VOCATIONAL SUSPENSIONS: Spec'ing for the job

Bain Disposal is a perfect example of a vocational fleet that needs to spec more than one type of suspension to get the job done right. Leonard Dietrich, president of the Pasco, WA-based refuse hauler, explains that Bain operates trucks for three distinct applications: OTR tractors that haul waste-filled trailers to the local landfill; roll-off trucks that handle commercial dumpsters; and rear-loader

Bain Disposal is a perfect example of a vocational fleet that needs to spec more than one type of suspension to get the job done right. Leonard Dietrich, president of the Pasco, WA-based refuse hauler, explains that Bain operates trucks for three distinct applications: OTR tractors that haul waste-filled trailers to the local landfill; roll-off trucks that handle commercial dumpsters; and rear-loader packer trucks for neighborhood refuse collection.

While the suspensions on these vehicles share some critical characteristics, the demands of the different tasks mean the suspensions used for each must also be quite unique.

“The first and foremost suspension trait for all of our trucks is simple: longevity,” says Dietrich, who switched his fleet to Peterbilt trucks in 1984. “All of our trucks do a lot of turning and maneuvering, so we need a robust suspension that helps us manage tire wear to a degree.”

After that, the differences start piling up fast. “Weight doesn't make a difference to us [on our rear-loader refuse trucks], but maintenance costs do, so we want something rugged that's going to take a lot of pounding,” he explains. That means using a rubber block suspension with 46,000-lb. rear axles and 18,000- to 20,000-lb. front spring axles. “That gives us a smoother ride over bumps — helping to smooth out the beating our trucks take.”

Dietrich is the first to admit that steel spring suspensions are very stiff and heavy, which explains why Basin specs air-ride suspensions for its roll-off trucks and OTR tractors. “They are equipped with Peterbilt's AirTrac air suspension because weight is a concern for us with our OTR trucks — the air ride suspension is lighter, so we can haul more payload and so make more money, while the durability we expect has been there for us.”

Gerry Remus, vocational segment market manager for Hendrickson Truck Suspension Systems, agrees that a truck's application is the main challenge when dealing with the vocational market. “There's just such a diverse number of vocational operations out there: concrete mixers, dump trucks, refuse haulers, truck-mounted cranes, you name it,” he says.

“All are similar, [but] all have very unique requirements. The basic cost of entry to the vocational market is that a suspension must be reliable and durable,” Remus points out. “Once you have fulfilled that, other aspects come into play, such as reducing the total cost of ownership and improving operating efficiency.”

David McKenna, powertrain sales and marketing manager for Mack Trucks, concurs. “No one suspension solution fits all vocational fleets,” he says. “Every suspension is a tool. Just like a carpenter carries a big toolbox with hammers, chisels, screwdrivers, saws etc, to handle a variety of different jobs, we OEMs offer specific suspensions for specific vocational applications. While you can [pound] a nail into a wall with a screwdriver, a hammer really works much better.”


Cutting weight out of the suspension so a vocational truck can haul more payload — and thus generate more revenue — is a growing trend, says Hendrickson's Remus. “Reducing weight can also help improve fuel economy,” he adds. “Due to changes in engine technology for 2007 and 2010 emission targets, we're now adding weight back into the truck. So we're trying to find more weight savings from the suspension.”

Matt Stevenson, director of marketing for Sterling Trucks, points out that you must be careful because “designs with lightweight properties can sacrifice durability; there's a trade-off here and you need to be a realist when you confront it.”

The tipping point seems to be the amount of off-road work a vocational truck is expected to perform, says Bob Miller, director of sales and project manager for ZF Group North American Operations. “That's where performance tends to take the lead over weight,” he notes. “For on-highway vocational operators, weight is king; but it takes a back seat when off-road work increases.”

According to Stevenson, the challenge is getting a lighter weight suspension that can still deliver durability, reliability, good ride and performance.

The key to achieving that is a greater strength-to-weight ratio, says Hendrickson's Remus. “You do that through materials and engineering — combining new fabrication techniques and components with new designs,” he explains. “In the old days, your suspension just had to be beefy enough to survive in the field. Now we are using computer programs and test tracks to design more efficient suspension systems.”


Jim Zito, manager of vocational sales for Peterbilt, sees an increase in the number of vocational fleets spec'ing air-ride suspensions. “Air ride is definitely taking its place in the vocational business cycle, but for a lot of different reasons — not just weight savings,” he adds. “Its road performance is one, and its cushioning ability can save wear and tear on truck bodies, especially dump trucks.”

Ivan Neblett, vocational segment manager for Freightliner Trucks, agrees. “Air-ride usage is increasing because it offers the driver a better ride and it saves the chassis/body/payload from the ill effects of a harsh ride,” he says. “Depending on the application, a high-capacity air suspension could weigh less than a spring suspension, which could result in more payload capability.”

While lack of articulation has traditionally made air-ride suspensions a less-than-perfect fit for off-road work, Zito notes new technology may be changing that. “There are ways to compensate for lack of articulation now, such as with a locking differential,” he says. “That increases suspension cost…but…you get your money's worth. It doesn't necessarily increase a truck's capability, but it keeps the box from getting beaten up and makes the driver happy.”

Remus attributes the growth in air-ride suspensions in vocational applications to driver satisfaction and a “better fit in the cost-of-ownership equation. It helps reduce damage to body and chassis, especially as there are more electronics and hydraulics on vocational trucks today.” Hendrickson is currently working on an air-ride 20,000-lb. steer axle that will be appropriate for almost all vocational applications, including rear discharge mixers.

Traditional spring suspensions also benefit from new technology. According to Brian Lindgren, vocational sales manager for Kenworth Truck Co. “We're offering a proprietary 22,000-lb., three-stage front taper leaf spring for customers who used 20,000- or 22,000-lb. multi-leaf versions in the past,” he says.

Lindgren describes it as a progressive-rate spring with five leaves. “The first stage, using the top three leaves, has a spring rate that's an estimated 12% softer than the previous 20,000-lb. multi-leaf, for a better empty ride. The second stage comes into play at around 16,000 lb., and has a spring rate that's about 30% stiffer…for improved steering feel when loaded,” he continues. “The third stage, which is essentially an overload spring, is approximately 85% stiffer, which is a benefit for pusher-equipped chassis when operated with the lift axles in the up position, providing greater stability in creep conditions.”

Little or no maintenance and minimal downtime are also high on a vocational fleet's suspension checklist. According to Bain Disposal's Dietrich, before adding torque arms to the fleet's walking beam suspensions (to keep them from “walking around”), the fleet was performing maintenance on the suspensions at least once a year. But now it can get eight or nine years out of the suspension before maintenance is necessary.

“Our biggest focus is keeping our trucks in service. At the price trucks cost today, we don't have spares — so we can't afford to have one sit around,” Dietrich adds.

Freightliner's Neblett points out that while there has been considerable improvement in terms of the maintenance requirements and reliability of suspensions for vocational applications, it's still important for fleets to spec their equipment appropriately if they want to maximize these benefits. “The key is understanding the application and not under-spec'ing the suspension,” he says.

The safety factor

Advancements in electronic technology have enabled manufacturers to put an even greater emphasis on safety as they develop new suspension systems for vocational trucks.

“There's a lot more potential for safety features via the electronic packages being built into today's vocational suspensions,” says Bill Sixsmith, director of severe service marketing at International Truck & Engine Corp. “[While] anti-rollover systems mostly involve the brakes and the engine, the suspension helps, too, in the sense that when a rollover situation is possible, it can shift air to the side that's rolling, augmenting vehicle stability,” he explains.

Jim Zito, manager of vocational sales for Peterbilt, says his company is testing several suspension safety systems, including stability controls on straight trucks and mixers. “Our focus is on early warning systems to alert drivers to a dangerous situation, communicating to the driver through a series of actions that he is in trouble,” he says.

The reason for this focus is simple: Vocational trucks typically have a high center of gravity (CG), leaving them very vulnerable to rollovers. “The higher the CG, the greater the risk of a rollover,” notes Zito.

“Any truck with a high center of gravity should have some type of stability or anti-rollover control system,” adds David McKenna, powertrain sales and marketing manager for Mack Trucks. “The big problem is that movement of the suspension is much greater in vocational trucks compared to highway tractors. We need to come up with software to handle that amount of variation to make the system work efficiently.”

Sixsmith says customers are particularly interested in suspension-based safety systems. “There are tremendous benefits to this technology because rollovers are so devastating,” he notes. “It's pretty easy to adapt safety systems like these to the suspension's existing electronic architecture; all the OEMs can do it. That's why we're looking at it for the vocational market.”

Preventing just one rollover pays for all of this additional technology, Sixsmith points out. “It prevents not only equipment damage, but also loss of life, vehicle downtime, cargo claims, insurance rating and costs,” he explains. “Saving one truck more than offsets the added cost [of putting this technology on] the suspension.”

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