Symmetrical All Wheel Drive

  What makes Symmetrical All Wheel Drive different from other all wheel drive systems? The answer is in the layout of the drivetrain. The word symmetry implies that one side of the drivetrain is a mirror image of the other and that is the concept behind the symmetrical all wheel drive system.
  Before I explain the method used to achieve the symmetry I will explain why the symmetry is important. When equals amounts of torque is sent to the four wheels but all of the wheels have different weights and elements acting on them the equality is reduced. Through years of research Subaru has developed the Symmetrical All Wheel Drive which is a perfectly (or very close to) balanced all wheel drive system.
symmetrical driveline
Symmetrical drivetrain of a Subaru Outback
  The Method
  Subaru uses the boxer engine and that starts the symmetrical layout as the left side of the engine is the same as the right side and it is longitudinally mounted (crankshaft goes from front to rear).
  The transaxle is also laid out longitudinally and two identical axle shafts emerge from the transmission housing, one on each side to drive the two front wheels. The driveshaft emerges from the rear of the transaxle as usual to drive the rear wheels.
  There are no external parts, axles or shafts on either the left or right to offset the symmetry as there are in other all wheel or four wheel drive systems. The entire drivetrain is along the center line of the vehicle and balances the weight and power distribution on both sides.
  The weight of the drivetrain is also spread out from front to rear. It may not be symmetrical from front to rear but it is better than many all wheel drive systems. Symmetrical all wheel wheel drive also detects wheelslip and the system directs power to the wheel or wheels with sufficient traction to further improve the system. This makes it an intelligent all wheel drive system.
  Its unfortunate that the entire vehicle can't be a perfect balance of symmetry as some things that you only need one of like the steering, steering rack, battery and so on is almost impossible to place in the center of the vehicle but the drivetrain being symmetrical is a vast improvement that has proven to be effective so far.
External Links
Detailed Explanation Discussion Video  
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