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I was looking at photos in another thread of top bracing in which the upper arms of the x-braces were shaped in a straight taper running almost from the x-brace joint to the brace ends at the kerfed lining. It got me thinking about what considerations go into the shape of the upper x-brace arm because I was taught to keep the upper arm full height for almost its full length and then have it scooped out close to the brace end to bring the height down to the lining. An image search for x-bracing showed that there are quite a variety of shapes for the upper x-brace arm on different guitars. It seems to me that the differences in shape would reflect a trade off between structural function and top movement. The rationale behind the full height upper arm was based on providing enough strength to resist the top folding under string tension in the sound hole area and the idea that the structural function is more important than whatever contribution the area of the top between the x-brace joint and the upper face brace might make to the overall sound. The fact that many guitars use upper x-brace arms that are tapered or even scalloped suggests that full height braces might be overkill. Maybe they unnecessarily limit the movement of the top in that area?

How do you all think about this part of bracing design and how do you approach this in your guitars?

Here are some examples of some upper x-brace arm shapes. There are other variations out there too.

Full height, scooped at end


Full height, but scooped well before the end


Gradual taper with scooped end


Gradual taper from x-brace joint to brace end


Scalloped upper arm


Even more scalloped upper arm


And, tapered to brace end plus reduced x-brace height at x-brace joint

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I'm in the "Full height, scooped at end" camp, although that X looks uncomfortably far set back for some reason.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Put me under image #7. I’ve been using this general pattern for 6 years so far, and least 2 of them from that period are in the hands of gigging musicians. It’s nice to get a chance to see how they are holding up. So far so good.

I view it as purely structural. There's not much 'tone' north of and around the sound hole. I would have thought that image 7 was the worst of them all but it seems to work for Michael. Just goes to show you there are no rules in this game.

We taper from X height (never higher than 0.625", and usually between 0.500" to 0.550") to 0.375" at the rim for structural efficiency and scallop to not less than 3/16" at rim (usually 1/4" for 000 and larger). Lower arms are scalloped to zero height at the lining. We've seen many old Gibsons and a few custom guitars come in with upper arms scalloped to well under 0.100", and the combination of a deeply scalloped X and UTB always shows a wrinkle or even a crack close to the binding as the top distorts over time. In cases where the UTB and landing of the upper arms is close together, a moderate (not less than 1/4" height) UTB scallop will mitigate the loss of stiffness with deeply scalloped upper arms. Where the UTB and upper arms are separated excessively (your Photo #1), both UTB and 1/4" will generate top wrinkles and sometimes top cracks proximate to their respective landings on the lining.

The X-braces shown in your photos #1-#6 look a bit stout in terms of height at the joint, so slight scalloping on the upper X arms as in Photos 5 and 6 likely will not have much of an impact as the instruments are likely quite stiff in any case. Where we have seen issues in custom-built instruments is where the X is of appropriate height and capped, but scalloped deeply as if it were a lower X arm. After what appears to be 4-5 years, these instruments show permanent top deformation both in front of and behind the bridge, as well as excessive bridge roll if not overbuilt in other areas (e.g., excessive top thickness, very high X height at joint, etc.).

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For the times they are a changin'

- Bob Dylan

My mind says that scalloping makes little sense. You are weakening the brace but leaving all of that unnessary mass arbitrarily hanging out all over the top. Do scalloped braces look the way they do because that’s as far as the person could reach their hand into the sound hole, leaving the peaks?

I have been working with a bracing scheme that is a fan - X combination. The X brace contacts the soundboard at the "X" and at the ends (against the sides) and some of the fans pass under it and end in the upper bout . So far I like it.

Braces are scalloped before the body is closed, so no - I don't believe that the brace shape is an artifact of human reach or access limitations. I suppose scalloped braces look that way because they reduce mass and stiffness where desired with less in the way of tooling and labor than some other approaches. The strong continued interest in both vintage and new instruments employing this strategy to control top stiffness also suggests that it works to provide a desirable result.

Re: weak versus strong (strength) and rigid versus flexible (stiffness) - few guitar parts are not massively over-strength by the time they are stiff enough to do the job, so reducing strength seems like it is a secondary concern versus controlling stiffness.

When I look at the progression from 1920's and 1930's scalloped and tapered bracing through mid-to-late 1940's tapered bracing (which is identical to much of what contemporary builders refer to as parabolic bracing), and finally onto 1950's and beyond straight bracing, it appears to me that Martin's goal was to reduce both skill required and labor necessary to achieve the result they desired. Pre-CNC, scalloping and tapering were both skill and labor-intensive, with tapered bracing requiring less of both, then finally straight bracing requiring little in the way of time or skill. Taylor and other manufacturers carried this further by leaving the bracing rectangular in section, further reducing labor and skill at the cost of a few grams of excess mass.

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For the times they are a changin'

- Bob Dylan

I am in the gradual taper camp leaving the end maybe 1/8" + thick, tucking it into the lining and triangulating the actual brace to decrease mass.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Arthur Benade wrote about using weights to find areas on the soundboard (violins) to adjust their sound, and then thinning those areas to make them less stiff. Adding weight, or reducing stiffness has a similar effect. It's possible scalloping X bracing was used in a like fashion.

In my limited experience I fee that a strait taper is fine since all the heavily bracing is above the sound hole which will mitigate much top movement. Now, if I were to do a fretboard that was suspended over the top and leave off the popsicle brace.... I would consider scalloping the upper ends of the x to maximize movement. To what degree? I’m not sure without feeling it :/


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I agree with Michael. To me scalloping braces makes no sense and if they are not too small in the scalloped areas than they are likely too tall (i.e. taller than needed so adding unnecessary mass) in the arches. Conversely, if they are the right height in the arched areas then they must be too short in the scalloped areas. I've seen pictures of braces that go down to almost nothing in the scalloped area and I find it hard to believe that would be much different from having two short braces where really you wanted one long one. My intuition says that braces are closer to joists than suspension bridges and that you get the required strength for the least amount of wood with a proper sized, uniform brace that tapers at the ends.
This is not to say that a brace that started off over-sized can't be improved by putting in some scallops. But what I think that is doing is just getting it closer to the correct size and that there is still excess wood left over at the peaks.

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There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

DO NOT scallop the upper arm of the braces you are asking to top deformation. There is no reason. That is a structural area. I have seen too many do this and wonder why the tops dip.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Pat,

I see your point, but since scalloped and tapered bracing clearly yield different tonal results, it's tough to say scalloping braces doesn't make sense, since it can yield such a good sound. All that 'extra' mass helps contribute to that sound, so is that mass really 'extra', or just part of what makes the system do what it does?

I admit that I am assuming that the best tone comes from having the lightest but structurally sound braces. I could be wrong about that. The other thing is that while what is "good sound" is a very subjective, there seems to be many bracing patterns out there that result in what most would say is "good sound". So there seems to be considerable lee-way in how a guitar is braced. So perhaps it should not be that surprising that some guitars with scalloped braces sound good. It could also be that those braces that are deeply scalloped would work as two separate braces - they seem that close to that anyway and if the guitar is holding up, well why not other than it seems counter-intuitive (at least to me). But I would consider this a different bracing pattern; that is, to me a guitar that has what we might call a standard "X" brace pattern but with very deep scallops in the "X"braces to me is no longer what I would call standard. It looks like an "X" brace pattern but the "X" brace has been divided up along its lengths into individual smaller braces hence it is a new pattern. Perhaps this could be structurally sound but my intuition is that the load from the bridge wouldn't get as distributed along the top.

My premise is that if the scalloped braces still have enough meat in the scalloped area then there must still be an excess in the peaks. It would be interesting to do an experiment where one gradually shaved the peaks off of those braces to make them more uniform to hear the impact on the sound.

I sort of think there are just three bracing possibilities and a myriad way of getting them: under-braced, over-braced and braced just right. Hitting that third one for whatever pattern you are using is the trick.

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There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

I like image 2: gradual taper with scooped end.

IMO, the structurally critical area of the top is between the sound hole and the bridge. Once that collapses under the combination of bridge torque and compression/buckling you've lost the top. I don't think it's accidental that the two most successful brace systems, the Torres 'fan' on Classical and the X brace scheme that Martin seems to have picked up from English makers, concentrate wood there. I would not make the X crossing lower then the braces on either side, and always use a patch on the open side of the box joint to keep it from opening up and cracking the brace at the level of the lap.

Scalloping is more for sound than strength, and, as others have pointed out, there's not much 'tone' north of the X crossing. Scalloping the upper arms seems to me to be asking for trouble with no prospect of benefit.