Suspension mods:
Heres a link to my B12 kit install
http://www.1addicts.com/forums/showthread.php?t=1245723
The handling mods on the car really make the car more enjoyable to drive. Straightline performance is nice but driving the car on a twisty road or racetrack is what is really exciting to me.
Stock suspension setup is garbage on a stock 135i. Totally inadequate for the power and just gets worse as the suspension goes past 100k kms.
I did the mods in this order:
Nolathane Subframe bushings - Locked down the rear end
M3 front control arms + pulled alignment pins - Very expensive for what you get, I only achieved 1 degree of negative camber on both sides on stock suspension. Ultimately these arms are only good with lowered suspension and camber plates.
Yokohama AD08R - Much more comfortable, grip was amazing
Bilstein B12 prokit + Dinan camber plates - Much less body roll, stiffer but more comfortable due to better shocks. Handling felt a lot sharper
After all these mods the car was very capable and nice to drive. But still suffered from massive understeer at the limit, even when not going that fast. I suspect that the Macpherson strut suspension was to blame. It always felt like all front end grip would fall off a cliff after the car leans beyond a certain point.
According to the theory of Macpherson front strut suspension, the camber changes when the suspension is compressed, initially you will gain a little bit of camber and beyond a certain point you will LOSE camber!
M3 Front sway bar
Therefore the common mod is to do an M3 front sway bar to prevent the wheels from reaching that point and losing grip. Scored a great deal on an M3 front sway bar and made a massive difference in the way the car handles. I can say my theory was confirmed and it was a big improvement. At the limit there is much more grip, still has an understeer bias but you have to be going very fast to reach this point. Before the limit, there is a reduction in front end grip, probably 5-10% less grip I'd say. But the tradeoff is worth it.
Downside is that the car is definitely harsher over bumps. Its not much but definitely noticeable. The B12 prokit is already pretty stiff, but just about the stiffest you'd want before getting on your nerves. The front bar just nudges it towards being slightly too stiff for me. I am quite picky and would prefer a more compliant car though. Over uneven roads is the main issue, if the wheels hit the bump at the same time its not as bad but still noticeable. I may change out the bushings in the sway bar with softer ones or polyurethane ones (which would allow the bar to freely rotate). The M3 bushings are very hard rock solid rubber and would resist the twisting of the bar quite a bit on their own.
I was afraid the front end would feel too stiff compared to the rear and did feel a little bit of it when I first put it on. I have gotten used to it but still feel a stiffer rear bar would benefit and even things out a bit. Stock bar is 12mm, the 22.5mm M3 rear bar seems like overkill, I may try an E93 15mm rear bar instead. I feel the twichiness of the 1M may be due to the fact it is using the 22.5mm rear bar which might be a bit too stiff.