The FFB is calculated from the tyre forces and moments and the force-path through the suspension/steering links.
Increasing the Caster angle increases the caster trail (and thus mechanical trail) which creates a leverage with the cornering force. This steering force is directly related to the cornering force of the tyres.
If you have too much caster you only feel the cornering force, so the FFB just increases towards the peak slip angle and then stays high (just a slight drop-off). So with high castor it's not easy to feel the peak or the "sweet-spot" and it can be hard work.
The other force comes from tyre contact patch. The variation of contact pressure and stick/slip creates a self-aligning moment. The moment is related to tyre load through pneumatic trail. The torque increases sharply and as the contact patch starts to slide it drops off sharply (just before peak cornering force). When the entire footprint is sliding the moment becomes zero. This gives you a very detailed feel of the peak, but note that the steering force can peak slightly earlier than the peak cornering force
-Mechanical trail gives you a feel for the cornering force
-Pneumatic trail gives you a feel for the when the contact patch starts sliding.
Pneumatic trail is much more informative so most racing cars are designed with low caster trail. Personally I rely on pneumatic trail in simracing but with some mechanical trail to bring the peak of the steering force a little closer to the peak slip angle and to have a certain feel/weight after the peak (some self-centering).
