"Air ride is the standard when it comes to ride quality, but suspension suppliers have really made strides on leaf spring and some of the rubber block torque rod designs," says Stevenson. "They are getting real close to air ride without having the worries that air ride can provide in terms of stability and also articulation."
In terms of weight savings, it is hard to match the air ride suspension. "There is just not as much metal in the air suspension as you would have in the rubber block or leaf spring," says Ferro.
"The challenge with an air suspension is getting good articulation and keeping adequate roll stiffness," says Lindgren. "You tend to trade those off against one another."
Articulation still favors the walking beam suspensions. "The best of the walking-beam suspensions, whether they are rubber or steel, do better than the best of the air suspensions so far," says Lindgren.
Stevenson claims the Hendrickson HAULMAAX, which is a rubber block suspension with a torque rod, and the Sterling TufTrac leaf-spring suspension with a torque rod are among the leaders in terms of articulation. They offer from 17.5 to 18 in. of cross-directional motion.
In terms of roll stiffness with air ride suspensions, it really varies by design. "There are some misconceptions about air suspensions and roll stiffness," says Lindgren. "Some air suspensions are very good and some are not very good, so you cannot generalize all of them into one category."
But an air suspension might not be the best choice for high center-of-gravity (cg) loads. "Certainly, air ride is by far the smoothest, but by its very nature it is not very stable with a high cg load," says McKenna. "That is exacerbated by uneven and loose ground cover."
One example is when a dump body is hoisted. "In dump truck applications, you have to dump the air bags before you get that body in the air," says Stevenson. "It just becomes another item that a construction fleet owner or supervisor has to worry about —making sure his guys are dumping the air out of the air bags before they dump a load. If you are running air ride, you definitely have to be confident in the operators of your units."
As with the rubber block suspension, cost can be an issue with the air ride. "It is in the same price range as rubber block for the initial cost," says Ferro.
On shorter wheelbase medium-duty trucks, there can also be an issue with the available space. "You are looking for locations for the air reservoir and the air ride itself, as well as the air brake. So real estate, especially underneath the truck, becomes a premium," says Ferro. "In the shorter wheelbases, it becomes very difficult to find that real estate."
But for the future, air suspension seems to hold the most promise. "The air suspensions, especially some of the later designs, are getting better and better," says Stevenson.