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

Derek Pulvino dbpulvino at hotmail.com
Fri Oct 26 22:35:02 EDT 2001


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.

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.  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.

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

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.  Yes, 
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...

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.  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.

Derek P

>From: QSHIPQ at aol.com
>To: dbpulvino at hotmail.com, jolly at zoomtown.com, s-car-list at yahoogroups.com
>CC: torsen at audifans.com
>Subject: Re: [s-cars] Tire mix em up?
>Date: Fri, 26 Oct 2001 08:50:59 EDT
>
>I wouldn't mix an match tire sizes ever.  The torsen center diff has brass
>shims which are very heat sensitive.  Different tire sizes will bias the
>torsen to something other than the 50/50 it constantly hunts for.  This can
>lead to premature center diff failure.
>
>Mixing and matching tire compounds/size adds another variable into a
>complicated traction matrix of the center diff.  I'm not at all convinced
>that a 60/40r quattro will 'O' changing tire tractive abilities.  Remember,
>once you exceed the tractive ability of a tire and the rear driveshaft
>attempts to spin faster than the front, 56% of engine torque will shift to
>the opposing driveshaft, 78% of engine torque to the steering tires of a
>60/40 quattro doesn't at all sound like O to me.
>
>Torsen equipped audis *can* oversteer.  But that is a moment in time, not a
>constant.  That would require a fixed split of something other than 50/50
>equilibrium the audi quattros center diff constantly tries to achieve.
>
>Also, a rear tire with less tractive ability will also reduce the
>effectiveness of the rear braking ability.  Those with bigger brakes may 
>not
>notice, but both bigger brakes and stock G60  will become less effective 
>(or
>ar more likely to get overloaded sooner).
>
>This isn't an area I'd mess with.  Keep tire size and compound constant.
>Change effective spring rates, ride height, and weight distribution.
>
>My .02
>
>Scott Justusson
>
>Derek writes:
>only thing I can add to this thread is just be careful with the mix and
>match.  Depending on the characteristics of the tire, the 
>handling/breakaway
>bias of your car can change accordingly.
>
>I see the question coming up from time to time, how to make the quattros
>oversteer more...well there you have it.  That or deeper braking.
>
>As it stands, I think I have one of those rare oversteering quattros. 
>Change of rear toe helped some, but those rear tires of mine are terrible. 
>Mildly put it can get um, interesting in the wet!
>
>Derek Pulvino
>"oversteering in seattle"
>
> >From: jolly at zoomtown.com
> >To: s-car-list at yahoogroups.com
> >Subject: [s-cars] Tire mix em up?
> >Date: Tue, 23 Oct 2001 01:04:04 -0000
> >
> >Hi,
> >Unfortunately, I had some prankster(s) poke pinholes in my pass side
> >sidewalls this weekend.  Which I did not "notice" for 2 miles driving-
> >   boy can the tires get hot.
> >
> >I'm using 235/45/17 Yoko AVS Z4's  which is no longer being sold in
> >that size.  So now I have 2 good tires which I cannot match.  Since I
> >have a set of tires for summer/track, and this is street use only,
> >i'm thinking of using another high-performance-All-season.  Any recs?
> >Light snow for about 4weeks total/yr. in my locale.
> >
> >Jolly
> >
> >BTW I am assuming it is bad to have 2 sizes on the car at the same
> >time.  Ie:  245/40/17 and 235/45/17 will affect the transmission, yes?
> >


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