Found these questions and answers on the Forza support page. Thought it might help others find the appropriate settings.
Forza Support: FFB FAQQ: Why is my car too understeery, or too oversteery with my wheel? Is this an FFB problem?
A: Many players incorrectly assume that FFB settings make the car understeer or oversteer more. This is not possible. Oversteer and understeer happen to the car at physics level and can be tuned by the player through the car’s tuning setup, not through the FFB. FFB is what you feel through your steering wheel from those physics effects. FFB helps you feel what is going on, it does not affect the car’s behavior. In short, is your car too oversteery when you lift the throttle? Raise the differential deceleration; is your car too oversteery at apex and exit of corner? Stiffen your front spring or antiroll bar or both. And so on…
If you have too much oversteer or understeer, change the setup of your car (spring, anti-roll bars, differential accel/decel, camber front/rear, toe, front/rear damper) to fix the issue and make the car drive as you like. You cannot tune a car by tuning FFB. FFB only helps you to feel understeer, oversteer and in general what is going on with the car, but it does not change the way the car drives from a physics point of view. You will find the car easier to drive with FFB tuning that suits your driving style best, but the car effectively is the same.
Q: What if I don’t want any FFB tricks like dampers and springs, just raw physics?
A: If your goal is as close to raw physics force as possible, try this:
Setting the damper and spring to 0 removes most of the tricks. Reduce rumble until you can barely just feel it. Reducing Force Feedback Understeer will effectively make you feel the understeer more, because it removes the mechanical trail effect, so you solely feel the pneumatic trail. If you run understeer to max you will feel less understeer (FFB drops when exceeds the peak), effectively it increases the mechanical trail effect, so post peak the force remains higher. Try leaving FFB minimum at the default setting. This will, roughly, give you pneumatic in raw. FFB Minimum alters the ramp-up of pneumatic trail (somewhat counterintuitively). Turning it down all the way is almost like having a flat tire; high forces at low lateral loads.