There are two special ribs that help support the main gear legs, the B and the BB ribs.  Study the photo below:

The B rib is fabricated from three different materials—1/4” phenolic, ¼” aircraft grade plywood, and ½” last-a-foam.  The BB rib is fabricated from just two of these materials, the ¼” phenolic and the ¼” aircraft grade plywood.  There is a large hole that is cut out toward the bottom of the ribs to allow the aileron push tube to pass through.  The BB rib has a series of holes to support the aileron idler assembly.
To begin assembly, print out full size copies of the following templates:

• B rib front section
• B rib full size drawing 1 of 2
• B rib full size drawing 2 of 2
• BB rib full size drawing

Cut the plywood and phenolic parts out and bond together with a thick micro.  Be careful not to mix up the parts or get them upside down.  Both the plywood and phenolic parts have a top and a bottom, are different sizes and they can be hard to tell apart.  Mark them clearly. Bond the foam aft rib to the B rib plywood centered with the plywood.  Using micro, fair the edge where the foam meets the plywood so we have a smooth transition. 

Apply a two layer BID and epoxy to both sides of both the BB and B ribs.  Let cure.  Once the fiberglass is well cured you can clear out any holes with a drill or dremel.  Then thoroughly sand the ribs with coarse grit sandpaper.  We will be applying many layers of fiberglass to these ribs later and we want a good bond. Remove a bit of the foam edge on the aft section of the B rib and fill with micro as you did with the other ribs.

The front section of the B rib is made in the same manner as the other (non-B ribs).