Caster Issue-searched and have #'s

HP d30, 4.56 gears, TNT truss so mounts aren't stock. Custom front shaft, RE adjustable uppers and lowers, drop brackets, 5" lift measured dirty way.

Ok so I've been fighting a drivability issue as I'm wanting to take jeep off trailer and drive to more trails-it drives like my caster is too little, but I can't figure out the cause. I've sat another jeep with the same setup next to it and pulled numbers and can't repeat my problem. Any insight would be great! Almost to the point to scrap the housing and build another.

Front diff reads 81 degrees, both sides, on flat ground. According to Go-Jeep that would put my caster at 0, which it drives like.

My pinion is at 7 degrees (or 83 however you want to slice it).

My driveshaft is at 9 degrees, and will vibe on hwy above or below this setting.

My ball joints read 4 degrees from the top.

My confusion is the correlation between my front cover readings and my pinion. I should be able to run 4.5-5.5 degrees of caster and still be within spec on my driveshaft angle, but I can't. I've tried rebuilding shaft, new bearings in tcase and running without a front shaft and what I assume to be the correct caster. Jeep drives great with caster set by Go-Jeeps method but leaves my driveshaft 6-7 degrees off of perfect which in my searching nobody is doing successfully.

So what am I not seeing? What needs to change? What route would you go to alleviate problem? I can't figure out on a d30 with 5" of lift why I can't get even closer to spec.
Also-I've thought about rotating knuckles but at this point that involves dissecting a truss, and doesn't seem to find the issue causing it.
Thanks for the help

Alignment specs are for stock XJ's.

Pinion angle is more important than caster angle, but you have to balance the two for best results, and neither will be ideal. Lifted XJ's with larger tires should run more toe-in, about 1/16 - 1/8th inch in.

What are the drivability issues ? I have 31" MT's, 5.5" of lift, RE drop brackets, fixed lower and adjustable upper CA, stock steering, and it drives just fine at 75 mph.

The only real fix is to rotate the knuckles. As Tim said you have to find the best compromise between pinion angle and castor.

Rotate the knuckles or do a hub conversion.

I have 4.5 inches of lift with 4:56s and 35s and can get about 4.5 degrees of caster with drive shaft vibes coming in around 75-80. Drives OK but could be better. I really wish I would have rotated my knuckles when I built my axle.

Tim_MN said:

Lifted XJ's with larger tires should run more toe-in, about 1/16 - 1/8th inch in.

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Tim Factory Specs are in Degrees, not inches, so what is "Stock" in inches?

If I get the toe in set anywhere between 1/16 and 1/8", which when doing it alone with a tape is about as accurate as one can get, I figure that's good, regardless of tire size.

-Ron

Offset bj's may help.

Thanks for the replies so far guys! I know that the problem is there-but im still wanting to know the "why" behind it-and if i missed anything. I know that a HP d30 SHOULD be able to run 4.5 degrees of caster on a 4.5" lift. I know there's a ton of variables in play with each housing, but before scrapping over and possibly having a problem that repeats itself I'd love to dig as deep as i can.

I've run different amounts of toe-in, and with less caster actually found as close to zero as possible drives the best. I put new spicer ball joints in when i rebuilt axle, so i know they arent offset (DeftwillP appreciate that suggestion and have thrown around going that route as a fix)

are there any variables effecting caster and pinion relationship i'm overlooking? I've opened up housing, checked backlash on gear set and pinion bearings, I'm looking for anything-to me this doesn't seem like a standard issue.

On a fixed Beam Axle, the relationship between Caster and Camber is set at the factory to the spec given to them by the car manufacture.

By lifting the vehicle, you moved the suspension though an arc to it's new resting normal. Doing so rotated the axle center, and changed caster, camber, and toe (Toe because the camber and caster changed),

As I tell people, measuring from the axle center is irrelevant, what matters is what is at the wheel.

Your choices are:
Suck it up-most folks choose that option and make lemon aid. Make sure the adjusters at the body are both in the same place, make the LCAs the same lenght, the UCAs the same length, and put it together. Look at the front pinion to DS relationship, adjust the LCAs equally, so the pinion is 2 deg below the DS. Adjust your tow and call it

Replace upper ball joints with adjustables. Set LCAs to equal length, and UCAs to equal length. Adjust Pinion in-line with the DS, then use the upper ball joints to dial in caster and camber. They will still mix, and you may still not be able to get what you want.

Cut off the outer "C"s, align the axle in the vehicle at resting height. weld the "C"s back on where you want them,

Me? I'm in the Suck it Up department. This is a 4WD truck, and a modified one at that.The second you lifted it, the OEM Caster, Camber, and Toe numbers became junk. It will never drive like stock again. Learn to live with it

-Ron

Ron

Excellent reply! My jeep has been lifted with this setup for going on 10yrs! Since I put the current axle in, around 2yrs ago, I haven't driven it much on the street-so I sat the pinion and pulled it out of the garage and onto the trailer to wheel it.

I know that in previous axles i've had, with this setup, what you said applies exactly. I do not find it to be a norm that at this lift height people are forced to run 0 degrees of caster and just suck it up-something is off. I know that on my jeep with this same suspension setup i've been able to run up to 5 before with the pinion being fine.

I'm at the point of grabbing another housing and starting over, I really believe something is off with the one i have. However before I invest the time and energy to do that I thought I would ask to see if there was any further trouble shooting methods I had overlooked. Maybe another measurement on the axle? Maybe the relationship to pinion and diff cover angles told a story of where the problem lies?

Hopefully you understand my hesitation to scrap a housing that's got an expensive truss on it.

I run 7 degrees of front caster and 1/8" toe-in on my stock '98 HP axle with 4 inches of lift and 31" tires. I get a very small amount of drive line vibrations but the tires wear perfect and it drives like stock. Anything less was causing cupped tires and wandering. U-joints are way cheaper than tires!

DeftwillP said:

Offset bj's may help.

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I have heard of may kinky things but can't picture this