I've read all the various "What is a Hybrid" threads I could find. There seems to be the consensus that a Flat Bar road bike isn't just a standard road bike with flat bars. How is the geometry different from a regular road bike? Can you tweak a standard road bike to fit like a flat bar or is better to just buy the correct frame/bike to start? I've got a group in pieces waiting for a donor frame.

My ultimate goal is something like a flat bar cyclocross rig. I'd like the ability to use large tires should the need arise. Which kind of kills the dual pivot road type frame. Disc brakes are nice but don't really fit into what I want on this project.