What exactly comprises a 'platform' in a car? When manufacturers talk about 2 cars sharing a platform, are they referring to the design of the frame, or the frame and suspension, or something more?

 

Well, no modern car has a "frame" as they're all unibody, but it generally refers to a general design of the unibody structure. You can get vastly different vehicles off of the same platform. Much of the time very little other than engineering time is actually shared between two cars sharing a platform. Usually they take the same processes to build and often are produced in the same factory requiring only comparatively minor changes to switch between production of models sharing a platform.



Generally if you looked at the sheet metal on the underside of two cars sharing a platform similarities would be pretty obvious.

 

I saw a concept platform once that really was a platform . . . a rolling flat bottom-of-the-car with a drivetrain. You want a sedan, put a sedan on top. You want a truck, put a truck on top. I don't think it ever was carried out to that extreme yet but that's the idea. A good platform can be versatile more than you might think. Hyundai made the RD Tiburon and like-year Elantras out of almost the same bits . . . but car companies have gone so far as to make MINIVANS starting from the guts of family cars.