RE: Limited slip

[Eaton Dave <dave.eaton@minedu.govt.nz>]

george, i wouldn't worry about making mistakes on the list.  we all do.  i
certainly find it amusing to see you being lectured about "open diffs
splitting torque 50:50" by someone who, not 3 weeks ago, was vehmently of
the opposite opinion :-)  so, clearly, we're all learning here....

fwiw (as a sort of summary), the open diff cannot do anything other than
split torque 50%:50%.  the amount of torque transmitted is determined by the
driveshaft with the least traction.  there is no "torque transfer" across
the diff, that is, both output shafts share torque equally.  the open diff
is a torque *equalising* device, but not a speed limiting one (i.e.. one
shaft can be stationery, the other spinning like crazy).

a locked diff will transmit 100% torque to either output shaft, as required
by tractive conditions (wheel lift etc).  however, it will not allow speed
difference between the shafts.  it functions as a torque distribution,
speed-difference-limiting device.  it's inability to allow speed
differentiation between output shafts means that, in the classic audi
arrangement, major understeer reigns.  this is also due to the fact that the
quattro with locked centre will take more torque to the front during
*normal* operation due to the chassis kinematics (i.e. being front heavy).
this means that the locked centre quattro will behave more like a fwd car,
will understeer more under power, and exhibit a greater tendency to snap
oversteer on throttle lift during cornering, than the open centre quattro.
interestingly, another side effect of this 100% torque distribution effect
is that you must now size your output shafts to accept 50% of the input
torque, whereas with an open diff, 25% is all that is required.  this has
obvious implications in weight.

a torsen otoh, will re-distribute torque based on the traction requirements
of the output shafts.  but the important fact is that (1) because of it's
internal gear friction, it will *limit* the maximum torque distributed to a
shaft to a % (the bias ratio) of the input torque (the internal friction is
providing a torque "drain"), and (2) it allows output shaft speed
differences.  the actual bias ratio varies depending upon the type of torsen
design (e.g. whether the diff is used in the centre or on an axle).  the
torsen starts to shift when it detects a torque imbalance between the
shafts, *not* a speed imbalance.  the amount of torque shifted will be
limited to the output shaft with the *most* traction.  so, the torsen is a
torque distribution device, but allows different shaft output speeds.  it
will shift torque up to the bias ratio (as will any limited slip
differential).

the vc (again there are a great many types with fundamentally differently
characteristics of torque transfer) uses axle speed differences to initiate
torque transfer and will continue to differentiate up to the bias ratio
(again dependent upon design), or until heat causes "hump".

there is a better paper of the torsen in awd setups (more descriptive than
the chocholek paper on audidudi's web site) from autotech '95 which i
circulated last year.  when i get some time, i'll get around to publishing
some of the other stuff i have on the torsen (yeah right)... 

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


-----Original Message-----
Date: Tue, 22 Jun 1999 00:43:33 -0400
From: "George W. Selby, III" <IsuzuG@prodigy.net>
Subject: RE: Limited slip

OK, I can buy that, thanks for the info, I and will try not to give 
technical explanations in the future, I will try to stick to the practical 
applications.  That was the reason I had to drop out of engineering school 
- - I could always do the projects, they worked correctly, and I was usually

the first one done.  However I could usually not theoretically explain what 
had just happened, my stuff always worked so I felt why should I explain 
it, it works, those whose projects don't work, they can explain how it was 
supposed to.