RE: Limited slip

["George W. Selby, III" <IsuzuG@prodigy.net>]

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.

After I started thinking about it, I realized I had left out the torque 
factor of the drive wheel with the most traction.   If you add it in to my 
scenario, it works just like you explain it, below, which is a quite 
succinct explanation (why didn't you post it to the list in the first 
place?)  What got me in trouble was when I started attaching torque values 
to the explanation.  I thought it would make things clearer, but in fact it 
just muddled it up. Plus I realized that the 50/150 split I had chose just 
happened to = the bias ratio possible with a Torsen

I saw your post on the q-list, and I really can't comment on the use of the 
brake trick on racing applications since my use of the brake trick has 
primarily been to free trucks in an off-road situation.  I believe that 
theoretically it would work, but the other consequences could be far worse 
(OH SHIT OH SHIT OH SHIT!!!!)  Having learned to drive on dune-buggies and 
VW bugs (both with rear swing axle suspension) I can assure you I know all 
about spins due to lift throttle or even worse braking during hard 
cornering.  I can now manage a oversteering car quite well, but can't 
really explain why (see paragraph 1).

In case some are wondering I have nothing against Torsen diffs and actually 
was already planning on getting a Detroit True Trac for the front of my 
F-150, to go with the Detroit Locker in the back.

George Selby
78 F-150 400M, 4 on floor, 4x4
86 Audi 4000CS Quattro
IsuzuG@prodigy.net

-----Original Message-----
From:	QSHIPQ@aol.com [SMTP:QSHIPQ@aol.com]
Sent:	Monday, June 21, 1999 4:13 PM
To:	IsuzuG@prodigy.net
Subject:	Limited slip
In a message dated 6/21/99 1:28:53 PM Central Daylight Time,
IsuzuG@prodigy.net writes:

> In the example I was trying to keep the numbers the same between the 
three
>  example of differential type:.  If you have 150 at one side and 50 at 
the
>  other, you will transmit 200.  If you have 150 at one side and 0 at the
>  other you will put 150 to the ground.
Understand your intention, but you got the operation wrong.

>
>  Please explain how I am wrong as I go thru each scenario:
>
>  The open diff - transmits 100% of torque to the wheel with the least
>  traction. - any excess not absorbed by the ground is wasted spinning.
You have decreased Trg, you didn't in the example you gave.  If one wheel 
can
hold 50lb/ft, the other 150lb/ft, then the total torque to the ground is
100lb/ft.  Engine torque in excess of 100lb/ft spins the wheel opposite the 
one that can track 150lb/ft.  An open diff equally divides torque until the 
wheel with the lowest traction is exceeded.  Torque is always double the
traction of the lowest cf wheel however.
>
>  The limited slip - transmits most torque to the spinning wheel, but
>  transfers a percentage (less then 1/2) thru a system of hydraulics,
>  clutches, or gears, to the non spinning wheel.  Notice it has to 
transmit
a
>  portion of the torque being apply to the ground or brake in order to 
have
>  torque at the wheel with the most traction.
No, the limited slip transmits most torque to the NON spinnning wheel. 
 It's
usually more than 1/2, in fact most LSD are in the 75-82% locking or 3:1>
Bias Ratio.  The audi torsen is 78:22 which is 3.6:1.  But remember the 
shift
is 78-22=56%.  The total torque to the ground can't exceed the maximum
traction of all four wheels.  The torque at any given axle can't exceed the 
Bias Ratio of the torque of the wheel with the least traction.  For your
200lb/ft example and a 3:1 Bias Ratio Torsen, the 50lb/ft wheel transmits
50lb/ft, the other 150lb/ft for a total supported torque output to the 
ground
of 200lbft.

>
>  The locker - transmits 100% of torque available to wheels  (any excess
>  still results in spinning, but by both tires).
The locker will transmit 50lb/ft to the one side 150lb/ft to the other. 
 Lift
the 50lb/ft  wheel the locker transmits 150lb/ft to the ground and will 
spin
that grounded wheel with any excess Trg.
>
>  My explanations may not have been to clear, although I tried, but it
>  explains the phenomena that I correctly described the fix for - that is, 
>  the non-movement of a vehicle equipped with torsen diff, due to wheel
being
>  in the air.  Once again just pull up on the e-brake and the vehicle will 
>  probably move.

Many a time has this been effective, and really it is what the audi EDL and 
merc 4ETS systems do.  The Merc can apply braking to 3 wheels with 3 open
diffs (F,C,R), since it's tcs matrix can cross the center diff.  The audi 
EDL
can only apply braking to 2 wheels, and hence uses a torsen center, because 
the front and rear TCS are independent of the other.

>
>  The guy who asked the original question tested my trick, and stated it
>  worked, and was happy with my explanation.  I was trying to water it 
down
>  to simplicty for a person who obviously wasn't familiar with 
differential
>  operation at all.

No question in my mind that brake application in torque distribution works.
Problem was you really didn't have the differentials operation theories 
right
yourself.  Makes it tough to explain.  What you are effectively doing is
increasing the Trg to the wheels with the braking trick.  This can work 
with
an open, locked or LSD differential.  The least effective is the locked, 
the
most effective the LSD, and the highest heat generates from the open.

HTH

Scott Justusson