Suspensions have a job that is both difficult and crucial. Think of them like the spine of the vehicle, supporting both the handling and the ride quality, keeping cargo in good shape while making sure the driver is shielded from the harshest bumps on the road. The importance of this component cannot be overlooked.
There are two major types of heavy-duty suspensions: air ride and leaf spring. Mechanically, leaf springs are used in steel suspensions and have a simpler design; air ride suspensions, on the other hand, are a bit more complex, being made up of a system of airlines, air-spring bags and valves. From an application standpoint, there are several other important differences to note that will impact your choice.
When asked how he would describe the differences between the two, Bill LaGue, director of trailer OE for Reyco Granning, responded with a one-word exclamation: “Extreme!”
LaGue went on to specify several of those extreme differences. “For instance,” he said, “air ride suspensions have a less abrasive ride when the trailer is not fully loaded or empty, helping to reduce maintenance to trailers and improving driver comfort. The most current air ride suspensions weigh less than conventional spring ride and also have better load handling capabilities—uneven distribution of loaded freight—and will compensate in road curves. In addition, they maintain ride height and dock height from empty to fully loaded.”
Roger Jansen, product manager of axle/suspension systems for the Americas with SAF-Holland, raised LaGue’s one-word exclamation by singling out two phrases that sum up the main differences between the two types of suspensions.
The first: simplicity, as the leaf spring suspensions do not require any air controls, shock absorbers or air springs. The second: ride quality. “The air ride will have a better overall ride quality when the trailer is empty or partially laden,” Jansen explained. “Half to fully laden ride quality between the two designs is much closer, however, and would likely not be discernible to the driver.”
“Vocational fleets with trucks equipped with expensive components, or special body types directly mounted to the chassis, look to air ride suspensions to protect and enhance the life of the equipment. Air ride also protects the life of tires and the infrastructure,” said Jason Heath, Neway product manager with SAF-Holland.
One disadvantage of air ride suspensions is their higher upfront cost compared to their leaf spring counterparts. Yet Ed Brown, director of engineering for trailer products with Meritor, notes that the reduced maintenance leads to a better total cost of ownership over time for the air ride suspensions, and traditionally, higher resale value for the trailer. Air ride suspensions also are known for providing superior cargo protection.
SAF-Holland’s Heath also pointed out that there is a third type of suspension, called rubber rides or walking beam suspensions, which are used in severe-duty, off-road applications. These make up a much smaller share of the market but have their place among fleets who need something that will hold up in those severe applications.
The driver factor
The ongoing driver shortage touches every aspect of the industry, and suspensions are no exception. The primary benefit of the smooth ride of air ride suspensions may be that it keeps cargo and equipment in good shape, but many fleets are seeing an additional benefit: that the smoother ride appeals to drivers. And as fleets are looking for every advantage they can find to attract and retain drivers, this is leading to an improved market for air ride suspensions.
“We are seeing a trend of fleets moving back to air ride suspensions to appease drivers,” Reyco Granning’s LaGue said. “We believe this trend will continue due to driver shortages and the change in types of drivers the fleets are seeking. Keeping the drivers happy is the key.”
“Air suspensions for the rear axle and steer axle tend to provide a superior ride for both driver and cargo,” said Stephen Hampson, director of marketing for Hendrickson. “In addition, air suspensions provide a constant ride height, which provides a consistent driveline angle.”
“In regard to drivers, air ride suspensions are preferred as they provide softer, more comfortable rides than leaf spring suspensions,” SAF-Holland’s Heath said. “Travel range is an important variable to consider. Longer routes can cause driver fatigue; the softer ride of an air ride suspension helps drivers to overcome fatigue issues. Jobsites are tough on your drivers, but they don’t have to be.”
Prioritizing your potential suspension selection pros and cons based on your fleet’s application is the best way of choosing between the two.
“Since there are many suspension designs to meet the varying needs of each fleet, it is important for the fleets to educate themselves on the many attributes and considerations that go into spec’ing the right suspension for their specific application,” Reyco Granning’s LaGue said. “The suspension manufacturers can help educate the fleet on the best suspension type for their application.”
Considerations include upfront and long-term maintenance costs, type of freight, expected tractor or trailer life, environmental conditions and, as mentioned in the previous section, driver acceptance.
“Air suspensions are used widely today for on-highway applications, with more than 80% of fleets specifying them for driver comfort and load protection,” Hendrickson’s Hampson noted.
Hampson went on to say that air suspensions provide an additional benefit for tractors with a fifth wheel, as the height can be adjusted for easier coupling and uncoupling of trailers.
Meritor’s Brown pointed out that air ride suspensions are preferred in vocational applications, especially those that travel in multiple regions with different regulations, as this will demand lift capability, axle load equalization, self-steer capability, etc.
“On the vocational side, we have many different trailer applications,” Brown said. “Some of these applications may need a mix of suspensions. For example, a Lowboy Double Drop trailer may need yoke mount, low mount, self-steer suspensions, and lift capability—all in one trailer. With air ride suspensions, load balancing on multiple axle trailers becomes very easy.”
Considering the important role that suspensions play for both trucks and trailers, it is equally important to keep them in good shape with good maintenance practices.
“No suspension design is maintenance-free, and the vehicle OEM or suspension suppliers service recommendations should be followed,” began Hendrickson’s Hampson. “Air suspensions require inspection of air springs and supporting pivot arm. In addition, air suspension leveling valves need to be inspected for correct ride height settings.”
As with all equipment, a good preventative maintenance schedule can extend the life of the parts and make sure you don’t run into any unwanted surprises on the road. SAF-Holland’s Heath recommended starting by looking for signs of loose connections, rust and cracks. Maintaining the proper torque on all component connections such as the pivot, axle, and torque rod connections is critical for extracting the maximum life of rubber bushings, he continues. Use a flashlight to closely inspect bushings and rubber parts looking for chafing and wear; if chafing and wear is visible, it is time to replace the part.
When dealing with air ride suspensions, the biggest difference as opposed to their spring counterparts is the added complexity.
“While air control components have evolved to become highly reliable, the fact of the matter is that such extra hardware will eventually require service over the life of the trailer,” SAF-Holland’s Jansen said.
“On the air ride,” Meritor’s Brown said, “you will need to inspect shocks, bushings, height control valves, check for any leaks, torque values, ride height, etc. We strongly advise that fleets get themselves educated and talk to service managers to make sure they are doing the right level of maintenance.”
The benefits of leaf spring suspensions
This story may focus on air ride suspensions, but we would be remiss if we did not talk about the other major type of truck and trailer suspension: leaf spring. As mentioned in the main article, leaf springs are used in steel suspensions that have a less complex design than their air ride counterparts. Both air ride and leaf spring suspensions have their applications that they are best suited to.
“Leaf spring suspensions are proven in many vocational applications,” said SAF-Holland’s Heath. “They are durable, affordable and require less maintenance than air suspensions, but they do not provide the same level of ride quality.”
Certain applications benefit more from leaf spring suspension than they do from their air ride counterparts.
“There is an industry-wide move to a broader use of steer axle monoleaf leaf spring suspensions for lighter weight designs with good ride performance,” Hendrickson’s Hampson pointed out.
“While air rides are the prevalent choice for many specialty trailers such as tanks, bulk, and platform, the mechanical suspension maintains a significant market share on the dry freight, on-highway segment,” SAF-Holland’s Jansen said.
From a maintenance perspective, Reyco Granning’s LaGue noted that there is plenty to keep an eye on with spring suspensions as well. As he pointed out, springs break, torque arms have bushings that require replacement, wear pads wear out and retainer bolts need replacing, to name a few.
LaGue advised fleets and technicians to:
- Check the alignment at each scheduled PM service;
- Check the torque of all the attaching hardware at scheduled service intervals especially the first service; and
- Perform a good visual inspection at each scheduled service.
In addition, Hendrickson’s Hampson noted that leaf springs require periodic inspection of axle to suspension connections and in some case greasing of the shackle pins.