Everyone knows there are control and capability benefits of all-wheel drive when you’re dealing with the elements—come sand or high water. Dropping an engine’s torque down to the ground with four tire patches instead of two would give any vehicle more traction. But headed to market in more and more performance cars are new systems that can seamlessly and instantaneously distribute torque to any single wheel at a time. Welcome to torque vectoring all-wheel drive.
Most modern all-wheel-drive cars and SUVs already offer some type of computer-controlled, part-time engagement to save fuel. When the computer detects that one or more wheels is rotating faster than the vehicle’s speed or that the vehicle is yawing off its intended path of travel, the system steps in. First, it engages the other drive axle and applies a proportion of the vehicle’s torque to it. If the wheels continue to spin, the computer reduces engine torque or even brakes one of the wheels, if necessary
In recent times, these systems have taken a fairly radical step forward. Automakers have reinvented front and rear differentials to the point where an engine’s torque can be passed around—or vectored—to each corner of the car. In other words, your torque can go from front to back like a traditional all-wheel-drive setup and distribute from left to right on a given axle—all very, very quickly. It’s like having a computer-controlled, super-speed limited slip differential in each axle. This means not only great foul-weather traction but also eerily competent handling performance on dry roads.
Acura, for instance, has offered its Super Handling All-Wheel-Drive (SH-AWD) system for several years. It monitors vehicle speed, wheel speed, gear position, steering angle, yaw rate, lateral G forces and other inputs, while automatically adding torque to the outside rear wheel in corners to make the car turn quicker. A set of electromagnetic clutches in the rear differential passes the torque from side to side. The system, which normally distributes torque 90 percent up front and 10 percent in the rear, quickly changes to a 50/50 split during acceleration or hard cornering. The system can then send some or all of that 50 percent going to the rear axle directly to the outside tire to make the vehicle bend into a corner more sharply. Mitsubishi, a torque vectoring pioneer, has used a similar system called Active Yaw Control in the rear axle of its high-performance Evolution sport sedan since the late ’90s.
Audi, BMW and others are taking it a step further: While SH-AWD only works on the rear axle of a normally front-drive vehicle, new systems from automotive suppliers Ricardo in Britain and ZF in Germany can vector torque to all four tires simultaneously
The Ricardo Cross-Axle Torque-Vectoring system uses wet clutches and planetary gearsets, in both the front and rear differentials, that are controlled by electrical, electromechanical or electrohydraulic control systems. Ricardo says the system’s response time, from the push of the accelerator to the delivery of up to 90 percent of available torque, is only about 0.1 seconds. If Ricardo’s vectoring is used only in an all-wheel-drive vehicle’s center differential, the engine torque effectively gets passed around front-to-rear and side-to-side—with split-second accuracy—for every driving condition. Look for it in the new Audi A4 and A5
German transmission and driveline company ZF has also developed a torque-vectoring system, called Vector Drive—and it’s ready for volume production in all-wheel and rear-wheel drive vehicles. The system distributes drive torque individually to each of the rear wheels, generating a yaw movement around the vertical axis. This improves both cornering performance and vehicle stability in less-than-ideal road conditions. When driving straight, the torque vectoring rear axle behaves like an ordinary open differential. Drive torque is distributed equally to the wheels. Torque is only distributed individually along both halfshafts on an axle during cornering, controlled by an electromechanically actuated multi-disk brake. The ZF system also generates wheel differential torque independently of the drive torque. When cornering through a downhill section off the throttle, the outer wheel receives more drive torque than the inner wheel, allowing crisper turn-in. The gears of the planetary gearset don’t turn when driving straight, so the system saves fuel too. The torque-vectoring drive also acts like a positive-traction or locking differential on dry or uneven traction startups, with torque going to the wheel with higher friction potential
These new torque vectoring systems will undoubtedly join forces with the pre-existing ABS brakes, traction control, stability control, steering and rollover mitigation systems. The result will be smarter, safer and quicker vehicles, whether it’s on a rain-soaked freeway, a snowy driveway or a racetrack.
Ricardo Torque Vectoring
ZF Torque Vectoring