When the car was fully finished, I set up the suspension using the works Hunter system and found the caster was a little less than expected - I just thought I must have been inaccurate in my fabrication (an ex pro welder fabricator, this was a bit of an admission, but thought little more about it).
I think I have fully sussed it now (ignoring KPI for now!) for the straight ahead position.
I have used my CAD program to draw a diagram to show what I mean. The upright ball centre dimension has to be the hypotenuse or it would have to stretch when leant back.
Luckily, my self centring was about ideal for normal driving and for passing the SVA without using the bodges of setting the pressures to 60psi and having 10° of toe out.
The Sierra builders seem to have a greater problem with the self centring than the cortina builders. I imagine a true castor of 6° should give an acceptable degree of self centre.
If any maths students could work out a formula for the true setback for a given balljoint centre dimension, I am sure this would help a lot of new builders. Similarly, if any suspension engineers could verify my thoughts...
Rescued attachment True Castor.jpg