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Re: Haldex and Viscous Couplings II - kinda long

In a message dated 9/12/99 4:55:55 AM Central Daylight Time, 
Dave.Eaton@clear.net.nz writes:

>  you should understand also that audi's edl works *across* the open front 
>  open rear differentials.  it does not work across the centre differential.
>  for example, it will not allow *both* wheels on an axle to be braked, but
>  will brake one wheel when the other is spinning up.  hence 100% torque to
>  the rear with the haldex.

Think about that statement again, Dave.  What is braking with EDL?  Braking 
Trg isn't it?  100% of Trg can't be transferred to the rear if ANY EDL 
braking is involved.  That statement can only apply to a locker with no 

>  or, perhaps this press release from vw will help?
>  (http://www.vwvortex.com/news/4motion/page3.html)
>  "With a multi-plate clutch running in the oil bath, it sets up a direct
>  drive between the front and rear wheels as soon as the input and output
>  shafts of the multi-plate clutch do not rotate at the same speed and a slip
>  occurs between the front driving wheels and the rear wheels. In this way,
>  the power of the V6 engine is always transferred to the wheels with the
>  maximum possible traction reserve. The Haldex coupling, depending on the
>  difference in rotational speed, infinitely regulates the distribution of 
>  drive between the front and rear wheels; under extreme conditions, the
>  **entire drive** is transferred to the rear wheels."

Kinda misleading.  ALL differentials that are physically locked "under 
extreme conditions", the **entire drive** is transferred to the rear wheels.  
Bottom Line is this, audi is making some marketing claims that are not the 
accepted practice.  A 4wd part time Haldex or VC is not capable of 
controlling torque beyond 50% rear.  Physically, 100% of available Trg can be 
at either axle, but it can happen in ANY 4wd/awd system, it has nothing to do 
with the differential.  Marketing doesn't change the facts, only distorts 
them some.  The above statement also doesn't account for the fact that, with 
EDL /TCS intervention, it is impossible to have the entire Trg transferred to 
the rear wheels.

>  anyway, now that we have established that, i'd be very intersted in your
>  driving experiences of the haldex.

Sounds like you are first.  Be critical, it's not awd.
>  fwiw, (and i certainly have no interest in getting back into the wrc cars
>  debate), the toyota wrc car had no centre differential, but a hang-on 
>  (a la haldex), mounted (surprise) at the rear.  not a centre vc.  in fact 
>  centre differential at all.  check out the "racecar engineering" article on
>  the "class of 98" wrc cars.  it's all in there. yes, the hang on clutch was
>  replaced, but only with a fully active setup.

Whoa up again Dave, how quickly we forget history.  In 1997 (for 1998 year) 
Toyota introduced the following system on the WRC car.  Front and center 
Hydraulic Torque Sensor (HTS -tm) differentials were installed with a Torsen 
(man I just can't believe you would have missed this:) rear diff.  This was 
the first application of the Hydro-electronic differential system.  Again, 
this was a full time awd system that was capable of transferring torque 
between both the front and rear axles.  Toyota obviously had a couple of 
issues with the "new" technology.  They spent countless hours getting the f/r 
static weight distribution correct, and were so exact that they included both 
the driver and copilot in the figures, but still had an overall weight 
problem to the front wheels and to the chassis itself.  The second issue, 
which was not introduced until sometime in the 1998 season, was the 
successful development of the hydro electronic rear diff.

How bout some audi history to complete the WRC conversion.  What Toyota found 
out, was that the center differential performed best on this chassis at a 
50/50 front rear torque split.  Let's wing back to 1983 when audi was 
rallying the center diff locker.  Bottom line is:  If the locker is locked 
all the time, you don't need a diff.  Flash forward 15 years.  Hello?

So, round about came the hydro-electronic rear diff, and the front stayed 
hydro-electronic, and the center (actually no center, just torque split) was 
50/50.  The advantage here was a few fold.  The rear mounting of the HTS, 
gained better f/r weight distribution, reduced overall weight, and allowed 
oversteer condition by "unlocking" the front diffs during cornering.  But 
bottom line again, the difference between the current Toyota, and the current 
audi Haldex is obvious:  Toyota's are awd the audis are 4wd. 

What's the difference?  Well, when you start a Toyota WRC car, the rear diff 
is always engaged during forward acceleration, so is the front, awd.  In the 
audi, you are always fwd, then during slip, you become awd, the definition of 

>From Haldex's Home Page VW press release:
..."Technically, the Haldex LSC can be described as a hydraulic pump with a 
wet disc package. The hydraulic pump is actuated immediately by the speed 
that occur when a wheel spins. *************>>>> The coupling is thus not 
activated during normal driving.<<<<<<<****************** With a relative 
speed difference, oil is pumped to a piston which pushes together the disc 
package, placed between the in- and outgoing axles and thereby slows down the 
speed difference and brakes the spinning."  

If the coupling at the rear is not activated during normal driving, during 
normal driving, it is FWD or RWD.  In the case of the Audi application, it is 

I'm smiling at the innovation that Toyota has refined.  In the FIA group A 
rally car I service, we have a choice of locked/open or torsen front, locked 
or torsen rear (center always locked).  It has run all rallyes locked f/c/r 
here, mostly because, unlike FIA rallyes, there isn't enough time to swap 
diffs in SCCA.  Having driven this beast hundreds of miles, both on and off 
road, my impression is this.  You better be ready to accept massive 
understeer in the tarmac stages, so much that she actually crabs while 
turning (but the 1.15 turns lock to lock helps some here).  On the 
dirt/gravel/snow, life gets better, but there are definitely times when an 
open front seems like a better and faster way to fly.

Introduce the hydro-electronic front diff (I'm a fan of it in the front of a 
car, center and/or rear seems pretty redundant to me).  Disengaged during 
braking and turning, engaged during post apex WOT.  Way cool.  Disengaged 
front you have oversteer, engaged you have maximum traction with understeer 
or oversteer up to maximum traction (before fully locked 50/50).

>  hence also my advice to take it easy on stating that the haldex is not a
>  hi-tech quattro system...

Your advice so noted.  The difference between audi Quattro and audi quattro 
Haldex is pretty obvious to me.  ALL true awd/"quattro" cars (including the 
PSK, TSK, VCDL,  Gen >all previous< quattro) have 4 driving wheels all the 
time under acceleration.  Haldex's own anouncement of the 4motion and 
"quattro" system, calls it what it is, a part time 4wd system.  It may be 
hi-tech, but it ain't quattro.  Not unless you want to redefine quattro as 
4wd.  I warn you not to do this, cuz then were going to see those Syncros 
start retroactively putting "quattro" badges on the Westphalica. :)

Looking forward to your test drive report Dave.

Scott Justusson