RE: Haldex clutch question (long)

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

well, opinions on the haldex vary.  while no-one on the list has yet driven
the tt (jouko??) or the a3q, we'll have to wait for personal experience.
we're about 3 months away from the ttq arriving in this neck of the woods,
and me getting a drive of it.

with regard to the technology, the important differences are that the haldex
is not a permanent awd system, it is like a viscous coupling (vc) in that
[axle] slip must occur before it acts (i.e. in normal operation, it
transfers no torque between the axles).  the torsen is a permanent system,
both axles are permanently driven (torque transferred) and [differential]
lock occurs when the internal friction of the device (i.e. the "feedback"
from the axles) exceeds the bias ratio.  what is bias ratio?  the amount of
torque "imbalance" between the driven axles.

the haldex is much quicker than the vc though, in that it acts within about
10 degrees of axle rotation (100ms for the wet clutch to receive pressure).
also unlike the vc, it is computer controlled (a computer controlled
throttle valve controls the hydraulic "bleed" pressure) which means that it
can be setup for various conditions (e.g. throttle lift on corner entry).
it also means that, while the vc has a non-linear locking ratio (unlike the
torsen also), the haldex's ratio can be controlled and varied for different
conditions.  in other words, the bleed valve is computer controlled and so
can be made to operate within a wide range of hydraulic pressure and axle
rotation delta.  the electronics can also be directly interfaced to the
vehicle electronics (lateral g, abs sensors, engine management system), and
it is this level of integration which offers the engineers much more finesse
in their desire to control vehicle dynamics in extremis.  purely mechanical
systems such as the torsen and vc don't have this degree of flexibility.

the haldex can also accept very large amounts of torque (2,000nm) which the
torsen and vc would struggle with.

so far, most road testers seem to like the haldex's ability to promote
oversteer on corner entry. (e.g. georg kacher on the a6q 4.2v8 in the latest
car magazine).  this is the proverbial rwd argument (i.e. unless you like
and can control oversteer you don't have big balls, and you're not a real
man).  however, you also need to bear in mind that the audi torsen
implementation has changed over the years and the latest systems attempt to
provide a "feedback loop" though the use of the abs sensors and individual
wheels brakes (called "edl") to provide another level of control over the
differential.

imho, the haldex-type system is the way the technology is developing due
primarily to the high degree of integration which is possible with an
electronic control system.  it is a simple and very elegant design, which
requires no high-cost ancillaries such as external pumps and actuators.  it
is also much closer to a commercial "active" system than anything else on
the market.  at the present point in time state-of-the-art active
transmission systems are unique to the world rally circus (wrc - little
joke) and what those guys know, they not telling.  interestingly though,
outfits such as xtrac are getting involved more and more and so the
technology spread will become much wider over time.  bear in mind though
that in the wrc, cost is no object ($usd150k for a transmission for the ford
focus), and so commercial vehicle manufacturers will have a whole set of
different cost criteria.  clearly the haldex meets these for vwag.

certainly the current vwag implementation of the haldex is the transverse
front-drive chassis (golf platform) which focuses on putting the haldex at
the rear for improved weight distribution and packaging, which would not
work for the longitudinal audi platform.  however, for the audi platform,
there would be little to stop the installation of haldex clutches on each of
the front and rear diffs with a simple clutch (or epicyclic diff) in the
centre (aka audi automatic transmissions).  this would give additional
benefits, be full-time, and allow an even greater ability to control vehicle
behaviour, at the expense of greater system weight.  whether audi will do
this or stay with the torsen for the longitudinal engines is presently
unknown...

interestingly, "evo" magazine tested a ttq against a 20v ur-quattro in their
first publication (they have a 20v ur-q as a part of their fleet).  the two
machines have a similar spec, the same weight, with the quattro edging the
tt for torque (228lb ft vs 206) but not having as much power (220hp vs
225hp).  the ur-quattro was preferred due to it's more consistent
on-the-limit behaviour, and better engine.

hth,
dave
'95 rs2
'90 ur-q

> -----Original Message-----
------------------------------
>
> Date: Sun, 18 Apr 1999 18:24:27 -0500
> From: ryanweatherby@juno.com
> Subject: Halex clutch question
>
> I'm hearing a lot about this Halex clutch Quattro system in
> the new TT,
> apparently "true" Quattro enthusiasts aren't big fans of it.
> Can someone
> please explain to me the differences between the Halex clutch
> Quattro and
> a Torsen diff. quattro?
>