I decided to update this first post with current options as well as increasing the detail of information. Let me know if you have any questions, comments, or critiques. I want to make sure this is as accurate as possible.
Step 1: Determine your needs
The first step is to determine how the car will be used. This will prevent you from needlessly spending thousands of dollars on components that will never be used to their full potential. More importantly, it will give you a goal to shoot for. With that goal in mind, you can set-up your suspension to best fit your needs. In an extreme example, a person who enjoys car shows and group runs will have very different needs than someone who routinely participates in time attacks or advanced HPDE’s.
Step 2: Improve your abilities
Consider this an extension of “Step 1” for those that choose to track or autocross their car. The more that you improve your ability to understand, manipulate, and control the car, the better you will be able to diagnose what issues you need to solve, and what methods would be the most practical (from a cost and time) stand point.
Step 3: Wheels/Tires
Now that you know your needs and the car, the first step to improving it should be to upgrade the wheels and tires. Tires are the single most important mod that you can do to your car (car, not driver). Choose wisely and carefully. Here are some factors to consider: proper size, wear rate, grip, sidewall rigidity, and heat management.
For the driver that won’t see the track, but only the occasional back road, something from the “Max Performance Summer” category (Hankook Ventus Evo, Yoko S.Drive, Sumi HTR-ZIII, etc) would fit the bill fine. For the split duty track/street, I’ve heard great things about Nitto NT05, Bridgestone RE11, Federal 595RS-R. These are all tires that should be able to take the heat that our heavy cars dish out on the track. For the driver that has moved onto R-Comps, there is really nothing I can tell you since you should be an advanced driver.
As far as wheels, the street driver can choose just about whatever size they want. It looks like the 1% rule is bogus...but make your decision wisely. The track driver really has no reason to go above 18”. If possible, stick to 17” due to the lower cost of tires.
There has been quite a discussion about staggered v. square. Hopefully, I can end that discussion. Keep in mind that there are 3 sections of every corner: entry, apex, and exit. A square setup increases front-end grip on turn-in (entry), but potentially leads to understeer later in the corner. The opposite is true for a staggered setup. Additionally, remember that tire pressure has a huge effect on the grip that each tire has. Manipulating tire pressure is a cheap and effective way of controlling the balance in your favor. Further, this ties into “Step 2”. If you know where your car understeers, and have adjusted your driving style, you should know which tire set-up fits your driving needs.
Step 4: Alignment & Suspension Geometry
This step is again extremely underrated in its ability to change the behavior of the car for a relatively low expense. Also, remember that if you change anything with your suspension, come back to this step and ensure that your alignment and geometry allows you to take full advantage of your modifications.
There are three parts to an alignment: camber, caster, and toe
. Additionally, it is worth your time to understand the types of suspension designs that the e90 has: Macpherson strut
. Understanding the dynamics of those two designs will greatly benefit you as you attempt to understand how your car reacts to changes in direction, bumps, or acceleration/deceleration. Additionally, some terms that might help further your knowledge are: scrub, king pin inclination angle/steering inclination angle, roll center, roll couple, and bumpsteer
For the street car, the stock alignment should suffice. If you choose to, you can drill out the camber pins for a bit more negative camber. For the track driver, camber plates are a must in terms of optimizing front end bite/grip/camber curves but also for the life of your front tires.
Step 5: Spring, Struts, Shocks, Coilovers
This step will likely make the single largest difference in how your car feels, rides, and handles. It can also be the single most expensive step. This is where understanding your needs, your car, and your driving style will be to your advantage. Be sure to read this link
as it will further explain the logic behind the choices below.
The first step is choosing your spring rates. This is determined by finding the target natural frequencies for the front and rear. Some will say that natural frequencies determine ride comfort, but it is my opinion that travel and proper valving have more of an effect on the ride quality. In any regard, your budget and your needs will determine which path you should pursue.
There are some complications with lowering the car. Remember the design of the front suspension? Although the e90 Mac-Strut is a much better design than previous 3 series’, you can over lower the car, increasing the roll-couple and thereby increasing the propensity of the car to roll. That’s right, the more you lower the car, the more it will want to roll. If performance is important to you, keep that in mind. I am hoping that a shop will be able to fab a roll center adjustment kit, but until then, camber will be the main weapon against wild roll centers.
For the street driver, the best option for spring rates looks to be the BMW Performance Springs. The rear bias is close to stock, the natural frequencies are within the “street-sport” range, and of course, they offer an attractive drop. Currently the only strut/shock for the “xi” are the Bilstein HD and Sports. Since you are lowering the car, I would advise the “sports”. AFAIK, the valving is exactly the same and the Performance Springs are within their range. The best part is the struts/shocks are easily rebuildable and revalvable.
For the street/track driver, currently there are 3 cost effective options for the –xi drivers: KW V3 w/ HPA Swift Conversion, AST’s, and TC Kline SA. All are good options, each with their pro’s & con’s.
For the needs of this driver, the natural frequencies should range from 1.8f-1.6r, to 2.0f-1.8r. I will caution that as you approach an 800# rear spring rate, you will need to replace the stock rear-subframe bushings. Using those natural frequencies your spring rate options are 350f/670r to 430f/840r. As I mentioned before, how well the shocks are valved in order to handle the spring rates will determine your ride quality.
For the track driver who routinely uses R-Comps, you will need rates about 2.0hz in order to take full advantage of the tires. In that case, 2.2f-2.0r is the highest that you will want to go due to stiction and the design of the rear suspension. Anything above 1000# you may want to look into a true full coilover setup, or just buying a Corvette!
Step 5: Sway bars
Sway bars should be used to balance out the chassis dynamics and reduce load transfer. They are a tool to fine tune how the car handles. For the xi, are front options are pretty limited. We have the stock bar or the UUC xi bar. All of the rear options for the other e9x will fit on the xi. Since the rear bar requires the rear subframe to be dropped, you might as well take advantage of the labor and drop in the M3 subframe bushings. They’ll have a huge effect on the chassis flex and springiness.
For the street drive that chose the Performance Springs, the best choice is the UUC front and H&R rear. This will bring the car more into balance and drop the roll down to 2.66.
For the track driver with the softer rates (336/672), the UUC rear or M3 rear will provide the best balance. For the higher rates (450/850), the UUC front and e93 M3 rear have great balance and roll below two degrees.
Step 6: Revisiting Suspension Geometry
Now that you have lowered, and stiffened your ride, we’ll likely need to take a look at how these changes have affected the geometry and alignment through the suspension’s new range of movement. If you followed the “street” advise, this will likely not have much relevance. However, those who track their car may want to look into this.
As I mentioned before, lowering the car will change the roll center, camber curve, and bump steer. Obviously, the further you lower, the more all of these will be affected. Bringing the rear back into factory/stock settings is fairly easy. Velocity Motorcars sells an adjustable arm/rod kit that allows you to fine tune the alignment and range of travel. The front is a little more difficult. The M3 arms do a great job improving the geometry for the RWD e90’s, but obviously, we cannot use them. I am hoping that a vendor will be able to press in new ball joints to the outboard end of the wishbone and shim it or potentially, change the inboard mounting points for the front lower arms. This will bring things back into line with stock without having to compromise camber settings. Making these adjustments will ensure that all of your hard work choosing, installing, and paying for your fancy new suspension doesn't go to waste. Getting rid of the stock bushing slop, stiction, and ensuring smooth travel will allow your suspension to work its best.