Cross grain classical soundboard bracing

[J Mack]

This is my first post on this forum. I have been playing guitar for 60 years and since I retired 6 years ago have been making guitars in order to better understand what makes them tick. Thus far I have built 5. It is a hobby and I like to experiment.

My 5th guitar got me thinking about the way we brace classical guitars. The redwood top I thicknessed according to Trevor Gore’s complicated spreadsheet formula (2.2mm). At this thickness it had good stiffness along the grain but it was unusually flexible across the grain. I opted to add some cross grain bracing to fairly standard 7 fan bracing. It sounds OK.

I am fortunate to own a Trevor Gore falcate braced classical. On thinking about the reasons for the way this guitar performs (it really does), it occurred to me that a lot of the falcate bracing runs across the grain. There is also a lot of cross grain bracing in lattice style guitars.

If we are aiming for a soundboard that is both strong enough, and has a desirable blend of weight and stiffness, then logically we will get a greater increment in stiffness by tackling the low cross grain stiffness. Keep the long grain bracing to the minimum required to counter the tension from the strings.

I have not heard a ladder braced classical guitar and wonder why we don’t see more guitars with some sort of a hybrid ladder bracing system. Conversely, I also wonder why some luthiers make guitars with a predominantly along the grain 9 or 11 fan pattern.

I think it was Robert Ruck who was quoted as saying that whenever he applied logic to his craft he was disappointed with the result. Am I being too logical here?

[johnparchem]

I found that the bridge itself is a very stiff cross grain brace of the lower bout in a classical guitar. My classical guitars are falcate braced without a lower transverse brace. I get a 10 Hz rise in the top resonance frequency when I glue on the bridge. I get a drop on my X braced steel string guitars. I assume for a classical guitar the bridges contribution to the top's stiffness overcomes the additional mass added to the top.

I think many as they look at a classical guitars bracing pattern miss that the bridge is a brace as well.

[Dave M]

First an analytical approach has to be a good thing for whatever craft.

John is entirely right that the bridge is a hugely stiff cross grain brace. Just hold one in your hands and try to flex it.

I can't find the reference but somewhere in the G & G books there is a discussion of cross grain stiffness. ( It's always a problem finding the bit you want given how much material is in the books!). My memory is that that they say that cross grain stiffness is not very important compared to long grain stiffness.

The traditional Torres type fan does of course give you a fair bit of cross grain stiffening given that the fans do cross the grain lines at a fair angle.

Good luck with the build.

[J Mack]

Thanks for your responses.
The topic concerning cross grain stiffness in the G&G books is Design 4.5.4. The analysis appears to be based on unbraced soundboards although this is not specifically stated. They go on to state that cross grain stiffness can easily be addressed with bracing. It would seem that falcate bracing does just that??

There is no argument about the stiffness added by the bridge. I have noted some of the most respected luthiers (notably Bouchet, Friedrich, Antonio Marin Montero) have opted to use a solid bridge bar under the bridge to add considerable stiffness. Garrett Lee uses a bridge bar and fairly hefty closing braces which are about half way between the bridge and the end block. The diagonal harmonic bar used by Fleta and others must add considerable cross grain stiffness to that part of the sound board although I find it difficult to discern the effect on sound. To me is seems there is a lot of anecodotal evidence that cross grain bracing is important and to a degree neglected.

Has anyone built or listened to a classical with a more ladder braced design? If not I might have to build one

J