suspended bracing

BMS · Post by **BMS** » Sat Jun 21, 2008 6:40 pm

Hello Everyone:
Here is a new topic about flying bracings: To start the topic I will do a very short history of them:
-To my knowledge the first to use such a thing was Scherzer in the 1850â€™s. He used a rod of steel from neck-block to tail-block secured by a screw in the tail-block; you could adjust the tension with a flat screwdriver to compensate string tension. Remember Scherzer made a lot of guitars with more than 6 strings (up to 12 floating basses!), you clearly can see the device on this photo:

A lot of makers from Germany and Austria followed this maker: Resinger, Brauer, Raab and much more. Surprisingly even Fender used a similar system in some folks from the 60â€™s! Some modern makers, like myself use them with one difference: Usually new makers just glue the bar in place (made ot of maple, spruce, fiberglass or graphite) , I prefer to leave it floating in between 2 supports with an Allen to adjust tension on the device. Matanya Ophee made an entire experiment with one of his Scherzers adjusting the tension to see the influence in the sound. You can read that on the Google newsgroup here:

http://groups.google.be/group/rec.music ... bar&lnk=ol&

-Another approach of that are the flying braces used by Steve Klein on his guitars but in that case the bars just hold the bridge-plate in positionâ€¦
Best regards
Benoit

Dave White · Post by **Dave White** » Sun Jun 22, 2008 3:08 am

Benoit,

Thanks for this - interesting stuff!! Do you know of any historical versions of the flying buttress braces that go from the neck block into the sides like Mike Doolin/Rick Turner use as opposed to straight front to back as you show - which I think the Larson brothers used as well.

These are the sort I mean:

Rick Turner · Post by **Rick Turner** » Sun Jun 22, 2008 5:29 am

There are "Tilton's Patent Improvement" guitars from Boston in the 1890s. The curved brace in the Baby Taylors doesn't work...it flexes too much. Ditto with that in the Klein/Taylor bass.

For a good look at what I do, check the brand new issue of Fretboard Journal and you'll see that I use the double A flying buttresses as well as carbon capped back braces and center seam reinforcement. I also double the sides up at the top 1 1/4" or so around the top joint and use Charles Fox's reverse kerfing. This all helps to create a very stable skeleton inside the guitar that will resist the long term stretching, torquing, and change that affects acoustic guitar bodies and particularly the set of the neck. All this coincidentally takes a lot of pressure off of the upper bout of the top, and with the cantilevered fingerboard, it allows at least the possibility of getting tone and volume out of the upper bout.

BMS · Post by **BMS** » Sun Jun 22, 2008 6:15 am

Thanks for the info!
Benoit

Rick Turner · Post by **Rick Turner** » Sun Jun 22, 2008 3:26 pm

The problem with the flying buttresses pictured above is that the bottom pair are going to want to work in tension as the top pair work in compression. Those better be really good glue joints in there. I run both sets up as high as I can get them so all four buttresses are in compression with the force of the upper portion if the heel pushing in on the neck block. Then the CF reinforced back strip and the back itself take the tension load, and the CF topped back braces help maintain the doming of the back itself. This is straight ahead engineering/architecture balancing the forces of compression and tension in the shape of the guitar. The double "A" line approach is very stable and also resists any twisting of the neck block. You actually have four triangulated strut members carrying forces down to the almost vertical line of the sides in the lower waist.

Rick Turner · Post by **Rick Turner** » Sun Jun 22, 2008 3:45 pm

Here's a pic:

Allen · Post by **Allen** » Sun Jun 22, 2008 4:58 pm

Thanks for that picture Rick. Makes it a hell of a lot clearer to see what your describing. It appears that the buttresses are dowels. Would you mind sharing with us where you source your carbon fiber from?

Dominic · Post by **Dominic** » Sun Jun 22, 2008 5:00 pm

Rick, Iv'e seen this picture before and it made me think about the CF on the back braces.

My understanding is that the thin CF strips on the top of the braces work better in tension than compression when stuck on top of a brace. That is, very hard to make them longer unless they crush the brace but they could corregate a bit by lifting of the brace under compression and get slightly shorter. You can pull a strinig but you can't push it.
If this is the case, the back in the pic has more chance of bending into a smaller diameter curve than of flattening.

If I think through the forces acting on the flying braces, would it not be better to have the CF on the bottom of the brace, against the back? Or on the bottom and the top with the actual brace doing the job of keeping the two bits apart and working structurally together like a truss?

Like wise if you use CF on the top. Wouldn't you want the CF on the top of the braces in front of the bridge where it wants to flatten out and underneath the braces behind the bridge where it wants to push up.

Dom

Dave White · Post by **Dave White** » Sun Jun 22, 2008 7:32 pm

Rick Turner wrote:The problem with the flying buttresses pictured above is that the bottom pair are going to want to work in tension as the top pair work in compression. Those better be really good glue joints in there.

Rick,

Well yes the joints are good as with all joints on my instruments - the earliest instrument I made with this system is rock solid after three and a half years (an 8 string) but I will have to wait another 120 years or so to be definitive I guess

. You and Mike Doolin were my inspiration fo adopting this style of building allowing me to concentrate on tonal bracing of the whole top and I owe you both a huge thankyou.

I am a little confused here though as you rely on a cf back-strip in tension as well to resist the forces on the bottom of the neck block. Are you saying in "engineering terms" that cf works better under tension your way? Also I see that your A frame doubling gives more support against the compression on the top of the neck-block but the lower leg takes this force down at an angle into the back rather than straight into the plane of the sides in the top one. Also in terms of neck-block twist resistance wouldn't the two parallel pairs give a better balanced response to these forces.

Your system is very elegant by the way and I'm not being argumentative here just interested in the engineering.

Rick Turner · Post by **Rick Turner** » Sun Jun 22, 2008 10:21 pm

Look at where the forces on the neck block change from compression to tension. It's at about the half way point of where the heel would be. Thus the top of the block under the top itself will be under compression, and the bottom of the block at the back will be under tension as the leverage of the neck from string tension works on the neck block and body of the guitar.

I'm using and helping the natural tensile strength of the back to resist the torquing of the neck block, and the CF on the back strip helps resist the tendency of the back to flatten out over time from the pull of the neck block. Using the back, back strip, and back braces for tensile strength puts that strength as far from the neutral axis of the neck block as possible given the torquing forces, and by "improving" upon the natural tensile strength of the back, the CF is being used in close to it's optimal location.

I discussed this issue of the back flattening out with age with Tim Teel at Martin over a nice lunch in Nazareth, PA one day. Tim's assessment was that if they could build guitars with truly flat backs, those guitars would never need neck resets because the overall shape of the body would stay OK and the neck block would not rotate forward.

Yes, I could make these even stronger by laminating CF to the underside of the braces where they are glued to the back, but I wanted to have a wood to wood glue joint there so any slight movement had a chance rather than building up too much tension. So look at it as a truss with CF on the top of the brace and the back on the bottom. With the load of the strings...or a person...on the back; the CF on the back braces is indeed in tension...just right for CF. Another way to look at it is that it's a partial I beam truss and a partial stressed skin panel. As it is, the CF on the top of the braces makes the backs strong enough to hold a 160 pound load on the center of the back...I know, because I stood on one, and in fact the forces acting on the back to flatten it out work the tops of the braces in tension.

The CF dowels in the picture are solid 1/4" rods. I'm switching to tubes because I think the rods are overkill, and now I'm trying to pull back a bit and lighten these up. I get the CF from "the Graphite Store" on-line.

If I were to truly optimize this design for just engineering, I'd probably redesign the body shape so the sides in the lower bout waist area were perfectly in line with the flying buttresses. That vertical wall of the sides constrained by the back and top and reverse kerfing is incredibly strong in compression. As it is, the angle of the buttresses meeting the sides isn't too radical, and I just like the shape as it is.

There are a lot of force vectors going on with guitars, and I'm trying to use the natural structure of the guitar's body shape as much as possible, and then give it a bit of help here and there.

There is a point where strong and stiff enough is strong and stiff enough...
As I said, I now know that these are probably overkill, and now I'm shaving weight and lightening the guitars up a bit. This all does give me a lot of flexibility with regard to how I deal with the top as it doesn't have to support the neck or fingerboard.

I just had back my first of the tilt neck guitars that I built (not the first with buttresses); I added a pair of side sound ports. That guitar was built in 2001, and it's as solid as the day it left my shop on its way way down South...

There are some other things I want to try in coming years; this design is a work in progress.

Dennis Leahy · Post by **Dennis Leahy** » Mon Jun 23, 2008 12:57 am

First of all, I'm a complete beginner, so realize that these are the ramblings of a beginner and experimenter.

Also, I realize that luthiers may have different engineering ideas for what suspended bracing or buttress bracing is supposed to do for an instrument. That is, it may be used primarily to keep the neck block from rotating or to handle anything from a percentage of the string tension (shear) to all of it. I'm trying to handle all of it, so my designs will appear to be overkill to many people. Even for what I'm trying to do, it is overkill, and would take a few experiments making the bracing members smaller and smaller to find the minimum structure that will handle all of the 180 pounds or so of string tension.

I'm using a bridge like a pinless bridge, but then the strings continue back to a tailpiece. So, I'm taking all the string shear force back to the tail of the guitar, and I want my suspended braces to take all of that string force back to the neck block. I also want to permanently prevent twisting of the neck and tail blocks. My whole reason to do this is so that I can "discard" the X-brace and standard Martin style bracing, and have a clean slate to experiment with very lightly braced "sonic" bracing patterns.

So, now that you know what I was trying to achieve, here are photos of my first and second designs:

"Angelina" Guitar
These triangulated White Spruce suspended braces also included a transverse suspended brace, touching the top only at the extremities. The idea was to help hold the soundboard arch. I found that mortising rectangular braces into the neck and tail blocks was time consuming and I'm not really confident that the neck and tail blocks can never twist. So far - 2 years - so good. (The body shape, especially at the butt, has nothing to do with this suspended bracing topic.)

"Basia" Guitar
This one is just started. At first, I designed just a pair of suspended compression rods as close to the soundboard bracing as possible, which would have handled the shear. But I decided to add the lower two tension rods, to ensure that the neck and tailblocks cannot twist. (The lower rods could have been much smaller diameter.) All four of the rods (Oak dowel rods) are epoxied into drilled mortises. I could save a few grams of body weight by using CF tubes, and may do that next time, but this is really not very heavy. (It looks especially heavy because this is a parlor guitar.)

So, that's where I am in my engineering development and experimentation.

Dennis

Rick Turner · Post by **Rick Turner** » Mon Jun 23, 2008 5:53 am

That will certainly work.

With the exception of the flying buttresses, I'm trying to keep the structure closer to the plates...the surfaces of the body...where less mass does more good. The closer to the neutral axis of stresses and forces you put structure, the less it does.

I'm also trying to free up more of the top to vibrate, and so I use a very small neck block and tail block...1/2" thick Baltic birch with about the same surface area as the kerfing for the top to glue to.

BMS · Post by **BMS** » Mon Jun 23, 2008 6:46 am

Here is the last harp guitar inside I did with 2 flying braces:
One long from tail to neck-block adjustable in tension by an Allen key and one glued Ã la Turner. The other side is supported by the BIG maple block who has a double use: Neckblock for the "strat mounted" neck and pin bridge for the 8 super treble strings.
Cheers
Benoit

Rick Turner · Post by **Rick Turner** » Mon Jun 23, 2008 7:13 am

Very cool, Benoit.

A friend suggested the adjustable rod idea for my buttresses, and I may just try it. Might be another example of overkill, though. The guitars I've made over the past 11 years or so that have either a pair or two pairs of buttresses don't seem to have moved at all.

BMS · Post by **BMS** » Mon Jun 23, 2008 7:33 am

Dear Rick:
Mine have 21 strings all attached to the top... So maybee I prefer to overkill a little

Here is Muriel Anderson playing it at a concert in Gent.
Benoit

Dennis Leahy · Post by **Dennis Leahy** » Mon Jun 23, 2008 7:45 am

Rick Turner wrote:That will certainly work.

With the exception of the flying buttresses, I'm trying to keep the structure closer to the plates...the surfaces of the body...where less mass does more good. The closer to the neutral axis of stresses and forces you put structure, the less it does.

I'm also trying to free up more of the top to vibrate, and so I use a very small neck block and tail block...1/2" thick Baltic birch with about the same surface area as the kerfing for the top to glue to.

Rick,
The back strip being an integral brace is a good idea rather than the lower suspended braces that I used. My first thought along that idea would be to use a tall, skinny, laminated back seam brace, with CF vertically down the middle. Seems like that could really hold the longitudinal arch, and I suppose if the back shape cannot distort the longitudinal arch of the back over time, then the neck and tail blocks cannot splay.

With my untrimmed sides shown above, it appears the Oak dowels are intentionally close to the center of the guitar, when in reality, they are as close to the top and back as I dared to go. I didn't want the soundboard to vibrate and have the soundboard braces hit the suspended braces. They would be slightly further apart on a deeper guitar.

I have the same thought with making as much of the top accessible to vibrate as possible, but my stubborn streak that does not want the truss rod erupting through the headstock means that I need to chop a hole in the top to allow the neck tenon to come through. (The end of the truss rod has to have something to push against. Plus, the furnituremaker in me does not want to use a butt neck with threaded inserts.) You can't see it in my photo above, but the parlor "Basia" guitar has a neck block that does not touch the top at all. I added small strips of wood at the beginning of the neck block, the same width as the lining. The heel block is tapered in a curve to provide only the width of contact that the lining does.

Benoit,
Tell me a little bit about the adjustable tension on the CF tube. Do you need to loosen it if you take all the strings off?

Dennis

BMS · Post by **BMS** » Mon Jun 23, 2008 7:57 am

Dennis:
Yes if it is for a long period like repairs... I can even compleatly remove the cf through the bass arm soundhole.
Benoit
PS: Another off topic: the necks is holded by 2 screws at the body and one at the headstock that adjust the neck angle.

Allen · Post by **Allen** » Mon Jun 23, 2008 5:45 pm

I was sent a link to this site to check out, and thought that it would be of interest to those of you considering suspended bracing.

http://www.brunner-guitars.com/flash/

Have a look at the "Flying Top" section.

BMS · Post by **BMS** » Mon Jun 23, 2008 6:15 pm

If you want to try them and not spend too much mooney IBANEZ just have them under licence... They are called the ACS.
Benoit

kiwigeo · Post by **kiwigeo** » Mon Jun 23, 2008 6:46 pm

I received the latest edition of The Fretboard Journal today....contains a nice article on Rick Turner and also Les Paul.

matthew · Post by **matthew** » Tue Jun 24, 2008 12:17 pm

This is the tone bar inside a double bass made by John Devereux in about 1860. The bar ran from block to block, not touching the top.

You guys are way behind the times ...

(BTW, AFAIK just about all Devereux's instruments have since had their tone bars removed ... )

Jim Howell · Post by **Jim Howell** » Wed Jun 25, 2008 7:00 am

Just received my copy of the Fretboard Journal. I have a quick question on the manner that the CF rods are attached. A close inspection of the photo's looks like the rods have hemispherical ends fabricated that then fit into a hemispherical hole in the wood. Is this joint glued? Also wanted to say that the top bracing pattern is very interesting.

Hesh1956 · Post by **Hesh1956** » Wed Jun 25, 2008 12:24 pm

Teuffel on his very cool Birdfish guitar using tone bars that are replaceable with different woods to provide a different tone.

Dennis bro when I looked at your four oak dowels I wondered what would result if you expanded the function of the the dowels to be tone producing members and not just structural members.

Perhaps using dowels of Honduran Rosewood, BRW, Mad rose etc. might be a tone enhancing mod?

Rick Turner · Post by **Rick Turner** » Wed Jun 25, 2008 3:51 pm

The CF rods are superglued into the sockets. As they are in compression, I don't see any issues with that. They could just as easily be epoxied in. They're installed at the same time the vertical waist post is glued to the sides.

The top bracing is a hybrid of "X" and fan, and the "X" braces have coved out sides to make them more or less "I" beams. I did some weight/stiffness measurements and that give a net gain of about 7% on the stiffness to weight factor. Every little bit helps. I'm tying the fans to the bridge plate with short lengths of CF. I'm not (yet) going to CF topped top braces because I still want to be able to go in through the soundhole and tweak/shave bracing if I want to revoice the top. I have learned to be patient with how these open up. They start off on the tight side, and then start really putting out after a couple of weeks. The first 24 hours and then 48 hours are dramatic. I don't want to have these go flabby and boomy, I'm looking for that real mid punch with a lot of harmonic sustain supported by a fast and solid low end. Projection is what I'm after, and I'm getting it with this design.

Jim Howell · Post by **Jim Howell** » Fri Jun 27, 2008 7:50 am

Thanks to all for being incredibly generous in posting this information. This has to be about the most informative and interesting thread I've read in quite a while. Wow! Lots of folks thinking outside the box.

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