Air suspensions provide a better ride, protect cargo
Air suspensions systems originally penetrated the marketover fifty years ago on cargo-sensitive applications like electronics andfurniture vans as well as auxiliary lift axles for vocational trucks. By oneestimate, air suspensions are applied to nearly two-thirds of all trailersbuilt today, and for some applications or situations, they are the onlysuspension offered by the OEM. Air springs have found their way into drive,steer, primary trailer and lift axle suspension components, as well as intoless obvious applications such as cab mounts and driver seats.
When trucks ride better cargo is protected, suffering lessdamage and fewer damage claims. Drivers also benefit from air springs, becausethey are more comfortable, more productive and less subject to accidents causedby fatigue, according to one manufacturer. In addition, there is less wear andtear on equipment, from reduced roadshock on mechanical and electrical components to decreased “trailer hop” whenempty. When used with leveling valves, air springs maintain consistent trailerheight regardless of the load, keeping trailer floors flush with loading dockfloors. Also, air springs are recommended for roll control and stabilitysystems because it is so convenient to measure the loading directly from thepressure in the air springs.
When to spec’?
Mike Lynch, product manager, North American Trailer Systems,ArvinMeritor, says air spring trailer suspensions should be spec’d forcustomers who require increased cargo and equipment protection. “A properlyspec’d air spring system will enhance trailer control and will equalize axleload distribution on tandem suspensions,” he notes.
Chris Helmer, manager, applications engineering, TrailerSuspension Systems Business Unit, SAF-Holland Inc. adds that air springsuspensions also offer higher resale value for their upfront price.
Rence Oliphant, senior director – North American Sales,Hendrickson, adds, “Whether you are hauling steel or fragile computers, an airsuspension gives you versatility.” For dry freight and refrigerated vans, heexplains that a sliding air spring tandem that is rated at least 34,000 lbs. isthe leading choice. But air suspensions aren’t just for dry vans. Integratedextreme-duty air suspension systems with each axle and suspension rated at30,000 lbs. are available to meet the demands of logging, forestry, mining,construction and other off-road and heavy hauling applications. There are otherair suspensions for fleets pulling platforms, tankers, grain or livestocktrailers that still want to maintain the ride, performance and durability theyhave come to expect with air suspensions and still save weight.
Ken Wall, senior design engineer for Reyco Granningsuspensions points out that other applications for air springs include vehiclessubject to long-term fatigue (aluminum trailers, for example), trailers thatrun often empty (tank trailers that return to the terminal empty), vehicleswith high center of gravity or vehicles where some axles need to be liftable.While fleets have lots of say in spec’ing a van slider suspension because it’seasy for the trailer manufacturer to substitute one brand for another, Wallsays, manufacturers of flat beds and dump trailers tend to engineer in aparticular brand and don’t give the fleets as much choice.
While air springs have many proponents, they may not benecessary in every case. A heavy-duty leaf spring trailer suspension is stillpopular for dump trailers and refuse trailers where rugged, reliable performanceis more important than ride quality. “Trailer suspension choice is more of an‘economic’ decision,” according to Mike White, Hutchens Industries Inc.vice-president of sales. Although the company manufactures sub-frames forvirtually all of the major air suspension manufacturers’ products, it doesn’tsell an air suspension itself. White summarizes, “If your trailer will not seesignificant empty or partially loaded miles, does not need to maintain aconstant floor height (or ground clearance) in all load conditions and yourshipper doesn’t demand an air suspension, then a leaf spring suspension makessense for your trailer.”
Additional benefits of air
SAF-Holland’s Helmer notes that air suspensions offerconsistent self-leveling trailer height that improves vehicle control. Airsuspensions also generally have higher roll stiffness, that is, resistance tocornering forces, than leaf spring suspensions, says Reyco Granning’s Wall, sothey can contribute to safety. They reduce road vibration inputs into thetrailer frame during unladen or half-laden conditions. And they offer theability to lift certain axles when running unladen or with less than a fullload.
There is little weight penalty nowadays, Wall notes. “Earlyair suspensions were usually heavier than similar capacity spring suspensions,but that gap has narrowed and has even disappeared in some air suspensionmodels.”
Spec’ or retrofit?
There is mixed opinion on whether it is necessary to spec’an integrated air suspension on new equipment to gain the benefits or if it ispossible to add air spring components as a retrofit. ArvinMeritor’s Lynch saysintegrated systems have advantages–– generally lower overall system weight,better durability, and sometimes fewer components. “Adding air spring componentsto a mechanical suspension really do not provide the benefits of a properlyspec’d air spring system, and do not help to reduce maintenance because most ofthe spring suspension components remain.”
Specifying air suspension on the trailer when it is built ismuch preferred to installing an aftermarket “add-on” solution, saysHendrickson’s Oliphant. In the past, there were separate vendors for eachcomponent and the trailer OEM assembled all the pieces. “By offering anintegrated system, we were able to maximize the weight savings, make theproduct more durable than separate components and take responsibility for theentire module,” he says. Fleets recognized these benefits and now specify thesystem approach, he notes.
Air suspension components can be added to an existing truckor trailer, but it is generally not cost effective to do so, says StefanSiarkiewicz, global brand product manager, Firestone Industrial Products,makers of Airide brand air springs. Adding air suspension components after thefact can cost up to four to five times more when compared to specifying anintegrated air suspension system.
While simply adding air springs to an existing suspensionisn’t practical because most suspension manufacturers have separate designs forair and mechanical spring suspensions, Reyco Granning’s Wall explains that vantrailers with slider suspensions can swap out a complete steel spring sliderfor an air slider.
His company has an air slider conversion kit that allows theaxles and components such as the air tank system to be removed from the steelspring slider and installed on an air slider sub-frame. He notes that somerepair shops can upgrade a truck or tractor equipped with a steel spring drivesuspension by completely replacing it with an air suspension, but this usuallyrequires specialized knowledge and specialized equipment such as frame drills.Auxiliary lift-axle air suspensions can be added to many trucks and trailers.These are complete axle/suspension units designed for the task, not simplyadd-ons.
Helmer at SAF-Holland agrees that the industry is moving tohighly integrated suspension systems that incorporate components engineered tooptimize performance, cost and assembly. “Axles, braking systems, suspensionsand key structural components, for example, are designed as a system,” heobserves. “Systems are typically designed to meet specific applications andtrailer geometry that require appropriate frame brackets, equalizing beams withbushings, air springs, and shock absorbers that must be matched correctly toensure proper axle travel and performance.” Another trend he sees, “In thefuture, new technologies such as advanced braking systems with roll controlthat are starting to capture market share will recommend an air suspension onthe trailer for optimum performance.”
Mix and match?
If trailers are spec’d with air suspensions, is it necessary(or recommended) to match them with air-suspension-equipped power units? Ingeneral, the answer is no. Because of the distance between the tractor driveaxles and the trailer axles, they operate just fine independently of eachother, says Wall of Reyco Granning. It should be noted that tractor and trailerair suspensions generally operate on separate air controls, so they do notautomatically load-equalize. Distribution across tractor and trailer is managedthrough load placement and the position of the fifth wheel and trailer bogie,says Lynch of ArvinMeritor.
Hendrickson’s Oliphant says there are some benefits tohaving a “matched” air suspension for the entire vehicle. His company offersair suspensions for trucks and tractors for both the front steer axle/
suspension and the truck drive axle/suspension as well asair spring systems for trailers. “Today, every major truck OEM offers an airspring steer axle suspension as an option and the drive axle suspension is airspring for the vast majority of on-highway tractors,” he notes.
Maintenance issues
Air spring maintenance is covered in detail in TMCRecommended Practices (RP) 634 and 643. You will note that many of theinspections are the same between a mechanical (leaf-spring) suspension and anair spring suspension. “A mechanical suspension will need to be inspected foralignment, bolt torques and broken or cracked springs. In comparison, an air springsuspension requires inspection of alignment, bolt torques, shocks, heightcontrol valves, air bags, and suspension bushings,” notes ArvinMeritor’s Lynch.
“The maintenance effort on steel leaf spring suspensionswill focus on the integrity of the spring, the tightness of the threadedfasteners and the alignment of the axles. Air suspensions add concerns forleveling valve and shock absorber performance,” says Hutchens’ White.
The biggest difference is that the air suspension has apneumatic control valve system that must be operational for effectiveperformance, says SAF’s Helmer. “The system is the brain of the suspension andmaintains the suspension’s ride height, which is critical, whereas a mechanicalsystem’s performance generally is determined by the spring characteristic.”
Major items to periodically check on a trailer airsuspension would be the shocks and the height control valve, and depending onhow the trailer is set up, there could be manual and/or automatic dump valves,pneumatic slider pin activation devices and other options, according toHendrickson’s Oliphant.
Air suspension systems require a minimal amount ofmaintenance that can be performed during other routine vehicle maintenanceservice. But, if an air spring fails, a trailer can still be pulled at slowspeeds, whereas a trailer with leaf springs becomes immediately inoperable,notes Firestone Industrial Products’ Siarkiewicz.
Air supply systems do require some maintenance, though.Problems often stem from moisture in the air system, so dryer systems andmoisture drain systems need regular attention, says Reyco Granning’s Wall. Thismaintenance is required anyway because these also supply air for brake systems.He notes air suspension items that may eventually need replacement include:bushings, leveling or height control valves, shock absorbers (not usually foundon steel spring suspensions) and the air springs themselves (usually from roadhazards more than wear).
Basic air suspension maintenance varies by suspensionmanufacturer and design but may include: pivot connection torque, U-bolttorque, alignment (visually check for unusual tire wear as part of regularpreventive maintenance), and a check for air system leaks, Wall advises.
Basic maintenance
Preventative maintenance on air suspensions can easily bedone when other routine maintenance, such as repairing brakes or installing newtires, is undertaken. By following these steps, the combined wisdom of severalair suspension manufacturers, you will achieve the best results for your airsuspension system.
Remember that a properly aligned trailer suspension willprovide increased tire life, increased suspension bushing life, andsignificantly reduced wear between components. An out-of-alignment suspensionwill decrease overall vehicle fuel economy. Suspensions of all types requireperiodic visual inspections for failed components and adjustments, but thesemaintenance items are unique to air suspensions:
Air springs – Inspection of the air suspension should beperformed with the air springs inflated. Make sure there is enough clearancearound all the air springs while at their maximum diameter. Check the airsprings for irregular wear or heat cracking. Look for signs of air leaks,abrasions to the flex member, or tears in the flex member. Check the pistonsfor any build-up of foreign materials. Is the air spring piston in one piecewith no chipped or missing edges? Clean and replace parts as necessary.Approved cleaning products are soap and water, methyl alcohol, ethyl alcohol andisopropyl alcohol.
Shock absorbers – Confirm that you have the proper shockabsorbers. Inspect to make sure they are in working condition with no oil leaksand the eyelet bushings and bolts are in good condition. Improperly functioningshocks will decrease tire life and may increase overall brake stoppingdistances. Check for loose, worn or broken components.
Bolt torques – Inspect suspension system for loose bolts orfasteners. Make sure all bolts and fasteners are torqued to the manufacturer’sspecifications.
Height Control Valve (HCV) and linkage – Inspect and measure the suspension ride height.Make sure the valve is set to the correct suspension height recommended by theair suspension manufacturer (see the serial tag). This is very important for performanceand long-term durability. Make sure the leveling valve or HCV functionsproperly and is not leaking. Refer to the manufacturer’s instructions on how toinspect and test HCVs.
Suspension bushing – Inspect suspension bushings forlooseness and wear. A worn out bushing may lead to improper alignment andincreased wear and subsequent damage to mating components. Refer to themanufacturer’s procedures for bushing inspection and replacement guidelines.
Given the broad range of benefits they provide, airsuspensions can add value to all types of truck and trailer applications.Additionally, advantages translate intoless maintenance, longer tire wear, protected cargo and higher trade-in value.
Air springs at the heart of the system
While the air suspension manufacturers make the metalcomponents and the rubber bushings, says Firestone Industrial Products’ StefanSiarkiewicz. “Without the air springs, you wouldn’t have an air suspension,period.” As a provider of air springs, his company works with air suspensionmanufacturers to custom design air springs for their individual products.
The number of different air spring products is staggering.“We have over 7,000 different air springs available,” Siarkiewicz says. Indesigning a product for a specific application, he explains that they try tomix and match existing components, such as the rubber bellows, the top plateand the piston, to satisfy OE customer needs. Where they cannot, they design acompletely new air spring.
What determines the shape of air spring used? The sleevetype is used for higher-pressure, mainly automobile applications, Siarkiewiczexplains. The rolling lobe type is used mainly for cargo protection and drivercomfort, while the bellows “convoluted” design (the one that looks like a shortstack of donuts) is used more for lift applications, not ride. In moston-highway applications, the air suspension protects cargo and offers drivercomfort. In off-highway applications, it also protects the chassis and all ofthe equipment carried onboard from vibration.
The air spring flexible members––the air bags––are made ofmulti-layer, fabric-reinforced natural rubber (or neoprene, for use at highertemperatures and pressures) about 1/4-in. thick. These air-filled bladders arecombined with a bead plate, piston, fasteners, an air fitting and sometimes aninternal bumper to make the finished air spring.
The life of an air spring can be anywhere from three to fiveyears, depending on the application, says Siarkiewicz. He reports that some airsprings have exceeded one million miles in on-highway applications.
Air springs work together with shock absorbers to giveoptimum ride, Siarkiewicz continues. Pressure is set through the height controlvalve (HCV) to maintain ride height, which is the distance from the center ofthe axle to the bottom of the chassis. As more load is put on, the HCV willadjust to allow in more air to maintain ride height. Air springs provide thesame amount of cushion as load decreases, because the HCV decreases the air pressurein the air springs (they usually operate at 80-85 PSIG loaded). “You still havethe same spring rate, empty to loaded,” he points out. “Compare that to amechanical spring suspension that is designed to perform best at maximum load,for example, 34,000 lbs. As you decrease the load on steel springs, the ridebecomes stiffer.”
Siarkiewicz notes that air springs are designed andoptimized to the specs’ of the truck/bus/trailer suspension manufacturer. “Theend user can change to stiffer or softer air springs, but that doesn’t changethe suspension’s load capacity.
“Air spring suspensions make for a more versatile vehiclethat can carry solid or fragile goods such as electronics, fruits andvegetables, even potato chips,” he concludes. He believes the versatility of anair suspension outweighs the initial cost difference versus buying a mechanicalsuspension.
