Ride comfort is writ large in the Audi A8: The new active chassis is the first electromechanical suspension system to significantly reduce vertical body movements on a wheel-selective basis.
Hermann J. Müller (copy) & Nils Kasiske (illustration)
Active body stabilization: Stabilizes the vehicle body, decoupling it from road excitations.
Active pitch control: During accelerating and braking, body pitch is reduced to achieve dynamic yet smooth handling.
Active roll control: During cornering, body roll is reduced to ensure dynamic yet safe handling.
Adding or subtracting newtons
Suspension systems are not normally a big factor in the way people look at cars. Style lovers focus on a vehicle’s shape and color, sporty types are primarily interested in horsepower and cylinders, while connoisseurs appreciate above all a car’s creature comforts. Sure, they all need a suspension system, but they rarely talk about it. This is a little surprising. After all, as engine power increases, so do the demands on road holding—and even the most comfortable seats cannot disguise an unbalanced suspension. This is the classic conflict in chassis design. A stiff suspension improves handling, while a softer suspension enhances comfort—and the best suspension is always a trade-off that comes closest to the vehicle’s overall character. A significant step forward in this regard was the introduction years ago of Audi drive select. Now available on most models, it lets the driver choose between comfort, auto, dynamic, efficiency and individual driving modes. Audi drive select has always been particularly versatile in the brand’s flagship vehicle: The Audi A8 combined adaptive air suspension with hydraulically controlled dampers that responded to shocks within millisecnds.
The electromechanical active chassis in the new Audi A8 is now even smoother than than the system used in its predecessor. In addition to the air struts (which allow ride level adjustment and ensure the vehicle always has the same ride height regardless of load) and the hydraulic dampers (which counter wheel hop over bumps), the new system significantly reduces vertical body movements, known as “heave” or “bounce”. Drawing on advanced electronics and mechanical systems, the Audi engineers have also minimized pitch movements around the vehicle’s transverse axis during accelerating and braking, as well as roll motions around the longitudinal axis during cornering.
The brain of the active chassis system is a controller—the electronic suspension platform—where all the handling data come together. On this smart, highly interconnected platform, data are instantaneously recorded, evaluated and translated into concrete commands, which are then executed by sophisticated technology installed on both axles. At the rear, for example, the inverted U-section of the subframe is packed full of the active suspension’s actuator and electronics. At each end is an electric motor that does the physical work. When Joachim Schmitt, Audi development engineer for suspension systems, talks about it, it sounds quite simple: “What we are doing is basically adding or subtracting newtons.” In other words, to reduce body movements, forces are transferred to the suspension as required, or the spring rate is adjusted accordingly.
The new Audi A8 raises the bar in ride comfort. Thanks to reduced bounce and pitch motions, the body seems to float on the road. What’s more, the deliberate roll allowed during cornering is hardly perceptible compared to conventionally sprung vehicles. The effects of bumps in the road are reduced significantly.
In practice, it is a complex system. Transmitting the force of the electric motors to the suspension struts calls for an elaborate mechanical system. Via a belt drive, the motor transfers its torque to a strain wave gear set. From there, the torque passes via a hollow torsion tube to a titanium torsion bar, which pierces the gearbox and finally sends the torque via a lever to the suspension/axle system. Yet the force is not used to provide additional propulsion power—the Audi A8 has plenty of that already. Rather, at each of the four wheels, it serves to individually control and influence the vertical forces occurring between wheel and body. For this to happen instantaneously, even under extreme conditions—for example, when driving over a level crossing—corresponding force levels are required. Up to 350 kilograms can be added or subtracted at each wheel, which in turn calls for a corresponding amount of drive power. Each of the four electric motors produces 2 kilowatts, which in tandem with the installed mechanical system creates a fabulous torque level of 1,100 newton-meters. Joachim Schmitt: “The only car engine in the Audi Group that develops more torque than our system is in the Bugatti.” To allow a fast, efficient response, the system is fed by a 48-volt electrical system that supplies the actuator motors with the voltage they need even when requirements are high. Despite this, the overall system’s average power consumption in rapid driving on country roads is surprisingly moderate at around 200 watts, not least because energy does not need to be supplied all the time. In damper function, the system actually recuperates energy, with the electric motors operating as generators and the 48-volt battery acting as an internal energy storage system.
The result is truly impressive. As all four wheels can be controlled individually, vertical body movements are reduced to almost imperceptible levels, delivering a completely new ride sensation. Bumps in the road are ironed out without any rocking of the vehicle. Plus, thanks to the individual ride modes selectable via Audi drive select, the system allows a very wide spread between comfort and dynamic handling without detracting from the suspension’s excellent overall impression. “That has never been possible before in this form with any other suspension system,” says Schmitt. However, the active chassis only fully compensates for pitch motions during accelerating and braking, whereas it allows a certain roll angle during cornering—and for good reason: “Drivers still need to get a feel for how fast they are traveling,” says Joachim Schmitt. But he is in no doubt at all about the car’s superb handling qualities: “In terms of roll angle, the Audi A8 handles like an Audi R8—as crisply as with a super sport suspension but much more comfortably.”
Thanks to lightning-fast, wheel-selective reactions to sudden changes in the road surface, the Audi A8 handles like a dream. The active chassis responds to emergency situations by leveraging numerous parameters instantaneously in order to mobilize extra safety reserves.
The new Audi A8 virtually adds a new dimension to ride quality. The sedan takes even large cobblestones and bumps easily in its stride. Vorsprung durch Technik clearly makes its presence felt in numerous handling situations and speed ranges.