For some reason, it appears that iRobot used two different rubber compounds on my Roomba 400 series vacuum. On one side, the tire has held up magnificently. The other side is as bald as [insert funny analogy here]. I'm not sure why.
It seems like the bald wheel has a softer rubber compound that shreds easily, leaving black rubber dust on the floor, mostly right near our one carpet - a small mat near the back door.
On top of that, it had a noticeable list to the left.
// if you're only interested in the process, and not the back-story, skip the rest of this step
Because I'm usually quite busy, I tried to do a quick-fix. I took a piece of bicycle inner-tube, and stretched it over the bald wheel. Then I swapped the drive wheels from either side, and stuck 'em back on the machine. (I thought that perhaps there was more of a frictional load on the left side wheel).
It seemed to work, because the tire wasn't shredding itself any more. Unfortunately, the drawbacks of this fix were brought up with a comment from my wife.
"What are these black marks all over the floor?"
Well, they seem to follow the pattern of some kind of automatic floor-cleaning apparatus with a limp. Yup, room-bot's spare tire was not up to the task.
However, he remained in this state for several months. Then came my wife's requested birthday gift - a stick vacuum that's really light and works pretty well. Also, it doesn't leave black tracks all over the floor.