Why Can't I slice Wheels or Rims face up without getting a plug/disc covering it?

[turbonium]

Has anyone tried to print wheels or wheels and tires face up? Why do three different slicing programs (MakerBot Desktop, Cura, and ReplicatorG) all put an extra disc or plug of material over the top of the wheel, completely blocking it?

See also photos...

Here's a photo of the STL

sketchup.jpg

These next images are progressive screen captures from Cura - MakerBot Desktop does the same thing.
cura1.jpg so far so good...

cura2.jpg.... wait, what? Where'd the disc come from? The settings are no support material!


cura4.jpg total fail.


Why does this happen when I select no raft and no supports?

Printer is a Flash Forge Creator Pro but I don't think its the printer.