Locked center diffs v. tire diameter...

[audidudi@mindspring.com (Jeffrey J. Goggin)]

>> Interesting ... Yes, I've driven it briefly on an icy/snowy road and yes, it
>> was loosey-goosey there, too.  I don't understand why there should be a
>> problem, though, since the measured difference in diameter was exactly 5.5mm
>> or about .2" ...
>
>Which is also something you shouldn't really do - run a set of new tires
>on one end and worn out ones on the other end.  Buddy of mine had a 4wd
>truck in for major fixin's, tranny was shot, gee I wonder why, 16 x 8.5
>on the front, 16 x 9 on the rear.

Hmmm ... in my case, the actual difference (which I've measured since my
previous post earlier today) is nil.  When I first installed the larger
width tires up front, the rear tires were about 3/4 worn ... however, this
meant the actual diameter of the two was pretty close since the rear tires
are theoretically taller than the front tires and the difference in wear
equalized this.

This afternoon, the difference in circumference of the now 3k-mile old front
tires and worn-out rear tires is just under .25" so far as I can tell from
my tape measure ... this works out to a .32% difference in diameter which is
about the same as a total tread depth difference of 1.4/32nds between two
identical size tires.  But since the tread is present at both the top *and*
bottom of the tire, you need to divide this figure by two to determine the
tread depth as typically measured, and this works out to a whopping .7/32nds
...  how many cars have front and rear tires that are this finely matched?
Even if you're careful to rotate them every 3k miles, I'll bet the
tread-depth difference front to rear is larger than that at every rotation
interval due to wear.
Is the quattro system -- or any AWD system -- really *this* finicky?  (And
if so, how about the ABS system?)

>When you lock the center diff or expect it to balance things perfectly,
>ie under slippery conditions, a 1% diameter difference is a lot.  An
>inch every 3 yards!  And if the smaller diamter tires happen to wear
>faster - even worse!

True but even identically sized radial tires will differ in diameter
somewhat, due to the air pressure, weight carried on each tire (which
changes the dynamic rolling radius) and plain ol' production tolerances.
Again, I wonder if the quattro system is really this finicky?  Of course, I
don't think Audi really intended for people like me to ever lock the center
diff on dry pavement but my testing has convincingly proven that, with my
car, setup as it is, it's by far the quickest way around an autocross
course.  And until I can keep the inside rear tire from lifting off the
ground in tight corners -- alas, the modifications this would require are
illegal in the class I compete in -- I can't see how this will change...

I realize this thread has the potential to become rather esoteric rather
quickly (I promise not to mention the Torsen center diff!) but I'm curious
about other people's experiences under similar circumstances ... everyone
locks their center diff for track use, right?
 
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   / |      _| o    | \       _| o   Jeffrey Goggin
  /__| | | / | | __ |  | | | / | |   audidudi@mindspring.com
 /   | |_| \_| |    |_/  |_| \_| |   http://people.delphi.com/audidudi
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