Re: A4 Electronic locking diffs...

[dan_masi@MENTORG.COM (Dan Masi)]

On Oct 18,  8:31am, PDQSHIP@aol.com wrote:
> Subject: Re: A4 Electronic locking diffs...
> In a message dated 96-10-18 01:41:30 EDT, you write:
> 
> > Nope.  You are confusing locked rotational speed with equal 
> > torque.  It ain't so.  A locked diff will apportion torque in 
> > relation to the traction available at the individual wheel. 
> > Torque distribution could vary from 0:100 to 100:0 and 
> > anywhere in between with a locked diff.
> > An open diff will always split the torque equally.  In low 
> > traction situations it simply won't be albe to split much of it.
>  
> .... A locked diff in the center will split torque 50/50....  An open diff
> will send more torque to the spinning wheel, hence the locking....  On a side
> to side diff, the lock is rarely 100%, usually more like 70%, even with a
> torsen, hi is 80%....  The locking mechanism on audi GENII is 100% or open,
> 100-0-100 to the spinning wheel. Open diffs send more power to the spinning
> wheel, limited slips "lock" the other side when wheel slip is sensed....   Me
> thinks you might be confused....

Actually, no, he's not confused... technically, he's got it
right.  Seems that torque is an often-confused subject, though.
Look at it this way: assume you've got a RWD car with the right
tire on ice, and the left tire on pavement, and that it'll take 
100 pounds of force to get the car moving.  Also assume that a
spinning tire on ice will generate at most 10 pounds of force.

Now, picture the car with an open diff.  Let out the clutch.  Right
tire spins, car goes nowhere.  If you measured the force that each
tire was contributing, you'd find 10 pounds from the right, and
(ever-so-slightly-less-than) 10 pounds from the left, which
combined is not enough to move the car.  Sure, the engine might
have been capable of opposing a 300 pound force, but there's no
such force to oppose in this case.  The open diff is supplying
equal force (i.e. torque) to each wheel.

Now, imagine the car has a locked (really locked, not viscous
or other pseudo-locking mechanism) diff.  Let out the clutch.
As the engine tries to move the car, the opposing force
builds.  If you measure the force at the left wheel, you'll
find it climbing (this is all in the instant before the car
moves), 10, 20, 40, 70, 99 pounds.  At the right wheel, there's
almost no opposing force.  Measure the torsional stress on each
axle at this point.  Is the left axle stressed?  You bet.  There's
a lot of *torque* on it.  Is the right axle stressed?
Hardly... it's not doing anything.  So, at the output of the
locked diff, we see a large force (torque) on the left side,
and almost none on the right... hence, you could say that the
locked diff allows for different torques, or has split the
torque.

Hope this helps...

Dan Masi
'96 A4Q