[Msquared]

To be more clear about SAI's definition, it's the inclination of the steering axis from a frontal view. The axis is defined by the upper and lower points on which the axis turns when the steering wheel is turned. Caster is the angle of that steering axis when viewed from the side. For an old-school double-A-arm suspension, those points are the upper and lower pivot points, and for an old-school strut they are the lower ball joint and the upper strut bearing. For our cars, the lower pivot point is a virtual one. It's defined by extending lines through the two lower lateral links to their intersection point.

As far as I know, most/all cars have some positive SAI (axis leans in toward the center of the car at the top). Some have a lot in order to minimize scrub radius (e.g. 88-96 Corvettes), but it seems like engineers have decided that 6 degrees or less is desirable. The problem with SAI is that it causes a loss of camber as you turn the steering wheel. The tighter you're turning, the more camber you lose. That sucks, but it can be counteracted by caster at a proportion of 1:1 (1 degree of caster counteracts the camber loss of 1 degree of SAI)...to a point. You can't realistically dial in 14 degrees of caster to counteract all the camber loss of the massive SAI in later C4s: all manner of weird things start happening if you try.

Anyway, adding negative camber by moving the top of the strut (or UCA) or moving the lower control arm(s) also increases SAI because that moves the pivot points for the steering axis. Changing camber at the strut:knuckle interface, like our cars do, does not increase SAI. So only some types of camber adjustment affect SAI. Caster does not affect SAI: you can change caster without changing SAI. AFAIK, SAI is measure by moving the steering lock to lock and the alignment machine measures camber change (I assume after it factors in caster).

But again, since SAI literally involves the steering axis, then I don't know why or how anybody would measure it in the rear suspension. There is an axis back there, but since the rear suspension has no active steering, then I don't know if we can really even call a "steering axis." And I definitely don't know why anyone would care! And finally, a changing SAI won't be caused by a bent knuckle. Somethings is moving/changing as you drive. atx_traveler is on to something with the eccentrics. The rear camber is also adjusted with eccentrics. These things suck, and they can move if not really tightened correctly. That's the first place I'd look.