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Air suspensions continue to grow in popularity and already account for about 80 to 85 per cent of Class 8 tractor suspensions and 55 to 60 per cent of trailer systems. It's easy to see why they're so ...

Air suspensions continue to grow in popularity and already account for about 80 to 85 per cent of Class 8 tractor suspensions and 55 to 60 per cent of trailer systems. It’s easy to see why they’re so popular. They offer a smooth ride for both cargo and drivers.

But while they’re fairly simple in design, many people fail to pay any attention to these systems until a problem occurs.

Mis-aligned rubber air bags can lead to holes. Another source of problems comes when the rubber separates or pulls away from either the top or bottom plates – a condition usually traced to an overextended air bag (spring). Both under-inflation (a low vehicle ride height) and over-inflation can cause ride problems and shorten the life of air springs.

In addition, the suspension’s contribution to driveline vibration is related to its ability to maintain specified and correct U-joint working angles. During acceleration, the twisting and turning forces of torque transferred to the driveline are at their maximum, particularly when you’re traveling in low gears. This same torque will force the ride height of the suspension to increase (a condition known as frame rise), affecting the driveline working angles.

These sharper working angles can generate – or even increase – torsional vibrations from the engine, and lead to U-joint failures. Torque windup will typically occur in trailing-arm systems as the drive axles twist from the torque transferred during braking and acceleration. But the proper selection of shock absorbers with internal limiting springs will prevent the shocks from overextending during these two operating conditions, preventing U-joint damage.

The pullout of shock absorber mounting eyes, and/or signs of elongated eyes, meanwhile, are generally signs of incorrect ride heights, the wrong shock absorbers or even loose mounting bolts.

How they work

Air springs or rubber bags are typically inflated by compressed air that comes from the vehicle’s secondary air reservoir through a pressure protection valve. A chassis-leveling valve controls the flow or exhaust of air from the individual air bags to provide a pre-determined chassis ride height.

Keep in mind that a dirty filter or crimped lines can interrupt the air supply, making leveling valves sluggish.

Ride-height adjustments

Mechanics will often adjust air suspension ride heights as a way to respond to complaints of poor rides that may in fact be caused by a damaged rod or other failed component. They also play with the ride height to lower the fifth wheel to offer easier access under the low nose of a trailer- particularly when a driver complains about hard coupling or uncoupling – while the problem may lie in the fifth wheel or linkage.

This adjustment, if outside the normal ride height, will lead to other problems linked to the driveline and U-joints, as well as affect the overall ride quality of the tractor.

Individual axle and suspension models determine the best ride height for any given vehicle. Limiting the amount of suspension travel in the rebound direction best controls this ride height (or frame rise), without negatively affecting the vertical travel of the suspension to absorb road shocks. The system must still deliver a comfortable ride and ensure that the axles travel enough to drive over obstacles, while maintaining tire contact with the road.

Adjusting ride height is a simple task that involves disconnecting the linkage from the leveling valve. You then have to exhaust the compressed air from the air bags (springs), proceeding to adjust the linkage according to the OEM’s specified ride height.

Follow this procedure as a generic example:

Park the vehicle on a paved, level surface. There should be no load on the tractor chassis.

Unhitch the trailer from the tractor, block the wheels on one axle of a tandem, and release the parking brakes.

Exhaust air from the air bags, and then re-inflate them. This can be done by simply disconnecting the leveling valve control lever, and moving the lever arm down to a 45-degree angle. Re-inflate the air springs by pulling up on this same lever to a 45-degree angle. (You’ll be able to hear the air loss or re-inflation of air springs.)

If the leveling valve does not supply or exhaust air, or the neutral position will not prevent the flow of compressed air, refer to the air suspension diagnostics table in the OEM truck service manual for possible causes before replacing the leveling valve. Check that when the ride-height lever is in the neutral position (horizontal) that there is the recommended freeplay in the dead-zone travel, in both up and down directions, as per the suspension system OEM’s specs. Typically, this can range between 1/8 inch (3.2 mm) and as much as 3/8 inch (9.5 mm). Ensure that you compare this to spec!

Reconnect the leveling valve vertical linkage, and proceed to check the vehicle’s ride height.

Start the engine and build up air pressure to at least 100 psi (690 kPa) or until the air compressor governor unloader cuts out. Then turn the engine off.

Check the length of the leveling valve control lever between its two pivot points, and compare it to that recommended in the system’s service manual. If incorrect, adjust it accordingly.

Check the published spec’s (or see the frame or in-cab decal) to determine the distance from the bottom of the axle stop to the top of the axle. If the distance is as it should be, then the leveling valve is adjusted correctly. If the ride-height is incorrect, it can usually be adjusted by loosening the leveling valve’s mounting screws to the bracket, and rotating the valve slightly to the right or left to alter the ride height. But keep in mind that this is a trial-and-error process. You need to exhaust the air (deflate the air bags), check the ride height and compare it to spec’s, then reapply the air and compare the ride height when the bags are inflated.

After every adjustment, test drive the unloaded vehicle for at least half a kilometre (1/4 mile).

Return the vehicle to the shop and recheck the ride height. Some change can be expected since the suspension system settled during the road test, but as long as the ride height is within the low and high tolerance spec’s, it is OK.

Correct ride height will ensure a smooth ride, ease coupling and uncoupling, ensure minimal driveline and U-joint problems, and eliminate vibration problems. n

– Bob Brady is the president of HiTech Consulting.

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