Hello. I currently own a Z4 35is since end of 2011. 67mkm now. I only changed the tires for non RF as soon i received it ! And i drive it with winter tires for about 6 to 8 monthes a year (i can explain...).
As i also own a E61 M5 for family displacing, and a single seater for my tracking itching, you will easily understand that i had no need to improve the handling of my Z4. I was satisfied, yes, but also i was not dupe : the steering is tilting, the front and rear bumpers work as if they do not know each other, the soft position of the adaptative susp leads to diving at each braking phasis, and the hardest position does not avoid tilt and lackness of lateral guiding. And is not apppropriate for french nor english country (but yes pleasant) roads...
And, last but not least, the brake pedal feeling is way soooooooooft and unconsistent (even if it brakes, yes).
But all of us know that, i guess.
So... What finaly hapened ?
Well, week after week, year after year, my expectation raised and i definitely could not bear my Z4 lacks.
Finally i decided 1 month ago to replace the springs and the adaptative dumpers for a carefull, cost friendly, comprehensive fix : i got a B12 pro-kit (eibach spring and bilstein B8), plus the Eibach Pro-Tronik module to neutralize the adaptative adjustable suspension (which is not useful with the B8).
And this is now a pure revelation ! i can't still believe it : amazingly more comfortable at midle and high sppeds (not at very low speed but who cares), so much more predictable at the limits, no more tilting phenomenon, the steering is sharp and easy (better feedback and no more adverse reaction on bad roads), and the DCT is less requested on greasy/wet. DCT off, it is easier to drift and keep it sliding, and the grip recovery is soft and predictable also. Braking distances are shorten.
I-CANNOT-BELIEVE-IT.
Yeas I am used to racing and to much more powerfull and lighter cars, but i am also able to distinguish between racing purpose and road needs : i was not searching for a track behavior for my Z4 : i only expected something sharp, predictable, efficient and daily manageable.
All the good aftermarket kits are surely of interest : Ohlins, KW, E/B... Inside all these famous solutions, my choice was finally based on a pricey consideration : if i have made a bad choice, it had to be painless to flow the kit in the garbage !
Now we are in november, already 2mkm with this B12 kit, and i mounted the winter tires (michelin PA4 in 18" 225 and 255) since one month.
It is (if possible) more amazing : corner handling and traction with these winter tires are improved to an unexpected level.
If i had to summerize my feeling with the B12 kit replacing the electronic dampers of my Z4 35is, i would talk about the steering feeling : sharp and easy, without any side effect.
(i can give advices or details by mail if wanted)