Air-suspension systems are still a long way from set-it-and-forget-it. They require
periodic maintenance and performance checks, and when a suspension begins to fall apart, the wear often shows up somewhere else.

Irregular tire wear – usually cupping – is often an indicator of a worn-out shock
absorber. But why wait until your tires are near ruin before addressing the cause
of the problem. Tears in air bags or misalignment of air bag parts are other
symptoms that one or more shocks aren’t doing their job.

Shock Absorbers
Check mounting bolts for tightness and the mounts themselves for integrity. Look for signs of leaking fluid. A mist or light coating of oil on the shock body indicates that the shocks are working. Streaks of oil running down the shock body are signs of worn or blown seals or other interior damage. Such shocks should be replaced.

A shock can look okay but still be shot. Touch the shocks after the truck’s come in off the road. Compared to the truck frame at ambient temperature, the shock
should feel warm. If the shock is cold, it’s probably not working.

Conduct a visual check for cracks in the upper and lower bushings, broken upper or lower mounts, broken dust tube, improper installation, and bent or dented tubes.

Pull any suspect shocks off the truck and check for broken parts inside and out (it shouldn’t rattle when you shake it), stroke the shock several times in an upright
position to determine if there’s resistance. The resistance should be stiff; if not, the shock is finished. Always replace the shock with something matching the original equipment spec.

Ride Height
Drivers frequently tamper with the ride-height adjustment arm trying to smooth out their ride, but it’s imperative that the factory-set height not be tampered with. Ride height can influence load equalization on tractors and trailers. Even small
adjustments in ride height (within the specified range) can impact the way the load
is distributed across the axles. This can reduce ride quality, damage cargo, and
increase suspension wear. Changing the ride height also changes the driveline angle, which can chew up U-joints at a great rate, and can wreck a drive shaft.

And if you’re noticing driveline vibrations or find you’re replacing U-joints
prematurely, the first place to look is air-suspension ride height.

Engineers at the Spicer Driveshaft Division of Dana Corporation warn that as
U-joint angles increase, joint life decreases. If you double the joint angle, U-joint life is halved. Their recommendation is not to exceed three degrees joint angle between the transmission and the lead axle, and not more than six degrees joint angle at the inter-axle positions.

Under ideal conditions, the frame slope (the difference in height above the pavement from either end of the frame) will be zero. Check ride height (the distance between the lower frame rail and the center of the front drive axle) against the manufacturer’s recommendation, and adjust if necessary to original specs.

During routine servicing, push and twist drive shafts looking for excessive
looseness and spine wear. It’s equally important to periodically check driveline
angles during maintenance inspections.

If the system won’t maintain proper ride height during operation, check the air
supply lines for clogs, kinks, or contamination. Check the mounts on the control valve for proper positioning, and check the exhaust valve for good airflow.

Suspension Bushings
Bushings are designed to keep pivoting metal parts from coming into direct contact. Because bushings also act as bearings to some extent, they’re susceptible to wear. Worn bushings still allow the metal parts to pivot, but the
movement will often exceed allowable limits. This could cause the inside walls of
the tires to rub on the suspension frame hangers. To avoid serious tire damage,
the bushings should be replaced. Signs of excessive axle walkout are a sure
signal of impending trouble. If axles fail to re-align themselves after maneuvering
through a curve, for instance, it’s a good bet that re-bushing is necessary.

Good bushings also improve roll stiffness and vehicle handling. Bad bushings will
give a sloppy-feeling ride and a sense of fishtailing in a turn. Check the outer
edges of the bushing for cracks in the material, or signs of excess movement
between the two metal parts. Bushings must be properly torqued when installed or
they will crush (when over-torqued) or move around and wear prematurely (if
under-torqued). Poor bushing materials and overloading of the truck will also cause crushing.

In its Recommended Practice (RP) 643, the Technology and Maintenance Council of the American Trucking Associations (TMC) suggests air-suspension maintenance intervals of 15,000 miles for the A and 50,000 miles for the more thorough B inspection.

The RP lists a number of things that should be inspected and adjusted regularly, or replaced. These include all fasteners, U-bolts and bushings, air bags, shock absorbers, the height control valve, and the various air lines and connections.

Drivers should eyeball these every day as part of their pre- and post-trip inspections.

To order a copy of the TMC
Recommended Practices Guide – THE maintenance bible – call 703-838-1763. It’s not cheap, but it could be money well spent.