2005 focus zx4. 115,000 miles. I was having a vibration/shimmy in the steering wheel,so thought it might be a tire out of balance. I checked the front tires first,looked fine,no weights missing. Back passenger side tire was the bad tire. I am no tire expert,so I have no idea what would cause this,but the tire has an almost bald spot all the way across the tread about 3 inches wide and is "wavy" in other parts of the tread. The driver's side rear tire is a little like this,but not near as bad. the tires are firestone precision sports and have about 20,000 miles on them. Is this just a tire failure or is something wrong with the suspension,and what would it be? The only thing I could find in a search was the inner tire wear on the rear.