“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.
LISTEN TO THE DRIVER
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.”