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haldex clutch details

i've recently received some more information on the haldex clutch which
vw/audi have decided to use in the new s3 a3 quattro, and the tt
quattro.  rumours have it being used in the upcoming s6 as well, where
it will function as both a front, centre and rear diff, but i have no
concrete details on this...

i am in the process of gaining further information on the system as
implemented by audi, and in particular, it's operational

the haldex is based around a wet multi-plate clutch which connects the
two driveshafts, by way of input and output flanges.  with the haldex
the clutch is generally *not* engaged  so, in the default application,
the two driveshafts are unconnected (although normally turning at the
same speed of course), and the vehicle is 2wd.  from an understanding of
the diff, there is no reason why, with some minor engineering and
software changes, the clutch could not be on (active, or locked) by
default, and progressively unlocked as required.  one suspects that this
is the method audi may have used for their quattro, but as i said, i
have no information on this yet.  as it stands the haldex operates
basically as a smarter vc...

the haldex has 3 pistons (really annular pumps) which are mounted
axially (think concentric cylinders) in the input shaft flange.  The
output flange runs against 2 of these pistons via a *series* of rollers
concentrically mounted in the output flange surface (facing the
pistons).  hence any *relative* movement of one shaft (flange) versus
the other will produce a pumping motion in the pistons (from the rollers
against the pump surface) and, from there, the action starts.  no
relative movement, and no pumping.  the 3rd piston is connected to the
output side of the 1st two pistons, and is also connected to the
multi-plate clutch.

the suction side of the 1st 2 pumps is connected to the system by way of
check valves.  the oil from these pistons is pumped (via another check
valve) to the 3rd piston where it compresses the clutch, which has the
effect of braking the speed difference.  the oil returns to a reservoir
by way of an adjustable throttle valve.  it is this computer controlled
valve that enables logic to be applied to the system, as it controls the
oil pressure and therefore the force on the clutch pack.

as basically a piston pump, it is claimed that there is no pumping loss
in the haldex and almost instant reaction to slip (reaction within
100ms).  the oil flow is proportional to the shaft speed difference, and
so is the oil pressure.  so you have a closed loop system for
controlling the relative shaft speed difference.  the 2nd piston pumps
at a 30 degree delay compared to piston #1 which, along with the design
of piston #3, allows a (claimed) even flow of oil, and an even torque
transfer betwene the shafts (not something you can claim for a vc).

there is an overload protection circuit designed to prevent excessive
torque from being transferred through the housing.  by default, it is
designed for 2,000 nm which means the rs2 and new s4 should be ok, but
the new s6 probably not.  apparently this protection circuit can be set
according to manufacturer preference...

to ensure fast response times, the pistons are continually in contact
with the rollers by means of an electric pump providing a low pressure
"feedback" loop.  this pump works only when the vehicle is running.

from this basic principle haldex have developed the coupling to provide
more proactive response.  for instance the electronic control system
takes into account the feedback from the abs system (wheel speed
sensors), brake light switch, engine speed, torque and throttle
position.  continuous monitoring of this information allows the software
to modify the characteristics of the coupling by way of the throttle
valve.  there is no suggestion of "active" control of these parameters
(e.g. tweaking the brakes a la a4/a6 quattro), but then this could then
still be applied over the top of the haldex.

the size of the unit is not huge, it's about the size of a normal diff.
in the a3 and tt, the unit is mounted ahead of the rear diff.

if anyone is interested, drop me a line and, when i get the time, i'll
scan some of the photos and diagrams...

'95 rs2
'90 ur-q