Rhino has a very hard time with coplanar and duplicate surfaces for boolean unioning and joining because the computer has no idea how to “unfold” the surface (oops… I mean, “unroll” in Rhinospeak)
The concept at work here—especially for creating watertight solids for 3D printing— is that every surface has a direction, and these all have to point in the same direction for optimal results (and proper unfolding)
Rhino has a VERY nifty feature that only a handful of (OK, only two other) programs I know of have—which is to color the backfaces. Why McNeel does not showcase this extremely powerful (and almost totally unique) feature, and turn on single color backfaces as the default remains a complete mystery. I used Rhino for two years before someone told me about this feature that I rely on like oxygen in my work. When I mention this to most people, their reaction is ZZZOOOOMMMMGGGGG!!!
Here’s how it works: When backfaces are on, and you have duplicate surfaces, you will often get a “moire” pattern, for lack of a better word. Like this, from your problem file:
When you get this sort of moire, you need to delete at least one of the surfaces. In your case, both actually need to be deleted if you’re after a 3D printable watertight solid using the outer “tire”. With both inner surfaces gone, you can then add the curved outer part and join it all up. Like so:
See how all the grey is on the outside and the orange is on the inside? Think of it as a candy-coated chocolate and you get the idea. When you bite into it, you can see the chocolate on the inside, and the candy coating on the outside.
Here’s how to turn on Backface coloring (which, is buried far, far deeper than it should be, methinks):
Menu > Rhinoceros > Preferences > Display Modes > Under Standard, pick all views that you want. In my case, I use Shaded and Ghosted display modes with back face coloring.
Then do the following separately for each display mode you’ve want to use this feature in:
On Right > General > Shading Settings > Backface Settings > Choose “single color for all backfaces” > Single object color > pick your favorite. (I like orange, can’t you tell?)
Completely obvious, right?
To check for watertightness, select all objects, Menu > Analyze > Edge Tools > Show Edges. Select Naked Edges. Look at the prompt. It should say “no naked edges, no non-manifold edges” (And this is also a killer feature!)