Share your thoughts with me on air ride on a W124

Hey all, new member here, been enjoying the site and particularly the gorgeous cars.  Planning a build of a W124 Mercedes, which I'd like to put on air.  W124s, like most older Benzes, have separate springs and struts, rather than coilovers.  This means there aren't any off-the-shelf kits available, so I'm planning something custom.

Best I can tell, there are two options here:

1. Replace the coil springs with bags, a la this guy: [url]http://www.youtube.com/watch?v=S35a8TcScRg[/url] .  This seems like a straightforward solution, but since bags perform differently than springs, I'm worried about losing the car's handling.

2. Add something like UAS Aero Sport bags to the shocks.  I know far less about this option, but it seems like I could have the ability to raise the car for getting into driveways / over speed bumps without losing the factory suspension balance.

What I wish was available was some kind of universal air cup kit that I could put atop my factory springs, but I'm not seeing anything like that in my searches.

Do you guys have any thoughts on the best way to go about this?  Thanks!

Well don't know much about MB's, but E36 coilovers work with the front and I think Bilstein etc does make stiffer dampers for the rear ( and front why not)... sooo I think you have to go the reg truck route. Bags instead of springs and stiffer dampers, your choice of parts to use... which ever gives the stiffest possible outcome. And if you make it 4 corner adjustable, the handling shouldn't worsen if enough pressure is in the bags. Or go full custom hydraulic, some of the MB guys here have gone that route :shrug:

Thank you for the response!  The full-air route has some appeal to me, as I would like the ability to drop the car for parking as well as raise it for getting over obstacles.  The air-over-shock option only offers going up, in this case, and makes the effective spring rate of the suspension extremely stiff when inflated.

I've been running through the principles of this system in my head, and I think I've got it figured out, but I'm struggling with wrapping my head around setting the ride height on this thing.

For example, on the front, I've calculated that the front corners each weigh 923 lbs / 419 kg.  The bags I'm looking at (Slam Specialties SS-5) have a 4.2" / 10.7 cm top cap.  This means, to raise 923 lbs / 419 kg, the bags have to be filled to 66.7 psi / 460 kPa.  Alright, fine that makes sense, and the numbers seem reasonable from what I've found around the internets.

At that point, going up in pressure will obviously raise the car, down will lower it.  However, in addition, adding [I]a larger volume of air[/I] at the same pressure will raise the car, correct?  This means that, since I'm wanting to be able to raise and lower the car from ride height, that a simple pressure-based height management system will not be sufficient, and I'll need something with height sensors (or I'll need to get out and look at where the suspension is sitting every time I set ride height).

Am I thinking about this correctly?

Well every applications varies, but most still use basic pressure gauges (analog or digital) and with enough testing find out what psi + cargo gives approx height. Height sensor management is also starting to get pretty common nowadays, and the plus side is you don't have to think about it too much. Easy preset height+psi settings and I more weight is added the system will compensate. With my setup (Toyota Aristo) I was able to roll around w/ 40psi / 1" lift, and max lift (6") was at 130-150psi, sweet spot comfort and ride wise was between 60-90psi w/ 2,5-4" of lift... give or take, this was back in -09.

you are correct in thinking of the relationship between the volume of air and the pressure of air. keep in mind volume usually is limited to the bag size and the bag size is limited to the car application.

check this vid out regarding height sensors vs pressure based sensors.
[url]https://www.youtube.com/watch?v=LR5Wfu2aaqs[/url]

That video is actually what got me thinking about pressure-based ride height management, and how I don't think it'll work for my application.  The problem being that, unless your ride height is at maximum bag extension, pressure isn't directly related to ride height.  More precisely, if your ride pressure is, say, 60psi, that pressure can be reached with [I]any[/I] extension of the bags.

My extremely tentative plan is to use these bags:
[URL="https://dl.dropbox.com/u/623607/sleeve-bag-1__34986_std.jpg"][IMG]https://dl.dropbox.com/u/623607/sleeve-bag-1__34986_std.jpg[/IMG][/URL]
[URL]http://bagriders.com/modlab/products/TAPERED-SLEEVE-BAG.html[/URL]

...managed by an AccuRide rocker switch system with some random compressor and tank.

I like these bags for a couple of reasons.  First, they're narrow, which should make them easier to fit in place of the stock springs, especially given how ludicrously complicated the W124 rear suspension is.  Second, my calculations suggest that this bag should offer a spring rate curve as follows:

[URL="https://dl.dropbox.com/u/623607/spring%20rate.png"][IMG]https://dl.dropbox.com/u/623607/spring%20rate.png[/IMG][/URL]

To me, that curve looks reasonable.  At ride height, the air spring will be a little softer than a set of lowering springs (310 lb/in versus 370), but it quickly stiffens as the spring compresses.  This also gives me a range of 4" up from ride height (which is the bare minimum I need to get in my garage) and 3" down, which should be absolutely slammed.  It's hard for me to say whether that curve could be better, though, because I'm far from an expert on how air bags behave.  Let me know what you guys think, because I can change some parameters to alter the spring's theoretical behavior.

Again, I really appreciate the help!

Well which model W124 do you have, 'cuz the weight of the car plays a significant role in how said bag works. Although that particular one does seem to have pretty nice amount of lift, but the same part is used with their Golf air kits... also do you know the length of the stock/lowering springs? 

Here's a few more links:
 [url]http://www.ridetech.com/store/components/air-springs/[/url]
 [url]http://www.universalairsuspension.com/store2/catalog/bags-c-22.html?page=1&sort=3a[/url]
 [url]http://shop.airliftcompany.com/products/634489/Air_Springs[/url]

It's a '95 E300D, the big heavy one.  I dug through the published specs for the car, and everything I could find indicates front and rear corner weights of 923 and 697 lbs respectively, though I need to wander over to a scale and check for sure.  The coil springs I'm designing against have free lengths of 15" and 12" respectively, with 357 lb/in rates front and rear.  A little simple math indicates that the front springs, installed, should be compressed to 12.4" front and 10.5" rear, which sounds right for a 1.3" drop.

I obviously want to ride lower than 1.3" below stock, which means I'm looking for something like 11-11.5" front and 9.25"-9.5" in the rear.  My rough calculations indicate that the example air bag I posted above will have reasonable spring rates at 3" extension / 8" total length in the front at around 75 psi and 2" extension / 7" total length in the rear at around 60.  Add a couple inches for the upper and lower mounting plates, and you're right within my target height ranges.  The bags partially compressed at ride height like that also gives a very reasonable set of height ranges.  I know a lot of this is based on math and physics, but W124s on air ride aren't exactly common, so there's no "standard solution," so to speak, nor is there much information available.  If I didn't care about maintaining some semblance of handling, I'd just stick the bags on and call it a day, but this will be a daily driver, and I don't want it to be sketchy to drive.

I'm going to dig through those links you've provided and see if there's anything else that catches my eye.  Thank you very much for the push in the right direction!

You also have to consider the width of the bag so theres no rubbing. I wanted to go double bellow bag for my W210 wagon but the clearance around the frame and the front shock was very tight and that car weighs about 3900lbs. I talked to John at bagriders.com and he suggested the AirLiftCompany tapered sleeve bag since it will provide the most clearance.