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When I went to my 4th set of PZero Corsa tires on the 2008 SL
a few days ago, I lost my tire pull virginity. Yep, that's right, my
first time! As it turns out, one of the two front tires did turn out
to be bad: minutely misaligned outer bead, and a seemingly more
soft side wall on one side, resulting in conicity, which in turn did
cause my car to pull left.

During my first drive I noticed that the silver stripe at the CF top
of my steering wheel suddenly pointed right... even though I was
going straight, and that the pull to the left was especially strong
under heavy "kick it" acceleration.

An initial visual inspection only suggested that the gap between
the top of the tire and the bottom of the fender seemed ever so
slightly smaller on the front left than on the front right. It looked
as if someone had lowered the left side by almost 1/2".

So... back to shop I went. Now, while the front wheels could be
moved to the rear (to see if the pull disappears) the back wheels
don't fit on the front (since they are wider). So we started with
a different experiment: a different set of wheels + tires. No pull.
So the suspension and alignment (i.e. the car) probably were not
the culprit. Next we swapped the two front tires. We could have
tried a simple left-vs-right swap, i.e. running them backwards for
a very short test drive, but that still wouldn't have told us which
one to blame. So instead we swapped with a full unmount/mount,
in the course of which we already started to suspect what used
to be the front left, and now became the front right -- that tire
just didn't look and feel right. And sure enough, once mounted on
the right, at least the lowered suspension Ghetto look was gone.
The car still pulled left of course, but I could begin to tell that it
was now coming from the front right side.

One day later... a replacement tire from TireRack was delivered,
went on the right front wheel, and voila, all is well again!

Now all that's left is for UPS to pick up the bad one, for return to
TireRack. So far I'm very pleased with how painless this has been
in terms of process: a phone call, describing the problem, and the
replacement was on its way.

Tire Tech Information - Diagnosing Tire Pull

Oh and no, the difference in circumference (left vs right side wall)
is too small to show up with my simple "piece of string" method. It
really only shows under load i.e. when on the car.

So, why did the shop not notice this during the test drive after the
initial installation? Because they were taking it easy: it was pouring
heavily during that week. (Insert bad memories of gvan going for an
alignment, and suffering from a bad "test drive" in the rain, followed
by many weeks without a car.)

Anyway.

So now I know how tire pull feels. :)

I probably wouldn't have noticed it, had it happened on some cheap
car with shitty suspension and less-than-perfect alignment. :-o

Drive safely!
 
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