[Edited - added rolling shot
I realize that I am not the most qualified person to provide a suspension review, but since I had the chance to do my purchase based on the opinions found on the forum, I decided to provide my contribution as well. Please take it as it is, a personal, subjective and not extremely technical take on the matter.
Back to the subject....we all know how the factory xi suspension rides and looks (in that order). Incented enough by this, I started looking at my options and discovered that there are not that many. Based on Doyle's generous input, I looked at Koni Sport and its derivates (TCK and HPA coilovers). I have also looked at AST and KW and of course, I have considered the shocks & springs combos. Due to a combination of mostly subjective factors (logistics - me=Canada based, price, stock levels, my ability to find vendors on the web, forum reading) I stopped at xDrive specific Bilstein Sport
(a brand I personally trust & a technology that has decent reviews) and H&R Sport
. I looked at Eibach ProKit as well, since it has good coverage on the forums and it seems to work very well for a good number of people, but being paranoid about using xDrive specific products (the ProKit that is available in NA #2092.140 seems to be RWD, in Europe they seem to have a different part for the AWD with different permissible axle load numbers), I settled on xDrive specific H&R Sport (#50494). The adjustability offered by coilovers (in terms of height & potentially damping) was not important for me and the price was good, better by far than any coilover option that works for us. As I said, I was pretty comfortable with the brands and the quality of the products I chose.
I am also fortunate to have some experience with the RWD platform in LCI E90 form (Sport/MSport suspension), so I have a good comparison base.
So, in terms of ride, what I noticed so far (suspension was installed yesterday, so I only have ~200km on it. I am also on RFT.):
- cornering is, as expected, flatter, with no drama and extremely low body roll. Much better than before and actually better that the Sport Suspension RWD car I had before.
- steering...oh, I am driving a BMW again. Seriously, when I switched to xDrive I was surprised to see that the tight steering that was the BMW signature was under that level of sedation. I thought that because of the front wheels being driven, the steering requires a different type of calibration and I was reluctantly accepting it...but boy, is it ever back! Tight, firm, responsive, the way it should be.
- comfort - it is obviously stiff. I know I have a good tolerance (given the advantages!) to a firm suspension and I do not mind it at all, but I can see why some people might not like it. A switch to non-RFT will alleviate the problem, but as I said, I am just fine with it. It feels clearly firmer than the sport suspension on the RWD.
- damping - appropriate. Keep in mind, Bilstein Sports are monotube. You feel the cracks in the pavement (no problems with that in my book). The bigger surface variations feel fine so far, with no crashing (for the lack of a better word). On my drive to work, there is a small ditch across the entire lane that used to be absorbed pretty well by my standard xDrive suspension. I am glad to report it does not feel significantly different. That is just a small example, obviously more time is needed to judge it.
- Rear looks slammed. Front is 1.2 inches lower than standard xDrive suspension (measured 15" from the center of the wheel to the fender before the install, I have 13.8" now) and I know it will settle further, not sure how much - so far, it is as advertised by H&R. It looks [subjectively] lower than the Sport Suspension I had on the RWD before in the front, and clearly lower in the back. I have 19X9 wheels in the back, wrapped in Bridgestone Potenza RE050 RFT (OEM) and there is very little room between the tire and the fender. Not planning on any spacers yet (I just don't like the idea) so it's all good in terms of clearance.
To conclude, I am happy. Waiting for the excitement to inevitably wear out but I am sure I will still enjoy the benefits of the upgrade later on. Will do an alignment after a month or so, as my daily driving is not that extensive and I want to make sure all is settled in properly before.