Air suspension appears on most large luxury cars and on a growing number of others. Its value is not simply a softer ride but the ability to change the spring itself while driving.
A steel spring has one fixed rate
A coil spring is defined by its geometry and material, and the force it produces is proportional to how far it is compressed. That relationship cannot be altered once installed.
Engineers must therefore choose a single compromise covering an empty car on a smooth road and a fully loaded one on a rough one.
Ride height is fixed in the same way, and it sags as load is added, which affects headlamp aim, suspension geometry and aerodynamics.
Air springs decouple height from stiffness
An air spring is a sealed rubber bellows containing compressed gas. Adding air raises the car; changing the volume the gas occupies changes how stiffly it resists compression.
Because a compressor and valves control both, ride height and effective spring rate can be adjusted independently and continuously while driving.
Some systems add a switchable auxiliary chamber, which enlarges the gas volume for a softer rate or isolates it for a firmer one within a fraction of a second.
Self-levelling removes the load compromise
The system measures ride height at each corner and adds or releases air to hold the specified height regardless of how many occupants or how much luggage is carried.
The suspension therefore operates in the middle of its travel at all times, preserving both bump absorption and rebound control under any load.
Towing benefits particularly, since the nose weight of a trailer no longer lifts the front of the vehicle and unsettles the steering.
Height changes serve several purposes at once
Lowering the body at motorway speed reduces frontal area and the amount of air passing beneath the car, which lowers drag and improves stability.
Raising it on demand increases ground clearance for rough surfaces or steep driveways, which is how one vehicle can claim both road manners and off-road capability.
Many systems also lower the car at rest to ease entry and to reduce the lift height when loading the boot.
The costs are complexity and repair
The system depends on a compressor, a reservoir, valve blocks, height sensors and the air springs themselves, and any of them can fail.
The rubber bellows is a wearing item exposed to road salt, grit and temperature extremes, and a leak causes the corner to settle overnight.
Replacement costs are substantially higher than for a coil spring, which is why some owners of older vehicles convert back to steel and accept the loss of function.