All-wheel-drive systems are becoming an increasingly popular feature for large sedans. They allow for more exciting driving dynamics than pure front-wheel-drive, but also provide for increased fair weather traction over pure rear-wheel-drive. Acura revealed their all-wheel drive system dubbed Super Handling All-Wheel-Drive (SH-AWD) in the 2005 RL. Since then, the system has been adapted to the MDX and RDX vehicles as well. It is a system with a name only a Japanese company could get away with, after all there is a Honda powered Formula 1 team named Super Best Friends. Acura is proud of their technological development. Their 2007 Los Angeles Auto Show booth featured a SH-AWD display front and center. We used the opportunity to snap some pictures of the complex unit in order to illustrate its unique features. Follow the jump to read more.
Acura's SH-AWD system for the RL is like any other system in that it distributes torque between the front and rear wheels. However, unlike other systems it can also distribute torque between both rear wheels. The front transfercase sends a maximum of 70-percent of the total torque output to the front wheels. This occurs in situations like straight-line cruising and low throttle cornering. Full-throttle straight-line acceleration sends a maximum of 40-percent of the total torque to the rear wheels. Hard cornering situations can see up to 70-percent of the total torque reach the rear wheels. This percentage of total torque can also be split between the two rear wheels, even sending all 70-percent to the outside wheel. The rear wheels can even be overdriven to spin up to five-percent faster than the front.
A rear view of the Acura RL underside and drivetrain. The rear differential is situated in the center, between the rear axles. Click the image for a high-res view.
The SH-AWD system's rear differential case houses three sets of planetary gear and clutch sets in a T configuration. The first set, coupled to the drive-shaft, is known as the Acceleration Device. This is the device that overdrives the rear wheels by spinning the output shaft at higher rpms than the input shaft. The other two sets are known as Direct Electromagnetic Clutch Systems. There is one for each of the rear wheels. They function as limited slip differentials, adjusting clutch plate pressure to control the amount of torque that reaches each wheel. To determine the clutch pressures for the rear differentials the engine ECU provides engine rpm, intake manifold pressure and transmission gear data and the traction control ECU provides lateral g, yaw rate, wheel speed and steering angle information to the SH-AWD ECU. The SH-AWD ECU then controls electronic solenoids that adjust hydraulic actuators to change torque split as necessary.
A close-up view of the rear SH-AWD differential case. The driver's side Direct Electromagnetic Clutch system can be seen through the acrylic cover on the left side of the photo. Click the image for a high-res view.
Acura's all-wheel-drive system is a design that actively fights understeer. The Acceleration Device can increase the wheel speed of the rear wheels so that they do not drag behind in cornering. This is because the rear wheels take a larger arc when cornering than the front wheels. For the front and rear wheels to complete an arc simultaneously the rear wheels need to be traveling at a faster rate. The Electromagnetic Clutch System allows for the SH-AWD system to also compensate for the different arcs the inside and outside wheels must take in a corner. The inside wheels take a shorter arc than the outside wheels, thus the rear outside wheel must spin the fastest to keep up. SH-AWD lets the outside rear wheel receive all rear torque in cornering conditions and it can turn at a higher rate of rotation than the other wheels, thus increasing vehicle stability.
Rumor has it that Acura will employ SH-AWD in the next generation NSX. However, it must be noted that the NSX has been a mid-engine, rear-wheel-drive package and the SH-AWD system is designed for a front-engine vehicle with a front-wheel-drive bias. The entire system is also extremely heavy. The current vehicles offering the system weight in at over 3,900 lbs, 4,000 lbs and 4,500 lbs for the RDX, RL and MDX respectively. It is apparent that every ounce of weight savings in the package is key, as the RL employs the use of a carbon fiber reinforced driveshaft. When the system shows up in the NSX design hopefully it will have the strength to operate 100-percent rear-wheel-drive with the SH-AWD components located in positions that make for the best front-rear weight distribution. It could create a package with the potential to give Nissan and its GT-R's ATTESA all-wheel-drive system a run for its money.
The above picture shows the lightweight carbon fiber reinforced driveshaft used on the Acura RL. Click the image for a high-res view.