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"Making an all-wheel-drive car from a front-driver is relatively simple: take power off the transmission, send it to the rears, and voila! You now have a car that will plow through a foot of snow.  &amp;quot;Plow&amp;quot; being the operative word. It won&amp;#039;t want to turn\u2014this layout often does nothing to help mitigate understeer\u2014and it&amp;#039;ll be both slower and less efficient than the car you started with.  It&amp;#039;s taken a lot of engineering to make front-biased all-wheel-drive cars want to turn, and Honda perhaps did it best with its SH-AWD system. The SH, of course, means Super Handling, and it wasn&amp;#039;t a misnomer\u2014the system&amp;#039;s torque-vectoring rear differential could distribute power to one rear wheel or the other, simultaneously helping the car turn and taking best advantage of the available traction. It&amp;#039;s a transformative system.  Go ahead, take an Acura ZDX onto a race track. You&amp;#039;ll be astonished as the big, front-heavy brute flings itself neutral on its way out of every corner. As long as you&amp;#039;re feeding power to the rear wheels, the moon rover-looking thing goes immediately neutral. Surprise, but the ZDX is more throttle-steerable than some sports cars.  Oh, so you&amp;#039;ve never considered taking an Acura ZDX on track? You&amp;#039;re not alone there, which is perhaps why so few people realize just how good the SH-AWD system is. The people who actually notice the benefits from it tend to be, say, drivers of the Acura MDX\u2014soccer moms who just want their car to get them around town in a snowstorm.     And it&amp;#039;s in the around-town driving where SH-AWD, like every AWD system, has its drawbacks. Its extra weight and drag means slower acceleration and more fuel consumption.  Until now. Honda has just solved those two dilemmas by taking the SH-AWD torque-vectoring philosophy and adding electricity. The system debuts on Acura&amp;#039;s new flagship, the RLX, and it&amp;#039;s called Sport Hybrid SH-AWD.  Up front, things change around a little. The front-drive RLX&amp;#039;s new 3.5-liter direct-injection V6 stays but earns revised cam profiles for smoother start/stop. The base car&amp;#039;s six-speed automatic is ditched in favor of Acura&amp;#039;s first dual-clutch auto. The unit, which is the same size as the automatic, also contains a 47 hp motor/generator that replaces both the starter and the alternator. And, of course, works to assist the V6&amp;#039;s output.  Energy is stored in a 66-lb, 1.3-kWh lithium-ion battery stored over the rear wheels (and reducing trunk space considerably.) The battery is also connected to the TMU at the rear, the Twin Motor Unit, which is the most important piece in the new RLX.  Weighing only around 130 lbs, the device contains two 36 hp electric motors, each connected to a rear wheel. The system can operate the two motors independently, which means they can both accelerate the car or slow it down and regenerate electricity in the process. But they can do one thing no other hybrid-electric system can: continually vary the power of the wheels independently from one another.     One wheel could, for example, be at full regen while the other is at full power up to a maximum of about 1000 lb-ft of differential torque. This creates a significant yaw moment on the car. That&amp;#039;s a fancy way of saying &amp;quot;it&amp;#039;ll turn the car using the rear wheels.&amp;quot;  Best of all, the system doesn&amp;#039;t need engine power to operate, so it can adjust the attitude of the rear of the car under acceleration, cruising, deceleration, or even with the gasoline engine switched off. (The ZF torque-vectoring differential in some BMWs and Audis accomplishes the same thing.)  Part of the beauty of the system is that the motors are fairly small, made possible by gearing them down. A planetary gearset couples the motors to their respective wheels at a 10.38:1 ratio. The problem with that is that at about 78 mph, as the motors reach 11,000 rpm, they begin to produce too much voltage for the electric systems to handle reliably.  The solution is to decouple the motors from the rear wheels at high speeds\u2014which is also a boon to efficiency. (And let&amp;#039;s be honest\u2014you don&amp;#039;t actually need four-wheel drive at high speeds. If you&amp;#039;re spinning your front wheels at 80 mph, you may as well start dialing 9-1-1.)  Decoupling the motors from their wheels, though, would eliminate the SH-AWD&amp;#039;s system from performing torque-vectoring\u2014and that would change the RLX&amp;#039;s cornering behavior dramatically. The solution is to remove the motors&amp;#039; ability to assist or regen (and thus slow their rotational speeds) but not remove their ability to remove torque from one side and add it to the other.     And that&amp;#039;s where things get crazy. The TMU&amp;#039;s two motors operate on a single planetary gearset with a single ring gear that&amp;#039;s held in place by an electrohydraulic clutch. Each motor&amp;#039;s torque enters through its own carrier gear, is multiplied, and then heads to the axle via that side&amp;#039;s sun gear.  At 78 mph, the clutch releases, allowing the ring gear to spin. However, thanks to the miracle of planetary gears, if the motors exert different levels of torque on their respective carrier gears, they experience equal and opposite forces. Meaning the system can transfer torque from one side to the other\u2014albeit only in equal measures. Key point: torque-vectoring is still possible.  Bigger point: the system is genius.  Even bigger point: You neither hear nor feel any of this happening from the driver&amp;#039;s seat. Hybrid-system operation continues, just using the front motor to assist and to regen, and when speed descends through 75 mph, the rears are recoupled.  The takeaway is that the all-wheel-drive RLX is more powerful than the base car (377 total system horsepower versus 310 hp), it remains impressively neutral in corners (and the base, front-wheel-drive car uses active rear steering for neutral cornering, so it wasn&amp;#039;t an understeering pig to begin with), and it gets better fuel economy. EPA city jumps from 20 to 28 mpg, and even highway mileage increases by 1 to 32 mph.     It&amp;#039;s a win-win.  Except for a few things. For one, the blended brake system has a grabby, non-linear and slow-to-respond brake pedal. Second, the battery pack takes up valuable trunk space, decreasing overall cargo capacity from 15.3 cu ft to 12, and there&amp;#039;s no pass-through for large objects.  Oh, and there&amp;#039;s one last point: This all-wheel-drive concept needs to make its way into cars where it matters. The forthcoming NSX will use a similar system. In the meantime, it&amp;#039;s a shame that hybrid SH-AWD, like its mechanical predecessor, will likely never makes its way into the lexicon of most enthusiasts. Because they probably won&amp;#039;t be buying a derivatively styled, twenty-headlight luxury sedan. Those who do buy the RLX SH-AWD will probably just tell their friends, &amp;quot;It goes well in snow and gets good gas mileage.&amp;quot;  Sure. But they&amp;#039;ll have missed the point. "^^<http://www.w3.org/2001/XMLSchema#string>
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