When you ask Bob Wood how refuse fleets view the suspension systems on their trucks, he gives you a very blunt answer: “They're invisible to them — and they want to keep them that way.”

By “invisible,” Wood, environmental/refuse sales manager of North America for Denton, Texas-based truck manufacturer Peterbilt Motors Co., doesn't mean that suspensions are ignored. “They want a suspension system that performs day in and day out without them having to pay a lot of attention to it in terms of maintenance,” he explains.

“From the refuse operator's perspective, the suspension isn't the major component on the truck they worry about,” says Melissa Gauger, waste segment manager for Warrenville, Ill.-based International Truck & Engine Corp. “However, they expect the suspension to be rugged and durable, to do a good job without them having to think about it.”

Yet Gauger emphasizes that the suspension plays a very critical role in allowing a refuse truck to perform its tasks. As the link between the chassis, the axles and the refuse body, it must be able to handle excess weight, such as when taking on a load of rain-soaked garbage. “The suspension is really the ‘bread and butter’ component that helps hold the truck together,” she says.

The Basics

Wood believes refuse fleets must consider four critical points when it comes time to choose a suspension for their vehicles:

“Ride and handling is really not a great concern for most refuse applications, simply because most trash trucks operate in stop-and-go patterns, covering very short distances,” says Wood, noting roll-off trucks as the one exception due to their tendency to operate at high speeds without a load much of the time.

Waste applications impact other suspension specifics as well, adds Brian Andrews, sales application specialist at Hagerstown, Ind.-based Autocar. “For trucks going into a landfill, a high articulation is required, whereas it is not as necessary if the truck is going into a transfer station,” he says. “Front suspensions typically vary dependent upon application. A front loader, for example, requires a heavy, stiff front suspension, usually with added load cushions, to keep the suspension from ‘bottoming out’ each time a dumpster is lifted.”

Andrews says other applications, such as rear loaders and roll-offs, typically are lighter on the front axles and therefore require lighter, softer springs to improve vehicle ride. A taper leaf front spring is best suited to these applications, he adds.

Making Choices

For the most part, the refuse industry remains wedded to rubber block, steel spring and steel taper leaf suspensions — especially for those trucks that drive into landfills every day, Gauger says. Air suspensions are, by contrast, used only in a few select applications, such as the aforementioned roll-offs as they must perform highway driving.

“Most refuse trucks back into the accumulated trash at a landfill site to dump their loads, which would expose the rubber ‘bags’ of an air suspension to damage from sharp objects,” she notes.

“That's why the truck's application is the main challenge when dealing with suspension selection in the refuse market,” explains Gerry Remus, vocational segment market manager for Hendrickson Truck Suspension Systems, Woodridge, Ill. He points out that the price tag for different suspensions must also be considered alongside other rising operating costs, including higher fuel prices, more costly tires and the rising expense of the trucks themselves due to more stringent emission reduction regulations.

Weight is another consideration. Remus says fleets walk a fine line between vehicle weight and durability. “You can take 100 to 200 pounds out of truck from suspension changes but you must also look at how that impacts off-road stability, longevity and maintenance cost.”

“A couple hundred pounds of weight savings adds up to quite a bit of additional payload revenue over time,” says Matt Stevenson, director of marketing for Redford, Mich.-based Sterling Trucks, before echoing Remus' admonition not to compromise durability. “There's a trade off here, so you need to be a realist when you confront it.”

David McKenna, powertrain sales and marketing manager for Allentown, Pa.-based Mack Trucks, says there's no one-size-fits-all solution. He urges fleet managers to think of suspension options like the contents of a toolbox. “Manufacturers offer specific suspensions for specific vocational applications,” he says. “Because, while you can pound a nail into a wall with a screwdriver, a hammer really works much better.”

Other issues impact suspension choice as well, from geographic location to concerns about safety, adds Andrews. “Fleets in California are always concerned about saving weight whenever possible. But for most of the rest of the country, particularly in the Northeast, fleets would rather go beefy with their suspensions.”

Boosting Performance

Truck manufacturers also are making changes to the suspension and related components to make trucks more maneuverable. Kirkland, Wash.-based Kenworth Truck Co., for example, moved the steering gear forward, ahead of the axle, and added longer front springs as part of a front end redesign of its W900S heavy-duty vocational truck model. This increased wheel cut by four degrees and reduced the overall turning radius.

Portland, Ore.-based Freightliner — the parent company of Sterling Trucks — has designed a new rack and pinion steering system for heavy trucks that interacts with the suspension to increase the nimbleness of the vehicle. “It is 45 pounds lighter than integral gear systems with spring suspensions, allowing for greater payloads,” says Jonathan Randall, director of product marketing at Freightliner.

Even air suspensions are getting a fresh look from truck makers, as they seek to help refuse fleets find ways to improve vehicle ride and handling to reduce driver fatigue.

“Air suspensions specifically designed for vocational truck applications addresssome of the ride issues certain fleets were looking to improve,” Andrews says. “We also completely redesigned the front steering geometry to optimize wheel-cut and turn radius.” He adds that the company is investigating additional front suspension options.

Those options may include new taper leaf front spring options, he says, noting that refuse operators will still need to balance their choices against the durability and longevity needs of their equipment. “The trade-offs that you have to keep in balance when it comes to choosing suspensions are weight, cost, ride, durability and ease of maintenance,” Andrews says. “Those concerns are never going to go away.”

Selection Tips

So, you're in the market for a new truck. What sort of suspension should you choose? Even if you have a time-tested spec, Gauger of International Truck & Engine Corp. recommends that you have your dealer perform a weight distribution analysis to confirm the front axle capacity and position needed for your truck. This analysis should include body weight, payload weight and any special equipment weights, such as a front-loading arm for dumpsters. Once axle capacity is determined, you can move on to suspension selection.

“As a general rule, it is best to spec your front suspension capacity equal to axle capacity to assure that vehicle performance is not compromised,” says Gauger, noting that International has developed a list of front- and rear-axle tips vocational fleets should consider as they put the suspension together.

A fleet manager must first determine the maximum payload the truck will be required to carry. Then, by adding the truck, body and equipment weight, the manager can arrive at the desired gross vehicle weight (GVW). Once the GVW is determined, a front and rear axle must be selected to accommodate the load.

A set-back front axle (typically 40 to 52 inches bumper-to-axle measurement) is recommended for maximizing load transfer to the front axle and optimizing the vehicle turn circle. A set-forward front axle (typically 28 to 32 inches bumper-to-axle measurement) is recommended to optimize weight distribution and improve vehicle handling for applications with front-mounted equipment such as plows and front-loading arms.

As for rear axles, the first question is: Tandem or single? Tandem rear axles typically will be required if the rear axle loading weight exceeds 30,000 pounds.

The next step is to select the proper front and rear suspensions. The selection most likely will be dictated by the intended use of the truck. Typical options include spring, air, rubber block and walking beam (for tandems only).

Spring suspensions are an economical suspension option for both tandem and single axles. Because tandem-axle springs (called four-springs) allow limited articulation, they are recommended for on-highway use. Single-axle springs come in several configurations: vari-rate, multileaf or extra-duty. Vari-rate spring characteristics become stiffer as the load increases, optimizing the ride depending on the payload. Multileaf springs provide high stability with a stiffer ride. Extra duty springs typically are more suitable for severe driving applications.

Parabolic taper-leaf suspensions have fewer “leaves” than other spring suspensions, which reduce inter-leaf friction, resulting in a smoother ride. Instead of stacking many thinner leaves, the parabolic leaves are thicker to handle the necessary capacity. The taper in the leaf thickness provides a variable deflection rate, which allows precise response. Tuned shock absorbers are standard to help optimize the performance. Parabolic taper-leaf front suspensions are recommended for all applications.

Rubber or steel auxiliary springs are available for single axle spring suspensions to provide additional stability with high center of gravity loads or on/off highway operation. Auxiliary springs should never be treated as a substitute for increasing suspension capacity.

Air suspensions provide excellent ride characteristics and lower weight. They are recommended for on-highway applications. Air suspensions are available for single or tandem axles. A front air auxiliary spring is recommended for applications where unequal side loading occurs, as with a wing plow. The air spring is driver controlled and is available on either the right or left side. This feature eliminates the need to beef-up the suspension on one side of the vehicle to balance uneven loads.

Walking-beam suspensions allow tandem rear axles to go up and down independently so that they can “walk” over uneven surfaces while maintaining traction. They are recommended tandem suspension for demanding off-road applications.

Finally, suspension torque rods are standard on some suspensions but optional on others. “They are recommended to increase axle and suspension durability for applications that have frequent starts and stops,” Gauger says.

Sean Kilcarr is senior editor for Fleet Owner, a sister publication of Waste Age.

TRUCK SUSPENSION GLOSSARY

Air Ride Suspension

A suspension that supports the load on air-filled rubber bags rather than steel springs. The compressed air is supplied by the same engine-driven air compressor and reservoir tanks, which provides air to the air brake system.

Walking Beam Suspension

Type of truck and tractor rear suspension consisting of two beams, one at each side of the chassis, which pivot in the center and connect at the front to one axle of a tandem and at the rear to the other axle.

Spring Suspension

Suspension that is comprised of a series of metal “leafs” that form a spring that absorbs road vibrations to keep the load and the vehicle from bouncing around. However, as they are made of metal, they are not as flexible as air suspensions so the springs must be designed to handle the specific load weight required of the vehicle.

Axle

Structural component to which wheels, brakes and suspension are attached. Drive axles are those with powered wheels. The front axle is usually called the steer axle. Pusher axles are unpowered and go ahead of drive axles. Rear axles may be drive, tag or pusher types. Tag axles are unpowered and go behind drive axles.

Source: Truck Writers of North America