martinbo
Staff member
As we approach the twilight of mainstream ICE passenger vehicles, it remains interesting to see new innovations within the legacy drivetrain concept. For the forthcoming RS3 performance model, Audi introduces a new take on the rear axle differential: one that eschews the traditional ring-and-pinion arrangement for a purely electronic torque distributing unit, simply titled "RS Torque Splitter". So, for us mechanically-oriented old fogies, here's a fresh take on an age old challenge.
06/22/21 | Ingolstadt | Models
TechFocus: torque splitter
The Audi RS 3 prototype represents the epitome of unadulterated driving dynamics. This is Audi’s first vehicle to feature the RS Torque Splitter, which distributes drive torque between the rear wheels in a fully variable manner. This results in optimal stability and maximum agility – especially when cornering at high speeds.
How does the RS Torque Splitter work?
The RS Torque Splitter makes active, fully variable torque vectoring between the rear wheels possible. Unlike the rear axle differential and the previous multiple disc clutch package on the rear axle, the torque splitter uses one electronically controlled multiple disc clutch each on the respective drive shaft. During dynamic driving, the torque splitter increases the drive torque to the outer rear wheel with the higher wheel load, which significantly reduces the tendency to understeer. In left-hand curves, it transmits the torque to the right rear wheel, in right-hand curves to the left rear wheel, and when driving straight ahead to both wheels. This results in optimal stability and maximum agility – especially when cornering at high speeds. When driving on closed roads, the torque splitter enables controlled drifts by applying all of the engine power at the rear axle to just one of the rear wheels – up to a maximum of 1,750 newton meters of torque. The exact distribution of drive torque always depends on the mode selected in Audi drive select and the respective driving situation.
Each of the two multiple disc clutches has its own control unit, which use the electronic stabilization control’s wheel speed sensors to measure the wheel speeds. Other influencing factors include longitudinal and lateral acceleration, the steering angle, the position of the gas pedal, the selected gear, and the yaw angle, i.e. the rotational movement around the vertical axis. In addition, the torque splitter is connected to the modular vehicle dynamics controller as a higher-level entity.
Why does the torque splitter improve driving performance?
Due to the difference in propulsive forces, the car turns into the curve even better and follows the steering angle more precisely. This results in less understeer, earlier and faster acceleration when exiting corners, and particularly precise and agile handling – for added safety in everyday use and faster lap times on the racetrack. The torque splitter also compensates for oversteer by directing the torque to the wheel on the inside of the curve or, if necessary, to both wheels.
How is the torque splitter integrated into the driving dynamics system?
Audi drive select modifies the characteristics of the torque splitter and thus the car’s handling depending on the selected mode. Five characteristic curves – Comfort/Efficiency, Auto, Dynamic, RS Performance, and RS Torque Rear – are saved in the system. In this context, engine power is distributed to all four wheels in the Comfort/Efficiency modes, with priority given to the front axle. In Auto mode, torque distribution is balanced, which means that the RS 3 prototype neither understeers nor oversteers. Dynamic mode, on the other hand, tends to transmit as much drive torque as possible to the rear axle – for maximum agility and increased dynamics. This is taken to perfection in RS Torque Rear mode, which allows drivers to perform controlled drifts on closed roads. The highly rear-heavy distribution of the engine power leads to oversteering behavior, with up to 100 percent of the drive torque directed to the rear ending up at the wheel on the outside of the curve. In addition, Audi has adjusted the engine and transmission characteristics. This specific setup is also used by RS Performance Mode, which is designed for the racetrack. It is specially tuned to the Pirelli P Zero “Trofeo R” performance semi-slick tires. In this mode, the torque splitter delivers a particularly dynamic, sporty ride along the longitudinal axis with as little understeer and oversteer as possible. This results in fast acceleration out of corners and thus better lap times.
What role does the modular vehicle dynamics controller play?
The modular vehicle dynamics controller (mVDC) ensures that the chassis systems interact more precisely and more quickly. To do so, this central system captures data from all the components relevant to lateral dynamics. The mVDC synchronizes the torque splitter’s two control units, the adaptive dampers, and the wheel-selective torque control for high-precision steering and handling. All in all, it increases the agility of the entire vehicle, especially on dynamic stretches of road.
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