Rear suspension sleeve install

[Oh Damn, it's Sam]

Keep in mind that the SS-5s are the same or larger in diameter than the Firestone 9000s. This means your bag pressure is going to be lower for a given ride height, which is probably not what you want if you're not getting a decent spring rate curve out of the 9000s.

A narrower bag might be what you need to get that bag pressure up and get the spring rate a bit stiffer.

[svcglobal]

After few miles of light test, very light indeed, one of the plastic bottom (with a fragile brass insert) of the left sleeve was broken.
Since I don't think sleeves should be mounted this way (my bad, I confess), I am not take in consideration mounting the short 9001 also.
They must be squeezed in an absolute vertical movement that my rear suspension will not give at all.

Now, I strong believe that Sam's idea of the SS-5 is for sure the best solution for trailing arm suspension.
Just bought a pair @ eBay for $190 shipped, in few weeks they will arrive.

[JayinMI]

Keep in mind that the SS-5s are the same or larger in diameter than the Firestone 9000s. This means your bag pressure is going to be lower for a given ride height, which is probably not what you want if you're not getting a decent spring rate curve out of the 9000s.

A narrower bag might be what you need to get that bag pressure up and get the spring rate a bit stiffer.

If I understand Sam correctly, the SS-5's are not a better choice based on spring rate curves.

Sam, do you have a link to something that might help explain spring rate curves?

Jay

[Oh Damn, it's Sam]

I've discussed the derivation of spring rate curves for air springs briefly in my build thread and elsewhere on here, but basically the idea is that an air cylinder (which an air bag is) behaves much like an extremely progressive spring with an exponential ramping of spring rate as the bag compresses. Unlike an ordinary coil spring, however, the behavior of the air cylinder depends not only on the cylinder itself, but also the conditions of its installation. Three installation-related factors come into play to affect how the bag responds to deflection: the weight of the vehicle, the motion ratio of the suspension, and how compressed the bag is at rest height. Using this data, you can actually do a lot of fine-tuning of your system to adjust spring rates to better suit your expectations. Installing the bags more compressed at ride height will put you in a stiffer, steeper section of the curve with the cost being limited uptravel before the bag compresses flat. Installing the bags more extended gives a softer, more linear response, at the cost of downtravel.

There is a lot that goes into how an air spring behaves in the real world, including some fairly advanced engineering concepts such as the material properties of the rubber in the spring as well as the compressibility and flow characteristics of the fluid inside the bag and attached line. The inclusion of these concepts into the discussion is far beyond what I'm capable of writing up (and, let's face it, I need a refresher course in fluid dynamics anyway), but barring that, you can actually come up with a pretty good approximation for most setups with the following simple equation:



where:

w is the weight of the corner in question
r is the motion ratio for the suspension on the corner in question (i.e., the ratio of how far your spring compresses to every inch your wheel moves up in the wheel well)
h the height of the air bag. You can use this to estimate the spring rate at rest (put in how tall the bag is at ride height) or to estimate the spring rate as the suspension compresses
hmin is the minimum height of the bag

Plot this equation for all meaningful values of h and you'll see the curve appear.

Do note that this equation is an approximation and really only works for fairly average systems. If your system runs extremely low bag pressure (just to pick one topical example), the effects of some of the more complex and subtle variables that I've abstracted away begin to become increasingly important, and the above equation will not give you even remotely correct results. Nevertheless, for the average system, it's a reasonable approximation.

EDIT: If you're interested in the derivation of the above equation, it just comes right out of Boyle's law, but I can post up how I derived it if desired.

[svcglobal]

Practically (non theoretically), I found the 9000 sleeve too long for my project.
In order to keep the light rear weight in the proper height that sleeve grows up to 8 inches. Too much.
The SS-5 (that appears practically to be more short) I think it will fit better.
Looking at SS-5 table above 5-6" will be achieved at 40 psi with 694 pounds.

[svcglobal]

Another point is that SS-5 have strong aluminum insert on both sides.
This is good since plastic bottom of the 9000 broke during road tests.

[JayinMI]

OK, so your saying at 694 lbs, and 40psi gives you about 5-6" of bag height, right?
And Sam is saying that 40psi might not give you the ride characteristics you want (more or less...) and that something with a higher pressure at that height might perform better. At least that's how I'm understanding it.

SS-5s are one of the bags I'm considering for the rear of my car, which uses a similar rear suspension design to yours, SVC.

My best estimates put my corners in the vicinity of (with driver):

LF 888 RF 750
LR 592 RR 500

I'll be curious to hear how they work out for you.

Jay

[Oh Damn, it's Sam]

Looking at SS-5 table above 5-6" will be achieved at 40 psi with 694 pounds.

Out of curiosity, where does the 694 lbs number come from?

I hope very much that the SS-5s work out for you, but let me just share with you a couple of concerns:

I've been working off the data you gave at the beginning of the thread, which suggests that your rear corner weights will be 473 lbs (2700 lbs total car weight * 35% rear weight distribution * 1/2). The cap area of an SS-5 bag is 16 square inches, which gives rear bag pressures of approximately 30 psi. I am concerned that this low of pressure will put you into the zone where my approximation is no longer adequate.

Even if we take the approximation as accurate, 30 psi with the bag mounted about 5.5" high at ride height will give you rear spring rates of between 150 and 200 lb/in. Even with how lightweight your car is, I do not think that spring rate will be sufficient to prevent the suspension from crashing on bumps. This is borne out further by the damage that occurred to the Firestone bags, which I suspect occurred as a result of the suspension bottoming out. SS-5s are more robustly-built than Firestone 9000s, but bottoming the bags out will destroy any air bag in short order.

Again, I hope they work out well for you, but do be prepared if they do not.

[svcglobal]

Sam, found another spec about the weight distribution and curb weight.
It is very difficult to find reliable data about an European car of 25 years ago.

This link shows the "new" weight distribution: http://www.carfolio.com/specificatio.../car/?car=2860

So, "new" curb weight of 2900 lbs (+- 400 lbs for turbo, new exhaust manifold, 2.5" exhaust piping, MPI injection, sound system, solar roof, air ride system, and so on) and 60% front.

I started to question my own figures after the test with the sleeves - for sure I have much more rear weight than expected.

So, 2900 * 40% = 1160 / 2 = 580 LBS on each side.

That 694 I found in the Lift Chart - Force in Pound in a topic above where I posted the SS-5 specs.

I strongly hope the SS-5 will perform as we need, can't see any other option...

[Oh Damn, it's Sam]

One important thing to remember, and a common misconception about how air ride works, is that you don't really get to set your bag pressures, so the 694 lbs thing in the chart above is irrelevant. Bag pressure is a function of primarily five variables: bag diameter, vehicle weight, suspension motion ratio, the angle of the bag's central axis relative to the control arm, and the angle of the lower control arm relative to the ground.* Your bag pressure is essentially set by these five variables, not so much by you yourself. The reason this is a common misconception is that the pressure inside the bag changes as the suspension cycles, since the lower suspension arm changes its angle relative to the ground, which affects how much of the force vector coming from the bag is translated into lifting force and how much just gets absorbed by the suspension bushings or subframe. This is why pressure-based management systems like V2 sort of work for managing ride height. To put it another way, the bag will have a given pressure at ride height - you don't get to just "pick" a bag pressure.

You can get a rough estimate of what your bag pressures are going to look like by dividing the corner weight of the car by the motion ratio of the suspension, then dividing the quotient by the cap area on the bag. This assumes the rear control arm is relatively flat, but it's a decent approximation. Given the new corner weight spec of 580 lbs, a motion ratio of approximately 1.0, and a cap area of 16 square inches for the SS-5, this produces rear bag pressures of around 35-40 psi, which is still very low, and I suspect your rear instantaneous spring rates are going to hover at the 200 lb/in level. All of this is if my spring rate equation can be even trusted at these low pressure levels, which I suspect it cannot.

If the SS-5 does not work out, the next step would be to examine bag options that will get the rear pressures up. This means a narrower bag. Firestone's 70xx-series bags might be an option, as they should get your rear pressures to at least 60 psi.

*Disclaimer: In reality, there are more than five variables at play here: the material properties of the bag, the port size of the fitting on the bag, how the bag's shape changes with pressure and compression, whether the bag is straight or bent, how much of the bag's lifting force is absorbed by rubber bushings in the suspension, etc. etc. etc. all play into this. The above are just the big five. In my experience, just taking these big 5 into account gets you an estimate that is within 3-5% or so of reality, which is pretty damn good.

[svcglobal]

Thanks so much Sam, you are a powerful bacon in the dark.
For now, I will wait for the SS-5 that were not shipped yet.
Seems Slam Specialties production is under stress. This email is from yesterday...

"Sorry for the delay. One of our material suppliers equipment broke down and we were back ordered for a while. We finally received it late yesterday so we are starting up production today. I will ship your orders out as soon as they are available.
Again, I'm sorry for the delay and any inconvenience this may have caused you or your customer."

In the mean time I started the work in the front suspension.

[svcglobal]

In order to give more room to the SS-5 I will cut the suspension inferior cup. In the place I will weld an adequate bracket so increasing 1" 1/2 in space.

[svcglobal]

There was no need to cut anything and the SS-5 bags worked just fine.
Pressure @40psi.