As autocrossers, we're all about negative camber and keeping our contact patch nice and flat in a turn. There are trade-offs, of course. Everything is a compromise, nothing is free. If you have "too much" camber on your drive wheels, you're potentially losing straight-line traction. And if you have "too much" camber on your front wheels, you're compromising straight-line braking traction. And if you have "too much" camber, especially coupled with a lot of toe (in or out), you'll munch a set of tires in a hurry. But, within those constraints, for a street car, we almost always want "as much negative camber as we can get", or as much as we can stand in consideration of the other variables.
But, what about Caster? We don't talk about it much. The general wisdom is "run as much as you can get". Again, on a street car, that's almost universally true because within the factory alignment range, we can't get "too much" caster. So, if you want the short answer: Run max caster, but don't sacrifice camber to get caster. (on some cars with fully adjustable alignment, you do have to make that choice to give up some caster to get more static camber) If your car has manual steering or steering that otherwise already feels "heavy", you'll want to continue reading.
Of course, we're not going to stop there without exploring the "why" and "how".
What IS Caster, and how do we adjust it? Caster is the tilt of the steering axis fore or aft. If it tilts forward, that's called "negative caster", and it makes our wheel potentially all sorts of unstable, like the front wheel of a shopping cart, following the whims of the road. So, we definitely want "some" positive caster, tilting the top of the steering axis toward the rear. If this is adjustable at all (it's not on a lot of cars), the adjustment will either be at the top of the strut (push it toward the rear), or at the rear of the lower control arm. (move the rear inner control arm bushing inward and it pulls the ball joint end rearward) Typical street cars are in the area of 4 degrees. Rear wheel drive and/or performance cars will run closer to 7 or 8, sometimes as much as 10 degrees.
Why do I want max Caster, and how much is too much? Glad you asked. Caster does two things for us. When you turn the steering wheel, the caster on the outside wheel becomes negative camber! (woohoo) On the inside wheel it becomes positive camber, but we're not loading that wheel as much, so that's okay. The other thing it does is introduce a jacking force on the outside tire. It's actively trying to LIFT that corner of the car a little bit. This is what gives us the most readily felt effect of caster, steering feel. If you have adjustable caster, you can easily experiment with this. Set your caster to something absurdly low, like 2-3 degrees. You'll find that your steering feels light and airy, and has almost no self-centering. You turn into a turn, you MUST turn out of it, the car wont' straighten itself. It feels weird. Then go the other way. Set it to 6 or 8 degrees, or whatever you can get. (on my wife's 2015 Miata, we got 10 degrees) You'll find that the steering feel is much more precise, and the wheel snaps back to center nicely. If you have manual steering, you'll want to temper your desire for "max caster" with your desire to be able to steer the car! I found that on a Miata with a manual rack, I like right around 3.8-4 degrees. Less than that is too vague. More than that is hard to turn in a parking lot. And 7 degrees was way too much of a workout.
Too much caster? Well, aside from the possibility of getting into "too much steering effort" on a manual steering car, you're not normally going to see it. However, if you get into modifying suspension in ways that will allow you to truly get "as much as you want", there are some other effects of extreme caster. People building custom hot rods, lo-cost race cars, or crazy off-road trucks will sometimes get into this. More than about 8-10 degrees is usually where things can get bad. (of course, there are a LOT of other variables) What can happen is that the wheel will get into an oscillation, just like your wonky shopping car wheel. You get over a certain speed, and it will just wobble! Again, you're not likely to get there on a street car without modification. But, if you happen to get more than 8 degrees of caster AND you experience a "speed wobble", you might try backing off on the caster and see if that fixes it.
How much difference does this REALLY make in our negative camber? Good question. If you've got a camber gauge or access to an alignment rack and some time, you can experiment with this. Set your caster to minimum, say 3 degrees. Then measure your camber... turn the wheel a half turn, and measure camber again. That's your caster-induced camber gain. Now set your caster to maximum, say 7 degrees. Do the same camber check. Congratulations, now you have some data!
In playing with Miatas, I found that every degree of caster gave about a tenth of a degree of camber. I was actually kind of disappointed. For an autocross car that's running 2 degrees of negative camber, and 5 degrees of caster... it was only giving me another half degree. So, for me, I think of caster mostly as a way to tune steering feel. More caster = crisper steering feel and more wheel self-centering and straight-line stability.
Where it becomes more relevant is when your'e designing or setting up a more street-oriented car. Something that you don't want 2 degrees of negative camber on. It's going to see a lot of highway driving, you want to limit it to, say -1 degrees of camber. But, the suspension will allow you to dial in 8 degrees of caster! So, in a turn, that 8 degrees of caster might be .8 degrees of camber... putting you a lot closer to where you want to be in a turn!
If you really want to muddy your brain, you can try to understand the difference between "king pin inclination" and "caster" and how they play together. KPI is sort of like camber on the steering axis. How much the steering axis tilts in or out. It's sort of related to camber, but there are ways that things can be designed or adjusted to where KPI at the steering axis and Camber at the wheel are not the same. I'm not going to go any further than that.
