SUPER HANDLING ALL-WHEEL DRIVE™ (SH-AWD®)
The TL SH-AWD® features Acura's acclaimed Super Handling All-Wheel Drive™, which progressively distributes optimum torque not only between the front and rear axles but also between the left and right rear wheels. The benefits of the system are not only superior all-weather traction but neutral, accurate steering while under power that is unmatched by front drive, rear drive or conventional all-wheel-drive systems. The SH-AWD® system used in the 6-speed manual transmission version of the TL features unique parameters to account for the manual transmission.
By utilizing torque vectoring, and by rotating the outside rear wheel faster than the front axle speed while cornering, SH-AWD® generates a yaw moment that helps turn the vehicle through the corner—thus reducing understeer and improving handling balance and controllability. With cornering forces more evenly distributed between front and rear tires, overall cornering grip is increased.
Vehicles with high power ratings using conventional front or rear drive systems commonly use a limited-slip differential to maintain traction under power. By linking inside and outside drive wheels, these systems tend to resist turning and to increase understeer—working against the front tires as they attempt to turn the vehicle. Conventional all-wheel-drive systems similarly work to link the inboard and outboard tires along with the front and rear axles that create resistance to turning. Using torque vectoring to help turn the vehicle, SH-AWD® creates a more responsive, neutral and predictable platform while retaining the usual benefits of all-wheel drive.
Electronic Controls and Parameters
The control logic for SH-AWD® is integrated with the TL's Vehicle Stability Assist™ (VSA®) ECU. The Powertrain Control Module (PCM) provides information on engine rpm, airflow and transmission gear-ratio selection, while the VSA® ECU provides wheel-speed data. For the 6-speed manual transmission TL, the VSA® incorporates revised parameters based on the more sporty driving nature of the vehicle. The SH-AWD® ECU also monitors steering angle, lateral G-forces, yaw rate, and electromagnetic clutch engagement for the right and left rear axle shafts. Drive torque is calculated from PCM information and then the acceleration situation, wheel spin, lateral G forces and steering angle are used to determine the front-to-rear torque distribution along with the torque split between right and left rear wheels.
SH-AWD® operating parameters include:
- Up to 90-percent of available torque can be transferred to the front wheels during normal cruising.
- In hard cornering and/or under acceleration, up to 70-percent of available torque can be directed to the rear wheels to enhance vehicle dynamics.
- Up to 100-percent of the torque sent to the rear axle can be applied to either rear wheel, depending on conditions.
SH-AWD® System Layout
The TL SH-AWD® is a full-time all-wheel-drive system that requires no driver interaction or monitoring. A torque-transfer unit is bolted directly to the front-mounted transaxle. 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.
The TL SH-AWD® rear drive unit is constantly overdriven by 1.7-percent and the resulting overdrive effect is regulated by left and right electromagnetic clutch packs which independently control the power delivered to each rear wheel.
Direct Electromagnetic Clutch Systems
Two direct electromagnetic clutch systems mounted on either side of the hypoid gear that drives the rear axle control the amount of torque sent to each rear wheel and provide a limited-slip differential function when needed. The clutches can be controlled as a pair to alter front/rear torque split or they can be controlled independently to allow 100-percent of rear axle torque to go to just one rear wheel.
An electromagnetic coil modulates the pressure controlling each clutch pack, which changes the speed of the sun gear within a planetary gear set and controls the torque sent to the wheel. Electromagnetic control of the clutches permits torque delivery to each rear wheel to be proportioned in an exceptionally quick and precise manner, which reduces wheel-slip in low-traction conditions.
The clutch packs and their friction material are carefully designed to withstand the small amount of continuous slip between front and rear axles created by the 1.7-percent speed differential while delivering the durability expected of an Acura product. Through an oil-temperature sensor, the PCM monitors the clutch-plate coefficient of friction (which changes with temperature) in each clutch pack and then adjusts the voltage to each electromagnetic coil to compensate. As miles accumulate, the PCM uses data from a feedback loop to adjust the voltage supplied to the electromagnetic coils to compensate for any clutch wear, ensuring optimal and consistent transmission of torque.