Think of it more as "ride height" than "stiffness".
Air is a much-more progressive spring than the coil spring that it assists. By shooting more air into the bag, you will raise the ride height and allow more travel for the rear wheel. By bleeding the air out of the bag, you will lower the ride height, reducing wheel travel.
Let's say there is a 2x4 in the road. If you have a total of 6" of travel (I have no idea how accurate that is for an RT), you will likely use up 2" of that just sitting there. As you go down the road, running over that board, you will use up half the remaing travel available. If you have a passenger on board, you might sink another inch, meaning that running over the board will take up 2" of just 3" available, increasing the possibility of bottoming out. Adding air could raise you up to having about 5" available travel, so running over that 2x4 will barely be felt. In other words, a MUCH smoother ride.
If it were up to me, I would remove the coil spring and go entirely with an air spring, but I'll settle for the way the CanAm did it.
Also note that the system is self-leveling. If you adjust your headlights to be just right for you, then load up the saddlebags and trunk AND have a passenger hop on board, the compressor will kick in and add air to the bag, bringing the rear suspension to the same height and your headlights will be aimed just right again.
It's a bit tricky to see the system actually work. Several things have to happen to see it. Bike must be running. Transmission NOT in neutral. Parking brake OFF. I put chocks ahead of and behind the front wheels so the bike would not roll. Had the bike on our back patio with the lights shining across the yard, with a fence in the distance. When the ride height stabilized, I stood on one of the passenger floorboards and saw the hedlights go high on the fence. As the compressor came on, the lights went back down to normal. My son then stood on the other passenger floorboard, the lights went high again, the process repeated. When we both got off, the lights were, of course very LOW. The valve opened up to release the extra air, the lights came back up to normal.
Nice system overall, but don't think of it as "hard" and "soft", think of it as "higher" and "lower".
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