2005 AWD Family Wagons AWD Systems

What's The Diff?...cont

The Volvo uses a system that requires the detection of wheelspin before redirecting driving power--with a difference. If the XC70, primarily a front-drive vehicle with 95-percent front-torque bias, detects as little as one-seventh of a front-wheel rotation (relative to the other wheels), it'll grab the spinning driveshaft with a clutch that in turn provides the rear axle with torque. This Haldex system doesn't rob the engine of unuseable power and convert it to heat (as in the Cadillac), but sends as much as 95 percent of it to the rear wheels where it can be used.

The Subaru B9 Tribeca features an electronically controlled clutch that acts on a planetary center differential to constantly fluctuate front/rear power distribution. Under normal conditions, the split is 45-percent front/55-percent rear. However, the system is predictive and, using a series of sensors to measure throttle input, steering angle, yaw rate, wheel speeds, and lateral g forces, compares the vehicle's stability with the driver's intended direction. When these two don't match, the first line of defense is to reroute power to a max of 50/50 front rear. If this still doesn't make the prediction match reality, the stability system begins to apply brakes selectively to keep the vehicle on its intended path.

The Subaru was the only vehicle here in which the stability system remained active when switched off and became the limiting factor in the slalom test. There was also a nanny lurking in the Dodge, but it was calibrated to activate well outside the limits of a skilled driver. The Subaru and Dodge stability systems were active (on or supposedly off) in the skidpad portion of the wet and dry figure-eight tests.