Tires: Tire cupping, rear struts, tire cupping

Question
I went to have my tires rotated the other day but the shop said the rear tires were cupped and would cause a lot of vibration if I did. The tires have aprox 15000 miles. They are cupped but my question is what could have caused it and should I get new ones or ride these out? They are Firestone, I should have known better. The shop said my rear struts were probably shot. I remember from my youth that the way we checked shocks was by bouncing the car. How do I know if the struts are bad? These are on a 99 Camry.

Answer
Roger,

"Cupping" is a term that is frequently misused and what I think you have is irregular wear.

The usually cause of irregular wear is misalignment and this is aggravated by insufficient inflation pressure and insufficient rotation practices.

With 15,000 miles of non-rotation, even a little misalignment will cause irregular wear.  And this is not because of the brand.  Any tire will do this.

"Cupping" - as opposed to irregular wear - is caused by bad shocks.  Unfortunately, shocks have gotten so complex that I don't think the "Bounce Test" works any more - particularly since I have seen shocks fail "hard" - too little movement, which would not be revealed in a "Bounce Test".

I suggest you get an alignment and get the tires rotated and live with the vibration for a couple thousand miles.  That way when you get new tires, you'll be sure they won't do this - provided you rotate regularly.