Solid axle caster angle measurements - different left to right? Huh?

[Garry in AZ]

So, I have been following a thread where the OP's new Jeep steers poorly. He took it to have the alignment checked, and found the caster angle was lower than the factory spec, but from his post, only on one side.
I can't wrap my head around this. We've got solid axles. Caster is set by changing the length of the lower control arms. Assuming the device making the measurement was reading correctly, how can you have a full degree or more difference in caster angle between the left and right sides? That to me, indicates the axle tube is being twisted. I understand there are manufacturing tolerances and such, but I would expect the tolerance to be within a tenth of a degree or so.
Maybe someone who knows more about this can set me straight.

Garry

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[GATORB8]

Caster is effected by all 4 control arms, upper and lower. A difference of a couple tenths could easily be in the mounts/ production tolerance, you're talking about a sixteenth of an inch over two feet on a lower to create that.

A full degree would be more concerning.

 

[Jim1964]

Your understanding is correct. The relative caster left vs right is fixed in place when the C’s are welded to the axle tubes. No one’s going to independently adjust left from right.

Except by the use of special ball joints with caster correction offsetting built in.

 

[Yellow Cake Kid]

As mentioned above, the placement of the pivots on the frame and axle can create a difference that is described as "Cross Caster" or "Caster Spread"

Jeep specifies the cross caster for the JL series as 0.00° +/- 0.50°.

Another thing to consider is the spring height at either wheel. The cross caster caused by differing spring heights will not amount to much but it will amount to something.

For example; The stock Sports and the Rubicons use the same control arms and different springs which provide different spring heights that place the control arms at a different angle of repose, so the nominal caster of either model is predicted to differ about 0.55°.

If the driver and passenger springs differ in rest height the control arms are going to be placed at slightly different angles and a small amount of cross caster will occur.

Here is a post from a thread that references the alignment specs: https://www.jlwranglerforums.com/fo...nt-specs-toe-caster-camber.17832/post-1397676

Also consider that many adjustable control arms are adjusted with 12tpi threading at increments of 1/12 of an inch. So, using the adjustments to compensate for variance in pivots' placements may not result in the precision one might infer from the term "adjustable". A 1/12th inch adjustment on an upper or a lower control arm amounts to just a little more than 0.5° so when you get the driver and passenger sides closer than 0.5° of cross caster further adjustment is unlikely to achieve a closer match.

The types of control arms that permit finer adjustment may seem desirable but introduce other concerns that may not seem appealing to everyone.

If you consider these influences you may begin to imagine why and how cross-caster occurs.

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