Watching Your Back

When it comes to suspension maintenance, "Tractors tend to get a lot more attention than trailers do," says Tim Fulkerson, technical service specialist, Dana Corporation. As a guy who spends a lot of his time poking around under trailers, Fulkerson sees the results of negligent trailer suspension maintenance all the time. "They just don't get the type of PM and inspection that the tractors do."

According to Fulkerson, there are two main reasons for this. First, fleets tend to bring their trailers in for PM inspections far less often than they do their tractors. Second, many fleet maintenance shops that have traditionally serviced older trailers with spring suspensions tend to consider trailer suspensions low-maintenance items, and that attitude carries over when the fleet switches over to air ride trailer suspensions.

"Back when you had the old spring ride trailers, they didn't get a whole lot of attention," he explains. "If you've got fleets or shops that are dealing with the new ones the same way, they're missing a lot of opportunities, because routine maintenance and inspection are really critical, because air suspensions are more complicated than the old springs."


Bruce Barton, director of engineering for Ridewell Corporation, agrees that trailer air suspensions are too often the victims of neglect. "It is very obvious to us as a manufacturer that, irregardless of the information that we put out—our service manuals, our drawings, any maintenance requirements, like periodic re-torqueing of the fasteners—nobody follows it," he says.

"If there's one thing you could get across, it's: ‘Read the manufacturer's service literature, and follow it,'" he adds. "I can't tell you how many times pheyeople say, ‘I didn't know I need to re-torque!' Well, it's right there in the service literature! I don't know why it is, we just continually get calls here, ‘Hey something's loosened up here.' Well, gee whiz, did you re-torque at 6,000 miles, like we recommend? ‘Oh no, I didn't know to do that.'

"Number one headache is the pivot bolt fastener maintenance," Barton says, "That is the single most common lack of maintenance we see. All fasteners will lose some of their torque over time... and the first re-torque is the most critical re-torque of that fastener.

"We like technicians to use a torque wrench," he adds, "but most of them will just use an air impact wrench and just guess at the torque level, and let the chips fall where they may."


Looking at the typical trailing-arm air ride trailer suspension, Barton breaks maintenance inspections down by the three main "consumable" components that need your technicians' attention: the air bellows, the pivot bush and the shock absorbers. "From a mechanic's standpoint, they have just got to do a visual inspection of the air suspension," he maintains.

Barton says that the first thing for technicians to look for is air spring abrasions. Because it is a rubber element, the air spring is subject to fatigue cracks in the areas where the flex member rolls over its mating part. Additionally, road debris could be thrown up and abrade or puncture the rubber bellows. "These rubber air bellows, they do have a finite life," he explains. "A lot depends on how rough the service they're in, but the air bellows needs to be inspected periodically."

The next main element in the suspension is the pivot bush. "This is typically a very large rubber bush that is up at the leading edge of the trailing arm," Barton explains. "That is a rubber element, and it is considered a serviceable item. The bush is typically pushed into the beam portion of the suspension: it slides into the hanger, then there's the pivot bolt that actually attaches the bushing to the hanger." To Barton, this may be the most important part to inspect. He recommends that technicians look for excessive play at that bush, especially if the driver has complained of the axle wandering or walking; both are pretty sure indications that the pivot bush needs to be replaced.

"If that is neglected," Barton warns, "the hanger typically has to be replaced, along with the bushing, the bushing fasteners, and in some cases the beam may have to be replaced. It gets to be a very expensive ordeal, especially when the hanger is welded on; you have to torch it off, grind it, and weld on a new hanger.

"Typically, these pivot bushes are tucked inside the hanger," he continues. Because of this, Barton says, in cases of suspected wear, the technician may need to actually break the pivot bolt and drop the beam and bushing down to perform a proper inspection. "You've got to make a judgement, because it's so hidden in the hanger. It's difficult to see, and you have to look for excessive play."

Finally, Barton explains, technicians should be inspecting shock absorbers, as they provide crucial damping in an air suspension. "If there are complaints about vibration, the first thing to look for is whether the shock absorbers are worn out," he says.


Dana's Fulkerson advises technicians to look for anything that might be rubbing on the air springs. That includes any line or wiring that might be working loose and rubbing or scrubbing on components. He also advises that technicians look for cracked or loose components, loose fasteners, or cracks in any of the structural members.

Like Barton, Fulkerson urges technicians to pay close attention to the condition of the air bellows and the shock absorbers.

"Good air springs, just like on the tractor, I'd say they'll go for three, four, five years of good use," he explains. "It all depends on the applications, too. You'll get into places where the roads are considerably worse, and if you've not done good maintenance, and you've got worn shocks, then your air bag is going to take a much worse beating than if you had good shocks. If the shocks are actually leaking, or starting to wear out, the bags tend to pound down a lot more."

Of course, a visible leak or a wet spot on the shock indicates a problem, but what if there is no external sign of wear?

Fulkerson offers a simple test: when the truck comes in from the road, the driver should feel the shock's casing for warmth.

"What we recommend to drivers is, after you've been out on a long haul, reach in and grab onto the bottom of the body of the shock; if the thing is the least bit warm, you know that the valving is working," he explains. "If the shock is actually stone cold, or ambient temperature, more than likely the valving is worn out inside or it's had some sort of problem and lost fluid."


According to Fulkerson, the driver plays a crucial role in spotting trouble signs from the trailer's suspension. "A driver ought to be catching loose fasteners, anything that looks like it's rubbing or chafing," he says.

And, once out on the road, the driver also has first watch over the trailer's suspension. All he or she needs to do is keep looking in the truck's mirrors.

"Probably the easiest thing for a driver to see is whether a trailer is dog tracking or not," Fulkerson says. "If I look in one mirror and I can see this side of the trailer, but I'm looking in the other mirror and I can't see the back corner, something's wrong. That's probably one of the most visual things that a driver can pick up."

Simple enough. But Fulkerson cautions that this visual inspection may not be effective with some newer trailers:

"It's gotten a little harder because of the wide-based trailers that have come out," he says. "The old 96-inch trailers, when you were looking in the mirrors you were looking right down the sides of the trailer. A lot of the trucks now, unless they've got the mirrors really stuck out there, you won't look straight down the sides of the trailer.

"More than anything, it's issues with alignment or tire wear," he says. "Somebody calls in complaining about tire wear and you start asking them questions about the alignment—do they do routine alignment checks or anything—and more than likely that's the thing that's really neglected. They'll take the time to align a tractor when they see steer tire wear, but they don't think about it on a trailer, but it's just as critical. When it comes to fuel economy and tire wear, make sure the trailer is following where it's supposed to be."


"One of the biggest issues is the dock walk limiting devices," says Dan Carter, project engineer for Great Dane Trailers. "Because of the inherent geometry of a trailing arm suspension, when you raise or lower that suspension when the brakes are locked, the trailer's going to roll away from the loading dock, and it tears up the landing gear. Well, there are a lot of devices that eliminate that dock walk, but do people know how to operate them?"

Carter suggests that trailers with spring suspensions can fall victim to road debris and other assorted impacts that can throw the suspension out of alignment and cause wear. He suggests that maintenance technicians pay close attention to cracks in the springs, as well as visible wear in the springs, the rocker arms, or any other mechanical parts. The same goes for inspection of welds.

Like Barton, Carter believes there is not nearly enough fastener re-torquing going on in fleet maintenance shops. "We say to check it after 3,000 to 5,000 miles in the break-in period, and then the torque should be re-checked every 25,000 miles," he says.

"Those are the things that can really pay off, if they're part of your regular maintenance," he says.

Carter saves his final tip for the upper running gear rail on a sliding suspension, which is often set once and then forgotten. He recommends that fleets slide the suspensions on a regular basis.

"If they order a slide suspension and they don't change their bogey position much, they come to the end of four or five years and they want to sell it, and somebody tries to operate this slide and it's stuck. You can really get into some issues trying to get that broken loose and trying to get it to slide again. So, regular exercise of that suspension goes a long way."

Bruce Barton, director of engineering for Ridewell Corporation, offers this checklist for trailer suspension inspection and maintenance:

Trailing arm air suspension:

  • Understand the suspension—read the service and installation literature that is supplied by the manufacturer.
  • Inspect for obvious wear situations:
  • Air spring abrasions, fatigue cracks in the areas where the flexmember rolls over mating parts
  • Worn pivot bush
  • Excessive play, complaints of axle walk
  • Fatigue cracks in rubber
  • Suspensions often use "wear" or thrust washers adjacent to the pivot bush. Wear can be expected on this washer, but if it is missing or worn through, it should be considered for replacement.
  • Worn shock absorbers—because shocks supply most of the damping, vibration complaints often occur when the shocks are worn.
  • Cracks and corrosion damage to general suspension structure, beams hangers and axle.
  • Perform periodic fastener maintenance—this is the single most commonly seen lack of maintenance
  • Torque to manufacturer's recommendations
  • First re-torque most critical
  • Bushing fastener (pivot bolt/nut): "mission critical" joint
  • U-bolts if part of the suspension
  • Shock absorber
  • Hanger attachment to frame, if bolted
  • Air spring fasteners
  • Leveling valve hardware/linkage
  • Check ride height
  • Proper functioning of suspension requires correct ride height setting and functioning of the leveling valve and linkage.
  • Check air supply
  • Air suspensions rely on an air supply. Ensure that all pneumatic components and plumbing upstream of the air springs function properly.
  • If an air spring is used as an actuator (i.e., as part of a liftable suspension that uses an air spring for lift force), use a pressure regulator to limit pressure to 100 psi. Often, the air spring top and bottom mounting surfaces don't remain parallel when inflated, and under full system pressures premature failure can be expected.
  • Tools
  • Normal tools suffice for most suspensions.
  • Ridewell offers an easily-changed clamped-in pivot bush, while some suspension manufacturers offer special tools to replace the bushing. Service literature should always be consulted in regard to special tool requirements.
  • New technology
  • Weight savings drives material selection in the form of steel, aluminum and plastics. Non traditional suspension materials cost more than steel, so emphasis must be placed on the cost vs. benefit (value proposition).
  • Expect increased durability of the rubber materials, such as improved compounds that better resist aging (ozone). Ridewell will shortly introduce a new composite pivot bush that is lighter and more cost-effective than present designs.
  • Single tires and disc brake applications will drive suspension and slider design changes.