Originally Posted by progre-ss
Looks to me like the tire was rubbing the chainstay. If it was flexing, the paint would look more like it was flaking rather than the polished look it has. A good question to ask Tony @ Iro.
What I meant was, is the chainstay flexing enough to *'cause* the tire to rub? If it was flexing along its entire length that wouldn't flake the paint, but might cause enough tire-movement to 'cause the tire to rub, would it not? The tire is true, and centered in the chainstay, so it's definitely not rubbing all the time.
On closer examination it is apparent that the spot is towards the back of the little divot in the chainstay that allows for tire clearance. I swapped the chain and took an additional link out of the new one (pulling the tire back, more towards the center of the divot). Maybe that will provide enough clearance to eliminate the problem.
-Trevor