Questions about sleeve style bags

[veedubbin]

Two questions:

1- Is there anything wrong with using sleeve style bags for front struts? I'm planning on using airlift's universal 75563 kit. I'm just worried that they won't freely rotate correctly.

2- How closely do you have to keep to the weight limit on these universal struts? There is a weight limit rating of 750lbs for the 75563 kit. There's about 800 pounds of pressure on the front corners of my car.

The reason I would prefer to have sleeves instead of the double bellow bags is because the double bellows are 7 inches wide at 100psi while the sleeves are only 4.6.

[veedubbin]

anyone?

[hamiltonshmamilton]

I have heard that sleeve bags for the front won't handle as nice as the double bellow bags. A friend of mine had an old set of air lift sleeve struts for his mk4 and it didn't handle nearly as well as their double bellow struts

[Jeremy@AirLift]

What kind of vehicle do you have? You have to run way more pressure in sleeve bags upfront to support the weight of the engine.

[veedubbin]

@Jeremy, It's all going into a 535 E28.

I don't really understand why it wouldn't handle as well. Just because of higher pressure? wouldn't that just make it stiffer/better if it changes it at all?

It's not going to be a race car. Just a low and slow daily driver. I'm really only concerned about damaging the bags by exceeding the weight limits.

[Oh Damn, it's Sam]

I don't really understand why it wouldn't handle as well. Just because of higher pressure? wouldn't that just make it stiffer/better if it changes it at all?

I would be very interested to see an adequate explanation of the physics behind the different handling characteristics between the two bag styles. So far, the explanations I've been able to find suggest that sleeve-style bags are less progressive than double-bellows, though I haven't been able to find why this is the case.

You are going to obviously have higher bag pressure with sleeve-style than with double-bellows since sleeve-style are smaller in diameter. In addition, you have the diameter of the shock/strut tube to subtract out as well. But, higher bag pressure does not translate directly into higher instantaneous spring rate, which is as much a function of bag pressure and the ratio between bag volume at rest vs. fully compressed as anything.

I would love for an industry expert who understands the properties of these bags to provide a deeper explanation of the disparate behavior of these two kinds of bags. In comparison to coil-spring suspension, the general understanding of the physics of air springs is pretty low in the car community, myself included.

[KyleAnderson]

/\/\/\

PV = nRT (pressure x volume = number of moles of gas x constant x temperature)

Bag sizes, starting height 6", compressed 5":
Sleeve: 3" dia
Bellows: 5" dia

We're assuming same starting ride height (6") and pressure (175psi) to illustrate the point.

Sleeve 6" starting height, this equation gets the number of moles
(1206573 x .000695) / (8.314 x 295) = n = .341

Sleeve 5" compressed height, output is final pressure
(.341 x 8.314 x 295)/ .000579 = P = 1444468 (210psi)

Bellows 6" starting height, this equation gets the number of moles
(1206573 x .001931) / (8.314 x 295) = n = .950

Bellows 5" compressed height, output is final pressure
(.950 x 8.314 x 295)/ .001609 = P = 1448103 (210psi)

So, this shows that any size bag, with the same starting pressure and compressed the same percent of its volume, will have identical final pressure (in theory). Basically, it was a lot of math to show bag size itself is not the issue, rather how much fits in that bag...

What this doesn't take into account is the compressibility of air, the entire reason it works as a spring. This is where the progressive nature of an air spring comes from. At the same pressure, any number of moles of air will have the same distance between the molecules (remember this!). This "free space" is where the spring effect comes from. The molecules want to be at a certain distance apart . Push them together, and they'll push back. As shown in the example, the sleeve bag has a lower number of moles of air (at the same height and starting pressure), compared to the bellows. Lower number of moles means lower amount of free space to compress, so when you apply a force, the force pushing back will increase at a faster rate than the bellows, which has a higher number of moles and more free space.

[Oh Damn, it's Sam]

I was hoping you'd join us!

See, ideal gas law and figuring out bag pressure I can do all day long (though those are very helpful calculations for those who can't!), and it's relatively simple to approximate instantaneous spring rate from bag-at-rest-volume to bag-collapsed-volume ratio and pressure.

What I'm curious about is that I've seen numerous, numerous threads with input from laypersons and industry professionals alike that suggest that there is something fundamentally different about the progressiveness of spring rates between double-bellows and sleeve-style bags, with the implication that this is an effect that cannot adequately be explained by the differences in cylinder diameter and resultant pressure. This has always sounded like bullshit to me from people who don't understand the basics of physics of compressible fluids, but I am very interested in being proven wrong.

[Oh Damn, it's Sam]

For the sake of discussion (and shitting up this thread further), let me pull a couple quotes that back what I am curious about above.

From Air Lift's website:

"Because of their size and design, bellows air bags can lift with greater force at lower air pressures [...]"

From Bret at Air Ride Technologies:

"A double convoluted airspring has a very progressive spring rate...it gets incrementally stiffer the farther it is compressed. A convoluted airspring might take, say, 400lbs to compress 1 inch, 850lbs to compress 2 inches, 1350 to compress 3 inches, etc. This progression, coupled with the ability to adjust the load capacity and spring rate via air pressure, can be a very powerful tool. At the same time a rookie can adjust it into misery. A convoluted airpsring is more sensitive to the installed or intended ride height of the airpsring than a sleeve style airspring.

A sleeve style airspring has a much more linear spring rate...meaning that it might take 200lbs to compress it 1 inch, 400lbs to compress 2 inches, 600 lbs to compress 3 inches, etc. [Keep in mind these are all arbitrary numbers for demonstration purposes only] A sleeve style airspring is more tolorant [sic] of a wide variety of ride heights. Also, with a sleeve style airspring, the linearity or the progressiveness can be manipulated by the profile of the lower piston [the part that the sleeve rolls down over]. With a convoluted airspring the only way to affect the progression is by changing the bias angle of the fabric during manufacture. [...]"

(emphasis mine for both)

This is what I'm getting at. This implies that if you created a double-bellows bag and a sleeve-style that had identical proportions, identical materials, and installed them identically, they would behave significantly differently. I'll buy some difference for sure from the additional force required to "roll" the sleeve bag as the suspension cycles, but that seems like it ought to (mildly) affect spring rate, not progressiveness. In addition, as Bret implies, making a sleeve-style taper on the lower piston can affect the diameter of the lower piston area, since you can make the sleeve roll outwards to varying degrees. That can obviously reduce progressiveness to a degree.

That, or there's a lot of poor wording going on that is exaggerating the degree to which design differences play into performance differences, as opposed to bag volume, pressure, etc, that precipitate out of sleeve-style tending to be smaller in diameter than double-bellows.

Basically, I are confus.

[veedubbin]

I've decided to settle for the middle and go with 75568s all around. I've heard that dampening adjustability isn't really as great or necessary as I thought previously.

[KyleAnderson]

Most of it sounds like bullshit to me too. Spring rate and progressiveness comes down to how large the container is and how much is in it. I don't think bellows vs sleeve is the issue.

Funny, I came across that quote from ART too. It makes some sense if you consider that the thread it comes from is talking about application-specific parts, and that sleeve bags need more pressure. Because they take more pressure, the amount of stroke you'll see in application is shorter than bellows. With a shorter stroke you can expect to see less of a change in pressure from the usable extended and compressed lengths. Since he's talking about application-specific, recommended ride height is a given, and he can tune the setup to get the desired spring rate at that height. Him saying it's a more linear spring rate than bellows, I think is complete, 100% BS. Like you said, we're assuming a sleeve and bellows in the same application would have different diameters. But even if they were the same diameter, I'm still not buying that it's a significant difference to rate progressiveness.

The lobe shape can change the characteristics by changing the available volume in the bag. A lobe taking up more space should result in more progressive rate because it takes less amount of air to reach the desired height.

Couple examples:

Hollow lobe allows more airspace, so it takes a great amount of air to fill. More air means more distance between molecules to compress, so the rate shouldn't rise as rapidly.


Solid lobe takes up more space, less available volume to compress, so more progressive rate.


I think it should be clear that the lower available volume, the more progressive the rate should be, regardless of bag type.

I think one good thing to bring up here (applicable to the OP) is the reason why sleeve bags need more pressure to lift to the same height. In my previous post I did the math to show that 2 bags of different diameters with same starting height and pressure have the same final pressure when compressed the same percentage of their available volume. What it doesn't show is how that pressure is converted to lifting force, which is how much pressure is exerted on the end cap of the bag.

The 5" diameter bag has an area of 19.625in^2. Multiplied by the given pressure of 175psi, the force exerted is 3434 lbs. The 3" bag has an area of 7.065in^2 for a force of 1236 lbs. As you can see, the sleeve bag exerts significantly less force than a bellows in a similar application. This is only due to the bag diameter, not construction type.

OP, Because sleeve bags require higher pressure, you run the risk of reaching the burst pressure. This is why they're not recommended on the front of many vehicles. Fronts usually see more weight, and therefore need more lifting force and even more pressure. The more weight you put on them, the higher your pressure needs to be, the higher your static spring rate will be, and due to the greater amount of air in them, the more progressive the rate will be.