RE: further on the s6 and rs-series & miscellaneous stuff

["Dave Eaton" <Dave.Eaton@clear.net.nz>]

there's nothing "poor mans" about subaru's current awd system, although it's
not state-of-the-art in pure technology terms, although no-one is
complaining about the performance, just like the current audi system...

however, note the plural ("viscous couplings") in the press report.  my
guess (emphasis on that word) is that audi might be debuting some sort of
yaw control using a controlled rear diff to control over/understeer.
mitsibushi use an "active" rear diff on the lancer evo 6 to do the same
thing.  however, mistibushi basically copied the idea from honda who used it
in a fwd application of theirs.  and honda probably got a few ideas from the
original carrera 4 awd system.

some history:
while generation 1 awd systems (audi locked centre, early subaru) where
all-or-nothing devices (100% torque shift front to rear when locked), the
next generation of awd systems (generation 2 shall we say) were passive
based with automated torque dispensation depending on either torque sensing
or speed sensing across the centre differential. the emphasis on generation
3 systems (audi's current technology) is to provide a feedback loop to the
passive awd system using the wheel speed sensors (abs) and the engine
(traction control system, tcs) to modify vehicle behaviour while cornering.
the new generation systems (generation 4) are increasingly dispensing with
the basic passive centre differential, and moving to technology which allows
more direct control of vehicle dynamics (yaw control) by directly
controlling the torque shift.

yaw control is certainly now the state of the art, building on the advances
made in coupling an awd system to tcs and abs systems.  for this to work you
need your usual compliment of abs sensors, a yaw rate sensor (usually
mounted behind the c of g, a steering angle sensor, and a lateral "g"
sensor).  the system works by comparing yaw with the steering angle and
figuring out if understeer or oversteer is happening.  the previous
generation systems would then brake the left rear to correct understeer and
the right front to correct oversteer (some systems apply all the brakes
depending on the exact circumstances).  the major disadvantages with tcs are
basically the lag in the system, the "cycle time" for the brakes, and the
requirement for torque to make it all continue to happen.  with an active
rear diff, the locking rate of the diff is changed to produce the right
"moment" across the rear (fully locked rear = understeer, unlocked rear =
oversteer).

btw, mitsibushi have used electric control for their active diffs (clutches)
due to concerns over the lag in hydraulic systems (and their serviceability
and weight).

hth,
dave
'95 rs2
'90 ur-q
'61 mb fintail

-----Original Message-----
From: FBFISH@aol.com [mailto:FBFISH@aol.com]
Sent: Friday, 21 May 1999 01:03
To: Dave.Eaton@clear.net.nz; quattro@audifans.com
Subject: Re: further on the s6 and rs-series & miscellaneous stuff


In a message dated 5/20/99 6:50:38 AM Eastern Daylight Time,
Dave.Eaton@clear.net.nz writes:

<< "viscous couplings" whatever that means.... >>
poor mans AWD like Subaru??
Frank Santoro