[torsen] Re: diffs and flaming.............sorry it's long....

QSHIPQ at aol.com QSHIPQ at aol.com
Tue Feb 13 21:52:17 EST 2001

I'm just not with the thinking here Johnny.  I'm happy to increase my 
librarians pay by ordering more SAE papers stat, so I can read and understand 
them.  Here, the 15mph cutoff you are *assuming* to be for the split u 
conditions found in this SAE paper.  I might argue with simple physics and 
real world testing, that split u conditions cause unexpected steering angles 
quite regularly, and if one looks at fig 12 of 880321, you can see that large 
steering inputs are required regardless of diff (try telling any driver that 
they will have an 80degree steering input required within the first second of 
braking, not without being *really* alert, the paper assumes all are in a 
position to do this, I don't).

That said, a couple of issues that don't come up in the paper are relevent 
here, including abs behavior in split u conditions.  Remember that the test 
methods the audi boys used in this paper had 1 side of the car on u.25 and 
the other on u.8.  2 wheels on each.  The real world in which we live in 
split u, means that argument might be guaranteed for exactly the wheelbase of 
the car, maybe (most likely) even less.  So the results are not only skewed, 
they are incomplete and unreal representations of real world split u

The control issue in 880321 was somewhat misrepresented by Mr E, you should 
get it if you are really interested in it's conclusions.  What he is 
referencing wrt to the torsen car is the rear diff lock only which requires 
180degrees of steering input to correct in the conditions stated.  What I am 
referencing wrt my experience with locking diffs is even more severe, with 
locked center and rear, 500degrees of steering angle is required for me to 
maintain control in the split u condition described in fig 12, within 1 
second.  I dare say I haven't had to do that, but then again, I tend to play 
in more predictable split u conditions as well, so I doubt I ever will.  

What this paper really brings forth, IMO, is that with a split u extreme, no 
driver could handle the input.  I don't disagree.  I only add, that split u 
extremes are rare, so rare in fact, that I argue that anyone caught unawares, 
abs on or off, rear diff lock on or off , center diff on or off: Is going to 
have a control problem.  The safety feature that audi added to address this 
really isn't the 15mph cutoff, it's a full 4 channel abs system.  3 channel 
abs sucks, and it's what they had when they published this test.  When audi 
was able to use 4channel abs, they took the diff lock out alltogether and 
gave it a traction control feature on the rear axle.  THAT, and ONLY THAT, 
assures that control might be retained on a split u of .25 and .8 
respectively.  It then would depend on when and where the driver hops to the 
other u.

I'm all for testing and methodology and proper conclusions.  Here I believe 
you and many others assume 880321 is the *reason* for 15mph disable of the 
rear diffs.  I don't buy that argument, cuz if it were true, then why 
isn't/wasn't a retrofit of the gen 1 rear locker (with even more benefit) 
available to quattro owners.

Naw, I believe the 15mph disable was cuz people forget to do stuff.  I can't 
tell you the number of customers I have that have no idea what the diff knob 
does in their Gen 1, or have no idea the limitations of (or even that they 
have) 3 channel abs.

Thanks for your post, I'd like to see more real world testing of the moderate 
split u conditions.  Cuz right now, I look at this paper, understand what was 
written, and it doesn't change my behavior towards the split u conditions 
they describe (isn't this test in fig 12 the same as dropping a wheel off the 
pavement and hitting the brakes.  I don't remember that warning to *not* 
include ALL cars), nor does it modify my execution of diff locking in 
performance driving.  

I advocate that anyone that wants to understand audi quattro chassis dynamics 
could get a better education than going to steamboat once a year.  The 
experience jazzes you on quattro, frazzles those with torsens, and teaches 
all the reality of physics applied to the real world


Scott Justusson 
In a message dated 2/13/01 8:03:01 PM Central Standard Time, J123fs at aol.com 

> hi,
>  Again with my .02 .
>  Regarding all this comment(s) on the diff position: Rear lock or unlocked, 
>  center Torsen, or early manually lockable.
>  I again have seen much opinion, and little hard fact, other then the 
>  SAE papers........
>  .................There is truth in both sides of the argument regarding 
>  discussion.........
>  SAE engineers and authors have vested interest in disseminating valid 
>  information regarding testing done in a scientific manner, and ergo, Scott 
>  has some good experience although "seat of the pants" and not really based 
> on 
>  actual testing with accelerometers that may or may not be valid. 
>  The key here is to address real world usage of the cars as anyone (Myself 
>  included) with a smidgen of knowledge of chassis tuning knows what works 
>  correctly at 10/10s DOES NOT WORK at anything less, and certainly would 
>  be acceptable for mass consummation or "run of the mill" driving styles or 
>  skills.
>  Hence the factory cutout or the locked rear diff at a predetermined (LOW) 
>  speed, so as not have massive oversteer at the INEVITABLE HIGH slip angles 
>  the cars will encounter ( the same as rally cars, off-road  live rear axle 
>  racing trucks, and welded diff (or  NO DIFF) dirt track cars experience). 
>  In my own feeble but informed opinion, what works on a packed snow surface 
>  or other low CF surface) where rear power (really power oversteer)  
>  and high correction factors ( slip angles) are both fun and the fast way 
>  go, that really have 0 bearing on any street driving, or high performance 
>  pavement driving both in the wet or dry.
>  I would challenge some of the posters to back up their arguments with 
>  physics and a bit of reading on the nature of what really holds a car to 
>  road, as I think both sides can learn valuable insight as to different 
>  of looking at things for different situations, conditions and driving 
>  not just to flame each other.
>  See Ya
>  Johnny

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