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[torsen] Re: EDL vs ELD explained.... Re: [s-cars] Exceptionally lame question

QSHIPQ at aol.com QSHIPQ at aol.com
Tue Sep 10 09:30:03 EDT 2002

A great read!  One minor correction in your "torsen" explanation.  Since a
torsen is mechanically attached to 2 driveshafts, with applied torque, a turn
WILL cause increase torque split UP TO the bias ratio, before it will allow
"speed" differences in the driveshafts (at that maximum bias ratio, unless
this torque shift causes a traction argument).  Which means (S-car content
here) that the center differential will allocate more torque to the rear of
the car during a turn, which, if that turning radius induced  torque
allocation then causes the rears to break traction, torque allocation then
shifts forward.

More simply put, a torsen, being "torque sensing" differential cannot allow
any speed differentials between driveshafts before performing it's maximum
designed shift in torque allocation first.


Scott Justusson

In a message dated 9/10/02 3:07:42 AM Central Daylight Time,
Keith.Maddock at trw.com writes:

Turns out my suggestion about the unit under the rear seat was bunk!  Oops,
sounds like it is probably the ABS unit per Calvin's comments.  I miss having
a UrS around to check my theories out before posting..  (Imagine sound of
Keith eating his words)

ABS-based EDL  (Electronic Differential Locking) replaced the
Electronically-Locking-Mechanical-Locker-Differential for 95.5 according to
urS4.com.  Im not sure of the exact changover date.

I could write several pages of details about the differences between the two,
as EDL functionality is one of my specialties here at TRW.

In a nutshell however, here are the basics...  I'll CC this to the list
incase anyone else is interested.

Both systems are based on a normal, "open differential".  The physics that
govern a open differential dictate that the torque output to each side of the
differential is exactly 50% of the input torque.  Open differentials can also
be built to have different torque splits for the two output sides, which is
often the case when they are used as a center differential.  (X5 for example
has a 38%/62% Front/Rear torque split on its open center differential).
However for front and rear differentials, (other than perhaps Nascar or
something), the torque split is always 50%/50% left/right.

So what does this mean  in the end?  Basically, the torque at both wheels
must be the same, so the maximum torque that can be applied to both wheels is
limited to the wheel with the least traction.  Consider a classic "Split Mu"
condition, where the left half of the car is on asphalt, and the right half
of the car is on glare ice.  (this is generally a condition only found on our
test tracks, but you might get close to it on the side of a snowy/icy road)
In this circumstance, both wheels will get exactly as much torque as can be
applied to the wheel on the ice.  Since the ice wheel now has about 10% of
the traction as the asphalt wheel, the asphalt wheel is also limited to 10%
of its normal torque.  The end result is pretty poor acceleration.  Another
example of this that most people can probably relate to is a car in a muddy
field, and one wheel just keeps on spinning and the car goes nowhere....

In order to get more torque at the asphalt wheel, there has to be some device
working with the differential.  These fall into three general categories:
A) Limited Slip Device (Torsen, Various active and passive clutch systems)
B) Locking Device (Mechanically or Electronically , passively or actively
C) ABS-based wheel braking device.

Below are the differences between these three, in a VERY simplified manner:

A) Limited slip device:  In some way through clutches of various types, or
friction between internal surfaces gears or clutches, these devices provide a
fractional mechanical coupling between the two sides of the differential
output, reducing the amount of speed difference between the two sides.  This
allows a fractional amount of torque transfer from one side of the
differential to the other.  Depending on the design, this amount of transfer
can either be proportional to the speed difference between the two sides of
the differential (traditional LSD or most active systems), or the torque
difference (Torsen).  The benefit to these types of devices is that they
still allow a speed differential between the two sides of the differential,
which is necessary for normal turning (why we have to have a differential in
the first place).  In our classic split mu case, these devices can increase
the torque of the aspahlt wheel up from the open-diff's 10% all the way up to
100%, though most would probably only be able to increase the torque capacity
to 50% or less.

B) Locking device:  These devices generate a full mechanical bridge between
the two sides of the differential, which means when activated, the
differential is considered closed or locked, rather than open.  In this case
(depending on the strength of this bridging mechanism), the output speeds of
both sides of the differential must be the same.  This allows up to 100% of
the input torque to be transfered to either side, as necessary by the torque
absorption potential of .   In this case 100% of the rear differential's
input torque can be transferred to the wheel on the asphalt.  This is great
until you start to turn...  This is the system on the pre 95.5 Audi's.

C) ABS-based wheel braking device.    The important thing to realize with
these systems is that they ABSORB torque rather than TRANSFER torque.
Remember when I said that the torque available to the asphalt wheel was
limited by the amount of torque that the ice wheel could absorb?  By applying
the brake to the spinning wheel on the ice side and absorbing some torque,
we've now increased the amount of torque that this side of the differential
is absorbing, therefore allowing more torque to go to the ashpalt wheel.  At
the "best case" in this system, absorbing all the excess torque on the ice
wheel, we can allow a maximum of 50% of the rear differential input torque to
be output to the ashpalt wheel.

Of course, in this process, 45% of the rear input differential torque is
being absorbed by the rear brake pad on the ice side, which generates heat
and removes material from the brake pad.  Do this for a while, and you'll
either light your brakes on fire, or (in most cases) activate a protection
mode in the ABS which will shut down this functionality and return you to
normal open-differential operation until it thinks your brakes are cool
again.  (in some units, this can be done by turning the car on and off, I
have no clue if the Audi unit remembers its brake temperature estimates while
its off)

So its obvious that the ABS-based braking solution is not the best from a
performance standpoint.   On the classic split mu surface, a S4 with locking
rear diff wil have nearly double the acceleration of a S6 with ABS-based EDL
(assuming all other factors are identical).  Then again this only applies for
0-15!  The S6 will catch up after 15 and leave the S4 in the dust, at least
in a staight line, depending on how fast you really want to be going in this
kind of a road condition!!

This points out, perhaps, the sole performance advantage of the ABS-based EDL
system, is that it will manage torque at speeds above 15mph  (though
generally the software will also shut off this functionality at a certain

The biggest advantage,however, is that ABS-based EDL is CHEAP to the
automaker.  For a few extra valves in the ABS body, a few KB of added code in
the micro, and a separate wheel speed sensor at each wheel, this
functionality can be provided. (alot of early ABS systems and most truck ABS
systems today only measure rear propshaft speed, not individual rear wheel

In most cases, this proves to be a lot cheaper than the cost of a limited
slip or locking mechanism.  From a longevity standpoint, this system
istendent to be more robust, with less warranty claims.  The wear items, the
brake pads and rotors, are now more the responsibility of the owner, rather
than the car maker via warranty.  (excluding many included-maintenance plans
offered by premium automakers such as Audi)

Also, the reality of this is that such a small fraction of owners ever need
this functionality, that the degraded performance of the cheaper ABS-based
braking system will never be noticed by most owners.

OK, since my "simple explanation" has filled more than a page now, I'll shut
up.. hopefully this will prove enlightening to a few people..  I welcome any
comments or questions.

To now answer Aaron's questions original questions (to avoid the classic
engineer or management response of talking for 30 minutes in response but
never actually answering the original question..)

Q1) So does this mean mine has ABS traction control and not locking diff?
A1) Yes, I'm pretty sure, according to urS4.com. I believe the surefire way
to tell is whether or not you have a diff lock switch on your center console.

Q2) Do the ABS systems work better?
A2) IMO, No.  They work cheaper though!!
(And they provide me with employment.. viva ABS, EDL, & DSC!!)

Q3) Could not possibly be as uch fun as a locker.
A3) Agreed

Keith "babbling about braking in der fatherland" Maddock

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