Funny thing: When you ask maintenance and engineering experts about air-ride suspension systems, the last thing they want to talk about is air. To them, the keys to maintaining modern air-ride suspensions are: shock absorbers, fasteners and bushings.
Oh, they get to the air system eventually, but the overriding message is that the suspension is only as good as the smallest component.
It’s fitting, then, to begin this look at air-ride suspension maintenance with a look at bushings and fasteners.
LOOKS GOOD, RIDES BAD
“The suspension is only as good as the torque rods, and the torque rods are only as good as the bushings,” says ATRO Engineering Systems founder George Sturmon. “The technicians don’t understand the forces and the mechanisms that those bushings are under, and that’s why (bushings) wear out relatively soon.”
Because of this lack of understanding, Sturmon feels that fleet technicians aren’t inspecting bushings properly. “They’ll grab ahold of the (radius) rod and try to move it, and you can’t move it by hand, but it is nevertheless shot,” he explains. “If you can move it by hand, it’s been shot for a long time.”
The problem as Sturmon sees it is that the radius rods themselves only operate—only see loads—whenever the truck is braking or accelerating, or perhaps going over rough road, creating a back-and-forth movement.
“They don’t have constant loads on them,” he says. “So, to check them, you should take a rod or a bar, and put it in there, and see how much you can move it with that. But that’s not done; the technicians will maybe look at them, and on a rubber bushing they might see some checking or cracks, but if they can’t move it they think, ‘Well, it’s good.’ Well, actually, it’s bad.
“Ride quality is affected by bad torque rod bushings,” he explains. “The back and forth movement; the driver can feel that. If he says, ‘This thing is moving back and forth,’ the first thing the technician should look at is torque rods. And then they should put them through a very rigorous inspection: Make sure you’ve got enough force on that bushing to move it. There’s not anybody I know who can grab ahold of a torque rod—they’re supposed to handle up to 30,000 lbs. back and forth—and move it by hand. If you can move that bushing—say you look at it and the material between the two sleeves is a quarter of an inch—if this thing moves upward of an eighth of an inch, it’s bad. If they move that much, they need to be replaced.”
John Morgan, product manager for ArvinMeritor Commercial Vehicle Systems, offers a few guidelines for replacing those bushings.
“When installing a new bushing, follow the manufacturer’s guidelines for lubrication,” he says. “Oil-based lubricants can damage natural rubber bushings.”
“Also, be sure to use the fasteners specified by the equipment manufacturer, and always torque to the indicated level,” Morgan adds. “Be sure the threads are free of contaminants, including paint overspray and dirt, and follow the manufacturer’s recommendations regarding lubricant. Failure to do so can give false torque readings, or yield improper clamp loads, which can cause the connection to fail and result in equipment damage.”
RUBBER VS. ELASTOMER
“Back when I started with this business, when the suspension manufacturer sold you a suspension, the torque rods normally came with it,” says ATRO’s Sturmon. “Now a lot of OEMs are getting their own torque rods made, and they take the rod, they put a rubber cylinder on it, they put a pin in it and they shove it in a hole—not realizing that these things have to be positioned at zero on ride height, so that they’re not stressed at ride height.
“They’re very hard to install, and the guy has to force them into position,” he says. “Well, if you get them in a little bit crooked, particularly on a rubber bushing, you’re immediately degrading the material: Rubber under constant stress degrades.