[torsen] RE: Haldex differential

David Eaton deaton at tranzrail.co.nz
Wed Nov 20 22:50:04 EST 2002

1) don't use hp in this discussion, you need to be talking torque.  in your
figures for the rs6 you make the same mistake, but as the numbers for nm and
hp are near enough, i thought it was a typo.
2) shift and capacity are one and the same.  if the diff is supporting a
particular bias ratio, then that is where the torque is going.  in a open
diff, the bias ratio is 1:1 so that input torque is differentiated evenly,
with the input torque determined by the traction of the least tractive axle.
in a locked diff, the bias ratio is 1:infinity, so that there is no limit to
the ability of the most tractive axle to support torque (100% in other
words) - to the limit of adhesion.  with the haldex you have the ability,
courtesy of the throttle valve stepper motor to have a variable bias ratio
determined by the ecu, based on input from the abs sensors, the throttle and
the "g" sensor etc - provided there is slip in the 1st place.  what you
*don't* have is a static torque split (bias ratio).

this is the reason that your "45% torque shift rearwards" scenario makes no
sense in a haldex.  at various times the ecu will be doing exactly that of
course, in reaction to road conditions, but at other times, it will be 80%
rearwards, other times (the "normal case") 0%.  the beauty of the haldex is
that, provided there is pressure in the system enough to create a desired
bais ratio, you can infinitely vary the bias ratio *below* this using the
throttle valve, *regardless* of the slip conditions, or even increase the
bias ratio if slip is continuing to happen.  

as i see it, there are 2 weaknesses with a haldex: there is no torque bias
in the normal case (i.e. 2wd until slip occurs).  to solve this you need a
differential (keith you are right about the visco lok - good and bad - i
have the sae paper).; and secondly that once slip occurs the haldex is
working to minimise the slip so your pump has increasingly less ability to
build pressure.  whether or not this is a real issue, i don't know - it
certainly appears that way on the surface.

the haldex gives the chassis engineer some interesting tools to play with.
i wasn't the only person wondering if the tail happy nature of the early tt
had something to do with the ecu opening the clutch on throttle-off
conditions where a more prudent approach would be to ensure that there is at
least a measure of rearwards torque bias, particularly at high speed.  again
the weakness of the haldex in these situations and a nice summary of the
above 2 points is that if slip hasn't occurred in a high speed corner, in
the throttle-lift scenario where you would want to apply rearwards torque
bias to limit oversteer, you can't do a thing.  with an active clutch you
could do what you liked.

the 2,000nm is a real figure for the torque limit of the current haldex
coupling.  i'm sure that it can (and is) modified for other applications.  i
have no idea what sort of capability the existing housing has to support
2-3x the torque capacity, but i imagine there would be a fair amount of
re-engineering involved...

'95 rs2
'90 ur-q

 -----Original Message-----
From: QSHIPQ at aol.com
Date: Wed, 20 Nov 2002 08:33:32 EST
Subject: Re: [torsen] RE: Haldex differential

[ Picked text/plain from multipart/alternative ]
In a message dated 11/20/02 3:55:55 AM Central Standard Time,
Dave.Eaton at clear.net.nz writes:

>errr no.  100% torque is 100% torque.  when the haldex clutch is locked,
>rear can and will support up to 100% of the driveline torque.  i have
>absolutely no idea how you come by your limit of 45%.  it is certainly not
>supported by any data from haldex.

Capacity vs torque shift. Torque Shift is expressed in terms of Torque Bias
Ratio.   If you look at SAE paper, it's formula for torque bias ratio is:

T1/T2 = Bias Ratio.  Solve for audi locked diff
T1/T2 = Bias ratio.  Solve for audi haldex VC

How do we solve these two to demonstrate they are equal, or are they?
we then really looking at capacity, not Bias Ratio?  Specifically, isn't a
TBR reflecting the ability to SHIFT torque in a given device, not support
torque within it.  What happens then, it we put the Haldex releif valve at
Trg of 150hp out of 200?  I'm liking this Haldex more and more.

>you might also want to figure that the haldex lsc can only support 2,000nm
>of torque prior to overload - somewhat short of 50% of that supported by
>torsen.  btw, it is not a "hadlex vc" either, it's got no viscous component
>to it - i assume that is a mistake?

Dave, isn't it possible to increase that torque support by adding more
clutches?  Haldex claims so.  Torsen does too, you can add more helix gears.
Demand of current applications?  1400lb/ft would support the heresay that
current haldex G1 can be run in full time mode on a 225hp TT (and just about
any other vehicle for the time being)?  Could it be with the relief valve
allowing slip (haldex) vs. shock load (torsen) dictates that a lower peak is
necessary?   The Haldex IS a VC by definition.  Wet multiplate clutches
however activated are "viscous couplings".  A magnetic lockup wet multiplate
clutch (ala scoobie) is a VC as well.

>anyway, wrt the 45%, put up the data, or lets drop it because the
>signal/noise ratio is getting pretty high here.

Solve for T1/T2.

>btw, the statement on the 3 different audi torsens i posted yesterday was
>directly from the zexel vp of engineering 3 years ago via email.

AND it doesn't support either the family album OR the Zexel published
applications.  Audis mysteries continue.  The most blatent issue appears to
be with the "revised bias ratio" on the S4tt (audi claims), when the family
album shows S4tt/RS4/Urs4/6 all carry the same torsen part number (5 OR
6speed O1E).  Stasis Engineering offers a 500USD shim service for the S4tt
"put it back to the RS4" TBR.  How can that be so?!?!

Thanks for the posts Dave.


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