For air suspension developers, tuners and repair technicians, air struts on the market come in highly varied constructions. To select and develop reliable components, you need to understand two core architectures: series-type air spring and parallel-type air spring.
1. Series-Type Air Spring
Series-type air springs are divided into rigid-mount (hard connection) and flexible-mount (soft connection) based on how the aluminum housing connects to the end fitting.
1.1 Rigid-Mount / Hard-Connection (Older generation, e.g. Mercedes-Benz W221 chassis)
The aluminum protective housing is rigidly clipped directly onto the strut end fitting. ✅ Advantages
- Eliminates the air bellow retaining ring and related assembly steps
- Rubber bellow can withstand higher internal air pressure; thinner aluminum housing is acceptable
- No risk of housing separation caused by poorly crimped retaining rings
❌ Disadvantages
- Extra stress-relieving joint must be added to absorb dynamic movement stress
- More complex assembly and higher manufacturing cost
- Multiple moving parts raise the risk of abnormal noise
Note: This design is rarely adopted on modern new passenger vehicles.
1.2 Flexible-Mount / Soft-Connection (Modern mainstream, e.g. Audi A8 D5)
The aluminum housing fastens onto the rubber air bellow, without rigid contact with the strut end fitting. ✅ Advantages
- No dedicated stress-relieving joint required, cutting component cost
- Reduced interference between moving parts, simpler overall design
❌ Disadvantages Higher performance requirements for the rubber bellow. Critical parameters including bellow expanded outer diameter, burst pressure and housing push-off force must be precisely controlled. Poor specification will result in bellow bulging or housing detachment failures.
2. Parallel-Type Air Spring
For parallel-type layout, the damper and air-spring bellow are installed side-by-side as separate assemblies. It demands larger packaging space and is mostly applied on rear suspensions of large-size SUVs such as Porsche Cayenne and Range Rover.
- Early-model parallel air spring: Simple construction with rubber bellow, two plastic end caps and locking metal rings, mainly for cost reduction.
- Updated modern version: Equipped with aluminum housing and stiffness valve, greatly improving suspension stiffness and driving performance. This upgraded solution is widely used among new-generation vehicles.
Summary
- Series-type: Damper and air spring are integrated into one strut. Flexible-mount soft connection dominates current market.
- Parallel-type: Damper and air spring are independent side-by-side units. Requires more space and common for rear suspension on large SUVs.
Important: No single design is perfect. Every structure has inherent pros & cons derived from its mechanical logic. Component parameter deviation will trigger failures regardless of architecture.
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