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.”