>But it is still true that changing rim diameter alone does nothing to
>improve stability and cornering.  It is the characteristics of tire that
>determine this.  Put an improper tire on a 15' or 16" rim and the van
>will drive and ride terribly.

Important also to remember is that a larger diameter wheel will generally
allow you a wider tire, which CAN affect stability. That, and a 15" rim
expands your tire choices quite a bit both for on and off road
applications. 16", not so much choice for off road apps because, as you
say, the sidewall will have to be too narrow for off road (at least without
jacking up the suspension).