At first blush, the rear tires on a front-driver seem kind of like the tail on an elephant; they seem to do so little, you almost wonder whether you'd notice if they were missing completely. And it is true that these tires are generally under far less stress than front tires, coupled to less-sophisticated suspensions and generally living a longer and happier life. All is not always well on the hindquarters of this pachyderm, though, for the snapping hyenas of driver error and mechanical failure are never far behind or below.
On the front or the rear, certain wear patterns almost always indicate certain things. Wear down the center of the tire means overinflation, and excessive wear on both outside edges indicates underinflation. If you have excessive wear along one edge or the other, first determine whether it is sharp, defined and covers less than half the tire, or whether it's softer, more tapered and stretches to about the tire's centerline or further. If it's the former -- sharp and defined -- it's due to the tire leaning in that direction at the top, aka "camber wear." If it's "feathered" -- smooth and stretches -- then it may be because the tire is angled outward or inward at the front. The "tie-in" or "toe-out" misalignment, as well as camber wear, can occur on one side or both. Regular divots along one side or along the middle of the tire are called "cupping" or "scalloping."
It's normal for tires to lean in and out a bit under use. Suspensions are designed so the the tire leans in at the top -- "gains camber" -- under compression to help dig into corners like a slalom skier. Sharp edge wear along the inside of the tire isn't uncommon for car with sagging or worn-out springs, those with aftermarket lowering springs, and cars that regularly carry around a lot of weight in the back seat or trunk. All of these will compress the suspension, causing a camber gain and excess wear. If you're confident that everything's working as it should be, the rear may just need an alignment.
Feathering is distinguishable from edge wear by running your hand over the tread from the inside out to the worn edge; you should feel the small ridges in the tire before you see them. Your left- and right-side tires are linked together by a toe bar, which keeps them pointed either straight down the road or at a slight inward or outward angle. Over time, toe settings can tend to wander out as end links and suspension components wear and suspension geometry changes slightly. You may just need an alignment if the problem is chronic; but if it happens more suddenly, you may have worn end links, ball joints or bushings, bent or broken suspension components, or some combination of them. It's also worth noting that many multi-link suspensions are designed to both camber in and toe-out slightly under compression. So, chronic toe and camber wear could also be due to sagging or aftermarket springs, or an overloaded trunk or back seat.
Cupping is a cyclical failure, meaning that something is either regularly wobbling or bouncing in the suspension. That means you're not going to get away with a simple alignment. Cupping along the center of the tire will more likely indicate a blown shock absorber or a badly out-of-balance wheel, but any component that connects the chassis of the car to the road is suspect. That includes bushings, shocks, springs, strut mounts, wheel bearings and bent or broken large components like the control arms, steering axle hub carriers or axle tube where applicable. Inner-edge scalloping is often a bit more indicative of a failed toe bar end-link, particularly if you see the same inner edge scalloping on both sides. Outer edge scalloping could be many things, though ball joints and wheel bearings are at the top of the list.
