Auraraptor's E31 Suspension

[Auraraptor]

Part I: Suspension Basics

So you want to upgrade your suspension. The 'jury' is far from out on what to do, and how to do it. Alll of this is pretty well established now.

Lowering Springs Combos:

Front Spring stiffness Guide: (Soft) Racing Dynamics < OEM < Dinan < Mtech* = H&R < CSi < Eibach (Stiff)
Rear Spring stiffness Guide: (Soft) Racing Dynamics < H&R Rear < OEM < Cut OEM Rear = Eibach Rear < Mtech Rear < CSi Rear < Dinan SIII rear (Stiff)

Springs Setups are listed with the proper F/R rate balance in order of softest to stiffest Front spring (realize soft is a relative term, all are stiffer then stock and I have not included those less then stock):

• H&R Front/ Cut OEM Rear = H&R Front/ Eibach Rear
• H&R Front/ Euro Mtech Rears = Euro Mtech Front*/ Euro Mtech Rears
• H&R Front/ CSi Rears
• Dinan Fronts/ CSi Rears
• Dinan Fronts/ Dinan SIII rears
• CSi Front/ CSi Rears
• Eibach Front/ Euro Mtech Rears**
• Eibach Front/ CSi Rears
• Eibach Front/ Dinan SIII rears

Note: Racing Dynamics and H&R Rear springs should be avoided!
*Same rate, but less drop then H&R fronts
**I have not worked out the balance on this setup of springs yet

Shocks (Softest to stiffest):

• OEM Standard
• Bilsteins Revalved with the "comfort" setting (NOTE still stiffer then stock)
• CSi Front/Rear (available from any dealer...last I checked with discount they come to around 1600 for a pair of just fronts)
• Bilsteins Sport

Make sure to get the CSi secondary shock absorber as well

Swaybars:
Cost no option:

• Finding a set of GregK bars (Stiffest made for the E31, at 28.6F/20.6R but VERY rare.)
• Kmac sway bars from OZ (28.5F/19R)

Cheaper options:

• Later cars: CSi 27Front/ 17Rear (available from any dealer)
• If you have an early car, then either E32 SPORT front bar or better yet, the E32 Dinan bar (available at discount currently from Dinan!); For rears you have to really hunt to find an upgrade bar. Talk to 8Eights, he knows who it was that made the upgrade rear bar....I think it was Gpower?

"Other"

• Either Mwrench or Kmac camber plates
• Strut bar** (either Strong strut or MK Motorsports; both function nearly equally)
• Rear shock tower supports from Wuffer (a must for rear shock tower integrity)
• Rear

shock bushings are in development with Phoenix motorsports

*The strongstrut/any other strut bar DOES work. First realize it is not to limit vertical flex, as many incorrectly assume, rather lateral flex which the E31 does suffer, albeit slightly, from. Even BMW realized this, and that is why later cars were given the "cross brace" lower front (and why early cars cannot use later swaybars and vice versa). As such, you will see the most improvement with early, pre-cross brace cars. Cars will the factory cross brace will also benefit, but not nearly as much.

Part II: Technical Data
Special Thanks to Mwrench and Rod)

The goal with a good suspension setup is to reduce understeer. Our cars have quite a bit "out of the box." Understeer can be addressed by either stiffer rear springs (relative to the front) or stiffer rear sway bar (again relative to front). Use the OE as a baseline, and work toward less understeer.

Spring Comparison %F-%R (again, higher the %Front value, more understeer)

• OEM: 43F-57R
• Euro MSprings: 44F-56R
• CSI: 40.5F-59.5R
• H&R(with H&R rear springs): 54.5-45.5 (yes, the frequency balances out backwards! This IS bad, given the stress from the undersprung spring!)
• Eibach: 54.5F-45.5R (yes, the frequency balances out backwards as well! This is not bad thing though, just balance it out with a proper swaybar step!)
• H&R(with OEM rear springs): 50F-50R
• H&R(with cut rear springs): 47F-53R

Sway Bar Comparison

The key here is to divide the Front/Rear and compare the ratio to the OEM setup. Use this with the F-R balance mentioned above to choose the best swaybar (over vice versa the best spring setup) for your desired driving:


*Adjustable bars that allow tuning of the actual F/R ratio, and as such the value can only be an approximation of the actual.

Sway Bar Discussion

27F/19R is the best setup for the commonly run spring setups, with (the better) 28.5F/19R the best setup ideal only for those with stiffer rear springs like the Euro Mtech or CSi (or higher stage Dinan as listed above).

The GregK GenII kit (28.5F/20.6R) was originally designed to compensate for the H&R incredibly weak rear spring (which should never be used today anyway). In short, the GenII was an answer to a question (soft H&R rears excessively increasing understeer) that has a better answer (using cut OE rear springs).
Thus, today a full GenII should only really be run with Eibachs, but even then I personally would rather 27F/19R, or even better, 28.5F/19R with the rear springs swapped for CSi rears.

Summary
What does this all mean? Based on the data, you cannot simply choose parts and put them together. Sure it will drive just fine, but you will be setting your car up for far to much oversteer if you are not careful. Best bet is to balance your suspension around your desires and expectations.

[1chiban]

brilliantly done bro, beautiful write up!

[Toy72]

Bravo!!! Throwing flowers at you right now, cool write up

[dudu850]

Looks fantastic.
Do you mind if i translate this, and post your data on the Spain E31 club ?
Any EDC clues ?

Cheers.

[Auraraptor]

Thanks guys! Feel Free to translate, and I will answer any questions I can. I will add S EDC info (what I have) later today.

[Ladislav]

I'm interested in hearing from other 8er owners who have practical experience with (front/)rear H&R and front/rear RD springs.

By practical I mean installed and driven (as in on the road or track, not some forum).

References to prior posts welcome. Remember, that's practical experience not hearsay. I am aware of Ed's (MWrench's) comparison.

Thx.

[Auraraptor]

Proper Scientific testing >> any anecdotal evidence.

Anecdotal evidence will be affected by everything from the poster's background/car experience to choice of overall suspension setup. (For example, you can mask the softness with aggressive F/R shocks, but this will not change the over/understeer balance; drivers more key on 100+ mph cruising would find the understeer comforting, etc)

If you don't like the facts, you can choose to ignore them, but that will not make them go away.

The weakness in the rear shock tower is documented. The excessive stress on the mount with the H&R rear spring is documented (ie riding on the bumpstops). A rear spring softer then H&R will be more stressful in the rear. That is basic physics. RD Fronts are "ok" in that you will not be over stressing any particular part of the car, but their soft damping will increase risk of damage to your front spoiler with the wrong shocks. I would still use RD fronts if the overall package (shocks/sways) was still balanced...and in fact such a setup would likely outhandle the OEM setup!

FYI, there is no heresy in my post. I actually own many of the springs (about half) and swaybars (later car versions) mentioned in the above (though I sold my GregK bar a short while ago).

[Iamnotkento]

you can't just install the front and rear of eibach?

[Auraraptor]

Yes, you can just fine. Great springs! The only thing I would do is try as stated above is find a swaybar setup that would best compliment the F-R spring balance. In other words, find/use a swaybar setup that is more aggressive in the rear relative to either OEM or the CSi setup. A 24F/17R or a 27F/19R (or even a 27F/20.6R or 28.5F/20.6R)) sway bar setup would be working very much "in the right direction."

[smokum]

You could always ditch the bump stops.

[LEFT OVER RICE]

Nice write up Auraraptor

I think this is a great start up for people looking for ways to to make the suspension a strong point for their standards.

[Auraraptor]

You could always ditch the bump stops.

If you get your Air Ride up and running, I would love to add it to the post!

[Chuck850]

Test any ACS springs? I assume those are rebranded from one of the major manufacturers...

[Auraraptor]

IIRC, ACS springs, Hartage Springs, and Alpina springs (and even some others) are all Eibach springs .

[rcrad6653]

A small insert you could make to the Bilstein shock options Aura is a revalve option to the Dinan spec that is slightly softer than comfort. I have confirmed this with a Bilstein rep.

[Ladislav]

Proper Scientific testing >> any anecdotal evidence.

Anecdotal evidence will be affected by everything from the poster's background/car experience to choice of overall suspension setup. (For example, you can mask the softness with aggressive F/R shocks, but this will not change the over/understeer balance; drivers more key on 100+ mph cruising would find the understeer comforting, etc)

If you don't like the facts, you can choose to ignore them, but that will not make them go away.

The weakness in the rear shock tower is documented. The excessive stress on the mount with the H&R rear spring is documented (ie riding on the bumpstops). A rear spring softer then H&R will be more stressful in the rear. That is basic physics. RD Fronts are "ok" in that you will not be over stressing any particular part of the car, but their soft damping will increase risk of damage to your front spoiler with the wrong shocks. I would still use RD fronts if the overall package (shocks/sways) was still balanced...and in fact such a setup would likely outhandle the OEM setup!

FYI, there is no heresy in my post. I actually own many of the springs (about half) and swaybars (later car versions) mentioned in the above (though I sold my GregK bar a short while ago).

Just saw this post after I just launched a PM in your direction. Don't know how I missed your followup.

I am not disputing the facts. The underlines were used to hopefully head off posts that detailed the experience of someone's uncle's cousin's nephew and such with a 3er. ;-)

I own stock, H&R and RD as well as Kbars (gen I) and am trying to decide which route to take. Don't have time to experiment.

I am after the subjective here. Just want to know what people think based on driving experience. Please read the PM and PM me back.

Thank you.

[Neuance]

I have the acs (eibich??) springs all around, and I am goning to put the blisten shocks in the summer. Good idea? I only put 3-5000km a year on the car. What sway bars do you guys recomend? And who would be a good source to aquire these wonderful items.

Thanks,

PS Auraraptor any luck with the steering box and temp controle unit mods. I have the oil filter canister and hydraulic resovoir out right now so if your buddy here in Edmonton needs some better pic of the steering box get him to let me know as it is going back together in a week or 2.

Cheers,

[Ahmed303]

[SIZE="3"]
Spring Comparison %F-%R (again, higher the %Front value, more understeer)

• OEM: 43F-57R
• Euro MSprings: 44F-56R
• CSI: 40.5F-59.5R
• H&R(with H&R rear springs): 54.5-45.5 (yes, the frequency balances out backwards! This IS bad, given the stress from the undersprung spring!)
• Eibach: 54.5F-45.5R (yes, the frequency balances out backwards as well! This is not bad thing though, just balance it out with a proper swaybar step!)
• H&R(with OEM rear springs): 50F-50R
• H&R(with cut rear springs): 47F-53R

If these 2 are the same why one is marked bad while the other has a positive note. I feel a bias towards Eibach in this post.

To me, for a post to be a sticky needs unbiased view.

[Neuance]

perhaps one spring is progressive and the other is not?? I don't know but it could be a reason.

[Ahmed303]

Nope. The are the same rate of progression but Eibach is Stiffer than H&R in both front and rear. But we are on the topic of frequency balances.

MWrench Front Spring test

MWrench Rear Spring test

[Koizumi]

Great write up! I'd like to get front & rear sway bars but I'm very unclear as to which direction to go. I've got the Strong Strut installed too, so I'm assuming this affects my choice(s) since I'm a little stiffer up front. And I'm also assuming that a rear sway bar, which is probably really all I'd need for my specific setup, is not available at all

Cheers

[Ahmed303]

I've got the Strong Strut installed too, so I'm assuming this affects my choice(s).

Nope. Strong Strut does not affect your anti-sway bar choices. I had both K-Bars installed (GenI and GenII). I autocrossed with the stiffest setting and left it in the softest setting for regular use. The use of Strong Strut did not change anything except for the front end tightness feel at the softest setting.

PS. I did have the MWrench Camber plates installed that helped my autocross times.

[Koizumi]

Nope. Strong Strut does not affect your anti-sway bar choices. I had both K-Bars installed (GenI and GenII). I autocrossed with the stiffest setting and left it in the softest setting for regular use. The use of Strong Strut did not change anything except for the front end tightness feel at the softest setting.

PS. I did have the MWrench Camber plates installed that helped my autocross times.

Excellent info, thanks. My only gripe would be that it is impossible to find K-Bars. Anybody have one laying around?

[Auraraptor]

If these 2 are the same why one is marked bad while the other has a positive note. I feel a bias towards Eibach in this post.

To me, for a post to be a sticky needs unbiased view.

No bias: I actually use and prefer and use H&R fronts!

Only reason Eibach is ok is since you can safely compensate for the rates with proper rear sway bars. This is not safely possible with an H&R springs. This was mentioned in the OP!

Neither is 100 % optimal...compensating with swaybars for the Eibach's still would reduce some of the natural independance if the rear suspension.

[Chuck850]

I have the acs (eibich??) springs all around, and I am goning to put the blisten shocks in the summer. Good idea? I only put 3-5000km a year on the car. What sway bars do you guys recomend? And who would be a good source to aquire these wonderful items.

Thanks,

I have the bilstein shocks with my ACS springs... nice firm, smooth ride.

I have the GregK bars (gen II), but they are not in production any longer).