RE: torsen tech 102

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

Dave,

>2) normal driving.  the torsen diff acts as a bevel diff with a 50:50
torque
>distribution.  note that the locker can't and doesn't do this but maintains
>a torque distribution which mirrors the weight distribution of the vehicle
-
>i.e. front biased due to the relative lack of tractive force at the rear
>(more weight up front).  this is the primary reason for the relative lack
of
>understeer for the torsen in contrast to the locker on corner entry.

The locker torque distribution doesn't mirror weight distribution until
the distributed torque to any given output exceeds the tractive force
of the output.


>3) cornering.  the torsen diff is *locked*.  read that again folks.  the
>diff does *not* allow *any* speed difference between the front and rear
>axles.  why is that?  well it's a friction device, and the internal
friction
>(i.e. stiction) prevents speed differences between the shafts.  while the
>diff is not at the bias ratio, the diff is locked.  however, the diff also
>allows torque to be transferred to the rear axle.  how?  well because it's
>acting as a locked diff at this point, and it is putting torque to where
the
>greater tractive force is.  based on front and rear (relative) wheel slip.
>in *exactly* the same way as the locker.  

Under your scenario, I am interested in the transition from "normal" 
driving to "cornering". According to your description it goes from
open differential mode (equalising torque output while allowing speed
differences) to its "locker" mode (equalising shaft speed while allowing
torque differences). So the torsen is cruising along at 50:50 torque
distribution in open mode while the shafts could be at different speeds,
then corner entry and it transitions to "locked", shaft speeds all become 
equal. To do this, there has got to be some "range" in which it can 
operate in this "open" mode. I am wonder how big that range is, is it 
+/- 5% about 50:50, or is it +/- 0.001%  about the 50:50 mark?


>4) cornering at the bias ratio.  at this point the diff is *unlocked* and
>will allow axle speed differences to occur.  when the bias ratio is reached
>in cornering (e.g. 75% torque to the rear), the diff will apportion more
>torque from the fast rotating axle to the slower one.  it is back to acting
>as a open diff.  how often do you corner at the bias ratio? very rarely in
>practice.  

I wouldn't jump to the "very rarely" conclusion so fast.

>audi's figures show that a 80q accelerating around a 40m circle
>with lateral acceleration of 4m/s/s ends up with a final torque ratio of
62%
>to the rear (torque is progressively moved to the rear).  so the diff is
>still locked.  according to audi it would take a circuit of 15m at the same
>speed to reach the bias ratio.  most often the bias ratio is encountered on
>low cf surfaces, or a low speedon high cf surfaces and tight turns (parking
>lot).

Given the conditions described above, the bias ratio is acheived when taking
a (15m)50 ft radius corner at 17.3 mph. To transition 62% of the torque to 
the rear, all we need to do is take the (40m)130 ft radius corner at 28.3
mph.
That is about the radius of an on ramp, and may people take these types
of corners much faster. Add more speed and we approach the bias ratio.


>thats it folks.  pretty simple really.  a nice little device.  does the
>torsen get confused by slip angle differences?  no it can't any more than
>the locker can.  because it doesn't see them.  the torsen and the locker
are
>identical in behaviour during cornering because they are both locked and
>allow no axle speed differences.  in both cases torque will go to where
>relative wheel slip dictates that it should go.  once the bias ratio is
>reached (and about when the locker goes into terminal understeer), the
>torsen will limit understeer by diverting torque to the front (i.e.
allowing
>axle speed differences).  it is at this point seeking to deny wheel spin on
>either axle.

What you have just described here is a behavior of the torsen to send
the bias ratio of torque to the rear of the car which sets up an oversteer
condition and then once the bias ratio is reached to divert torque to the 
front which sets up an understeer condition.

-
Dave Lawson