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

[Frank Deutschmann <fhd@interport.net>]

Ken Bell sez:
>    o  What is meant by "locking the rear differential"?  Is this
>       required for AWD, or is this simply one way of using AWD?
>       Is it done manually or automatically?

If the rear diff is an open design (a normal diff), locking it causes the
two output shafts to be locked such that they always turn at the same
rate -- the diff is effectively eliminated, and becomes what is sometimes
called a spool.

>    o  Why would locking the rear differential make the ABS speed
>       sensing mechanism (just a magnetic sensor and gear-like hub
>       disk) less accurate?

The computer that controls the ABS system uses information on the lateral
and longitudnal acceleration of the car, and the rotational speeds of all
four wheels to control brake pressure release.  With the rear two wheels
locked to the same rotational speed, regardless of actual tire slip, the
computer has much less information to go on; in particular, it becomes 
impossible for the computer to accomodate what is know as "split-coefficient
braking surfaces" (left wheel on pavement, right wheel on dirt, for example).
If the computer improperly controls ABS in such a situation, it may lead to a 
spin.

>    o  Do other AWD cars retain ABS functionality when AWD is
>       enabled?  For example, is ABS disabled automatically on the
>       Audi Quattro when the rear differential is locked, or is
>       this requirement specific to the Pontiac design?

Quattros do this, too.

>    o  Is there such a thing as AWD with an automatic transmission?
>       All the Audi Quattro ads I've seen indicate a 5-speed tranny,
>       but I think I've seen reference somewhere to automatic.  Other
>       than the "enthusiast" aspect of manual shifting, are there any
>       AWD automatic cars that work well?

AWD and choice of transmission type are not interdependent, though not all
AWD vehicles are offered in both manual and auto boxes.  The Subaru SVX is 
another semi-popular AWD sports car, and it is only available in automatic.

>    o  Finally, is AWD a full-time thing, or is it something that one
>       manually turns on and off?

Convention these days seems to be that AWD refers to full-time AWD, while
4WD refers to part-time four wheel drive, as in Jeeps and other "serious"
off-road pounders.  (The difference is that AWD requires a center diff,
while 4WD relys on a simple transfer case and wheel slip.)

-frank
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