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Viscous Torsens

In a message dated 96-01-08 02:15:58 EST, you write:

>>in my ur-quattro, i found that to avoid this situation when driving
>>fast, i'd always lock the diff.  as a result the car lost some feel
>>for the road, and understeered more etc. etc.
>i always thought that locking the center diff led to oversteer.

No, the locking center leads to understeer cuz instead of an open diff you
have a fixed front rear the axle rotation which, with it locked it's just
like having no diff at all which translates to tire slip to make of the
rotational speed front to rear, leads to understeer....

>> i've found the ur-quattro with the torsen understeered a lot less than the
>ur-quattro with the open centre diff.
>because the torque split in a turn was rear biased.  the torsen would
>sense that the front axle was "slipping" wrt to the rear and thus
>transfer power to the rear. this is because the front wheels travel a
>greater distance than the rears in a turn.  the center torsen would
>treat it as "slip".
>> as i understand it, the big advantage of the torsen centre diff as
>>opposed to the viscous variety is:
>> 1) the spped of the reaction to low-traction situations
>> 2) the ease of implementation of abs.
>more specifically:
>the torsen (and computer controlled clutches) have a linear relationship
>between its locking action and the amount of slip.

Most of the new VC systems allow less than a half rotation rotation of slip
before "locking", and the mitsus have that feature as well, in fact they
pioneered it......  I'm not sure that the difference between the v8 computer
controlled EC and the new VC systems are not transparent in their
applications, and the Torsen is not miles above it becuz of the limitations
as to the true locking capabilities, it's advantage is transparent
application......  Assuming a 75% slip torsen (which is production spec), the
driveshafts only lock to 75% of 50/50 torque split, or more a range of 37.5
to 62.5 fr/rf of driveshaft rotations in relation to each other.....  In a VC
or EVC system you can have a 100% lock, as well as in a manual lock center
diff....  Lots of folks describe the "hunt" in a torsen gleason in the tricky
stuff, and usually not in a good way.....  In the rear and front diffs a
torsen with full time ECV or VC center diff AWD is prolly desrireable over a
"delayed (power off)" manual lock that is found in the early q's......
 However a truly locked front rear center diff will do just as well as a
torsen in preventing the 1 drivewheel spin associated with driving in the
slick......  Everywhere but the turns, on turns the understeer will be
slightly more on the 100% locked center differential....  Remember folks,
that eliots V8 has a different differential setup than the "torsens" in the
89> q's....  

Lot;s of folks I've talked to think that the v8 has torsens....  Well it has
one, not two....  It is in the rear, a 75% LSD torsen worm type....  The
center is ONLY EVC and has the true capability of 100% drive shaft rotational
lock the torsens don't enjoy.....  The early thinking is that the abs was the
advantage to it, but advances in the VC technologies has made that argument
less convincing......  but it is much better, IMHO, than a torsen is in stock
>the VC has a far more exponential characteristic.  it makes the
>chassis trickier to design.  for the original full time 4wd 911C4, it
>was rejected for this reason.

I'm not sure I agree with this statement anymore tho, eliot......
>the VC is rotational sensing, while the other two are torque sensing.
>actually it is just a fancy term to describe the fact that VC locks
>both in acceleration and braking, while torsen and CCC lock only
>during power application.  immense effect on ABS implementation
>because ABS requires independently rotating wheels to make its

If a VC operates in less than a 1/2 of a full rotation, I'm not sure this is
true....  The ECV, has to have a sensor and computer control to it, and I
would guess that the rotation is more than 1/2 turn for that to enable, and
regardless will have a delay to it......  The ABS argument with the "newer"
style VC's doesn't hold true anymore either....  The old style VC was a heat
build up, not a silicone shear type the newer ones are.....  The shear type
will rival the best of the EVC systems.....  And are instant on application
on and application off......   Once equal driveshaft rotation is sensed the
lock is "disabled" and that occurs in the same 1/4 to 1/2 turn.....
>the cars that feature VC full time 4wd and ABS have to slacken the
>couplings so that ABS can work properly.  it is a cheap and ugly
>hack. i believe that the latest porsche c4 and turbo disconnect
>drive to the front altogether during braking.

I think you are referring to two different systems eliot, a part time AWD
system with VC and a full time AWD system with VC.....  The part time
systems, like found in dorF trucks, use a VC to engage and disengage the
driveshaft to the front wheels, so it is considered a part time 4 wd system
vs a VC to control the driveshaft speed differential of an AWD vehicle....
  If that IS indeed the system the C4 is using, it is sub par, but I don't
think that is the system it has......