RE: limited slip - cool trick & interesting analysis

[George Selby <IsuzuG@prodigy.net>]

I have used the brake trick, the poor mans locker, on both two and four 
whel drive, limited and unlimted slip differnetials.  (Never owend a locker 
equitped vehicle until my present 86 4kcsq.)  You have to apply the brake a 
lot more on an unlocked differential as opposed to a limited slip.  On 
Hummers, which use a 2 torsens and a manually lockable center (but 
unlimited) diff.  The procedure if one starts to lose traction is to apply 
the brakes gently to redistribute the torque to all four wheels, so I guess 
it is a matter of careful application.

I believe the statement in your first paragraph is correct, if you have 
three torsens then you would be no better off than with two wheel unlimited 
slip diffs.

Basically all the limited slip or Torsen diff does is to promote stability 
in moderate slippage conditions.  When you have a 0 torque spinning wheel, 
however, they can't do nothin' for ya man.

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


-----Original Message-----
From:	Avram Dorfman [SMTP:dorfman@est.org]
Sent:	Saturday, June 19, 1999 2:32 PM
To:	IsuzuG@prodigy.net
Cc:	quattro@audifans.com
Subject:	Re: limited slip - cool trick & interesting analysis

That's an interesting analysis - let me see if I have this straight: you're
saying that a Torsen differential can only redistribute a portion of the
torque that is being "used" by the contact patch at the lowest friction
wheel. If that's true, it sounds like if you had 3 Torsen differentials, 
and
one wheel in an tiny rutt on wet slick ice, you'd be no better off than a
loser with 2 wheel drive and a full-slip diff. Is that true?

We tried your trick of applying the hand brake with the rear wheel in the
air. Worked like a charm! The airborne wheel spun a little, and the car
completed the hill until all 4 were on the ground again.

However, this doesn't say anything about Torsen diffs. If I'm understanding
correctly, this trick works equally well with full-slip diffs. It's the 
same
principle as Electronic Differential Locking, except it's manual - if you
apply a resistive force to a spinning wheel, then by definition of a diff,
that force causes an opposite reaction at the other end of the diff.

We confirmed this in my '87 4kcsq: with both diff locks *off*, I couldn't
get up the hill at all once the rear wheel went airborne. But then with the
parking brake on about 1/3 force, I tried again, still w/ diffs unlocked,
and I got up the hill too, airborn rear wheel and all. A very cool trick to
say the least!

Here's a follow on question: Does this only work b/c the parking brake only
applies force to the rear wheels? I coudn't seem to make it happen with the
brake peddal. Is it just a question of skill & finding the right pressue? 
Or
is it that the brake peddal applies equal pressure to all 4 wheels, and
therefore the grip equation is equivalent, just with a higher coefficient?
It sounds like you're saying this works in 2 wheel drive cars - is that 
ones
with full slip diffs, or only ones w/ limited slip?

-Avram

Original message:
> Date: Fri, 18 Jun 1999 18:11:17 -0400
> From: "George W. Selby, III" <IsuzuG@prodigy.net>
> Subject: RE: Limited slip
>
> Limited slip is not locked.  Locked is what is takes if you lift one 
wheel
> off the ground.  The limited slip sends (usually with American Domestic 
V8
> - - not the same but it illustrates the point I'm about to make)  approx. 
3
> times as much torque to one wheel as the other.  The spinning wheel gets
> the most, the stable one gets the least.  So if you have say 100 ft-lbs. 
of
> torque to move the vehicle, 25 is going to stable side and 75 to spinning
> side.  The car moves.  Now lift that spinning tire off the ground.  Now 0
> torque is spinning the tire.  The other side gets 1/3 of 0, which is 0.
>  The car does not move.   This is what happened to your buddy.  The
> spinning wheel took 0 torque to spin, so that's what the other axle got
> too.  When you spin a tire on the pavement, or off-road, it usually 
allows
> some torque to the ground, and the limited slip distributes some of this 
to
> the non spinning side.
>
> Now a quick fix.  Apply the brake (the e-brake if its the rear wheel 
that's
> spinning) this will put a little drag on the wheel, requiring torque to
> overcome, and voila, torque to the non-spinning wheel and you just might
> move.
>
> This trick has saved me numerous times off-road in two wheel drive 
trucks.
>
> Or if you have diff locks, just lock them.
>
> George Selby
> 78 F-150 400M, 4 on floor, 4x4
> 86 Audi 4000CS Quattro
> IsuzuG@prodigy.net
>