Originally Posted by
Fixitman
The front wheel tracks when you turn, the back wheel does not so as you make a turn the portion of the rear tire contacting the road has to pivot on the contact patch. That is more likely the cause of the flat spot and why you see it on the rear and not the front. Also more of your weight is typically on your rear wheel so that tire is going to wear more. Anyway that’s two bit theory on it.
During a turn, though, the tires are tilted, and won't wear a flat spot -- they should wear evenly across the tread instead. You could probably say that the front is creating more turning force because it's changing the heading of the bike, so it should scrub a little more.
The rear does take more weight, but it's also handling acceleration and braking duties (and, because it's so easy to lock up, it sees more skidding than the front ever does). I'd think that that's what's creating the flattened tread.