Earlier in my career, I managed the North American operations for a German truck components manufacturer. One day, we received a warranty claim from a fleet customer. Our product was suffering premature failure. After a quick examination, I determined that it was being destroyed by road shock and vibration when operating off-road.

To determine the cause, the engineering director came over from Germany. The first place we took him was to a heavy truck dealership. We knew the dealer was preparing a chassis to mount a mixer. The chassis had a Hendrickson walking beam suspension with rubber block springs.

“Where are the springs?” the engineering director asked. We pointed and described the rubber block to him. “In America,” he asked, “is everything so heavy duty?”

It turned out our product had never been designed to resist the forces it was subjected to. In the concrete industry, many of the suspensions are designed to be “so heavy-duty.”

First, why do our trucks have suspensions? A suspension's primary purpose is to provide ride quality. They isolate the unevenness of the road (or off-road) surface from the truck's frame.

The suspensions absorb shock and isolate vibration so drivers and machinery aren't bounced around too badly and are protected from damage and injury.

Suspensions also keep tires on the road instead of letting them bounce off the surface. Without the suspension to keep wheels and tires in place, drivers could barely maintain and control trucks. Tires transfer the forces of acceleration, braking and turning from the truck to the road. But to do it all, they have to be kept on the ground in contact with the road under roughly the same load at all times.

Suspensions equalize the gross vehicle weight among all axles. If an axle and its wheels and tires and overloaded, the tires wear prematurely and are prone to blowouts. Axles could break. And without sufficient load on the tires, traction is reduced and the tires can't function properly.

They determine roll stability. If the roll stability factor is too low, a truck will tip over easily. Steering and handling may be adversely affected. But if roll stability is too high, the ride is rough and road shock transfers to the chassis.

Three varieties

Manufacturers have addressed each of these roles a suspension plays by offering three types of spring media. Each type you will see has its own advantages and disadvantages.

Rubber blocks, either in compression or shear, are rugged. They operate best when fully loaded, and offer a rough ride, bouncing tires off the pavement when running empty.

Multi-leaf steel springs 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 only as increased weight brings them into contact. At vehicle capacity, all leaves are in use and flexed.

When air suspensions were first introduced, fleet managers considered them too delicate for ready-mix trucks. In the last few years, they have become a viable alternative, but getting them accepted hasn't been easy.

In the ensuing decade, a great deal of progress resulted in new, rugged designs for air suspensions. “Our Primaax has high articulation and high 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 very good articulation. We also made the air springs very durable. There's even a weight savings of about 300 pounds between a Primaax and a 46,000-pound-rated steel leaf walking beam.”

Today, air suspensions are quite popular on front-discharge units. And air suspensions have been gaining attention on ready-mix trucks due to their characteristics when empty. Air is easier on equipment, more stable, and it provides a comfortable ride.

Pump suspensions

Concrete pumps—which weigh virtually the same, loaded or empty, and could probably operate quite well with leaf springs specifically designed for the vehicle—are most often specified with air suspensions.

“Pumps get a better ride using air suspensions, and at Mack, we are successfully using air on mixers, too,” says Steve Ginter, Mack Trucks' marketing manager, vocational products, Air ride is available to 52,000-pound capacity, from Neway. “What is most important is the bags are self-equalizing,” Ginter explains.

Producers have been slow to accept air suspensions for rear-discharge mixers because of the perception of less roll-over resistance. Remus says today's air suspensions are far superior to those of a decade ago in stability and ground clearance. Both are serious concerns when running both on-and-off-highway.

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

Producers who have mixers with Hendrickson, Neway or Ridewell walking beam suspensions can switch to air with Raydan Manufacturing's Air Link retrofit kits. Raydan air suspensions can also be specified from Mack, International and Condor.

— Paul Abelson is a former director of the Technology and Maintenance Council of the American Trucking Association and is currently on the Board of Truck Writers of North America. E-mail[email protected].

Here is a sampling of vehicle suspension products. Circle the reader service number for more information.

Rugged air-ride model

The Primaax air-ride rear suspension features an upgraded main suspension beam that reduces the unit's weight and improves reliability in severe-duty applications. The retooled suspension geometry improves roll stiffness and handling, suitable for rear-discharge mixers. Capacity reaches up to 26,000 lbs. for single-axle configurations, up to 78,000 lbs. for tridem-axle applications. Hendrickson. 866-743-3247. www.hendrickson-intl.com.

Lightweight construction

The Meritor Air Link suspension combines light suspension weight with stability and smooth ride. Suitable for on- or off-road applications with GVWR up to 52,000 lbs., the product can save as much as 450 lbs. per tandem compared to competitive suspensions. The walking-beam configuration permits equal weight distribution between axles, and high wheel articulation. Arvin-Meritor. 248-435-1000. www.arvinmeritor.com.

Tandem-drive suspension

The RD-202S Dynalastic suspension features rubber spring-cushioned torque beams, individually jointed for traction and stability. The design transmits the entire axle load from the frame to the center trunnion, where the load is distributed equally along the compensator. Top load capacity is 65,000 lbs. Ridewell Corp. 877-434-8088. www.ridewellcorp.com.

Air-ride front tandem suspension

This air-ride steering suspension continually equalizes weight, even in the event of a loss of air pressure. The unit offers up to 46,000 lbs. load capacity per tandle axle grouping. The four-corner axle alignment helps extend tire life. The suspension works with minimal air pressure, resulting in a softer ride and reduced driver fatigue. Raydan Manufacturing Inc. 888-472-9326. www.raydanmfg.com.

Tandem truck suspension

The 800-46 suspension provides a maximum capacity of 46,000 lbs. It comes in low-, high-, extra-low-, and extra-high-mount configurations. The vertical or beam-type shock kits are optional. The HS model offers the driver high stability on high center of gravity applications. Shock kits, which are recommended for all tractor applications, are also available. Chalmers Suspensions. 905-362-6400. www.chalmerssuspensions.com.

Front-tandem suspension

The AMS40TAA (air-assisted) front-tandem suspension combines the durability of springs with the comfortable ride of an air suspension. Load capacity reaches up to 40,000 lbs.; top axle load capacity is 20,000 lbs. Simard Suspensions Inc. 800-423-5347. www.simardsuspensions.com.