“Air suspension isn't rugged enough for public works vehicles.”
“They're good for on-highway vehicles, but not for off-road use.”
“Going to work sites in severe service, they're too delicate and subject to damage.”
These were some of the comments I heard at a recent trade show – all of which may have been true a decade ago. But air suspensions for vocational trucks have come a long way, shedding pounds while improving ruggedness and durability.
Today every manufacturer offers air suspensions specifically engineered for rugged conditions. Even “walking beam” suspensions, designed for maximum articulation over rocks, are available with air bag springs instead of the more common elastomer (rubber block) or multiple steel leaf springs common to severe-duty trucks.
Air suspensions need shock absorbers to control bumps and road forces, whereas multileaf springs use interleaf friction. But increased complexity is offset by numerous benefits including reduced tire wear and fuel consumption. They also eliminate up to 60% of vibration, which makes drivers more comfortable and protects emissions control devices, sensitive electronic equipment, and other components including hydraulic, electrical, and mechanical devices.
Like all component systems, the new air suspensions should be inspected regularly. Maintenance involves three areas: shock absorbers, air springs and related components, and fasteners and bushings. For a detailed plan, see Recommended Practice RP 643, Air-Ride Suspension Maintenance Guidelines, issued by the American Trucking Associations' Technology and Maintenance Council (TMC).
Shock absorbers – or more accurately, motion dampers – operate by controlling the flow of fluid through valves within storage tubes. Pistons react to the suspension's motion, forcing an oil-based fluid through control valves. The fluid travels through the valves, converting kinetic energy to heat and removing energy from the system to damp the oscillations.
Change shocks before they wear out, which is usually at or before 100,000 miles. Inspect them every 10,000 miles (A-interval) or the closest regularly scheduled maintenance. Replace a shock if the end mounts or dust tube are cracked or the body is dented. Check bushings for wear and replace if necessary. If a worn bushing isn't replaceable, replace the shock. Check for leaks (light misting on the body is acceptable).
Every 20,000 to 30,000 miles (B-interval), check heat levels with an infrared remote thermometer using a chassis rail's temperature as the baseline. Remember, the shock works by converting motion into heat. If one is 20% cooler than the others, remove and thoroughly inspect the shock. Replace it if necessary.
Other signs of worn shocks are out-of-balance tires, loose suspension components, ride deterioration, alignment problems, and excessive vibration.
The air system starts at the air compressor, which can develop internal leaks that allow oil into the compressed air. Oil will shorten air dryer life and allow moisture to corrode or freeze valves. Make sure operators or technicians drain air tanks daily and check for oil mist. Federal Motor Carrier Safety Administration regulations require that air system hoses and fittings be checked daily.
During each B-interval inspection, check all air bags for physical damage and replace a bag if you find cuts, gouges, deformed upper plates, or other obvious defects.
The RP 643 guidelines include photographs and probable causes of typical damage as well as other inspection procedures, including tips on checking the entire system.
Fasteners and bushings
Suspension hardware should also be checked regularly.
Do a visual check as part of your A-interval maintenance and check fastener torque at each B-interval. Bushings allow temporary misalignment of suspension components and return them to proper alignment when the stress or load is removed. They shouldn't be stressed when air-system adjustments, especially in ride height, are made. RP 643 describes proper bushing inspection.
Fasteners maintain proper component alignment, but are subject to the most direct road shock loads.
Re-torque all fasteners after 1,000 to 2,000 miles and during regular maintenance thereafter. The same goes for U-bolts; they have the critical job of joining the axles to the suspension.
An increasing number of suspensions of all types are built with huck bolts. These look like threaded fasteners, but the nuts are swaged on, compressed around ribs – not threads – on the bolts. They don't lose torque, but you'll need special tools to apply and remove them.
— Paul Abelson (firstname.lastname@example.org) is a former director of the Technology and Maintenance Council (TMC) of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.