[A4] tire sizes

T. Jackson mancubus99 at hotmail.com
Fri Mar 9 16:48:24 EST 2007

Actually, I've had 2 diffs break on me.  This was in my '95 90 Quattro
Sport.  The first one went at around 180,000km and it was replaced with a
used one which had ~ 110,000km on it.  That one went at 220,000km at which
point I sold the car dirt cheap and moved into an A4.  The first diff made a
horrible noise which got worse progressively over time to the point where
the car was an embarrassment to drive.  The problem was worn bearings.  The
fluid levels were ok so I'm not sure what happened.  The second one was just
starting to make the same noise I was all too familiar with so I decided to
let it go before it got worse.  

Bad luck?  Maybe, but I think that car was a lemon in every sense of the
word.  I had some very strange things happen to that car, i.e. the accessory
tensioner bolt going into the block sheared off not once but twice and had
to be drilled out. The rear main seal was replaced twice due to excessive
leaking. The Cat material broke apart suddenly while I was out on a date
causing the exhaust to clog and rendering the car inoperable. (needless to
say I didn't get a second date). And so many other problems.  Some would say
I am crazy for getting back into another Audi after what I went through with
that car but by comparison my A4 has been more like a Honda compared to my
90 (knock on wood).

-----Original Message-----
From: a4-bounces at audifans.com [mailto:a4-bounces at audifans.com] On Behalf Of
Sent: Friday, March 09, 2007 2:30 PM
To: a4 at audifans.com
Subject: Re: [A4] tire sizes

hey guys, just have to bring a little common sense to the party. different
tire sizes would cause the wheels to rotate at different speeds, right?
which also happens when?

every time you turn the steering wheel. and the inside wheels are travelling
slower than the outside wheels. that's why the diffs are open not locked. if
minor deviations in rolling diameter were an issue, the diffs wouldn't last
very long at all.

besides - it's not a "perfect" world. recalling werner and his fabulously
principles of uncertainty; no 2 sides of the road, no 2 tires, no 2 diffs,
no 2 mating surfaces, etc etc etc are perfect so there is always going to be
some 'slop' in the system. audi knows this, and while sure, it's a good idea
to keep tire sizes as similar as possible, the difference between 2 same
sized tires with different wear is SO tiny it's just not going to have any
meaningful effect. it would be the same as running 1 tire a few psi low. not
"ideal" but not going to break anything.

that said, if the other tire is close to the end of it's life, sure get a
new pair of tires since you'' be at the shop anyway. but if it's got more
than 30% life left  i'd just get 1 new tire and be done with it. im sure the
audi guys factored in some % deviation in tire diameter in the differentials
to account for spare tires, odd tires, different tire pressures and the rest
of the 'slop' in the system. if they hadn't there would be a trail of broken
audi diffs littering the roadways.

and i've NEVER heard of an audi diff breaking. it's about the only part that
doesn't break.
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