Flatback?

The braces run their grain at a right angle to the back, so they work against each other when there is moisture induced movement. This causes cracks and or the braces to come unglued. Eventually the back will shrink to the point that the braces are too long and will not fit inside the ribs, requiring the whole bracing to be redone. Some modern luthiers are doing different things with their bracing that could address this issue, but most of those basses are too young to say for sure how well they work.

 

My flatback is 100 years old and hasn't shrunk much. Many old ones have strips of new wood added to the center seam to add width. Mine has one that's approximately 1/8" wide. In the three years I've owned it I haven't had any problems at all, other than the large hole in an upper rib that's being fixed this week. The back is fine.

 

Funny that even furniture makers know that you don't glue or otherwise tightly fasten a cross-grain brace to a wide plank or glue-up, but instrument makers persist in doing just that. When the plank tries to shrink, the brace restrains it from moving, and the plank has no choice but to crack.

 

It depends on the weather.

The less fluctuation in humidity, the more stable the wood i.e., the less expansion and contraction. There's no precise way to time the process.

 

While the voice of an instrument is one part of the flat / carved back conversation, simple economics plays a big role in building as a flatback: out of the same set of wood, I can get one carved back or four flat backs if I have a good saw and the skills to cut it. Bass backs in the rough tend to be very large and very expensive to a builder.

 

I think the reason luthiers glue up the braces to the back, is that if you did not do so, the back and braces would set up a buzzing. (I could be wrong.)