In the tuning menus. The game says running more positive caster allows the tires to be more straight up during straight line acceleration and provides camber during cornering, but the game doesn’t list any downsides to running too much positive caster like it does with most other tuning options.
Sooo, is there any reason not to run Max caster?
Too much caster makes the steering wheel harder to turn. If you’re playing on a gamepad, then I’m not sure how it would affect you.
Less caster angle is faster in a straight line. You can see a difference is you make a drag tune and try with a lot of caster angle and with very little caster angle. More caster angle mitigates having to run with extreme negative camber, so it’s a trade off to keep some of the straight line speed while keeping some of the cornering advantages. But if your car’s grip is more than sufficient for what you want to do with it, then you can trade some of that caster angle for more speed. Street races is a good example. There’s usually a lot more wide open throttle than cornering in the street races.
Leave caster on default really.
The handling seems to be the best with default caster at least for all the cars I’ve tested so far.