The first question you need to ask when spec’ing trucking equipment is a simple one: What’s it going to be doing? Understanding the application and what’s necessary for the equipment to perform properly is the most important thing to know before making any purchasing decision.

Take vocational suspensions, for example. They have different needs than over-the-road suspensions because they will be tackling different jobs, and so there are a few application factors unique to vocational suspensions that fleet managers will need to keep in mind.

To start with, there are three different types of suspensions: air ride, leaf spring and walking beam.

Of the three, market share in the vocational space is divided between air ride and walking beam, dependent on the particular target segment. Many fleets tackling severe-duty applications might prefer a walking beam suspension, for instance, while other vocational segments will prioritize driver comfort and spec an air ride suspension.

Peter Schimunek, marketing segment manager for Western Star Trucks, says that many vocational fleets will choose air ride suspensions because of the stability and cushioning that they offer, which reduces freight damage and driver fatigue. However, he notes, “some air suspension models are best suited for highway applications, so we recommend adhering to manufacturer recommendations for your specific application and weight carrying capacity.”

Of course, “vocational” is a wide umbrella that covers quite a few different types of trucks, and suspension needs will be different for each of them.

“Capacity, stability, ride quality and durability are some of the main considerations when spec’ing a suspension system for a vocational truck,” says Kurt Swihart, Kenworth’s marketing director. “Vocations with a high center of gravity, such as mixers and dump trucks, require suspensions that provide maximum roll stability. In these applications, we typically recommend a walking beam style suspension system. Air suspensions are recommended when ride quality is one of the most important considerations. While air suspensions don’t typically have the same stability as beam style suspensions, there are several air suspension offerings that are specifically designed for vocational applications.”

“The needs of the vocational market are very specialized, and each application focuses on a different aspect of the suspension as the primary requirement,” says Sean Whitfield, director of marketing for Hendrickson.

For example, he notes that the key attributes needed for a suspension in concrete mixer and refuse applications are that it has the ability to maximize carrying capacity while still preserving stability and that it provides lower maintenance costs.

“Weight is an important factor when spec’ing a concrete mixer,” Western Star’s Schimunek notes. “The lighter the truck, the more concrete you can haul, which affects productivity. Choosing the right rear suspension for the job may also result in additional weight savings. However, mixers can get into some rough jobsites, so be sure to spec a suspension with good articulation, ride quality and durability.

“Chassis height is also an important factor as the mixer body must be able to fit beneath the hopper,” he adds. “A lower frame height results in a lower center of gravity, which provides increased vehicle stability. Customers should work closely with their dealer to spec the right suspension for their specific job demands.”

As for dump and crane trucks, Hendrickson’s Whitfield says that loaded stability and empty ride performance must be paired together to survive the terrain and loading cycle of these applications.

“When the vehicle is empty and/or traveling on-road, the equipment and driver must be protected from excessive road inputs,” he says. “When the truck is on-site and either being loaded or being used to lift a load, it must be supported by a suspension with high roll stability.”

Lastly, in heavy-haul applications, Whitfield says that equipment protection and ride quality are crucial to help ensure safe transport of cargo and driver. This, he says, must be done without sacrificing durability and roll stability, demanding a true vocational suspension, and he mentions Hendrickson’s Primaax EX as an example.

Additionally, there are some factors that apply across the board for vocational suspensions, regardless of the vocation.

“Vehicle weight, axle capacity, loaded and empty CG height, creep rating, and the operating environment are important application factors to consider,” says Bryan Redeker, powered vehicle systems product manager for SAF-Holland. “These factors are equally important regardless of the type of vehicle.”

Redeker says that it is important to know if outriggers will be present and where they are on the truck, as they may play a role in packaging. It’s also important, he mentions, to know whether there will be lift axles on the truck, how many, and how they will impact loading of the suspension when they are up or down. Additionally, frame rail spacing and package size of the lift axle assembly should be considered, he added.

Vocational suspension maintenance

With specific application considerations for equipment come specific maintenance considerations, and you’ll need to keep them in mind, especially those that are unique to vocational segments.

“Maintenance of vocational suspensions is similar to other suspensions—visual inspection of components and bushings. For those vocational suspensions installed with U-bolts, follow the OEM recommended practices for checking torque,” Kenworth’s Swihart says.

“A key to maintaining a vocational suspension is following proper inspection intervals,” Hendrickson’s Whitfield says. “For these applications, inspections should follow the vehicle OEM and suspension manufacturer’s service instructions, which usually list recommended inspection intervals based on hours and/or miles of operation.”

According to Whitfield, some essential items to check for, especially on vocational suspensions, include potential signs of overloading such as bent or cracked steel components.

“Reviewing the transverse torque rod (TVTR) bushing wear and replacing the TVTR when necessary is particularly important in vocational suspensions,” he adds. “The transverse torque rod keeps the axle aligned laterally on rubber-based suspensions and plays a large factor in supporting the other suspension components. Once that torque rod is fully worn, it is important that it is replaced in order to properly maintain the suspension as a whole.”

SAF-Holland’s Redeker says that it is important to monitor bushings, shock, air springs and fastener torque per the routine maintenance schedule.

“These components are always important to check, regardless of the application,” he notes, while adding, “A fleet operating in severe vocational applications may wish to increase the frequency of checks. Performing the initial 5,000 mile (100 hour) re-torque is critical to suspension longevity—especially the pivot bolt connection.”