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Re: AutoX tire pressure over

Buster Ashmore lamented:

>I ran the 325i in an autox this past weekend with somewhat
>disappointing results (the 2002 is _much_ faster :-) 

Must be the wheels!  ;)  (sorry, inside joke)

>Am I correct that by lowering the front tire pressures relative
>to the back by about 3-4psi I can reduce the understeer
>tendencies? I would much prefer that the car oversteer
>slightly. I want to leave the car stock, so tires/pressure seem
>to be my only options.

I've been hastily scanning the digest the last couple weeks and have seen =
lots of questions about correct tire pressures for street tires during =
autocrossing and track events.  I thought I'd toss in my $.02 FWIW.

First, remember that street tires -- even so-called "high performance" =
tires like the A509 -- are not designed for racing or competitive =
autocrossing, but rather for high mileage and safe operation in a *wide =
variety* of operating conditions.  Hence, no matter what you do to them =
they may never feel "right" if you're used to the feel of R-compound tires=
.  That being said...

Assume there is an optimum pressure for a tire -- a single pressure that =
provides the most grip.  Going up *or* down with pressure from this =
optimum pressure will *reduce* that tire's grip, and therefore the grip =
at that end of the car.

So, if we assume 40psi is the optimum for an A509, then *lowering* the =
front will *reduce* the grip at that end -- more understeer.  By the same =
token, *raising* the *rear* by 3-4 pounds will reduce the grip at that =
end -- more oversteer.

If you lower the front and get *oversteer*, then 40psi wasn't the optimum =
pressure because now you're getting more grip there than you did at =
40psi.

So it's not only the differential between front and rear that matters, =
it's the combination of pressure differential *and* knowing the optimum =
pressure that makes it all work.

Street tires tend to react differently to different surfaces more than =
autocross or race rubber and it can take a lot of experimentation to =
determine their optimum pressure under extreme (race) conditions.  But it =
sure is fun figuring it all out, isn't it?!

Hope that helps.  Back to lurk mode,

Bob Tunnell
bobt@domain.elided
95 M3

P.S.  Buster, Goo Gone will work wonders removing all that cone blood. ;)