I want to raise issue with the idea that the Martin popsicle brace or any other brace across the upper bout supports the fret board extension preventing upper bout distortion.
The extension has very little longitudinal strength. I have broken off a few on removed necks, and know how weak they are. Keep in mind that the fret board is partially sawn through at every fret slot. The wood remainder below these cuts is very thin and therefore weak. So the extension has insufficient strength to prevent neck block rotation towards the sound hole. Braced any way you like, the extension cannot prevent and has nothing to do with neck rotation towards the sound hole.
String tension generates the force causing upper bout distortion. The important force vector acting to break the top is parallel to the top, not downward into the sound hole. The neck and therefore the neck block and the wood under the fret board extension do want to rotate towards and into the sound hole. But the most significant problem happens before that.
If there is no bracing at all the upper bout will collapse very quickly under string tension. The first and most important failure occurs in the top wood adjacent to the fret board extension. A poorly designed upper bout structure allows this area to slip or even fracture along the grain. An example is the neck block extension only design with no cross braces. This design fails to prevent wood adjacent to the extension from splitting. It is true that the result is downward rotation of the extension, but the most important distortion is the initial breakage of top wood beside the extension. Therefore, compared to cracking or deformation of the top wood adjacent to the extension, rotation of the neck and extension into the sound hole is a secondary problem.
To solve the problem, Martin designed the excellent popsicle brace. It does a good job of preventing upper bout distortion. Note that they made it relatively thin, but nice and wide with plenty of glue surface. This brace works because it prevents the top wood on either side of the extension from cracking and allowing the extension to slide forward. It is interesting to note that in the 1970s Martin structural designers thought it might be OK to stop installing the popsicle brace. This was not a good idea.
I contend that in addition to a beam like cross brace just above the sound hole, builders should install a thin, wide and long spruce brace adjacent to the neck block. This suggests a brace similar to the popsicle brace, but wider, longer and placed next to the neck block. Spruce wood from an old top is a good candidate. I cut up low grade top planks for this purpose. Designers and builders should stop relying only on a large block glued to the neck block and under the extension. Guitars built that way will experience a high frequency of upper bout failures, or at least upper bout distortion and increased play action over time.
The image below shows what I am talking about. Though this is a 12 string, it happens to 6 string guitars too.
©2022 D.R. Hanna