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Sometimes I have seen people saying that the X brace angle should not be 90 degrees.
Why not? Is there a reason, or is it just one of these accepted and unchallenged "truths"?
Might it actually have advantages?
For example keeping a 12 fret guitar bridge in the "normal" relationship to the X bracing on a 14 fret one.

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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.

No real knowledgeable insight, but I adjust the angle, based on space between the sound hole and the cross and where the X braces hit the bridge. So I will change the angle on the same body moving between 12 fret and 14 fret body joins. I do not know what might be magical about 90° that makes it bad. The linear stiffness of a brace falls off with the cosine off of the angle that it is at. So if I am doing it right a 90° angle has the braces at 45° so .707 of the stiffness vs lets say an 80° X .76 of the linear stiffness. It does open the lower bout more but there are generally a couple of tone bars there.

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I didn't realize there were people saying it should be 90*.

In the 70s when I was at Charles Fox's school in Vermont, he was teaching use of a 90 degree X brace angle.

Dave

Same. I don't consider the X angle to be a primary design parameter, just something you can measure after laying out the pattern based on other considerations. If it happens to land at 90 degrees, that's fine by me I can't think of any reason it would be special.

One of my guitar models uses a 90deg angle and it just so happened to come out that way based on it's shape and as mentioned, getting the braces to line up with the bridge wing tips.

I generally use 97.5, but I did one unique design that turned out to be 90. It worked just fine. I like for the X braces to intersect the lower corners of the bridge, regardless. That is the aim of Martin forward X patterns.

Sent from my SM-G950U1 using Tapatalk

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John

I do wonder if that is the more important question. How the braces are coincident with the bridge ends or not.

I went round this loop with a classical bracing design. Should the fan braces 2 and 6 sit under the bridge ends. I couldn’t actually reach a conclusion.

I had been using a less wide bridge than the traditional one to reduce mass but would not sit over these fan braces. My question was is this structurally significant or sound wise important. I don’t have the knowledge/experience to answer the question.

The x brace angle is pretty much the same question I think.

Dave

I have seen many combinations of angles and position in relation to the bridge. I think bridge location is more important than angle , I have seen Martins at 99 degrees and as low as 95 degrees. The brace angle changes on martins in 39.
I have made them from 85 to 99 and not one of those guitars sounded like a tube.
My best advice is to make one and see if you like it.

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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

Already have, Jumbo nicknamed "Fat Boy",.
I like it fine.

_________________
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.

I build 8 different sized guitars, and every one gets the X pattern that "it wants" when I'm laying out the bracing. I would quit building guitars if someone else told me what I had to do with my X's.

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Happily the only "X's" I have are in some of my guitars.
When I lay out the X brace I have it kiss the ends of the bridge and extend up into the waist far enough to reinforce the area of the top weakened by the sound hole. I like the intersection of the X brace to fall on the centerline and relatively close to the sound hole. I've never measured or worried about the angle of the X.
I think the principle virtue of the X brace scheme is that it puts the greatest support on the areas where the strongest force is exerted and that the braces run obliquely to the grain of the top rather than cross grain (at least in the lower bout). I never considered the angle of the X to be important.

Bracing is used mostly to add stiffness without adding all the mass you'd get from a really thick top. I agree with Mark Blanchard when he says that 'the sound is in the top': you can't do much with the bracing that will improve a poor (or poorly chosen) top, but you can easily ruin a good one with bad bracing. The factories do that all the time.

The two most popular brace patterns are 'fan' bracing on Classicals, and some version of 'X' bracing on steel strings. Both concentrate stiffness in the critical area between the bridge and the sound hole. X bracing adds cross grain stiffness that you don't get with fan bracing, so the vibration modes of the top that involve a lot of crosswise bending are relatively higher pitched on a steel string than a nylon string guitar with fan bracing. There can be other differences depending on what you do with the braces behind the bridge, too.

I've often thought that there might be an 'optimum' X-brace angle for any given top, depending in part on the ratio of long-grain to cross-grain stiffness, and probably on the outline of the guitar too. The trick is to figure out what that might be in any given case! Mark has done work that convinced him that changing the X angle hardly matters, and prefers to match the stiffness ratio of the top to the outline; using wood with high cross stiffness on wide tops, like Jumbos, and wood that's more flexible in the cross direction on narrower things like Parlors. It seems to me that once you've done that, almost any reasonable brace angle will work, particularly if you have some method of adjusting the brace profiles, such as tap tuning, load testing, or Chladni patterns.

Factories don't have the luxury of making those sorts of adjustments, so they have to do the best they can with standard layouts. Of course they do everything they can to convince buyers that the standard they use is somehow 'perfect' for every top, and any deviation from it is, at best, futile, if not an actual sin...

Certainly, 90 degrees would be an easy angle to jig up for cutting, and I'll speculate that it may be one of the reasons behind Mr. Fox's teachings, but unless in a factory setting, that would seem like a waste of time for a joint which consumes perhaps 4-5 minutes or so to cut and fit, I'm not sure why a jig would be needed or whether it actually reduces time required or accuracy.

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For the times they are a changin'

- Bob Dylan

I agree that ease of cutting for first time builders probably had a lot to do with it. But, as you say, cutting an X brace to any angle isn't a big deal.

Dave

I jigged up with a miter fence on a router table.
Just for accuracy, it beats my saw hand down, a little excess glue and the joint hydraulics when I try to fit it

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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.

I use the band saw. No jig needed. Tilt the table down to the desired angle. Mark the half way depth on the side of the 2 brace legs, stack them and use the miter fence to hold the two legs at 90 degrees to the blade. Pop out the cut pieces with a small chisel. I like to make the notch just a tad tight and then sand down the side of each leg on a strip of sticky back sand paper stuck to my table.
And to answer the initial question, mine are usually 97 degrees or approx. 100 degrees, depending on the style guitar. The important thing is where the legs cross under the bridge ends. So the wider angle just means the X has to be rear shifted a small amount to get the legs in the proper location.

I lay them on the top, where I want them to go, one across the other, mark them with a pencil, cut one side with a dovetail saw, then put them back together, mark the other side and cut with the dovetail saw. Cutting one brace at a time helps me not screw-up and cut it wrong. I never measure the X angle because I'm putting the X brace where I want it to go.
If I always built multiples of the same model guitar I suppose jigging up might save a couple minutes per X, but since I don't I think my way is quicker than machine methods.