How does AWD work?

[Creative]

This may seem a stupid question, but can someone explain how AWD on my 99
Outback works.
Does AWD mean full time 4wd? or FWD with rear cutting in when required? etc
etc etc.


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Ian Wharton
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[John Reece]

http://www.subaru-global.com/about/awd/index.html

http://www.drive.subaru.com/SubaruDrive-Sum02/AWD-Demystified.asp

 

[ceraboy]

Those links are the typical "transfer power from wheels that slip to
wheels that grip" TV commercial information. The AWD system depends
greatly on the type of transmission, manual vs automatic. The manual
transmission has a viscous coupled center differntial giving 50/50
power to the front and back wheels (capable of approximately 80/20 or
20/80 depending on tire slippage). The front and rear differential are
still slip type which means in the end only one front wheel and one
rear wheel will get the power. A rear limited slip differential was
available on some models. The automatic transmission uses a wet
transfer clutch integrated in the transmission which supplies 90% power
to front wheels and 10% to the back under normal conditions (this can
change from 90/10 or 10/90 given certain conditions). I did hear
somewhere that the newer subie autos changed the normal transfer ratio
from 90/10 to 60/40, but am not sure on that one. The auto uses
sensors on the each of the wheels to detect tire slip and adjust power
transfer whereas the manual is purely mechanical. Over time the
transfer clutch can wear out and fail to engage when front wheel
slippage is detected (rendering your AWD vehicle a mere front wheel
drive), I have also heard some stories of the transfer clutch binding
and causing troubles with cornering.

 

[David]

The front and rear differential are
still slip type which means in the end only one front wheel and one
rear wheel will get the power.

In an open front or rear differential, torque is applied evenly to both sides, until one starts to slip.
Then it becomes 100/0 in favor of the spinning side.

 

[Rob Munach]

In an open front or rear differential, torque is applied evenly to both sides, until one starts to slip.
Then it becomes 100/0 in favor of the spinning side.

... but the torque is still applied equally. The spinning wheel gets
essentially no torque and neither does the stationary wheel - otherwise
you would be accelerating. By definition, an open diff always applies
equal torque to both wheels. A locked diff or solid axle provides torque
to each wheel in proportion to the traction.

 

[David]

.. but the torque is still applied equally.

I think you got me there.