Re: Torsens and recovery...

[audidudi@mindspring.com (Jeffrey J. Goggin)]

>He was sure that unless my car, non'torsen, was in a locked center diff
>setting that it wasn't 50/50.  He said his old 4kq was only 50/50 when
>locked. When it wasn't locked it was ? 

50/50, of course ... until one of the four wheels starts to spin, at which
point nearly all of the available torque is directed to it.  Due to internal
friction of the spider gears, etc., a very small amount of torque gets sent
to the other end of the car but it's effectively zero.  Under perfect
conditions, an open diff can and will allow BOTH wheels (in a side-to-side
app) to spin ... it took me a while to figure this out since I couldn't
understand how a car without an LSD (Dodge Shadow Turbo) could spin BOTH
tires during a launch at a dragstrip! 

>I said no, they are always 50/50 locked or not.  The center diff and rear
>diff are open diffs. They don't have any sensing abilties as far as where
>to put the power.  They are open or locked.

As explained above, you are correct but only up to the point that any one of
the four wheels starts to spin.
A quattro with one wheel on glare ice and both the center and rear diffs
unlocked isn't going anywhere, just the same as a FWD or RWD car...

>His response was then why lock them.  What would be the advantage?  Why
>would a locked 50/50 be better than an unlocked 50/50.  I didn't have a
>good answer to that.

By locking the diff, you guarantee the torque split will ALWAYS be 50/50,
regardless of how much traction is available to each of the tires.  This
means you will always have at least 1WD (one front or one rear wheel!)
instead of 0WD; with the rear diff also locked, you have 2WD and only very
rarely do you ever have 3-4WD.  A Torsen center diff guarantees that you
always have at least 1WD and with the rear diff also locked, you have 3WD,
although under extreme conditions, one end of the car gets only 22% of the
available torque ... when it comes to getting unstuck in mud or snow, this
is about the same as zero.  That's why Audi still includes the rear diff
lock on all of its quattros...

In the case of my Ur-Q, when it lifts the inside rear tire during an
autocross, all torque is directed to it and the car is effectively
"coasting" until the wheel returns to the ground ... locking the center diff
will allow at least one of the front wheels to continue driving the car out
of the corner.  Despite the tradeoffs that come with a locked center diff
(more understeer, slightly sluggish turn-in, less responsive to throttle
modulation, slower recovery from overdriving, et al), I've proven to my
satisfaction that it is consistently quicker (an average of .5 to .6 seconds
on a typical course) with it locked than unlocked.

On the positive side, Torsens freewheel during decceleration (that's why the
ABS works) and this also allows the car to have a noticeably sharper turn-in
response, similar to that of a car with an open center diff, as well as the
same guaranteed minimum of 1WD under all conditions without all of the other
problems associated with a locked center diff for street use (harsh ride,
excessive tire wear, etc.).

Driven at the limit, though, the continously variable distribution of torque
makes the car very difficult to balance.  In my opinion, this makes the car
slower in the hands of anyone but a hero driver (which leaves me out) since
it takes a remarkably delicate touch to keep it on the absolute edge of the
friction circle.  For normal street use, though, this doesn't matter and the
benefits listed above (especially the ability to work with ABS) are the
primary reasons why Audi is now using Torsen center diffs in its current
range of cars...

(P.S.: I think Torsen diffs in side-to-side apps -- especially FWD cars --
are wonderful things.  The key to their success in these apps is that all
the shuffling of torque takes place at the SAME end of the car, thus making
it possible to trim the balance of the car by modulating the throttle as
well as the steering.) 

>If you cc'd the answer to the list it may clear up some confusion among
>others as well.

Done.  :^)
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   / |      _| o    | \       _| o   Jeffrey Goggin
  /__| | | / | | __ |  | | | / | |   audidudi@mindspring.com
 /   | |_| \_| |    |_/  |_| \_| |   http://people.delphi.com/audidudi
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