Suspended on air

Air suspension is becoming increasingly popular for premium and luxury vehicles, with nearly all the major carmakers replacing all four steel spring systems on the corners with air springs. Features that add value for customers include optimum ride height that is independent of vehicle load, which can be achieved with a less complex rear axle air suspension. BMW has achieved this advancement and decided to equip its new 5 Series Touring and Gran Turismo models with rear axle air suspension as a standard feature. 

Automotive Industries (AI) asked Helge Westerkamp, sales and marketing leader of WABCO Car Systems, about WABCO’s product portfolio for electronic suspension systems as well as the content delivered into new BMW 5 Series Touring and Gran Turismo vehicles.

Westerkamp: WABCO introduced its first electronic air suspension system in 1986 for commercial vehicles. Meanwhile, it has become a standard in Europe on buses and the majority of trucks and trailers. The breakthrough for passenger cars in Europe happened in 1998 with the introduction of air suspension in the Mercedes S-class where WABCO developed its first generation high-power, oil free compressor as the heart of a system that could pressurize air up to 16 bar in a single stage. In the following years, additional system components were added to the portfolio including complex electronic control units for air suspension in combination with damper control. The delivery content for BMW is the electronic control unit plus the complete air supply unit including compressor, air dryer, valveblock, brackets, isolation and air intake.  

AI: What is the technical difference between rear axle air suspension versus normal steel suspension?

Westerkamp: The problem with steel suspended vehicles is always the compromise between comfort and limited overall wheel travel. The effect is that a vehicle with soft suspension, which is fully loaded, has very limited wheel travel left to compensate for road undulations. This becomes critical, particularly for vehicles with high payload such as station wagons or minivans. These vehicles need a stronger suspension to be able to carry heavier payloads and not kneel too deep when fully loaded. The downside is an uncomfortable ride when the car is empty. Air suspension is the ideal solution because the air spring pressure is automatically adjusted by a compressor. The effect is always optimum ride height independent of vehicle load and a spring characteristic that is softer at lower loads and stiffer at higher payload. Nature has made it simple for us. 

AI: What is today’s market penetration and what future trend do you see for air suspension?

Westerkamp: Today only around two per cent of all light vehicles are equipped with air suspension. The share between rear axle and 4-corner systems is similar. Nearly all carmakers offer air suspension on their premium vehicles. The functionality is increasingly enhanced through additional features such as damper control or switchable air spring characteristics. Rear axle air suspension offers some of the most important features while being less complex than a 4-corner system. We are convinced that this feature will become more and more standard on station wagons and minivans. The time will soon pass when families are riding low toward summer vacation with fully loaded vehicles. Today’s high-tech vehicles can no longer permit a hanging appearance. BMW has recognized this early and decided to put air suspension as standard on its recently launched 5 Series Touring and Gran Turismo.

AI: Which features of air suspension do customers value most?

Westerkamp: WABCO has carried out a global consumer survey, asking passenger car drivers about their perception of air suspension. An interesting outcome is how end-customers rank the different features of air suspension. The analysis of the data allowed us to set up a top 10 list of air suspension features; it clearly identified improved comfort and vehicle ride height independent of the load situation. These features can be realized with both the rear axle air suspension and 4-corner system approaches.

AI: What competing technologies are available to compensate height change due to load variation?

Westerkamp: Theoretically, there are four different physical principles available. First, air suspension; second, hydraulic suspension; third, mechanical adjustment of the spring base plate with an electric drive; and fourth, electromechanical suspension following the principle of a loudspeaker. Only the first two have made it into automotive series production; the others are too complex and expensive. For rear axle systems, the main competing technologies have been air suspension and self-leveling shock absorbers.

AI: What are the main differences between rear air suspension and self-leveling shock absorbers and what effect does it have for the end-customer?

Westerkamp: Air suspension directly compensates height change when the vehicle is loaded. The height sensors measure the deviation from optimal ride height and the electric control unit operates the compressor in order to inflate the air springs accordingly. The driver gets direct feedback. The self-leveling shock absorber needs movement of the vehicle’s body relative to the road in order to pump hydraulic oil within the damper into a load-carrying volume. Consequently, the vehicle must drive to create body movement and pump itself up. On even roads, this can take a couple of hundred meters and the driver gets very limited feedback. Also, active features – like trailer connect or easy loading with on-demand height reduction while parked – can only be realized with an air suspension system. We see the market moving toward air suspension for new vehicle generations.

AI: What have been the major breakthrough achievements in WABCO´s air suspension history for passenger cars?

Westerkamp: The first application was the VW Passat in 1986, where WABCO introduced its oil free compressor technology plus the electronic control module for a rear levelling system. After a series of further applications i.e. for Landrover, Mercedes and Audi, we launched in 2000 the first mechatronic air supply unit for several General Motors SUVs. The innovation was to integrate all air supply components including electronic control into one device in order to realize a plug-and-play solution for the OEMs. In 2006, we introduced air suspension into the compact class for PSA C4 Picasso vehicles as the full system supplier. Another highlight was the launch of the Audi A8 in 2010, where WABCO developed the complete air supply system including the first electronic control unit for air suspension plus damper control with FlexRay technology.

AI: What other technologies is WABCO focusing on for the future?

Westerkamp: As a technology leader for vehicle control systems for commercial vehicles, WABCO is experienced in all kinds of safety related systems like ABS, rollover prevention, driver assistance systems, and many more. We have development and production capabilities across the globe and we are expanding our capabilities to embrace several new technologies. This expertise can also be adapted to the passenger car world. For commercial vehicles, we have a clear path to enhance vehicle efficiency and environmental sustainability by 20% through WABCO products. Our vision is to improve advanced safety and driver effectiveness, and reduce the number of commercial vehicle accidents by 50% by 2020.