[torsen] Re: "Care to Comment" -> differential t...

QSHIPQ at aol.com QSHIPQ at aol.com
Tue Apr 9 21:03:23 EDT 2002


Rob:
Please feel free to either send Corey's address directly or get him on the 
torsen list.  For now, I'll respond to the post to all lists BUT the main 
list because T*rsen is banned there.   If I overextend my welcome nerding on 
the S car list, please give me fair warning....
In a message dated 4/9/02 5:27:50 PM Central Daylight Time, 
randrews at sbcglobal.net writes:


>Here's one response to that Scott...I'd like to see what you have to say...
>"First of all, you've got a couple things wrong. You are assuming the 
traction of >the rear gets exceeded first....which isn't hardly true. 

I don't assume that at all (a better statement would be "which isn't ALWAYS 
true").  What I do understand, is that the torsen changes engine torque 
allocation based on exactly two inputs of which it can't differentiate:  
Turning radius and traction.

>I will agree that when you transition from trail-braking turn-in, to 
on-throttle, the >lightest corner of the car is the inside rear, and there a 
chance that you can spool >up that wheel because of the lack of grip. That 
can be overcome with technique of >throttle application, and with suspension 
tuning.

I don't understand that argument at all.  The torsen acts based on turning 
radius or traction, that's all.  Power up oversteer isn't a suspension 
problem, it's a torque allocation problem.
 
>Once you get past that point, however, you begin to transfer weight back to 
the >rear of the car, therefore making the tractibility of the rear higher 
than the front. >Especially when you get to full throttle, trying to get to 
the apex, then tracking out >past the apex, the tractibility of the rear is 
much greater than the front.

You are making traction arguments and turning radius arguments as a single 
property.  They are independent variables that *can* affect each other.  
 
>How do I know? How about countless amounts of data and seat time from 42 >
autocrosses in 2001? 

The problem I had with going from autocrossing to 'on track' driving, is that 
they are different animals in terms of handling evaluation.  And effects of U 
and O are usually at a slower speed, and they don't usually become a problem 
until you track them.  YMMV

>When we finally got a high bias Torsen to put in the car, the benefit was >
immediately noticeable. Now that the power-on understeer has been greatly >
reduced, we can apex earlier and get on the throttle earlier, which mean 
higher >corner exit speeds. 

Again, basic understanding of the device is necessary.  IF you use a higher 
bias ratio you potentially shift more torque fore and aft.  AT the limit of 
adhesion shifting 60% of engine torque (4:1 BR) sounds....  Overwhelming and 
oxymoronic.

>How about 4 wheel speed sensors on the Stasis A4 touring car? I've seen 
their >data, and the advantages of the higher bias Torsen were very apparent. 


I don't buy into the argument that a wheel sensor will give you the data you 
are looking for for wrt the torsen, necessarily.  Remember, the torsen is 
TORQUE SENSING, which means by the time you measure wheel speed differences, 
torque has shifted already.  

>You could see the high bias Torsen doing its work, transfering power back 
and >forth as traction levels were changing (which including heavy throttle 
at apex, >bouncing over berms, and heavy throttle at track out). With just 
the 2:1 Torsen in >their car, the car always had some amount of wheelspin in 
the FRONT of the car, >which resulted in on-throttle understeer.

Again, I'm not convinced you understand WHAT the device does while turning.  
IF the torsen was transferring power back and forth, with a 4:1 BR, that 
means when TRACTION is exceeded, then 60% of the torque goes to the front 
wheels, which results in power on U.  Once that condition causes the front 
wheels to spin faster than the rears (traction and turning argument), 60% of 
the power goes to the rears = power on oversteer.  The net effect of this 
oscillation is that A) you aren't at the limit of adhesion or B) you have to 
take a corrective action > steering/throttle/brake to change it, all 
resulting in a slower corner.  One could accept your O argument if you could 
keep the rear bias all the time, but that's not what you are claiming with a 
4:1 BR.
 
>You bring up a good point about locking the diffs, but if you really think 
about it, >you'd be limiting yourself. 

A thorough review of audi racing would contradict that statement.  Solid 
center diffs were found in just about every rally car, and still found in 
racing audis today, rally or track.  A front weight biased car U's under 
throttle, why fight it.  You aren't limiting yourself unless you try to make 
a front weight bias car try to O under certain conditions.  The torsen IS the 
limiting problem.

>A biasing system is dynamic, and can handle changing situations.

Only optimally in a straight line, the torsen follows weight distribution 
EXACTLY.  The PROBLEM with a torsen, is it doesn't know the difference 
between a traction argument and a turn.  This is WELL documented.

> Also, the "championship" winning S4's (the Champion S4's) are running a 
viscous >center (which is also very expensive!). Although a viscous center 
doesn't seem >like the best solution for road racing (widely used for rally), 
it is working for them >because they've made the car bias most of the power 
to the rear of the car at all >times.

Interesting conflict to Dave Lawson's interviews posted last year Corey. And 
if I remember the rules correctly (if memory serves from my search last 
year), diffs are NOT free, but can be welded, which means VC's are not 
allowed(?)

Before you data log playing with the Bias Ratio, I'd suggest a thorough read 
of WHAT the device is and isn't capable of.   Then I'd suggest a simple 
comparo of lap times (on a TRACK) between the torsen high bias ratio car and 
baseline (welded center).  I think you might be surprised (I'm not). The 
problem with racing torsens comes down to predictability.  Too many variables 
affect it's torque allocation. 

HTH

Scott Justusson
 


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