DETROIT --- The 2010 Cadillac SRX is launching with an advanced all-wheel-drive system (AWD) that delivers higher levels of driver control.
The intelligent, active system continuously distributes engine drive torque between the front and rear axles, improving handling, stability and grip in all driving conditions -- from fast corners to slower curves, in dry or wet weather, General Motors said.
"The 2010 SRX will deliver excellent, balanced driving dynamics in all types of weather," said Bob Reuter, SRX global vehicle chief engineer. "The all-wheel-drive system with electronic limited-slip effectively transfers torque not only from front to rear, but also along the rear axle."
In honing the integration and performance of the system, Cadillac engineers completed more than 600,000 miles of testing in all weather conditions across some of the most challenging roads in Europe, as well as test tracks in Italy and Spain and the famed Nurburgring circuit in Germany, where Cadillac has developed and validated new vehicle features in recent years. The result is an active, all-wheel-drive system --specifically tuned for the SRX -- which exploits the power of the vehicle's 3.0L direct injected V-6 engine that delivers 265 horsepower (198 kW) at 6,950 rpm and 223 lb.-ft of torque (302 Nm) at 5,100 rpm.
To optimize traction at take-off, GM said the system incorporates a pre-emptive engagement of the rear wheels. Unlike conventional all-wheel-drive systems, this system eliminates the need to detect front wheel slip before rear drive is activated.
On the road, the AWD system operates seamlessly, GM said, and is programmed to help the driver keep the car stable, splitting drive torque to counteract oversteer or understeer situations when cornering.
The electronic control unit receives data inputs 200 times every second from more than 20 sensors in the vehicle, including those for the ABS and ESC (electronic stability control) systems. The data processed includes vehicle speed, wheel rotation speeds, lateral acceleration, throttle setting, engine torque, yaw rate and the steering angle. Those inputs are used to continually adjust the torque distribution in response to the driver's input of steering and throttle, the traction that is available at the individual corners of the vehicle or the intended versus the actual path of the car.
For example, on dry pavement during wide-open throttle, 50 percent of the torque is delivered to the rear, providing acceleration with no wheelspin. On slippery surfaces, torque is delivered to the tires with traction, up to 100 percent in cases where one end of the vehicle is on glare ice. Highway cruising conditions will reduce torque delivery to the rear to 5- to 10-percent, decreasing driveline drag and optimizing fuel economy. In most other driving conditions, the torque varies between 20 percent and 50 percent to the rear axle.
The precise balancing of the drive torque reduces ESC throttle and braking interventions, providing smoother and more predictable performance, GM said.
Perhaps the most significant aspect of this system, however, is the addition of an active rear limited-slip differential (eLSD). The eLSD can transfer up to 85 percent of maximum rear torque between the rear wheels, to whichever has more grip. Under hard cornering, or when completing a high-speed maneuver, such as a lane change, the application of more or less torque to either wheel helps the rear of the car more closely follow the direction of the front wheels.
The new 2010 SRX's AWD system is integrated with new chassis settings and revised self-leveling rear suspension geometry that delivers a 50-percent increase in camber stiffness. Springs, dampers and the steering system have been specifically tuned for pleasing ride dynamics and good feedback, GM said. In addition, the 2010 SRX features tires with a higher speed rating.
The SRX all-wheel-drive hardware consists of a power take-off unit (PTU) in the front final-drive that transmits engine torque through a prop-shaft to the rear drive module (RDM) that includes a torque transfer device (TTD) and the optional eLSD. Both are wet, multi-plate clutch units from Haldex.
The TTD is activated as soon as the vehicle is placed in gear. RDM clutches are applied and ready to transfer torque before acceleration begins. This pre-emptive function leapfrogs many current technologies that require the detection of wheel slip or driveline rotation before the rear drive is activated, GM said. The enhanced functionality provides better traction immediately for smooth, strong acceleration from a standstill without the possibility of any drivetrain hesitation.
During driving, torque delivery between the axles is varied by a valve in the TTD, which increases or reduces the hydraulic pressure on the wet clutch plates to progressively engage or disengage rear drive. The degree of slip determines the amount of torque transmitted. The eLSD, located alongside the RDM, operates on the same principle as the larger TTD. The electronic control unit functions in concert with the engine, transmission and ABS/ESC control modules.
The 2010 SRX’s AWD system requires the factory installation of a specific rear sub-frame to carry the RDM, as well as the revised rear suspension geometry and new wheel hubs for the drive shafts. The three-piece prop-shaft runs through two bearings with constant velocity joints for smooth running with minimal "wind up," GM said. With the AWD system, the SRX's wheelbase and rear track dimensions are unaltered from FWD models.