72bluNblu said

Joe covered it pretty well. It comes down to the camber and caster settings in the original specs. The factory specs call for +.25* to +.75* for camber, and 0 to -1* of caster. But that's horrible for radials and handling. It's basically the opposite of what you want.

Caster adds a self centering effect to the steering, basically, the more positive caster a car has the more the wheels want to go straight down the road. So, more positive caster means more stability at speed. It can also make the car harder to steer if you add too much (it resists steering change), so that's why you see different specs for power and manual steering. But you definitely want positive caster. The wheels tend to return to center after a turn or bump, so, the car tracks straight. Negative caster will increase the tendency of the car to wander, especially with input (like a bump, turn, etc).

Camber effects the contact patch. When you go around a corner the weight of the car transfers to the outside wheel, and the suspension compresses. That tends to tip the top of the tire out, away from the car. The more angle on the tire the smaller the contact patch, and that tire is carrying more of the load so it's more important for traction. Negative camber tips the top of the wheel toward the car. When you're going straight it will put more pressure on the inside of the tread, but, when your turn the tire will tend to straighten up, evening the contact patch and giving the best traction. The more aggressive a car is set up for handling the more negative camber it will have. If the car handles well and has a lot of grip, it will transfer more weight and tip the tire more. For a street driver you can run up to around -1* of camber before you start to see abnormal tire wear (unless you're driving the car hard in the corners! Then you can get away with a little more). Full on autoX and road race cars will run as much as -3*. Needless to say, with positive camber like the factory specs the tire is already tipped out, so, when you start cornering it tips out even more, reducing the contact patch.

This is a good tool for alignment with radial tires, the ranges are pretty good. It's a little conservative, especially for the amount of positive caster. It works decent for manual steering, but for power you can add a couple degrees to all the categories if you can get it. Modern cars tend to run as much at +7 to +9* of caster, and I know Peter Bergman runs like +8 on his '71 GT. I have +7* on my Duster, and it has 16:1 manual steering. I may dial it back a little, as the steering effect is noticeable. The amount of caster needed does depend a bit on suspension design, you can see on the bottom of the chart the FMJ cars should have more.

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