RE: Torsens: The Trojan Horse...

[Chris Maresca <ckm@eainet.com>]

At 1:12 PM -0700 5/4/98, Buchholz, Steven wrote:
[snip]

>This brings me back to a question I asked a couple weeks back ... ABS
>algorithms.  Does anyone have any info on them (FWD, AWD)?  Does the ABS
>system that supports EDL have any more sophisticated electromechanical
>devices than does the old ABS system in the '86-'88 5000s?

I don't know all that much about any of this, but here is what I know:

What EDL essentially does is take advantage of a normal _free spinning_
diff.  It essentially does the reverse of ABS, and applies the brakes to
the corner losing traction.  The speed differential between both wheels (on
the same axel) causes more power to go to the wheel that has "more
traction", eg. the un-braked one, in the same exact way as power is
distributed while going around a corner.

This can also be applied in a front-rear fashion as well.  The rear brakes
will be applied and more power will go to the front, as if the driveshaft
was an axel going around a corner.  "EDL" is a misnomer, since the
differentials do _nothing_, other than what all diffs. do...  Only the
brakes are affecting power distribution by simulating cornering (from the
diffs. view...).

Some of this behavior would probably not fit with a Torsen, which
interprets a slower axel rotational speed to mean _more_ traction, whereas
this system (EDL) works the other way around...

I don't think that cars with EDL have any more 'devices' than normal ABS,
other than a modulator that can apply as well as relieve pressure.  I also
think that ABS systems are pretty dumb in general.  Some cars have lateral
acceleration sensors which give ABS systems more information to process,
but most are simple four-wheel feedback sensors.

I think that the major difference in ABS systems has to do with the
granualrity at which they can analyze sensor feedback.  Early ABS systems
had highly noticable pedal pulses, while later systems have virutally
eliminated this...

The Quattro's have a slightly different problem since the ABS has to take
into account the fact that _Torsen_ diffs are capable of locking up, and
this could confuse the ABS, which is why most earlier Quattro systems are
designed to have ABS off when they are under acceleration or locked diffs.
IMHO, this is why Audi is moving to EDL/non-Torsen, it integrates much
better with ABS, since it works in roughly the same fashion.

Which brings up another point, in hard driving in bad conditions, with
plenty of EDL action, would that have an impact on brake fade or rotor
life?  It seems to me that a lot of this (esp. w/traction control on a 2WD
car) might be detrimental to brake performance/durability...

Chris.
'90 CQ

P.S. I felt U-O-U last friday in light rain (oily, slick roads), no loss of
control, but wierd.  Light steering corrections took care of it...