Re: Questions on AWD, ABS, and locking the rear differential

[Frank Deutschmann <fhd@interport.net>]

Alan Dowless/SYBASE sez:
> AWD also uses limited slip differentials, this means that the wheels do not 
> have to turn at the same rate enabling power to be moved to the wheels with the 
> most traction.

Actually, all diffs (open and limited slip) allow different rates of rotation
on the two output shafts -- that is the purpose of the diff.  AWD does not
depend on a limited slip -- in fact, most Quattros throught the years were not
equipped with a limited slip on either the front or rear axle; most Quattros
were/are equipped with open diffs front and rear.

An open diff will actually transmit more torque to the wheel with the least
traction; a limited slip diff limits the applied torque differential.  A third
type of diff, the TORSEN (for TORque SENsing, patented by the Gleason Gearworks)
actually does direct more torque to the wheel with more traction.  The Audi
Quattro system uses a Torsen between the front and rear axles to optimally
split the torque front/rear.  (Actually the above is a massively simplified
explanation of torque distribution, so no physics lessons, please!)

AWD differs from part-time 4WD in that AWD employs a diff -- typically a Torsen
or other (viscious [sic] coupling) torque-sensing diff between the front and
rear axles, whereas 4WD uses a fully locked diff (then reffered to as a transfer
case) between the two axles.  with no diff between front and rear, the 4WD
depends on wheel slip during cornering to allow different rotational speeds for
the front and rear axles.

>  Some 4-wheel drive trucks 
> have the wheel lock mechanism on the center hub of the wheel.  

On a 4WD, the hub lock mechanism is *not* a diff lock.  (Most 4WD's are equipped with
ordinary open diffs on both ends, sometimes with limited slips on the rear, but never
on the front, at least from the factory.)  The hub lock mechanism actually is 
used to disconnect the front wheels completely from the front diff (the wheels
freewheel, regardless of what the diff is doing).  This saves wear and tear on
the front diff and forward drivetrain; it is not strictly necessary to disengage
the hub locks, but leaving them engaged will cause wear and waste fuel.  4WD's
without external hub locks typically use a single vacuum-actuated hub lock to
disconnect one wheel from the front diff; while easier to operate (no need to leave
the warm and cozy cab), they are prone to problems, and cause additional wear.  I
actually replaced the entire front axle of my '87 Jeep with a Dana unit that had
external hub locks for this reason (among others), but thats another story...

-frank
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