Suspension design's a deceptive thing. There's some very basic principals at work, but some of them don't really show unless you're looking for them. At a first glance, the PM files in rFactor look pretty simple, and I'd say they are very simple (though I'll be going over them anyway just to make sure everything's clear).
I'll try to quickly go over the main details to keep in mind during suspension details. This is written primarily for use with rFactor. I think the functionality is pretty much the same in GTR2, GTL, GSC, Race 07, etc. and to some extent, rF2 aswell, but there may be some differences I haven't mentioned here.