Lucid AWD/4WD explanation

[dmiller@iea.com (Doug Miller)]

Rabid Wolves from Mars made Mike Marion say:

>4WD requires that the vehicle be taken out of 4WD on dry pavement.  In this
>case, the difference in tire speed is NOT allowed for within the drivetrain
>so must be accomodated EXTERNALLY by actually allowing the tires to slip
>(like when on ice, snow, or mud).
>
>DEEPER:
>
>In 4WD there is no center differential to allow for front to back speed
>differences, in AWD there is.  Most good AWD systems also allow for
>transmitting power to the wheels that have the most traction (a regular
>open differential actually sends most of the power to the wheel with the
>LEAST traction!).  Two good systems (my own judgement) are the viscous
>coupling and the Torsen (torque sensing) - both work fine and are reliable.

Mike, I am sure you are getting similar responses to this knotty issue, but
4WD vs AWD has blurred considerably over the years.  In particular, many
current 4WD systems can be run on pavement, and there IS allowance for
slippage both between tires on the same axle and tires on different axles.
Clouding the issue are differences between part time (often a no-slip
mechanical linkage in the center is engaged called a transfer case), and
full time systems (using a center differential in use full time).

Its a tough issue, and the two systems probably are better characterized by
their intended use.  Offroad systems tend to be heavily built, with gear
casings designed for dragging across rocks, generate significant friction
and mileage penalties, and found on high clearance vehicles.  Onroad
systems tend to be lighter, more compact, designed with more focus on
minimal friction, and found on vehicles intended for road usage. Beyond
that, there are so many combinations it defies categorization.  For
instance, our LandCruiser is a full time system like an AWD car, yet it was
designed for offroad use and has factory locking front, center and rear
differentials.

Just my two centavos.

Doug Miller
dmiller@iea.com
97A6Q Wagon, 93 LandCruiser w/diff locks (and well used skidplates), 71
LeMans Sport V8 Convertible. Cannondale Super V 900 Comp.