Bracing Question (Yes that old chestnut!)

[Nick]

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"Originally Posted on:Thu Feb 04, 2010 9:11 am"

I've been asked to make a bit of a 'show' guitar for a friend of mine who's in a band that is just starting to get a name for themselves in this tiny land. My past efforts have been archtops and Selmers. The archies just have two braces and the Selmer's are of course ladder braced so this will be my first 'conventional' Martin style X braced twanger.
As part of my 'pre-build' research I've been looking at bracing patterns, shapes (parabollic vs Scalloped) e.t.c. As well as several different plans I have of guitars braced this way I've also been trawling on the internet and I have a few idea's floating around in the primordial stew of my brain already! One question I have, is I seem to remember there was some discussion some time ago in the luthier community (wasn't this forum) about the two lower bout braces
img~

& what effect changing them from being parallel to splayed made (if any). Only thing is I can't remember the outcome so I was wondering with all the collective knowledge floating about on this forum, if people would add their thoughts on the subject so that I can make an informed choice when it comes to laying out my bracing.
The size of the guitar will be around about a OOO size (15.25" lower bout) and in true Oliver style I have an idea that is slightly different for a soundhole, so I've decided if my idea didn't work I would make two at once, one conventional and one with my fingerprint on it, so I'm looking forward to getting stuck in!
As I was trawling the net I also came across this & wondered if anybody had any thoughts on it ?
http://www.kinscherff.com/BracingSys.html
The guy seems to think it works with benefits, but then he is trying to sell the idea and guitars to his customers so may weight his findings somewhat.
Any thoughts/suggestions gratefully considered

[jeffhigh]

Drilling the braces like that SEEMS like a good idea and if done properly could save you a little weight.
However-

-it may make the braced more prone to splitting with impact
-It limits your ability to do more voicing after drilling
-His whole bracing looks rather substantial anyway, cancelling out any weight loss and perhaps being too stiff anyway.

[Matt_Blacka]

I can't help with the main question here, but I find the swiss cheese bracing interesting.

Definately one way to reduce mass. Remembering back to my structural engineering courses, I think there is some rule about the maximum size of holes that can be put through structural members (such as beams), without having any real effect on their strength. However, steel and concrete beams are somewhat less prone to splitting along the grain than little timber guitar braces...... It also looks like there is a whole load of extra mass on those braces with their rectangular cross section, compared to nicely tapered triangular or trapezoidal cross section braces. While puting holes in the braces may not reduce their strength (?) I'm sure it would affect the tune/tap tone of the braces dramatically.

Matt

[sebastiaan56]

Ive looked at this a number of times as well. Jeff and Matt's points are what I have heard before and are very valid. Just to throw the cat amongst the pigeons, what about a bit of CF on the swiss cheese? Would help with the splitting risk and add that new hi-tech mojo that seems to be the point of this bracing.

[John Steele]

I remember back in my construction days that drilling any hole in glu-lams
(laminated engineered beams) was a big deal. The engineers would come running with load calcs and such showing how much a simple 1 inch hole for a conduit degraded the beam's integrity. My simple understanding of wood is that it is composed of layers (rings/grain, etc) which is the basis of it's strength. Disrupting them can have a big impact on the piece's overall strength, whereas metal, such as in a bridge support, not so much depending on the load
point(s) in relation to the holes. Sounds like he is getting good results so maybe the benefits out weigh the risk. As for angling the tone bars, I dunno. Open (wider angle) = loose, while more acute angle = tight ??? Mine were always parallel to the lower part of the X if that means anything.
Is the bridge plate in your pic ebony or ebonized maple?
John

[Nick]

John Steele wrote:
Is the bridge plate in your pic ebony or ebonized maple?
John

Wouldn't have a clue John, I nicked this picture off the internet Just to illustrate the braces I was talking about (I believe they are refered to as Lower Finger Braces?). My plates are far from being in that state, they still only occupy a large empty space in my head at the moment. Thanks for your comments regarding them. I must admit all the image searches on Google have rendered only parallel finger braces so I'm begining to think that it either wasn't a goer or the discussion never happened & I only imagined it.

[John Steele]

Check out the OM drawing here:
http://www.grellier.fr/plans.php?lang=en
Definately not parallel...
J

[jeffhigh]

Most refer to those braces as "Tone Bars"
A pretty meaningless name but commonly understood.

[seeaxe]

Nick,

Can't offer any advice on where they should go, but I can comment on the stiffness versus mass issue and the rectuagular versus shaped brace.

The brace must acting as a T-beam of sorts, with part of the soundboard to which it is glued. The fibres controlling what happens are the extreme fibres on the outer edges of the beam and soundboard (once its glued it is acting as a composite section). Thus a hole drilled in the effective midpoint will make little difference. However if you drill a hole at the outer edge of the beam you will make a huge difference.

A good analogy is a truss which is a beam with a lot of holes in it. When you look at really big span beams where mass is a problem, they are usually trusses - aircraft hangars for example. Stiffness and low mass = truss. You can poke all sorts of ducts and crap through the truss, but cut the top or bottom chord and it all falls down.

We have seen some guitars with truss braces I think. The endpoint of this is whether or not the highest possble stifness to mass ratio actually gives you a better result! I certainly dont know but I suspect if this was the holy grail of soundboard bracing, we would all be drilling holes in our braces. There are enough experimenters out there to have found out by now.

So if you want higher stiffness and less mass, then don't shape your braces, just drill holes in them. The Kinscherff rectangular braces may not look right but they will be stiffer than the equivalent height shaped brace, purely because there is more material at the outer extreme of the brace/beam. If they look beefy its because they can afford to be - they probably don't weigh as much as a shaped brace. It looks cool to me.

Thats my two pennorth! Good luck Nick.

[Dominic]

Nick, Just watched the Ervin DVD and he explained the best way to lay out braces is so they evenly distribute the forces and support the top evenly. That means spread them evenly over the face. Thinking this through, dividing up the top differently would mean some small unsupported areas and some larger ones. The larger larger areas would be looser but there would be other relatively stiffer areas nearby on the lower bout area.

The stiffness mapped top that Ervin shows in the DVD (and in the books)is interesting. Using the cube rule for stiffness of tops and braces he suggests that the braces create a very complex structure consisting of very stiff areas that respond very differently to string energy transfer than the far less stiff area next to it. Difficult terrain for sounds. Very interesting point.


The holy braces make sense (and look kinda cool) but most of the weight is in the top isn't it? Charles Fox uses something like this on his Ergo guitars last time I looked.

As seeaxe says, modern truss engineering can probably shed a lot of interesting light on what we are trying to achieve.

Cheers
Dom

[vandenboom]

Nick O wrote:
......the two lower bout braces & what effect changing them from being parallel to splayed made (if any)

I have heard some analysis of this aspect on a John Mayes DVD on voicing the top. It's a while since I have looked at it but the gist of it that remains with me is the old tradeoff between the bass response you are after, stiffness of the top and stiffness of braces being used. eg. if you want more bass response and the top is quite stiff, you would splay those two 'tone bars' more to loosen things up a bit more. Another variable here is how close you bring the braces to the linings or in fact whether you run them under the lining.

I do like strong bass response in my dreadnought sound so I have really tried to focus on this for the last couple of guitars. I think it has paid off.

Frank

[Nick]

Thanks for the input Richard. I found that "holy brace" site whilst ogling pictures of braced soundboards. I wasn't planing on using that style of bracing, it's intersting but damn ugly to my eyeball. Just thought I'd throw it up for Members to wonder about and discuss merits/ faults.
I can see the design factors behind it after being in the engineering field for 31 years! (christ has it been that long?!!) and involved in designing structural/load bearing members for probably a quarter of that time, coming at it more from a practical experience point of view rather than a slide rule in hand standpoint. Just not sure I'd want to turn the bracing into swiss cheese as opposed to a nice flowing brace that looks like it belongs on the back of a soundboard. Asthetics and finish inside the box are as important to me as the outside.

Thanks Dom and Frank, some really interesting comments for me to consider. I can see Ervin's standpoint of support across the whole plate whilst still allowing resonance and I guess that is why these two are angled rather than straight across, the angle is allowing some movement across the plate between braces(especially if the ends aren't tucked into the linings) whereas going straight across would just tie all the lateral stiffness in together. I can also see Franks point about compensation between the top and braces, if the top is stiffer open these up abit more to compensate. Hmmmmm Simple stuff this Luthierininin ain't it?

[Dominic]

An alternative to opening the braces is to think carefully about their size and shaping etc once the top is together. More heavy scalloping in certain areas can give you all the bass you are after. Factories may choose opening up braces in order get more bass instead of more labour intensive hand carving braces but hand builders aren't so constrained by the need to rush guitars out the door.

A good i picked up from Kent Everett’s dvd was where he suggested we not change too many variables at once and concentrate on making small controlled changes to determine their impact on sound.
Cheers
Dom

[matthew]

does anyone scallop braces very slightly on the underside so that the glue contact surface is reduced? this technique i've seen done on basses to try to reduce the back braces tendency to come unglued when the back shrinks or expands.

[woodrat]

Nick,With regard to the Holey Brace picture I dont think that he would have very much scope to tune the soundboard because if he reduced the height to decrease stiffness then he would quite easily carve into a hole and that would mean an instantaneous and uncontrolled decrease in stiffness as soon as he cut the last fibres over the hole. It looks like a nice piece of Art though.

John

[Allen]

matthew wrote:
does anyone scallop braces very slightly on the underside so that the glue contact surface is reduced? this technique i've seen done on basses to try to reduce the back braces tendency to come unglued when the back shrinks or expands.

I've never heard anyone doing that Matthew. I honestly don't know if you'd see an advantage to doing something like that on a guitar. The top and back plates are just so much smaller, so less movement than the double bass.

[jeffhigh]

Why do we shape the cross section of braces to triangular or rounded?

-because Martin does it?
-because it looks better?
-because it is a sign of a fine handmade guitar and I would face ridicule if I left them square?
-to reduce the mass of the brace? (going fully triangular reduces the mass to 50%)
-to reduce the stiffness of the brace? (going fully triangular reduces the stiffness of the brace to 33.333%)
-because****### says to do it?

[Nick]

jeffhigh wrote:
Why do we shape the cross section of braces to triangular or rounded?

-because Martin does it?
-because it looks better?
-because it is a sign of a fine handmade guitar and I would face ridicule if I left them square?
-to reduce the mass of the brace? (going fully triangular reduces the mass to 50%)
-to reduce the stiffness of the brace? (going fully triangular reduces the stiffness of the brace to 33.333%)
-because****### says to do it?

All of the above?

[matthew]

Not a guitar maker, but instictively the aim to maximise stiffness and glue surface, and minimise weight leads to a pointed brace shape, unless you are using "t-beam" theory and CF.

[Nick]

Actually it's interesting timing, but my latest GAL magazine arrived and bugger me there's an article on bracing and the different sectional shapes and what factors to consider. Haven't read it yet (looks like there's alot of numbers and mathematical symbols in it ) but will try and digest at the weekend.
I'm sure somebody at GAL is getting into my mind, first when I was designing an adjustable bridge for the Macca, now the timing of this article! Hmmm scary. Should I be getting around with a tin foil hat on?

[matthew]

Nick O wrote:
Should I be getting around with a tin foil hat on?

Definitely. It should be slightly pointy, and properly earthed.

[J.F. Custom]

Hey Nick.

It is interesting timing for you to have touched on the whole holey brace issue too. I've just finished some work of this style and here is my take on the how and why and conclusion.

Here is what I have done on the back plate of this guitar bodied bouzouki -
img~


And on the top - a hybrid 'X' brace/lattice, not too dissimilar to that used by Graham McDonald... who we haven't heard from for some time...
img~


OK so how and why - has pretty much been covered above. I was in essence trying to reduce weight while maintaining most stiffness in what I hoped was a controlled or controllable manner.

I used a combination of styles in the 'holey' bracing top though. Firstly, I only used it for the main 'X' brace. These were 5mm wide and I profiled them to 3mm wide at the top - so still similar to a standard brace. Then instead of scalloping the braces between the X crossover and the bridge plate, I curved the entire brace gradually from the 'X' out to the edge, before scalloping the final 20-30mm. My theory for this was to gradually reduce the stiffness as I approached the holes out toward the edges. This was carried through to the point where I completely and deliberately broke the surface of the last few holes by varying amounts to significantly increase the 'flex'. I then capped the lot in 1mm x 3mm carbon fiber to regain some overall structural stability with the hope that the carved spruce below dictated the amount of stiffness across the whole 'X'. Much as a pure lattice has the most stiffness in the centre and tapers to all edges. I hoped the X would be most stiff in the middle, and more 'responsive' at the edge.

Thereafter I intended to use the 2.7mm wide lattice to control the stiffness of the rest of the plate and adjust the tone - carving and tapping as usual. The overall shape of the lattice is rounded with further scalloping at the edges and adjoining the X.

I hoped the design would allow the plate as a whole to vibrate via the reduced stiffness in the perimeter. Then I hoped it would 'rock' side to side (cross-dipole?) in the narrow X brace, with reduced stiffness at the edges meaning it could 'rock' around the centre stiff point. And finally possibly 'rock' end to end (long-dipole?) for much the same reason. But also the fact that the X is a fairly central stiff point. I considered the upper bout heavily braced and resisting most movement, so I left the lower section of the lattice uncoupled to the X to allow a bit more movement below.

So that was the thinking - flawed engineering and design? Quite possibly Very Happy Wink I'm no engineer so possibly yourself or Jeffhigh will enlighten me to my mistakes... Anyway, I like to try new thoughts and see how they go - I learn that way.

Now the conclusion - in a nutshell, I doubt I'll do it again!
I found by the end that I still felt the central area to be too stiff and found I have very little ability to adjust that any further. I didn't want to scallop this area as thought that would really weaken the structure too much. Therefore I felt the whole was probably still over-braced. Anyway, I'll see how it pans out when the strings are on.

I think if you were to use this design, you would need to have your bracing at a point where you know exactly what it is doing in a repeatable manner, and thereafter what each size hole drilled where would do in turn. I don't really see this as likely as you need to adjust for each piece of timber so I feel it limits your ability to adjust as you go. My 2 c.

Finally Matthew, did you mean something like this design by Charles Fox? If so, I've seen this type of removed section/floating brace many times.
img~


Cheers,

Jeremy.

[Dave White]

Sometimes I think that guitar makers "invent" new bracing patterns and constructions as much to stop themselves becoming bored as to find the Holy Grail of tone. I know I'm guilty and did this on my first uke build when I discovered that the trick is to not actually brace it at all. My solution was to make a uke with twelve strings on so that I could invent some bracing to keep me occupied


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