But wait! There's more.
The top (non air line end) of the bladder uses flathead screws, so it's flush. The top mount would be simple, a flat bracket with holes at each for the spring mounts on your seat pan, and the mount pattern for the air bladder in the middle. I'm using and old Bates seat pan. so a mount like that wouldn't sit exactly flush on the seat pan, due to the pan's center brace. It could either be spaced at the spring mounts of a little creative bending would be in order.
The bladder itself is about 4 1/4" tall unloaded, and about 3 3/4" with 80 psi of air in it. Combined with the thickness of the mounts and spacers, that makes for a pretty tall seat spring. The plan was to make the bottom mount like a small cup, with straight sides. Put the inflated bladder in a vise and crank it down, it never gets more than 2 7/8" in diameter, so if the "cup" had an i.d. of 3" or so, you're good. This cup gets spliced into the center of the cross brace, so how deep you want it depends on a lot of things, how low you want the seat, clearance for fender and oil tank, stuff like that. The bladder, with all the air out of it, will compress to 1 3/4" but your effective minimum with air in it is probably closer to 2 1/2" The flat bottom of the cup will need the mount holes for the bladder, a hole for the air line hookup, and don't forget a drain hole at the lowest point so it doesn't collect rain or wash water.
You'd probably want to define the lowest point for your seat's travel with rubber bumper or two, in case of a leak or blowout.
What about a seat hinge with only a single bladder? here's an email exchange I had with Fab Kevin a while back:
Kevin,
Will one of your seat hinges work with a single spring or HD police air bag centered in the rear of the seat, without too much side to side rocking?
Thanks, Chuck
Chuck,
My hinge is the tightest and strongest hinge available, so if it won't do the trick, you will need to engineer something from scratch.
For sure, if you use an airbag, it will work. If you weigh over 250 pounds, and use a single, wobbly spring, then I can't guarantee anything. I have never had a single failure using my hinges, so you take that for what it is worth.
Feel free to call me, if you want to go over your ideas.
Respectfully, Kevin
One more thing. Filling it with air. If you've got a compressor with a good regulator on the air line and an accurate gauge, that'll work. I use a hand pump, the one HD sells for air shocks. Also, you're probably gonna go a little more on the air pressure than you would on a police bike. On the police bike the point at which the rider's weight is centered is about 2/3 or 3/4 of the distance between the hinge point and the air bladder, which means the air bladder only carries 2/3 to 3/4 of the rider's weight. On a chopper with a solo seat, that distance is probably closer to 90%, so you'd have to adjust accordingly.