Choosing the best suspension for a fleet’s application is vital for vehicle longevity and driver satisfaction. Applications can change slowly over time, and there is always new technology to consider. For those reasons, periodically reviewing suspension specs to make sure they match current needs is a good way to make sure a fleet is giving its drivers and the loads they carry the smoothest ride possible.
There are three main suspension types for Class 8 vehicles, says Mark Molitor, senior product manager for Link Manufacturing: spring, rubber, and air-ride suspensions. Each one has distinct features that lend each one to specific types of trucks in specific applications.
“A lot of times, the differences between these suspension types can be minimal. In those cases, personal preference is usually the deciding factor,” Molitor says. “I’ve seen dry van fleets go from air ride to springs because they really don’t see that big a difference in ride quality. And their thinking is they’d rather take the upcharge for air-ride suspensions and put it toward other specs on their trucks.”
Schneider, for instance, in a 2020 report to shippers, said it ran an extensive series of tests between various air- and spring-ride suspension systems and found that air springs do not guarantee superior ride quality.
Dig Into Suspension Differences
When looking at on-highway van trailer applications, the main differences can be summed up in two attributes, explains Lance Gage, product manager for SAF-Holland.
“If you’re interested in simplicity, then you’ll probably want to spec leaf spring suspensions, as they do not require any air controls, shock absorbers, or air springs,” he says. “[But] air-ride suspensions will have a better overall ride quality when the trailer is empty or partially laden.”
Things get a bit trickier for fleets that routinely run half- to fully-laden.
“Ride quality between the two design types is much closer in these applications and would likely not be discernible to the driver,” Gage says. “Generally, a fleet’s specific vocation and target customer applications (freight) will be paramount in determining which suspension system is the ‘best’ choice. While air rides are the prevalent choice for many specialty trailers such as tank, bulk, and platform, the mechanical suspension maintains a significant market share in the dry freight, on-highway segment.”
Some differences between air and leaf spring suspensions include trailer isolation, load distribution, and enhanced operational capabilities, says Melanie Elliott, marketing manager for Hendrickson. She explains that air suspensions isolate the trailer from road inputs because the suspension reacts to variations in the road surface without transmitting those variations into the trailer or the cargo. The result is less damage to the trailer and the cargo.
“Air suspensions also evenly distribute the trailer load to all trailer axles,” Elliott says. “On a spring ride, however, the leaf spring suspension nearest to the trailer load will carry more of that load. While wear is inevitable on components like tires and brakes, with an air suspension, these components wear at a more consistent rate, which helps extend component life. It also provides a more even distribution of brake force, helping to improve vehicle stopping distances.”
Another advantage offered by air-ride suspensions are their adjustable spring rate, which can be altered while maintaining a constant ride height via a height control valve. Pressure inside the air springs is adjusted based on the payload to ensure optimal suspension dynamics and ride comfort whether loaded or empty.
“Air suspensions can also be lifted when the vehicle load is reduced,” Elliott adds. “Lifting a suspension can increase the vehicle’s fuel economy and help extend the life of the brakes and tires on the lifted axle. A lifted axle can also reduce toll road costs.”
Suspensions For Vocational Applications
Most vocational fleets tend to opt for mechanical suspensions, which come in two types: leaf spring or rubber-mounted.
Hendrickson’s Elliott notes that these are generally simpler in design with less parts — which means they’re less expensive to spec and easier to maintain. And they are generally better suited to more demanding operations, especially off-road.
“Air-ride suspensions are becoming more popular in vocational applications, as they offer a better ride when unloaded,” says Tim Wrinkle, senior product manager for vocational and medium-duty trucks at Mack Trucks. “However, these suspensions can be limited in articulation and stability.”
Driver knowledge and training are also important with air suspensions because drivers need to evacuate air bags before dumping the load. Depending on the setup and a driver’s experience, this can also slow the dumping process for dump trucks.
“If you’re in a vocational application, and your load changes frequently during the day on routes with lots of stops and starts — refuse or dump operations, for example — you might want to at least try an air-ride suspension,” says Link’s Molitor. “That’s because an air-ride suspension is non-torque reactive. That means it works better to protect the drivetrain from shocks and vibrations generated by bumpy roads and simply starting and stopping the truck all day long. So you’ll get much better life out of your drivetrain components.”
On the other hand, he says, “mechanical suspensions — particularly leaf springs — offer you the lowest maintenance costs. As long as the metal in them is properly designed and not overstressed, they’ll last forever. That’s one advantage they have over rubber suspensions, which can deteriorate over time. Rubber does have an approximately seven-year life span before it starts to break down. So that’s something to consider.”
Mack’s Wrinkle stresses that the suspension spec is dependent on the application.
“Air suspensions do well in vocational applications,” he says. “But they don’t offer as much articulation and stability, and require additional training. If you want the smoothest ride, an air suspension is your best option. If you want maximum articulation and traction on jobsites, then a spring leaf over rubber block suspension, like the Mack mRide, is a better choice.”
Another point to consider is that vocational air-ride suspensions do have a few different maintenance procedures than leaf spring suspensions, adds SAF-Holland’s Gage.
“Both air-ride and leaf spring suspension styles, especially in vocational applications, are the muscle under the backbone of the truck,” he says. “They carry heavy loads and traverse rough terrain found at worksites. Thus it is critical, whether a spring or an air-ride suspension, to establish routine maintenance practices. Air rides do have more wear parts like bushings, air springs, shock absorbers and air controls that require servicing.”
Adhering to a routine preventative maintenance inspection for air-ride suspensions can go a long way towards extending the life of the suspension,” Gage adds.
Elliott cautions that there is not a “one-size-fits-all” solution for fleets when it comes to suspensions.
Fleet’s still have to zero in on the correct type and setup to get the performance they need.
This article first appeared in the October 2022 issue of Heavy Duty Trucking.