Re: limited slip - cool trick & interesting analysis

["Avram Dorfman" <dorfman@est.org>]

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
>