RE: torsen tech 102

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

ok thanks for all the cards and letters.  most accept the baseline, time to
move on....

jack raises key points which require explanation...

first, some basic understanding explanations are required.  you need to
understand torque and speed, and the differences between them.  you need to
understand that an open bevel diff is a fixed torque proportioning device
where 50% of input torque is apportioned to either output shaft, regardless
of the either shaft's output *speed*.  a locked diff (called a locker
herein) on the other hand is a torque proportioning device where up to 100%
of input torque can be apportioned to either output shaft because output
shaft speeds are the *same* (locked) (if it would help, think of a locker
with the front driveshaft disconnected - 100% torque to the rear and you can
move).  what is a "bias ratio"?  simply this is the maximum torque imbalance
between output shafts that a differential can support.  defined as tbr =
t(max)/t(min).  an oft-quoted audi torsen tbr is 75/25=3.0, but other audi
torsens have different figures (80/20=4). for completeness, an open
differential has a tbr of 1:1.

anyway, the torsen.  a marvellous little device.

it has 4 basic modes of operation.

1) slow speed, tight turns.  kinematics are the biggest factor.  the diff is
unlocked (generally) because it is operating at the bias ratio.  it is
operates as a bevel diff and allows different output shaft speeds.  this is
useful for slow speed good traction turns (car parks) where it is important
to allow speed difference between the shafts.  depending upon the bias ratio
and the diameter of your turn, you will or will not hear tyre squeal as the
diff compensates for different axles speeds or not.

2) normal driving.  the torsen diff acts as a bevel diff with a 50:50 torque
distribution.  note that the locker can't and doesn't do this but maintains
a torque distribution which mirrors the weight distribution of the vehicle -
i.e. front biased due to the relative lack of tractive force at the rear
(more weight up front).  this is the primary reason for the relative lack of
understeer for the torsen in contrast to the locker on corner entry.

3) cornering.  the torsen diff is *locked*.  read that again folks.  the
diff does *not* allow *any* speed difference between the front and rear
axles.  why is that?  well it's a friction device, and the internal friction
(i.e. stiction) prevents speed differences between the shafts.  while the
diff is not at the bias ratio, the diff is locked.  however, the diff also
allows torque to be transferred to the rear axle.  how?  well because it's
acting as a locked diff at this point, and it is putting torque to where the
greater tractive force is.  based on front and rear (relative) wheel slip.
in *exactly* the same way as the locker.  in the light of scott's and jeff's
arguments about the torsen getting confused by slip angles, when in fact it
is operating in the same fashion as a locker at this point, it would mean
that if they are right, then the locker has exactly the same problem.  heavy
irony alert.

4) cornering at the bias ratio.  at this point the diff is *unlocked* and
will allow axle speed differences to occur.  when the bias ratio is reached
in cornering (e.g. 75% torque to the rear), the diff will apportion more
torque from the fast rotating axle to the slower one.  it is back to acting
as a open diff.  how often do you corner at the bias ratio?  very rarely in
practice.  audi's figures show that a 80q accelerating around a 40m circle
with lateral acceleration of 4m/s/s ends up with a final torque ratio of 62%
to the rear (torque is progressively moved to the rear).  so the diff is
still locked.  according to audi it would take a circuit of 15m at the same
speed to reach the bias ratio.  most often the bias ratio is encountered on
low cf surfaces, or a low speedon high cf surfaces and tight turns (parking
lot).

thats it folks.  pretty simple really.  a nice little device.  does the
torsen get confused by slip angle differences?  no it can't any more than
the locker can.  because it doesn't see them.  the torsen and the locker are
identical in behaviour during cornering because they are both locked and
allow no axle speed differences.  in both cases torque will go to where
relative wheel slip dictates that it should go.  once the bias ratio is
reached (and about when the locker goes into terminal understeer), the
torsen will limit understeer by diverting torque to the front (i.e. allowing
axle speed differences).  it is at this point seeking to deny wheel spin on
either axle.

torsen behaviour braking in a turn is not the subject for debate, but i'll
explain this (and how it differs and improves on the generation 1
technology) if required.

in "torsen tech 103", i'll look at comparative behaviour of the qudi
generation1, 2 and 3 systems.

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

-----Original Message-----

Date: Thu, 21 Oct 1999 10:59:12 -0400
From: "Rich, Jack" <JRich@Kronos.com>
Subject: Re: torsen tech 101

I can't believe I'm butting into the thread from hell, which I was really
quite tired of, but...

Dave,

When you refer to a "locker", are you talking about the Gen1 diff being run
locked, or unlocked?  If you are referring to unlocked, then it distributes
torque to the end of the car with the least traction.  This is a standard
differential, which I usually refer to as an "unlimited slip" differential.
If you are referring to locked, then I have to assert that this will force
the driveshaft to the front differential to spin at exactly the same speed
as the driveshaft to the rear differential.  Wheel slip is *forced* to occur
in a turn.

You say:
>when cornering within the bias ratio of torque distribution the torsen diff
>is *locked*.  you may need to re-read that statement.

Sorry, but re-reading it hasn't helped me.  You cannot say that a torsen is
*locked* in a turn within its bias ratio.  If this were so, it would not be
possible to turn a torsen-center-diff'd car at low speeds in a parking lot
without major drivetrain shudder and chatter as the wheels were forced to
slip.  (Never mind.  You can say it.  I mean that I don't understand it and
don't believe it.)  "Locked" means that there is no slip occurring, and that
the two outputs are spinning at the same speed.  I believe you may mean the
opposite -- that beyond the bias ratio (say 75:25) it "locks" into that
percentage, i.e. one output can never spin less than 1/3 the speed of the
other output drive shaft.  (I'm guessing at your meaning here.  I don't
claim to know poop from shoe polish about torsens.)

>let me say it again in a different form.  the torsen operates as a bevel
>differential until relative slip between the axles occurs.
There will be relative slip the instant you turn in a parking lot -- the
front wheels will be