Braces with adjustable Youngs Modulus?

[Deems Davis]

When I was younger and more nimble, I dreamed of a way to manufacture snow skis with variable stiffness to allow for a 'soft' ski when skiing powder snow and a 'stiff' ski for hard pack and going fast. The idea was to vary the stiffness of the top skin relative to the base to alter the overall flexibility of the ski.

Fast forward too many years (and joint replacements) and once again I am wondering if it might be possible to manufacture a falcate type brace that could be adjusted to assist in modal tuning. To illustrate take 2 pieces of paper of equal size and staple them together on one edge. Then place them between your thumb and forefinger and apply tension in opposite direction, The result is that the papers will bend one way or the other based upon the way tension is applied.

I'm looking for any feedback from any and all as to whether the concept has any merit. If anything I've learned, is that there is a LOT more experience on this forum in instrument making than I will ever acquire 1st hand myself.

With recent advances in 3d printing which allows for printing with new materials (carbon fiber, wood, metal) I believe it is possible to fabricate a prototype brace to experiment with and gather some data.

I've had to pack up the shop while selling the home and waiting on the new one to be built, so I have 'idle' time and a 3d CAD program to dream about this.

Deems

[DarwinStrings]

Don't ask me Deems I can't even get what your doing with the two pieces of paper.

[seeaxe]

Hi Deems

Seeing as we have given another recent poster a hard time for not using the correct terms, we had better make sure you are!

For a start, unless you glued your thumb and forefinger to the paper, you cannot apply tension to the plies of a laminated brace. You can apply pressure and you could apply shear force. If you use your thumb and two fingers, you might be able to apply bending.

Will any of this help make you an adjustable brace?.....nope...

The function of a brace is to stiffen something ... in this case the soundboard. A GG falcate brace achieves high stiffness with low mass by using hi tech materials like carbon fibre. The books explain the details but once built its stiffness is not adjustable because.....

The stiffness of any structural member is EI/L, where E is the Youngs modulus of the material, I is the second moment of area (a function of the dimensions of the brace), and L is the length of the brace. So given that the material properties cannot change, then the only way to change the stiffness of the brace is to change the dimensions.

So how could you change the dimensions of brace? You would need to use some sort of truss mechanism that can vary the distance between the top and bottom chord. I have seen pictures of guitar braces cut out of wood using CNC, they should theoretically have a more efficient section (more stiffness relative to mass) than just a simple rectangular brace. And you would need to check each of the struts and ties have adequate capacity. If you cut it out of a single piece of wood a lot of truss members will not have the grain in the right orientation. It would probably still not be as effective as a CF timber composite brace, a la GG.

A better idea would be a simple bowstring truss/brace with an adjustable mid span post.

It would be an interesting exercise.

The other thing to take into account is that the brace has to survive for the life of the guitar. The truss approach looks like becoming a balsa wood aeroplane guitar..I'm not sure that would survive very long.

Good luck with it. Look forward to seeing the prototypes...unless you patent it of course.

Cheers

[kiwigeo]

Your biggest problem is adding in ways of adjusting the brace (eg truss rod) without seriously increasing the mass of the brace.

[jeffhigh]

The title is a little misleading
Adjusting the stiffness of a brace is one thing, but Young's modulus is a material property which is not readily changed.
It gets a bit confusing sometimes when we use the term stiffness in a generic way for Youngs modulus, crossectional stiffness (EI), and also for the overall stiffness of a component or complete structure.

[Deems Davis]

Hi Deems

Seeing as we have given another recent poster a hard time for not using the correct terms, we had better make sure you are!

You are quite right, and thanks for the nudge.

A better idea would be a simple bowstring truss/brace with an adjustable mid span post.

A bowstring is a much better analogy of what I was envisioning, rather than a brace carved from a solid, I was thinking of something that could be 3d printed with carbon fiber, kept to a very low mass. It would consist of 3-4 major components:
a. A base ( less than 1 mm thick by 3-5 mm wide) that would follow the same contour as the bottom of a falcate brace. This would be used to glue/attach to the soundboard
b. A micro adjustment mechanism attached to one end of the base. (this WILL be a challenge as Martin noted)
c. A series of triangular pylons placed at intervals along the lengthwise span of the base. these would be 3d printed and integral to the base
i. The pylons would vary in height from near zero at the end to 7-10mm at the thickest (bridge) point and then decrease in height towards the opposite end.
ii. The top of each pylon would incorporate a 'saddle' or an enclosed ring whose purpose is to keep the 'bowstring' material positioned properly. (The curve of a falcate brace presents an added complexity. a prototype might start with a straight brace)
d. A bowstring (material to be determined) that is firmly attached to one end of the brace, passing over each pylon and then attached to an adjusting mechanism on the other end.

by shortening the length of the bowstring I believe it would increase the stiffness of the brace.

If I can find some time, I'll try and draw up a 3d CAD model as a picture would no doubt add some clarity ( and some additional questions).


Thanks for the comments, and counsel.

Deems

[Max Taylor]

Deem,

Hello neighbor (Las Vegas here). What your describing is what Buckminster Fuller called a Tensegrity structure, a combination of tension and compression.

Your last description is like a bridge spanning a gorge with no structure above the road but rather pylons sticking down with a single cable running under all of them. Tighten the cable and the roadway rises, loosen the cable and the roadway sags.

This concept would work as an adjustable structure but only in a straight line where Falcates are curves, hence the problem I see implementing such a design. Introducing adjustability into such a structure would have to have something like an adjustable screw (think truss rod here) that can be tightened or loosened from inside the body. Seems like it would have to have at minimum a very fine steel wire spanning the pylons, fastened at one end and adjustable at the other. The base (or roadway as exampled above) would have to be rock solid like one half of a truss rod with the other side adjustable. I may be wrong in interpreting your concept but that is how I see it from here based upon your descriptions.

As you make no actual claims for the concept it may still not be out of the realm of possibility. Keep us updated on your progress.

—Max

[Deems Davis]

Max

Thanks for the Tensegrity link, I'll need to digest that a bit and see where it leads. The curve in the Falcate brace is a complication and may be prohibitive. I think you've got the concept correct

[seeaxe]

Hi again Deems

Just to say that by altering the tension in the "string" of the bow you are not changing the stiffness. No dimensional change in the brace. What you will be doing is pre stressing the soundboard and possibly bending it, as someone else said, like truss rod. Given the amount of restraint there is in a glued up box, I think the results are difficult to predict.

Cheers
Richard

[Max Taylor]

This topic has me thinking but not so much in the realm of adjustability after the fact but more with the idea of making each of the Falcate braces with a different stiffness based upon the need of the location under the bridge that they will occupy.

For instance, and I might be wrong here as this is just a guess, but it seems that the braces under the 5th and 6th strings need to be stiffer than those under the 1st and 2nd strings because of the difference in the tensions of those strings. The 1st string is not going to excite the top as much as the 6th string can so having uniformly stiff braces reflected on the 1, 2, 3 string the same as those for the 4, 5, 6 strings. My logic (even after reading the “Books”) says the 4, 5, 6 side bracing could be stiffer.

That said, has anyone experimented with making the Falcate braces laminated using two strips with a thin sheet of Carbon Fiber standing vertically between them instead of on the top and bottom as is commonly done?

If not with a lamination of CF then maybe a three piece lamination with a center thin (varying thickness) core using some hard wood like Maple sandwiched in between. By simply varying the thickness (width) of the Maple core from say 1mm on the first string side to 3mm +/- on the 6th string side this could provide the varying degree of stiffness across the soundboard providing the adjustability Deem is looking for but not after the fact (once they are glued on). Substitute any preferred hardwood of your choice from Maple above. These braces could possibly still be shaved internally after the top is glues on if they are found to be to stiff.

Although they would not be adjustable after the fact one could soon figure out how the different stiffnesses affect the outcome that one might be looking for and what might be needed to get a specific output ahead of time so they could be adjustable that way too.

Just my thoughts on this today.

—Max

[DarwinStrings]

That said, has anyone experimented with making the Falcate braces laminated using two strips with a thin sheet of Carbon Fiber standing vertically between them instead of on the top and bottom as is commonly done?

Max if you go to page 4-46 you will see that Trevor discusses this type of set up. In Fig 4.4-17 with the caption "how not to design composite braces" he show the design you are referring to and explains the whys and why nots.

[DarwinStrings]

While ideas like this may seem like good ideas intuitively, you may end up (if your not using good spruce) adding more mass by using CF products. Trevor does mention somewhere in the book the the use of the CF/spruce braces does not save much weight and at best with really good technique when adding your CF the best you could do is maybe save 10%. The use of CF on the brace is more about stopping problems with creep than reducing the weight of the braces.

[Max Taylor]

DarwinStrings, you are absolutely correct and thanks for jogging my memory. I just re-read that section of “Design”. Has been some time since I read that section.

Trevor does state that the total reduction is about 10% in weight for the same flexural rigidity, as stated. Not a huge savings as you stated but it all adds up in the end.

Well, it’s dark on this side of the world and time to call it a day here so g’day to you all and thanks for all the input.

–Max

[DarwinStrings]

Jim, Max.

[kiwigeo]

That said, has anyone experimented with making the Falcate braces laminated using two strips with a thin sheet of Carbon Fiber standing vertically between them instead of on the top and bottom as is commonly done?

Covered in full detail in Trevor and Gerard's books....you do have "The Books" don't you?? The optimum set up is a horizontal sandwich - CF tow/laminated brace/another layer of CF tow on top of the brace.

[kiwigeo]

After weeks of hardly any posts here on the forum we suddenly get an impressive explosion of brain storming and thinking outside the square with this thread and Franks discussion on his bridge. Must be spring...

[routout]

Many Ideas are a good thing no matter how strange they sound I guess ,never know where things will lead me it's all about the fun.

[Max Taylor]

Must be spring...

Not for us on the other side. Winter is on its way for Deems and I although winter is nice in Arizona but cold in Vegas..

—Max

[kiwigeo]

Must be spring...

Not for us on the other side. Winter is on its way for Deems and I although winter is nice in Arizona but cold in Vegas..

—Max

You've got a nice hot election to keep you warm

[johnparchem]

...
You've got a nice hot election to keep you warm

I go to these guitar sites to avoid thinking of the election

[frankfalbo]

After weeks of hardly any posts here on the forum we suddenly get an impressive explosion of brain storming and thinking outside the square with this thread and Franks discussion on his bridge. Must be spring...

Hah! I do like the idea of adjustable tension on braces, maybe even on the back, along where the backstrip is to manipulate the neck angle and projection a little? Trouble with manipulating the laminate axis of the falcate braces is they'll be pushing in odd directions across the wood grain, could result in bulging or just go right ahead and rip open a crack in the spruce?

Maybe a thin brake cable type cord through the center, or on an arc like a single action truss rod...Bound and threaded at one end, adjustable with an allen wrench...

[Deems Davis]

Hi again Deems

Just to say that by altering the tension in the "string" of the bow you are not changing the stiffness. No dimensional change in the brace. What you will be doing is pre stressing the soundboard and possibly bending it

Richard, thanks for shining a light on this, I've been thinking about it a lot, and I think it points out something I hadn't seen. You're correct it doesn't change the overall stiffness of the brace it only adds tension in one side/direction, since vibrations are cyclical, I think this would be a serious problem with what I had envisioned. And might act more as a dampener.

The idea of having a means to adjust (fine tune) the soundboard other than shaving braces still intrigues me, but I'm not sure what I had in mind will work. Thanks to all who took the time to think about this along with me, it's why I posted the idea in the 1st place.

Deems

as to our eminent election, God help us!

[Trevor Gore]

The idea of having a means to adjust (fine tune) the soundboard other than shaving braces still intrigues me....

There are many ways. You can adjust the mass or stiffness of anything the soundboard couples with and that will alter the coupled main top resonant frequency, which is usually the one you want to fine tune.

[kiwigeo]

The idea of having a means to adjust (fine tune) the soundboard other than shaving braces still intrigues me....

There are many ways. You can adjust the mass or stiffness of anything the soundboard couples with and that will alter the coupled main top resonant frequency, which is usually the one you want to fine tune.

Eg. weights attached to sides.