Date:         Fri, 12 Apr 1996 16:48:28 -0400
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         SyncroHead@aol.com
Subject:      Differentials: open, limited-slip, & locking (was: limited slip rear end)

In a message dated 96-04-12 06:28:01 EDT, raulb@iglou.com (Raul Barreras)
writes:
>
> Maybe I lost something in the translation but....
>
> Correct me if I'm wrong but...installing this locking mechanism to the rear
>differential does not a 4 wheel drive make, maybe a REAL 2 wheel drive or in
>some other worlds also called positraction. My ex-Jeep had 2 differentials!

Here's the differential deal,

Vehicles have differentials to allow wheels to operate at different rates of
speed.  Typical cars, vans, & trucks all have differentals that are called
"open".  These allow for no more torque to be supplied to a wheel with
traction than to the wheel that is slipping.  This works fine when conditions
allow traction at both drive wheels, but when either drive wheel looses
traction the trouble begins.  A "2WD" vehicle with an open differential could
arguably be characterized as a "one-wheel-drive" when either drive wheel
loses traction.

2WD with open differential:  When one drive wheel loses traction the vehicle
cannot move since no more torque can be supplied to the wheel with traction
than to the slipping wheel.

4WD with two open differentials:  Same as on 2WD except that each axle has a
chance at getting traction.  In this case if either front wheel AND either
rear wheel loses traction, the car cannot move.  This often happens in an
off-road situation when the vehicle becomes supported diagonally on one front
wheel and the opposite side rear wheel.

LIMITED SLIP DIFFERENTIALS
A small step up from the "open" differential is a "limited-slip"
differential.  In the case of the limited slip differential, some useful
torque may be applied to the non-slipping wheel and motion is possible.
 These are called limited slip because they allow a limited ammount of
slippage between the wheels.  Typically this amount of slippage that they
allow is quite high and are of little value in off-road or truely slippery
conditions.  Limited-slip differentials typically employ some form of clutch
mechanism to create the limited-slip effect.  The best use of a limited-slip
differential is on pavement where one is attempting to accelerate very
quickly and both tires have decent but slightly different traction.  The most
effective variety of limited-slip differential is the TORSEN (TORque SENsing)
differential.

2WD with limited-slip differential: If one drive wheel completely loses
traction the vehilce will not move.  The amount of torque needed to be passed
to the wheel with traction will exceed the differential's "limit" and you go
nowhere.  If one drive wheel has marginal traction, you might be able to move
if the traction is sufficient enough as to not exceed the slip limit of the
differential.

4WD with two limited-slip differential: This is a highly unusual arangement.
 I believe that late model Audi Quattros use TORSEN differentials at both the
front and rear (center also perhaps).  If one wheel on each axle completely
loses traction the vehilce will not move.  However if you have no traction at
one axle and marginal traction at the other, this arrangement can get you
moving if the traction is sufficient enough.

LOCKING DIFFERENTIALS
In off-road or snow conditions, this is a BIG step up from an open
differential.  The locking mechanism "locks" both sides of the axle together
and eliminates any side to side slippage.  The down side to this is that on
the street this is a real problem allowing you to turn corners since the left
& right wheels MUST travel at different speeds around any corner.  If the
left & right wheels are locked together turning a corner on the street would
be a tramatic experience.  You would tend to be pushed straight ahead
(instead of turning) until one wheel started skidding on the pavement.  To
alleviate this on-street problem, locking differentials must be un-locked.
 There are two primary ways this is done.  In the  4X4 community this is most
often done by using an automattically unlocking differential that will unlock
when you trun a corner with traction on both wheels.  Available from several
manufacturers, the best known of this type of "locker" (as they are called)
is the Detroit Locker.  Many 4X4 enthusiasts have one of these installed in
their rear differentials.  These once unlocked & relocked with quite some
drama requireing drivers to be careful turning corners on the street.  Newer
ones offer a nearly unnoticeable transition. 
Another type of locker is one that will lock & unlock at the command of the
driver.  The aftermarket variety of this system is called the ARB Air Locker
& uses compressed air and driver selector switches to lock & unlock either
the front or rear differential.  Typically the differentials are left
unlocked during all street driving and are locked when one goes off-road or
perhaps travels on a snowy street.  The Vanagon Syncro uses a similar
mechanism to lock & unlock it's differential(s).  In the Vanagon's system, an
air vacuum (instead of compressed air) is used to lock & unlock the
differential(s).  In the US and Canada, Vanagons were only available with
this locker in the rear differential.  In some other countries (South
Africa?, Austrailia?) a locker was also available in the front differential.
 This explains the unused light & knob positions on your diff lock control
panel.  I have heard that the center position on the control panel was used
to lock & unlock the connection between the rear axle and the viscous
coupling.

2WD with a locking differential: If either one of the two drive wheels has
traction the vehilce will move.  Both drive wheels always will rotate at the
same speed.

4WD with two locking differentials: If any one of the four drive wheels has
traction the vehilce will move.  All four drive wheels always will rotate at
the same speed.  You must loose traction on all four wheels to stop the
vehicle.

CENTER DIFFERENTIALS
Typical 4x4 vehicle (Jeep for instance) does not have a center differential.
 Instead the front & rear axles are connected directly by gears when 4WD is
engaged.  This direct connection is what prevents them form using 4WD on the
street - again the going around corners problem & wheels needing to spin at
different(ial) speeds.  4WD vehicles such as the Vanagon Syncro employ a
center differential which allows for some degree of different(ial) axle
speeds.  These vehicles are often refered to as All Wheel Drive (AWD) to
distinguish them from the 4X4 variety of 4WD vehicles.  The Syncro uses a
viscous coupling as its center differential which acts kind of like a
limited-slip/locking differential.  I use those normally seperate terms
together hear to indicate the nature of the viscous coupling operation.
 While it allows for some slippage, the greater the slippage, the greater the
degree of lock-up, to a point where the coupling is nearly locked, and
transfering as much torque to the front wheels as possible.  Things have
gotten blurry in the 4WD / AWD market lately as many manufacturers are now
installing center differentials in traditional 4X4 vehicles.  This allows
them to operate in 4WD all the time, yet most have the ability to lock the
center differential for maximum off-road performance.  The viscous coupling
in the Vanagon Syncro does not have an engagable 100% lockup feature.

Wow, that was longer than I thought it would be!  I would rate drive &
differential systems in the following order of traction preformance (without
regard for a center differential, or lack of):

2WD open differential ------ Typical Vanagon
2WD L/S differental
2WD locking differential
4WD open differential ------ Vanagon Syncro
4WD one L/S differentials
4WD rear locking differential ------ Vanagon Syncro with optional rear diff
lock
4WD two locking differentials

Hope this helps,
Jim Davis
87 GL Syncro
88 GL Wolfsburg