A Corvette Forum member asked me to provide some in site for the reason we added rear caster settings on the C7, so here goes……
If you recall the camber adjustment on the C6 was done by moving the rear lower control arm rear inboard attachment only. There was no adjustment on the front attachment. So, when the rear attachment is moved outboard to increase negative camber, the rear lower control arm ball joint also moves forward creating rear caster. This also put an undesirable preload on the real lower control arm inboard front bushing.
The further we moved the rear lower control arm rear attachment outboard and the rear lower ball joint forward to increase negative camber, also increased rear caster, and produced ride and roll curves that were undesirable. I no longer have access to these curves, but I do remember we wanted/needed to improve them for the C7. The toe change in ride and roll on the C6 when the rear negative camber/caster was increased produced some motions that were subjectively unfavorable.
So, for the C7, we made both inboard attachments on the rear lower control arms adjustable. This kept the ride and roll curves linear. However, there was a bi-product of adjusting both lower control arm inboard attachments which was a rear caster setting that needed to be monitored as we increased rear negative camber.
Therefore, the rear caster adjustment was never included to be some kind of tuning setting, but it was necessary to maintain 0 degrees as rear negative camber is increased.
When we were at the Nürburgring, several times I worked with our technicians doing alignments on C7’s. Maintaining 0 degrees caster in the rear was not that difficult when increasing rear negative camber. An extra set of hands helped but was not necessary. I believe there is a special tool that goes into holes in the rear knuckle, but all we used was a digital level. There are 2 machined surfaces on the inboard side of the knuckle that the level can be placed against. As we adjusted rear negative camber, we always ensured the rear caster was 0 degrees.
Because of the undesirable toe curves generated when the rear caster was not 0 degrees, we never deviated from that setting.
Finally, I know there are production tolerances on the alignment settings that are way more than I like. Decreasing them was a battle I continually lost. Also, I recently was been told dealers do not monitor rear caster when performing an alignment, I never knew that, but this is also unfortunate.
I’m not sure there is much more I can add, but I hope this helps.