Maintaining Air Springs and Shock Absorbers

Suspension performance suffers if they’re not in working order.

December 2015, - Feature

by Tom Berg, Senior Contributing Editor - Also by this author

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This brand-new air spring won’t need much attention until it’s worked for many miles and its rubber bag ages through contact with the elements. Its operating range is 40 to 100 psi. Photo: ContiTech North America
This brand-new air spring won’t need much attention until it’s worked for many miles and its rubber bag ages through contact with the elements. Its operating range is 40 to 100 psi. Photo: ContiTech North America

Air suspensions cushion the ride for drivers, vehicles and loads. Among their components are the springs themselves and shock absorbers, which help control the movement of the bags and the rest of the suspension. These parts are subject to wear and aging, and must be regularly inspected and repaired or replaced when necessary.

The American Trucking Associations’ Technology & Maintenance Council recently revised its Recommended Practice 643A, Air Ride Suspension Maintenance Guidelines.

“Generally speaking, air springs are very durable components,” says John Healy with Stemco, which offers Goodyear brand air springs.

Because they’re made of rubber, air bags usually “age out” before they wear out, notes Dave Vanette, new business development manager at Firestone Industrial Products. “This aging out process has a lot to do with the heat history of the part and exposure to ozone,” he explains. If an air spring is run predominately where the average temperature is high, it accelerates this process.

“The typical failure mode for an aged-out air spring is not catastrophic, but would more likely be a slow leak that eventually becomes too much for the compressor to keep up with,” Vanette notes.

As it ages, you will see cracking or horizontal splits in the rolling lobe portion of the air spring, which is under the highest stress. By itself, cracking on the surface does not mean a leak. When the cracking actually starts happening on the inner liner inside of the air spring is when leaks occur. However, Vanette says, “this horizontal cracking of the rubber in the rolling lobe area is a good indication that the aging process is pretty well advanced and it will just be a matter of time before the air spring starts to leak, so it is probably a good time to change out your air springs.”

Try to track the age of the air spring by the purchase date of the truck or trailer, the purchase date of a replacement air spring, or by reading the date code on the air spring, he says. Knowing that the air spring is, say, more than five years old might prompt a closer inspection.

Damage can also result from tough operations, and RP 643 includes an inspection guide that lists various types of damage to air bags.

Are shocks working?

TMC guidelines say misting oil is OK for a shock absorber, but leakage isn’t. ART:  ATA Technology & Maintenance Council
TMC guidelines say misting oil is OK for a shock absorber, but leakage isn’t. ART:  ATA Technology & Maintenance Council

Today’s low friction Class 3-8 suspensions require high-functioning shock absorbers to minimize wear and protect suspension components from vibration damage, according to shock maker Gabriel.

“The number one cause of air spring failure is overextension caused by an improperly functioning shock absorber,” says Stemco’s Healy.

But the shocks themselves can be harmed if a suspension bumper or stop is damaged or missing, according to Joe Bacarella, manager, product training and technical resource centers at Tenneco North American Aftermarket, which markets Monroe shock absorbers.

“Bumpers keep the suspension from articulating too far and the shocks from stretching beyond their limits, to where internal parts are damaged,” he says. “Some bumpers are rubber and some are metal; they can be compressed by sharp contact or knocked off. If they’re missing, there’s nothing to keep the vertical movement under control.”

The folks at Gabriel say you should check your shocks for possible replacement if you experience uneven tire wear, ride deterioration, excess vibration, sagging taper leaf springs, premature wear, and broken or torn air springs.

“It is very important to ensure that your shocks are providing dampening and not just going along for the ride,” says Jason Heath, product manager, Powered Vehicle Systems at SAF-Holland. “We’ve all seen tractor-trailers bobbing up and down endlessly while going down the highway. This needless undulation results in unnatural, accelerated tire wear. 

“A simple way to check if your shocks are working is to take the truck for a spin and check the temperature of the shock. If it is warm or hot, it’s working,” he says. 

RP 643A says shocks on both sides of the vehicle should be checked in this manner, one right after the other. Both shocks should be warmer than the reference point (a frame rail, for instance) when touched, and be similar in temperature to each other. A cooler or cold shock may have failed. Stemco recommends using an infrared temperature gauge. When doing so, TMC’s RP says, there may be up to a 20% temperature difference between the two compared shocks. If the difference is greater than 20%, it may indicate a shock failure and should be removed for closer examination.

For that, check for visible signs of wear. Turn the shock upside down and shake while listening for any rattling that would indicate a broken internal piece. Also stroke the shock several times in an upright position to determine if there is any resistance. Stroke gas-charged shocks in the same way; it should return to a full-height position after release. A gas-cell shock needs to be removed, held in a horizontal position and stroked several times. If a lag develops, replace the shock. Unless a shock has been damaged by an external blow, replace both shocks of a pair so performance is equal on both sides of a vehicle.

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