Our mechanical engineer explains the science behind air suspension, which equips many upmarket cars.
RIDE comfort and handling have an awkwardly inverse relationship. For instance, with the fitment of lower and stiff er springs that improve a car’s cornering capability, it’s almost impossible to improve its cruising comfort at the same time. In fact, suspension modifications in certain cases could cause a deterioration in both ride comfort and handling. Rarely, if ever, can these two dynamic characteristics be improved simultaneously. The two most critical components in a suspension are the spring and damper. In the most common design, the spring is made from a spring-steel rod formed into a helical coil – hence its name “coil spring”.
The damper is an oil-filled tube with rod and piston, and serves to attenuate any oscillatory motion. Both these components are physically constrained to just one setting, which the engineers have determined to be the best compromise between comfort and handling. Substituting the steel coil spring with a cushion of air introduces a useful degree of adjustability. This is what has been done with some suspension systems. In place of the spring is a bellow (usually made from rubber) filled with air. Under load, the bellow is compressed, causing a decrease in volume and an increase in pressure.
If more air is pumped in, the bellow is reinstated to its normal volume or, more crucially, its normal height. As an air spring, the increased pressure means increased stiff ness, which is necessary for the added weight, while the vehicle’s ride height remains constant. Air suspension systems now feature in numerous upmarket saloons, sports utility vehicles and station wagons. Some of them have air springs only at the rear, so that even when the boot (or load bay) is fully laden, the vehicle stays level. Supporting the air-filled bellows are a number of components not present in the regular steel-sprung suspension system. One of them is an electric or beltdriven compressor to supply air into the bellows when called for. What “instructs” the compressor is a level sensor.
In early models such as the 1968 Mercedes- Benz 300SEL 6.3, a lever between the body and suspension arm directly controls a valve to either fill or release air from the bellows. This sounds more complicated than it is – the setup is actually simple and sturdy, with reliable operation that only requires an overhaul after 100,000km or so. The latest air suspension systems, such as those used in flagship Range Rovers and Rolls-Royce Phantom limousines, rely on plenty of electronic hardware and a solid-state controller (or an ECU).
As a result, level-sensing and ride-height corrections are accomplished in a much shorter time, while on-the-go adjustments are made continuously. One major advantage of air suspension is its ability to keep the car’s ride height constant, since there is control over the volume of air. This means that despite varying passenger or luggage loads, the vehicle’s wheel geometry (camber, castor, toe-in/out) is unaff ected, all to the benefit of driving comfort and overall handling.
Ride height control is also selectable on some models, usually sports utility vehicles. For the purpose of off -roading, selecting a raised ride height sends a signal to the compressor to pump more air into the bellows. Conversely, releasing the air lowers the vehicle, which will enhance aerodynamic effi ciency at high velocities. Speeddependent controllability is usually part of the modern air suspension system. Compared to steel coil springs, air springs off er far superior ride comfort, while their self-levelling capability enables consistent handling regardless of the occupancy/cargo load on board.
