How do you give a large SUV sporty handling properties and minimal body roll without compromising driving comfort? Audi has resolved this conflict of objectives with electromechanical roll stabilization (eAWS – elektromechanische Aktive WankStabilisierung). Assisted by the 48-volt on-board electrical system and powerful actuators, the stabilizers on the front and rear axles can be actively controlled depending on the driving situation. As a result, the models maintain their high level of comfort when driving in a straight line. In contrast, in cornering and load-shifting situations, they impress with improved lateral dynamics combined with minimal body roll. The technical advantages of Audi's electromechanical solution are that it is energy efficient, operates in near real time and requires no maintenance due to the absence of hydraulic components.
What challenges do large SUV models pose for chassis engineers?
Large SUV customers are delighted with the many practicalities they offer, from a generous cabin space, advanced chassis technologies to powerful engines and advanced control and assistance systems. In addition, an SUV offers superb off-road performance. Due to their design, these vehicles have a higher curb weight and a higher center of gravity. This means that an SUV's body leans more outward in the corners than models with a lower center of gravity.
What technology counterbalances the body roll and movement of the car?
When cornering, the car tilts outward due to centrifugal forces, i.e. the wheel outside the bend compresses while the one inside the bend relaxes, the vehicle rolls around its longitudinal axis. Torsionally flexible anti-roll bars between the left and right sides of the axle are proven to compensate for this effect. They reduce the tendency to roll by applying reverse torque to the suspension on the outside and inside of the turn, neutralizing the tendency to roll. This passive suspension component has the same effect when cornering and driving in a straight line. However, the desired effect when cornering can adversely affect straight-line driving comfort on roads with bumps or potholes on one side of the road. While passive solutions are reaching their limits here, Audi has resolved this conflict of objectives with electromechanical roll stabilization. By using sensors to measure and detect the driving situation, the system will only intervene when less body roll is desired, with pinpoint accuracy. Thus, the rate of flexibility of the stabilizers on uneven and straight roads is lowered to a basic level, the spring and shock absorber forces act largely independently on the left and right wheels.
How does electromechanical roll stabilization work?
A conventional stabilizer works passively, that is, it simply balances the suspension movements on both sides by means of a mechanical coupling. On the other hand, the electromechanical stabilization of the roll can be specifically controlled. The system consists of two axle stabilizer halves, with an electric motor running between them on the front and rear axles. It can rotate the stabilizer halves in opposite directions from each other and thereby generate torque that counterbalances the roll torque, individually and on each wheel. Therefore, it reduces roll angles and actively supports them against the physical effects of the driving situation. The system receives its commands via control units on the front and rear axles, which are part of the Electronic Chassis Platform (ECP). The ECP is the central brain of the chassis. Within milliseconds, it harmonizes a variety of parameters such as speed, ride height, the car's roll and pitch movements, the coefficient of friction of the road surface, current driving conditions such as understeer or oversteer, as well as the chassis system data involved. From this input, the system calculates the ideal responses for the integrated components and adjusts them quickly and precisely to each other. The necessary electrical power is supplied to the eAWS by a powerful 48-volt on-board electrical system. Within milliseconds, the system calculates the correct actuation values for the stabilizers. The electric motors deliver their power through three-stage planetary gears, with torque levels of up to 1,200 Nm generated at the stabilizers.
What is "Vorsprung durch Technik" in the case of an electromechanical solution?
The 48 volt system allows for immediate system response, even at low speeds. The lag time between when the sensors detect body roll and the response of the electric motors is only a few milliseconds. Unlike hydraulic solutions, the environmentally friendly electromechanical system does not require oil circuits and requires no maintenance. It is even able to recover energy by picking up the suspension pulses on its electric motor, converting them into electrical energy, and storing it in the on-board electrical system's lithium-ion battery. The electromechanical solution also uses energy more efficiently. Unlike hydraulic systems, it does not need to store and supply pressure.
How does the driver benefit from the system?
The system reduces the tendency to roll, gives a sportier and more confident driving impression and thus underlines the versatile character of the wide range of Q models. It can actively distribute the roll torque to the front and rear wheels and thus influence the characteristics. intrinsic steering characteristics such as the tendency to understeer or oversteer. The Audi drive select dynamic driving system offers various configuration options for this purpose. The electromechanical active roll stabilization gives the driver a dynamic and precise feel in all situations and offers improved handling characteristics. This is one of several systems that perfect the dynamism of the top-of-the-range Q-series models. The Q7, SQ7, SQ8 and RS Q8 models with their controllable stabilizers always respond precisely to the driving situation intended by the driver. On uneven roads, vehicle movements are reduced while driving comfort increases. In sporty driving and at high speeds in the corners, the car is more stable and comfortable. It takes place in the turns. Audi has deliberately chosen a configuration which does not completely neutralize the roll angle but which continues to give an authentic feeling of the driving situation.
How is the system accepted in reality?
Proof of the benefits of controllable stabilizers was provided by test and racing driver Frank Stippler in the fall of 2019. As part of the development work of the Audi RS Q8, the professional racing driver, who in 2019 won the 24 hour Nürburgring race for the second time with Audi, set a new lap record for production SUVs. He managed to cover the 20.832 kilometer track in the Eifel region in just 7:42 minutes. The system is also in great demand on a daily basis. 40% of all Audi customers who ordered a model from the Q series equipped their model with active electromechanical roll stabilization.