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This Hendrickson air suspension has multiple torque arms to properly locate the axles.
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Those who have trucks with Hendrickson, Neway, or Ridewell walking beam suspensions can switch to air suspensions with Raydan Manufacturing's Air Link retrofit kits. Raydan air suspensions can also be specified from Mack, International, and Condor (American LeFrance LLC) trucks. Other medium-duty suppliers of air suspensions include Kelderman Manufacturing, Silent Drive Inc., Reyco Granning Suspensions, and Watson & Chalin Manufacturing Inc. Photo: Raydan Manufacturing

Multileaf steel springs, on the other hand, are more easily tailored to varying loads. The first few leaves are bundled to take the weight of an empty truck. Additional leaves bear on the first group, coming into play as increased weight brings them into contact. At full vehicle capacity, all leaves are used.

Single leaf springs, popular for steer axles where load is more constant, are mostly made of steel; but fiberglass is also available providing weight savings, a softer ride, and a gradual failure mode.

When they were first introduced in the 1950s, fleet managers considered air suspensions too delicate for off-road operations. The more rugged designs of the past decade are slowly changing this perception.

“Our Primaax has high articulation and roll stability,” says Jerry Remus, marketing manager for Hendrickson. “Customers get the benefits of a walking beam to equalize load between the axles while giving good articulation. We also made the air springs very durable. There's a weight savings of about 300 pounds between a Primaax and a 46,000-pound-rated steel leaf walking beam.”

UTILITY SUSPENSIONS

Utility trucks with hydraulic booms weigh virtually the same loaded or empty. They could probably operate quite well with leaf springs specifically designed for the vehicle, but are often specified with air suspensions. Plus, air suspensions are becoming increasingly available on medium-duty trucks.

“All vehicles get a better ride using air suspensions, and we're successfully using air on all classes,” says Steve Ginter, Mack Trucks Inc.'s marketing manager for vocational products.

Users have been slow to accept air suspensions because of the perception of less rollover resistance. But today's air suspensions are far superior to those of a decade ago in stability and ground clearance, both serious concerns when running both on- and off-highway.

With the growth in horsepower and torque, suspensions have been designed to be nontorque reactive. They have more torque rods to transfer forces. Less suspension twist reduces pinion angle variation. That, in turn, reduces driveline vibration.

— Paul Abelson is a former director of the Technology and Maintenance Council of the American Trucking Association, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.