Heavy Duty: Smoothing Out the Rough Spots

In our last issue, we looked at the inspection & maintenance of air-ride suspensions, with an emphasis on bushings and fasteners. Turned out that fleet maintenance technicians aren’t always catching wear and damage the way they should. This time around, we look at shock absorbers, air springs and ride height adjusters to see if they’re getting enough attention from your techs.


The correct shock absorber provides the proper damping of suspension oscillation needed to bring the suspension back to its “neutral” position after encountering a road input, according to John Morgan, product manager, ArvinMeritor Commercial Vehicle Systems.

“This helps to keep the tires in more constant contact with the road surface,” he says. “The result is a more controlled ride, less wear on tires, and potentially better braking. On trailer air suspensions, shock absorbers also act as rebound limiters to protect air springs from damage due to overextension.

“Because each shock absorber has its own damping characteristics and travel, installing anything other than the specific shock called for on a particular suspension can result in premature failure and component damage,” he says. “Shocks with incorrect damping can cause poor ride quality and suspension control. Those with different extended or collapsed lengths can lead to damage to the shock itself or to the air spring.”

“Shock absorbers require very little in terms of maintenance,” Morgan says. “Simple visual inspections will help to maximize component life. Check for leaking fluid—that’s evidence of a seal failure. A slight sheen of fluid is normal, but dripping or signs of heavy leaking indicate that the shock should be replaced.

“Also, look for loose fasteners, a bent or otherwise damaged shock body, or deformed bushings,” he continues. “These can be from one-time incidents, or could be symptoms of equipment being out of adjustment, or even misuse. A working shock absorber will heat up with use. One that does not generate heat has likely lost its damping ability and should be replaced.”

And of course, Morgan recommends that technicians check with the suspension manufacturer for troubleshooting assistance.


“Most people just don’t understand shock absorbers,” says William G. Nash, manager of technical services for Hendrickson Truck Systems Group.

“Once you get to know the system,” says Nash, “you can just tell when it needs shocks. Even by the ride, an operator ought to be able to tell that there’s something wrong with his truck. There are a lot of tell-tale signs that a technicians should be able to see at a PM. He really doesn’t need to do anything special, just glance up if he’s under the truck greasing it and look at the shock. Look to see if it’s leaking oil, look to see if it’s dented from road debris hitting it. It’s really just an observation thing as you’re doing something else; when you’re under the vehicle you ought to be looking at everything: fasteners, shocks, the suspension components.”

“One of the primary specifications of a shock is to have the correct travel to meet the requirements of the suspension,” says Greg Richardson, vice president of engineering for Reyco Granning. “On most air ride suspensions, the ‘jounce’ (up travel) is restricted by a stop of some type, maybe internal to the air spring or external, possibly directly over the axle. If the shock bottoms out prior to the full jounce of the suspension, it will wind up being the stop. Shocks generally are not designed to handle such loads and will be damaged internally if forced to do so.”

Richardson points out that the majority of shocks used on Reyco Granning air-ride suspensions are designed with high pull-apart strength, so they can be used as down stops for the suspension at full rebound. He cautions that if an aftermarket shock is used without the required pull-apart strength, and the shock fails, it will generally result in the failure of other suspension components, such as the air springs.”

“Because air-ride suspensions typically don’t provide much damping, you must get most of the damping—if not nearly all—from the shock absorber,” says Bruce Barton, director of engineering, Ridewell Suspensions. “Proper damping will control vibration, improve the road-friendliness of the suspension, reduce tire wear, and help damp vibrations that might preserve the cargo that’s carried above the suspension. So, by spec’ing the wrong shock absorbers you may be improperly damping the system.”

“Proper dampening of a shock can be equally as important as having the proper travel and pull-apart strength,” Richardson stresses. “A properly dampened (tuned) shock will do a better job at keeping the tire in contact with the road as the vehicle is driven over road imperfections, which can reduce tire wear and increase vehicle component life as well as reduce cargo damage. Vehicle handling is also affected by the use of properly tuned shocks by reducing wheel hop, vibration, bouncing and pulsating of the suspension.”

Shocks are cycled continuously as the vehicle is in motion, creating heat and pressure inside the shock. An aftermarket shock that is not engineered for the application may not be able to handle the duty cycle. According to Richardson, this may result in seal failure, causing oil to leak around the seal which in turn reduces the dampening of the shock.


Do your technicians consider air springs to be “maintenance-free?” Although the description is technically accurate, ArvinMeritor’s Nash warns that air springs still need to be inspected, and, occasionally, replaced.

“An air spring is pretty much maintenance-free, but they do wear out,” he says. “They’re not a lifetime component.”
Nash’s advice is for technicians to listen closely to drivers’ ride complaints.

“What’s the operator saying about the vehicle? If you don’t have the right ride height, one of the signs you’re going to have is the air spring is going to give you a harsh ride,” he says.

Just in case the drivers in your fleet aren’t that talkative, Nash recommends that technicians get into the habit of conducting visual inspections whenever they’re under a vehicle.

“When they’re under the vehicle, doing something else, they’re back there probably greasing the vehicle: look at the air springs,” he says. “Look at the rubber: is the rubber deteriorating? Is it cracked? A big part is the piston—it’s usually trumpet-shaped, at the bottom of the air spring—look and see if it’s cracked. They used to make them out of aluminum, but most of them now are plastic or some kind of synthetic, and they can get broken.”

If there is damage to an air spring, can the cause of the damage be easily spotted?

“If you’ve got a failure in an air spring because of something that’s been rubbing against it, it’s pretty important you find out what’s been rubbing against it,” Nash says.

Nash’s company, Hendrickson, has a suspension system that has been specifically designed for dump trucks, and he points out that before a dump truck driver raises the dump body to unload, he or she lets all the air out of the truck’s suspension. As a result, those air springs are cycling up and down, changing shape, many times a day, over weeks and months and years of duty.

“Well, when you do that, the air bags go flat and they get up against the frame rail or whatever… so they start rubbing,” he says. “Well, when the technician’s looking at the springs, he doesn’t see all that. He sees the air spring full of air, he sees an air spring that’s failed and he puts a new one on.”

And then it fails again.

“In those cases, you need to be able to identify (the cause), so you can fix it,” Nash stresses. “But, when the fix is that there’s a design change that needs to be done to the air spring, there’s really not anything you can do about that.”

“Fleets generally do a thorough job during inspection and maintenance,” Richardson says, “but it is critical for proper function of the suspension that the ride height of the suspensions is set properly by the height control valve. Improper setting could result in premature air spring failure, suspension failure or even vehicle damage.”


Randy Petresh, vice president of technical services for Haldex, discusses the complexities of ride-height adjustment:

“It’s part of a system; you can’t set it on a bench before you put it in,” he cautions. “The valve selection and the linkage subsystem and the specifications associated with the valve—the ride height and the linkage—are all part of the total suspension package. The OEM has the final call as far as what valve he chooses, the linkage he chooses, and that’s appropriate for determining what the ride height is at, based on where and how the valve, linkages and lines are installed.”

The trouble is, a lot of things can affect the adjustment of the ride-height valve and linkage.

“It could be just basic wear of the components over time,” says Petresh. “The valve wears, the linkage arms that are attached to the valve assembly—you’ve got one rod between the lever attached to the valve and another attachment point on the axle, or suspension arm—they wear over time, get sloppy, lazy. The linkage connections are vulnerable to damage: road contamination, ice, snow—it gets wiped out, damaged by the environment quite often.

“The technician also should have some idea as to what his ride height should be, within limits, without going through some exhaustive, complicated diagnostic procedure; that’s not necessary until he suspects he’s got an issue,” he says. “He should be able to determine from experience, by the way the vehicle is set up and spec’ed, to know whether it’s sitting properly or not. And that’s really a routine PM check, based on his operation.

“A lot of times the driver should pick it up,” Petresh continues. “If you’ve got a reasonably competent driver who knows what he’s doing, he’ll pick up that vibration. Because when your ride height goes to hell, your driveline angles are out of whack. So then he starts feeling vibration problems and a halfway decent driver is going to feel that. It’s obvious: that’s the biggest clue and the first clue.
“Of course, if you’ve got a camber side-to-side, or the one axle drops down, then you know you’ve got a major problem,” Petresh says. “And usually one of the other things, the adjustment or the height, slips out of position, prior to having a major meltdown—a broken arm, a total valve malfunction or a linkage failure—it’s either been busted or knocked off or it’s worn to the point that you no longer get any decent engagement.”


Clearly, there’s a lot here that the technician needs to be aware of.

“This is critical,” says Hendrickson’s Nash. “A technician really needs to periodically check the height of the suspension. If you don’t…

“I came out of the transmission business over at Meritor, and one of our biggest problems there was synchronizers getting knocked out, and they get knocked out because the driveline angle is off,” he says. “And the biggest culprit for having the driveline angle off is the air-ride height being wrong. You can lose u-joints, you can lose transmission synchronizers, drivers complain of driveline noise.”
“The first step,” says Ridewell’s Barton, “is to make sure they DO inspect. That should be part of the PM schedule.

“They should be looking at the leveling valve,” he says. “Air suspensions will typically have a leveling valve in the system—perhaps just one leveling valve for a series of suspensions. They will need to check that to make sure that it’s set properly, at the right ride height, that it’s functioning correctly. The other things to look at are the air springs on the suspension, to ensure that they’re not cracked or need replacement. I would say that typically you need to take a look at air lines; are they chafing any part of the frame structure, where perhaps you could have a failure going down the road, where a line would bust through because of chafing?”


“The pneumatic controls are probably the most vulnerable, the most susceptible to damage and abuse and wear of anything on the suspension. I don’t think people recognize that,” says Haldex’s Petresh. “I’m in charge of our nationwide field service staff, and I get all my guys together quarterly to review things like this and try to figure out the best way to go after it, new manuals or instruction sheets; we’re always looking for a better way to get that message across. But technicians don’t realize that the controls are not boilerplate like the rest of the suspension is, and as a result they don’t pay any attention to it. It’s just neglected because of this awareness thing. They’re just not aware of it.”

How are those awareness levels on your shop floor? Are your technicians inspecting air-ride suspension components every time they’re under a truck? Are they lisening to ride complaints from drivers? If not, today may be the day for a walk-around on the shop floor, to make “awareness” the word of the day.