Everything is a compromise, something that is particularly true when it comes to making a car go around a corner. Whether front-wheel, rear-wheel, all-wheel or four-wheel drive, getting the right amount of power to each of a vehicle’s four wheels to get it to push, pull, twist, drag or persuade that vehicle to go around a curve is limited by one design element of another.
Without getting into a dry and long-winded diatribe on the good, bad and ugly of each configuration, Acura’s SH-AWD system attempts to optimize each wheel’s contribution to the cause by causing the outside rear tire to turn faster than the inside, creating a yaw moment by torque vectoring. Or more simply, making the vehicle rotate by pushing the rear corner forward.
Here’s how it works. Acura’s SH-AWD (for “super handling all-wheel drive”)is a full-time all-wheel-drive system, nominally a front-drive arrangement with a torque-transfer unit bolted directly to the front-mounted transaxle. For the tech savvy, Acura explains it this way:
“The torque-transfer unit receives torque from a helical gear that is attached to the front differential’s ring gear, and a short horizontal shaft and hypoid gear set within the torque-transfer unit’s case send power to the rear propeller shaft, which in turn transfers power to the rear drive unit.”
Without the techno-mumbo-jumbo, the driveshaft to the rear axle turns 1.7 percent faster than the front. There are electromagnetic clutches within the rear differential, one on each side, that basically allow the rear axle to (almost) freewheel when they’re not engaged. In practice, 90 percent of available torque goes to the front wheels under that condition.
In hard cornering and under acceleration, the clutches controlled as a pair can fully engage to send 70 percent of the torque to the rear axle, the faster turning rear axle overriding the front. By fully engaging the clutch on one side of the rear differential, 100 percent of the torque reaching the rear end can be sent to that side. The clutches also provide a limited-slip function side-to-side in low traction situations.
The Acura SH-AWD system is optional with the Acura RDX crossover and Acura TL sport sedan, and standard equipment with the Acura MDX sport-utility vehicle and Acura RL luxury sedan. The Acura RL, however, adds planetary gearsets in the rear to overspeed the outside rear wheel by 5.7 percent, increasing super handling effect, as it were, while the SH-AWD in the Acura MDX has “cooperative VSA”–it’s integrated into the MDX’s stability control system.
By combining the basic all-wheel drive function with the ability to overdrive the outside rear wheel, the Acura SH-AWD system merits not only the AWD portion of its name, but also the super handling of the SH. It’s more electronically/mechanically involved than ordinary all-wheel drive and therefore more expensive, but like we said in the first place, everything’s a compromise