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      11-24-2010, 10:01 AM   #1
Doyle
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End-All xi Performance Suspension Thread

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 and multilink. 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.
STREET
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.
STREET/TRACK
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.
TRACK
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.
Attached Images
File Type: pdf E90 SUSPENSION & CHASSIS[1].pdf (535.2 KB, 2462 views)
Attached Files
File Type: txt XI Suspension Worksheet.txt (284.0 KB, 383 views)

Last edited by Doyle; 10-23-2011 at 12:14 PM.
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      11-24-2010, 11:02 AM   #2
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In my understanding, there isn't a spring with matched dampers. You have to go after market springs but dampers remain OEM. There are mix reviews with this setup.

You might want to check this thread http://www.e90post.com/forums/showthread.php?t=457355 - #13 post. He mentioned that a vendor is actually going to work on a specific configuration.

Last edited by blimey; 11-24-2010 at 11:07 AM.
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      11-24-2010, 11:09 AM   #3
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You are correct.

By "matched dampers" I meant getting a set of aftermarket shocks revalved to match the spring rates. Like a Bilstein B8/Swift Combo with said spring rates. This appears to be very popular with the miata autocross crowd.

I saw the AST/Swift combo and think that that could be a great option for "Driver 2". However, finances or need may dictate that that combo is excessive.
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      11-24-2010, 05:21 PM   #4
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I went with the AST 4100 and Harold at HP Autowerks because I like the flexibility of adding the right spring for your driving needs. After tons of research I decided to go with the AST 4100's because they are well built and have a great reputation. It is a custom order and takes 4 - 6 weeks but to me it is worth it! I plan to post pics of the install process and reviews after I try them....I'm going with 336lbs springs in the front and 672lbs in the rear. The stock 335 has 145lbs front and 460lbs rear. You could go with 400lbs in the front and 784lbs in the rear but those are more for real track use....
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      11-25-2010, 10:04 AM   #5
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Thanks for posting the spring rate options.

The AST c/o's should be the option for anyone looking for serious performance.

I am curious why the spring rates were advised to be so high? It was my understanding that most of the body roll and rough ride issues that the xi has comes mostly for poorly matched dampers to the springs. For example, the e9x M3 still has fairly low spring rates compared to what Harold recommended (190/550). It would be great if Harold could provide some knowledge to help us all out.
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      11-25-2010, 10:51 AM   #6
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Ride comfort in relation to spring rate is a factor of damper construction and valving. You can run high springs rates with a properly setup damper with good ride comfort.

I have ast 4100s on my 335 with 450# front and 900# rear springs and it actually rides pretty well. Certainly better than the hr sport springs on stock shocks that I used to have.
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      11-25-2010, 10:57 AM   #7
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It seems like the spring rates are being changed from stock ratio of 3/1 rear vs front to 2/1 rear vs front with the upgraded coilover package.

Spring Rates....

MJR went with the 336 and 672

Subie went with 450 and 900.

Stock 145 and 460 = 3.2/1

I wonder if this is the proper ratio with the 2/1.

Subie do you have an X-drive or rwd?
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      11-25-2010, 11:20 AM   #8
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Quote:
t seems like the spring rates are being changed from stock ratio of 3/1 rear vs front to 2/1 rear vs front with the upgraded coilover package.
You need to look at the actual wheel rates, not the spring rates. It is not a linear relationship. Additionally, the higher the spring rates you run, the higher the ride frequency goes. The higher ride frequency the more you want to shift the bias to neutral or even front biased. It also depends on what sway bar set up you are running as this well affect the total load transfer and effective wheel rates. I don't have my worksheet, so I can't provide the actual ratios.

Quote:
Ride comfort in relation to spring rate is a factor of damper construction and valving. You can run high springs rates with a properly setup damper with good ride comfort.
This is what I was getting at in regard to a proper spring/shock combo. With the adjustability of shock travel, compression/rebound in coilovers I think it is possible to get away with running higher spring rates.

It seems like the common "economy" trend to cure poor ride and/or body roll is to swap springs. This would only exacerbate the problem as the shocks seem to be underdamped from the factory. By adding shorter/stiffer springs you are further limiting the travel that the stock dampers can work in, as well as stiffening up the quick response characteristics of the ride. The end result? A rough ride that constantly bounces off the bumpstops. For economic performance upgrades, it may make sense to look at upgrading the shocks before springs.
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      11-25-2010, 11:53 AM   #9
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Glad to hear someone else talking about the crappy travel in the stock suspension. I feel that lowering springs in any way on the stock shocks or shocks that don't use shortened bodies is a horrible idea that will result in poor ride quality and worse handling.

Comparing ratios is not the proper way of figuring out spring rates for the car. Weight distribution and natural frequencies play into the equation in such a way that makes the ratio method not reliable.
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      11-25-2010, 12:32 PM   #10
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Quote:
Originally Posted by subieworx View Post
Glad to hear someone else talking about the crappy travel in the stock suspension. I feel that lowering springs in any way on the stock shocks or shocks that don't use shortened bodies is a horrible idea that will result in poor ride quality and worse handling.

Comparing ratios is not the proper way of figuring out spring rates for the car. Weight distribution and natural frequencies play into the equation in such a way that makes the ratio method not reliable.
Yeah. I think people confuse bouncing off bump stops with bad spring rates...the shocks aren't allowing the springs to do their job.

Natural frequencies! This is key, and highly overlooked.

Another thing that I was wondering: HPAutowerks did a great job R&D'ing M3 conversions for e9x. They also seem to be cool with getting AST to do custom coilovers. So I wonder how difficult it would be to get someone to fabricate a xi variant of the M3 front suspension bits? Essentially, taking the xi wishbones etc, adding higher durometer bushings, making them from aluminum, and maybe adding some built-in negative camber? Basically, taking the aspects of the M3 suspension that better the steering "feel" and adapting it to the xi layout.
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      11-25-2010, 06:16 PM   #11
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I've thought about making xi parts but am not sure there is a large enough market.
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      11-26-2010, 09:13 AM   #12
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Quote:
Originally Posted by subieworx View Post
I've thought about making xi parts but am not sure there is a large enough market.
Seeing as the front control arms and tension rods are the most recommended pieces from the M3 kit, I'm amazed nobody has done it yet. However, with the front drive wheels, would it even make a difference on an xi?

I have no background in fabrication, so I was just thinking out loud. This is why we need more informed people than myself commenting in this thread!
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      11-29-2010, 01:45 PM   #13
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See Post 1.

Last edited by Doyle; 07-28-2011 at 09:55 AM. Reason: Redundant info
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      11-30-2010, 12:03 PM   #14
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Quote:
Originally Posted by subieworx View Post
I've thought about making xi parts but am not sure there is a large enough market.
How many people would you need interested in this in order to produce them?
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      04-03-2011, 07:54 PM   #15
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thx for the info
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      04-04-2011, 09:27 AM   #16
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Great technical info about suspension setups. Now I don't claim to know the level of detail that you guys might have discussing the details in this thread but I was under the impression that beefing up the rear sway bar (replacing stock with M3 version) would only add to the under steer problem evident in AWD setups? Am I missing something?
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      04-04-2011, 11:21 AM   #17
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Things get real complicated with suspension set-ups. There is a textbook answer, then there is the "art". One will get you close, the other takes a skilled driver and engineering team to figure out. Moral of the story -- don't touch your car's suspension until you figure out how you drive and where your car is lacking. Drive the piss out of it during HPDE's/Auto-X and have instructors relay their impressions of your car.

Quick answer is there is no quick answer.

The textbook answer is that a stiffer rear bar will provide less load transfer and make the rear stiffer, thereby producing more oversteer.

BMW's have a MacPherson strut front suspension type, and the rear is a multi-link. This means many things, but in terms of this conversation it means that the inside rear gains negative camber in a turn, while the inside front loses negative camber. While cornering, this means that an already understeer prone car loses front end grip, and gains rear end grip. Thus exacerbating understeer the harder you corner. So, in an effort to maintain camber up front alot of track going e4x's and e8x's are running fairly large front bars and huge amounts of negative camber (-3.5'+). This maintains the contact patch of the front wheels and prevents rollover. More front grip = less understeer. This is why people rave that adding huge amounts of negative front camber drastically changes the handling of their BMW. Some guys all out delete their rear bar. It is backwards, but it works.

Things are further complicated when you take into account the fact that x-drive can direct up to 100% of torque to either axle depending on traction:

http://www.bmw.com/com/en/insights/t...mm_xdrive.html

So, for our cars, it would be best to treat them as primarily RWD platforms. That means that an xi, would be best suited to higher spring rates all around with the front natural frequency being 5-10% higher than the rear. Additionally, camber plates would be a must. Especially if you plan on tracking the car. You can go pretty far with just that. After that, run a stiffer bar to fine tune things. An adjustable blade type rear bar would be ideal, but there is nothing on the market. If you really want to get fancy, hit up Velocity Motorcars and get their full rear link kit to make sure you are maintaining your geometery. We are still at a standstill for what to do for the front arms/bushings -- hint, hint...anyone?

In the end, it is best to trust teams that actively track their products and platforms. Since HPA seems to be the most vocal about their setups and what works, I would trust them. If you feel like doing some thinking for yourself, dig around. There is a lot of information here, but the noise to signal ratio is pretty high.

These are good primers for suspension stuff:

http://www.motoiq.com/tech/the_ultim..._handling.aspx

http://www.optimumg.com/OptimumGWebS.../TechTips.html
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      04-04-2011, 01:09 PM   #18
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Doyle- great read, thanks. I am fairly new to all of this as well. Based upon my little knowledge, I chose to go AST 400/800 with the Vorshlage adjustable plates up front. I have yet to really test out the car. For daily use, I am running -1 camber up front and -2.5 in back. For track duty I plan on bumping the fronts to -2.5-3.5. I have to see what I can get using the plates. Otherwise, I haven't changed anyting out yet. I want to see how this setup performs and get the feel of it before making further adjustments.
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      04-04-2011, 01:31 PM   #19
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Thanks for the info Doyle...good reads
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      04-04-2011, 01:54 PM   #20
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TX-

Thanks!

Yup. Just start figuring out the little details. Preferred alignment, tire pressures, favorite aspects in a tire, etc... The good news is the only way to do those is to spend some quality time behind the wheel.

Enjoy your car, man. I bet it's a hoot.


MeeTsenPu-

No problem. This is why I started this thread. Just glad that it is finally getting some traffic.

There is no reason why you can't have a car that looks great, pulls 1+g on streets, and hits 0-60 in 3.6. All this while tackling terrible weather.
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      04-04-2011, 04:56 PM   #21
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nice thread, I thought the m3 rear sway would not work on an XI though?
I know other companies like UUC make one
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      04-04-2011, 05:42 PM   #22
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I have a set of UUC sway bars for my xi but have yet to install them. They are 27F and 19R. How will these calculate/work with stock springs and dampners?
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