You may want to check out Kevin Ryan's Acoustic Parallel Plate - http://www.ryanguitars.com/Innovations/KRG-Innovations-APP.html

The lower bout still needs some reinforcement, but that can be achieved a variety of ways as you suggest.

Josh

As always this forum is an amazing source of information.

I'm experimenting with a curved bridge plate with matching curved PMTE (Proulx Magic Tone Enhancer) that seems very, very strong and should, in my mind, add a lot of resistance to any lift behind the bridge and I just got to thinking that if that area were strong enough the lower bout braces would not serve much purpose.

Further thinking makes me wonder what kind of structural help lower bout braces add any way? Fan braces I can clearly see the benefit - tone bars? Always wondered.

Thanks for posting that, I was just having the same thoughts as the OP as I am building an OM and I'm wondering if I really need both tone bars.

Thanks Larry I have been leaning towards two finger and one tone bar for mine as well. Looks like that's the way I'll go, especially after seeing yours.

This is on my to-do list Small steel string with only the upper transverse brace and a very light X brace. Probably 14" lower bout at most. Anything more would probably get too heavy, since the whole point is to leave the top a lot thicker than usual and study the opposite extreme from the thin plate/heavy bracing style. I suspect I'll be doing a lot of perimeter thinning.

Mine will probably be 12 fret, so the bridge-to-tail span is shorter.

The bridge plate is a good variable to play with in this style. Mine will probably just be a small strip to protect the top from the string ball ends; no stiffening purpose. The thick top itself should be plenty to prevent bellying. But going thinner on the top, and then stiffening the bridge area by means of a large plate, should be good too. I guess mine will be more or less the same stiffness distribution as that anyway, after perimeter thinning.

And of course there's the variable of string-height-at-bridge. 1/2" is the generally accepted sweet spot for the usual designs, but I'll probably do .375-.400" on my X-only guitar so I can do lower stiffness.
...dang it, now I want to get started on this thing, but I already have too many other builds I'm procrastinating on Not to mention it's still too humid to glue anything anyway.

EDIT: Oh, and it definitely wouldn't hurt to include a crack catcher or two. I'll probably just do one going across the lower quadrant, similar to my fan-less nylon string guitar:

I built an OM with no bracing. That was in '95 and I still have it. Long story though because it is sandwich construction and technically more like "contintuous bracing". It rings like a piano.

Thanks Ken. I've thought about that approach for many many years, I had no questions structurally, just concerned about how it would sound. Your post helps me a lot.

No bracing? Just go sandwich...pretty decent record on this approach.

Minimalist bracing? Martin has built the 15 series, MMV, etc. with flat, single finger per side and a single tone bar right behind the bridge plate for decades...really no significant bracing between the lower legs of the X. Larrivee has a similar, labor-saving tonebar and finger system which is almost identical to the Martin scheme.

I've hotrodded a few 15's by adding an additional tonebar between the existing brace and the tail block, which tightens up the bass response (flabby in 15 series dreads), and adding another flat finger on each side. The 000's sound pretty good with just the usual tweaks, although I added a partial TB on one...and ended up taking a good part of it off in tuning, which told me that the Auds and GCs were pretty good without mods...a little looser bass and less focus trebles, but an overall warmer tone that works pretty well for live music even if it sounded wolfy in the studio.

In the top brace system, most of the weight is in the top and most of the stiffness is in the braces. Get rid of the braces and the top has to get thicker. This makes the system weigh more for the same stiffness. Responsiveness is generally reverse proportion to the weight. More weight, less responsive. Certainly no free lunch and how you build depends on what you want to get out of the guitar. Smaller guitars can have less bracing, but there is a limit. Just my opinion of course.
Tom