"So how do you then fix the castor?"
i'm not sure..... i've gotten a bit lost btw the pinion angle and caster correction. I always thought the two were not mutually exclusive. but somehow i think i'm not fully understanding this. at this point i just want the pinion pointed at the transfer case and this seems like a good solution considering i've drilled the heck out of my arms doing the LT plates. if somehow i give up a bit of caster and achieve optimal operating angle for a DC shaft that's a small compromise i'm willing to make.
if someone can really break down how caster and pinion angle are mutually exclusive i'd love to hear it.
Caster is determined by the angle of the steering arm on the knuckle - you can roughly measure it straight from the arm if you have a flat surface to measure from and the rig is on a flat surface.
Therefore, caster is fixed to the axle, as is the pinion, and when you rotate the axle for any reason both caster and pinion angle have to change together.
Rotate up by lifting, and you will lose caster. Rotate down with caster correction and you will gain caster but lose alignment of the stock driveshaft u-joint angles.
It is a stroke of pure luck that the stock driveshaft is a "broken back" design, meaning that increasing caster lowers the pinion to align for a DC shaft. In most rigs this is not possible - I spent years trying to deal with caster loss on a stock DC front driveshaft Jeep Cherokee with AWD, and in the end the only solution was a new D44 front angle with axle brackets aligned for the lift I had (same function as cutting and turning the knuckles).
The stock 80 series front driveshaft also seems to defy u-joint physics. It runs relatively clean up to about 5 degrees of variance in t-case to pinion u-joint operating angles, which it simply should not do but yet achieves time and time again. This is because a u-joint operating at a zero degree angle rotates in a perfect circle, but as you add angle the joint moves in an elliptical arc.
If both ends are not moving in the same arc, the entire assembly is forced to rotate out of phase, so when one joint is operating at 11 degrees and the other at 6 degrees, you should always have vibrations. Max tolerance is supposed to be a 3 degree variance between the operating angles of the two ends, but the 80 series front can often go beyond this. Again, nobody here knows why, although I suspect there is something in the overall design that is masking the effects - physically the variances in elliptical angles have to be there, and there going to be impacts of running a driveshaft in a materially different arc on its two ends somewhere in the system (draw a lower arc and higher arc ellipse overlapping and see what you think about that in terms of what the driveshaft is connected to on each end regardless of how much noise it makes).
This is why I personally prefer the DC for the front of an AWD 4x4 running on a decent amount of lift. An 8 degree driveshaft angle split between two joints in the DC (4 degrees each) with zero angle on the pinion end leaves everything operating at maximum life expectancy.
Yes, there is the added complexity of the ball joint and you have to keep up on lubrication, but this is small compared to what I think becomes a frog boiling in water syndrome of misaligned stock shaft. Slowly over time it will get a bit noisier, rumble just a bit, and you won't really notice how hot the water has gotten but it's not nice and cool any more, and you may be shortening bearing life in the t-case and/or pinion if you put a lot of miles on it.
And so I agree. The first measurement in front alignment is pinion angle, and caster follows. Aligning for a DC on a 3.5" lift should give you stock range caster figures. Post up your final numbers if you don't mind.
If anyone else would like to chime in on my original question I'd appreciate that, as well.
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