Marketing 101

[QSHIPQ <QSHIPQ@aol.com>]

Jim Simone writes
>Being a new owner of an A4 1.8T qms and  not an engineer (immunologist), I am
>trying to understand your arguments for VC and against Torsen.
>But, if I may ask an ingenuous question, what are the "advantages" of a
Torsen
>diff and why does Audi use them if viscous diffs appear to be the better
choice?
>Is this a question of racing versus street? Or, is it just good marketing?
>Thanks,
Not sure, maybe the idea that in a straight line, the torsen wins.  Darn
turns.  The biggest issue against the torsen is predictability (and you can do
an archive thread, or post me privately for the book version).  It assumes all
inputs to the device to be traction inputs.  Even when they aren't (slip angle
f&r and relative f&r slip angles) traction inputs.  

AND, ABS works (this is in deference to the old lockers, which disabled the
abs).  To jump and say that ABS works 'better' in a torsen than in a VC needs
a lot more data than has been published.  Given Trg at a minimum, I don't
think either claims a win, both are 'linked' to some extent (in techno terms,
both have a decel Bias Ratio).  Racing, the Torsen was not used to any great
extent (singular exceptions so noted) in the audis.  

What the torsen EXACTLY lacks, that a VC doesn't, is that you can get U-O-U
(understeer = u, oversteer = o) in the same turn with no 'traction' reasons
for Tshift.  A torsen audi then exactly doesn't maintain chassis character
thru a turn, since at 50/50 static distribution it understeers, at 78r/22f it
oversteers, and at 22r/78f it understeers.  A VC (3kgtVr4 for instance) has a
primary set of drive wheels, and the lock is at a specific ratio, that can be
made to keep the inherent chassis dynamics, here O-O-O.  Even with slip angle
variables, the chassis dynamics doesn't change, it's still oversteer.  Just as
predictable as a locked 50/50 audi diff at U-U-U, only better (racers tend to
use a 65r/35f split as preferred, btw) 

The argument FOR torsens is in terms of absolute traction devices.  In a
straight line, they can't be beat.  Darn turns.  Good marketing, and the
beaners were happy to.  A torsen center, with open front and rear is cheaper
than a locker and all it's hardware.  The customer is happy, cuz 'ABS OFF'
doesn't appear when they fear in need of it the most in a locker.  Sounds like
a win win.  Yup, until you go to steamboat, or drive with Jeff and I at a
track, or step out a little in the rain or snow.  Then spiders land in your
lap.  An absolute traction device fooled, has some (upside-)downsides
associated with it, when turning.  

Not a single other manufacturer uses torsen centers.  I argue for good reason.
And believe the audi racers knew it too.  Too bad they didn't take those
marketing boys for a ride.....

HTH

Scott Justusson