RE: Limited slip diffs

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

>IME, in slow speeds the braking does affect the torque shift of a torsen,
>center or otherwise.  I have also found that in a classic race turn 
scenario,
>applying the brakes during a torsen shift scenario, is far from comforting 
to
>the driver instinct, and increases the chance that you will lose it all
>together.  This would apply to all center limited slip differentials with
>less than 100% locking.


Apparently someone didn't read my original post.  Here I have reposted it, 
because I have clearly explained how a torsen (TORque SENsing differential) 
operates, as well as other limited slip devices, both under torque and 0 
torque conditions.  The first paragraph explains a differential 
distributing torque on the ground, and then describes how you can make it a 
0 torque spinning wheel (by lifting it off the ground)  The original 
question post asked about a wheel in the air, so that is basically what my 
response to was.  The main difference between a Torsen and a regular 
limited slip are how it accomplishes the task.  A Torsen uses a special 
fluid inside the special diff.  A regular limited slip uses small clutch 
packs and springs.  Some high performance ones use gears that bind together 
under slippage conditions.

Application of the brake trick at higher speed will only result in the loss 
of speed, not increased traction.  The brake trick is primarily to move a 
nonmoving vehicle, not to stabilize an already moving one.  Note the 
e-brake trick doesn't work in a front wheel drive car, for what should be 
obvious reasons (except Subaru which strangely use the parking brake on the 
front (drive) wheels).

I am going to restate the diff explanation in terms of a torque applying 
wheel here, both this explanation and the one in the post below, are with a 
two wheel drive, rear wheel drive car.  The same principles apply to the 
center diff in a four-wheel drive car.

Lets say the ground on one side of the car can support 50 ft-lbs of torque 
to move the vehicle, and the other side can support 150 ft-lbs.  A vehicle 
with an open diff comes to a stop on this surface, and then proceeds on. 
The wheel on the loose surface side transmits 50 ft-lbs, and the other side 
transmits nothing, and the rest is wasted in spinning the free tire.  Now a 
vehicle with a limited slip approaches the same surface and does the same 
thing.  The spinning tire still transmits 50 ft-lbs to the ground, but an 
addition 1/3 of 50 ft-lbs is transmitted to the other (non loose side), and 
the vehicle moves a little faster, and with better stability.  Now an car 
with diff locks rolls up to this surface and engages his rear diff lock, 
The axles are locked together now, and the firm side will transmit 150 
ft-lbs to the surface, and in addition the loose side will transmit 50, and 
then spin a little (but no faster than the other axle).  Thus in this 
example, a open dif will transmit 50 ft-lbs, a limited slip will distribute 
67 ft lbs, and the locker will transmit 200 ft-lbs.

Now do this with a wheel off the ground.  A open diff will just spin the 
wheel - 0 torque to the ground.  A limited slip will just spin the tire off 
the ground - 0 + 1/3 of 0.  The locker will transmit the 150 ft-lbs from 
the side on the ground+ 0 for a total of 150, and the vehicle moves.  The 
application of the e-brake provides a little torque for the limited slip to 
work with.



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

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