PROBLEM: Bagged E93

[Jackalbeee]

Hey guys,

Have my bagged E93 with D2 full set up with v2 management.
kit here: http://d2airsuspension.com/Complete%20Kits.html but i have the airlift 4 gallon tank also.

My fronts work perfectly, the double bellows are GREAT with a superb ride....

But the rear.. well they actually don't work much.. it raises around 2 inches going from 0-150 PSI...

I have calibrated it and the height is never consistent with the PSI in the rear bags.

Could anyone lighten the situation? Is it because they are sleeve bags or is there something else i don't know about when it comes to sleeve bags and the e9x set up.

Friend of mine had the aired kit with rear sleeve bags and that didn't work for him either. Anyone experience the d2 set up for e9x or resolve any sleeve bag related issues?

I can provide photos and videos if that helps?
I have 39 days to the first show of the season and I am pulling my air out so bad!

Many Thanks!
Jack

[Oh Damn, it's Sam]

Is your setup with a sleeve-style bag over the rear shock, or is your rear bag independent?

Also, how inconsistent are we talking about? PSI is only a loose approximation of ride height, after all, which depends also on load and so forth. If your bags are inboard like the stock springs are on the E9X chassis, a small difference in load could produce very noticeable changes in ride height.

[Jackalbeee]

Thanks for your reply Sam, the bags are independent from the shocks and are located on the lower arm.

The height is inconsistent when I am standing outside the car, with no added weight and i will only air up when i have full 175 PSI in the tank. The rear does not move more than an inch unless i have my fronts fully aired out and the rear does no raise at all when i am sitting in the car.

When calibrating it seems to only go up to like 125 on the rear bags but when i have to air fully up it goes to 150 and i will hold for 5 seconds on each button (in manual mode) and it may just raise above the tyre.

For a long period of time I have to someone lift the rear of the car up for the car to clear the tyres when airing up simultaneously.

The manufacturer sent me an email today saying "Our engineer had further research on E90 and we happen to have a 2~3-year old E90 here.
We install the air bags on the old E90, we found that there is problem with E90 old car:
The end of lower control arm where near the hub comes with pillowball design; while the other end where near car body comes with rubber bushing.
And when the rubber bushing getting old, the whole lower control arm will sway.
From pictures you sent before, the lower mount is leaning to the rear, our manager said please ask the customer to replace the rubber bushing to solve the problem."

The english is pretty broken, but do you think that this could be the case?

Excuse me for the long response, i have spent nearly £3000 on the air set up and just want it to work haha

[Oh Damn, it's Sam]

I'm beginning to suspect that the bag in the kit doesn't have enough cap area to lift the car at the pressures the V2 system is capable of supplying, at least not as far inboard as it sits on the E9X chassis. A soft bushing at the pivot might exacerbate the problem by absorbing some of the lifting force of the bag, but if the system is that sensitive, it's poorly designed.

Here's the issue. The E93 is a heavy pig. It just is. The figures I can find suggest a weight of between 3700 lbs / 1700 kg and 4000 lbs / 1800 kg for the convertible, depending on model. Since it's a BMW we're talking about, the weight split is just about 50/50, so taking the upper of those figures, let's say each corner is supporting 1000 lbs / 450 kg of weight.

Now, the springs are wayyyy inboard on an E9X, which means we have to account for the motion ratio of the suspension when figuring out how much the bag has to lift. The figures I can find suggest a rear motion ratio of 57%, which means that the rear bags have exert a force of 1000 lbs / .57 = 1754 lbs each to even begin to lift the rear of the car.

Plus, we have a final problem, the tiny cap area of the rear sleeve bags. The lifting power of a bag is related to two things, the pressure inside the bag and the cap area. Thus, if we know how much force the bag has to exert and we know its cap area, we can figure out how much pressure is needed to get the car to start to raise.

Now, I don't know what bags you've got nor their cap area, but let's take a typical, but relatively large, sleeve-style bag as an example, say, the Firestone 9000. Its top cap is 4.76" in diameter, the bottom 3.59". Since the sizes are different, we take the average of the two and get 4.175", for a cap area of 13.69 in^2.

1754 lbs / 13.69 in^2 = 128.12 psi just to begin to lift the car.

It's then not super surprising to me that the system struggles to lift the car, since you're not far from max pressure just beginning to lift. As the rear LCA's angle becomes increasingly shallow relative to the axis of the bag, the amount of air pressure required to hold the car up increases, (which is why pressure management systems sort of work in the first place) and it doesn't take much change in height to add another 25 psi onto that figure. Add a driver and a tank of fuel, and that number goes even higher.

I suspect the rear bags they've provided are undersized and working at the absolute maximum of their pressure capabilities. In addition, the deeply inboarded spring location on an E9X is going to be exceptionally sensitive to variations in pressure and load, meaning it's an additional challenge to be consistent in ride height with a given pressure.

This is neither a good application for sleeve bags, nor a good situation for pressure-based management, in my opinion.

[Jackalbeee]

Thank you for your in-depth reply Sam.
As you can tell I am not very technical therefore find some of it hard to understand, but most of which is common sense and by the sounds of it, i need to get a refund on this rear setup, possibly the whole thing and have it replaced with a new set up? potentially the airlift kit that supplies with new lower sup arms that will relocate the position of the bag so it is not so narrow (compared to where the stock springs are)

If anyone could please advise me on a kit I can get within the UK that actually works I will have to see if I can fit it into my budget, as I have 5 or so weeks until the first show and I would like to get this fixed.

Once again Sam, really do appreciate your help!
EDIT: Seriously Sam, i need to buy you a beer/non-alcoholic beverage for your help.

[Oh Damn, it's Sam]

It sounds like your fronts are fine, so I'd probably leave them alone and just worry about the rear.

The fact that AirLift even makes such a product suggests that they too know there's an issue there, otherwise why bother making such a thing? I think the first step will be to get a larger diameter bag back there so it can lift the rear of the car at a lower pressure, preferably around 100 psi at ride height (or less, if you can pull it off) to give you lots of headroom.

With how far inboard the springs are on an E9X suspension, you may find a pressure management system will always struggle to be perfectly consistent, but getting that rear bag pressure down out of the stratosphere will help a lot.

[Jackalbeee]

+1 with the worryingly high pressure in the rear bags.

Going to have a search around tomorrow to see if there is something i can do with the retailer to switch to that rear airlift set up.
I agree with you, if one company only sells e9x kits with new lower arms, why doesn't everyone? because there is a persistent problem. Just wish manufacturers would be truthful with there products.

[R1 Joey]

Hi I have air lift struts on v2 on my e60

As said above I have the air lift kit on my e60, They don't make a kit for the e60 so ive fitted a e39 kit same rear problem however my bag is on the strut not trying to hijac thread just stating that airlift bag might also not work mines goin back sat to see if it's manifold problem my symptoms are from the ground rise on start to driving hight, The front goes to where it needs to be the bag just about lifts.
How ever off I fill them all up to 100 psi then come back down to driving hight it seems to stay at the right hight, when it first attempts to lift and does by an Inch it reads the same pressure as what it does when you go fully up and come back down but totally diff levels are your problems similar?

[Low Expectations]

I have the full airlift bags/v2 on my e90 and my rear lifts just fine. Their kit comes with a new rear LCA, which I believe moves the bag to a different position. I recall reading something about someone else having a similar problem and they switched to the airlift kit and it solved it. I know I can raise mine to about stock height at around 110-120 psi, I'd never drive at that high but it can easily get there. Maybe check into their kit, I know you can get it thru bagriders and they'll help you with anything you need.