What should the caster be set to if ya were runnin 37"-39" tires??
Plannin on dropping in a new front end and only plan on doin it once. Figured with all the drive train swappin goin on in here somebodies gotta have some numbers for me. :D
Runnin coils w radius arms if it matters.

what size springs you going to run that matters.:confused:

I hope your jokin. 17" ride height if your serious (very confused if ya are).
Heres the low down
I went to a suspension / aliagnment shop today to ask said question.
They had a 'big' 4x4 with swapped in axles sitting in thier lot so I checked it out (looked kinda cool). After checkin out this 'abortion' of a swap, I decided not to ask any questions, or stick around. So here I am........ at the mercy of the PBB.
Figured I'ld start here and weed out the crap later. :D

Is it a custom built axle you're speaking of or just a "new" one for your rig? If you want to give the specs to the axle builder for caster setup and you already have a set amount of suspension lift on the vehicle, just measure the length of the front shaft and draw a line out from the lowest part of the shaft (probably at the U-joint) keeping it parallel with the ground and measure up to just below the t-case (horizontal measurement). Then from there measure up to the t-case for the vertical measurement (or use the eq: d-shaft^2 = horizontal^2 + vertical^2 to find whatever measurement you don't have). Then once you have the vertical measurement, just add whatever amount of lift that the tires will be adding in height, and put it in here:

new angle = arcsin (vertical measure + extra tire height / d-shaft length)

Now this will just tell you how much more you need to rotate the pinion if you want to merely adjust it for the height that the tires will add. From this you'll know how much to adjust the caster. It's just simple geometry, hope I didn't make it sound more complex than it is.

Pinion angle and caster angle will be dealt with independantly.
My front axle has no knuckles on it right now. So I plan on adjusting the pinion angle to D-shaft & tranny loacations, then I can adjust the caster to whatever it should be. So in my case the driveshaft angle/distance off the diff is irrelivant, right?

4-6 degrees seems to work well.