Alright, somebody explain self-leveling suspensions to me please...

[todor]

I recently bought a 2001 540iT (E39 touring wagon) that has self-leveling rear suspension. I have the following questions:

1) Does it have coil springs or does it just have an air-pressure-operated equivalent of springs? I didn't see any springs in there, just two plastic-looking concentric cylinders, which I'm assuming are the "air springs". But then I did some searching here on bf.c and found people talking about how they replaced the springs on their self-leveling suspension with lowering springs, and then the self-leveling suspension kept increasing the ride height to compensate... wtf???
2) Self-leveling suspensions (SLS) only affect the rear, right? There's nothing self-leveling about the front suspensions?
3) Are the shocks on an SLS car any different from regular shocks for cars without the self-leveling suspension?
4) Does a self-leveling suspension have anything to do with EDC (electronic damping control)? The way I understand it, EDC affects only the shocks, while SLS affects only the springs (or the spring equivalent), so EDC and SLS are completely separate and you can have either of them without the other, or both together. For the record, my car doesn't have EDC.
5) Everyone here on bf.c keeps saying that cars with SLS have a different power steering reservoir. Well, mine doesn't - it has the regular reservoir with the dipstick on the cap, and it takes ATF. I'm not 100% sure about this though - I'll have to check tomorrow.

Please clear these things up for me. Thanks.

[OlyVR]

Trying to be short, as I'm at work...

1) Front: Regular coils springs no different from sedan. Rear: Airbags inflated by a compressor under the spare tire. To lower an iT, people often get Eibach springs for the front and then go to the dealer to have them electronically lower the rear.
2) Yes.
3) Yes.
4) Your are correct, they are separate systems and EDC was not offered in the US.
5) Don't know about that one.

[Qsilver7]

5) Everyone here on bf.c keeps saying that cars with SLS have a different power steering reservoir. Well, mine doesn't - it has the regular reservoir with the dipstick on the cap, and it takes ATF. I'm not 100% sure about this though - I'll have to check tomorrow.

Please clear these things up for me. Thanks.

Since OlyVR covered 1-4...I'll take care of number 5.

The important thing to remember is that when you are looking up info...be aware of what model and year car you're reading about.

SLS on the e39 touring is different than the earlier SLS systems on the e34 Touring. The early systems were hydraulic and used Pentosiin CHF 7.1 or CHF 11 S hydraulic fluid...which shared the same reservoir with the power steering system. The SLS in the e39 is pneumatic (air)...so that's why you are getting confused.

Did you pull up the diagrams in the online parts database so that you could see all of the parts etc for the SLS system for the Touring? It may help bring some clarity...specially when compared to the regular suspension...or the SLS suspension of older cars like the e34 Tourings or e32 & e38 7 series that use hydraulics.

E39 SLS: http://www.realoem.com/bmw/partgrp.d...96&hg=33&fg=50

[todor]

I see, thanks for the replies OlyVR and Qsilver7. Yeah, I was assuming that the self-leveling on E38s is the same as on E39s, that's why I had gotten confused about the power steering reservoir.

This leaves only 1 question unanswered:
Since you said "yes" to #3, then my next question is what is the difference and why do they have to be different? Is it because the air spring has a different effective spring rate from a coil spring, and therefore needs to be matched with different shocks? Do the shocks have anything electronic on them?

[Qsilver7]

You've pretty much answered your own question really....and though I can't speak specifically to the pnuematic systems in the e39 Touring...I am familiar with the SLS (or LAD) hydraulic system of old.

BMW uses conventional springs that have various spring rates. A car/model may be identical on the outside but could have different spring (part nbrs) based on something as simple as the way the car is optioned. If you ever get a chance to see (or you've already seen) the BMW spring table...you will notice this.

Also, in the hydraulic system, the rear shocks and springs do not have to bear up the greater share of the rear end like the conventionally optioned cars...in that the hydraulic system with its pressure regulator and accumulators perform this perfunctory job...(and this probably goes for the pneumatic systems, too). The shocks and springs serve in an ancillory capacity in stabilizing the car.

Think of it in the same way as a building...it has a conventional roof that has an expanse that requires the use of a center beam, rafters, ceiling joists, and pillars to support it. If you'd like to open up the ceiling and vault it...you can remove many of the pillars, joists, and rafters... and not have the roof collaspe under its own weight if you replace it with another system that uses trusses that will spread the weight of the roof in a different manner.

Essentially, that's what these hydraulic and pnuematic systems do...they take on the support role for suspending the rear end of the car...which in turn doesn't require conventional springs or springs with the same rate as the conventional system.

PS...the only "electrical" shocks that I'm aware of are the EDC shocks. They constantly receive data from the EDC control unit which is receiving info from various sensors (steering angle, lateral & vertical sensors, and speed sensors) around the car that determine to what degree they should dampen. There are electrical solonoids within the EDC shocks that open and close which allow the transfer of oil contained within the shocks to flow from an upper chamber to a lower chamber...which will either give a soft, medium, or hard (sport) setting. All of this is done automatically as the car is being driven. Then it also has a SPORT setting that locks out the "comfort" damping parameters.

For more info about EDC (electronic damping control) click this link: http://www.bimmerboard.com/members/W...3+B3715_01.pdf

[todor]

Thanks for the info, that makes sense, and it's pretty much what I thought.