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Thread: Tell me why not: Carbon fiber bracing.

  1. #26

    Default Re: Tell me why not: Carbon fiber bracing.

    Peter, If you use carbon fiber tow on top of very thin wood braces, are you reducing the size of the wood braces?

  2. #27
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    Default Re: Tell me why not: Carbon fiber bracing.

    Peter, If you use carbon fiber tow on top of very thin wood braces, are you reducing the size of the wood braces?
    Yes. The idea is to make the braces stiffer so I can make the braces smaller and also can make the top a bit thinner as well. Carbon fiber gets you into another dimension as far as stiffness to weight ratio is concerned, way higher than any wood. The end result is a top that is lighter, and that will make a louder and more responsive mandolin. The other thing that happened is it sounds quite significantly better than an X brace, and I was not expecting that much of an improvement in sound. Note that carbon fiber is not light, but you need a lot less of it to get the same stiffness. I can't remember the exact figures, it is in the Gore and Gilet design book, but it is something like a 2% increase in weight of the carbon fiber tow will give a 30% increase in stiffness. Big difference for very little weight gain. Note that the Gore and Gilet design book is heavy on maths so it can be heavy going if you want to understand the maths behind it all. I started reading it and going through the maths so I could understand it all, but it gave me a head ache so I skimmed over most of the maths. It is a very long time since I was a student at University and not too bad at maths, now mostly forgotten. You don't need to wade through all the maths, but Trever Gore has said he left the maths in the book because it explained and provided proof of the reasoning behind the concepts in the book. It is difficult to argue against that. Most luthiers won't understand the maths, but that is ok.

    One thing that is tricky if you use a completely different bracing pattern is how high to make the braces. You can use trial and error to get the answer, but that is awfully inefficient. I sat down with a calculator and used the fact that stiffness of a beam is directly proportional to the width and proportional to the cube of the height. Using a carved top mandolin as the reference and assuming 3 braces on a top 2.7mm thick I did the calculations. Came up with 5mm x 8mm and added 1mm safety margin since the braces will be planed down a bit after installation. Very rough approximation, but it was spot on. The free plate modal frequencies were spot on what I was aiming for - i.e. about the same as an X brace with CF but the new bracing pattern has no CF yet so the bracing pattern is more efficient. With the CF, the frequencies are higher and overlap what I measure on carved tops. We all use trial and error to make improvements to our instruments and use educated guessing to decide what to trial. Some people call it intuition, but I call it educated guessing because that is what it really is. What the Gore and Gilet books (and the other papers) do is to increase the educated part of educated guessing so the probability of the guesses being wrong is much less - i.e. the whole process becomes more efficient and hence faster.
    Last edited by peter.coombe; Nov-24-2022 at 6:54pm.
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  4. #28

    Default Re: Tell me why not: Carbon fiber bracing.

    Peter, Iím wondering why not use, say, 1mm x 3 mm CF strips directly glued to the soundboard? Iíd love to try an all CF bracing pattern, but obviously donít want it so stiff that the top wonít move. If the braces float and donít attach to the edge, I can also add a vertical CF rod anchored to the neck block and tail block. Instead of spruce braces and toe thread, which does strike me as prone to separating, what would be the thinnest CF I could get away with for a CF only bracing pattern?

    Dave


    The mandola is one of the best sounding I have made, flat top or carved top, and the mandolins are by far the best sounding flat top mandolins so far. I can hardly wait to try it on an octave mandolin. The braces are only 5mm wide and max 9mm (10mm on the mandola) high (main cross brace and center brace under the bridge). The braces taper down in height as you go out from the center. Now, the bracing of the top is only part of the story. How you make the back is just as important. Mine are made as a "dead back" system (see Gore and Gilet book), so think heavy, high internal damping woods, and hefty braces. Maple is not a particularly good wood for this system, but I have access to many species of heavy native woods. They are not actually "dead" backs, it is not black and white, but the contribution of the back to the sound is much reduced. The top and back are matched according to what is recommended by Gore and Gilet, they got it right. It is somewhat counter intuitive, but the heavier backs make a louder mandolin (and guitar). That is because the top moves more. Think of a see saw with one man heavier than the other. If the see saw is balanced by moving the fulcrum, the lighter man will move more. That is exactly what happens. The main top mode node (the fulcrum) moves out to the rim of the body. With a light back, the node is about 1inch in from the rim so the rims are moving and wasting energy not producing sound. A heavy back has a similar effect as adding mass to the sides of guitars as is described in the Gore and Gilet books. If the back is too stiff I can adjust it by filing down the center brace on back through the end pin hole. I measure the frequency of the main top and back modes with the strings on, but you can also determine if you need to reduce the stiffness by sticking some blue tack to the back and listening to the sound. If the sound improves with the blue tack then the back is too stiff. Blue tack adds mass and will lower the main back mode frequency. To get the recommended frequency interval, the back does need to be really really stiff and I can get it into the ball park from the frequency of the free plate modes, but the relationship to the assembled instrument is not exact, there is a lot of scatter if you plot the frequencies so I don't get it spot on every time.

    Note that as the mass of the top is reduced and stiffness is maintained, there will be more higher frequencies so the instrument will sound brighter and louder. This is simple physics and is explained in the Gore and Gilet design book. So, a warmer sounding back wood helps to make a better balanced sounding mandolin. Jarrah and Red Gum work really well, as does Macassar Ebony. I play a Macassar Ebony mandolin in a band and it has a massive dynamic range and can be heard. Note that these mandolins are all oval hole mandolins and oval hole mandolins usually get drowned by loud fiddles etc. This one does not.

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  5. #29
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    Default Re: Tell me why not: Carbon fiber bracing.

    Your desire to try something new, and hopefully innovative, is admirable. What is problematic is that you seem to want to do so without first gaining the basic knowledge pertinent to what you want to do.

    One of the themes I sense in your posts is that you want to try CF composite materials because they are much stiffer. In instruments plates, what is needed is not MORE stiffness, but rather the RIGHT AMOUNT of stiffness, whatever that is. That was pointed out by others earlier in this thread. Luthiers usually try to arrive at that right amount of stiffness by trial-and-error. Second, you want that RIGHT stiffness, whatever it is, to come at a cost of the least amount of mass. CF composite is significantly stiffer than wood, but also about 2 to 2.5 times as dense as woods typically used for top plates and braces. By just using CF composite bars of the dimensions you have described in your posts for your bracing, you are going to end up with a plate which is not only too stiff, but it will also be heavier than it needs to be.

    Wood is orthotropic, meaning it has different stiffnesses - actually Young's moduli, aka elastic moduli, in different directions. Wood is stiffest in the direction parallel to the grain, or growth ring, lines. It is anywhere from about 7x, to as much as 30x or greater, LESS stiff in the other two cartesian directions. The elastic moduli parallel to the grain tend to follow the density of of the wood, albeit with some variation. The perpendicular moduli, on the other hand, can be all over the place. Back in 1974 or so, Graham Caldersmith showed that to a good approximation, the overall flexural stiffness of a plate.is given by the square root of the product of the parallel and perpendicular elastic moduli. You can ignore the modulus in the third dimension because of the thinness of the plate in that dimension.

    In most wood instrument plates, the overall flexural stiffness is NOT defined by the braces alone (except for the plates built by the Australia luthiers mentioned earlier in this thread by Graham McDonald). Rather, the bracing tweaks the overall stiffness of the assembly. So with bracing, as with the plates, you want to get to that RIGHT stiffness, whatever it is, and it is important to be able to adjust until you get to that stiffness. Another factor is the ratio of the perpendicular stiffness to the parallel stiffness. Some of your wood plates will have a much lower ratio of the perpendicular modulus to the parallel modulus. In those plates, the bracing pattern can be varied to increase the perpendicular stiffness without significantly raising the mass. Ladder bracing, X-bracing, and lattice bracing patterns can all be used to increase the perpendicular or cross-grain stiffness. Similarly, some of your plates will have a higher ratio of perpendicular modulus to parallel modulus. In those plates, you may not need to add much cross-grain stiffness, so patterns like "tone bars" or a more elongated X, or fan bracing such as Peter's, will be appropriate. Finally, it was shown about 50 yrs ago now that the exact position of braces in classical guitars did not make a difference in. sound that could be discerned by experienced listeners. What I take from that is that we should use bracing to complement the properties of the plate, rather than as some kind of secret sauce. Hope this helps.

    So, before you begin to mess with the stiffness of your bracing, make some measurements of the properties (especially parallel and perpendicular moduli) of your plate woods. James Blylie had some articles in American Lutherie (#s 125, 128, & 133) on how to make those measurements. Particularly in AL #128, he showed how to make those measurements simply in your own garage. You could even improve on those a bit, e.g., by using a dial indicator with a magnetic mount instead of a ruler.

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  7. #30

    Default Re: Tell me why not: Carbon fiber bracing.

    Thank you Dave. I will look that up. The soundboard on this old Washburn is very thin, so thin that the cracked areas I’m going to glue can move an inch up and down. Ideally, after conventionally gluing and cleating, I’d like use very thin CF bracing to prevent it from collapsing but not stop it from vibrating. I know that the wheel has been invented with spruce braces, but I’d like to try a new type of wheel…. The area around the sound hole had the most warp but it’s now flattened with steam and clamping. I want to put a ring of CF sheet around the hole with a couple of vertical bars, because that’s how it deformed the last time. If I can get the top to vibrate properly with 1mm x3mm CF fan or asymmetrical X. I’ll risk it. If not, I’ll try PETERS spruce and CF toe. Will keep you posted.

  8. #31
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    Default Re: Tell me why not: Carbon fiber bracing.

    What exactly do you mean by "vertical bars"? If you are talking about something like the soundpost in a violin, there is reams of material and experience here and elsewhere that should convince you that it is not a good ideal for any plucked string instruments.

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    Default Re: Tell me why not: Carbon fiber bracing.

    I am also having difficulty understanding what you have in mind. Note that with CF, it needs to be away from the axis of bending to be effective. if it is close to, or on the axis of bending you will just be adding weight. That is why my fan braces are highest under the bridge and the main cross brace is highest directly behind the sound hole. CF is not light weight, but you don't need to use much of it. Too much and it will be heavy and way too stiff for good sound as already pointed out by Dr Cohen several times. My fan brace pattern has only 2gm of CF composite. That is actually made up of much less than a gram of actual CF tow, the rest is West Systems epoxy resin. So, tiny amounts of CF is enough.
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  11. #33

    Default Re: Tell me why not: Carbon fiber bracing.

    Quote Originally Posted by Dave Cohen View Post
    What exactly do you mean by "vertical bars"? If you are talking about something like the soundpost in a violin, there is reams of material and experience here and elsewhere that should convince you that it is not a good ideal for any plucked string instruments.
    I meant a couple of small popsicle sticks from the oval hole to the neck block area. But maybe I will just cut some extensions from the annular ring of CF sheet surrounding the hole , like the ears of Batman cowl. When I get my package of 1mm x 3mm CF strips in the mail, Iím going to see if they have some spring and rigidity. If so, maybe a floating fan pattern of three strips, plus a 3x3mm rod connecting the next and tail block. My idea is to provide, as close as possible, some weightless stiffening to the soundboard, allowing it to pump air like a speaker. Iíll try some tapping acoustic analysis afterward and see what the pattern looks like. Since the soundboard was never take off, I canít rely on the research on plate patterns. Iím going to look into whether there is any recognized way of evaluating soundboards attached to the instrument. Dave

  12. #34
    Registered User sunburst's Avatar
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    Default Re: Tell me why not: Carbon fiber bracing.

    Quote Originally Posted by DMand View Post
    ...allowing it to pump air like a speaker...
    While the fundamental mode of motion of a top or back is sort of like a speaker cone, there are more modes of motion as well, and all are involved in the mandolin making sound. In other words, the speaker cone analogy, while not completely off base, is very simplistic when it comes to top and back plate dynamics. There are papers and images, notably by Dr.s Cohen and Rossing, that describe and illustrate the lower frequency modes of motion of mandolin plates so the info is available.

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  14. #35

    Default Re: Tell me why not: Carbon fiber bracing.

    Peter,

    I think I may try your fan bracing, perhaps with a few 1mmx3mm rods. My old Washburn is now completely rigid at the neck, after gluing a thin, twill carbon fiber sheet around the soundhole, up around the tail block. I’m thinking of doing a fan pattern below that with maybe 3 CF rods. The fan pattern is interesting…just inside my absolute favorite guitar, a Skytop, and discovered that it is fan braced as well. So there seems to be something about that arrangement. Keep you posted. Have some more repair work before bracing. Dave

  15. #36
    Registered User j. condino's Avatar
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    Default Re: Tell me why not: Carbon fiber bracing.

    It seems like no matter what suggestions folks in this thread are making, the original poster is not satisfied. You are asking for definitive answers to a problem that only exists in your own head. Nobody else can help you with that. Pick a method, build it, and proceed. If you look at the depth of knowledge and literally thousands of instruments built by the combined people who have answered, we all have made dozens of instruments (at a minimum, likely hundreds combined) whose sole purpose was to answer more complex material and design questions that had no generally accepted answer.

    The answer from my end is always the same: turn off the $#@&ing computer & build some kind of test rig with a removable back like Dave and I have. You can answer all of your own questions and try out 20 different bracing patterns in the course of a day or two and move on. Even if I could give you an exact tested response from a previous build, creative & curious people like you & I never follow it fully. We always modify and experiment, often nullifying the previous tested results, unbeknown to our curiosity.

    If you want solid answers for traditional instruments, do your best to make a clone of a some old baby diaper brown 1923 classic design and materials and drop the unicorns and fairy dust. When you or I or the rest dream up crazy $#!t, own it and build the crazy $#!t......

    As a side note, the math was my favorite part of the book!
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  17. #37

    Default Re: Tell me why not: Carbon fiber bracing.

    Thanks. You’re ignite, I’m listening to ideas and want to try something new. How do you create temporary bracing without glueing and ungluing that might damage the old soundboard ? Dave

  18. #38
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    Default Re: Tell me why not: Carbon fiber bracing.

    Use paper in the joint between top and brace. Strong but easily removable.
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  19. #39
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    Default Re: Tell me why not: Carbon fiber bracing.

    Many people simply carve the braces off rather than release the glue joint.

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  21. #40

    Default Re: Tell me why not: Carbon fiber bracing.

    Quote Originally Posted by Vernon Hughes View Post
    Use paper in the joint between top and brace. Strong but easily removable.
    Brilliant. Will try that. Thanks.

  22. #41

    Default Re: Tell me why not: Carbon fiber bracing.

    Quote Originally Posted by sunburst View Post
    Many people simply carve the braces off rather than release the glue joint.
    That works with wood, but not if I experiment with carbon fiber. I’m going to try Vernon’s paper suggestion with a few CF braces.

  23. #42
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    Default Re: Tell me why not: Carbon fiber bracing.

    Vernon's suggestion is not gonna work out very well for you either. That suggestion works very well with WOOD braces, and especially when using hide glue. CF/epoxy components are usually glued with epoxy. Further, for good joints, the CF/epoxy composites usually need to have the surfaces to be glued roughened, usually with 220-grit sandpaper. If you use epoxy glue, your CF composite bracing will NOT be easily removable, even with the piece of paper in the joint. If you use any other type of glue, the joint will not be very good.

  24. #43
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    Default Re: Tell me why not: Carbon fiber bracing.

    Might be able to release epoxy joints by heating the CF brace.
    Many years ago, I epoxied oversize bone point protectors onto an F5-style mandolin. When the epoxy fully cured I started grinding the bone to shape on the belt sander when suddenly the protector fell off! I didn't understand why the epoxy didn't hold until I picked up the piece of bone and found that it was hot from the friction of the belt sander.
    That experience taught me (among other things) that commonly available epoxy is easily released with heat.

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  26. #44
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    Default Re: Tell me why not: Carbon fiber bracing.

    Yep. Heat also helps w/ release of PVA glues. Also, not all epoxies are of similar quality. And, joints made with epoxies that have been sitting around too long before use (like a year or two) may fail easily.

  27. #45
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    Default Re: Tell me why not: Carbon fiber bracing.

    I got to thinking that I didn't make something clear enough for the OP. As sunburst pointed out, the lowest normal mode in plucked string instruments is somewhat like what people think of as a loudspeaker cone motion. Or put another way, what people think of as the loudspeaker cone motion is similar to the lowest mode (aka, the (0,0) mode) in plucked string instruments, But a common misconception is that that motion in plucked string instruments occurs mostly due to flexibility near the edges of the plates. In reality, shear and bending has to occur throughout the area of the plates. If one makes a plate that is very stiff throughout most of the plate area, but flexible in a smaller area near the edges of the plate, the effective stiffness of the plate will be very high - and yes, even at the edges. In fact, you can think of plate edges glued to a ribset as being clamped to a very large brace. So the stiffness there is relatively high, even if the plate is made thinner there. If you read my papers, or for that matter, any of the many papers using interferometry to image the motions of plates, you will see that the amplitudes of the lowest plate mode are GREATEST at the center of the plates, and smallest at the edges of the plates. If one makes plates that are very stiff except near the edges, the normal modes of the plates will occur at frequencies too high to be of any use in driving the lowest air modes of the instrument body cavity. The plate modes have to be close in frequency to body air modes in order for what is called coupling to occur.

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  29. #46
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    Default Re: Tell me why not: Carbon fiber bracing.

    I think I may try your fan bracing, perhaps with a few 1mmx3mm rods.
    That may not work. In fact it probably won't unless there is some form of incredible stroke of luck that the stiffness ends up the same.
    1x3mm CF rod is not the same as a wooden brace topped with CF tow. However, trial and error is part of the learning process of building instruments so if you want to try it, why not. However, if it does not work it will be difficult to reverse since CF composite can only really be effectively glued with epoxy.
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  30. #47
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    Default Re: Tell me why not: Carbon fiber bracing.

    To the OP: You have asked several questions describing your ideas for making non-standard modifications to a standard-built mandolin. It makes sense to ask experienced builders and repairers to review your suggestions, especially if your goal is to avoid a train wreck with unfamiliar techniques. That has resulted in a lot of information regarding carbon fiber bracing being collected in this thread, so thank you for that. Still, no one apparently has tried the specific things you have suggested, so in the end, as always, you are on your own to try your ideas and see how they work. If they work, then you will have contributed to this growing body of knowledge. If they don't, then you will have a first hand answer to your question – Tell me why not. Either way, I think we all look forward to learning something from your experience.
    Tom

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  31. #48

    Default Re: Tell me why not: Carbon fiber bracing.

    Thanks TOM. I’m giving a lot of thought to the bracing, since I’m gluing the soundboard cracks now, and bracing is next. I’m thinking of putting a spruce brace under the bridge or maybe a 3x3 cf rod. I’m going to but another lateral cf rod across the bottom of the cf annular ring amount the sound hole, wrapped up the tail box, which has made the neck nice and rigid. I’m thing of gluing 1/4ßinch cf tow flat strips of cf in a fan pattern in lieu of other braces so I’m hoping for a stiff very light strong bracing pattern. As you suggest, it’s a new idea and if it works you ‘ll be the first to know. Dave

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