[torsen] Re: [s-cars] Tire mix em up?

QSHIPQ at aol.com QSHIPQ at aol.com
Fri Oct 26 23:27:02 EDT 2001

As the ascribed torsen nerd, I'll insert my comments below.  I don't doubt 
that you might have increased oversteer moments, it's just not possible for 
the device to consistently give them to you.

>Couple of points to clarify.  First off, I was refering to mixing tire 
>types, not tire sizes for the same reasons sighted.  I wanted to make sure 
>that the potential purchaser was aware that the chassis bias may be changed 
>through mixing and matching tire types.

Ok.  So noted.  But if the overall diameter is the same, it doesn't matter.  

>As to oversteering situations, in my case, I'm not talking about theoretical 
>situations, this knowledge is garnered by actual experience.  My car will 
>and does oversteer, and more than anything I credit this behavior to having 
>two different types of tires in the same size from front to rear.  

I'm not a theorist either, in fact Derek, I went from practical back to 
theory to try to explain the O-U-O-U phenomenon I experienced.  The problem 
I'm really having is that O can happen, but the device wants 50-50 f/r 
distribution, and constantly strives for that equilibrium.  On a 60f/40r 
weight biased chassis, I don't see O happening alot.  If the rears lose 
tractive ability, the torque goes to the front, more U.  It's the nature of 
the device.  I guess how much O moment you have depends on how you approach a 
corner.  In the traditional in like a lamb out like a lion, your O moment 
won't last long.

>To add 
>credence to this point, I have in the past switched rear to front, and found 
>the back end nigh imposible to break loose.  I did not stay with this setup 
>as along with having a lower level of grip, the tires now on the rear of my 
>car have much worse straight line tracking as well.  Specific/details 
>concerning breakaway characteristics can follow as necessary.

The problem is this Derek.  The torsen doesn't know if you are turning or 
accelerating.  It treats both conditions equally.

>Good point with the torsen biasing.  I had though about that, but from the 
>opposite perspective; as the front tires start loosing traction it becomes 
>easier for the tires to also spin so power is transfered to the rear.  

Then the rears are overloaded because of lower tractive ability, power goes 
back to th front.  Starting the o-u-o ocillations that is the definition of 
the torsen cars design tendency.  As I've proposed before, major driver 
correction is necessary mid turn when 56% of the torque is going forward and 
backward.  By the definition of "driving at the limit" you don't have the 
ability to allow/control this.  Motorola/Speedvision S4's weld the torsen for 
faster times, all teams.  They can't change the diffs to VC (like audi sport 
seems to prefer) because of the rules, but existing diffs can be welded....  
Guilt by association would tell me, if the race car guys go faster welded....

>However, as I thought about it today I think yes equilibrium really is 
>hovering right at 50/50 range. 

That is the nature of the device as equipped in your car from the factory.

 >Any transfer of power as related to wheel 
>spin will be short lived, thus we'd wind up more or less at an even split.  
>So yes, the quattro drive system itself is not inherently oversteering 

The quattro driveline is U biased.  60f/40r weight distribution with 50/50 
torque split is U by definition.  A tight turn and a lot of power can change 
that, but you are back to U by either straightening out the wheel or breaking 
traction rear.

>I would however argue that different tire tractions is a legitimate tuning 
>technique, just may not be the best place to start on the quattros.  

No problem as stated.

>adding by subtraction seems self defeating.  Situations where it does work 
>as a portion of the chasis tuning? Porsche 911 or Chevrolet Corvette.  I 
>know these cars also have the added burden of rear drive (or rear weight 
>bias), but...

Again no problem as stated Derek.  In a torsen quattro, just Not Applicable.

>I think where the confusion comes in is you are reading the situation as 
>I've experienced it to be a rear driver like power-on oversteer.  Remove the 
>variable of power delivery for a moment and think merely of the chasis.

The chassis inherently understeers because of the steering wheels supporting 
60% of the weight.  Take the power out and think of merely the chassis, you 
have U.  I'd be suspect of finding another tire that would compensate for 
that kind of weight transfer.  Regardless, as soon as you add power, you have 
that variable of power back.

 > As 
>I am seeing it, the rear of the car has less traction than the front, and 
>thus has a lower breakaway speed.  Yes, this situation can likely be 
>mitigated by doing all of your braking before the corner and powering 
>>through, but what fun is that.

Faster lap time?  If you are coasting thru a turn, you are praying - my 
favorite passalong line from Mr Woodner hisself when I went to prorally 
school years ago.
The problem is Derek, as you add power, you invite those spiders into the 
party.  cof is only part of a very large matrix in the torsen torque 
allocation while turning.  The very reason you will be hard pressed to find 
torsens in audi sport's various racing theatres.

The documentation of the device and quattro chassis don't seem to support 
your claim as presented.  For more, get SAE 885140.  Happy to carry this 
discussion wholly to the torsen list if you have more than casual interest.

Scott J
Your torsen antichrist

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