I think the air bumps are suposed to give you a more progressive stop so a kind of emergency shoch absorber.Instead of the axel hitting a solid lump of rubber it comes to a softer stop.On the IH8MUD home page there is an article about parabolic leaf springs on their site there is some pics and info on all bumps .Hope this helps.
P.S thank you for the hj porn ,coil springs are one of my fantasys!
