Shape of Braces - Is conical cross-section the best shape?

[Simon126]

I am building a dreadnought with traditional x-bracing. I want to explore newer construction techniques for this build. I understand why braces are traditionally thinned so them have conical cross-sections. They are fat at the bottom and taper to a point at the top. The traditional explanation is that height of the brace is important and thinning the sides reduces the weight while preserving strength.

But if a brace is a beam underneath the soundboard, wouldn't an I-Shape cross-section be better? It would still be low weight but provide more strength. Does anyone have any experience with changing the shape of bracing to an I-Shape cross section?

[Dave M]

In principal you are correct that an I beam shape should be useful. However when you look at the dimensions in practice it is simply not practical to make in timber.

There is a clear exposition of brace shapes in the Gore/Gilet design book section 4.4.7 if you can scrounge a look at it. The end result is essentially that a tall triangular brace gives you most bang for your buck in terms stiffness against mass. Within limits, too narrow a base will be difficult to glue securely.

Cheers Dave

[peter.coombe]

The only really practical way to make an I beam brace is to use carbon fibre tow on the top and bottom of the brace. That is how falcate bracing is implemented and is very well covered in the Gore/Gilet books. As you say it does make braces that a lighter in weight for the same stiffness. If have not read the books then you need to get a copy and read them.

[kiwigeo]

The end result is essentially that a tall triangular brace gives you most bang for your buck in terms stiffness against mass.

And changing height of brace is a quicker way of adjusting stiffness than taking meat off the sides.

[TomBeltran]

In principal you are correct that an I beam shape should be useful. However when you look at the dimensions in practice it is simply not practical to make in timber.

Stuart Mossman, in the later 1970's used an I-Beam construction in his steel string guitars that was very well regarded. You might be able to find details and a picture on the web.

Also, John Gilbert's classical guitars had fan braces in a "T" shape, which is sort of a one-half I-Beam. David Schramm has an article with pictures of the brace.

[Dave M]

Thanks Tom That's interesting.

That fan bracing by Gilbert does however look pretty big compared say to a typical Torres. It would certainly improve the gluing area of the brace.

[kiwigeo]

One makers take on I beam braces.......comments anyone?

tumblr_lqg6ludN4a1qkrsrz.jpg (45.61 KiB) Viewed 11661 times

[Dave M]

That’s interesting Martin.

On a classical the brace height is so small that putting holes in it probably won’t do much.

However the big x brace on a steel string may well be worth thinking about. As I understand it the stiffness of a beam is not reduced by much if you take material out of the centre. (And please correct me if I am wrong) so one may be able to get a significant reduction in mass for not too much loss of stiffness for these big braces.

A proper engineer could set me right on this. Is Trevor perhaps listening in?

[kiwigeo]

As I understand it the stiffness of a beam is not reduced by much if you take material out of the centre. (And please correct me if I am wrong) so one may be able to get a significant reduction in mass for not too much loss of stiffness for these big braces.

I believe that is correct but Trevor will be along to the pulpit at some stage to preach the truth

[kiwigeo]

That whopping great X brace...surely a tall conventional brace would have far less mass and just as much if not more stiffness?

[TomBeltran]

On youtube, there is a video called "Guitar Physics and the i-beam bracing," which shows the construction of the I-Brace, using a router. It is at the 16 minute mark. I had heard somewhere that Mossman had changed from the I-Beam brace to holes in the braces to lighten them. A few builders are doing what Kiwigeo shows. Kinscherff guitars does them sort of like that. It might be fun to experiment with, but it seems to me that the I-beam could be the same height as a regular X brace, so it might work better.

[WJ Guitars]

Here is another Youtube video to add into the mix of comments regarding guitar 'I' beam braces.

https://www.youtube.com/watch?v=k-VeV4n6X-M

As for myself, I will continue using Carbon Fibre top and toe in accordance with the Gore / Gerald books.

Wayne

[peter.coombe]

As described above, you can reduce mass by making braces tall and thin, or drilling holes in them. However, carbon fiber has a very high stiffness to weight ratio, no wood even comes close, so the most effective way to make braces stiffer and lighter is to use carbon fiber tow. It is no coincidence that falcate bracing and lattice bracing using carbon fiber makes lighter tops than any other method. Carbon fiber also gives you stability because it does not have memory like wood does. This is all covered in the Gore/Gilet book. The disadvantage is you need to use epoxy which is messy, smelly, and toxic stuff to work with, but the end result is worth it. Just take precautions, gloves, good ventilation and don't sniff it. I am with Wayne on this.

[Taffy Evans]

Hi, over the years I have often added a brace to a top to stabilize a repair on certain guitars. To limit the amount of extra mass introduced to the top I have at times put holes in the brace. Here is a photo of an early repair using this idea.

Sorry for photo quality

DSCF5107.JPG (38.05 KiB) Viewed 11155 times

Cheers Taff