I replaced both TR ends the other day, and since I was going to take it in for an alignment I thought I’d dial in my caster. My driver’s side had been 4.3*, but the passenger side was a touch lower at 4.0*. Mind you….no issues with the way the jeep drove. Well, as I was standing there I decided to measure from the body mount to the ball joint on each side. The measurement was close to .5” longer on the driver’s side. That would seem to mean my front axle was a bit ****eyed. I would have thought that would produce a pretty crappy driving experience…..but it drove just fine and I have no odd tire wear. So, I lengthened the passenger LCA 2 rotations and the measurement was about dead on side to side after that. Ran it in for alignment and ended up with caster 4.6* left and 4.5* right. All seemed good, went home and parked it. Drove the jeep to work today and feels like I’m pulling left pretty good. Looked at my alignment specs again and noticed the thrust angle is out of spec. Ugh, heading to Colorado tomorrow morning and wishing I hadn’t jacked with this at all.

So, am I right in thinking that the measurement from body mount bolt to ball joint zerk should be the same on each side, signifying that the front axle is square with the frame? I don’t have factory inner liners so I’m measuring from the body mount that is readily accessible at the rear of my wheel well to the ball joint. I would note that previously my front lower arms were the exact same measurement, but my frame side bracket on the passenger side is tweaked just slightly, and I have replaced the axle side brackets as well…..so was thinking there is a small bit of possibility those two things were changing how my axle was sitting. I thought I recalled in one other thread I was in someone suggesting the body mount to ball joint as a measurement, but can’t locate that now.

 

More or less all solid rear axle vehicles have a thrust angle that is slightly off center, hence the need for a thrust alignment. The other factor involved is that frames are far from perfect as they roll down the assembly line, otherwise the need for alignment would not exist. Having a .5 degree higher caster reading on the driver side is relatively normal even when aligning a vehicle with sla independent suspension. This helps with compensating for road crown. Most likely you have inadvertently changed the relationship between your rear axles thrust angle (or as some say "dog track" and what was previously the center line of your front axle.

 

I have old print out at home that I need to look at to see what it was previously. Spec is +/-.25*, and I am currently .42*.

I figured it was an inadvertant effect of what I did up front, but that along with pulling left now made me wonder about the measurement I'm questioning. Kinda feel like it's a dumb question, but wasn't finding a great answer online weeding through threads upon threads of everyone asking about axle centering measurements.

 

Not questioning the spec, however from an alignment tech standpoint (i have been an automotive technician for 20 years, a few of which i spent time on an alignment rack) the two biggest concerns are tire wear and driver feel. .25* isn't going to have a whole lot of effect on either. Caster comes into play on tire wear while turning, it is referred to as "toe out on turns" it is another measurement all together that you wont find on an alignment sheet. It is generally calculated by the engineers and is what sets the spec for caster. My bet at this point your pull to the left is the rear axle pushing you left (back to the thrust angle).
Also... Dont forget.... no such thing as a dumb question.

 

Wouldn't positive thrust angle be pushing me right though? That was another thing that was perplexing me unless I have that backwards. I thought negative would be left and positive right. Print out shows me at .42*.

 

I guess my real concern is did I take a square front axle and make it unsquare? It makes sense to me that the measurement should be the same side to side.

 

That was what I was getting at before only the other way around... obviously I am merely speculating based on my experience, but... I bet it was "unsquare" before, yet adjusted to make it drive correctly, you (however well intended) made the front "square" with an unsquare rear... resulting in a "Rhombus"... It has been 10+ years since I have done an alignment, but as I remember vehicles do respond opposite of what logic would tell you with thrust.

 

The only thing I'd say about that theory is that my rear has full set of MC arms. They are dead on the same measurements from left to right, and there's no reason for me to think that the rear axle is c0ckeyed just a bit.

 

Have you tried measuring from hub to hub between front and rear and comparing this measurement between driver and passenger? At least it would tell you if you have a square.

 

Agree completely... Didnt realize previously that you had adjustable rear arms.. even still, doesn't change the frame, and I would bet dollars to donuts it isn't square.