Like many of you, I generally use an old tire as well. Although I do own a trainer tire, and will install it once the other is completely trashed.

We've held trainer rides here in the shop for over ten years, and I have seen a couple of people in that time that would leave black all over the wall. At least with these people, the reason that happened was that their rear wheel was out of round. When they would adjust their flywheel so that it made contact all the way around the circumference of the wheel, of course there were areas where it really dragged... hence the rubber coming off. So I'm not sure whether this is the issue here as well. You could always check out the rear tire and it will definitely show evidence of that if it is true for you.

Merry Christmas everyone!!