Thinking about this thread, I just checked my 6-speed bike and spotted that I had come perilously close to eroding its righthand chainstay. The edge of the tyre was only about 1mm from the stay at the outermost part of the lateral wobble of the slightly untrue wheel's rotation. I hadn't quite seated the wheel axle properly before doing up the quick release. After sorting that out, there is now an extra few mm of clearance.