Please instruct me on air springs

I was looking at an interesting 1968 BMW 2800 sedan, and the owner had added air springs. Now in theory that should have advantages:

Being able to add pressure to  respond to changing loads and tuning suspension (If you have separate front/rear bias)

Increasing ride height on bad roads (con: messed up geometry)

Being able to hunker down to a cool stance 

Concerns:

How do you change your effective spring rate at a given ride height?

How much abuse will these systems take? I was planning on using the car for a three-day 1000 mile rally full of windy roads 

Citroen (eek)

What else should I think about?

You don't get to change your spring rate, that's the fun part. Well, you do - that's what changes the ride height. So you can either have a soft rate and a low ride height, or a high rate and a high ride height. I'm pretty sure Citroen got a lot more clever with their setup, they were hydraulic and not pneumatic which means you're dealing with non-compressible fluids.

They're great for my pickup when I need to compensate for heavy loads. They're not a good option for a passenger car IMO.

When you pump in the extra air, the *volume* raises the car, not the pressure. Unless the air spring is extending against some resistance (like the airbag itself if it gets stretched and is trying to find the air), raising the car uniformly won't change the pressure. The area of the bag or piston didn't change, and the weight on the corner didn't change, so pressure doesn't need to change to raise it. 

Air springs will act very progressively once you start loading them though unless they balloon out (like bags are tuned for). Air cylinders don't balloon, so they do give a very strong increase in spring rate as you compress them without letting air volume out. 

Lincoln used to have air suspension in all of their cars in the 80s and 90s. Rear only in the Town Cars but at all four corners on everything else.  I know some of them (Taurus based Continentals) used a negative rate mechanical spring in addition to the air - the mechanical spring forced the rear suspension to compress, and the air pressure had to go up to compensate.

What effect this had, however, makes my brain hurt to contemplate 

One curious thing about air springs is that they don't quite respond the same way as coil springs.  They can be quite nonlinear in response, and their volume at full compression vs full extension will affect THAT.  So maybe the negative spring was there to alter this nonlinearity curve.  But, again, brain aches when contemplating it.

In reply to cyow5 :

How does that relate to air shocks? I'm pretty sure my pressure went up when I added air as did my spring rate, although who knows it was almost 50 years ago. 

Look up Julian Edgar on YouTube,  it's been a while but I know he had some videos about air springs in the past.  It peaked my interests but I have yet to buy his books to read the full story. 

Keith Tanner said:

You don't get to change your spring rate, that's the fun part. Well, you do - that's what changes the ride height. So you can either have a soft rate and a low ride height, or a high rate and a high ride height. I'm pretty sure Citroen got a lot more clever with their setup, they were hydraulic and not pneumatic which means you're dealing with non-compressible fluids.

They're great for my pickup when I need to compensate for heavy loads. They're not a good option for a passenger car IMO.

Modern OEM air suspension is pretty sophisticated, aftermarket ones are a lot less so.  Both my C7 S6 and the C8 RS6 (which replaced it) have air suspension and it doesn't hurt the handling enough that you'd notice.  Now, both of them are big heavy German sedans with too much weight over the front axle so they weren't exactly autox champions to begin with, but what they have works very well for the class of car.

Aftermarket car air suspension really seems to be aimed at the how "how can I get a car with less than half an inch of ground clearance but still be able to handle speed bumps" crowd.

I did air ride on my '61 Chevy Pickup.

Set it up so that the air bag is installed at ride height sitting just as tall as the bag sits on a counter at rest with no air in it at all.  If the "free air" rest height is 8", set up the suspension that at your desired ride height, the bag is also 8".

The less air pressure you put in, the lower it sits, BUT the softer it rides.  The more air pressure you put in, the higher it sits AND the rougher it rides.

Diameter of the bag plays a significant role in how much pressure you need to reach ride height.  A smaller bag will need higher pressure, AND ride rougher.  Depending on how low you want to go, this could be an advantage, or not.

I went against the norm, and did smaller diameter single-convoluted bags on all four corners.  I run 125psi in the front, and 45psi in the back, and it rides like an expensive sports sedan.  125psi for me has the corssmember 5" off the ground.  At 110psi I run the risk of hitting the ground with the control arms on big bumps.  I run 150/75psi to get the truck on my 2-post hoist.

I run manual valves, two compressors, and a 5-gallon air tank.

So far I have 2500miles on the suspension, and it's been the most reliable part of the whole truck.