Air Suspensions
As Weight Comes Down, Popularity Goes Up
Tom Berg
Senior Equipment Editor
It's certainly not news that the majority of over-the-road tractors have been riding on air for many years. In fact, nine out of 10 are now built with air-ride tandems.
Long-haul truckload carriers discovered back in the 1980s that they had to adopt air suspension if they wanted to hire drivers, who insisted on decent-riding vehicles.
What may be news to some is that air-ride has been spreading in popularity to other types of trucks, including those in construction and trash hauling. Why?
Gravel and piles of garbage don't need a cushy ride, but drivers do, and so do the trucks themselves.
To be sure, there will probably always be a market for steel-spring and other types of mechanical suspensions. Cost is the primary reason. More than a third of dry-van trailers, for instance, still are sold with steel suspensions because their buyers are satisfied with their performance and watch life-cycle costs closely.
Steel suspensions are priced less and contribute to lower life-cycle costs, especially for vehicles that are kept a long time and don't run a lot of miles. The price premium for a tandem air-ride can be several thousand dollars. That can be recouped if a trailer is sold in a few years, but not if it's run until it's worn out. There are highly capable mechanical suspensions, especially for vocational trucks, that deliver good ride and performance, and composite leaf springs offer light weight.
But the more a vehicle costs and the greater its annual mileage, the more air-ride pays off, suppliers insist. A typical dry van trailer might run 20,000 to 30,000 miles a year, while a refrigerated trailer covers 100,000 miles. The dry van is rather simple, but the reefer has the expensive refrigeration/heating unit, fuel system and insulated walls that deteriorate with vibration and road shock. This is why close to 80 percent of reefer trailers get air-ride.
This is true of other trailer types, too. More than 70 percent of tank trailers are on air-ride, and suppliers say some buyers get free extended warranties because the air-ride protects thin aluminum walls from shock and subsequent expensive welding repairs. Likewise for aluminum or steel dumps; almost two-thirds of dump trailers are air-ride-equipped.
As with tractors, drivers like trailer air suspensions for their smoother ride. Compared to a semitrailer with a steel-spring suspension, an air-ride-equipped trailer sends less shock through the fifth wheel into the cab. Drivers can feel the difference. Smooth ride, of course, cushions the cargo. As a result, some shippers demand air-ride for their products, and truckload carriers that have converted their van fleets to air-bag suspensions figure they've got a competitive edge.
Air-ride has become more popular among straight-truck users. Recent converts are fire departments, whose commanders have found that air-ride protects the complex chassis and bodies and adds stability, according to suspension suppliers. Special versions of on-road air suspensions include hangers that keep air bags in place while a fire truck goes up on outriggers during fire-fighting operations.
In less spectacular applications, dump and trash-packer bodies see less damage to hydraulic systems and other parts when riding on air, and heavy haulers like the reliability and stability.
Air-ride lift axles are increasingly common on heavy straight trucks whose owners must cope with stringent weight laws in various states. Steerable versions are becoming more popular because truckers realize that it's better for the rest of the chassis to keep the auxiliary axles down and shouldering their share of weight during turns. Air pressure on the bags is automatically released when drivers shift into reverse, but reverse-steering lift axles change geometry to allow reverse caster-steering, too. These have gained acceptance among milk-truck operators who often maneuver at farms with soft driveways and yards.
Good load equalization among all wheels is also a virtue of air suspension. This is why most flatbed and reefer trailers with spread tandems have it. Equalized weight on all wheels helps air-ride keep the wheels firmly planted, which aids traction. That helps with braking, and in getting power to the ground for trucks and tractors. Some air-ride products claim high stability and are used on trucks and trailers with high centers of gravity, such as with fire trucks, concrete pumper chassis and bulk tankers.
Air-ride also can save weight and reduce maintenance. Compared to a walking-beam tandem suspension on a vocational truck, an air suspension weighs 300 to 350 pounds less, about the same as a modern rubber-type mechanical suspension. Front air-ride on a highway tractor weighs about 100 pounds less than a leaf suspension.
In trailer suspensions, suppliers have wrung out weight to where some air-ride products weigh about the same as four-spring steel suspensions.
Vocational versions of steer-axle air-ride are coming out in a few years, say the makers of the two on-road products now available.
Maintenance-wise, some air suspensions need little attention, and feature easy adjustments for axle alignment. Air bags themselves are a replacement item, but last for hundreds of thousands of miles. Shocks, bushings, valves and other items also wear out, but are fairly easy to replace, enabling owners to do most work themselves, suppliers say. By contrast, some mechanical suspensions require special tools and skills, which is why specialty shops exist. Users who have converted from steel-spring to air-ride do need education and help, and suppliers pride themselves on publishing manuals and audio-visual aids, and employing field reps to provide on-site assistance.
Most truck builders offer proprietary air-ride products, and these are especially popular in road tractors and certain other trucks. Vendor suppliers say they are used to co-existing with the truck makers, usually as complements in the data book to builders' own suspensions and sometimes as standard. Vendors say they make parts for proprietary suspensions and also supply complete units carrying a truck builder's name.
As with any other truck and trailer component, aftermarket replacement parts are available, sometimes for less money than original-equipment items and sometimes claiming greater durability. Low-priced shock absorbers and long-life urethane bushings are examples, but the OE makers note that an air suspension is a system with all parts designed to work together. Engineers spend many hours working toward that goal, but users can upset it by installing pieces not tuned to perform as original parts.
Suppliers usually use rubber bushings because they absorb vibration and flex at certain rates. Urethane bushings are harder, so pass along those vibrations and keep parts from moving as they should. While urethane bushings may indeed last longer, the result is poorer performance from the suspension, the OE makers claim. Shocks, too, need to be tuned to work with the suspension. Installing non-OE shocks can result in too much or too little resistance, or in improper stroke length. So, as with anything, you get what you choose and pay for.
HDT thanks these people for their help in preparing this article:
• John Burch, on-highway segment manager, Hendrickson International
• Jim Crowcroft, director of marketing, Hendrickson trailer suspensions
• Roger Elkins, product manager, Holland Neway
• Ken Griswold, trailer product manager, Holland Neway
• Gerry Remus, vocational market segment manager, Hendrickson International
