15mph diff lock off - torsen content

[QSHIPQ at aol.com]

Dave E writes:

>according to audi (sae 880321), if you lock the rear diff, it can or will
>lead to instability under braking, particularly if you hit mixed traction
>surfaces.

Nope.  880321 gives the advantages of ABS with torsen vs locked 
differentials.  The torsen with abs disengaged is actually going to perform 
worse under braking with rear locked than a gen 1 with both center and rear 
locked.

Not sure you don't have that anyway.  If you have a rear locker with a 
torsen, you have 3 channel abs.  With 3 channel abs, if one wheel locks up, 
both are released to the wheel with the least amount of traction, so rear 
braking ability decreases.  And, you have a problem with mixed traction 
conditions with ABS off anyhow.  Isn't instability in braking alluding to ABS 
defeat, not so much locker dis/engage?  If one rear tire locks up in braking 
with a locked diff, both do.  Being one who's intimately familiar with rear 
diff locking under mixed traction surfaces (center diff locked or unlocked), 
I'm not really sure what the concern is.   Also the test methods in 880321 
don't at all include a steamboat type or performance type event (thinking 
scandinavian flick here), this paper only exactly addresses a given cf and 
breakaway, not intentional performance driving.


>the assumption obviously here is that abs is disabled.

>with the generation 1 quattro, as soon as the rear diff is locked and you
>brake on mixed traction surfaces, a lot of steering input is required.  with
>rear locked and centre free, 180 degrees of steering is required.  audi
>considers this outside the limits of controllability ("for a driver of
>average ability").

That appears to be a steering ratio problem to me.  

>with both diffs locked, over 600 degrees of steering input is required.  in
>other words, kyagb.

Just got back from steamboat, it wasn't the locker guys using a lot of 
steering input.  In fact, all stuffs with lockers were associated with lift 
and steer problems creating massive understeer under power in-lift off turns. 
 Not one locker off the straight due to loss of control with diffs locked.  I 
can recall several torsen cars (1 200 almost kept going into the field) with 
this problem however.  

>with a torsen (which unlocks on overrun, and so is very similar in this
>regard to a unlocked differential), i would expect that you would be looking
>at something close to the 180 degree steering case.  not easy to control,
>not pleasant, and not safe in other words.

Not with you here Dave.  Remember, were are speaking of performance driving 
here, not a street driving situation.  An ABS defeated torsen car with rear 
lock will have better acceleration, and maybe not better braking (depends on 
how you brake).  It might equal a gen 1 with the center unlocked, rear 
locked, but will be inferior if the Gen 1 center and rear are locked.  

All this paper tells me, is that greater control in turns on low friction 
surfaces can be had by driving center diff locked all the time, and exiting 
the turns (at any speed) with the rear diff locked, giving the greatest 
performance (btpostedt in former Steamboat reports) in terms of consistent 
lap times.  That also would tell me that this procedure would also benefit 
the torsen drivers too.  I personally don't find any difference approaching a 
corner with the rear diff locked or not under braking, cuz it is done with 
either the flick in mind or in a straight line.  

Interesting to note too  that the center diff locked only car in fig 12 has 
lower steering angles than the open diff.

CONCLUSION:
Gen 1 lockers should lock the center diff always during spirited driving.  
Both Gen 1 and Gen II (torsen with lockers) would benefit greatly from being 
able to engage the rear diff lock at any speed, using it as a post apex 
traction aid in all performance driving.  The loss of ABS in spirited driving 
might be the "cost" of this mod, but certainly at the Steamboat venue, one 
could argue worthwhile.

The posts were wrt to overriding the 15mph shutoff.  880321 doesn't change 
the advantage of doing so, given the parameters most are looking for.  IF you 
have locked the rear diff on any abs audi quattro, you have no abs.  I argue 
the traction benefit outweighs the braking benefit in this scenario.  Few 
with the choice run abs enabled at steamboat.  Non ABS or ABS OFF locker Gen 
1's consistently indicate that the best performance is attained by the 
driver, not the abs on, or the locker disabled.  Quite the contrary....

HTH

Scott Justusson
Diff lockin dude.