MAINTAINING AIR SUSPENSIONS

There are more and more highway tractors and trailers being ordered with air suspensions. One reason is the softer ride due to "air bags", protecting the driver and load from damage and shock. Drivers enjoy and smoother ride, shippers cargoes are damaged less, and fleet managers have fewer damage claims to deal with.

The components of these systems remain a mystery still. Yet manufacturers and maintenance crews feel they can explain what components fail and why. It is important to use correct parts, have them installed properly and keep them properly adjusted. The "mortality items" in air suspensions, and what to do about them, include:

Bushings - Must be properly torqued when installed or they will crush (when overtorqued) or move around and wear prematurely (undertorqued). Poor materials and overloading of the truck will also cause crushing.

Air Springs - Holes appearing in the rubber walls are the most common type of failure and are usually caused by misalignment of the bag. If the rubber is torn or pulled from top or bottom plates, it is due to the spring being overextended.

Shocks - Too often these are the wrong item for the installation. Check carefully that the correct shocks are bought and installed. If shocks are warm after the truck has been driven, they are working. If they're cold, they are not. It is normal for a bit of fluid to leak right after installation because gaskets inside are setting. But persistent leaks are the result of improper installation or adjustment of the suspension's ride height. External failures include pulling out or elongation of mounting eyes. These usually indicate that ride height's wrong, it's the wrong shock for the suspension, or mounting bolts are loose.

Height Control Valves - These have three positions; open, closed and neutral. Many of these are returned with nothing wrong with them, suggesting that mechanics are pulling them off and sending them in under warranty. Some analysis should be done before assuming the valve is to blame for a failure. Valves work sluggishly if the air supply is interrupted from a dirty filter or other cause. Improper installation or the wrong application can cause linkage to tear or pull out, in which case you should check alignment or the part number. If you can't hook up the linkage when air is dumped from the springs, either the linkage is the wrong length or the valve is not the correct one.

The Edge System
On the "trailing-arm" of air suspension, improper ride height will result in improper angles within the driveline. Sharper angles can cause vibrations, exacerbate any torsional vibrations emitted by the engine and lead to U-joint failure.

Driveline angles should also be checked - if angles are beyond a few degrees at each U-joint, corrections should be made. This can get involved because axles may be incorrectly mounted.

"Torque windup" occurs in trailing arm-type suspensions when the axles are twisted during braking or acceleration. To combat this, there are special shock absorbers with internal limiting springs. The springs keep the shocks from extending during dynamic forces that can lead to windup.

Adjusting Ride Height
What is the proper way to adjust ride height? There are various types of air suspensions on the market and each has procedures.

First, be aware that ride height is defined as the difference between two measurements taken on the chassis. It is recommended that this number be posted on the frame, somewhere in the cab, or in the operator's manual or other maintenance manual that came with the truck.

The correct ride height for the suspension on a truck will be expressed in inches or millimetres. Find this number and write it down. Next:

• Park the vehicle on level pavement, chock its wheels and release its brakes.
• Exhaust air from the springs, and then reinflate them.
• Pick the axle closest to the height control valve and measure the distance from the centre of its spindle and the pavement (use a level and plumb line to ensure correct measurement here.)
• Measure the distance between the pavement and the bottom of the frame near this axle.
• Subtract the smaller number from the larger to get the actual ride height. If it differs from the correct number by more than ¼ inch, adjust the suspension until it is within that tolerance.

Adjustment involves disconnecting the linkage from the height valve, dumping air from springs, and then adjusting linkage according to the manufacturer's specifications. Consult the proper manuals for this.

Mechanics and drivers often adjust ride height to deal with poor ride because of a damaged rod or other parts failure, to lower the fifth wheel to get under a trailer's low nose, or to squeeze under a low overpass.

Drivers often do not know the correct way to adjust height (aside from using a dump valve if the truck is so equipped). This information will be in service or operator's manuals, and should be consulted before any field adjustments are made.

With proper maintenance, air suspensions will give the smooth ride they're designed for. Knowledge and training are the keys to the needed maintenance.