Measuring axle position

[glockman]

I noticed this weekend that the front passenger coil on my LJ is hitting the front of the coil bucket. Like this picture but worse.


A little background.

The jeep has a Teraflex LCG long arm kit that has the long front lowers but uses the stock uppers. My wife hit a rock in someone's mow strip this summer, (long story). The rock punctured the front diff and bent the drag link but I don't think it bent the axle. It hit square on the diff cover. I had Teraflex stock length uppers installed and the shop said my front axle was too far back so they lengthened my arms to push it forward. This makes sense because my upper bump stops would hit the lower bump stop at 1:00 indicating the axle could move forward.
They replaced the drag link too and had it aligned so I am assuming the axle is not bent. It is not hitting on the drivers side so I measured the axle placement.
The lower passenger control arm is 1/4" longer and the axle is located the same amount more forward when measured from the lower ball joint to the closest body mount bolt on each side. I know this is not an exact way to measure but I can't think of a better place to use as a reference.
If the axle was not straight or perpendicular to the frame, wouldn't it cup the tires? Or would it be fine as long as the tow was set with the axle slanted?
I have always aligned this jeep with the tape measure method setting tow at 1/8 to 1/4" but never set caster. If I adjust the lower arms to be the same length I'd need to get it aligned to correct caster or is there an easy way to set that on a TJ with adjustable arms.

 

[LJTim]

I have a similar issue on both sides of my LJ. I just rotated my coil isolator, and it fixed the rattling I was getting. This doesn't provide a solution to your problem, but all your control arms should measure the same. I wasn't getting the results I wanted from any alignment shop other than the Alignment Specialists in SLC. But before I went to them, I snapped. I could't get any shop to give me a reason my thrust angle was off. I ended up with new adjustable front and rear control arms and centering the jeep on my own. Took it back in for a computer reading, and the thrust angle was spot on. My jeep however, still had passenger pull, and I was getting bad tire wear - even though the computer readings were good. I snapped again. I pulled it in my garage, got out the plumb bob, tape measure and pencil, and started dropping measurements from all 4 corners. I found my passenger side was 0.25" shorter than my driver's side wheel base. I made the appropriate adjustment in my rear end and took it to the alignment Specialists for the first time. They said it was perfect. I also no longer had any passenger pull, and felt comfortable to purchase new tires.

I realize this isn't the same issue you're having, but I would start by making sure all the control arms are the same length. While you're at it, make sure none of your control arm mounts have been reamed out of round, and obviously set to proper torque. I would think because your passenger side is longer, that is the reason you are only seeing this on your passenger coil. I'm wondering if this has been done to compensate for something else being out of whack? I would take it to the Alignment Specialists, and request a 4-wheel alignment with the specific instructions of checking the straightness of the axle. I'm not sure the best way to check if it is straight on your own. I know people use a straight edge all the time, but that only checks the axle tubes. Keep us posted

 

[Gravy]

I'd mimic what Tim said.

Start by measuring your wheelbase on each side.
Then measure the distance from hub centerline to the LCA and UCA boot holes on the frame.

Ideally adjustable uppers and lowers would let you adjust the wheelbase while keeping that optimum caster angle.

 

[glockman]

I have a similar issue on both sides of my LJ. I just rotated my coil isolator, and it fixed the rattling I was getting. This doesn't provide a solution to your problem, but all your control arms should measure the same. I wasn't getting the results I wanted from any alignment shop other than the Alignment Specialists in SLC. But before I went to them, I snapped. I could't get any shop to give me a reason my thrust angle was off. I ended up with new adjustable front and rear control arms and centering the jeep on my own. Took it back in for a computer reading, and the thrust angle was spot on. My jeep however, still had passenger pull, and I was getting bad tire wear - even though the computer readings were good. I snapped again. I pulled it in my garage, got out the plumb bob, tape measure and pencil, and started dropping measurements from all 4 corners. I found my passenger side was 0.25" shorter than my driver's side wheel base. I made the appropriate adjustment in my rear end and took it to the alignment Specialists for the first time. They said it was perfect. I also no longer had any passenger pull, and felt comfortable to purchase new tires.

I realize this isn't the same issue you're having, but I would start by making sure all the control arms are the same length. While you're at it, make sure none of your control arm mounts have been reamed out of round, and obviously set to proper torque. I would think because your passenger side is longer, that is the reason you are only seeing this on your passenger coil. I'm wondering if this has been done to compensate for something else being out of whack? I would take it to the Alignment Specialists, and request a 4-wheel alignment with the specific instructions of checking the straightness of the axle. I'm not sure the best way to check if it is straight on your own. I know people use a straight edge all the time, but that only checks the axle tubes. Keep us posted

This is both fantastic and depressing. I absolutely hate it when I do all the work a shop can't and still have to pay them to tell me I did it right. LOL.

I love your ideal of a plumb bob and lines on the floor. I'm going to try this. What I was struggling the most with was where to measure to and from. Like you said, if the frame is not straight that will through everything off. If I drop a line from each LBJ to the floor and from rear axle centerline to the floor that will allow me to adjust all 4 corners exactly.
It drives pretty straight and I have zero tire cupping or odd wear and the wheel returns to center well. Due to these factors I am assuming the castor is set correctly. I am going to measure the exposed threads on the upper and lower passenger control arms and move them both the same distance back in theory keeping the castor as it is now.

 

[Gravy]

Pretty easy to measure your caster (or more accurately, mimic the current caster angle) by placing a magnetic angle guage on the flat of the knuckle (you might need to use the flat of the lower bj) and keep it the same.
I'll bet you're at -5 or -5.5* (with 35s right?).

Consider with that inverted "Y" steering that pushing the axle forward will very very slightly increase toe in (or could off center your steering wheel) as the distance from the passenger knuckle to pitman arm increases.

I'd recommend 0-1/8" toe in on yours with the er towards 1/8".

 

[astjp2]

I would consider the pinion angle to be more important than caster...the ajustable control arms are really good at making up for manufacturing tolerances, if every control arm is a different length, so what if it tracks straight. The lower arms are what dictates your track, the uppers affect the pinion angle and caster, that is why the lowers are so much larger. Tim