GKN’s DR Ray Kuczera discusses the Twinster AWD system on the Ford Focus RS
For the Mk3 Focus RS, driveline expert GKN developed a new version of its Twinster torque vectoring all-wheel drive (AWD) system. GKN Driveline’s Vice President of Global Product Technology, Dr Ray Kuczera, explains what drivers need to know about the system.
What’s so special about the Ford Focus RS AWD system?
The Ford Focus RS uses a GKN AWD system with a rear drive module (RDM) that utilises our Twinster twin clutch system. The Twinster can apply torque to one or both of the rear wheels independently, enabling the vehicle’s dynamic torque vectoring functions across its entire speed range.
What’s different is that the Twinster in the Focus RS uses new gearing ratios that deliver more torque to the rear wheels. When AWD is engaged, the Twinster drives the rear wheels faster than the front.
Over-speeding the rear wheels alters the way the car feels and handles. In curves, the Twinster makes the vehicle turn in more sharply, responding more immediately to the driver’s inputs.
In Ford’s “drift mode”, the AWD system delivers even more torque to the rear axle, delivering enough torque to the rear wheels for the RS to achieve a controlled “drift” through corners.
How does the RS system compare to brake-based torque vectoring?
Some vehicles use the brake system to achieve simple vectoring effects but these can actually slow the vehicle down. For fully dynamic torque vectoring, you need to be able to apply increasing tractive force directly to individual wheels. This produces purer performance and feel – and also saves your brakes.
How does the Twinster work?
The clutches in the Twinster RDM are wet and are actuated hydraulically. GKN’s AWD software sits on a dedicated electronic control unit, controlling the Twinster’s hydraulics and solenoid valves to continuously vary the pressure at each clutch pack.
The software updates the hydraulic control settings 500 times per second to provide quick, accurate torque control. As a result, the clutches are continuously moving to controlled states between fully open and fully locked.
How do the drive modes of the Focus RS affect the AWD?
The RS has four different drive modes – Normal, Sport, Track and Drift – and these determine the AWD system’s torque control profile. The system responds to each mode using some unique mode-dependent subsystems and a lot of multi-axis calibration tables, developed by GKN’s vehicle dynamics experts.
When the driver changes mode, the software enables some new subsystems, disables others, and references new columns and rows in our calibration tables. These change the level of torque, the rate at which torque increases or decreases, and the left-right torque biasing.
How exactly does Drift mode work?
The fine details of the Focus RS Drift mode are closely guarded intellectual property. All I can say is that our software contains a unique subsystem that is only enabled in Drift mode.
The software helps modify the torque controls in ways that make it easier to induce on-throttle over-steer and then to maintain side-slip angle while counter steering. Creating the desired yaw behaviour is a combination of calibration changes that modify the level of torque applied, how quickly it gets applied, and how much delta / difference is created between the inner and outer rear wheels.
What does the future hold for AWD and torque vectoring?
AWD is becoming the driveline of choice for performance cars because it’s the most effective way of putting power onto the road. Iconic vehicles like the Focus RS, the Porsche 918 Spyder and the BMW i8 show how innovative drivelines are rewriting the rules. We expect torque vectoring and the Twinster to feature in more and more vehicles in the coming years – drive the Focus RS and you’ll understand why.