These cars were targeted to family, little old lady, shopping vehicles, like the Novas, Comets comparables. Performance wasn't the target.
The factory caster specs were near 0, I believe some were Neg caster, like a shopping buggy front wheel, you know how they track.
You crank in a buncha caster into a little old ladies car with manual steering, and she won't be able to maneuver in a parking lot, it just takes so much more strength to turn the wheel at slow speed, and she'll let you know about it ! ! - lol
This ignores the effect of the bias ply tires that these cars were designed with. The factory caster settings for these cars had just as much to do with the OE bias ply tires as they did the use of the cars at the time and the target audience.
It's already been said, but the construction of the bias ply tire actually adds a re-centering effect to the steering. They don't need as much static caster because of the dynamic effects of the tire's construction. The moment you change to radial tires you lose that effect, and you get a car that wanders around. It doesn't matter who the target audience is, you need more positive caster with radial tires to maintain stability. Which, if you look at the specs for the later F/M/J cars that came with radial tires ('77+), you see they called for much more positive caster than the bias ply cars did and they had the same exact target audience.
Dodge Diplomat alignment specs...
The offset bushings let you get to + 3*ish degrees where +2 was generally max, if lucky .
3* is such a vast improvement .
5* is kinda like putting a blower on a grocery getter, but with power steering who cares .
+3° is really the minimum that should be used with any modern radial on these cars to keep the car from wandering. And that's really just for the narrower kind of tires you'd see on 15" rims. Go much wider than a 235 up front and you need more to counter the tracking effects you get as the tires get wider.
+5° is no big deal at all, even with manual steering. Most modern cars run +8° or more. SRT Challenger specs are like +10°. Yes, the design of the suspension does matter, so some of that comes down to suspension design characteristics. But a lot of it is tire construction and size, and the wide front tires on modern performance cars are a big part of the large positive caster numbers they have for alignment specs. Wide front tires tend to track, and more positive caster counteracts that.
I've run everything from about +3.5° up to +8° on my Duster. With 275/35/18's up front the sweet spot I've found is between +6° and +6.5° of caster. Less than that and the 275's start to take you where they want to go. More than that and the steering effort starts to increase exponentially, much faster than the improvement in stability you get. But between +6° and +6.5° the steering is very stable, the steering effort isn't terrible, and the jacking effect from the additional caster on turn in isn't disruptive to the handling. And I run 16:1 manual steering.
