Is there an established time or mileage for replacing air bags on air suspensions?” Kevin Tomlinson, maintenance manager at South Shore Transportation in Sandusky, Ohio, posed that question as a panelist during an industry meeting in September. The format of the session, which focused on decision-making, did not allow for an answer — but it’s a good question.

Because trucks, tractors and trailers operate in a wide range of load and operating conditions, there seems to be no predictable life for air springs, shock absorbers, height and pressure valves, bushings, fasteners and the many other parts of an air-ride suspension. Tomlinson, who’s also first vice chairman of the Technology & Maintenance Council of American Trucking Associations, understands that, and also that the session, during TMC’s Fall Meeting, was not designed to get him any answers. One day last month he reported that he had a dry-bulk tanker in the shop to change out its air bags, and discussed the subject.

“We’ve had vehicles that have run a million miles with the original bags, others that went 400,000,” he said. “You never know. This trailer hauls out of a limestone quarry just a few miles down the turnpike and back to a shingle plant right in town. A round trip is maybe 30 miles, and it’ll make five trips a day. It spends more time loading and unloading than running, so doesn’t get many miles. On something like that, miles don’t mean much.”

Exposure to ozone in the atmosphere and the sun’s ultraviolet rays can age air bags’ rubber, but that doesn’t show up as it does in old tires, Tomlinson said. Much more important are wear and tear from road conditions as the bags compress and stretch over high and low points in pavement, trail or uneven off-road terrain, and debris slung at the springs as they travel at high speeds.

Regular inspections combined with record-keeping will track wear and dictate when a bag, or a set of them, need to be replaced. Vehicles in the same kind of service in the same areas might establish a pattern that could suggest a change-out time for all of them, or might not. TMC’s Recommended Practice 643, Air-Ride Suspension Maintenance Guidelines, said air bags should be inspected every 50,000 miles, but that might be too long for the short-haul trailer that the South Shore crew dealt with here, even if it sometimes goes on longer runs.

This trailer is a 2000 Heil pneumatic aluminum tanker that’s been running “forever,” he said. “The bags are probably the originals,” and eventually wear showed up that brought the trailer into the shop. Mechanics changed out all four bags because there was enough time. Sometimes a trailer must get back into service ASAP, so only one damaged or worn-out bag will be changed.

There are many ways that rubber air bags can be damaged, and RP 643 lists five:

• Misalignment, where the upper and lower ends of the bag are out of line vertically.

• A loose girdle loop.

• Bottom-out abrasion, from the bag compressing too much.

• Over extension, from wheels

• dropping too low too many times.

• Circumferential cuts, from improper pressure or height adjustment.

Cracks, holes, tears and chafing in the bags or at the top or bottom are among the types of damage that can occur. Cleaning the bags with a proper solution will give the mechanic a clear view of the rubber and other parts, the RP said.

Other components, including the height adjustment and pressure limiting valves, air filter, and bushings and fasteners, should also be regularly inspected during preventive maintenance or annual inspections.

There is virtually no standardization in air-spring mountings, even though only two manufacturers, Firestone and Goodyear, make most of them — a fact of life that irks Tomlinson. “It seems like every set is different,” he said. “The bolt patterns change with the vehicle manufacturers, and are just enough different that you can’t swap parts from one to another.”

He gets many of his parts from the TruckPro store in Greenwich, Ohio, where Jeremy Carruthers is the manager. “No, God no,” he said when asked if there is any standardization. “You can have two trailers built side by side and they can have two kinds of air bags. There are 80 different part numbers carried just in this store. In front of me is a Firestone book that’s got 150 pages of just air bags, with pictures, and there are four or five part numbers on each page. There are so many applications — drive axles on trucks, trailer axles, lift axles.”

Air springs cost from about $100 each for high-volume parts to $220 for more unusual types, he said. Of a predictable life cycle for them, Carruthers comments, “I wish there were an actual time line the guys could use before the bags blow up in their faces. But there are too many different uses. Normally, they replace ’em when they fail.”