After years of wanting to convert to air suspension all the way around, I finally decided to practice what I was preaching and do it. The appeal of the light weight, load correcting, side hill levelling, and creek crossing elevation abilities had appealed to me for a long time. I bought the springs from Everett Brake Supply which is a semi parts place. I hemmed and hawed about doing the conversion all summer but went ahead and put it together 3 weeks before the big SOB IV event last September. After 4 trails runs and some limited street driving, I'm still wondering about the decision to put these on. It has not been without problems.

First off, one of the main reasons I was reluctant to mount the springs I had selected was that they have a plastic upper plate and a plastic bell. They are pretty unique in the Firestone line of springs and are not even listed in the main catalog. All of the other ones have aluminum where mine are plastic and have 2 mounting studs and a threaded port on the bottom. Mine have a single offset plastic stud that serves double duty as the mounting stud and the air inlet. And the bottom has a non-threaded port. It was tough trying to decide how to mount them.

I finally decided to do it once I got some good technical information from a Firestone engineer. It took numerous phone calls to get to talk to someone who had some good info and was not afraid to share it. The springs were originally intended for use as helper springs on the front axle of a Peterbuilt tractor. They have a load capacity of 3000 lbs each at the normal pressure of 100 psi. So once I knew that the capacity was more than enough, I decided to go for it.  I run mine at around 40 psi in the front and about 32-35 psi in the back. I also did some crude spring rate experiments with a hydraulic press and some circumferential measurments and found that the spring rate was pretty similar to my existing BC Bronco front coils. Well that sealed the deal and the grinder came out in full force.
 

At the top of the pic, you can see the combo mounting stud and air inlet. There is also a ring around the top plate to keep it from shifting back and forth. Also visible in the pic is the limiting chain that I installed to keep the spring from coming out of its lower cup. Unfortunately, this was not enough to keep one from coming out of the lower locating cup.

So far it has not all been a bed of roses. I broke one of the spring bases at SOB IV when it came out of the lower cup and upon reimpact, broke the base. This is the story of the infamous "Rag Bag". There was a large hole in the bottom of the spring that we decided was plenty big enough to stuff rags into to get me off the trail. I still need to get a beach towel out of there and cleaned up and returned to John Lindblom. This incident prompted the use of limiting chains to prevent pull out and it seems to be working good on the rear, but the front is another story.

The exact same failure mode happened on one of the front springs, but I believe for a slightly different reason. With my 3 link rear locating bars, the spring pads stay fairly level (parallel with upper mount) throughout the range of suspension movment. But on the front, with the radius arms, the axle rotates a lot with the suspension. I believe that the springs kind of "tipped" out of the lower cup on the second failure. Before I take it out again, I will be gluing and screwing the bases to the locating cups.

My tolerance on this swap is being tested. My vow is that if I have one more failure, I will be putting the coils back in. I refuse to buy another air spring (I have bought 2 replacments beyond the original 4). I have even gone so far as to get some stock upper coil spring brackets to try to scare the air springs that are on there. I am not pursuing the in cab controller (which would make it really trick) until I know they are on for the long haul.

FWIW, I ramped about the same (or slightly less) than I had previously with the air springs set at normal lift height and pressure. I went about 43" vertical height which is around a 1335 20 degree RTI score. I had hoped that they would do slightly better, but without the in cab controller, it does not appear to be the case. I could live with that level of articulation since there are lots of other benefits too, but I am not willing to live with the lack of reliability my mounting system has demonstrated so far.