RE: Controlling torque v Marketing

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

sorry dave,  when the diff is locked, you do *not* have 50:50% torque
front/rear.  by definition.  ditto with the generation 1 system.  with a
locked diff (haldex, or manual quattro locker whatever), you have 0:100:0%
torque front/rear depending upon traction.  you are misunderstanding torque.
if it helps, with an open differential, you have 50:50% f:r torque
distribution by definition.  all the time.

this was discussed in detail on the list a few months ago.  what this means
in reality is that the locked centre diff means a *static* f/r torque
distribution which effectively mirrors the cars weight distribution.  this
exagerates understeer (in addition to the obvious f/r effects of the locker
in a turn).  a good reference in a discusison on this is the book "chassis
dynamics" by jeff daniels.  he devotes a chapter to awd technology's effects
on a chassis, and covers the 1st generation quattro in detail.

with regards the haldex and it's torque distribution potential, check out
the graph on page 6 of the white paper i sent you.

to re-iterate, when a diff is locked, torque flows either way through it
without hinderance.  up to 100% or down to 0% by definition.  this is true
with any centre diff technology which locks (vc, haldex).

the only exception to this rule is when a computer decides that this has
occured enough and starts to open the clutch.  for purely mechanical
differentials (such as a vc, or torsen) obviously there is no computer to
get involved, and until the fundamental condition disappears, the diff lock
remains, or some other form of computer to take over (e.g. edl with the
latest audi quattro technology).  the torsen also, by design, will limit the
torque transfer due to it's spider gears.  hence the haldex is under
computer control, but it is not "active", which seems to be what you are
wanting.

i wonder about the reasons behind the haldex.  the vc is known to have
difficulty in interfacing with traction control systems (the vc is a speed
difference orientated device), whereas with the haldex, you can program it
to respond to panic situations requiring tcs easily.

hth,
dave
'95 rs2
'90 ur-q
'88 mb 2.3-16



-----Original Message-----
From: Lawson, Dave [mailto:dlawson@ball.com]
Sent: Wednesday, 22 September 1999 03:51
To: Dave.Eaton@clear.net.nz
Cc: quattro list
Subject: RE: Controlling torque v Marketing


Dave,

>perhaps you misunderstood my question?  given that the haldex literature
>makes it plain that 100% drive to either axle is a function of the diff
(and
>the computer), i fail to understand your continued assertions that anything
>beyond 50% rear "can't be controlled by the computer".

I can't find a reference in the Haldex literature where it says that 100%
torque transfer is a function of this device and possible with this unit.

As far as >50% distribution to the rear "can't be controlled by the
computer", it's true. People appear to be using different baseline
scenarios when thinking through the problem which is causing
misunderstandings. A great starting point is all 4 wheels on the
same surface. The most that this device can do is a 100%
lock, at which the front and rear axles are turning at the same
speed and the torque distribution is 50/50 f/r. Because the
haldex is a hang on clutch on the rear axle, it can't control the
torque distribution to the front axle to get more distributed to
rear axle, hence we arrive at the 50% number.

Now, if for some reason the front wheels have less tractive
force than the rear (hard acceleration causing tire slip or
the fronts are on a surface which has a smaller coefficient
of friction i.e. ice), a torque distribution of > 50% to the rear
can happen, but the haldex or it's computer is not controlling
it, the most the haldex can do is 100% lock which gets the
distribution to 50/50. It's because of the environmental tractive
force of the front tires that allow more than 50% distribution
to go to the rear axle, the haldex isn't responsible for the
torque distribution >50% to the rear.

-
Dave Lawson