[torsen] 15mph diff lock off - torsen content
QSHIPQ at aol.com
QSHIPQ at aol.com
Thu Feb 8 15:49:12 EST 2001
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
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
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
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.
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....
Diff lockin dude.
More information about the Torsen