[torsen] RE: 15mph diff lock off - torsen content

Dave Eaton Dave.Eaton at clear.net.nz
Fri Feb 9 15:53:14 EST 2001

scott,  the original post was asking about the effect of maintaining the
rear diff lock on a torsen car above the cut-off speed.  the audi paper
addresses this.

ego to one side, this is a safety issue - not a (torsen) pissing contest.

if you want to tell people that it's ok to lock the rear diff under the
conditions where, according to audi's engineers: "the driver is forced to
correct the steering quickly and accurately.  during the first second, the
rate of increase of the steering angle exceeds the limit of
controllability", then i would assume that you are not giving this advice
"professionally"?  if so, then i hope that you have liability insurance.  if
you're not, then i will categorise this in the usual file...

'95 rs2
'90 ur-q

for those who are interested, make up your own mind - what the relevant
section in the audi paper (sae 880321) actually says is:

"The following description describes some of the problems arising from the
effect of differential locks on the ABS control process.

With differential locks giving a fixed mechanical connection between the
wheel brakes it is not possible to regulate the wheel speeds individually,
which naturally impairs braking behaviour.  Fig. 10 Shows the variations in
wheel speed when braking on a very low friction surface (µF 0.1) with the
central differential locked and the clutch to the engine engaged.  This
clearly shows the deviation of the wheel speeds and, as a result, the
reference speed.  The control performance is severely impaired.

Fig. 11 shows the deviations with and without differential locks during
cornering.  With a surface friction value of µF 0.25 the vehicle's speed
very nearly reaches the maximum possible cornering speed.  With the centre
differential free there is only an insignificant deviation in the control
function, and the cornering radius increases by about 2 metres.
When the centre differential is locked, this causes considerable deviations,
resulting in an increase in the cornering radius of about 12 metres.  If
both the centre and rear differential locks were engaged, the resulting high
slip values could cause the vehicle to leave the corner at a tangent.

With high surface friction values, again driving at the maximum cornering
speed, no serious loss of stability or performance impairment is observed.
When there are different surface friction values on either side of the
vehicle, this places greater demands on control performance and reliability.
These conditions, known as split µ, can occur quite often in spring, with
friction values differing by anything up to a factor of 10.

Fig. 12 shows a test series with split µ conditions of 0.25 and 0.8; the
graph shows the steering wheel angle as a function of time.  It is easy to
see that there are no stability problems without differential locks or with
the centre lock engaged.  The required steering angle does not exceed 180
degrees.  With the rear differential locked and the centre differential
free, the driver is forced to correct the steering quickly and accurately.
During the first second, the rate of increase of the steering angle exceeds
the limit of controllability.  A driver of average ability would encounter
problems at this stage.  The effect of locking both differentials hardly
needs comment: 500 degrees of steering angle in one second is beyond the
capabilities of even the best drivers."

-----Original Message-----
From: QSHIPQ at aol.com
Date: Thu, 8 Feb 2001 15:40:20 EST
Subject: 15mph diff lock off - torsen content
To: quattro at audifans.com
CC: torsen at audifans.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

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
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.
>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.

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