Tires: tire cupping, Tire Irregular Wear


Question
I have a 2005 Ford Focus with Potenza G009 tires on it.  The tires were put on less than 15000 miles ago...to replace worn (same kind of tire) tires...I got a rebate on
those, because there was too much wear for the miles on those tires.  I have 84,000 miles on the car & am on the 3rd set of tires.  
the car was vibrating a lot & I took it to Firestone to find out what was wrong with it.  I told them it seemed to be vibrating in the front.  They balanced the front tires.
That didn't help, so I took it to another Firestone ctr. a few days later to get the tires rotated & an alignment.
They did it.  It did not help...they said the rear tires were badly cupped and I need to get 2 new tires. (now those tires are on the front). Yes, they are baddly cupped, not just in one spot but around the whole tire, and only the rear tires.  The ones that were on the front look like new.  They told me I needed new shocks & struts all around.  Why, if only the rear tires were affected?  I had a bunch of people look at the rear shocks & said nothing leaking & the car doesn't bounce when the trunk is pushed down on.  I didn't rotate the tires in the 15,000 miles (my fault) but is there something else wrong here.  I read something about the "toe", is it an alignment problem? Do you think I can get some kind of recourse from Firestone, as they sold me the tires & did the alignments?
I don't know who to trust when it comes to fixing my car.
Thanks.


Answer
Gin,

What you have is irregular wear, which is caused by misalignment and aggravated by insufficient inflation pressure and insufficient rotation practices.  The problem here is that you have lots of gaps that are not their fault.

Even though F/S did the alignments, it is quite possible to have the alignment gradually go out due to sagging springs, worn parts, etc.  This is to be expected as the mileage on the vehicle accumulates.

Also, the interval with the rotation.

Overall I think you need to accept that part of the blame is yours - and part of the blame is "just one of those things".