Stiffness of X bracing

[Dominic]

An interesting discussion came up on another forum about the structure and stiffness of X bracing. Unfortunately that discussion turned into one of the most personal and distasteful threads I have ever seen on a guitar makers forum. I thought I would pose the question here for two reasons, 1. to see what people think and 2. to see if us Aussies can conduct ourselves in a more mature and constructive manner. I suspect the answer to 2 is of course we can so I am keen to hear what people think.

OK the proposition is, the junction of the X brace is the stiffest, strongest part of the X structure.

The answer is may seem counter intuitive but is no, the junction of the X brace is the weakest part of the structure.

A few caveats to ensure we are talking about the same thing.
Here we are talking about the X braces ability to resist string tension which means we are examining the stiffness at 90 degrees to the grain and we are looking at sections taken through the X brace and the top to identify the weakest section through which the braced top might fail. We are not talking about the influence of scalloping or secondary braces and the X can be capped as long as it remains the same size as the X legs. We assume the joint is perfect and introduces no structural weaknesses into the junction. These are to ensure we are only talking about the contribution of the X brace to the structures stiffness. OK

Here is my explanation.
With braces 10mm high and 6mm wide, the junction as a horizontal cross section through the top will have the top and a section of bracing 10mm high by around 8mm wide depending on the X angle. As you move back or forward of the junction, the total area of bracing increases until a cross section reveals 2 brace sections 10mm high by 8mm wide. That is, twice the brace area as found at the junction of the X brace. Here is a picture to show what I mean.
Cheers
Dom

X Pic.png (2.58 KiB) Viewed 24297 times

PS, try to stick to technical explanations of your argument and stay away from anedotes.

[ozwood]

Hi Dominic,

I'm Not a Technical Person by Nature , But I'll put In Lamans terms and see if I get your point , at the Piont that the two Braces intersect at the X there is a halving joint , so at that Point the two braces become one, not just as a Joint , but also as the total width and thickness are equal to one brace , and singularly they are really only half their height at that pioint , so either side of the brace is the strongest , but as they intersect they are at their weekest !.

and if everyone where as enlightened as I , they would use Trevors Falcate bracing and this thread would be superfluous.

Cheers,

[jeffhigh]

Yes that was a nasty thread, not just because of differing opinions or even ignorance, but because of a huge amount of malice on the part of one individual.

Structural analysis is often a difficult and involved process, I am rusty on it now and scientists and even engineers trained in other disciplines such as electrical engineering often have a very limited understanding.

The diagram Dom shows has it's limitations for a material like timber with directional fibres.

Look at an older gibson where they ran one brace through and butted the other braces against it.The butted glue joints from the end grain would normally be considered to have no tensile strength and so the crossection at the joint would only retain the Flexural rigidity of the one continuous brace
A lapped Martin Style brace would be slightly stiffer, and a capped lapped joint even more so but still not up to the stiffness of two individual braces

So I guess the answer from my viewpoint would be that it is pretty hard to detail an intersection in timber so that it is not the weak point.

[Bruce McC]

Good afternoon Dominic

My mature and constructive contribution to the debate you have initiated is so what?
The X brace design has been with us for some time and it seems to be favoured by
the majority of luthiers on steel string guitars.

As well as the patch on top of the X joint why not add a couple of reinforcing blocks
in the angles of the cross joint or alternatively reinforce with carbon fibre?

I have seen promo material from McPherson Guitars showing a cross bracing system
where the X braces are not half lapped but one passes over the top of the other, like a
bridge and the cross sections of the braces are not reduced in size at the cross over point.

[Philstix]

The point at which any piece of wood will flex the most is at its center. Try bending just the end of a long brace rather than the whole thing to prove it. The midpoint of any brace would be its most flexible point. But the brace is not a free span, it is glued to the top. The top and the brace ends are glued to the sides. For the brace to flex inward the sides must push apart. Each point along a brace must pull down the top in a widening area around it as it flexes. At the point where the x crosses the glue connection between the top and the braces is at its greatest. Whatever size piece you use to define the area of stiffness this will always be true. Thus the point of crossing should be the stiffest area of the top. The point farthest from any brace and the sides should be the weakest.

[Philstix]

Just to clarify things I am assuming an arch built into the braces(otherwise its flexing inward would require the sides to pull in not push apart) and leaving out any effect of the bridge, which would add to stiffness and help spread load if it is fixed, simply spread the load of the strings if it is floating. The most flexible point on the braces would be somewhere between the sides and the x crossing on the longer lower arms of the braces. Where exactly would be determined by the stiffnes of the sides relative to the braces. Just my opinion. I am in no way a structural engineer or makinig any such claims.

[jeffhigh]

The other thing to take into acount is that in resisting bridge rotation, the effectiveness of the X braces decreases with an increase in the included angle rather than increasing along with the area revealed in a crossection.

[Dominic]

The point is to fully understand the structure we are using. I was not trying to say don't use an X or any other motive. Its just always a good idea to know why we do things and the properties of the structures we are using. I've seen this come up a few times lately so I thought I would see what we think.
Philstix, you are talking about how much the brace deflects and you are right, it will almost always be closer the middle than the ends. But this is different to the stiffness at every point down the lenght of the top.

Here is another pic from above. Even is we remove any structural impairment from having to lap the X the result is the same.
The first plate corresponds with the top pic above. So the longetudinal stiffness at any line other than A is supported by twice as much brace material than at A. If we accept this then we must accept that A is the weakest point.
Cheers
Dom

X Pic2.png (8.13 KiB) Viewed 24206 times

[ozwood]

Hi Dominic,

Just to clarify I agree, and I see your point , the last bit was just a bit of fun!

Cheers,

[Allen]

I see you're point, and you've detailed it well enough that anyone should be able to understand it. I didn't see the thread that you were referring to as I don't frequent that forum just because of such goings on....so I'm not sure what the point of the discussion is.

The way I've come to understand the X-brace pattern is that we don't want, or expect the junction of the X to be the strongest / stiffest point if we expect the bridge to be able to "pump" the largest area of top possible using this pattern. To my understanding it's more of a hinge point (perhaps not the best description but make sense to me).

[Dominic]

No worries Ozwood. The fact that we were able to conduct this conversation without it descending into a free for all says a lot about the maturity of ANZLF members compared with some other forums.

Actually, the most disturbing thing about that discussion was how aggressive and absolutely nasty some of those have been towards Trevor as he patiently tried to explain various concepts. They have literally taken it upon themselves to wage a personal campaign against Trevor in his absence.
But if you feel left out of all the fun, don't fret. The attack seems to have expanded to include the incompetence of all Australian's.
Cheers
Dom

[ozwood]

Hi Dominic,

They are Crazy , I have The Highest Respect for Trevor, not only is he one of the Few inovators that is able to back up his claims with hard and fast data, Not hokus pockus and a don't question me
Mentality, but his generosity of that knowledge is truly amazing , I have been Using his Ideas long before the book came out becuse he shared them with the Students at the AGMS I am at the Point now after reading the Book , that I'm going too Just Go with it , Trevor and Gerards methods that is, it all makes sense to me, we are Fortunate to have Trevor as a Member of our forum , and to have him be so Generous with his Knowledge , I would hate To think that anyone would treat him , or anyone on this forum who is willing to share, for that matter, with that level of Contempt . I think the Yanks have the Attitude that They Invented the Modern Guitar , well that may be true , but Smallman ,Biffin, Gore and a few others Dragged it out of the dark ages , exposed the Superstition for being just that , and Tested it with Honest facts , not Hype.
I only use this Forum , I find the generosity of Knowledge , and the calibre of Person on this forum to be Execeptional , as John the woodrat recently Stated , he's found pretty much all on the Forum to be Honest and Decent people , and I agree.

So A big clap for Aussie/Kiwi luthiers , and our few Adopted members form OS .

Cheers,

[jeffhigh]

Yes that thread has been particularly nasty and has shown up the character of one santa fe individual pretty well to any reasonable observer.

Anyhow it should be obvious that banjos are an American invention therefore American guitars sound like banjos, whilst Australian guitars all sound like Didgeriroos. (or at least that is the standard of logic being displayed)

I still disagree with how you are looking at the stiffness at those two crossections, But at least here we can discuss things with respect for each other.
cheers
Jeff

[Dominic]

I see you're point, and you've detailed it well enough that anyone should be able to understand it. I didn't see the thread that you were referring to as I don't frequent that forum just because of such goings on....so I'm not sure what the point of the discussion is.

The way I've come to understand the X-brace pattern is that we don't want, or expect the junction of the X to be the strongest / stiffest point if we expect the bridge to be able to "pump" the largest area of top possible using this pattern. To my understanding it's more of a hinge point (perhaps not the best description but make sense to me).

Exactly Allen, and understanding this means we better understand the limits of the structure we are working with and better informs design tweeks we come up with. The issue came up because someone claimed emphatically that the X junction was the stiffest point in the structure, so we gave a similar explanation as above. Well, never let facts get between an idiot and his ego. I don’t care if individuals want to believe this stuff, but I think we owe it to the next wave of guitar makers not to leave these forums littered with false or misleading information. If the loudest most aggressive people endure the longest in these debates, sense and reason must step aside. There is enough myth and voodoo in guitar making already.
Dom
PS, Allen, I had not had much to do with this forum before either but I am finishing a couple of classical guitars so thought I would join and see what people had to say. And you have to post everyday or they cancel your membership so I got into a couple of discussions. Well, I think there would be less conflict and name calling on a forum where Tea-party and Occupy Wall Street protestors debate the merits of capitalism.

[Dominic]

Ozwood, agree with all your points. I am immensely proud that it was an antipodean who wrote such amazing books. To me, US guitar making seems to be all about driving while looking in the rear view mirror. Whereas we drive looking way up the road ahead, peering into the future. It also makes me immensely proud of the innovations that have come out of this relatively tiny population of guitar makers. We punch well above our weight. I only hope I can add to that reputation of innovation and quality as time goes by.
Cheers
Dom

[Dominic]

I still disagree with how you are looking at the stiffness at those two crossections, But at least here we can discuss things with respect for each other.
cheers
Jeff

OK Jeff, go for it. Where did my analysis go wrong?
Cheers Dom

[jeffhigh]

Well one of the main issues is that at crossections B and C, the braces pass through the crossection at an angle.They are not symetrical across the section.
They are actually less stiff against bridge rotation around this crossection than if they went straight through.
Your diagram shows an increase in brace crossection here and directly relates that to stiffness.
It doesn't quite work that way

[kiwigeo]

Um, nobody's mentioned finger braces........

[Dominic]

Hi Jeff, I thought we all selected brace sizes quite deliberately and then carved them to change their area of the brace exactly because it is directly related to stiffness. We reduce brace sizes to reduce the stiffness of the top. The cube rule relates brace dimensions to stiffness.
And, the angle of the brace legs at B and C to the string pull is the same as at A. If the junction is cut perfectly, the cross section at A will also be exactly the same area as one of the legs at B or C and have exactly the same grain direction.
So how does it work and why is section A, which has half the brace area as at B or C somehow stronger/stiffer?
Cheers
Dom

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[Dominic]

Um, nobody's mentioned finger braces........

That's because we are looking at the X brace in isolation, rather than a fully X braced top. But even so, it would be near impossible for a finger brace to stiffen the junction but not also stiffen the legs of the brace either side of the junction so all you really get is a proportional increase in stiffness of both the junction and the legs of the X in the vicinity of the junction. Even if you could, you would only need to do this if the junction is inherently weak.
Dom

[Kim]

Just a quick word with the mods hat on.

Can we please avoid making any generalisations based upon geographical location...It's great we are able to discuss things here in a mature way but we don't want to find ourselves bathing in the dirty water pour out by one boorish participant on another forum. Keep in mind that we have some very respected and 'forward thinking' members of this forum who are kind enough to share their thoughts with us from their home in the USA and I am certain that every single one of them are just patriotic as the rest of us.

So please, let's not wave any red rags with the potential to commit the very same offence we complain of. I know that would never be done intentionally by anyone at this forum, but we need to be aware that when ever we get nationalistic in a topic, things can go south 'very' quickly...its emotive and right up there with religion for causing good people to fall out so it has no place in a good discussion.

Hat off

Cheers

Kim

[graham mcdonald]

And where was this nasty attack on Trev?
Obviously not reading enough forums regularly enough

[Nick]

Um, nobody's mentioned finger braces........

That's because we are looking at the X brace in isolation, rather than a fully X braced top. But even so, it would be near impossible for a finger brace to stiffen the junction but not also stiffen the legs of the brace either side of the junction so all you really get is a proportional increase in stiffness of both the junction and the legs of the X in the vicinity of the junction. Even if you could, you would only need to do this if the junction is inherently weak.
Dom

Hope I'm reading Martin correctly & forgive me if Im not but I think Martin's point is valid. I know you are questioning specifically the strength point of intersection of the X braces and that is a good thing to think about but it is only a part of the whole structure which, I believe, has to be looked at as a whole, take away some parts of this structure & the whole top is useless as a stressed member. Finger braces play one part in spreading the load thereby reducing the stress at the X junction. I'm not thinking solely of stiffness when Im shaving braces e.t.c, granted I want it to be strong enough but are we wanting to stiffen this area up or is the whole idea of the current joint a balanced trade off that has been tried & found sufficient down through the years?
Just my two bob's worth.

[Kim]

Finger braces play one part in spreading the load thereby reducing the stress at the X junction.

Must agree Nick,

I see the finger braces (1 or 2 a side) as a kind of pipehead on a dam...(Yeah OK, its the best I could come up with @ such short notice ) The fingers angle from the X to spread rotational load emitting from the outter edges of the bridge plate into the waist and 'sides'. Sort of a reverse flying buttress if you will. The more substantial the fingers, (The more open the pipehead) the less load will be transferred into the X brace and as a result, the less tension will be 'pulled' into the top behind the bridge (the dam wall).

The trick is to carve away the fingers to close up the pipehead 'just' enough so the top behind the bridge will be loaded or stretched into responsiveness. The X of course is the framework for that exchange so it must be strong enough to withstand that constant tension, but also flexible enough to allow that tension to be put to good use and allow large oscillation of the top otherwise there will be a lack of bass response. I have no data on this, it just not how I work 'but' I am yet to find a nasally trebley sounding guitar that did not improve with shaved fingers.

You can build without any finger braces at all of course but you would then need to beef up the X to handle the roll of the bridge plate and I am not certain that would work well structurally in the long term even if you did get it to sound OK. Much better to have the finger there to balance things out and allow adjustment of where the load goes.

Cheers

Kim

[jeffhigh]

Hi Jeff, I thought we all selected brace sizes quite deliberately and then carved them to change their area of the brace exactly because it is directly related to stiffness. We reduce brace sizes to reduce the stiffness of the top. The cube rule relates brace dimensions to stiffness.
And, the angle of the brace legs at B and C to the string pull is the same as at A. If the junction is cut perfectly, the cross section at A will also be exactly the same area as one of the legs at B or C and have exactly the same grain direction.
So how does it work and why is section A, which has half the brace area as at B or C somehow stronger/stiffer?
Cheers
Dom

X Pic3.png

Definitely not stronger or stiffer at section A, just not down at 1/2 the stiffness, somewhere above that depending on whether you have a close fitting joint and use a cap you can approach equal. You have forces acting in two directions along the braces where they meet.
Think of it this way=
If you had an old style Gibson brace where one goes through continuously and the others butt up against it.
In that situation you would have exactly half the stiffness at the joint because one brace is discontinuous
Now cap that discontiuous brace and you have restored most of it's stiffness without in any way affecting the other brace.
You can now sum the stiffness from the two braces at that point.