Increase rear rollbar and decrease front rollbar. Set front bump 4 rebound 6 Set rear bump 8 rebound 12 Reduce rear wing, depending on track. Default setup is tight (understeering) to my taste
It is the compression and return rate for the dampers, the speed or rate at which they return default\level when going over uneven surfaces, cant remember the exact terminology or explain it better. Bump is the compression and rebound is the return. Edit - Quick google search, this site is easy on the eye and decent explanations. http://www.rapid-racer.com/suspension-tuning.php#Damping: Bump and Rebound.
more preload on the rear diff make the car want to go straight, and resist turning, resulting in understeer less preload on the rear diff allows the car to turn easier, resulting in less understeer I found lowering the default diff preload setting a few clicks, helps reduce understeer on GT3 cars