Well since I'm a newbie, I don't have really any tip to share but as I was looking for some advice on the Internet I've found this guide: http://www3.sympatico.ca/cbarnett/SetupMatrix.html
It explains what are each setting effects on the car. A good read. It's exactly what I was looking for as some settings are rather obscure to me.
This guide is referenced in the rfactor central website Setup Resources: http://www.rfactorcentral.com/setups/resources.cfm
If the members have other interesting guides, please post them here
PS for the admin: I think this kind of topic would be better in a "rfactor" section rather than kopitiam but either there is no such section or I can't see it ...
