z4z4z4's pic I think is a good illustration of that plane going from top of suspension to top of roll hoops. I can't get that clearance no matter how I sit and I think from that pic z4z4z4 doesn't have it either
I wondered why that plane mattered and not just the broomstick test and got my answer / reminder from one of our older threads where a Z4M coupe rolled
http://www.zpost.com/forums/showthread.php?t=262344