Its not so much that you HAVE to have Progressive because there are other brands out there, but, the fluid dampening and coil spring with the progressive suspension is far superior to the air. these are o my list of mods. Guy i ride with had em installed in Laconia on his SG. We rode all AM, and at lunch stopped at Meredith HD. he had new shocks put on and then we rode all afternoon. He said it is the #1 mod!! i have ridden his SG, and its like a caddy compared to the "clunky" feel i get from air. He had his lowered 3/4 witht he shocks as well. Then had the front Progressive monotubes put in which lowered front 1". it looks nice and rides nice. No nose dive when you hit front brake. I hate that!!!
2011 RGC, Sedona Orange 103" w/ 48H cams,PM Icon 21" wheel,Kury Mach 2 intake,Jackpot 3.5, PM brake and shift levers, CB 14" TRSSC shield, Sinister 14" apes, custom levers.Black exhaust,floorboards,pegs and crash bar.