I have 18" on my Z3, when i purchased the car it had 16" on and used to tramline, it was quite scary. I too followed advice and fitted a strut brace, which helped to a certain degree, Then looking for an answer, was lead down the path of stiffer bushes, but IMHO the car would not have been like this off the production line, so decided to investigate further. During an MOT it was noted that the rubber dust cover on one lower wish bone was broken, so i purchased new lower arms and changed them, what i noticed was that the front suspension is actually made up of ball joints, two at the hub and one arm to subframe. what i found was that the ball joints were worn and probably overall had 1mm play across them. I also found that the suspension top mount was loose. So i set about the change, the ball joints on the new arms are quite stiff na dt the same on the track rods and track rod ends (all got replaced), the bearing on the suspension top mount was also tightened correctly. The up shot is that the tram-lining and steering wander has all disappeared and the car is a dream to drive. I would suggest checking these parts out first before changing out to stiffer bushes and strut braces, all this will do is mask the issue of wear until it becomes unmanageable.
