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Hi

Is there any advantage/ disadvantages in selecting a different material for the UTB, Mahogany for example. I have also seem laminated UTB spruce - mahogany - spruce ?

Regards

John

spruce is stronger and lighter than Mahogany Laminated spruce may be a bit stronger but will be stiffer

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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

I agree on mahogany. It is a bit heavier than spruce, but no stiffer.
I have used black locust on occasion. It is about twice as stiff as spruce, but it is almost twice as heavy.
With bracing, the best way to increase the stiffness to weight ratio is to make it taller. Making braces wider increases the gluing surface, but the stiffness to weight ratio remains the same.

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John

carbon fiber laminated spruce...is an option

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MK5acoustics.com

I've pondered this as well... I see no reason not to use heavier woods for that brace. It's not supposed to move anyway... or at least not on my guitars, because I don't like needing neck resets. So the weight of it shouldn't have much/any effect on tone.

But spruce is already strong and tough and cheap and easy to work with, so the only reason I ever use anything else is for appearance.

What are you trying to accomplish? If you were to make a spruce brace and a mh brace of the the same dimensions, the mh one would be weaker. To make it as strong as spruce you'd need to make it larger and therefore heavier. I also don't worry about weight for the utb, but it seems like it should be a very strong brace. So why not use a wood that will pack more strength into a smaller space, ie spruce?

I doubt weight in this area would have much sonic impact in the completed guitar, but, it will affect the taptone and pitches of the free soundboard when you do your tuning - if you do it that is You might expect slightly lower pitches than usual after beefing up the UB.

What I think is one important thing about it is not to have it prone to cold creep. Under load wood tends to bend permanently (think about lower bout caving in front of the bridge). Light spruce is very prone to this so it will benefit from being beefed up from the standard sizes. At the same density some spruce cold bends easier than other samples as I noticed from testing braces. If you use random spruce but the same brace size you will end up with random degrees of creep resistance.

I currently use 2 UTBs, main one made from the densest spruce I have and with not much scalloping at the ends, and this is for nylon strings. A spine or cap of CF or hardwood would be the next step in making the UTB stronger.

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Build log

If you were to make a spruce brace and a mh brace of the the same dimensions, the mh one would be weaker.

Incorrect.

A mahogany brace of the same dimensions as a spruce brace will be slightly stronger and stiffer, but will weigh more. To make it weigh the same or less than a spruce brace, we would need to make the mahogany brace smaller, thus, -then- it would be weaker.

From my very first guitar, I made my transverse brace 5/8" tall VS the more normal 1/2"(by 1/2" wide). Some time back, I kept the 5/8" height, but narrowed it up to about 3/8". No failures, no CF... No need to go heavier than that.

Ah, ok, thanks...

Many differences of opinion then ?

My main reason to laminate was

1, To stiffen the UTB
2, To laminate it MH - Spruce - MH - just for the aesthetics as the hole for the truss rod looks neater

In my mind MH is stronger than Spruce, but that is only how I see it I don't base it on any evidence

J

Might be true of some pieces of spruce and some mahogany.
However published average modulus of rupture (strength) is slightly higher for mahogany, but modulus of elasticity(stiffness) is less than for spruce

Jeff, check your typing!

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks Trev. Fixed now

absolutely on height influences . you can increase stiffness by shape. Like Mario I like to tape the braces and add height. I still feel lighter is better but that is my preference as we all as builders hear what our ear tells us and that is why one builders guitar may be different than another.
I am a Martin traditionalist in my building and Mario sold me on Hot Hide and Fish glue. I Seldom use tite bond anymore. When you look at a cross section of the brace and figure the square inch you can make it higher with less cross section and make it stiffer. A 1 in square is so much more weaker that a 1/2 by 2 in . Same amount of material same weight but it becomes more stiff.
When in doubt make a model and observe. To me it is all about the weight to strength ratio. Lots of good info and a nice discussion.

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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

Normal for a Martin dread or 000 is 5/8" tall. Before mid-1939, the UTB was 5/16" wide. After that, it became 1/2" wide. A 5/8" tall brace is almost twice as stiff as one that is 1/2" tall. That is because stiffness is proportional to the cube of the height. Strength is proportional to the square of the height.

I know the terms 'stiffness 'and 'strength' are used interchangeably, but they do mean different things. In designing bracing, stiffness is the major consideration.

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John

On my first 23 guitars I used a simple 5/8" (tall) by 3/8" spruce brace, with no problems...........but I got to thinking too much, as well as reading and looking at pictures..........so I've been using all kinds of Spruce/ Carbon Fiber combinations. Because of stuff I read I kept worrying about the UTB being weakened by the hole I drill in it for truss rod access.
...................................So, sometime next week #54 is going to get a simple 5/8" X 3/8" spruce UTB.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Since the hole is near the sound hole edge of the UTB, it affects the stiffness very little.

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Formerly known as Adaboy.......

Since the top web is undisturbed, drilling the truss rod hole does not reduce stiffness appreciably. But the hole does reduce strength because it is a stress riser.
My solution is to not drill a hole, and make the truss rod shorter so it can be adjusted behind the brace. That appeals to my aesthetic sense, and discourages tampering.

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John