idea. My soundboards flatten out at the upperbout so there harmonic bars are flat, and the lower harmonic bar has just a slight radius to it.

The back braces, for instace, I pre-shape to their radius.

Thats great to hear that you've noticed improvement in the tone because of this. I don't doubt it. I think its important for classical makers.

_________________
Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

A different approach with similar results has been used by piano makers
for a very long time. The technique is to dry out the soundboard to a
certain degree just before bracing it in to a very slight radius. Then as it
reacclimates to a normal moisture content the soundboard will swell in to
a more significant radius to form its "crown". Using changes in moisture
content determine the final shape of the board may seem at first glance
like introducing another wild card in to the deck, but when you get your
techniques down it can prove to give incredibly consistant results. This
method also builds in a bit of a buffer zone for the end owner before
things start to collapse and split when they neglect to fill the humidifier in
the winter.

Different technique, but very similar philosophy to springing in the
braces. Adding these internal stresses and tensions to the top can do an
incredible amount for the tone of the instrument. Many folks quickly
forget that the majority of the braces on a guitar top are not there
primarily for structural reasons, but tonal. Things like the compound
radius (or semispherical, parabolic, etc.,) dome we force tops in to and
sprung braces are different approaches to stiffening areas of the top, and
shaping the tone away from being floppy or shapeless.

In my personal experience, I saw a notable difference when switching
from a top braced in a 28' dish with 28' radiused braces, to using a 60ish'
dish and braces and allowing it to swell to an end 28'.David Collins38951.4568402778

_________________
Eschew obfuscation, espouse elucidation.

[QUOTE=David Collins]In my personal experience, I saw a notable difference when switching
from a top braced in a 28' dish with 28' radiused braces, to using a 60ish'
dish and braces and allowing it to swell to an end 28'.[/QUOTE]

It seems like that should work in a humid environment, but here in dry-as-a-bone denver (down to 8% in the winter in the shop at times), the dome has a habit of flattening out anyway. Starting out with a 60' dome/dish may end up concave within a day or two 'round these parts.

-j.

_________________
“If God dwells inside us like some people say, I sure hope He likes enchiladas, because that's what He's getting”
-jack handy

[QUOTE=Hesh1956] Benedetto says in his excellent book that pre-stressing the top plate makes for a more easily stimulated plate meaning a more responsive guitar.[/QUOTE]

If that's the case, wouldn't gluing a flat bottomed bridge to a domed top also pre-stress the top? I wonder if the effect would be similar to what Benedetto is talking about? Does this mean that there's an argument against matching the tops plates radius on the bottom of the bridge?

J., You must have misunderstood my posting. The 60' radius is what the
top would be braced at right out of the oven and bone dry. By the time
humidity reaches 50% you will have around a 25'. So actually, the method
described above would stand a better chance of fairing than pretty much
any other method. If a top is braced to a 60' radius when dried with the
intention of achieving a 25' radius at 50%, you would see it flatten to as
low as aroundn 60' in extreme dryness. At this point the guitar will be
buzzing all over if you could play it, but your fingers would be bleeding
too much to play due to the barbed wire fret ends poking out.
The Martins and Gibsons would be buzzing like mad by 35%, cracked all
over at 30%, and virtually irrepairable collapsed by 20%. Hopefully no one
has any Olsons out thier because they would be splinters by about 40%.


Seriously though, 8-10% is far outside of a reasonable range for pretty
much anything made of wood. Homes can get pretty close to that here in
Michigan in the winter, and I tell people if they don't keep thier
instruments humidified then expect to have major problems and void all
warranties.David Collins38951.9038078704

_________________
Eschew obfuscation, espouse elucidation.