[V8] Different Tire Sizes

[John Bysinger]

I was just reading through this thread and of coursehad to toss in my $0.02.
One of my points is illustrated here, the 'something has to give' part.
This is definately correct, there will be give at the tread contact point,
which means excessive wear.  The part that was touched on earlier that I
wanted to bring up was with regards to the diffs on the v8q's.  Now I'm new
to v8's but from what I've gathered here and elsewhere, the v8 has a torsen
rear, an actuated diff in the center, and an open front.

With the torsen, it's a limited slip differential, and a pretty clever one
at that, it uses the natural friction in a gear meshing as the limiting
factor.  But the important part is it is a limited slip, and while tire
variation across the axle isn't desired, it is likely within it's slipping
range, and not likely to cause catestrophic damage.  BUT, and this I'm sure
is the motivation behind the recomendations, considering it uses gear
surface friction for torque splitting, this will cause it to wear faster.

Similarly, I'd imagine that the center diff is either friction band or
clutch based, both a limited slip arrangement as well, also allowing for
differentiation, but again subject to higher wear if in a different tire
size situation.

It's also cool to see others here with the 4x4 addiction as well, my other
addiction is an '83 jeep cj-7 set up for rock crawling.  It has full locking
diffs front and rear (lockright and a detroit) so it can get up the rocks.
In a full locking situation diameters and axle ratios are critical if you're
on a hard surface.  And if you have doubts about the stresses in a locked
axle trying to differentiate, try driving with a locked front diff sometime,
with the wheels trying to differentiate the turn it will literally fight you
from turning the steering wheel, and that's on gravel too.

Now will your v8q blow up on the freeway from different tire sizes?  I'd say
that's incredibly unlikely, however you might find that down the road
there's an odd 'looseness' in the torsen and end up replacing it because of
the wear causing slop in the gears.  With odd sizes that day may arrive in 7
years rather than 10.

Ok, so maybe that was more than $0.02, I just hoped this doesn't turn into
similar discussions I've seen on off road boards/lists about newer fulltime
4x4 vehicles, and rumors of drivelines exploding on the highway.  Nothings
gonna blow up. :)

-John
'90v8q

-----Original Message-----
From: v8-bounces at audifans.com [mailto:v8-bounces at audifans.com]On Behalf
Of Michael Thomas
Sent: Wednesday, October 26, 2005 11:23 PM
To: v8 at audifans.com
Subject: [V8] Different Tire Sizes


Without getting into what the differentials do, if
different tire sizes are used at the same time then
something has to give. When someone says that the
difference between 65% and 60% of a sidewall is tiny,
they haven't done the math.

215	215
60	65
15	15

25.16	26.00    diameter in inches
79.03	81.69    circumference in inches
801	775      revolutions per mile

If the driveline/differentials do not slip then the
slippage must be between the tires and the road. Even
if not perceptable to the driver, there is a lot of
tire scuffing going on.

(Yes, I know the actual revs per mile is higher
because the tire is not truly round, but the 25
revolution per mile difference is still the same.)

It wouldn't matter to me what size you decide to put
on your quattro, as long as all 4 were the same size.

Michael Thomas
'93 Pearl w/ 17" BBS RX






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