[torsen] (T*rsen content warning): Some thoughts on traction - kinda long

QSHIPQ at aol.com QSHIPQ at aol.com
Thu Nov 30 08:28:20 EST 2000

Hi Jani et. al.:
For those looking for a btdt <delete> key, please do so now, as I ask 
forgiveness for my tresspasses (and maybe torsen archives?:).  My experience 
on center diffs comes from driving the first T*equipped car Audi made the 88 
90q back in 12-87.  I was doing some rally-type driving in Northern MI in 4in 
of fresh snow with my ex-navigators 1100mile new car, when I got bit in the 
butt, and hard.  No damage (other than ego), but sliding sideways at 70 was 
never quite the same, and I couldn't explain why.

Flash forward some 13years, a 100 or so track events, and a bunch of research 
on the T center diff, as well as routing audis racing specs, has provided me 
with some better understanding of what's happening.  My comments inserted to 
your questions below:

>  -Why is the 20V urquattro (T.....) considered to be one of the finest 
>  handling examples of its breed as a performance ROAD car? 

Because awd is considered a great asset to a road car.  That said, one must 
understand a couple of things in rag tests.  Rarely do these tests use 
drivers intimately familiar with quattro and it's limitations (all exceptions 
so noted).  Several (including Piech hisself) will share that driving quattro 
at or near the limit needs both skill and experience.  To understand the 
limitations of which quattro driveline gives the best performance only needs 
a look to audis racing program.  The difference, IME, is that fine awd 
chassis and handling can be had with any iteration of quattro, when you start 
to prefer the predictability of the lockers, you are driving at a threshold 
of quattro performance most drivers never experience.

>  -How is it possible *if* the T..... is hopelessly flawed as a 4wd solution 
>  for the ROAD?

Jani, driving a quattro in adverse road conditions *is* performance driving 
by definition.  Remember cf and HP are really the same thing in terms of the 
traction argument.  Dry road or the most common reason most buy quattro road?

Thanks to Chris Miller, please read:

I think if you read Chris Miller's first site, Bensinger himself:
"' He reckons it would take a 34:66 front-to- rear torque split to provoke 
the 90bhp 80 quattro into a oversteer."
Well, since a T car has a center diff that really isn't considered limited 
slip, it's a no slip diff (it can't disengage), you will have 65/35 (O) at 
25mph at a 40m radius turn (also in a paper coauthered by Mr. B).  In that 
same paper, Mr. B et. al. proposed that *IF* this O condition resulted in the 
loss of traction at the rear, the T would transmit power to the front 
regardless of the slip angle O created by turning.  The definition of this is 
U (a 90q with 50f/50 to 75f/25).  So, dynamically (turning) the T car can O 
during a turn, and if during this O condition, cf or engine torque exceeds 
the traction of the rear wheels, it will U.  Predicting that matrix is 
overwhelming, even for the race guys.  The results can be more than exciting, 
what I call the now infamous spider bite.  Remember, the above scenario 
repeats itself ad nauseum, until you actively do something, like steer, or 
throttle or lift (no) or stop suddenly (cough).  Here's hoping what you do is 
the 'right' thing, cuz I'm not sure (no one appears to be) what the 'right' 
thing is, too many variables to *predict* perhaps?

>  -Why didn't Audi just stick to the wonderful lockable diffs in the ROAD 
>  cars?

Go to either Chris Millers second site or read Goggins interview with 
Chocholek.  For *road* cars, ABS with awd was considered a paramount target 
by the audi marketing>engineers.  T* gave them that.  Which works fine, up to 
a point.  Once exceeded, Benzingers arguments get real, btdt.  In the second 
site Chris put up, one can see that customers routinely forgot to unlock 
diffs as well, they aren't user friendly to most drivers, and this oversight 
also means no ABS.  Hard to vonTechnik when your target isn't so minded.  
Accept the compromises, give the customer what they 'want', hey it sells 
cars.  It's those compromises in target acquisition and their affects that 
many of us are discovering.

>  -Since Audi abandonded lockable diffs, why didn't Subaru or someone else 
>  start using them in their ROAD performance models? Land Rover not counted.

Subaru was committed to VC's very early, and has proven that VC's are a quite 
acceptable quattro system (even audi used them in racing alot, including the 
btcc cars).  I think a more pertinent question is (and Jeff gives you some 
insight in the Chocholek interview):  "Why didn't subaru or someone else 
start using the T* in their ROAD performance models?  Land Rover counted."  
Audi isn't the only awd player out there, why is it that in 13 years, they 
are the only ones still hooked on T* center differentials?

>  And please no pissing contest, I know that Mr. Rohrl for one has said that 
>  Tors.. is unpredictable on the limit, in racing. I also know that carbon 
>  brake discs are great in F1 but suck on the street. So answer my question 
>  seriously considering the ROAD use.

StigB has said that as well.   Jani, it depends on where and how you drive.  
IMO/E, the T* diff is good (I also think it's best... In a straight line 
traction argument).  Up to a point.  When that is exceeded, I think it 
becomes unpredictable (I wanted to use a stronger word, but this will work).  
That does include adverse condition road use.  When I can find the faults in 
a T* center diff, lower traction, my wife isn't going to find a lot of 
advantages in a situation of skid control.  Watching some really good quattro 
drivers muscling the wildling T*'s at steamboat springs over the last 7years 
puts "quattro advantage" in real perspective for me at least once a year.  
And, to date, no one has come to that venue, and "proven" T* advantages.  The 
locked diff guys just keep passing them, even without ice tires.
>  BTW, I earlier had a VW Passat G60 Synchro, 160 hp with VC 4wd, and I much 
>  prefer the Torsen cars. The VC always understeered (predictable yes, fun 
>  no),the torsen oversteers most of the time on throttle. Same negative 
>  experience from GM's Calibra turbo 4x4, also VC.

Well, I disagree that the T* oversteers most of the time on throttle.  It 
passes thru oversteer moments, but it understeers, that is the nature of it's 
design (it's constantly striving for a 50/50 split in traction or turns, 
that's U, and baseline chassis is U).  If Benzinger indicates that 65/35 on 
power would be needed for a 90q to oversteer, you must be taking a lot of 
turns.  First rule of thumb, quattros don't oversteer, in T* context, they 
only have their moments, very rare ones.

>  RS2 -95 (oops with Torsen, am I banned?)

Banned?  Naw, just having more fun (or not;) at the wheel than those of us 
without.  I work on quattros for a living, and have a passion for driving 
them very hard.  I also have discussions all the time (just did last night 
with a v8 guy) with customers who can't explain the 'unease' they felt in 
such and such slippery conditions, guess what diff those conversations 
revolve around.  I also have a passion for practical engineering, finding 
faults in what was done and correcting them.  Unfortunately, here, I don't 
think there is a "correction" to the device (short of a heliarc welder) in 
the *performance" arena.  

The real concern I have is how to teach the proper techniques to Qclub 
students in over their heads. With all the chip mods etc folks are doing to 
their cars, and audi seems to be addicted to adding more baseline to that 
equation, we are now seeing road car exceeding race car performance. Chip a 
new S4 or a RS4 out of the box, you have more torque and HP than what Buffum 
drove .  The big problem with that Jani, is that the race car guys have 
already put forth their opinions and preference wrt the center diff.  Great, 
what about the rest of us?

Jump to the Motorola/Speedvision series report by Mr. Lawson.  In the 
paddock, all championship audis converted road cars (clean sheet designed for 
T*) with welded diffs.  Although the reasons are obvious to me, somehow I 
just don't feel much better about it.  I'm much more optimistic about the 
Haldex, however, not as part time synchro, but as a full time quattro type 
design, ala WRX cars, and I believe that will be the next generation quattro 
(the abs problem will be solved, hopefully retaining the ability for left 
foot braking).  That leaves a 15year gap still, with used car tweeksters 
picking up T* equipped cars and going to town.

That's gonna be one 'happenin' town...

My "biassed" (tm- Kirby) .02

Scott Justusson
QSHIPQ Performance Tuning
'Chicago Lockers'
'87 5ktqw
'84 Urq
'83 Urq

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