2 degrees makes a HUGE difference (Pics)

I think that's backwards - here's a link: http://www.familycar.com/alignment.htm

According to this, the important aspect is the intersection of the vector through the steering pivot with the ground. If the intersection is ahead of the wheel (fat side to the front), that's stable.

 

Yes, the fat side to the back would add caster. When viewed from the side positive caster (or added, or "good") is when the upper ball joint is further back than the lower ball joint, or kingpin bearing in our cases!! An anology is to think of riding a bicycle with no hands... the front fork is angled back slightly on a bike, right? Now imagine if it was straight up and down, would you be able to ride with no hands? Nope! The "angle" causes the wheel to come back to center.

With lifted 4x4s, the problem arises with front driveshaft angle. When the front driveshaft angle gets to the point of vibration or binding, the quick fix is to shim the front axle with the "fat" side to the front, which eases the angle on the front driveshaft u-joints, BUT reduces caster, and vague steering results, often blamed on the lift or bigger tires. When I put the 2.5" rancho on my CJ7, it came with 2.5 degree shims already on the front leaf packs. I removed them and found no vibrations or binding, and my steering stayed the same.

The real fix if you have front driveshaft problems is to cut the welds holding the knuckles, and rotate the pinion (and diff housing) up, easing the driveshaft angles, while leaving the knuckles at the stock angle (or even adding a degree or two) and then rewelding them. This is a big consideration when having a custom axle built. The pinion angle and caster angle can be optimized for the lift/tire combination on the rig.

 

:? Typo? Fat side to the front would tilt the top of the spindle back (and the yoke down), giving positive caster. The axle is on top of the spring.

 

Tim's absolutely right. I was picturing my Chevy in my head (spring over). Spring under would require fat side to the front for more positive caster.