RE: how does an open differential work

["Lawson, Dave" <dlawson@ball.com>]

>
... just because the other wheel is not spinning does not mean that
there is no torque applied to it ... the amount of torque that is being
used to accelerate the spinning wheel simply isn't sufficient to cause
the other to turn.  
>
I can accept that there is a small torque applied to the side gear of
the wheel which isn't turning. And it is like you say, it is enough
torque to accelerate an unloaded wheel which is off the ground, but
can't over come the forces on the loaded tire to make it propel the car
forward. Extrapolating this, if the engine could produce a massive
amount of torque, the spinning wheel would keep accelerating and
eventually the loaded tire could exceed the forces and move propel the
car. Now how much torque this would take, i don't know. I would guess
that our internal combustion engines 
don't have this dynamic range of torque output to do this in the time
that wheel leaves the ground until it re-contacts the ground. It is
easier to bolt in a limited slip differential in place of the open diff
or to design the suspension so you don't lift a wheel.

Hmmmm, I wonder about flywheels attached your drive axles with some nice
stored momentum, and engage it on the loaded wheel...

>
Now, let's start adding a bit of resistance on that
freely spinning wheel ... let's say by applying the brake just slightly
(not enough to actually cause it to stop spinning).  Eventually you will
get to the point where enough torque is supplied to the "stationary"
wheel to get it to start turning.  
>

Now your conditions have changed and in this condition torque is sent to
both wheels. This is equivalent to the lifted wheel on a car in corner
coming back into contact with the ground. The car can now accelerate out
of the corner.

>
Do you know why very few if any quattros have torsens any more?  They
now use "EDL" which really is the same thing I described above ... use
the brakes on slipping wheels to allow more torque to the wheels that
have the grip ...
>
All the A4 and A6 models have a torsen center diff, don't know about the
A8. And they do have EDL traction control. They say EDL stands for
electronic differential lock, but there is no electronic control of a
differential. It works as you describe, they use the brakes to
limit/control wheel spin. And it only works below 25mph. Now when
applied to a torsen, this is one solution that Scott and Jeff might
accept. I was nerding out with some office mates and discussiing this
subject and one guy is a jeeper, you know, the slow 4wd kind that likes
to climb 4ft rocks and such. BTW he said the the offroad 4wd guys like
torsens, but when using one with tires bigger than 30in diameter, the
torsens break. Also said that you can buy aftermerket torsens to fit a
variety of differential housings used on these trucks and they are
priced around $600. When presented with the torsen hunting description
which Scott detailed, he said pull the parking brake. And thinking about
it that is what the EDL is doing. It tries to stop the spinning axle
from spinning at such a velocity to where the torsen will try to
redistribute the torque to the slower rotating axle, which is the front
in Scotts example. So if you can eliminate that transfer of torque to
the front, you eliminate the body rotation about the cg and the car
should maintain its oversteer attitude through the corner with the
majority of the torque being sent to the back axle, which is what Scott
desired.

Now, how to retrofit this EDL onto a 200 torsen chassis and allow it to
work at all speeds... We got the ABS and axle speed signals, we know how
to defeat the speed input to current rear locker, sounds like you just
need the control unit. 

Now who will undertake this science project. Hello... Shokan... About
those 20 wrecked A4s out back... How much for the EDL box....

-
Dave