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Paul - I suspect that there is a theory out there that quartersawn wood is inherently stiffer which results in a stiffer more reflective finished back assembly, i.e. improved sound.  It's hard to believe that there could be any measurable difference, but I suppose it's possible that someone may have test data on the issue.

Previous posts have already pointed out that the main benefit of quartersawn is the stability of the wood in service.  Another significant issue is the ability to safely bend the sides.  Quartersawn should bend more consistantly with fewer failures.  If you look carefully at the BRW for sale on line, you will notice that many vendors will tend toward quartersawn stock for the sides paired up with more wild figure for the backs. 

Hi Andy,

The stability issue has been mentioned by others. Here's a diagram to show how wood moves in drying:

To a lesser degree, the same cut of wood will react to changes in relative humidity throughout all the seasons of the life of the guitar. But, bear in mind, that diagram is sort of an average, and not all species will have the same movement.

If you want to know how any one particular wood will move seasonally, look up the radial (quartersawn) and tangential (flatsawn) shrinkage values.

Here's one place: Wood Info (Look for figures representing wood at 6% to 8% MC, which is about where most of our wood will be when we work with it.)

For example, Australian Blackwood, Acacia melanoxylon, is: radial 3.4%; tangential 9.0% That tells me that Aussie Blackwood is very stable quartersawn, but very unstable flatsawn. An extreme example: A guitar with a 16" wide back, built from flatsawn Blackwood in extreme humidity is taken to the desert, and the back shrinks 9%, which is 1.44". The guitar back cracks badly, in several places. OK, we don't build at high humidity or send guitars into the desert without humidifiers, but even 1/4 of that amount of movement would crack the back.

There are other woods, like Mesquite, with extremely low rates of shrinkage both radially and tangentially, as well as having the radial and tangential numbers in a ratio close to 1:1. Mesquite is bulletproof, when it comes to stability, whether completely flatsawn, completely quartersawn, or anywhere in between. Most woods are not. A very good example of a common guitar tonewood that is very stable is Cocobolo, at about 3% radial and 4.6% tangential shrinkage. So, if you like the look, you could build a stable guitar of flatsawn Cocobolo, and it would be about as stable as perfectly quartersawn Maple.

One wood that I try to remember is flatsawn Bigleaf Maple, Acer macrophyllum, because it is sometimes available as gorgeous quilt figured wood, and a very well sliced set of quilted lumber is very close to perfectly flatsawn. Tangential shrinkage is 5.7% at 6%MC. There are plenty of quilted Maple guitars in service, that are holding up just fine, so I figure that any wood with a tangential shrinkage rate up to about 6% will be OK to use flatsawn. If higher than about 6%, the guitar better be a narrow body, like a parlor, and precautions taken to make sure the instrument does not suffer extreme humidity swings.

Remember also that many boards are not completely flatsawn. It takes a monster of a tree to cut 8" wide flatsawn boards. So, look at the grain, look at the shrinkage table, and sort of extrapolate for each board whether it is below 6% shrinkage. If yes, it should work, if not, keep looking.

An aesthetic factor: while perfectly quartersawn boards can be sliced to yield mirror-matched backs (and butts), flatsawn wood cannot. So, if you're a symmetry freak, like me, flatsawn wood may not trip your trigger. Just a personal choice, but if a back is pretty well quartersawn at the joint, then flares out to rift or even flatsawn, that satisfies my desire for symmetry.

Doug mentions another factor of using flatsawn wood: bendability issues. Not by my own experience, but from what I have read, flatsawn sides are much more likely to cup and ripple. That may not be true of the flatsawn material like Cocobolo, that is nice and stable even flatsawn.

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Dennis, many thanks for your technical explanation. You wipe off a couple of doubts that I still had on this matter.

Thanks to everybody, too.

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Andy

Italian spruce if you look for the best!!

[QUOTE=DennisLeahy]
...If you want to know how any one particular wood will move seasonally, look up the radial (quartersawn) and tangential (flatsawn) shrinkage values.

For example, Australian Blackwood, Acacia melanoxylon, is: radial 3.4%; tangential 9.0% That tells me that Aussie Blackwood is very stable quartersawn, but very unstable flatsawn. An extreme example: A guitar with a 16" wide back, built from flatsawn Blackwood in extreme humidity is taken to the desert, and the back shrinks 9%, which is 1.44"... [/QUOTE]

Dennis,
I am not sure where you got these figures, but they appear to me to be green to oven dry figures, which are useful for the kiln operator but not too meaningful for end users. I looked in the Wood Handbook, published by the Forest Products Laboratory, at pages 14-10 and 14-11, and found tables for "Coeffecients for dimensional change due to shrinkage or swelling within moisture content limits of 6 to 14 percent".
There were no figures even approaching 1% for any wood, so your 9% figure for blackwood is indicative of measuring something other than stability of wood in service.
Let's look at the figures for Big Leaf Maple. Radial coefficient is .00126 and tangential is .00248. Multiply the coeffecients by a 16" wide back and you get .020" for radial and .040" for tangential. This means we are talking about a .020" difference between flatsawn and quartersawn wood from 6 to 14% moisture content.
Stability in a guitar back is not the same as stability of a table top. Guitar backs are glued across the grain, in theory stopping any movement across the grain with changes in moisture content. We all know that the back can cave and swell in an effort to move with moisture content, but the braces lessen the amount that the wood would swell if unhindered.
As some have mentioned, bending quartersawn wood is more predictable than flatsawn wood. It is usually not difficult to obtain 5" wide quartered or rift sawn lumber, so you might as well make your sides quartered to rift. The suitability of a particular wood for using a flatsawn back has more to do with the inherent elasticity and tenacity of a the species, I believe. Some woods are more brittle than others and probably cannot withstand the tension placed on a braced back with moisture changes as well as others. I have seen hundreds of guitars with flatsawn Brazilian rosewood backs. Most have held up very well with no cracks. Conversely, I have seen many guitars with quartersawn Brazilian rosewood backs that have cracks and checks.
I believe the builder's skill, judgement, and experience are the biggest factors in determining the ultimate success of a guitar, from the mundane structural details (like whether a flatsawn back is a good candidate), to the ultimate and more sublime use as a tool for making music.

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Stay with the happy people.
--Reynolds Large