Timbren and Sumo Springs offer similar products for the same purpose. They are auxiliary springs that add to spring capacity of an axle.
See Timbren Springs
See Sumo Springs
I have Sumo Springs on my 2003 Workhorse Chassis. I have no opinion on which product is better. I presume that are so close to being the same that it is a toss up.
If I had it to do over I might install inflatable air bags instead. About the same price and adjustable to a specific application.
They will not give a smoother ride. The tend to make the ride firmer.