Heavy-Duty Suspension

Yes and no. Height control valves are used to keep the air ride suspension at ride height thereby offering the optimum performance of the suspension and axles. There is usually one height control valve per trailer setup, not one per axle.

The air ride suspensions are all linked to the same air system so the height control valve actually controls the ride height in all the linked axles.

For example; a tridem axle trailer will have a height control valve on the middle of the three axles.

It has more jounce than rebound (opposite to the primary suspension). In most cases, the trailing arm has an additional weldment, whether a lift bracket or a lift arm. Also an additional weldment on the hanger or into the trailer's frame. 

A liftable suspension and a primary suspension of the same ride heights are engineered differently. In most cases, they will have different components such as hangers, shock brackets and upper air bag spacers.

Non-liftable, non-steerable suspension. It usually has more rebound than jounce. In some cases, it may have an equal jounce and rebound. It may also come in different configurations.

All the axles on a trailer are load equalized. When liftables are included in the trailer design, this may lead to the use of a proportioning valve due to different suspension geometries.

The distance between the centerlines of the suspension's LH and RH hangers, or LH and RH of the trailing arms.

The upward travel of the suspension. It is usually more than the "bumper contact" travel. It includes the compression of the air bag's bumper and the pivot bushing.

The downward travel of the suspension. It is limited by the allowable maximum height of the air bags and controlled by the extended length of the shock absorber. The shock absorber physically stops the suspension at the full rebound position.

Ride height is the measurement from the bottom of the frame to the center of the axle; or top of the axle plus 2.5" for 5" OD tube or 2.875" for a 5-3/4" OD.

The vertical load/capacity the suspension is designed and tested for. In many applications the capacity is not dictated by the actual, required vertical load.

Good examples are:
Using 30,000 lb. suspensions on highway trailers such as dump trailers.
Using 25,000 lb. liftable suspensions due to higher axle up travel.

A  suspension where the trailing arm is mounted on top of the axle. It is usually for tall ride heights of 10" or higher.

A suspension where the trailing arm is mounted under the axle. It is usually for ride heights of 12" and less.