"Regarding BMW's front MacP strut designs, I'm not familiar with its steering axis characteristics, but I can tell you a little bit about an interesting construction feature. These BMW suspension systems feature double pivot suspension arms. Check out this web site for more on this:http://www.pml.com.sg/showroom/glossary_detail.asp?start_letter=d&id=37
Apart from the advantages mentioned in the link above, the double pivot lower arms allow better tunning of the control arm bushings so that it allows abundant recession (elastic rearward travel of the front wheel when it hits a bump, which improves suspension confort and NVH), but without loss of steering precision."
I have a 695 page report on designing state of the art double ball joint suspensions. No, you can't have a copy.
The easiest way to think about them is to look at the lower arm arrangement in plan view. The body, two arms, and the spindle form a 4 bar linkage. From this you can work out the instantaneous centre of rotation of the spindle, and since you know the other centre of roattion (upper ball joint for an SLA, strut top for a MacP) then you can work out the king pin axis, and so the scrub radius, etc.
That's easy isn't it? No.
The two lower ball joints are offset vertically so the plan view is not an accurate guide to the geometry.
The big advantage of these suspensions is that you can keep the ball joints towards the inner face of the wheel, without sacrificing kp location, giving you lots of room fro brakes and half-shafts.