Class 8 truck suspensions serve multiple functions. It is important to understand the trade-offs inherent with all of the available choices since these suspensions directly influence the overall productivity, comfort and cost of ownership of the truck.
"It is the intermediary between the ground and the truck chassis," says Gerry Remus, vocational market segment manager, Hendrickson Truck Suspension Systems. "It helps control the axle in order to give you good ride quality, stability and off-road mobility."
Traction and roll stiffness vary between the different offerings. "The truck suspension plays an essential role in vehicle support, stability and mobility," says Al Zwicky, senior applications engineer, Peterbilt. "It is also an important consideration in determining vehicle weight and payload capacity."
In addition, the suspension isolates the shock loads. "It protects the driver, the body and the payload from damage going over different types of road surfaces," says Steve Ginter, vocational product manager, Mack Trucks.
Each type of suspension offers particular strengths and weaknesses. "Since each type of suspension provides a mix of different operating characteristics, it should be carefully matched to a customer's application and business requirements," Zwicky advises.
The weight of the suspension system will influence the overall payload capacity, and therefore deserves close examination. "In most cases, payload capacity continues to drive spec'ing decisions," says Zwicky. "The lighter the suspension, the more payload capacity the vehicle has and the more potential for productivity and profitability. There is also the potential for fuel economy advantages in lighter-weight vehicles, which can be an important consideration as customers look to lower operating expenses during times of elevated diesel fuel prices."
Durability and weight not related
"Durability is most critical in severe applications where heavy loads are carried over aggressive terrain," says Shawn Waterman, Construction Segment Manager for Sterling Truck Corp. "In such circumstances, it is important that the suspension and related chassis components be equipped to handle the job. This ultimately entails the use of heavy-duty crossmembers, suspension liners, additional springs and reinforced mounting connections. All of these factors certainly add weight to the vehicle. But with today's designs and materials, the weight increases are held to a minimum."
In many cases, weight has actually been reduced. "With the newer, advanced designs, there does not necessarily need to be a trade-off between weight and durability," says Zwicky. "The more efficient designs concentrate the suspension's mass at high stress points rather than an overall bulky design, optimizing strength where it is needed most."
Ginter adds, "Improved durability can be achieved without weight penalty by improving the design and optimum choice of materials. But you do add cost because some of the materials that provide weight savings are more expensive."
Balance ride quality with mobility
Ride quality really differentiates vocational suspensions. "Most suspensions will ride good loaded," says Remus. But unladen ride quality varies.
"It is pretty safe to say most vocational trucks are loaded half of the time and unloaded the other half," says Remus. "So what you really need is a suspension that gives you a good unloaded ride. Typically, when you get complaints about ride, it is when the trucks are unloaded."
The trick is to find a suspension system that offers the best mix of stability, traction and articulation with the best ride quality.
In off-road applications with rough terrain, this mix is skewed toward maximum articulation. "There is a balancing of the different characteristics, and articulation is most important because it provides equalization of the ground loads to each of the drive wheels," says Ginter. "If you encounter a road terrain where you have to exceed the articulation built into the suspension, then one of your wheel ends is going to be free. You probably will get stuck. There is a possibility you would overload the wheel end that is on the ground."