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

[MEL1523@ACS.TAMU.EDU]

;   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?
;
;AWD is full-time operation, and cannot be disabled.   As a wheel begins to lose 
;traction, power is moved from that wheel to a wheel that has more traction.  
;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.  When the differential is locked, this forces the wheels on that 

Not totally true.  A normal differential forces both output shafts (whether 
they are going to the front and rear, or to two wheels) to transmit the same 
torque.  If one wheel is on ice, and the other on pavement...tough cheese, 
they both get the same torque as the one one ice which spins.  This is the 
reason for locking diffs.  Two relationships characterize a differential.  
1. the torque going to the two output shafts is equal.
2. for a given input shaft speed, the sum of the two outputs speeds must be 
constant.

That having been said, strictly speaking, power goes to the wheel with the 
least traction.   Again the reason for lockable diffs.  A lockable diff is 
like no diff at all.  Both outputs rotate at the same speed regardless of 
the torque which may be different.  This enables a wheel with more traction 
(the one on pavement) to get some more torque.  Limited slip diffs have 
viscous couplings between the two output shafts.  This transfers torque from 
one shaft to the other when there is a large difference in their speeds.  
Problem is...it may not transmit enough and energy is dissipated in the 
process.

A torsen dif works by allowing slip when the transmitted torque is equal, 
and not allowing slip when there is a difference.  So when you are in a 
tight turn in a parking lot, (same torque) there can be slip, but stuck in 
the snow, with one spinning, no slip.  It does this by transferring the 
power from the input shaft through worm gears instead of pinion gears.  
Power can only go one way through worm gears.  This is why an electric winch 
can pull the cable, but when the motor is off, no amount of force on the 
cable can spin the motor.  Torsen stands for torque sensing.  New Audi's 
have these as the center diffs.  Last time I checked, Audi was the only 
maker using torsen diffs in their cars.  

So why not on the rear or front diff?  You wouldn't want different torque on 
the two sides resulting in yaw on the car.  Why on sports cars?  The driver 
has to steer to counteract the yaw...not the best design for a sedan you 
want to be stable...an engineering compromise.

;   o  Why would locking the rear differential make the ABS speed
;      sensing mechanism (just a magnetic sensor and gear-like hub
;      disk) less accurate?
;
;This I'm not sure of, but it must be related to the limited-slip diff.  BTW, 
;Audi's disable the ABS upon locking the diffs also.

The speed sensors themselves work just fine lock or no lock.

   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?

Nope.  On mine...lock the center or rear, and you lose the ABS...it's an 
electronic interlock.  It's not the AWD that necessitates the ABS cutout, 
it's the locking.  Why?  ABS prevents wheel slip.  It does not cut out when 
the wheel stops.  Read that last sentence again.  ABS does not look for the 
wheel to stop.  It looks for acceleration.  As you apply the brakes in a 
panic stop, the tire transmits more and more braking force.  As you exceed 
the traction of the tire, it doesn't stop, it slows down real fast 
(acc(de)elerates) and slips relative to the ground.  When the ABS senses the 
acceleration, it lets off the brake pressure.  It doesn't wait until the 
wheel stops.  That's why you can apply the brakes when the car is stopped, 
and the ABS doesn't let it all out.  

Why the conflict with the diffs?...When one wheel of an axle starts locking, 
you want the ABS to go off (I mean trigger) right?  But what if it can't 
lock because it's tied to the other wheel through the locked diff.  It 
can't.  The whole axle (both wheels) have to start locking before the ABS 
would trigger.  Plus, you'd be transmitting braking forces through the 
driveline which you don't want to do.  You want the ABS to trigger if either 
wheel starts locking.  All of the above also applies to the front/rear 
situation, which is why you lose the ABS upon locking the center diff.  Why 
is a torsen diff in the center ok?...it allow slip under no torque diff 
condition and does not allow torque to be transmitted front/rear because of 
the worm gears.  Therefore the two ABS channels will not crosstalk, and you 
can have the best of both worlds, ABS and differential torque delivery.  
Worm gears are like mechanical check valves or diodes.  Torque can 
be delivered from the engine differently, but cannot be transferred from the 
wheels or brakes to other wheels.  

;   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?
;
;Now, that you mention it, I don't recall having ever seen an automatic 
;Quattro.  I don't see any mechanical reason why they couldn't manufacture one 
;though.

There is no engineering reason why you can't have AWD and an automatic 
trans.  Lots of AWD cars have them both.

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

On Audi's you always have it if you have AWD.

;   o  Thanks :-)

 - Mitch Loescher